Search

In many temperate regions of the world, conifer species have been planted to stabilize soils and promote site recovery on former hardwood sites that were cleared for agriculture. In many areas of eastern North America, these conifer plantings consisted of introduced (non-native) Pinus species planted on abandoned agricultural land once dominated by mesophytic hardwood species. These plantings constitute a shift in overstory composition away from native hardwood species with nutrient-rich litter that decomposes more quickly towards Pinus species with recalcitrant litter that may alter soil chemistry and nutrient availability. To examine how edaphic conditions associated with long-term introduced Pinus species occupancy are related to forest regeneration and herbaceous-layer diversity and composition, we sampled a total of 97 plots in planted Pinus echinata and Pinus strobus stands and naturally regenerated hardwood stands growing on two ecological landtype phases (ELTPs) of southern Indiana, USA forests, Fagus-Acer saccharum/Arisaema Mesic Ridges, and Acer saccharinum/Boehmeria Bottomlands. We collected vegetation data and analyzed soil samples to examine herbaceous-layer species distribution across gradients using non-metric multidimensional scaling (NMS) ordination. Two-way ANOVA was used to examine differences in individual species and species functional groups across stand types (P. echinata , P. strobus , and native hardwoods) and ELTPs. Our results show that differences in soil chemistry resulting from Pinus spp. occupancy were associated with differences in the composition and distribution of herbaceous-layer species in ordination space. Species across stand types and ELTPs were distributed across dominant gradients related to litter depth, cation exchange capacity, cation content, and soil aluminum concentration. Hardwood sites had significantly greater herbaceous-layer cover (139.6 ± 8.0 %) than P. echinata (48.5 ± 6.2 %) or P. strobus sites (81.7 ± 7.7 %), as well as greater herbaceous-layer species richness and diversity (mean species richness was 46.9 ± 2.1 on hardwood stands vs. 33.3 ± 1.6 and 36.4 ± 2.0 on P. echinata and P. strobus stands, respectively). Pinus echinata stands contained a greater density of woody regeneration, including Quercus spp. (201 ± 48 saplings ha−1) and Fagus grandifolia stems (415 ± 82 saplings ha−1), both of which occurred in greater density than Acer saccharum (163 ± 93 saplings ha−1) and A. rubrum (70 ± 64 saplings ha−1). Our results suggest that pine occupancy has created divergent successional trajectories in comparison to hardwood stands. These differing trajectories may offer both challenges and opportunities for restoration efforts. For example, the greater abundance of Quercus reproduction under P. echinata on ridges, combined with less productive soils, may allow Quercus stems to be promoted into the canopy with less competition from mesophytic competitors. • We studied vegetation and soil in non-native P. strobus and P. echinata plantations. • Plantations were compared to native hardwood stands on ridges and in bottoms. • Herbaceous species were distributed along edaphic gradients related to Pinus occupancy. • Hardwood sites had greater herbaceous-layer cover, particularly in bottoms. • P. echinata stands contained more woody regeneration, including Quercus spp. [ABSTRACT FROM AUTHOR]

Tree harvesting practices that enhance genetic diversity are essential for the long-term sustainability of our forests. Using eSSRs, which are markers within expressed genes and thus, potentially subjected to natural selection and reveal adaptive potential, this study examines the effects of selection (SE) and shelterwood (SW) management systems using unmanaged (UM) stands as reference on the genetic diversity of sugar maples in the Northern Forest, USA. Comparison between management types shows no difference between SE and SW, but compared to UM, both have lower allelic richness (A R) while only SE has lower expected heterozygosity (H E). Size class comparison shows that seedlings have higher A R and H E than poles and mature trees. Two-way interactions between management types, size classes, and study stands from each of the four states that comprise the Northern Forest region (Maine, New Hampshire, Vermont and New York) were also examined. Results show that seedlings in UM and SW have higher A R compared to all size classes in SE and poles and mature trees in SW, while only seedlings from SW have higher H E compared to poles and mature trees in SE and SW. Only in Maine was there a difference in A R between SW and SE stands per state. Seedlings in New Hampshire and Maine have higher H E , as well as seedlings and mature trees in New York compared to the poles in New Hampshire and mature trees in Maine. Overall, the results indicate genetic erosion in sugar maple stands in the Northern Forest region. This genetic impact may be due to several factors including the intensity of cutting associated with SE and SW systems and size of the canopy gaps created. Higher seedling genetic diversity in SW compared to SE may be due to their more open stand structure that facilitates gene flow and seedling recruitment. When using SE, longer cutting cycle paired with lower level of residual basal area is recommended. This approach allows for a more open stand structure that could facilitate pollen and seed exchanges from the stands, improving their genetic diversity. In both SE and SW systems, leaving high-quality growing stocks will enhance their genetic diversity and ensure their sustainability. This study increases our awareness of the potential genetic impacts of forest harvesting practices and guides the development of recommendations to alleviate genetic erosion in sugar maple stands. • The effects of selection and shelterwood management on genetic diversity were examined using unmanaged stands as reference. • Selection and shelterwood generally have lower genetic diversity than unmanaged, with no difference between them. • Seedlings have higher genetic diversity than poles and mature trees, with no difference between these two. • Shelterwood seedlings have higher genetic diversity than its poles and mature trees and most size classes in selection. • Lower seedling genetic diversity in selection compared to shelterwood is a genetic erosion associated with management. [ABSTRACT FROM AUTHOR]

The regeneration dynamics of sugar maple-dominated stands in northeastern North America are intricately influenced by global changes such as soil acidification, invasive species, forest management, and climate shifts. A notable challenge is the expansion of American beech and its bark disease, threatening mature trees and ecosystem services. Although sylvicultural practices aim to control beech regeneration and enhance high-value species, their impact on understory plant communities remains poorly documented. In Valcartier, eastern Canada, we conducted a field experiment to assess the short-term effects of soil scarification and diverse soil amendments (nitrogen fertilization, liming, wood ash) on understory plant communities in a beech-invaded sugar maple stand. Over four years, we inventoried vegetation and measured various biotic and abiotic variables. Results showed that soil scarification significantly reduced diversity metrics like Simpson dominance, functional diversity, and redundancy (a metric correlated to community stability and resilience to disturbance). Negative effects of scarification were exacerbated by nitrogen, lime, or wood ash additions. Lime amendment without scarification enhanced diversity, suggesting its potential to support understory community stability. While scarification and amendments can promote sugar maple, yellow birch, and pin cherry seedling establishment, they have collateral effects on the understory, impacting productivity, diversity, and resilience in the short term. Land managers should exercise caution when combining these treatments, as they may transiently alter the stability and resilience of the understory and other forest functions. A more sustainable approach involves applying scarification and amendments on small, well-distributed patches within the targeted stands for treatment, avoiding extensive application over large areas. • Scarification reduced the stability and resilience of the plant community. • Negative effects of scarification are amplified by fertilizer, lime, or ash. • Lime has the potential to support plant community stability and resilience. • Seasonality and current site conditions shape plant diversity structure. • Scarification would be advisable to use only on well-dispersed patches. [ABSTRACT FROM AUTHOR]

Litterfall is a major pathway for transferring aboveground biomass to the forest floor and thus plays an important role in building forest soil carbon stocks. However, inter- and intra-annual variability of litterfall remains poorly documented, especially in North American temperate and boreal forests, due to the lack of recent long-term studies at high sampling frequencies. This potentially creates uncertainties in estimates of forest carbon budget models. The objectives of the present study were to 1) quantify the mean annual flux, interannual variability, and seasonality of litterfall in three sites (dominated respectively by sugar maple (Acer saccharum Marsh.), balsam fir (Abies balsamea (L.) Mill. 1768), and black spruce (Picea mariana (Mill.) B.S.P.)) in eastern Canada over a period of 22–32 years, 2) relate the litterfall amounts and temporal variations to the changes in the size of major organic matter pools in these ecosystems, and 3) compare our litterfall estimates with reference values used in national greenhouse gas inventories. Litterfall production decreased from the sugar maple to the balsam fir and black spruce sites, preponderantly due to species composition. Litterfall evolution was related to the aboveground biomass of live trees in both conifer sites; in contrast, in the broadleaf site, changes in forest composition and structure were apparently the main drivers. The litterfall seasonality varied between broadleaf and conifer sites and could be explained by a sigmoidal model. Substantial departures from the seasonality for some given years were likely due to important climatic anomalies. Forest floor biomass remained stable over time at all three sites despite the increase in litterfall at the balsam fir and sugar maple sites and rapid forest floor turnover at the latter site. Our analyses of litterfall suggest that reference values from the literature used for national greenhouse gas inventories underestimate annual litterfall and forest floor carbon stocks for temperate and boreal forests. [Display omitted] • Litterfall evolution was related to aboveground biomass only in conifer sites • Mean annual flux and seasonality of litterfall varied by forest type • Climatic anomalies altered annual litterfall production and seasonality • Carbon budget model underestimated carbon stock of litterfall and forest floor [ABSTRACT FROM AUTHOR]

Temperate forests face a "regeneration debt" reflecting limited tree seedling survival and/or growth. Monitoring demographic processes allows us to identify when and how these debts are incurred and can help identify the factors responsible. If rates of survival and growth are steady, we can easily estimate cohort half-lives and the times it takes cohorts to transit through a range of heights. Dividing the estimated half-life by the transit time yields the Regeneration Ratio (RR). This new metric efficiently integrates survival and growth data into one dimensionless index, facilitating comparisons among regions, treatments, and species. Demographic data from 17 cohorts of white pine (Pinus strobus), sugar maple (Acer saccharum), and eastern hemlock (Tsuga canadensis) seedlings tracked for 7–20 years in the upper Midwest, USA, yielded different half-lives, transit times, and RRs among sites, experimental treatments, and species. Overall, RRs ranged from 0 to 4.8, showing higher sensitivity to local conditions than half-lives or transit times. Half-lives varied significantly among species, dominated variation in RR, and were uncorrelated with growth. Thus, increasing survival may enhance regeneration more than forming gaps to spur growth. Species differ in how RR covaries with transit time and half-life reflecting their distinct physiologies and life-history traits. Within hemlocks, regional differences in white-tailed deer density and forest and wildlife management increasingly affected survival and growth between 1991 and 2011. Tracking at least 50 seedlings for at least 3 years may yield useful estimates. These metrics give managers tools to evaluate and track regeneration success over time and across landscapes, helping them to identify and respond to factors threatening tree regeneration. • Demographic data on tree seedling surival and growth predict recruitment success. • We analyze seedling survival as a cohort's half-life and growth as its transit time through vulnerable sizes. • Dividing a cohort's half-life by its transit time yields a useful comparative metric we term the Regeneration Ratio (RR). • We compare half-lives, transit times, and RR's using field data from white pine, eastern hemlock, and sugar maple. • The Regeneration Ratio provides a convenient, versatile, and sensitive measure of regeneration success. [ABSTRACT FROM AUTHOR]

Climate, soil, and competition factors jointly drive tree growth variability at local and regional scale. However, the comprehensive interaction of these factors and their combined effects on tree responses within their environment remains poorly explored in current research. Using a detailed forest inventory dataset in Québec, we examined tree growth of balsam fir (Abies balsamea Mill), black spruce (Picea mariana Mill), red maple (Acer rubrum L.), sugar maple (Acer saccharum Marshall), white birch (Betula papyrifera Marshall), and yellow birch (Betula alleghaniensis Britton), as a function of competition for light and space with neighboring trees, climate and soil-related variables. Interactions between all these variables were considered in a Bayesian model predicting tree growth. The amount of light received by trees was the main variables explaining tree growth except for coniferous tree species which was influenced mostly by climate variables. Among the studied species, only red maple and white birch exhibited increased growth under warmer conditions. Intraspecific competition had strong species-specific impacts, varying from negative effects for balsam fir, to positive for red maple and yellow birch. Interactions between climate, soil, and competition played a crucial role in shaping growth patterns, especially for sugar maple, and black spruce that strongly responded to a combination of climate and competition factors. In general, tree growth also increased with the soil cation-exchange capacity (CEC), especially when higher CEC is coupled with higher temperatures and precipitation, except for black spruce. While anticipated climate conditions in Quebec, even under the most optimistic scenarios, will have a strong negative impact on the tree growth of most tree species, management can mitigate this impact by promoting tree diversity with more complex stand structures. [Display omitted] • Shade emerges as a prominent factor explaining tree growth for deciduous tree species, while climate plays a significant role for conifers. • Competition shape tree growth responses within climate and soil gradients but the direction and strength varies with the tree species. • Soil-climate interactions impact tree growth; red maple show positive growth rate to higher temperatures but declines under low soil fertility. • Even in optimistic climate scenarios, climate change may impact tree growth, especially for black spruce, which has its southern limit in Quebec. [ABSTRACT FROM AUTHOR]

Sugar maple (Acer saccharum Marsh) is a key species in the northern hardwood forests of North America and management has historically focused on promoting its regeneration and yielding high-quality timber. A decrease in the abundance of sugar maple to the advantage of American beech (Fagus grandifolia Ehrh.) has been observed over the last decades. Together with the spread of beech bark disease, the understory dominance of American beech has often resulted in depleted, low vigour forest stands (i.e., with a low proportion of healthy trees). To favour regeneration and growth of sugar maple, increasing harvest intensity of partial treatments has been suggested, but the harvest intensity required remains unclear. In this study, we monitored the radial growth of saplings in 12 northern hardwood stands located in the Province of Quebec, Canada. These were commercially harvested with removal intensities ranging from 0% to 80% of the basal area. Using generalized additive mixed models, we investigated the effect of harvest intensity on the growth response of both species. Our results revealed that American beech saplings consistently had a much stronger radial growth response than sugar maple after harvest, regardless of treatment intensity. Consequently, modulating the intensity of partial harvests cannot singlehandedly favour the growth of sugar maple regeneration where American beech saplings dominate the understory. Additional silvicultural interventions are thus needed to control advance understory beech. • Partial harvests result in growth releases for both maple and beech saplings. • Beech has a stronger growth response than maple regardless of harvest intensity. • Irregular shelterwoods don't meet their goal of favouring maple growth over beech. • Other silvicultural interventions are needed to control beech understory dominance. [ABSTRACT FROM AUTHOR]

• American beech sprout density increases with increasing percentage of the harvest. • Beech sprouts grow faster than seedlings of all three species. • Beech seedlings and sprouts have higher survival than the other two species. • High growth and survival, and low palatability allow beech to dominate the saplings. In northern hardwood stands, silvicultural treatments aimed at regenerating sugar maple and yellow birch, such as site preparation and mechanical control of American beech saplings, often results in the formation of dense thickets of beech regeneration. The objective of this study was to compare the growth and survival of beech regeneration originating from stump sprouts, root suckers and seedlings, with yellow birch and sugar maple regeneration in partially harvested stands under cervid browsing pressure. Over an eight-year period, 997 individuals of the three species were monitored in three northern hardwood stands dominated by sugar maple, yellow birch, and American beech in Quebec, Canada. Each stand included replicates of four partial harvesting treatments and a control, providing a gradient of light and competition conditions. Beech stump sprouts and root suckers had higher growth and survival probabilities than seedlings of all three species under all light and competition conditions. Results suggest that treatments triggering beech sprouts should be applied with caution and under conditions that ensure sufficient survival and development of desired species. This study also provides evidence that selective browsing by cervids could be an important overriding factor exacerbating the effect of beech competition on yellow birch regeneration. [ABSTRACT FROM AUTHOR]

• Group-selection openings modestly increase tree diversity in uneven-aged hardwoods. • Large openings (22 m radius) have highest regeneration richness and diversity. • Shade-tolerant Acer saccharum dominant in regeneration layers 15 years post-harvest. Group selection has been proposed as a complementary system to conventional, uneven-aged management in temperate hardwoods (i.e. single-tree selection) to counteract declines in overstory diversity and growing importance of shade-tolerant species caused largely by past management activities. Mixed results from experimental group selection harvests in eastern North America and growing interest in retention forestry have led some scientists and managers to apply ideas from retention forestry into group selection systems. The Yellow Birch Legacy-Tree Project is a group selection experiment established in 2003, which incorporates single, dominant/co-dominant yellow birch (Betula alleghaniensis Britt.) legacy-trees into the centers of harvested canopy gaps in a northern hardwoods forest in the Upper Peninsula of Michigan. Group selection openings (n = 49) in three size classes based upon radii—small (11 m, n = 16), medium (16.5 m, n = 17), and large (22 m, n = 16)—were surveyed alongside single-tree selection reference sites (n = 20) 15 years post-harvest. Our goals were to (1) examine long-term trends in seedling and sapling abundance and diversity, (2) evaluate the initial and contemporary effects of gap size on regeneration, and (3) assess the overall efficacy of group-selection with legacy-tree retention in regenerating mid-tolerant and intolerant species, especially yellow birch. At 15 years post-harvest, we found that shade-tolerant sugar maple (Acer saccharum Marsh.) dominated regeneration layers across all treatments, but we observed modest increases in seedling and sapling diversity within openings that may alter long-term canopy composition over several cutting cycles. We found that gains in diversity and evenness in canopy gaps persisted through time and that large gaps (22 m radius) had the most species-rich and diverse regeneration compared to reference sites. Canopy gaps contained significantly higher densities of shade-intolerant and mid-tolerant seedlings and saplings compared to reference sites, namely red maple (Acer rubrum L.), black cherry (Prunus serotina Ehrh.), and yellow birch; however, most diversity occurred in shorter height classes. Nevertheless, legacy retention within group selection systems holds promise for maintaining ecological memory and structural complexity through time, but further treatments such as scarification may be necessary to shift compositional trajectories in favor of shade-intolerant and mid-tolerant species. [ABSTRACT FROM AUTHOR]

We used data on a forest-based ecosystem service, sugar maple (Acer saccharum) sap, spanning sugar maple's latitudinal range to predict the role of climate variation on two aspects of sap productivity: sugar content and sap flow. The sugar in maple sap is derived from carbohydrate stores influenced by growing season conditions in the prior year, while sap flow is tied to freeze/thaw cycles during early spring. Our findings suggest optimal regions for sap production will move northward, with lower average production across nearly the entire range except for the far north, and provide spatially-explicit projections for forest managers and syrup producers making decisions on the location and viability of management for maple syrup production. Climate change is affecting the benefits society derives from forests. One such forest ecosystem service is maple syrup, which is primarily derived from Acer saccharum (sugar maple), currently an abundant and widespread tree species in eastern North America. Two climate sensitive components of sap affect syrup production: sugar content and sap flow. The sugar in maple sap derives from carbohydrate stores influenced by prior year growing season conditions. Sap flow is tied to freeze/thaw cycles during early spring. Predicting climate effects on syrup production thus requires integrating observations across scales and biological processes. We observed sap at 6 sugar maple stands spanning sugar maple's latitudinal range over 2–6 years to predict the role of climate variation on sugar content and sap flow. We found that the timing of sap collection advanced by 4.3 days for every 1 °C increase in March mean temperature, sap volume peaked at a January-May mean temperature of 1 °C, and sap sugar content declined by 0.1 °Brix for every 1 °C increase in previous May-October mean temperature. Using these empirical relationships, we projected that the sap collection season midpoint will be 1 month earlier and sap sugar content will decline by 0.7 °Brix across sugar maple's range by the year 2100 in an RCP 8.5 climate change scenario. The region of maximum sap flow is expected to shift northward by 400 km, from near the 43rd parallel to the 48th parallel by 2100. Our findings suggest climate change will have profound effects on syrup yield across most of sugar maple's range; drastic shifts in the timing of the tapping season accompanied by flat to moderate increases in syrup yield per tap in Canada contrast with declines in syrup yield and higher frequencies of poor syrup production years across most of the U.S. range. [ABSTRACT FROM AUTHOR]

• High harvest intensity reduced American beech sapling density at well-drained sites. • Overstory tree mortality and depth-to-water had a positive effect on sugar maple sapling density. • Depth-to-water changed the magnitude to which tree species diversity respond to harvest intensity. • Implications for timber production and species diversity management objectives. The northern hardwoods forest ecosystem of eastern North America provides a wealth of products and services to human society. These forests have been managed under partial harvesting treatments to meet wood production objectives of desired species while maintaining or increasing structural and compositional heterogeneity. Few studies have examined how disturbance and site factors interactions may influence sapling dynamics and species diversity in northern hardwood forests. We used a retrospective approach to examine the impacts of disturbance and site factors on sapling (trees > 130 cm in height and <10.0 cm dbh) dynamics and species diversity across 37 stands with time since harvest ranging from 6 to 20 years. We assessed American beech (Fagus grandifolia Ehrh), sugar maple (Acer saccharum Marsh.), red maple (Acer rubrum L.) and yellow birch (Betula alleghaniensis Britt.) saplings density as functions of disturbance intensity (% basal area cut and overstory tree mortality rate (%)) and site factors. We also examined the effects of disturbance, site factors and relative density of American beech, red maple, sugar maple and yellow birch saplings on species diversity at the sapling layer. The density of American beech saplings decreased significantly with harvest intensity. High harvest intensity significantly reduced American beech sapling density at high depth-to-water sites. However, the densities of red maple and yellow birch saplings increased significantly with increasing harvest intensity. We found that high harvest intensities had less negative effect on sugar maple sapling density compared with American beech. Our results also showed that tree species diversity (Shannon diversity index (H ′)) increased with harvest intensity up to about 75% and then declined. However, depth-to-water (DTW) significantly changed the magnitude of tree species diversity response to harvest intensity. Red maple appeared to have contributed more to species diversity than the other hardwood tree species. Given that sugar maple sapling density was generally higher than American beech at high harvest intensities in this study, management regimes that promote mid-tolerant species in American beech-presence stands may eventually benefit sugar maple more than American beech in these forest types. These strategies may also increase species diversity and improve forest ecosystems functions and services. [ABSTRACT FROM AUTHOR]

• Group-selection gaps greatly increased understorey yellow birch density in 10 years. • Autumn-cut gaps recruited 10× more stems than winter gaps or single-tree selection. • Abundance linked to soil mixing, seed-tree proximity, and reduced sugar maple cover. • Autumn-harvesting and targeted habitat selection for gaps optimised these conditions. • Winter-harvesting reduced soil disturbance, favoured sugar maple recruitment. Wide-spread use of single-tree harvest methods has inhibited regeneration of yellow birch in shade-tolerant hardwood forests. Shelterwood harvesting can improve recruitment, but incurs higher costs, longer harvesting cycles, and produces an even-aged structure. Group selection (GS) harvesting provides an alternative method with the benefits of single-tree selection (STS; mixed-ages, shorter cycles), yet produces larger gaps that may favour recruitment of mid/intolerant tree species. Recent GS trial studies have shown promise, but results vary and differences in edaphic conditions and competition dynamics associated with climate, historical management, and season of harvesting may influence responses. We evaluated the yellow birch recruitment efficacy of GS in an Ontario shade-tolerant hardwood forest by comparing change in regeneration as percent ground cover (stems <50 cm) and large stem density (>50 cm) over ten years in STS harvested and unharvested forest relative to two types of GS gaps: "typical" gaps placed in suitable microsites, and indiscriminate "systematic" interval gaps harvested on a grid. Autumn-harvested gaps established 2–4× more yellow birch cover and recruited 7–10× more large stems by year ten than STS and winter harvested gaps; unharvested controls remained unchanged. Typical gaps recruited 3× more stems than systematic gaps in autumn-only. Soil disturbance and seed tree proximity were correlated to establishment of yellow birch cover in year 1, which predicted large stem recruitment by year 10. Winter-harvesting favoured sugar maple cover in year 1, which predicted lower yellow birch recruitment by year 10. These results indicate that autumn-harvested typical gaps optimised germination and growth conditions for yellow birch (light availability, soil disturbance, reduced competition, seed proximity/dispersal). Consequently, GS harvesting in autumn can be considered an effective alternative to shelterwood harvesting for yellow birch recruitment in poorly-represented shade-tolerant hardwood stands. [ABSTRACT FROM AUTHOR]

Highlights • Prediction of post-cut survival is improved by considering long-term growth trends. • Fewer growth release events occurred after cut than after natural disturbances. • The growth release potential of trees is related to the presence of stem defects. • The growth release potential is not related to prior long-term growth trends. Abstract We investigated the growth and mortality dynamics of 22 northern hardwood stands with well-known disturbance histories in southern Quebec, Canada. The main objective of this study was to evaluate the effect of short- and long-term growth trends on both the probability of mortality and the radial growth response of trees following partial cutting. This was first achieved by identifying growth release episodes and characterizing growth patterns prior to tree death from entire tree-ring series of 86 live and 26 dead sugar maple (Acer saccharum Marsh.) trees. Using mixed-effects logistic regression, we quantified the effects of short- and long-term growth trends, as well as external variables such as neighbourhood competition and tree vigour status on the probability of mortality and growth release events. Results showed that long periods of continuous growth decline or long periods of slow radial growth after a severe decrease occurred in 88% of dead trees. These growth patterns were associated with a progressive decline in tree vigour that was mostly initiated by drought and insect defoliations during the 1980s, thereby supporting the relevance of studying long-term tree-ring chronologies to deduce causes of post-harvest mortality. Our model showed that predictions of post-cut mortality events could be significantly improved by considering long-term radial growth trends (25 years before cutting) in individual trees. In terms of growth response, 24% of live trees had a release event occurring after the selection cut, which is slightly lower than the response induced by natural disturbance in previous decades. None of the tested long-term growth variables were significantly related to the occurrence of growth release events. Consequently, suppressed sugar maple trees appear to maintain their capacity to respond positively to gap creation, regardless of the length of the suppression period. We conclude that while decreasing trends in long-term radial growth increase mortality probabilities, the growth release potential of trees is negatively impacted by the presence of defects affecting tree vigour (e.g. fungal infections and cambial necroses) and positively related to the removal of neighbouring trees, rather than to prior growth trends. [ABSTRACT FROM AUTHOR]

Highlights • Selection harvesting has minimal impact on bulk soil chemistry. • Large differences in community composition among skid trails, gaps and uncut areas. • Selection harvesting has no impact on foliar Ca in maple seedlings. • Maple seedlings in disturbed areas have significantly lower foliar N, P and K. • Fewer maple seedlings and low nutrients may have implications for regeneration on skid trails. Abstract Sugar maple (Acer saccharum Marsh.) is a commercially important species throughout much of eastern North America and concerns have been raised regarding its successful regeneration owing to nutrient limitation, especially calcium (Ca) caused by acidic deposition. Timber harvesting can alter nutrient availability in soil, but the short-term effects of selection harvesting on sugar maple seedling chemistry have not been studied. In this study, sugar maple seedlings were collected for elemental analysis from unharvested areas, canopy gaps and skid trails one and two years post selection harvest in a mixed hardwood forest in central Ontario, Canada. Plant community composition and sugar maple seedling morphology were measured along with soil physical and chemical properties. While there were few measurable differences in soil physical or chemical characteristics amongst sites, understory vegetation was markedly different and Carex spp. were dominant in skid trails. In addition, sugar maple seedlings in the disturbed sites (gaps and skid trails) were morphologically different (decreased height:diameter ratio) and had lower foliar concentrations of essential plant nutrients including nitrogen (N), magnesium (Mg), potassium (K), and phosphorus (P) compared with unharvested sites. Calcium concentrations in seedlings were unaffected by harvesting, perhaps due to low soil Ca levels across all sites. In contrast to Ca, foliar concentrations of K and P in seedlings growing in gaps and skid trails were below or approached reported nutrient deficiency thresholds for mature trees, which may have potential consequences for forest regeneration that have not been previously considered. [ABSTRACT FROM AUTHOR]

• Quantified sugar maple (SM) age structure in selection-managed northern hardwoods. • Most SM canopy trees in Michigan stands were 90–120 years old. • SM saplings <60 years were understocked. • Using diameter as a proxy for age can underestimate age of small SM trees. • Limited SM sapling recruitment occurred during selection management. Single-tree selection (STS) silviculture has dominated management of northern hardwood forests (NHFs) in the Great Lakes region since introduction the 1960′s. Periodic partial cuttings are assumed to promote and recruit well-stocked natural regeneration, resulting in balanced uneven-aged stand structure with sustainable harvest volumes. However, low sapling density of tree species desirable for management, including typically dominant and economically valuable sugar maple (Acer saccharum), suggests STS may not be effectively recruiting sugar maple in some regions. Few studies have analyzed sugar maple size-age structure to determine whether recruitment failure has been chronic since partial cutting began or is a more recent phenomenon. Here, we analyzed sugar maple age structure, regional patterns, and possible drivers for 51 selection-managed NHF stands located in northern Michigan, USA. Our data were 1499 aged basal discs collected from stumps of sugar maple trees >5 cm diameter at breast height (DBH) harvested winter 2017–2018. Our results offer little evidence to suggest that partial cuttings have promoted well-stocked ingrowth of sugar maple saplings. For most stands, age and diameter were non-linearly related, with a wide range of larger-diameter trees having similar ages. Among stands, sugar maple sawtimber (>24 cm DBH) averaged 106 years old and poletimber (11.4–24 cm DBH) 91 years, along with older than expected saplings (5–11.4 cm DBH) averaging 67 years. Most stands were dominated by a ∼100-year-old upper canopy cohort, with a majority having ≤2 younger cohorts typically of low density and >50 years old. Areas with greater snow water equivalent and lower January minimum temperatures, where deer populations have been historically low, generally have younger sugar maple saplings. Low density and generally old age of smaller size classes led to much lower stocking of age classes <70 years old than expected under STS. Together, this suggests long-term STS management for sugar maple may be unsustainable over large portions of the study region if current trends in forest and deer management continue. Our age structure analysis supports the growing consensus that alternative management may be needed for NHF in parts of the Great Lakes region. [ABSTRACT FROM AUTHOR]

• Maple growth decline occurs widely in its distribution range. • Severity of maple growth decline varies by location. • High vapour pressure deficit mostly drives the decline. Sugar maple (Acer saccharum Marshall) growth in the species' southern range has been declining since the 1980s, putting at risk a variety of ecosystem services that the species provides. Heatwaves, drought, frosts, acidic deposition, and insect defoliation, all reducing photosynthetic activity, have been suggested to be behind the phenomenon. Because the geographic scope of previous studies on maple growth is limited to the southern temperate biome, it is not currently understood whether the same negative trends and factors affecting growth rates apply to the species in more northern regions of its distribution range. Here we used annual ring-width data of 1675 trees from a network of 21 sites in Quebec and Ontario between 45˚N and 48˚N to reconstruct maple growth and to analyze its trends and climatic drivers since 1950 CE. We developed a retrospective tree growth index by employing a Generalized Additive Mixed Model (GAMM) that accounted for tree age and size, terrain altitude, slope, competition and mean site climatology. Our reconstruction suggested considerable geographical variations in maple growth trends, with declining growth being less pronounced in the central portion of the study region. The growth decline persisted at the time of sampling in 2019 and 2020. Maple growth negatively correlated with the summer vapour pressure deficit (VPD) during both the previous and current growth seasons. As the decline in sugar maple growth is observed at the northern limit of its distribution, climate change may not favor the expansion of sugar maples to the north. The observed decline along with increasing VPD will likely have a negative impact on the ecosystem services supported by this species and calls for management adaptation measures. [ABSTRACT FROM AUTHOR]

• Drought driven seedling mortality helps explain temperate forest composition. • Drought more important than shade for seedling mortality in temperate forest. • Canopy trees facilitate tree seedling survival to drought in temperate forest. • Species with larger seeds and seedlings and deeper roots avoid drought mortality. • Plant large stock of drought-adapted species beneath partial overstory canopies. Periodic drought may interact with spatially patterned soil moisture regimes, canopy disturbance and species seedling traits to influence tree regeneration processes in northern temperate forests. In northern Michigan, USA, we assessed the effects of regional upland soil moisture regimes (six sites classified from dry to mesic) and tree canopy shading (% light) on planted-seedling dry mass, probability of mortality and functional traits for eight tree species varying in shade tolerance and natural distribution over soil moisture gradients (i.e., drought tolerance): Quercus velutina, Quercus alba, Quercus rubra, Prunus serotina, Acer rubrum, Acer saccharum, Fraxinus americana, and Betula alleghaniensis. The assessment period included droughts during the seedlings' 2nd and 3rd growing seasons. High mortality (especially on drier-classified sites and in more open, high-light environments) and negative associations of soil water minima with mortality collectively suggest drought-driven mortality across sites. In contrast, canopy shade driven mortality occurred only on mesic sites at <10 % light. Overstory canopies facilitated seedling survival relative to more open environments, with survival maxima ranging from 3 to 20 % light among sites. Among species, probability of survival was 1) highest for Quercus species (∼0.70) and lowest for B. alleghaniensis (0.10), 2) positively and generally associated with drought tolerance, but not associated with shade tolerance, and 3) positively and strongly associated with species seed-size related differences in total dry mass, root dry mass and rooting depth. In turn, rooting depth (range 3–68 cm) was positively related to seedling water status, and water status to survival during drought. In summary, site soil moisture regimes, facilitation vs. competition via variation in canopy density, and size-related species traits drove patterns of young seedling mortality during an interval that included droughts. Results suggests current drought regimes likely have strong influence on canopy composition across the region's spatially patterned dry to mesic soil gradients, and may have larger impacts if drought intensifies with climate change. Our results can aid development of planting strategies for assisted migration and other emerging needs, including planting 1) species more drought adapted than those dominating current overstories, 2) large stock with well-developed root systems, and 3) in microsites that maximize canopy facilitation. [ABSTRACT FROM AUTHOR]

This study compared 141 ecologically relevant climate metrics to field assessments of sugar maple ( Acer saccharum Marsh.) canopy condition across Vermont, USA from 1988 to 2012. After removing the influence of disturbance events during this time period to isolate the impact of climate, we identified five climate metrics that were significantly related to sugar maple crown condition. While three of these are monthly summary metrics commonly used in climate analyses (minimum April, August and October temperatures), two are novel metrics designed to capture extreme climate events (periods of unusual warmth in January and August). The proportion of climate-driven variability in canopy condition is comparable to the proportion accounted for by defoliating pests and other disturbance events. This indicates that climate conditions, though rarely included in sugar maple decline studies, may be of equal importance as more traditionally studied stress agents. Modeled across the state, results indicate that changes in historical climatic conditions have negatively impacted sugar maple health over the 25 year study period, and are likely to degrade further over time. Climate projections under a low emissions scenario indicated that by 2071 55% of sugar maple across the state would likely experience moderate to severe climate-driven stress relative to historic baselines, increasing to 84% under a high emissions scenario. However, geographic variability in projected climate impacts indicates that while conditions for sugar maple will likely deteriorate across the state, climate refugia should also be available to maintain sugar maple in spite of changing climatic conditions. Considering the predominant role of sugar maple in Vermont’s economy and culture, managing this resource into the future could pose a considerable challenge. [ABSTRACT FROM AUTHOR]

Maximum stand density index (SDImax) is an important factor controlling stand dynamics that varies by species and region, but less is understood how it differs within a region for a given species. In this analysis, linear quantile mixed modeling (LQMM) and an extensive network of permanent plots were utilized to examine regional variation in the SDImax of 15 species (7 softwoods and 8 hardwoods) across the complex forests of the Acadian Region in North America. Observed plot-level SDImax was then linked to various stand, plant trait, site, and climatic factors and the spatial patterns throughout the region examined. Results indicated high variability of SDImax for a given species with northern hardwood species like yellow birch ( Betula alleghaniensis Britt.), sugar maple ( Acer saccharum Marsh.), and American beech ( Fagus grandifolia Ehrh.) having the lowest values (555 – 627), while softwood species such as northern white cedar ( Thuja occidentalis L.; 1014), eastern hemlock ( Tsuga canadensis (L.) Carr.; 1026), and white pine ( Pinus strobus L.; 967) had the highest values. Compared to the other stand and site factors examined, climate showed the strongest relationship with SDImax with R 2 ranging from 82 to 98%. Of the climatic variables examined, those representing growing season length and the timing of precipitation were most influential. For the majority of the species examined, reductions in SDImax were forecasted due to changing climatic conditions. Across species, mean SDImax was found to linearly decline with wood specific gravity and increase with leaf longevity, but showed limited relationships with other species-level functional traits. Overall, the analysis highlights the strong variability of SDImax within and between species as well as the important role that climate has on this attribute within a region. [ABSTRACT FROM AUTHOR]

The most direct way of deciphering the dynamics of an ecosystem is to examine its biotic and abiotic components based on analysis of living and dead organisms distributed above ground. The surface analysis method presented here provides a centennial to millennial stand-scale composition and disturbance history and is applicable in any wood-dominated ecosystem. A meticulous analysis of living and dead trees, and macroremains (charcoal, leaves and insect) laying above mineral soil was performed in a virgin and an anthropic sugar maple ( Acer saccharum ) forests. Sugar maple ecosystems provide an appropriate setting for testing this method as they are impacted by several natural and human disturbances. The living and dead components in both sites indicate an increase of the species abundance independent of human interventions, although accelerated by logging in the anthropic forest. The stands were affected by recent insect outbreaks and by fire over the last 2000 years. Charcoal remains indicate that a mixed forest occupied both sites with sugar maple as companion species for more than 1000 years. Surface analysis is a direct method for improving our understanding of current, past and future forest dynamics in natural and anthropic conditions, in this case highlighting how a structuring species of eastern North America thrives in different successional states and disturbance regimes. Novel tools that give insight into pre-colonial ecosystems are greatly needed as a proper understanding of species current distributions and behavior relative to allogenic disturbances is of crucial importance for restoration purposes and accurate prediction of future changes. [ABSTRACT FROM AUTHOR]

• Lime promotes sugar maple and limits beech re-establishment. • Soil scarification is limited in promoting sugar maple and controlling beech re-establishment. • The key factor influencing sugar maple establishment is seedling competition with other species. • Sugar maple seedlings are more sensitive to moisture than beech seedlings. • Beech seedlings are more sensitive to light than sugar maple under small overstory gaps. A combination of global changes (e.g., soil acidification, invasive species, forest management, climate) impacts the regeneration dynamics of sugar maple-dominated stands in northeastern North America. More specifically, there are concerns about the regeneration of sugar maple and other high-value species due to the expansion of American beech and its bark disease, killing mature trees, compromising fiber supply and other ecosystem services. Here, we set up a field trial to study the effect of soil scarification, amendments (nitrogen fertilization, liming, and wood ash), and their interactions, on tree species establishment following understory vegetation removal in a beech-invaded sugar maple stand to promote the establishment of high-value species (i.e. sugar maple and yellow birch), to control beech regeneration, and help reduce the impacts of beech bark disease on stand development. After four years of monitoring, we found that maple-beech stands with well-drained soils were promising sites for the success of sugar maple and yellow birch establishment. Soil scarification had a limited effect on sugar maple and beech establishment while it enhanced that of yellow birch, pin cherry and red maple. The combination of soil scarification and amendments was more effective in promoting the recovery of yellow birch and pin cherry and controlling the recovery of beech and red maple than their separated effects. Lime amendment promoted sugar maple establishment without favouring competitors such as yellow birch, pin cherry and red maple, and limited beech establishment. Overall, our results suggest that combining soil scarification and amendments on targeted site types could help fine-tune tree establishment conditions in northern hardwood forests. [ABSTRACT FROM AUTHOR]

We resampled a research trial 20 years after applying acidifying or liming compounds (−16 to +16 kmol(+) ha −1 of acid-neutralizing capacity [ANC]) in a northern hardwood stand. Although soil properties showed no statistical changes after this period, foliar calcium (Ca) concentrations of sugar maple ( Acer saccharum Marsh.), yellow birch ( Betula alleghaniensis Britt.) and American beech ( Fagus grandifolia Ehrh.) were still related to the treatment ANC. The three species showed distinct growth responses to treatments. Over the two decades following treatment, basal area increment (BAI) of sugar maple increased by 138% in limed plots compared with controls, while it decreased by 25% in acidified plots. The BAI of yellow birch in limed plots increased mainly during the second decade, while in acidified plots, it remained comparable to that of the controls. As for American beech, the BAI increased by 133% over two decades in acidified plots, but not significantly in limed plots. Results support the hypothesis that liming improves the foliar calcium status of these hardwood species, and that acid input: (1) leads to long-term nutritional imbalance for calcium that could explain, at least in part, the positive growth response of sugar maple and yellow birch to liming, and (2) contributes to slow the growth of sugar maple relative to that of American beech in northern hardwood stands located on acid soils in northeastern North America. [ABSTRACT FROM AUTHOR]

The Eastern Deciduous Forest (EDF) region is undergoing a shift in species composition, the most dramatic trend being an increase in mesic taxa such as maple ( Acer spp.) and American beech ( Fagus grandifolia ), and a subsequent decline in xeric species such as oaks ( Quercus spp.). While several hypotheses have been proposed to explain these trends, most focus on allogenic processes such as disturbance and climate change, and little attention has been given to local species interactions. To help bridge this gap, we first quantify long-term changes in community composition of an old-growth forest using census data collected over a fifteen-year interval. Within two 50 × 70 m plots, all stems ⩾2.5 cm DBH were tagged and measured in 1996, and their spatial coordinates were mapped. Seedlings were measured in smaller subplots, and soil cores were sampled at these locations. Hemispherical photographs were then taken to quantify the light environment, and these procedures were replicated in 2011. As expected, we find an increase in the dominance of both American beech and sugar maple ( Acer saccharum ), and a marked decline for most others, particularly white oak ( Quercus alba ). Neighborhood models incorporating the density, basal area, and composition (i.e., conspecific versus heterospecific) of stems surrounding focal individual trees reveal that local density-dependent interactions were indeed weak for both American beech and sugar maple. At the same time, however, all other species experienced similarly weak effects. Likewise, soil resource gradients had little influence on long-term demographics, contrary to our expectations. Together, our results indicate that the changes observed across the Eastern Deciduous Forest region may indeed be most strongly impacted by allogenic disturbance processes, although we still encourage further research on the potential influence of local species interactions at mediating these dynamics, and expect neighborhood models to become an important tool in understanding the causes of compositional shifts in the EDF region. [ABSTRACT FROM AUTHOR]

• American chestnuts established across a range of managed forests conditions. • American chestnut growth rates increased with available light in the understory. • All species exhibited light-dependent plasticity in growth and photosynthetic traits. • American chestnut appeared to have higher plasticity than other comparative species. The development of a blight-tolerant transgenic American chestnut (Castanea dentata (Marsh.) Borkh.) would provide the potential for reintroducing this species into forests it historically dominated. However, we lack a silvicultural understanding of the environmental conditions most favorable to the growth and survival of American chestnut relative to species with which it historically co-existed. We examined effects of shelterwoods with variable residual densities on the growth and physiology of blight-tolerant chestnut seedlings relative to sugar maple (Acer saccharum Marsh.), and several oak (Quercus spp.) and hickory (Carya spp.) species across a broad range of light availabilities (gap light index; GLI; of 4–50%). To assess the hypothesis that American chestnuts are more physiologically plastic and responsive to light than oaks and hickories, we measured seedling relative diameter and height growth rates and photosynthetic light-response curves across a GLI gradient. The diameter and height growth of chestnut increased with GLI, while oaks and hickories did not. All species varied aspects of their photosynthetic physiology across the GLI gradient in an approximately equivalent manner; the dark respiration rate and light compensation point increased equivalently across species in response to increases in light availability. A multivariate analysis identified three major axes of variation across ten measured traits, largely reflecting differences in species averages and phenotypic plasticity across the GLI gradient. American chestnut seedlings were spread across these multivariate dimensions more than other species, consistent with the hypothesis of higher phenotypic plasticity in this focal species. These results suggest that low residual shelterwood densities may better promote the establishment of underplanted chestnut seedlings relative to associated tree species provided it does not result in detrimental increases in the productivity of competing understory species. [ABSTRACT FROM AUTHOR]

• Cumulative impact of atmospheric nitrogen (N) deposition is a concern for forest ecosystems. • N doses (0, 3, 10 times ambient rate) were applied during a 13-year period in a sugar maple stand. • N concentrations in foliage and organic soil remained similar between treatments after 13 years. • Soil nitrate increased and Ca, Mg, and K (top organic layers) decreased in the high N treatment. • Foliar N:P ratio increased while Ca and Mn decreased with increasing levels of N addition. • Sugar maple dieback rate increased over time in the high N treatment. Cumulative effects of nitrogen (N) deposition are a matter of concern in temperate forests of northeastern North America. Increased N deposition may stimulate forest growth and carbon sequestration while also causing N saturation, which may lead to soil cation depletion, particularly in base-poor systems. We applied ammonium nitrate for 13 years in a sugar maple (Acer saccharum; SM) stand at the rates of 3 (Low N, LN) and 10 times (High N, HN) the ambient N deposition rate. N concentrations in soil layers were similar among treatments, except for a nitrate increase in the top B horizon in the HN treatment. Exchangeable Ca2+, Mg2+, and K+ were significantly lower than for the Control, at least in one of the top organic soil layers in the HN treatment. The temporal dynamics of foliar chemistry revealed that N:P ratio increased and both Ca and Mn decreased in the early years following treatment initiation, but that they did not change much thereafter. Foliar Ca values in the HN treatment are the lowest reported in the literature. This probably explains why the proportion of crown dieback steadily increased over time, reaching values as high as 80% in the HN treatment in 2017. [ABSTRACT FROM AUTHOR]

• Maple sugaring is an important and rapidly expanding industry in eastern North America with a long and rich history, yet there is to-date no review on the mechanisms of sap runs and how they are affected by tree and tap characteristics. • Here, we review the mechanisms underlying sap runs in the leafless-state of maple species. • We also review knowledge on the importance of tap and tree characteristics, such as tap hole depth and tree size in maple sugaring. • Additionally, we conducted a meta -analysis on compiled data including about 15 000 data points on sap yield and the sap's sugar content from eleven tapping years across ten sites to test various hypothesis. • The meta -analysis revealed that tree and site characteristics are the primary sources of variability in both sap yield and the sap's sugar content. Maple sugaring mainly uses sugar and red maples (Acer saccharum and Acer rubrum) by tapping them for sap in the leafless-state across large portions of their ranges. How much sap exudes from a tap hole and how sweet this sap is, can vary substantially. Year-to-year variation in sap yield and sugar content can be primarily traced to differences in meteorological conditions that drive sap runs. Yet, how much of the total variation in sap yield and sugar content is linked to the year, site, species, tree, or tap has not been investigated systematically. Here, we reviewed the literature and also compiled a dataset of sap yield and sugar content from gravity taps on 324 red and sugar maples. The compiled data originates from multiple studies at ten sites across a large proportion of the ranges of sugar and red maple and stretches over eleven years. Using about 15 000 data points on sap yield and sap sugar content, we analysed the importance of tap and tree characteristics, such as height of the tap hole on the stem or diameter at breast height. We also review previous research on the importance of tap and tree characteristics in maple sugaring. Moreover, we partition variability in the data to attribute it to species, site, tree, year, and tap characteristics. Our results indicate that species, site and tree characteristics are the three largest sources of variability with regards to sap yield and the sap's sucrose concentration. However, differences between years and tap characteristics, which were found to be comparatively minor sources of variability in sap yield and the sap's sucrose concentration, have attracted far more attention in the past. We advocate for the continuation and expansion of systematic measurements of sap characteristics across a network of sites to further improve our understanding of maple sugaring. Such an understanding will be instrumental to prepare maple sugaring operations against the imminent effects of the climate and biodiversity crises and ensure their sustainability to perpetuate this traditional activity. [ABSTRACT FROM AUTHOR]

• Crown dieback is by far the best indicator of tree vigour. • Stem defects did not contribute much to explaining vigour. • Existing classification systems should be simplified. • The rate that vigorous trees decline increased with diameter. Northern hardwood forests include many degraded stands dominated by trees of low vigour due to past management. To facilitate the implementation of stand improvement, several classification systems have been developed to help tree markers visually assess tree vigour at time of harvest based on the presence of individual defects. Because very few studies have tried to empirically validate such systems, it remains uncertain whether many of these defects should be used to guide the tree marking process. In this study, we assess tree vigour using repeated measurements collected as part of long-term silvicultural trials conducted in 615 permanent plots throughout the northern hardwood forest of Quebec, Canada. We aimed to determine whether the defects that are commonly used for classification have a significant effect on both growth and survival over three decades, using 9,338 sugar maple and 1,316 yellow birch trees. We also conducted a retrospective analysis to quantify the rate at which vigorous trees develop defects. Our results confirmed that crown dieback is by far the best indicator of vigour for both sugar maple and yellow birch trees. Conversely, our results revealed that stem defects did not contribute much to explaining the variation in vigour, except for the presence of cankers and fungi, which had modest effects. Consequently, stem defects should not be used as the main indicators of tree vigour, and existing classification systems should be simplified by reducing the number of stem defects under consideration for this purpose. Lastly, our results showed that the rate that vigorous trees develop defects increased with increasing diameter, more so than the probability of surviving. Thus, assessing the risk of retaining large trees should not only be based on survival, but also on the risk of developing defects that reduce the growth and value of trees. [ABSTRACT FROM AUTHOR]

• Regeneration responses of eight species to partial harvests (PH) were examined. • Response patterns in terms of direction, immediacy, consistency was evaluated. • Different lag-periods across species were detected. • PH increased sapling growth of most of the species except sugar maple. • PH increased mortality of balsam fir, American beech, red maple and white-cedar. Ecosystem-based forest management associated with partial harvesting (PH) is intended to balance ecological and economic values of sustainable forest management. The potential for delayed growth response and elevated mortality of advance regeneration following PH remains a critical concern, and may present a barrier to more widespread implementation of this approach. We used 835 permanent continuous forest inventory plots to examine the rate and time course of species-specific regeneration growth and mortality of eight tree species in the first fifteen years following operational partial harvests in the mixed-species forests of Maine, United States. We aimed to provide a quantitative understanding on how regeneration of different species responded to PH in terms of growth and mortality. In addition, we evaluated how the patterns and magnitudes of growth and mortality responses developed over time, if these responses occur gradually or suddenly, and if the patterns of the responses were persistent. We found that the response magnitude, temporal trajectories of responses, and the length of initial lag-period largely varied across species, PH treatments, and the variables examined. For sapling diameter growth, paper birch (Betula papyrifera Marshall) and red maple (Acer rubrum L.) showed immediate responses to high-intensity PH, while a five-year lag-period was observed in balsam fir (Abies balsamea (L.) Mill.), American beech (Fagus grandifolia Ehrh.), red spruce (Picea rubens Sarg.) and eastern hemlock (Tsuga canadensis (L.) Carrière) and a 10-year lag period in northern white-cedar (Thuja occidentalis L.). The initial increase in sapling mortality was observed in balsam fir, American beech, red maple and northern white-cedar, but not in other species. Sapling survival reached a stable state irrespective of species after the initial five-years following harvests. In partially harvested stands, identifying preharvest conditions related to postharvest density, growth, and mortality was complex and interacted with time since harvest. Our results suggest that broad application of PH only results in species-specific gains, losses, and delays in regeneration responses within mixed-species stands. Future research should consider PH in combination with other treatments to initiate immediate responses to a wider range of species. [ABSTRACT FROM AUTHOR]

• Deer had only minor impact on growth and survival of sugar maple and ash seedlings. • Seedlings responded primarily to light availability, soil N, and initial size. • Measures of root collar diameter:height detected some effect of deer herbivory. Deer herbivory has a reputation for suppressing tree seedling development in Northern hardwood forests. We examined survival and growth of sugar maple (Acer saccharum) and ash (Fraxinus spp.) seedlings in a controlled factorial experiment with differing light conditions and levels of deer access in a Northern hardwoods forest in Wisconsin, USA. Measurements were made in ∼380 m3 harvest gaps, in transition zones adjacent to gaps, and under closed canopy conditions, both inside and outside of deer exclosures. Browse incidence was initially greater in unfenced treatments, but a general decline through time eliminated this difference. Seven-year survival of both species groups was correlated positively with initial root collar diameter (RCD) and was greater in transition zones. Ash seedling survival was greater in plots with greater overall seedling aggregate height. Soils were primarily differentiated by available nitrogen, which positively influenced height growth of sugar maple in transition zones but did not influence ash growth. Although sugar maple height growth was correlated positively with initial RCD, greater initial height reduced growth rates, both as a simple effect (sugar maple) and in combination with initial RCD (ash). Ash growth correlated negatively with seedling aggregate height in gaps but was unaffected in transition or canopy zones. Allometric coefficients of RCD:height indicated some influence of deer herbivory which was not detected in other analyses. Coefficient values trended downward from year 2 to year 9, but wide confidence intervals limit the value of this metric as an indicator of seedling community resilience with regard to deer. [ABSTRACT FROM AUTHOR]

[Display omitted] • Edge, thinning, and site-quality effects shape regeneration in red pine plantations. • Canopy openness often greater in red pine plantations than adjacent hardwoods. • Thinning and increasing distance from hardwood edge decreased regeneration density. • Red pine plantation interiors favor oaks and black cherry over undesirable species. • More intentional plantation management may further promote desirable regeneration. Red pine plantations may promote more desirable hardwood regeneration than hardwood stands because of differences in: 1) the composition and quantity of tree seeds available, and 2) abiotic and biotic properties of growth and establishment environments. However, information on how distance from hardwood edges, thinning of overstory pine, and site quality shapes hardwood regeneration properties in red pine plantations is scarce. Such information would aid in understanding and managing red pine plantation effects on hardwood regeneration processes and guide spatial and temporal rotations of such effects across landscapes. To elucidate relationships, we monitored hardwood seedlings and canopy openness across transitions from hardwood edges to red pine plantation interiors at 40 sites spanning a site quality gradient in northwestern-lower Michigan. Hardwood seedling density: 1) declined substantially from edges to interiors at all but low-quality sites before stabilizing 30-m into plantations, 2) was reduced, or not affected, by thinning despite positive effects on seedling light environments, and 3) was greater at intermediate than high- and low- quality sites. From edges to interiors, species composition shifted towards Acer rubrum , Quercus species, and Prunus serotina and away from Acer saccharum , Fagus grandifolia , Fraxinus americana , and Ostrya virginiana. Species increasing in relative abundance and/or density from edges to interiors were generally more drought-tolerant, shade-intolerant, large-seeded, and animal-dispersed than decreasing species. Shifts towards less shade-tolerant species from edges to interiors were consistent with increases in canopy openness and were most pronounced where changes in light were greatest (i.e. , thinned plantations on high-quality sites). Changes in hardwood seedling density and composition were likely driven by differences across species in dispersal traits, seed characteristics, and relative adaptation to interior conditions (likely hotter, drier, and nutrient poor). To promote commercially valuable hardwoods, increase mast for wildlife, or improve climatic resiliency, the most suitable targets for conversion to hardwoods may be plantations with significant interiors (i.e. , area > 30-m from hardwood edges) on higher quality sites adjacent to desirable seed sources (e.g. , Quercus rubra , Prunus serotina , Acer saccharum). More desirable hardwood regeneration may result from reevaluating thinning timings and intensities originally designed to maximize red pine production. Our results also indicate that intentional management of surrounding hardwood forest composition (i.e. , seed sources), within plantation retention of hardwood islands (e.g. , mature oak stems), and greater control of Acer rubrum competition may further increase the desirability of hardwood regeneration in red pine plantations. [ABSTRACT FROM AUTHOR]

Characterizing tree crown structure and quantifying its relationships with the surrounding environment is of critical importance in forest ecology and management. These relationships depend heavily on species plasticity and local competition for light. It is however difficult to study forest canopies due to their inaccessibility and complexity. The objective of this study was thus to use terrestrial laser scanning (TLS) data to overcome the limits of traditional canopy studies and to quantify the differences in sugar maple ( Acer saccharum , Marsh.) tree crowns between stands with different composition and developmental stage. A total of 72 sugar maple trees and their immediate surroundings were scanned using a TLS. We developed eight crown structure metrics and three competition indices using TLS data. We found that competitive pressure is higher in pure stands, when compared to mixed ones. Sugar maple crowns were bigger, less dense, and more sinuous and open in mixed stands. Moreover, differences between trees were generally more pronounced for trees in mature stands. Finally, TLS competition indices are better predictors of crown metric variability than stand type, highlighting the potential of TLS data to quantify tree competition and space occupancy. Our competition indices are good proxies of the canopy structure and thus seem promising to predict tree growth. [ABSTRACT FROM AUTHOR]

In uneven-aged stands, tolerant hardwood trees often sustain various degrees of suppression before they recruit to the merchantable tree layer. Whether this suppression influences stem quality development and growth potential of the trees in the future is still largely unknown. Using a retrospective approach, we assessed the influence of suppression history on tree growth response and the probability of stem damage following different intensities of partial harvest. Sugar maple and yellow birch samples were obtained from uneven-aged stands in northwestern New Brunswick, Canada. Results indicated that the likelihood of a tree possessing stem damage increased with length of suppression but decreased with increasing residual stand basal area (SBA) and site quality. Basal area increment (BAI) of both sugar maple and yellow birch was negatively influenced by period of suppression. BAI of yellow birch was higher than for sugar maple. Also, BAI increased with year since treatment but decreased with SBA for trees with diameter at breast height (DBH) larger than the subject tree’s DBH for both species. BAI increased in small-to medium-sized trees (10–35 cm DBH), reached a maximum at 35 cm DBH, and then declined for larger trees (DBH > 35 cm) in both species. Generally, trees on better quality sites had higher growth. However, we observed a significant effect of site quality on BAI for sugar maple only. Hence, we conclude that the partial harvesting system that is designed to provide growing space for intermediate or sub-canopy trees that have shorter suppression history would increase growth rate of residual trees while maintaining stem quality at the northern limit of tolerant hardwoods. [ABSTRACT FROM AUTHOR]

Partial harvesting methods are generally more similar to the natural dynamics of broad-leaved forests than clear cutting. However, their effects on biodiversity are still poorly understood. We investigated the effects of selection cutting on the occurrence of a large epiphytic bryophyte, Neckera pennata , in a northern hardwood forest of New Brunswick, Canada. Twenty-eight forest stands were selected, representing two contrasting forest management practices: 5–9 years old, first-entry selection cuts and untreated stands that had been subjected to low-intensity single-tree cutting at least 35 years earlier. Within each stand, we quantified the presence–absence of N. pennata on 36 trees and measured selected forest stand variables. Although N. pennata had persisted in post-harvest stands, its frequency of occurrence on maple trees was only 7% there, compared to 39% in untreated stands. The density of large-diameter sugar maple trees and crown cover were the most important factors predicting the frequency of N. pennata at the stand level. Tree diameter was also a strong predictor of N. pennata ’s presence at the tree level and the occupancy of large-diameter maples was almost twice as high in untreated stands as in selection cuts. However, the occupancy of large-diameter maples relative to smaller maple trees was much higher in selection cuts, possibly due to dispersal limitations resulting from reduced connectivity of large-diameter host trees. Taken together, our findings suggest that (1) large trees from older seral stages are a prerequisite for the long-term persistence of N. pennata in managed forests and that (2) they are therefore particularly important for managers to retain in selection cuts. Further, since our results indicate that reduced crown cover in selection cuts has a negative effect on N. pennata , the benefit of retaining large host trees would probably increase if buffered within retention patches of maturing trees. Finally, since host tree diameter clearly is a very important factor for the presence of N. pennata , any extension of the harvest rotation would be beneficial. [ABSTRACT FROM AUTHOR]

We used long-term data collected from 22 study sites in northern hardwood stands comprised of sugar maple ( Acer saccharum Marsh.), yellow birch ( Betula alleghaniensis Britt.), and American beech ( Fagus grandifolia Ehrh.) to establish relationships between sapling abundance and tree ingrowth. After 10 years, postharvest sapling density in the 6 cm diameter class (5.1–7.0 cm) showed linear relationships with ingrowth. Proportion of variation explained ( r 2 ) varied from 36% to 83% depending upon tree species and silvicultural treatment (partial cutting vs. uncut control). After 20 years, linear relationships were also established ( r 2 = 24–65%) between ingrowth and sapling density in the 2 cm diameter class (1.1–3.0 cm). From a wide pool of variables related to stand species composition, climate, physiography, and soil nutrients, postharvest sapling density was most strongly correlated to merchantable tree density ( r = 0.43–0.75). Sugar maple sapling density was also positively correlated with base saturation and calcium saturation of the B horizon ( r = 0.56 and 0.58). Over a 30-year period, the increase in American beech sapling basal area was substantial compared to mitigated increases found in sugar maple and yellow birch depending upon treatment. Our results provide useful information on integration of sapling data into forest management. [ABSTRACT FROM AUTHOR]

Understanding how soil nutrients affect sap sweetness of sugar maples ( Acer saccharum Marsh.) is important for producing maple syrup, an economically important non-timber forest product in the northeastern USA and southeastern Canada. Sugar maples were sampled for sap sweetness in 21 plots distributed across five stands in the White Mountains of New Hampshire. Sugar concentrations in maple sap were higher in plots with greater native soil nitrogen availability, indicated by N mineralization in laboratory incubations ( p = 0.01). To test whether nutrient additions can improve sap sweetness, treatment plots were fertilized with N, P, N and P, or Ca. Addition of 30 kg N ha −1 yr −1 increased sap sweetness two years after initial treatment. Foliar P had a negative correlation with sap sweetness ( p = 0.02) while trees with higher foliar N:P had sweeter sap ( p < 0.001). By selecting sites with higher soil nitrogen or fertilizing N-limited sites with N, maple sugar producers may be able to collect sweeter maple sap. [ABSTRACT FROM AUTHOR]

Maple syrup is a non-timber forest resource, for which yield depends strongly on natural processes. Past research has heavily emphasized the role of weather and climate in determining sap flow, and the relationship between sap flow and syrup yield. However, syrup yield depends on sap sugar content, as well as sap flow. Although sap sugar content varies widely among years, less is known about the causes of this variation. Drawing on ecological theories for causes of mast-seeding in trees, we hypothesized that a trees’ carbohydrate stores would fluctuate through time in concert with seed production, and that this fluctuation would affect sap sugar and syrup yield. We evaluated weather variables and past seed production as possible causes of inter-annual variation in maple syrup yield in Vermont, USA. Past seed production was strongly correlated with current syrup yield, suggesting that carbohydrate costs of reproduction affect stores. Climate variables were also important for syrup yield, but were only statistically significant predictors after accounting for variation in seed production. Seed production occurs several months before syrup production, and can be used as a way to forecast expected syrup yields, and prepare harvest plans accordingly. [ABSTRACT FROM AUTHOR]

Highlights: [•] Two calcium fertilizers were applied to an acidic, base-poor sugar maple stand. [•] Sugar maple nutrition, vigor and growth were evaluated 7years after treatment. [•] Both fertilizers improved calcium nutrition, tree crown vigor, and radial growth. [•] Growth response was far less than in a nearby experiment where CaCO3 was also used. [•] Results suggest that magnesium nutrition is not a limiting factor in this ecosystem. [ABSTRACT FROM AUTHOR]

Highlights: [•] NH4NO3 was applied to a base-poor sugar maple stand. [•] Soils and sugar maple vitality were evaluated 8years after treatment. [•] N fertilizer negatively affected the Ca nutrition of sugar maple. [•] No treatment effects were observed on crown dieback and basal area growth. [•] This study raises concerns about the sustainability of sugar maple on this station. [Copyright &y& Elsevier]

• Non-oak leaf litter generally decomposed faster than that of oaks. • Total leaf litter fuel loads were similar beneath non-oak and oaks. • Non-oak leaf litter was twice as abundant underneath non-oaks compared to oaks. • Non-oak leaf litter alteration to fuelbeds could limit fire restoration. Without periodic fire, historically open-canopied, oak-dominated upland woodlands of the central and eastern United States are shifting to closed-canopied forests with increased abundance of shade-tolerant and typically fire-sensitive species. Once established, these encroaching species (i.e., mesophytes) are hypothesized to initiate a positive feedback termed mesophication where mesophytes perpetuate conditions that foster their own proliferation at the expense of oaks (Quercus spp.). One potential mechanism of mesophication is reduced fuel loads through faster decomposition rates of mesophyte leaf litter, as leaf litter is the primary fuel in closed canopy forests. To better understand how different tree species impact fuel loads, we compared initial leaf litter chemistry and one-year (2016–2017) decomposition rates of four non-oak species exhibiting increased abundance in the region and/or relatively high sapling/midstory abundance relative to the overstory on our sites (red maple [ Acer rubrum ], sugar maple [ A. saccharum ], American beech [ Fagus grandifolia ], and hickory [ Carya spp.]) and three oak species (black oak [ Q. velutina ], chestnut oak [ Q. montana ], white oak [ Q. alba ]) in an upland oak forest in north-central Kentucky. We also evaluated fuelbed mass, composition, and bulk density beneath overstory (20–60 cm DBH) individuals of each species following leaf fall in December 2016. Except for American beech (66% mass remaining), we found non-oak leaf litter, especially that of red and sugar maple, decomposed the fastest (45 and 48% mass remaining), and oak leaf litter decomposed the slowest (54–64% mass remaining). Further, although total leaf litter fuel loads in December were similar beneath individuals of different tree species, fuel composition differed. Under non-oak crowns, the proportion of leaf litter from non-oaks was 22–35%, while under oak crowns, the proportion of leaf litter from non-oaks was only 10–12%. This suggests that individual trees of non-oaks are impacting fuel composition beneath their own crowns despite continued oak dominance at the stand scale. Considering differences between species in both leaf litter inputs based on allometric equations and decomposition rates, modeled leaf litter fuel loads in a forest composed entirely of red maple, sugar maple, or Carya spp. were ∼20% lower than an oak forest after one year of decomposition. While limited to the species examined in this study, these findings confirm that the leaf litter of non-oaks, excluding American beech, decomposes more rapidly than oak and that individual non-oak trees alter fuelbed composition beneath their crowns, suggesting that oak woodlands will become less-flammable with increasing mesophytic dominance. [ABSTRACT FROM AUTHOR]

• We studied regeneration of northern hardwoods 10 years after irregular shelterwood. • Soil was scarified and American beech saplings were controlled after harvesting. • Yellow birch established successfully, but was repeatedly browsed by herbivores. • Cervid browsing exacerbated invasion of the understory by American beech. • Sugar maple remained only as a minor component of the regeneration. In northern hardwood stands of the northeastern USA and southeastern Canada, the abundance and occurrence of American beech (Fagus grandifolia Ehrh) regeneration have increased substantially over the past decades, to the detriment of other deciduous species such as sugar maple (Acer saccharum Marshall) and yellow birch (Betula alleghaniensis Britton). To promote the regeneration of these declining species, we established an experimental study near Québec City (Quebec, Canada) to compare the effects of various partial cutting treatments and mechanical control of understory American beech. The design comprises 4 randomized complete blocks, each containing an uncut control, a hybrid single-tree and group selection cutting (SC), two intensities of continuous cover irregular shelterwood (CCIS) and an extended irregular shelterwood (EIS). American beech regeneration, which dominated the sapling layer before cutting, was removed with brush saws in gaps created in the SC and CCIS treatments as well as in the entire understory of the EIS treatment. In addition, gaps and the EIS understory were scarified to favor yellow birch regeneration, and 12 exclosures were built to prevent cervid browsing in CCIS gaps and in entire understory of the EIS treatment. Ten years after cutting, yellow birch regeneration was more abundant in gaps and in the EIS treatment than in the uncut control. These results confirm that increasing the understory light level, eliminating American beech saplings and scarifying the soil can successfully favor yellow birch establishment. However, preferential browsing of yellow birch outside exclosures maintained the overall dominance of American beech in the long term. Thus, under these conditions, damage from repetitive browsing outweighed the positive effects of actions aimed at promoting the regeneration of yellow birch. In the end, despite abundant seed availability, sugar maple remained only a minor component of the regeneration. [ABSTRACT FROM AUTHOR]

• Hemlock recruitment peaked in 1825–1875, ended; beech and maple recruitment continued. • Beech, maple, and hemlock increased in growth since the 1800s, especially after 1980. • Gap dynamics predominated, with <10% of trees responding to disturbance most decades. • Chestnut blight and beech bark disease transformed forest structure in the 1900s. • Over 30% of trees experienced growth release in 1980s–1990s after beech bark disease. Investigating the disturbance regimes of unharvested forests helps us understand their past, present, and future trajectory and gives us a model for forest management. It can also clarify the relative importance of small-scale gap dynamics versus more severe disturbances. Here we used tree rings to examine the recruitment patterns, growth dynamics, and disturbance chronologies of American beech (Fagus grandifolia), sugar maple (Acer saccharum), and eastern hemlock (Tsuga canadensis) in A.B. Williams Woods, an old-growth forest in Ohio, USA, over the past 250 years. We found that beech and sugar maple recruitment peaked around 1900 and continued through the 1900s, while hemlock recruitment peaked during 1825–1875, then declined and effectively ended in the early 1900s. Hemlock grew fastest during the 1800s according to ring width and basal area increment, while sugar maple ring width surpassed beech and hemlock in the 1900s. All three species showed a dramatic increase in growth from 1980 to 2010. Beech and sugar maple established regardless of canopy gaps, but 73% of hemlocks originated in gaps. In most decades, <10% of trees experienced gap recruitment or growth release, suggesting that ongoing, endogenous canopy mortality was the primary disturbance shaping this forest. However, a more severe forest-wide disturbance occurred during the 1980s–1990s when the scale insect causing beech bark disease was introduced, with greater than 30% of living trees showing growth releases in those decades. Another synchronous release occurred in the 1930s when blight-killed chestnuts were removed; 16% of trees showed releases. Both of these intermediate-severity disturbances involved human introduction of invasive species. Thus, we documented a natural disturbance regime of small-scale gap dynamics, punctuated by more severe anthropogenic disturbances in the twentieth century. These relatively frequent, intermediate-severity events probably mean that the forest's current composition is non-equilibrial. Hemlock may continue to decline while beech maintains its dominance by constant regeneration in both gaps and shade, and by responding to disturbance with root suckering and growth pulses. The codominance of sugar maple may be relatively recent, and perhaps temporary, as we found little sugar maple recruitment before 1875 or after 1950, and three times fewer sugar maple saplings now than in the early 1900s. Despite being protected as a park, the development of this old-growth forest has been shaped more by disease-causing invasive species than natural disturbances over the past century. This result emphasizes the pervasiveness of human impacts even in communities we look to as examples of natural pattern and process. [ABSTRACT FROM AUTHOR]

Highlights: [•] We studied tree recruitment patterns among six harvest gap sizes over a 12-year period. [•] Small gaps maintain an understory layer dominated by shade-tolerant tree species. [•] Medium to large gaps were open with a thick shrub layer and few saplings 12years after harvest. [•] Tree composition and diversity were similar among gap sizes 12years after harvest. [•] We found no optimum gap size, but competing vegetation removal or tree release treatments may increase their success. [Copyright &y& Elsevier]

Abstract: Humans have altered the global nitrogen (N) cycle, greatly increasing atmospheric nitrogen deposition in industrialized regions of the world. Groundcover plants can be sensitive indicators of nitrogen deposition impacts. Here, we report results from repeated measurements over a 7year period of groundcover (plants<1.4m tall) and understory (plants with a diameter<5cm at 1.4m in height) vegetation in four mature northern hardwood forests in the north-central United States receiving experimental additions of N (3gm−2 year−1 as NaNO3 for 18years). Experimental N deposition reduced the average abundance of sugar maple (Acer saccharum Marsh.) seedlings in the groundcover by>50% (P <0.001). This reduction occurred at all four sites, but was only statistically significant at the two sites where these seedlings were most abundant (site×nitrogen: P <0.001). Our observations of mortality within a large cohort of sugar maple seedlings across three sites provide further evidence of this effect. For these seedlings, experimental N deposition significantly (P <0.05) increased mortality in the weeks following germination, as well as over the longer term, reducing overall survival after 5years by almost 90%. Although groundcover plants accounted for<0.5% of aboveground plant biomass, they contributed up to 10% of ecosystem leaf area and 5% of aboveground litter. At the two sites where sugar maple seedlings were infrequent, understory vegetation was more abundant and dominated by hop-hornbeam (Ostrya virginiana (Miller) K. Koch; 42% of all stems). At these two sites, experimental N deposition significantly reduced the abundance of understory hop-hornbeam by more than 75% (site×nitrogen: P =0.008). The effects of experimental N deposition on the understory and groundcover vegetation occurred without significant decreases in reproductive litter or increases in canopy leaf area. Instead, the negative effects are more likely a by-product of other documented changes caused by the experimental N deposition: increased forest floor mass, decreased mycorrhizal abundance, and increased production of potentially alleopathic phenolic compounds. Because the late-successional species in these forests rely upon groundcover and understory plants for regeneration, the effects of added N on this vegetation have potential implications for future forest composition, particularly given the likely loss of some species in these forests due to exotic pests and pathogens. [Copyright &y& Elsevier]

Abstract: Dolomitic lime (CaMg(CO3)2) was applied in 1994 at rates of 0–50Mgha−1 to sugar maples (SMs) (Acer saccharum Marsh.) in a base-poor and declining northern hardwood stand subjected to a high level of acid deposition in Quebec. The soil chemistry and the SM nutrition, growth, crown vigor, and regeneration status were evaluated 15years after treatment. The soil chemical properties still responded strongly to lime after 15years. Similarly, the foliar Ca and Mg concentrations were still higher for treated trees relative to the control trees. After 15years, the mean crown dieback of trees decreased quadratically with the lime rate, from 39% for the control trees to a value of 1–3% for the lime rates of 5Mgha−1 and higher. Additionally, the stem basal area increment for the limed trees was nearly double that of the unlimed trees in 2009. The lime application was also beneficial to the SM regeneration. The overall SM seedling density increased with the lime rate, being nearly twice as much in the 50Mgha−1 (32 seedlings m−2) compared with the controls (16 seedlings m−2). The proportion of the SM seedlings to all of the other species increased quadratically from 22% in controls to more than 55% in the 5–50Mgha−1 treatments. In contrast, the proportion of competitive species decreased quadratically with the lime rate, including American beech (Fagus grandifolia Ehrh.) for which the proportion in the treated plots (24%) was nearly half the proportion observed in the controls (46%). However, increase in stem density of regeneration and canopy closure in response to lime application limit the development of the regeneration which did not benefit in terms of diameter and height. These results show that a single lime addition has long-term beneficial effects on the soil chemistry and the SM nutrition, vigor, growth, and regeneration in base-poor and declining northern hardwood stands. Moreover, the results confirm that liming is an essential tool to restore the SM representation and health in acidic and base-poor soils. [Copyright &y& Elsevier]

Abstract: Crown shyness is thought to influence both the productivity and dynamics of forests, but few studies have examined the underlying causes of this common phenomenon. The few studies that exist suggest that crown shyness is caused by the reciprocal abrasion of neighboring tree crowns, resulting in the death of peripheral buds and/or the breakage of peripheral twigs (referred to here as twig dieback). However, twig dieback has not been directly observed due to the difficulty of accessing the crowns of mature canopy trees. In this study, we used a mobile canopy lift to obtain in-situ measurements of twig dieback in mature sugar maple (Acer saccharum Marsh.) trees. We measured: (1) percent dieback (%year−1) – the proportion of twigs that broke off or died; (2) dieback (cmyear−1) – the length of the broken or dead portion; and (3) regrowth (cmyear−1) – the length of the new twigs that sprouted from axillary buds. One third of the twigs suffered dieback over the course of 1year, resulting in 1.41cm of dieback, on average. Percent dieback was significantly higher in tree crowns located within 3m of another crown, indicating that dieback is the result of the reciprocal abrasion of neighboring crowns. Percent dieback also increased with tree size, suggesting that tree sway increases as trees grow larger, resulting in more frequent and more intense abrasion. These trends were exacerbated by similar trends in regrowth, which was slower both in large trees and in trees located with 3m of another crown. Our results suggest dieback may represent a substantial drain on both tree growth and stand productivity – a drain that increases as trees grow larger and stands mature. [Copyright &y& Elsevier]

Abstract: The use of silvicultural systems based on the natural disturbance patterns for a given region has been suggested as an approach for promoting late-successional forest characteristics and maintaining native biodiversity in managed forests. To this end, we examined the effectiveness of natural disturbance-based harvest gaps in maintaining and restoring native tree species diversity within second-growth northern hardwood systems in northeastern Minnesota, USA. In particular, tree regeneration and downed coarse woody debris (CWD) were measured 6- and 7-years post-treatment in 46 gaps that emulated the historic range of natural canopy opening sizes for northern hardwood forests in the upper Great Lakes region. Measurements were compared across gap size-classes (>0–0.02, >0.02–0.04, >0.04ha) and with closed canopy portions of these systems. In addition, the factors affecting the recruitment of Betula alleghaniensis across gaps and within closed canopy were investigated due to the historical importance of this species in these forests. Seedling and sapling densities increased in harvest gaps; however, results indicated that these gaps did little to increase tree diversity, including the recruitment of shade mid-tolerant species, such as B. alleghaniensis. Gaps were dominated primarily by Acer saccharum and age distributions of dominant saplings within gaps indicated that most individuals established 10–40years prior to gap formation. Establishment of B. alleghaniensis was strongly related to highly decayed, large coniferous pieces of CWD with little recruitment occurring on the undisturbed forest floor. Age distributions of B. alleghaniensis established on CWD suggest this species can persist on this substrate for at least a decade. Levels of CWD increased with increasing gap size; however, all gaps had lower levels of CWD compared to the surrounding intact forest. Our results suggest that management regimes based on natural canopy gap sizes within northern hardwood systems will do little to restore native tree diversity if provisions for creating the suitable seedling microsites historically generated by natural disturbance (e.g., exposed mineral soil, highly decayed wood) are not included within management prescriptions. [Copyright &y& Elsevier]

Abstract: Soil nitrification rates in northeastern USA forested ecosystems appear to be regulated by a number of factors and are likely influenced by continuing N deposition. Among other factors, rates across small watersheds have been found to relate to the soil C/N ratio and tree species composition. We measured potential net nitrification rates in the Ranch Brook watershed, a relatively large (9.6km2) forested basin in north central Vermont, to determine if relationships found in smaller catchments were evident at a larger scale. The stream network was divided into eight reaches to determine the variability within the watershed. Sampling points (6–15 along each reach, total of 74) were established along transects that paralleled the major watershed tributaries. At each point, we measured net rates of nitrification and ammonification in the uppermost humified soil horizon (Oa or A), using a one-day lab incubation. The basal area and density of all tree species were measured in a 10-m radius plot, along with a number of topographic metrics such as slope, aspect and elevation. In a stepwise regression, 39% of the variability in net nitrification rates was explained by the density of Picea rubens, elevation and the thickness of the forest floor. When net nitrification rates were normalized to soil C concentration, 60% of the variability was explained by soil N concentration, C/N ratio, elevation and the density of P. rubens. The significant negative influence of P. rubens density, and not basal area, was consistent with a previous cross-site study of 10 smaller northeastern USA watersheds. No influence of sugar maple basal area or density was found. Other relationships, similar to those found in smaller watersheds, were net nitrification rates predicted by the fraction of inorganic N as , net nitrification predicted by the C/N ratio and the C/N ratio predicted by tree species. The consistent influence of tree species on potential net nitrification rates demonstrates a role for future forest management in influencing ecosystem processes. [Copyright &y& Elsevier]

Abstract: Natural regeneration in canopy gaps is a key process affecting long-term dynamics of many forests, including northern hardwood forests. The density and composition of regenerating trees are often highly variable, reflecting sensitivity to a suite of driving factors operating at different scales (e.g., harvest gap to regional landscape), including production of seeds, physical characteristics of gaps and stands, competition with non-tree vegetation, and browsing by animals. Multivariate analyses over broad geographic areas provide insights into the relative effects of these factors and permit exploration of spatial patterns in regeneration. We examined the effects of gap-, stand-, and landscape-scale factors on densities of tree seedlings (<1m tall) and saplings (1–2m tall) in 59 selection-harvested northern hardwood stands located across a 4500km2 region of Michigan''s Upper Peninsula. We used Bayesian multilevel modeling to account for the hierarchical structure of the data and assess uncertainty in parameter estimates. Sugar maple (Acer saccharum) saplings were absent from 61% of 154m2 plots centered in harvest gaps (n =347) despite its high shade tolerance and overstory dominance, but densities were high in other gaps. Densities of sugar maple seedlings and/or saplings were negatively associated with a combination of greater stand-scale densities of white-tailed deer (Odocoileus virginianus), greater gap-scale cover of non-tree vegetation, and lower gap-scale light availability, with deer density having the greatest effect. Densities of unpalatable and commercially less valuable ironwood (Ostrya virginiana), the second most common regeneration species, were positively related to gap-scale seed-production potential but were unrelated to factors affecting sugar maple. Ironwood tended to replace sugar maple saplings in areas with high deer density. At the landscape scale, densities of sugar maple seedlings and saplings decreased with decreasing latitude and snow depth and increasing winter deer densities. These inverse spatial patterns suggest that deer herbivory can lead to landscape-scale variation in regeneration success. However, the spatial distribution of habitat types (a proxy for soil moisture and nutrient conditions) confound this observation, with higher densities of sugar maple generally located on stands with less nutrient-rich habitat types. Results demonstrate that combinations of factors operating at different scales, and with different relative magnitudes of impact, contribute to high variation in regeneration composition and density following timber harvest. Selection silvicultural practices, as currently applied, do not ensure regeneration of desirable species; practices might require modifications in general (e.g., increasing gap size) and to match them to regionally varying factors like deer density. [Copyright &y& Elsevier]

Abstract: Anthropogenic factors such as elevated deer populations, invasive earthworms or climate change may alter old-growth forests of the Upper Midwest region of the United States. We examined demographic trends of woody species across all size classes over 35 years in a late-successional forest dominated by hemlock (Tsuga canadensis), sugar maple (Acer saccharum) and yellow birch (Betula alleghaniensis) in Michigan''s Upper Peninsula using two sets of permanent plots. For the duration of the study period, species that were less-preferred white-tailed deer (Odocoileus virginianus) forage, especially sugar maple, comprised a much higher fraction of all seedlings and saplings compared to overstory trees. The density of small sugar maple declined across the study period, but no other species became more abundant, creating a more open forest understory. By the most recent census, preferred species for deer browse had been nearly eliminated from the understory, and declines in unpreferred species such as sugar maple were also apparent. We found small changes in temperature (<0.5–1°C rise in minimum and maximum temperatures depending on season) and precipitation (±28mm depending on season) and little evidence of invasive earthworms impacts. Our results suggest that the sustained elevated deer density is shifting the structure and composition of this old-growth forest. A demographic model showed that if current recruitment, growth and mortality rates were to continue for 500 years the forest would eventually reach a new equilibrium with virtually no hemlock or yellow birch remaining. [Copyright &y& Elsevier]

Abstract: Competition is a major determinant of plant growth and is often used in studies of tree growth and species coexistence. However, these approaches are usually temporally static, i.e., assessed at a single point or period in time. While constantly changing forest conditions due to natural and human-induced disturbances potentially alter competition among individuals, static approaches cannot qualify the temporal variability of competitive interactions. Here we present a longitudinal analysis of competitive interactions among trees and discuss the implication of our results for ecological interpretation. Spatially-explicit tree growth data were obtained from 18 study plots (0.4ha each) in sugar maple (Acer saccharum Marsh.) stands in Quebec, Canada. During the studied period (1980–2003), these stands had been disturbed by insect outbreaks (forest tent caterpillar, Malacosoma disstria Hubner) and by commercial partial harvest. We analyzed radial growth rates (outcome of competition) on an annual basis and as a function of tree biology (bole diameter, crown position), competition (above- and belowground competition from neighbours) and environmental conditions (light availability, harvest disturbance). Competitive interactions changed throughout the studied period. Canopy disturbance from partial harvest interacted with defoliators and influenced competition symmetry by favoring smaller trees. Competitive interactions seemed to have switched from below- to above-ground following canopy recovery after harvest. Release from competition due to partial harvest increase neighbourhood size (radius of effective competition) and enhanced the competitive pressure from larger individuals. The temporal variability in parameter estimates may be used for setting confidence intervals on competitive success (growth rates), thereby yielding a more robust basis for ecological interpretation. Our results also show that temporal variability in competitive interactions could contribute to the maintenance of high tree species diversity and structural complexity in some ecosystems by temporally altering species-specific responses to environmental change and disturbance. [Copyright &y& Elsevier]