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Although northern white-cedar (Thuja occidentalis; henceforth cedar) is thought to have few insect pests, arborvitae leafminers (primarily Argyresthia thuiella) have been known to cause leaf necrosis. Yet, historical evidence for leafminer outbreaks is limited. We combined leafminer larval surveys conducted between 1950 and 1992 with tree-ring analyses from eight cedar stands to reconstruct a history of leafminer outbreaks in Maine, USA. Our tree-ring data show distinctive 2- to 3-year growth reductions that we attribute to leafminers. Several such growth reductions correspond to peak leafminer larval abundances, providing evidence that the reductions are reliable indicators of leafminer activity. Outbreak severity within a site was unrelated to cedar abundance. Outbreak periods thus identified (beginning ca. 1919, 1937, 1950, 1962, mid-1970s, but not at all sites) suggest that leafminer damage may have been more prevalent (albeit patchy) than previously thought. This historical information is relevant given current outbreaks in Maine and elsewhere. [ABSTRACT FROM AUTHOR]

Understanding the influence of post‐disturbance forest management on tree regeneration is critical for assessing ecosystem recovery and guiding future responses. In particular, the influx of elevated coarse woody material (CWM) following wind disturbance, if left in situ, may impede herbivore access, thereby protecting saplings from browsing damage through a natural 'exclosure effect'.In 2013, a tornado in northcentral Maine, United States and subsequent salvage logging operations created three clear 'treatments' for evaluation of the exclosure effect: blowdown, blowdown plus salvage logging and an undamaged control. Nine years post‐tornado, we inventoried tree regeneration within these treatments to evaluate differences in sapling abundance, species composition, size structure and browsing intensity. We also inventoried CWM, including the height above forest floor.Results revealed significant differences in sapling composition and browsing intensity among treatments. The salvage treatment had the highest proportion of browsed saplings (56 ± 28%; mean ± standard error), followed by the control (9 ± 10%) and blowdown (5 ± 8%). Blowdown had by far the greatest mean (50 ± 9 cm) and average maximum (169 ± 43 cm) heights for CWM. Binomial generalized linear models revealed that browsing probability was a function of mean CWM height and an interaction between sapling density and proportion of sapling hardwoods. Thus, browsing damage was less likely in plots with greater CWM heights and more likely in plots with greater sapling density and more hardwood saplings.Synthesis and applications. This study furthers our understanding of ecosystem recovery following blowdown and salvage logging. Results suggest that salvage logging created important differences in coarse woody material (CWM) abundance and height distribution, when compared to un‐salvaged areas, and that these differences in turn altered sapling size structure and browsing intensity. These findings highlight the potential long‐term effects of successive disturbances, as the differences evident in these early stages may persist for decades or longer. Importantly, we provide evidence of the exclosure effect, suggesting that CWM retained in the un‐salvaged area protected saplings from moose browsing. Thus, in post‐disturbance areas where browsing threatens regeneration, we recommend that managers consider retaining CWM to serve as a natural exclosure. [ABSTRACT FROM AUTHOR]

Stand density management is central to achieving diverse silvicultural objectives. Decision-support tools in this domain range from expert opinion to sophisticated computer models that vary by forest type, region, and organization. The graphical frameworks represented by density management diagrams (DMDs) and stocking guides (SGs), collectively called size-density management charts (SDMCs), are well established and balance quantitative rigor with user accessibility. Regional differences in species composition and site quality are known to influence maximum size-density relationships, historically limiting the development of reliable charts. Advances in statistical modeling and data availability are overcoming this issue and should encourage more widespread use. We briefly review the history of SDMCs and propose the adoption of relative density based on stand density index as a logical metric for linking the DMD and SG formats. Examples of SDMC construction and use are illustrated for spruce-fir stands in Maine, USA. Study Implications: Recognition of the importance of stand density management has increased in recent years owing to its potential to address threats such as susceptibility to catastrophic wildfires and forest health issues exacerbated by environmental stress. Natural resource managers will benefit from adopting a language around density management that is both biologically grounded and consistent. A relative density framework is compatible with the two primary types of size-density management charts and thus meets this objective well. Relative density provides an ideal launching point for articulating levels of stand density, or stocking, to meet diverse management objectives determined under relevant socioecological constraints. [ABSTRACT FROM AUTHOR]

Understanding forest structural changes resulting from postdisturbance management practices such as salvage logging is critical for predicting forest recovery and developing appropriate management strategies. In 2013, a tornado and subsequent salvage operations in northern Maine, USA, created three conditions (i.e., treatments) with contrasting forest structure: blowdown, blowdown + salvage, and control (undisturbed). We sampled forest structure in five stands representing each of these three treatments. Our results document obvious and predictable changes to forest structure caused by the blowdown and salvage operations; however, they also include unexpected findings: downed coarse woody debris volume remained quite high in the salvaged areas, although its vertical distribution was markedly reduced; salvage operations did not reduce fine woody debris volume; and the salvage operation itself reduced the abundance of upturned root masses. Our study contributes to a growing body of literature highlighting the fact that outcomes of salvage operations vary considerably from situation to situation. Nevertheless, they suggest that salvage logging has important implications for residual stand structure and regeneration potential and that these implications should be considered carefully when weighing postdisturbance management options. [ABSTRACT FROM AUTHOR]

• CO 2 emissions were measured from a spruce stump chronosequence that spanned 0 to 32 years post-harvest. • CO 2 emissions increased from 0 years, peaked ca. 6–8 years, and decreased by 23 years post-harvest. • No clear relationship was found between CO 2 emissions and environmental variables. Understanding the forest carbon cycle has become increasingly important as carbon dioxide (CO 2) emissions contribute to the changing climate. Decomposition is a major component of the forest carbon cycle; however, aspects of wood decomposition remain poorly understood, especially for stumps. To fill this knowledge gap, we examined the change in CO 2 emissions over time from Picea rubens Sarg. (red spruce) stumps using a 32-year chronosequence (i.e., 0, 2, 4, 8, 23, and 32 years since harvest) derived from detailed harvesting records in a northern conifer forest in central Maine, USA, that has experienced repeated partial harvests. We found low initial CO 2 flux (3.2 μmol CO 2 m−2 s−1 at year 0) followed by a rapid increase, peaking 8 years post-harvest (24.3 μmol CO 2 m−2 s−1) followed in turn by a decrease to very low rates by years 23 and 32 (1.4 and 1.7 μmol CO 2 m−2 s−1, respectively). We found no clear relationship between CO 2 emissions and any of the environmental or stump variables tested (wood temperature, wood moisture, soil moisture, and/or stump volume), suggesting that time since harvest was the overriding influence on CO 2 flux rates. The large variability in CO 2 flux rates among stumps of the same time since harvest points to the need for future research that includes larger sample sizes and covers a wider range of environmental and stump variables to better capture potential sources of variation. Our results add to the growing body of research on carbon emissions from deadwood that can inform forest carbon-cycle models. In addition, forest managers, who are increasingly interested in carbon management, can use these results to assess harvesting impacts on forest carbon emissions. [ABSTRACT FROM AUTHOR]

The overall goal of this study was to evaluate the correlation of multiple abiotic and biotic factors with organic-horizon (O-horizon) carbon (C) content on the Penobscot Experimental Forest in central Maine, USA. O-horizon samples were collected and their associated depths were recorded from stands managed with a range of silvicultural and harvesting treatments (i.e., selection, shelterwood, and commercial clearcut) and an unmanaged control. The overall mean for O-horizon C content from all samples was 25.6 ± 16.1 Mg ha −1 (mean ± SD). The samples were used to develop a pedotransfer function for predicting O-horizon C content from O-horizon depth ( R 2 = 0.47, RMSE = 1.6 Mg ha −1 ) so that an average of O-horizon C content could be calculated for permanent sample plots on which abiotic and biotic factors were quantified. O-horizon depth measurements recorded along transects on permanent sample plots were used to calculate plot average O-horizon C content. There were no significant differences in average predicted O-horizon C content among selection, shelterwood, and commercial clearcut treatments. However, variation in predicted O-horizon C content between stands where the same treatment was applied was statistically significant and was likely due to the timing of harvests and abundance of dead wood buried within O horizons. Depth to redoximorphic features, cartographic depth to water table or saturated zone, drainage class, fine woody debris mass, downed woody debris volume, tree basal area, and the relative basal area of conifer species were not significant predictors of predicted O-horizon C content at the plot level. When the individual predicted values of O-horizon C content were modeled, within-plot variation accounted for 83.8% of the variance. Bryophyte mass, which was predicted from bryophyte cover, only explained 1.2% of the variation in O-horizon C content at the microsite level. These results highlight the sizeable variability in O-horizon C content within and among these mixed-species stands with various forest management and natural disturbance histories. [ABSTRACT FROM AUTHOR]

1. Disturbance histories derived from old-growth forest remnants in Europe and eastern North America have shaped many of our current theories of forest dynamics and succession. Yet the small size typical of these remnants suggests they might not capture the full range of variability that may emerge at larger scales. 2. We investigated the frequency and severity of natural disturbance in a 2000-ha old-growth landscape (Big Reed Forest Reserve) in northern Maine, USA. Given its size, the Reserve provides an ideal opportunity to study, at multiple scales, natural forest processes in a region that has otherwise been dramatically altered by human activities. Using dendrochronological methods, we reconstructed disturbance histories for 37 randomly located plots stratified by five forest types (hardwood forests, mixed woods forests, red spruce forests, northern white-cedar seepage forests and northern white-cedar swamps). 3. We found no evidence of stand replacing disturbance on any plot during the last 120–280 years (depending on plot). The overall mean disturbance rate was 9.6% canopy loss per decade (median 6.5%, maximum 55%, plots pooled), yet the distribution was strongly skewed toward the lower rates. 4. We found little differences in disturbance rates between forest types, save a slightly lower rate in the northern white-cedar swamps. However, if we ignore forest-type classifications, we see that disturbance rates are clearly influenced by gradients in the relative abundance of component tree species, owing to species’ relative susceptibilities to particular disturbance agents. 5. Synthesis. Relatively low rates of canopy disturbance allow the accrual of shade-tolerant saplings. The abundance of this advance regeneration, coupled with the absence of stand-replacing disturbance, has maintained canopy dominance by shade-tolerant species in all plots, all forest types and throughout the entire landscape. Disturbance histories from individual plots coalesce to form a picture of a landscape in which pulses of moderate-severity disturbance are interposed upon a background of scattered small-scale canopy gaps. The landscape-level mosaic resulting from this disturbance regime consists of patches in various stages of structural development, not various stages of compositional succession. [ABSTRACT FROM AUTHOR]

• Northern white-cedar is not recruiting well from seedling to sapling stage in northeastern lowland forests. • Substrate moisture in lowland sites varied predicably among microtopographic features. • Cedar regeneration was found more often than expected on mounds, and less often in pits. • Canopy openness was most strongly associated with cedar sapling success. • Findings support managing for diverse microtopography, expanding gaps, and control of browsing. Throughout much of its range, northern white-cedar (Thuja occidentalis L., hereafter cedar) has experienced a bottleneck in recruitment: cedar seedlings are often abundant in these stands, particularly in lowland settings, yet cedar sapling densities are quite low, leading to concerns that cedar canopy trees are not being replaced. Several barriers to cedar recruitment have been suggested; however, findings from previous studies have been inconsistent with regard to limiting factors. Our objective was to characterize the microsite conditions associated with the establishment of cedar seedlings and saplings. We achieved this objective by mapping the location of seedlings, saplings, and overstory trees in 15 lowland cedar stands at five sites in Maine, USA, and examining the fine-scale site conditions (microtopographic features, canopy openness) in which cedar seedlings and saplings occurred. In particular, we recorded the occurrence of seedlings and saplings on microtopographic mounds, pits (small depressions), and flats (transitional features between mounds and pits). Substrate moisture content in these features decreased in the order pits > flats > mounds. Contingency-table results demonstrated that live cedar seedlings, and to a slightly lesser extent saplings, were found more often than expected by chance on mounds, and less often on flats and pits. Logistic regressions using status (live vs. dead) as the response variable generally supported these findings: dead seedlings were strongly associated with pits; however, the occurrence of live seedlings and saplings did not differ between flats and mounds. A companion planted-seedling experiment strongly supported these results, showing that after two growing seasons, survival was significantly lower in pits (12%) when compared to flats and mounds, which had similarly high levels of survival (62 and 80% respectively). Logistic regressions also showed live seedlings and saplings to be prevalent under more open canopy conditions (mean 19%, range 6 to 42% openness); though results from a planted-seedling experiment suggested that greater canopy openness (mean 32%, range 13 to 57%) was detrimental to survival. Regressions also showed that browsed seedlings were more likely to be found dead. These findings point to management prescriptions that maintain microtopographic diversity, create moderately open canopy conditions, and protect stands from browsing to promote viable cedar populations in these ecologically and economically important forests. [ABSTRACT FROM AUTHOR]

Using dendrochronological analyses, we reconstructed a 300 year history of eastern spruce budworm (Choristoneura fumiferana (Clem.)) outbreaks in northern interior Maine. By analyzing radial growth patterns from the budworm host, red spruce (Picea rubens Sarg.), and nonhost, northern white cedar (Thuja occidentalis L.), we identified five outbreaks beginning ca. 1709, 1762, 1808, 1914, and 1976, all of which have been documented from eastern Canada. However, little or no evidence was found in our study for the 1830s, 1870s, or 1940s outbreaks also documented there. The mean outbreak return interval in our study (67 years) was roughly twice that postulated for eastern Canada. Differences in forest types, and associated stand dynamics, between the regions may explain the longer return intervals, and consequently the absence of these three outbreaks in Maine. Results also indicate that small, slow-growing trees exhibit a budworm signal very similar to that of overstory trees, once tree-ring series have been properly standardized. [ABSTRACT FROM AUTHOR]

This article evaluates the control associated with a 55-year-old experiment on the Penobscot Experimental Forest (PEF) in Maine. The PEF experiment provides an excellent example of the usefulness and limitations of controls. The experiment includes an untreated or "natural" area, which is used as a control for the study. Spruce composition suggests that selection cutting resulted in an increase in the proportion of spruce. The PEF untreated area was repeatedly partially harvested before the 20th century. One example of potential benchmark or reference condition for the PEF silviculture study is the Big Reed Forest Reserve. It has diverse topography and composition ranging from forested wetlands to ridge hardwoods and includes many of the stand and site types on which forestry is practiced in northern Maine. Density and diameter distribution data of theses sites serve as examples of valuable comparisons that can be made between the PEF, Big Reed, and Ecological Reserve data.

We examined the dynamics of down coarse woody debris (CWD) under an expanding-gap harvesting system in the Acadian forest of Maine. Gap harvesting treatments included 20% basal area removal, 10% basal area removal, and a control. We compared volume, biomass, diameter-class, and decay-class distributions of CWD in permanent plots before and 3 years after harvest. We also determined wood density and moisture content by species and decay class. Mean pre-harvest CWD volume was 108.9 m3/ha, and biomass was 23.22 Mg/ha. Both harvesting treatments increased the volume and biomass of non-decayed, small-diameter CWD (i.e., logging slash), with the 20% treatment showing a greater increase than the 10% treatment and both treatments showing greater increases than the control. Post-harvest reduction of advanced-decay CWD due to mechanical crushing was not evident. A mean of 18.48 m3 water/ha (1.85 L/m2) demonstrates substantial water storage in CWD, even during an exceptionally dry sampling period. The U-shaped temporal trend in CWD volume or biomass seen in even-aged stands may not apply to these uneven-aged stands; here, the trend is likely more complex because of the superimposition of small-scale natural disturbances and repeated silvicultural entries. [ABSTRACT FROM AUTHOR]

• 63% of cedars contained internal decay, precluding the use of age distributions. • Mean disturbance rate was 6.4% of canopy area per decade; pulses rarely exceeded 25%. • Most canopy trees showed one to three growth release before achieving canopy status. • Structure similar to other regional old-growth forests, but with abundant woody debris. Natural disturbance histories and stand structures derived from old-growth forests are increasingly used to guide forest management prescriptions. Although such information is readily available for a number of forest types, it is lacking for others, such as northern white-cedar (Thuja occidentalis) forests, despite this forest type's wide distribution, ecological value, and economic importance in northeastern North America. We applied standard dendrochronological methods to six old-growth northern white-cedar stands within the Big Reed Forest Reserve of northern Maine, USA, to reconstruct the frequency and severity of past natural disturbances. The prevalence of internal decay (well-known for this species) precluded the construction of robust age-class distributions. Overall, 63% of cedar trees contained internal decay, and the probability of rot increased with increasing diameter. Evidence from growth releases reveals sporadic pulses of low- to moderate-severity canopy disturbances. The mean disturbance rate was 6.4% of canopy area disturbed per decade, and pulses rarely exceeded 25% per decade. Based on the subset of complete cores (i.e., those without internal decay), 44% of current canopy trees showed one growth release before achieving canopy status, 26% showed two releases, and 7% showed three releases. Of the 23% that showed no release, most showed persistent slow growth that eventually placed them in the canopy. However, an apparent hiatus in cedar recruitment in recent decades (albeit based on a subset of complete cores), as well as low cedar abundance in the sapling layer, suggest that cedar may not maintain dominance in the future overstory. Current structure in these stands is similar to that reported from other old-growth conifer forests in the region: mean living tree basal area was 44.3 m2 ha−1, density of large (>40 cm dbh) living trees was 130 trees ha−1, and coarse woody debris volume was 183 m3 ha−1. Taken together, these findings suggest that multi-aged silvicultural treatments incorporating periodic harvests of low to moderate intensity, retention trees or patches, and protection of coarse woody debris would be appropriate for sustaining or restoring lowland northern white-cedar forests. [ABSTRACT FROM AUTHOR]

• The responses of saproxylic insects to Tornado damaged forests were studied. • Saproxylic insects responded more strongly to blowdown and salvage logged areas. • No concerning increase in Dendroctonus rufipennis populations were noted. • Residual basal area and stem density influenced saproxylic insect communities. In July 2013, an EF1 tornado and straight line winds caused extensive forest damage in the Scientific Forest Management Area of Baxter State Park in northern Maine, USA. Subsequent salvage operations removed damaged trees from some areas, while leaving blowdown in other areas. Undisturbed control, blowdown, and blowdown + salvage (hereafter, salvage) treatments were established within and adjacent to damaged forests. Bark beetles (Coleoptera: Curculionidae: Scolytinae) and woodborers (Coleoptera: Buprestidae, Cerambycidae; Hymenoptera: Siricidae) were trapped using semiochemical-baited traps from within each of these areas over a three-year period (2014–2016). Disturbance-initiated outbreaks of Dendroctonus rufipennis were a major concern after the windstorm. However , D. rufipennis populations were low throughout the three-year experiment, and were captured more often in controls than in the two disturbed treatments. Overall, bark beetles and woodborers were captured at higher numbers in the two disturbed treatments, relative to controls. Individual species responses varied, with many species found more often in one of the disturbed treatments compared to controls. Species richness and abundance were generally higher in disturbed treatments than in controls. Blowdown and salvage treatments were also more similar to one another than to controls. Large amounts of dead wood provided easily exploitable habitat for wood-inhabiting insects in treatments; however, no evidence of alarming population increases of primary bark beetles were observed, suggesting that salvage operations are not always necessary to protect residual trees from attack by damaging bark beetles. [ABSTRACT FROM AUTHOR]

• Tree species, crowding, and diameter were top predictors of growth and survival. • Tree growth improved if neighbor species differed from that of the focal tree. • Crowding from saplings did not influence canopy tree growth. • Within a stand, tree success may be limited by insufficient and excess moisture. Individual tree growth and mortality drive forest stand dynamics and are universally important metrics of tree success. Studying factors that affect growth and mortality is particularly challenging in mixed-species, uneven-aged systems due to their defining heterogeneity and strong temporal and spatial variability. The goal of this study was to determine the relative importance of individual tree attributes (i.e., species, size, neighborhood crowding, crown position) and environmental characteristics (i.e., soil moisture) in driving tree growth and survival in an uneven-aged, mixed species forest. In particular we tested if the factors regulating growth were the same as those regulating mortality, as is often assumed. Due to its large size and intensive sampling, the 3-ha, stem-mapped plot (established in 1989) at Howland Research Forest in central Maine, USA, allowed us to address additional questions regarding the influence of sapling crowding, neighbor species identity, and legacies of past disturbance. Growth and survival of over 3000 plot trees was assessed after 25 years and modeled using multiple linear regression (growth) and binary logistic regression (survival). As expected, species, neighborhood crowding, and tree diameter were top predictors of growth and survival. Specifically, growth and survival decreased with greater crowding, and increased with larger diameters. We also found that the identity of neighbors influenced focal tree growth: growth generally improved in neighborhoods comprised of species different from that of the focal. However, this general finding did not hold for all species: eastern hemlock grew better in hemlock neighborhoods, and northern white-cedar showed no response related to neighbor identities. In contrast to growth, neighborhood identity was not related to survival. Crowding from saplings did not explain any additional variability in growth; however, unexpectedly, individuals with greater sapling crowding were more likely to survive. For both growth and survival, we found an interaction between crowding and soil moisture, suggesting that within a single stand, individual success can be limited by both excess and insufficient moisture. We found no relationship between neighborhood cut stumps (legacy of past disturbance) and recent growth or survival. These results highlight the many variables driving growth and survival in uneven-aged, mixed-species forests. The top predictors for growth were identical to those for survival; however, other predictors differed in their relative importance. Given the recent emphasis on promoting uneven-aged, mixed-species forests, we suggest that studies addressing a full range of predictors of individual tree success are necessary to better manage and maintain these complex systems. [ABSTRACT FROM AUTHOR]