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The Tibetan Plateau holds some of the world's highest undisturbed natural treelines and timberlines. Such extreme environments constitute potentially valuable monitoring sites of the effects of climate warming on high-elevation forests. Here, we analyze a network of 21 Smith fir forests situated in the Sygera Mountains, southeastern Tibetan Plateau, using tree-ring width (TRW) and basal area increment (BAI) chronologies. Sampled sites encompassed a wide elevation gradient, from 3600 to 4400 m, including some treeline sites and diverse aspects and tree ages. In comparison with TRW series, BAI series better capture the long-term warming signal. Previous November and current April and summer temperatures are the dominant climatic factors controlling Smith fir radial growth. The mean inter-series correlations of TRW increased upwards, but the forest limit presented the highest potential to reconstruct past temperature variability. Moreover, the growth responses of young trees were less stable than those of trees older than 100 years. Climate warming is accelerating radial growth of Smith fir forest subjected to mesic conditions. Collectively, these findings confirm that the effects of site elevation and tree age should be considered when quantifying climate-growth relationships. The type of tree-ring data (BAI vs. TRW) is also relevant since BAI indices seem to be a better climatic proxy of low-frequency temperature signals than TRW indices. Therefore, site (e.g., elevation) and tree (e.g., age) features should be considered to properly evaluate the effects of climate warming on growth of high-elevation forests. [ABSTRACT FROM AUTHOR]

Forest diversity-productivity relationships have been intensively investigated in recent decades. However, few studies have considered the interplay between species and structural diversity in driving productivity. We analyzed these factors using data from 52 permanent plots in southwestern Germany with more than 53,000 repeated tree measurements. We used basal area increment as a proxy for productivity and hypothesized that: (1) structural diversity would increase tree and stand productivity, (2) diversity-productivity relationships would be weaker for species diversity than for structural diversity, and (3) species diversity would also indirectly impact stand productivity via changes in size structure. We measured diversity using distance-independent indices. We fitted separate linear mixed-effects models for fir, spruce and beech at the tree level, whereas at the stand level we pooled all available data. We tested our third hypothesis using structural equation modeling. Structural and species diversity acted as direct and independent drivers of stand productivity, with structural diversity being a slightly better predictor. Structural diversity, but not species diversity, had a significant, albeit asymmetric, effect on tree productivity. The functioning of structurally diverse, mixed forests is influenced by both structural and species diversity. These sources of trait diversity contribute to increased vertical stratification and crown plasticity, which in turn diminish competitive interferences and lead to more densely packed canopies per unit area. Our research highlights the positive effects of species diversity and structural diversity on forest productivity and ecosystem dynamics. [ABSTRACT FROM AUTHOR]

Droughts and their negative effects on forest ecosystems are projected to increase under climate change for many regions. It has been suggested that intensive thinning could reduce drought impacts on established forests in the short-term. Most previous studies on the effect of thinning on drought impacts, however, have been confined to single forest sites. It is therefore still unclear how general and persisting the benefits of thinning are. This study assesses the potential of thinning to increase drought tolerance of the wide spread Scots pine ( Pinus sylvestris) in Central Europe. We hypothesized (1) that increasing thinning intensity benefits the maintenance of radial growth of crop trees during drought (resistance) and its recovery following drought, (2) that those benefits to growth decrease with time elapsed since the last thinning and with stand age, and (3) that they may depend on drought severity as well as water limitations in pre- and post-drought periods. To test these hypotheses, we assessed the effects of thinning regime, stand age, and drought severity on radial growth of 129 Scots pine trees during and after drought events in four long-term thinning experiments in Germany. We found that thinning improved the recovery of radial growth following drought and to a lesser extent the growth resistance during a drought event. Growth recovery following drought was highest after the first thinning intervention and in recently and heavily thinned stands. With time since the last thinning, however, this effect decreased and could even become negative when compared to unthinned stands. Further, thinning helped to avoid an age-related decline in growth resistance (and recovery) following drought. The recovery following drought, but not the resistance during drought, was related to water limitations in the drought period. This is the first study that analyzed drought-related radial growth in trees of one species across several stands of different age. The interaction between thinning intensity and time since the last thinning underline the importance to distinguish between short- and long-term effects of thinning. According to our analysis, only thinning regimes, with relatively heavy and frequent thinning interventions would increase drought tolerance in pine stands. [ABSTRACT FROM AUTHOR]

Drought is killing an increasing number of trees globally, yet mortality risk remains difficult to predict at fine spatial scales. We sought to identify metrics of living individuals that could be used to estimate mortality risk of Ashe juniper ( Juniperus ashei) trees and eventually to estimate the fraction of juniper populations at risk from drought. Ashe juniper is a keystone species in the Edwards Plateau region in central Texas, USA. We analyzed tree rings from both living and dead trees to determine growth rate prior to an historic drought in 2011 and measured morphological, physiological, and stand-level variables hypothesized to link growth rate and mortality risk. Slowly growing trees were disproportionately vulnerable to mortality. Fractional mortality of sampled trees was correlated to the difference between the mean predrought basal area increment (BAI) per tree and the predrought BAI of minimally stressed trees growing on deep soil (=BAI − BAI). Slowly growing trees had sparsely foliated canopies. BAI − BAI was positively correlated to the difference between: (1) leaf area per unit of projected canopy area per tree (LA) and the LA of minimally stressed trees and (2) projected canopy area (CA) and the CA of comparably sized trees. By contrast, there was no correlation between growth of living trees and light interception by neighboring trees, soil depth, or two functional metrics, the stem-leaf Ψ gradient and leaf light use efficiency. Mortality risk in Ashe juniper populations can be estimated from nondestructive measurements of leaf and canopy area of individual trees using relationships among risk, growth, and leaf and canopy area. [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]

It is expected that extreme climate events such as droughts will increase in both severity and intensity as a consequence of forecasted climate change. Complementarity among tree species in resource acquisition strategy may reduce interspecific competition and increase the occurrence of facilitative interactions, resulting in an improved tree growth and resilience to extreme climatic events. However, the response of individual trees growing in more functionally diverse stands to extreme events is still under debate., We investigate the growth response of silver fir ( Abies alba) to an extreme drought event which occurred in 1985-1986 in the Central Spanish Pyrenees and how the growth and resilience of trees relate to their neighbourhood functional diversity, stand heterogeneity and intraspecific and interspecific competition. Dendroecological methods were used to reconstruct radial growth. We recorded all tree species living around each sampled tree and calculated indices of intra- and interspecific competition among coexisting trees. Functional diversity around each focal tree was calculated by using the Rao's quadratic entropy., Silver fir growth decreased significantly in response to the drought in the vast majority of trees, but most of them showed a growth recovery 3 years later. Despite most trees showed negative growth trends following the drought event (70%), those trees growing in more functionally diverse stands were more resilient, recovered more quickly and displayed greater growth and growth trends., Synthesis. The presented findings support the growing body of evidence that shows a positive impact of diversity on forest function. In addition, we provide a direct linkage between tree growth response to climate at the individual tree scale and a direct quantification of neighbourhood functional diversity and competition. An enhanced functional diversity facilitating a more efficient use of resources at the interspecific level and potentially increasing facilitative interactions contributes to lessening the negative impacts of extreme events as droughts on forest growth. [ABSTRACT FROM AUTHOR]

Pinus halepensis plantations are widespread throughout semiarid-subhumid landscapes of the Mediterranean. Recently, drought-induced decline has often been reported raising concerns with regard to the future of these man-made ecosystems. The study was set out to investigate thinning as a means to alleviate water stress and improve performance of mature P. halepensis plantations experiencing prolonged drought. The study was conducted in a 40-year-old P. halepensis forest in the Jerusalem Mountains of Israel (rainfall: 550 mm year). Declining stands (stand basal area increment, BAI ≈ −3 % year) were treated by thinning: (1) intense thinning-tree density, BA, and leaf area index (LAI) reduced by 81, 68, and 59 %, respectively; (2) moderate-56, 48, and 26 %, respectively; (3) control (~560 tree ha). Plots of 70 × 70 m were used in four replicates. Individual tree- to stand-level variables were monitored during 4 years through stem and leaf area metrics alongside with predawn shoot water potential ( ψ) and tree mortality. Thinning ameliorated drought stress, reduced mortality, and improved individual tree growth ( ψ = −1.7, −1.8, and −2.0 MPa; mortality = 0.2, 2, and 5 % year; BAI = 3.4, 2.0, and 1.4 % year in intense, moderate, and control treatments, respectively). Thinning effects became more pronounced with time. LAI and individual tree leaf area (LA) fluctuated with association to annual rainfall. Higher LA caused by thinning reflected a 'selection effect' while increased leaf area efficiency (BAI per unit LA) was attributed to a 'release effect' of thinning. [ABSTRACT FROM AUTHOR]

Recent studies have revealed that European beech ( Fagus sylvatica L.) has significantly increased its growth in Central Europe during the last century but recently started to decline at the edge of its southern distribution. Climate scenarios predict an increased frequency of severe drought events in the future, which is supposed to cause a decline of beech forests even towards the northern edge. Whether the management has the potential to mitigate the negative effects of climate warming has not been fully addressed yet. In order to fill the knowledge gap, we compiled data from 29 long-term research plots (LTRP) at 8 sites across the western Carpathian Mountains (Eastern Europe). The LTRP were established in 1958–84 and measured every 4–5 years till 2015. Development of forest stand attributes including top height (h top ), mean quadratic diameter (d q ), mean annual tree volume increment (iv̅), periodic annual volume increment (PAIV), mean annual basal area increment (BAI), and total yield production (TY) was compared with the simulations by the Slovakian yield models developed in the 1980s based on data from the period before recent climate change. Results were additionally confronted with the growth of beech forests in a larger Central European region. Results showed an increase of TY since the 1960s compared to the simulated TY, starting from the same value, ranging from 5% to 40% and mainly depending on site quality and average annual temperature. The largest increase was found on less productive sites, which was in line with the previous findings in recent literature. Interestingly, beech TY in the Western Carpathians was found to be lower by −11% on average compared to beech forests in Central Europe (Germany). Moreover, while an increase in the BAI continues in unmanaged forests, it has recently slowed down in forests managed by “free crown thinning” and it even started to decrease in less productive forests where heavy thinning from below was applied. Finally, our results showed that the responses of beech BAI to climate variation significantly depended on tree class. [ABSTRACT FROM AUTHOR]

Funderburk, W.R.; Carter, G.A., and Anderson C.P., 2016. Evaluating the Influence of elevation and impact of hurricane Katrina on radial growth in slash pine ( Pinus elliottii var. elliottii Engelm) on Cat Island, Mississippi, U.S.A. Hurricane Katrina heavily affected the Mississippi barrier islands, in the northern Gulf of Mexico, on 29 August 2005. On Cat Island, Mississippi, field observations of geographic patterns in poststorm survival of slash pine ( Pinus elliottii var. elliottii Engelm) suggested that its growth and resiliency to storm impact may depend substantially on ground elevation. This hypothesis was tested by evaluating the lifetime mean radial growth rate (in millimeters per year), stem (trunk) diameter (in centimeters), the percentage of change in the radial growth rate for the initial 5-y period after the storm (2006-10) vs. the 5-y period before the storm (2001-05), and the yearly percentage increase in stem radius and basal area for 2001-10 with respect to high-accuracy GPS measurements of growth-site elevation. Radial growth declined after the storm and remained suppressed in 2006-10. However, simple regressions indicated no relationship between growth and elevation, either over the lifetime of the trees or on a yearly basis in 2001-10. Instead, radial growth was related to tree age and, most strongly, to reciprocal age. Notably, the relationship with age diminished while growth was suppressed in 2005-10. Although tree growth was not related to elevation, a future study will explore the mechanisms underlying an observed 0.5-m elevation threshold for slash pine existence on Cat Island. [ABSTRACT FROM AUTHOR]

Soil properties can buffer forest response to global climate change. However, it is unclear how soil characteristics, water availability and their interactions can affect drought response of trees. The aim of this study was to assess the influence of soil nutrients and physical soil properties on the growth sensitivity of Fagus sylvatica, Quercus spp., Fraxinus excelsior, Abies alba, Picea abies and Pinus sylvestris to drought in Central Europe., Yearly growth data from increment cores were obtained from 538 trees and combined with forest inventory and soil data at 52 sites covering a large gradient of water availability and C/N ratios in soil. Linear mixed-effects models were used to assess the species-specific growth responses to climate and soil properties for the period 1957-2006. The growth of the species was further projected across the full range of C/N and water availability observed at 1029 sites where soil and species cover-abundance data were available., Temperature, water and nutrient availability (C/N) were the most important factors for tree growth. Drought and low nutrient availability significantly reduced the growth of beech, ash, fir and spruce along the gradient. In contrast, the growth of pine and oak was little reduced on poor and dry sites, hence showing their competitive advantage over nutrient-demanding species under such conditions. The growth of ash and pine was enhanced at sites with high species abundance, whereas an opposite response was found for spruce. No clear relationships between growth and species abundance were found for beech, oak and fir., Synthesis. Our results suggest that assessing tree responses to climate change without considering simultaneously soil properties and climate may be misleading since soil nutrients can influence growth response of trees to drought. A detailed analysis of the influence of the soil characteristics on growth responses of trees is necessary to understand the sensitivity of tree species to global climate change. [ABSTRACT FROM AUTHOR]

The amount of sap that can be extracted annually from trees for maple syrup production using current equipment and practices is more than double the typical yields achievable when current maple industry tapping guidelines were developed. The growth rates of trees tapped with these "high-yield" practices at 18 sites in Vermont were measured and evaluated to determine whether they were sufficient for the replenishment of conductive wood to remain at sustainable levels when current tapping guidelines are followed. The basal area increments of healthy codominant or dominant trees across the sites ranged from 1.8 (± 0.2) in.²/year in 10-in. diameter trees, to 3.5 (± 0.3) in.²/year in 18-in. diameter trees. The estimated minimum growth rates required ranged from 1.4 in.²/year in 10-in. trees to 2.6 in.²/year in 18-in. trees. These results suggest that the growth rates of many trees tapped with high-yield sap collection practices are sufficient for this activity to remain sustainable when current tapping guidelines are followed. However, an average of 35% of sampled trees had growth rates below the estimated minimums. This indicates that tapping practices must be modified for some trees to ensure that adequate replenishment of conductive wood is maintained and that growth rates must be measured to be certain sustainable tapping practices are implemented. [ABSTRACT FROM AUTHOR]

Tree growth is strongly influenced, among other factors, by climate. Much knowledge regarding climate–growth relationships has been gained by studying tree rings. However, sufficient tree-ring data is rarely available if climate effects are required to be representative for large spatial scales, for example, in order to be used in scenario models to estimate forest development under climate change. Alternative data sources include large-scale forest inventories, although these usually provide lower temporal resolutions than tree rings. When working with temporally sparsely-resolved growth data, the question of how climate–growth relationships aggregate over time becomes relevant. To overcome this trade-off between spatial representativeness and temporal resolution, this study aims at optimally using the information contained in the annual resolution of tree rings to derive recommendations regarding the choice of climate variables for modeling tree growth based on forest inventories. We evaluated for Picea abies and Fagus sylvatica , which part of the year (spring, summer, vegetation period, whole year) and whether mean or extreme climatic conditions within inventory intervals should be taken into consideration. A three-step approach was used: (1) we used response functions to quantify the effect of monthly precipitation and temperature on annual basal area increments, (2) we temporally aggregated the annual basal area increments to hypothetical intervals of five and ten years, and correlated them with climate means and extremes – from different parts of the year – within the aggregated intervals, and (3) we fitted linear mixed-effects models to simultaneously quantify the effects of the climate variables, site characteristics and the years of the hypothetical inventories. The results did not generally differ between both species. Variables based on conditions during the whole year and partly during spring performed better than variables based on conditions during summer or the vegetation period. Defining the year as the period between October of the previous year and September of the current year allows possible lag effects of previous autumn and winter conditions to be taken into consideration. Mean climatic conditions reached or exceeded the correlations of the extremes and mostly performed similar to or better than the extremes in the models. Our results indicate that these relationships are insensitive to the often arbitrarily determined years, in which inventories take place. These findings can serve as basic recommendations for the choice of climate variables when modeling climate effects on multi-year growth of P. abies and F. sylvatica in the European lowlands. [ABSTRACT FROM AUTHOR]

Acidic sulfur (S) and nitrogen (N) deposition depletes cations such as calcium (Ca) from forest soils and has been linked to increases in foliar winter injury that led to the decline of red spruce ( Picea rubens Sarg.) in the northeastern United States. We used results from a 30 m resolution steady-state S and N critical load exceedance model for New England to better understand the spatial connections between Ca depletion and red spruce productivity. Atmospheric deposition and other inputs were estimated for a 5-year period (1984–1988) in order to smooth year-to-year variations in climate and patterns of atmospheric transport. Deposition levels prior to the reductions that followed the 1990 Amendments to the Clean Air Act were used because tree health and productivity declines were expected to be most responsive to high acid loading. We examined how radial growth (basal area increment) of 441 dominant and co-dominant red spruce trees from 37 sites across Vermont and New Hampshire was related to modeled estimates of S and N critical load exceedance. We assessed growth using statistical models with exceedance as a source of variation, but which also included “year” and “elevation class” (to help account for climatic variability) and interactions among factors. As expected, yearly climate-related sources of variation accounted for most of the differences in growth. However, exceedance was significantly and negatively associated with mean growth for the study period (1951–2010) overall, and particularly for the 1980s and 2000s – periods of numerous and/or severe foliar winter injury events. Because high winter injury reflects the convergence of predisposing (cation depletion) and inciting (weather) factors, exceedance alone appears insufficient to define associated patterns of growth reduction. Significant interactions indicated that exceedance had little influence on growth at low elevations (where intrinsic conditions for growth were generally good) or high elevations (where growth was uniformly poor), whereas exceedance was significantly associated with reduced growth at mid elevations over long periods of time. Exceedance was also linked to reduced growth rebounds following a region-wide foliar winter injury event in 2003. Overall, our analyses suggest that modeled S and N critical load exceedance can help account for red spruce growth and rebound from injury in the field. Interestingly, recent growth for red spruce is above average for the 20th to 21st century dendrochronological record – indicating that the factors shaping growth may be changing. The influence of reduced pollution inputs on this recent growth surge is under investigation. [ABSTRACT FROM AUTHOR]

Understanding the relative contributions of competition and climate on individual tree growth is critical to project realistic forest dynamics under projected climate scenarios. Furthermore, present competition levels may reflect legacies of past use. Here, we analyze the effects of climate, site conditions and competition on radial growth in three Scots pine stands located along an altitudinal gradient in central Spain. Current stand structure and retrospective analyses of radial growth (basal area increment, BAI) were used to model changes in tree growth as a function of a spatially-explicit competition index (CI) and climate. Linear mixed-effects models were employed to model BAI and to quantify the growth responses to climate of trees under low and high competition levels. Competition effects on growth were steady over time regardless of tree age. High competition levels negatively affected growth since negative exponential functions characterized the CI–BAI relationships. Tree growth sensitivity to climate increased with decreasing competition intensity. Growth at high elevations was mainly limited by low winter temperatures, whereas warm spring enhanced growth at middle elevations and warm late summer temperatures constrained growth at low elevation. Growth responsiveness to climate is enhanced under low competition levels. Overall, current competition is a more relevant driver of recent growth than climate. Proactive forest management should be adopted to reduce the vulnerability of Scots pine forests currently subjected to higher competition levels and warmer and drier conditions. [ABSTRACT FROM AUTHOR]

Fire severity and extent are expected to increase in many regions worldwide due to climate change. Therefore, it is crucial to assess the relative importance of deterministic vs. stochastic factors producing remnant vegetation to understand their function in the persistence of fire-sensitive plants. Vegetation remnants (areas within the landscape that have not burned for a considerable amount of time) may occur stochastically or in more predictable locations (fire refuges) where physical conditions decrease fire severity. Our aim was to determine if remnant forests of the fire-sensitive conifer Austrocedrus chilensis are associated with biophysical attributes that allow persistence in a fire-prone Patagonian landscape. We conducted a multi-scale approach, determining attributes of forest remnants and their surroundings (matrices) through remote sensing and field-based biophysical and functional characteristics, and quantifying how tree survival probability relates to microsite conditions. Trees within remnants displayed abundant fire scars, were twofold older and had threefold larger growth rates than matrix trees. Remnants were associated with high rocky cover and elevated topographical positions. Tree survival increased in hilltops, eastern aspects, and with sparse vegetation. Trees within remnants experienced severe reductions in growth during droughts. Our results suggest that A. chilensis remnants are mainly the result of refuges, where environmental conditions increase fire survival, but also increase susceptibility to drought. A trade-off between fire survival and drought vulnerability may imply that under increasing drought and fire severity, locations that in the past have served as refuges may reduce their ability to allow the persistence of fire-sensitive taxa. [ABSTRACT FROM AUTHOR]

Key message: Radial growth responses of gap-edge trees to GT intensity greatly depends on microclimate condition (soil temperature and PAR) within gaps and their abilities in competing and utilizing the limited resources. Abstract: Gap-model thinning (GT) is an effective management technique to improve the productivity of dense monoculture plantation. However, how GT intensity influence inter- and intra-annual radial growth of the gap-edge trees remains an open question. In order to explore this question, we implemented an experiment with four treatments (control and gap sizes of 74, 109, and 196 m) in a 30-year-old spruce ( Picea asperata) plantation in 2008. We used dendrochronological method to explore dynamic of the basal area increment (BAI) of gap-edge trees ( n = 127) from 2009 to 2012 after GT treatments. We found that effects of GT on the BAI of gap-edge trees highly depended on GT intensity and their crown classes. The large gaps significantly increased the BAI of gap-edge trees and further the latewood growth responded more sensitively than the earlywood, whereas the influences of the small and intermediate gaps were weak; the large gaps effectively improved the BAI of the dominant trees, following by the intermediate, but not for the suppressed trees. Our results also displayed that the earlywood increment was strongly related to the soil temperature in spring and the latewood formation was influenced by the photosynthetically active radiation in autumn. It was eventually concluded that dynamic radial growth of gap-edge trees were driven by the improved microclimate caused by GT and their abilities in competing and utilizing resources. The present study highlights the importance of setting appropriate GT intensity and choosing the felled trees in GT operation. [ABSTRACT FROM AUTHOR]

Many forest ecosystems with a large pine component in the western United States have experienced environmental stress associated with climate change and increased competition with forest densification in the absence of fire. Information on how changes in climate and competition affect carbon allocation to tree growth and defense is needed to anticipate changes to tree vigor and, ultimately, stand structure. This study retrospectively examined the influence of annual climate and competition measures on the growth and defense of 113 large sugar pines (Pinus lambertiana) in a mixed-conifer forest of the central Sierra Nevada of California. We found that growth in large sugar pine was positively associated with higher January temperatures and lower intraspecific competition. Resin duct size was negatively associated with climatic water deficit and total competition, while resin duct area contrastingly showed a positive relationship with total competition. From 1979 to 2012, the rates of growth increased, while resin duct size decreased. Our results suggest that tree vigor measures can respond differently to climate and competition factors that may lead to separate growth and defense trends over time. Stress associated with warmer temperatures and higher competition may distinctly influence individual tree and stand-level vigor with potential implications for future forest dynamics.

• Mingling effect in forest responses to drought was conducted in semiarid Siberia. • Conifers showed remarkable consistency in drought tolerance regardless of mingling. • Broad-leaved trees suffer less drought stress in mixed forests. • Decreasing duration of legacy effect with increasing mingling was found. Hotter droughts are becoming a major threat to forest growth in water-limited regions. Species interactions have long been acknowledged as an important component modulating ecosystem functions, including mediating their vulnerability to climate-related stresses. However, quantifying the impact of species interactions on forest growth in response to drought remains a challenge. Using the tree ring data of two species in two pure forests and two coniferous and broad-leaved mixed forests with different degrees of mingling in southern Siberia within the period of 1960–2017, we found that mixed forests with higher mingling have a higher ability to withstand the effect of extreme drought events, which is especially beneficial to the resistance and resilience of broad-leaved trees. Larch trees showed a remarkable consistency in their resistance and resilience to extreme drought events in both pure and mixed stands, suggesting their ability to promote the growth of neighboring white birch without reducing their own growth. The growth of white birch trees was enhanced in mixed forests and showed a significant trend of increasing resistance and resilience with increasing mingling. Our results provide further evidence of a decreasing duration of the legacy effect with increasing mingling for both coniferous and broad-leaved trees, which might explain the improved tolerance to drought of trees in mixed forests compared to those in pure forests in water-limited hemiboreal southern Siberia. Our study implies that changes in forest structure might help forests respond to hotter droughts in water-limited regions. [ABSTRACT FROM AUTHOR]

Aim Forest responses to global-change drivers such as rising atmospheric CO2 concentrations ( Ca), warming temperatures and increased aridification will depend on tree species and site characteristics. We aim to determine if rising Ca enhances growth of coexisting pine species along broad ecological gradients in a drought-prone area. Location Iberian Range, Spain. Methods We sampled 557 trees of five pine species encompassing a wide climatic gradient and measured their radial growth. We used nonlinear flexible statistics (generalized additive mixed models) to characterize growth trends and relate them to Ca, temperature and water balance. Results The sites most responsive to the growing-season water balance were dominated by Pinus pinaster and Pinus nigra at low elevations, whereas those most responsive to temperatures were high-elevation Pinus sylvestris and Pinus uncinata stands. From 1950 onwards, most sites and species showed decreasing radial growth trends. Growth trends were coherent with a CO2-related fertilization effect only in one P. sylvestris site. Main conclusions We found little evidence of growth stimulation of Iberian pine forests due to rising Ca. The results indicated that any positive effect of a Ca-induced growth increase was unlikely to reverse or cancel out the drought-driven trends of reduced growth in most Mediterranean pine forests. Further assessments of CO2-fertilization effects on forest growth should be carried out in sites where climatic stressors such as drought do not override the effects of rising Ca on forest growth. [ABSTRACT FROM AUTHOR]

Douglas-fir (Pseudotsuga menziesii [Mirb.] Franco var. menziesii) and Douglas-fir/western hemlock (Tsuga heterophylla [Raf.] Sarg.) stands were thinned at age 50 to different levels of residual stocking, with and without gaps on medium or better (McDonald) and excellent (Blodgett) sites in western Oregon. Postthinning basal areas ranged from 75 to 140 ft2/acre in 35-80 trees/acre. The range of densities led to periodic Scribner and cubic volume growth peaking at ages 60-90 with mean annual increment peaking after age 100 based on Organon model projections. Postthinning stand growth was greatest with the highest residual basal area; percent growth was highest in the lowest densities. Projections indicated that allocating 20% of plot area in 1/6-1/4 acre gaps with stand basal areas comparable to uniform plots led to 1-10% reductions in growth. Reductions were not statistically significant. Diameter increment and average size exceeded normal yield tables at one site and were inversely related to basal area. At age 103, the difference in projected quadratic mean diameter across the densities was about 6 inches. By age 65, some tree diameters were already late-seral sizes. Crown cover increased rapidly during 15 years after thinning. Mortality has been negligible in McDonald, but windthrow has removed trees, especially hemlock at Blodgett. [ABSTRACT FROM AUTHOR]

Aim Climate change is expected to modify growth trends of forests around the world. However, this modification may vary in strength and intensity across a species' biogeographical range. Here, we study European populations of silver fir ( Abies alba) across its southern distribution limits in Spain, Italy and Romania. We hypothesized that growth trends of silver fir will differ across its distribution range, with a marked decline in growth in drought-prone regions near the species' southernmost biogeographical limits. Location Europe (Spain, Italy, Romania). Methods We collected tree-ring data from at least 1300 silver fir trees located in 111 sites. The dataset was used to assess and model growth trends, quantified as changes in basal area increment, and to determine how growth responds to climate. Results We found contrasting patterns of basal area increments among countries and sites. Populations of silver fir located outside the Mediterranean area (e.g. northern Italy, Romania) have shown a clear increase in growth over the last two decades, whereas most populations in Spain and southern Italy have displayed a marked decline in growth since the 1980s. The growth of silver fir forests at the south-western distribution limit is severely constrained by low spring-summer water availability, whereas growth of silver fir forests in non-Mediterranean areas is limited by cold conditions in late winter to early spring. Main conclusions Climate warming is distinctly modifying growth patterns and responses to climate in silver fir across most of the species' European distribution area. In south-western Europe the reduction in growth of many populations is related to an observed increase in aridity, whereas in more temperate areas warming is enhancing growth. Our results confirm a decline in the growth of silver fir at its south-western distribution limits as a consequence of climate warming. [ABSTRACT FROM AUTHOR]

Aims: Few direct tests of phosphorus (P) limitation on highly-weathered soils have been conducted, especially in mature, native Eucalyptus stands. We tested whether growth in a mature >80-year old stand of Eucalyptus tereticornis in Cumberland Plain Woodland was limited by P, and whether this P-limitation affected leaf photosynthetic capacity. Methods: P was added to trees at the native woodland site at 50 kg ha year in each of 3 years, and stem and leaf responses were measured. Results: Leaf P concentrations before fertilisation were < 1 mg g and N:P ratios ranged between 16 and 23. Addition of 50 kg ha year of P increased leaf P concentration significantly (+50 %) compared to non-fertilised trees, for two but not for the 3 years. Despite higher leaf P in fertilised trees, photosynthetic capacity was unaffected. However, there was a 54 % increase in tree stem basal area growth during the first and second years of P fertilisation, statistically significant in the second year of the experiment. Conclusions: Our evidence shows that E. tereticornis is P-limited on Cumberland Plain soils. This has implications for forest responses to rising atmospheric [CO], because photosynthesis in elevated [CO] may become further constrained by required phosphate pools within the photosynthetic apparatus. [ABSTRACT FROM AUTHOR]

Key Message: In the Pyrenees low temperature and scarce precipitation limit radial growth at treeline. The climatic drivers of treeline formation in mid-latitude mountains are temperature and also moisture variability. Abstract: Alpine treelines are often induced by a threshold of minimum temperatures above which tree growth is generally not possible anymore. However, this worldwide pattern may not account for more drought-prone mountains, where changes in precipitation can also affect ecosystem functioning and productivity at higher elevations. Here, we aim to determine if tree-ring formation in ecotones along the Pyrenees indeed primarily depends on temperature means or also reflects hydroclimatic changes. The mean duration of the growing season at Pyrenean treelines was 177 days, lasting from mid-May to early November, which corresponds to a mean root-zone temperature of 8.2 °C. Temperatures during the early and late growing season were most critical for mountain pine growth at highest elevations, and above average spring temperatures also contributed to radial enlargement of tracheids. Sites showing more similar soil temperatures also presented more similar long-term growth trends indicating that local conditions trace regional influences on growth. Nevertheless, more wet conditions in summer enhanced growth at treelines subjected to more Mediterranean influence and therefore having a higher summer water deficit. A broader perspective reveals that soil moisture availability during spring and summer, however, also influence ring width formation at other treelines located in mid-latitude mountains separating temperate and semi-arid biomes. Our results suggest re-evaluating the idea of purely temperature-limited treelines, especially in mid-latitude mountains that experience moisture seasonality or where precipitation decreases with increasing elevation. [ABSTRACT FROM AUTHOR]

Rising atmospheric CO2 concentration (Ca) is expected to accelerate tree growth by enhancing photosynthesis and increasing intrinsic water-use efficiency (iWUE). However, the extent of this effect on long-term iWUE and its interactions with climate remains unclear in trees along an elevation gradient. Therefore, we investigated the variation in the radial growth and iWUE of mature Picea schrenkiana trees located in the upper tree-line (A1: 2700 m a.s.l.), middle elevation (A2: 2400 m a.s.l.), and lower forest limit (A3: 2200 m a.s.l.), in relation to the rising Ca and changing climate in the Wusun Mountains of northwestern China, based on the basal area increment (BAI) and tree-ring δ13C chronologies from 1960 to 2010. We used the CRU TS3.22 dataset to analyze the general response of tree growth to interannual variability of regional climate, and found that BAI and δ13C are less sensitive to climate at A1 than at A2 and A3. The temporal trends of iWUE were calculated under three theoretical scenarios, as a baseline for interpreting the observed gas exchange at increasing Ca. We found that iWUE increased by 12–32% from A1 to A3 over the last 50 years, and showed an elevation-dependent variation in physiological response. The significant negative relationship between BAI and iWUE at A2 and A3 showed that tree growth has been decreasing despite long-term increases in iWUE. However, BAI remained largely stable throughout the study period despite the strongest iWUE increase [at constant intercellular CO2 concentration (Ci) before 1980] at A1. Our results indicate a drought-induced limitation of tree growth response to rising CO2 at lower elevations, and no apparent change in tree growth and diminished iWUE improvement since 1980 in the upper tree-line. This study may contradict the expectation that combined effects of elevated Ca and rising temperatures have increased forest productivity, especially in high-elevation forests. [ABSTRACT FROM AUTHOR]

We investigated forest responses to global warming by observing: (1) planted Pinus halepensis forests, (2) an aridity gradient-with annual precipitation ( P) ranging from ~300 to ~700 mm, and (3) periods of wet and dry climate that included the driest period during at least the last 110 years. We examined: (1) how the length of climatic integration periods to which trees are most responsive varies in space and time, (2) the extent to which competition modulates growth decline during drought (2011) and subsequent recovery (2012) years. The temporal scale of rainfall that was most influential on growth shortened in progressing southward, and in the drier than in the wetter period. Long-term underground water storage, as reflected in the relationship of growth to multiple-year rainfall, remained significant up to the point where P ≈ 500 mm. Under drier conditions ( P < 500 mm) in both space and time, influential rainfall scales shortened, probably reflecting a diminishing role of water storage. These drier locations are the first from which the species would be likely to retreat if global warming intensified. Competition appeared to set an upper limit to growth, while growth variation among individual trees increased as competition-intensity decreased. That upper limit increased in 2012 compared with 2011. The observed insensitivity of slow-growing trees to competition implies that mortality risk may be density independent, when even any potential for higher soil moisture availability in open stands is lost to evapotranspiration before it can benefit tree growth. [ABSTRACT FROM AUTHOR]

There is a rising interest in the role of species diversity in ecosystem functioning and services, including productivity. Yet, how the diversity-productivity relationship depends on species identity and abiotic conditions remains a challenging issue., We analysed mixture effects on species productivity along site productivity gradients, calculated from a set of abiotic factors, in two biogeographic contexts (highlands and lowlands). We compared the productivity of 5 two-species mixtures (i.e. 10 cases of mixed species) with that of monocultures of the same species. Five main European tree species were considered: sessile oak ( Quercus petraea Liebl.), Scots pine ( Pinus sylvestris L.), European beech ( Fagus sylvatica L.), silver fir ( Abies alba Mill.) and Norway spruce ( Picea abies ( L.) H. Karst)., Our data set was compiled from the 2006 to 2010 French National Forest Inventory data base and covers 2361 plots including pure and mixed stands., Overall productivity of mixtures in highlands, that is European beech- Norway spruce, European beech-silver fir and to a lesser extent, silver fir- Norway spruce, was found to be higher than expected from the productivity of corresponding monospecific stands. Overyielding was mainly due to European beech for the first two mixtures and to silver fir for the third one., No effect of mixture was found for sessile oak- Scots pine and sessile oak- European beech stands in lowlands. Overyielding of sessile oak mixed with Scots pine was not strong enough to significantly increase overall stand productivity. Overyielding of European beech was balanced by an underyielding of sessile oak., The mixture effect changed along site productivity gradients for six cases out of the 10 studied, with a stronger and positive effect on sites with low productivity. The magnitude of this change along site productivity gradients varied up to 89% depending on the tree species., Synthesis. The nature of species interaction in mixtures with regard to productivity changes with species assemblage and abiotic conditions. Overyielding is strongest when species grow in highlands on less productive sites. A negative link between mixture effect and site productivity was found, in line with the stress-gradient hypothesis. [ABSTRACT FROM AUTHOR]

Theory predicts that the postindustrial rise in the concentration of CO2 in the atmosphere ( ca) should enhance tree growth either through a direct fertilization effect or indirectly by improving water use efficiency in dry areas. However, this hypothesis has received little support in cold-limited and subalpine forests where positive growth responses to either rising ca or warmer temperatures are still under debate. In this study, we address this issue by analyzing an extensive dendrochronological network of high-elevation Pinus uncinata forests in Spain (28 sites, 544 trees) encompassing the whole biogeographical extent of the species. We determine if the basal area increment ( BAI) trends are linked to climate warming and increased ca by focusing on region- and age-dependent responses. The largest improvement in BAI over the past six centuries occurred during the last 150 years affecting young trees and being driven by recent warming. Indeed, most studied regions and age classes presented BAI patterns mainly controlled by temperature trends, while growing-season precipitation was only relevant in the driest sites. Growth enhancement was linked to rising ca in mature (151-300 year-old trees) and old-mature trees (301-450 year-old trees) from the wettest sites only. This finding implies that any potential fertilization effect of elevated ca on forest growth is contingent on tree features that vary with ontogeny and it depends on site conditions (for instance water availability). Furthermore, we found widespread growth decline in drought-prone sites probably indicating that the rise in ca did not compensate for the reduction in water availability. Thus, warming-triggered drought stress may become a more important direct driver of growth than rising ca in similar subalpine forests. We argue that broad approaches in biogeographical and temporal terms are required to adequately evaluate any effect of rising ca on forest growth. [ABSTRACT FROM AUTHOR]

Tree growth can be influenced by a wide variety of complex interacting factors. The contribution of soil to site productivity is confounded by the interactions between other site factors and silviculture. The contribution of soil properties to the growth of silver fir ( Abies alba Mill.) was studied in the Dinaric Mountains, which are characterised by abundant sinkholes and limestone outcrops, resulting in diverse micro topography and different soil development at small spatial scales. Basal area increment and height growth data were estimated based on stem analysis of dominant silver fir trees ( n = 65) on a 50 m × 50 m sampling grid, and competition intensity was determined. The soil development around each tree was defined using morphological properties of the genetic horizons based on soil probing ( n = 65 × 12). In the study area, the chemical and physical soil characteristics (based on 21 soil profiles) were favourable for plant growth. Soil parameters e.g. soil depth, thickness of genetic soil horizons, share of soil types around each tree and soil associations were the factors controlling tree growth. Tree age and competition intensity were also influential factors in the case of height and radial tree growth respectively. Positive effects on height and radial growth were confirmed also for available water capacity of soil and location of tree in slope position (in the sinkhole, out of the sinkhole). The decrease in specific basal area increment with increasing competition intensity was most evident for trees growing on leached soils. The coefficient of determination and the statistical significance of the relationship between height growth and soil association over the last 100 years emphasised the cumulative effect of the soil condition on tree height growth. [ABSTRACT FROM AUTHOR]

Aim Increasing intrinsic water-use efficiency ( iWUE) in trees is a global-scale response to increasing levels of atmospheric carbon dioxide (Ca). Our primary goal was to determine whether significant differences in either iWUE or radial growth exist between locally co-occurring ponderosa pine ( Pinus ponderosa var. ponderosa; PIPO) and Douglas-fir ( Pseudotsuga menziesii var. glauca; PSME) trees growing in minimally disturbed environments and exposed to gradually increasing levels of Ca. Location Northern Rocky Mountains, USA. Methods We developed 14 tree-ring chronologies, calculated basal area increment (BAI) and examined stable carbon isotope ratios (δ13C) contained in the tree cores to determine: (1) the consistency of climate/BAI responses and iWUE between the two species and the magnitude of climate change for the primary drivers of BAI; (2) changes in iWUE and BAI values during ad 1850-present; and (3) the extent of pan-regional consistency in iWUE, BAI and climatic responses. Results PIPO and PSME experienced exponentially increasing iWUE rates during ad 1850-present suggesting either increased net photosynthesis or decreased stomatal conductance, or both. There was pan-regional consistency, with similar temporal trends in iWUE and climate/growth relationships between species. Both species experienced above-average BAI in the latter half of the 20th century despite no favourable changes in climate. We found substantive differences between species as PIPO has experienced greater increases in iWUE and BAI relative to PSME. The trends in BAI and iWUE were significantly correlated for PIPO, but not for PSME. Main conclusions Differential radial growth and iWUE responses exist among co-occurring species and are most pronounced during the period of highest Ca. As PIPO and PSME forests have extensive geographical ranges and are critical to North American forestry, any substantive change in forest dynamics related to water use and growth may have ecological and economic implications. [ABSTRACT FROM AUTHOR]

Actual status of mountain forests especialy, of Norway spruce (Picea abies Karst. (L)) stands is a result of combined action between forest management measures and biotic and abiotic risk factors. In this context, tree growth dynamics is a result of multifactor interaction (competition, disturbance, management practice, climate etc.). Wind damage affect the stand structure and normal ecosystem functions including the relationship between individual tree growth and competition processes. Though, it is recorded a diminishing of productive capacity for affected stands as well as a significant diameter increment for remaining standing trees. The correlation between tree growth (cumulate basal area increment in the last 10 years) and competition (Hegyi and Schutz competition indices) was analysed in stands with different windfall intensity. No relationship between tree growth and competition was observed in highly affected stands. Schutz competition index explains better individual tree growth comparing with Hegyi index in low or no-affected stand by windthrow. The variability of the basal area increment (In△g10), explained by competition indices, is rather low (<2%) in the stand that had been highly affected by windfall, regardless of the selected competition index. Thus, significant reduction of the stand density related to number of trees (low density), as a consequence of high intensity windfalls, crown competition has a very small influence on the basal area increment comparing with situations met in low (high density) and moderate (medium density) affected stands. [ABSTRACT FROM AUTHOR]

European beech forest with a variable admixture is one of the most important forest types in Central Europe. Growing evidence has demonstrated the positive effect of increased biodiversity on vital forest ecosystem functions and services such as productivity and nutrient cycling. Both complementarity in resource use and species identity are known to influence tree productivity but they have received relatively little attention in observational studies. Using a large dataset of repeat inventory trees in a near-natural deciduous forest in Central Germany we test whether tree diversity enhances tree productivity at the tree and the stand level, whilst accounting for tree size, tree vitality, local topography and the potentially confounding effects of spatial autocorrelation and negative growth estimates. Beech and hornbeam individual tree growth was sensitive to their neighbourhood diversity and composition whilst ash trees were only sensitive to the neighbourhood tree density. Neighbourhood complementarity effects were driven by differences in species’ competitive strengths, whilst at the stand level productivity gains were primarily attributable to the density of ash and diversity effects were less prominent. We conclude that small-scale admixture with patches of different species promotes tree growth in European beech forest; congruent with current management plans for beech and hardwood forests. [ABSTRACT FROM AUTHOR]

To better understand how the radial growth of beech ( Fagus sylvatica L.) high forests has responded to climate change, we selected 12 sites (137 trees) with optimal growing conditions along a W–E altitudinal gradient (67–590 m) in Belgium. We evaluated temporal changes in growth response to climate by using pointer year analysis, moving mean sensitivities (1860–2011), and moving bootstrapped correlation coefficients (1952–2011). The strongest driving climatic variables were identified by using the partial least squares method. The common patterns of growth trends, pointer years, and mean sensitivities among sites provided evidences for the impact of environmental changes operating at a regional scale. The results of growth–climate analysis indicated that these changes were strongly influenced by the climatic conditions of the previous year. The climate sensitivity of beech increased progressively in response to more frequent and intense heat waves and warming-related droughts, especially during recent decades, leading to remarkable inter-site synchronization. The changes were much more pronounced for sites located in lowlands (<300 m). The differences in growth responses along the altitudinal gradient and the consequences of warming for beech growth and physiology are discussed. [ABSTRACT FROM AUTHOR]

Our knowledge on tree responses to drought is mainly based on short-term manipulation experiments which do not capture any possible long-term adjustments in this response. Therefore, historical water channels in inner-Alpine dry valleys were used as century-long irrigation experiments to investigate adjustments in tree growth to contrasting water supply. This involved quantifying the tree-ring growth of irrigated and non-irrigated (control) Scots pine ( Pinus sylvestris L.) in Valais (Switzerland), as well as European larch ( Larix decidua Mill.) and black pine ( Pinus nigra Arnold) in Vinschgau (Italy). Furthermore, the adjustments in radial growth of Scots pine and European larch to an abrupt stop in irrigation were analyzed. Irrigation promoted the radial growth of all tree species investigated compared to the control: (1) directly through increased soil water availability, and (2) indirectly through increased soil nutrients and humus contents in the irrigated plots. Irrigation led to a full elimination of growth responses to climate for European larch and black pine, but not for Scots pine, which might become more sensitive to drought with increasing tree size in Valais. For the control trees, the response of the latewood increment to water availability in July/August has decreased in recent decades for all species, but increased in May for Scots pine only. The sudden irrigation stop caused a drop in radial growth to a lower level for Scots pine or similar level for larch compared to the control for up to ten years. However, both tree species were then able to adjust to the new conditions and subsequently grew with similar (Scots pine) or even higher growth rates (larch) than the control. To estimate the impact of climate change on future forest development, the duration of manipulation experiments should be on longer time scales in order to capture adjustment processes and feedback mechanisms of forest ecosystems. [ABSTRACT FROM AUTHOR]

Tropical forests play an important role in the global carbon cycle and climate regulation. However, our understanding of how Asian tropical forest growth responds to climatic variations is still limited. We developed tree ring-width chronologies of Toona ciliata from 90 trees (139 cores) from two study regions in the tropical/subtropical forests in Yunnan, southwestern China. Bootstrapped correlation analysis revealed positive moisture sensitivity (precipitation, self-calibrated Palmer drought severity index, relative humidity, and soil moisture) and negative temperature sensitivity of T. ciliata, and the relationship was strongest during dry and/or dry-to-wet transition months, indicating that radial growth of T. ciliata is primarily limited by moisture availability during early growing season. Furthermore, radial growth of T. ciliata was significantly and negatively correlated with the vapor pressure deficit and potential evapotranspiration during dry and/or dry-to-wet transition months. We analyzed long-term growth trends of T. ciliata using 'size class isolation' (SCI) and 'generalized additive mixed models' (GAMM) approaches which remove the effects of tree size on tree growth. We detected decreasing growth trend for both approaches at both study regions, indicating that the growth decline of T. ciliata stands in southwestern China is likely due to global warming-induced moisture deficit. The growth of T. ciliata trees is likely to continually decline under projected warming and drying conditions. The observed growth declines of T. ciliata raised concerns about developing sustainable management and conservation programs for tropical/subtropical forests in China. • Growth of T. ciliata is limited by moisture availability during early growing season. • Growth rates of T. ciliata in southwest China were declining over the past decades. • Growth declines are likely related to the current warming and drying conditions. [ABSTRACT FROM AUTHOR]