Tree Radial Growth Research Articles

Geographical, Climatological, and Biological Characteristics of Tree Radial Growth Response to Autumn Climate Change

Terrestrial forest ecosystems are crucial to the global carbon cycle and climate system; however, these ecosystems have experienced significant warming rates in recent decades, whose impact remains uncertain. This study investigated radial tree growth using the tree-ring width index (RWI) for forest ecosystems throughout the Northern Hemisphere to determine tree growth responses to autumn climate change, a season which remains considerably understudied compared to spring and summer, using response function and random forest machine learning methods. Results showed that autumn climate conditions significantly impact the RWI throughout the Northern Hemisphere. Spatial variations in the RWI response were influenced by geography (latitude, longitude, and elevation), climatology, and biology (tree genera); however, geographical and/or climatological characteristics explained more of the response compared to biological characteristics. Higher autumn temperatures tended to negatively impact tree radial growth south of 40° N in regions of western Asia, southern Europe, United State of America and Mexico, which was similar to the summer temperature response found in previous studies, which was attributed to temperature-induced water stress.

Trunk radial growth, water and carbon relations of mature apple trees on two size-controlling rootstocks during severe summer drought.

The use of size-controlling rootstocks is central to modern high-density fruit production systems. While biological mechanisms responsible for vigor control are not fully understood, differences in water relations and carbohydrate storage ability have been suggested as two potential factors. To better understand the processes that control growth vigor, we analyzed the trunk radial variation at seasonal and diurnal timescales and measured the midday leaf water potential (ΨMD), leaf gas exchange and concentrations of non-structural carbohydrates (NSC) in apple trees of variety 'Jonagold' grafted on two rootstocks of contrasting growth vigor (dwarfing J-TE-G vs invigorating J-TE-H). The measurements were conducted during an exceptionally hot and dry summer. We found that smaller annual trunk radial increments in dwarfed trees were primarily due to an earlier cessation of trunk secondary growth. The interdiurnal trunk circumference changes (ΔC) were slightly lower in dwarfed trees, and these trees also had fewer days with positive ΔC values, particularly during the driest summer months. The trunks of dwarfed trees shrank gradually during the drought, showed less pronounced diurnal variation of trunk circumference and the maximum trunk daily shrinkage was only weakly responsive to the vapor pressure deficit. These results indicated that lower turgidity in the cambial region may have limited the trunk radial expansion in dwarfed trees during the hot and dry days. Dwarfed trees also maintained lower ΨMD and leaf gas exchange rates during the summer drought. These parameters decreased in parallel for both rootstock combinations, suggesting their similar drought sensitivity. Similar concentrations and seasonal dynamics of NSC in both rootstock combinations, together with their similar spring growth rates, suggest that NSC reserves were not directly limiting for growth. Our results support the prominent role of water relations in rootstock-induced size-controlling mechanisms and highlight the complexity of this topic.

Dryness decreases average growth rate and increases drought sensitivity of Mongolia oak trees in North China

Mongolian oak is a widely distributed tree species in broadleaf forests of North China, which are susceptible to increasing drought. Expansion of oak species ranges has been reported in Europe, America, and Asia with drought and heat stress, and thus, oak species are often deemed to be drought-resistant. As an expanding species in our study region, the Mongolian oak (Quercus mongolica Fisch. ex Ledeb) was used to investigate how tree growth rate and drought resistance change along a moisture gradient. In this study, we used a network of Mongolian oak tree-ring samples across a wide range of moisture gradients in North China to assess oak tree growth and its response to drought under different moisture conditions. Our results revealed that spatial variation in tree radial growth rate was highly related to the regions’ moisture index, an index indicating moisture condition. Oak trees in drier portions grew slower than those in wetter portions and their growth was more sensitive to drought events. We used an ensemble of climate projections from general circulation models to predict future changes in the Ivanov moisture index, and then estimated the tree growth rate and drought-growth correlation for the period 2021–2100. Increased dryness is likely to reduce tree average growth rate markedly and increase the drought sensitivity of radial growth in the expanding dry portions of our study region, leading to substantial overall drought suppression of growth. Our results suggest that dryness will present an increasing challenge for the continuing expansion of Mongolia oak in already dry areas of North China, whereas oak trees will thrive better in high moisture conditions. The distribution of Mongolia oak trees will likely shift to regions with increased moisture availability, especially under the increasing drought.

Douglas-fir encroachment reduces drought resistance in Oregon white oak of northern California

Oregon white oak (Quercus garryana Douglas ex Hook.) is experiencing increasing competition from Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) across much of its range at the same time as climate variability is increasing, including more frequent and extreme droughts. These combined factors suggest that Douglas-fir encroachment is not only leading to range reduction from competition, but also likely exacerbating drought stress for a keystone oak species in the Pacific West of North America. Our research examines this by evaluating radial tree growth and drought response in Oregon white oak (n = 104 trees) and Douglas-fir (n = 104 trees) along a gradient of encroachment in an oak woodland in the North Coast region of California. Linear mixed-effects models were used to evaluate the effects of tree diameter, Douglas-fir competition, oak competition, and climate on drought resistance (2013–2015) and recent tree growth (2002–2016). Oregon white oak growth was more resistant to prolonged drought than Douglas-fir. However, Oregon white oak drought resistance declined with increasing Douglas-fir competition and increased with increasing oak competition. Oregon white oak and Douglas-fir growth related to different seasonal climate factors, but both species were more strongly limited by Douglas-fir competition than climate. Oregon white oak may be better suited to future climate conditions than Douglas-fir, although Douglas-fir encroachment will likely continue to reduce Oregon white oak resistance to future drought. These results present a strong case for the need to release Oregon white oak by controlling Douglas-fir in areas that were historically oak woodlands with diverse understory species, wildlife, and important traditional ecological values and services.

Comparison of the Radial Growth Response of Picea crassifolia to Climate Change in Different Regions of the Central and Eastern Qilian Mountains

It is important to explore the responses of radial tree growth in different regions to understand growth patterns and to enhance forest management and protection with climate change. We constructed tree ring width chronologies of Picea crassifolia from different regions of the Qilian Mountains of northwest China. We used Pearson correlation and moving correlation to analyze the main climate factors limiting radial growth of trees and the temporal stability of the growth–climate relationship, while spatial correlation is the result of further testing the first two terms in space. The conclusions were as follows: (1) Radial growth had different trends, showing an increasing followed by a decreasing trend in the central region, a continuously increasing trend in the eastern region, and a gradually decreasing trend in the isolated mountain. (2) Radial tree growth in the central region and isolated mountains was constrained by drought stress, and tree growth in the central region was significantly negatively correlated with growing season temperature. Isolated mountains showed a significant negative correlation with mean minimum of growing season and a significant positive correlation with total precipitation. (3) Temporal dynamic responses of radial growth in the central region to the temperatures and SPEI (the standardized precipitation evapotranspiration index) in the growing season were unstable, the isolated mountains to total precipitation was unstable, and that to SPEI was stable. The results of this study suggest that scientific management and maintenance plans of the forest ecosystem should be developed according to the response and growth patterns of the Qinghai spruce to climate change in different regions of the Qilian Mountains.

Modeling tree radial growth in a warming climate: where, when, and how much do potential evapotranspiration models matter?

Process-based models of tree-ring width are used both for reconstructing past climates and for projecting changes in growth due to climate change. Since soil moisture observations are unavailable at appropriate spatial and temporal scales, these models generally rely on simple water budgets driven in part by temperature-based potential evapotranspiration (PET) estimates, but the choice of PET model could have large effects on simulated soil moisture, moisture stress, and radial growth. Here, I use four different PET models to drive the VS-Lite model and evaluate the extent to which they differ in both their ability to replicate observed growth variability and their simulated responses to projected 21st century warming. Across more than 1200 tree-ring width chronologies in the conterminous United States, there were no significant differences among the four PET models in their ability to replicate observed radial growth, but the models differed in their responses to 21st century warming. The temperature-driven empirical PET models (Thornthwaite and Hargreaves) simulated much larger warming-induced increases in PET and decreases in soil moisture than the more physically realistic PET models (Priestley–Taylor and Penman–Monteith). In cooler and more mesic regions with relatively minimal moisture constraints to growth, the models simulated similarly small reductions in growth with increased warming. However, in dry regions, the Thornthwaite- and Hargreaves-driven VS-Lite models simulated an increase in moisture stress roughly double that of the Priestley–Taylor and Penman–Monteith models, which also translated to larger simulated declines in radial growth under warming. While the lack of difference in the models’ ability to replicate observed radial growth variability is an encouraging sign for some applications (e.g. attributing changes in growth to specific climatic drivers), the large differences in model responses to warming suggest that caution is needed when applying the temperature-driven PET models to climatic conditions with large trends in temperature.

Contrasting patterns of radial growth rate between Larix principis-rupprechtii and Pinus sylvestris var. mongolica along an elevational gradient are mediated by differences in xylem hydraulics

Species-specific environmental adaptability results in different species having different suitable distribution ranges in nature. Investigating the change of relative advantages in growth performances among tree species along elevational gradients and the underlying mechanism are essential for selecting elevation-specific optimal species in forest management in mountainous regions. Xylem hydraulics has been widely reported to play a key role in tree growth and survival under water-limited environments, but whether and how it regulates tree growth performances at different elevations with contrasting water availability remain unclear. Here we combined tree-ring analyses and measurements of xylem hydraulics to investigate the patterns of tree radial growth and their linkages with water-related physiology for Prince Rupprecht's larch (Larix principis-rupprechtii Mayr) and Mongolian pine (Pinus sylvestris var. mongolica Litvinov) along a 500-m elevational gradient between 1336 and 1825 m a.s.l. in a semi-arid area of northern China where both species were widely used for afforestation. Our results showed that the radial growth rate, reflected by the slope of linear regression line for cumulative basal area increments and tree age, was higher in Prince Rupprecht's larch than in Mongolian pine at higher elevations, which coincided with the advantages in xylem hydraulic efficiency, i.e. higher sapwood area-specific hydraulic conductivity and leaf area to sapwood area ratio for the former species relative to the latter one at the middle and high elevation sites. However, the reversed pattern appeared in comparing radial growth rate for the two conifers at the low elevation sites that might be due to obviously greater loss of hydraulic conductivity caused by drought-induced xylem embolism for Prince Rupprecht's larch. This is corroborated by strong correlations between the tree-ring width indices and water-related climate variables during the growing season in Prince Rupprecht's larch but not in Mongolian pine at low elevations. Our study provides a mechanismic explanation for contrasting growth performances between the two species across elevations and implies a potential risk of growth decline under the influence of climate change in Prince Rupprecht's larch, the currently most widely used species for afforestation in this region, particularly under the drought-prone environment in the low-elevation sites.

How wind acclimation impacts Pinus pinaster growth in comparison to resource availability

Abstract Key message Anthropic and natural disturbances in Pine pinaster forests may induce changes in tree wind exposure and resource availability. The wind acclimation has consequences on P. pinaster growth comparable to resource acquisition after sudden changes in micro-environmental conditions. Context More disturbances occur within the forest, resulting in the creation of inter-tree space. This increases resource availability for the remaining trees but also their wind exposure. Trees have the ability to acclimate to these changes in their micro-environmental conditions by modifying their development. Increase in both resource availability and wind-induced deformations (strains) are known to promote tree radial growth but their relative importance has not been previously analysed in field conditions. Aims This paper aims to study the Pinus pinaster growth in response to a sudden change in wind loading in comparison to a rapid increase in resource availability. Methods An experiment was established in a 16-year-old P. pinaster to create different conditions of wind loading and resources by guying trees with wires and removing their neighbours. The increase in trunk volume below 4 m was monitored for 3 years by measuring the stem radial growth at three stem heights. Results Acclimation to wind has important consequences for stem growth, comparable to that of the effect of greater resource availability after an increase in inter-tree spacing. The increase in trunk volume below 4 m induced by wind and by having more resources were quantitatively close and were equivalent after 3 years of treatment, to the trunk volume increase during the year before experiment. The observed wind acclimation was interpreted in terms of tree mechanical reinforcement by calculating the gain in the stem flexural rigidity: mechanical strength gains of 13.8 and 20.3% of swaying trees were observed in comparison to guyed trees for both high and low resource levels, respectively. Our analysis suggests that the relative effects of wind loading or higher resource are not modified over time by seasonable growth modulation. Both the effects of the wind acclimation and the higher resource were found to be independent and are likely to be cumulative. Conclusion This study highlights the importance of considering wind acclimation in the regulation of growth in forests after a sudden change in environmental conditions.

Comparison of the response stability of Siberian larch to climate change in the Altai and Tianshan

There is a spatial, geographical pattern of different tree growth responses to climate change, and global warming also leads to temporal nonlinear responses of tree radial growth to climatic factors. Therefore, we evaluated response stabilities to climate change and compared the growth trends of Siberian larch in the Altai and Tianshan Mountains of Northwest China. The main conclusion was as follows: 1) temperature was the dominant limiting climatic factor for Siberian larch in the two study regions. The radial growth in the Altai Mountains was mainly controlled by the minimum and maximum temperatures in July of the previous year, in May, June, and July of the current year from 1958 to 1985, and in January, February, and April of the current year from 1986 to 2012. In addition, the radial growth in the Tianshan Mountains was mainly controlled by the minimum temperature in July of the current year from 1958 to 1985, and in June of the previous year from 1986 to 2012. 2) Siberian larch in the Altai and Tianshan Mountains all had divergent responses to climate change. The relationships between radial growth of the Siberian larch in the Altai Mountains and the minimum temperature in July, September of the previous year and February, March, May, July of the current year showed an unstable correlation. The relationships between radial growth of the Siberian larch in the Tianshan Mountains and the minimum temperature in November of the previous year and May, July, September of the current year showed unstable correlation. 3) The trend in the interannual change of the basal area increment (BAI) of Siberian larch in the Altai Mountains was not significant (p > 0.05), but there was first an increasing and then a decreasing trend in the Tianshan Mountains (p < 0.05) with climate change. Siberian larch trees in the Tianshan Mountains were more susceptible to drought stress than those in the Altai Mountains.

Thinning decreases above-ground biomass increment in central European beech forests but does not change individual tree resistance to climate events

European beech plays a prominent role in the adaptation of European forests to and mitigation of climate change. Forest management may increase the mitigation potential of beech forests by accelerating carbon accumulation in tree biomass, but little is known about the interaction between the rate of biomass expansion in beech and its sensitivity to climate variation or its resistance to extreme drought episodes. A 60-year thinning experiment in beech forests in Central Europe was used to generate tree-ring width series describing past radial growth of dominant, co-dominant and sub-dominant beech trees. Randomisation applied to daily climate data was used to find the period of the year during which climate best explains beech growth. Results show that carbon uptake by above-ground biomass is higher in unmanaged stands and that thinning does not affect beech growth sensitivity to climate. Further, this study shows that average daily temperature amplitude and precipitation in March-July are the best predictors of radial growth in beech at lower-elevation sites. In a key finding, this research shows that site quality and thinning intensity increase tree size, which in turn lowers their resistance to drought. Using forest management to increase the productivity of European beech may thus increase its vulnerability to climate change.

Climate sensitivity and drought seasonality determine post-drought growth recovery of Quercus petraea and Quercus robur in Europe

Recent studies have identified strong relationships between delayed recovery of tree growth after drought and tree mortality caused by subsequent droughts. These observations raise concerns about forest ecosystem services and post-drought growth recovery given the projected increase in drought frequency and extremes. For quantifying the impact of extreme droughts on tree radial growth, we used a network of tree-ring width data of 1689 trees from 100 sites representing most of the distribution of two drought tolerant, deciduous oak species (Quercus petraea and Quercus robur). We first examined which climatic factors and seasons control growth of the two species and if there is any latitudinal, longitudinal or elevational trend. We then quantified the relative departure from pre-drought growth during droughts, and how fast trees were able to recover the pre-drought growth level. Our results showed that growth was more related to precipitation and climatic water balance (precipitation minus potential evapotranspiration) than to temperature. However, we did not detect any clear latitudinal, longitudinal or elevational trends except a decreasing influence of summer water balance on growth of Q. petraea with latitude. Neither species was able to maintain the pre-drought growth level during droughts. However, both species showed rapid recovery or even growth compensation after summer droughts but displayed slow recovery in response to spring droughts where none of the two species was able to fully recover the pre-drought growth-level over the three post-drought years. Collectively, our results indicate that oaks which are considered resilient to extreme droughts have also shown vulnerability when droughts occurred in spring especially at sites where long-term growth is not significantly correlated with climatic factors. This improved understanding of the role of drought seasonality and climate sensitivity of sites is key to better predict trajectories of post-drought growth recovery in response to the drier climate projected for Europe.

Dynamic response to climate change in the radial growth of Picea schrenkiana in western Tien Shan, China

Forests are important ecosystems for economic and social development. However, the response of tree radial growth to climate has produced ‘divergent problems’ at high latitudes under global warming. In this study, the response stability and trend of Picea schrenkiana radial growth to variability in climate factors were analyzed in the mid-latitudes of the western Tien Shan Mountains. Radial growth of P. schrenkiana was mainly limited by minimum and mean temperatures. The divergent responses of radial growth occurred in response to the minimum and mean temperatures at the beginning of the growing season (April–May) of the current year, but responses to drought occurred in July–September of the previous year. And the mean and minimum temperatures in June–September of the current year were both stable. Radial growth first increased and then decreased according to the basal area increment, with a gradual increase in temperature. Therefore, forest ecosystems in mountainous arid areas will be increasingly affected by future climate warming.

Radial growth of Pinus massoniana is influenced by temperature, precipitation, and site conditions on the regional scale: A meta-analysis based on tree-ring width index

The effects of temperature and precipitation on tree growth of given species may vary across its distribution range. To determine how temperature and precipitation influence tree radial growth, we conducted a dendrochronology-based meta-analysis using a network of 113 tree-ring width chronologies of Pinus massoniana trees from the geographical range of the species in southern China. Our results revealed that the difference in the sensitivity of P. massoniana growth to temperature and precipitation was not observed in the entire tree species distribution and each sub-region (all P > 0.05), indicating that there are similarities with respect to the effects of temperature and precipitation on P. massoniana growth among different regions. January–March temperature (T1–3), June–August temperature (T6–8), and July–September precipitation (P7–9) were the key climate signals influencing P. massoniana growth. T1–3 and P7–9 generally enhanced tree growth, whereas T6–8 lead to a decrease in tree growth. Sites with significant negative correlations between tree-ring width index (RWI) and T1–3 showed a northward shift (P < 0.05). The mean latitude of sites with negative correlations was 29.98 ± 0.77°N, whereas the mean latitudes of sites with a non-significant or positive correlation were 27.27 ± 0.28° and 27.34 ± 0.46°N, respectively. The three correlation classes (i.e., positive, negative and non-significant) between T6–8 and RWI showed no significant shifts in longitude, latitude, and elevation (P > 0.05), indicating that high summer temperatures are widespread climate stressors for this species. The growth-climate relationship was determined by combinations of both temperature and precipitation condition of the sampling sites. Temperature seasonality, temperature annual range, and mean temperature of driest quarter of the sampling sites were major contributors to this relationship. These findings have crucial implications for predicting tree growth, introducing the selection of appropriate provenance for afforestation, as well as for climate reconstruction at the regional scale.

Climate warming predispose sessile oak forests to drought-induced tree mortality regardless of management legacies

Climate warming-related drought could become a major driver of large-scale forest dieback. However, little is known about how past management legacies modulate the climate-growth responses during recent dieback episodes in central European oak forests. Here, we examine the role played by past management –unmanaged old-growth vs. managed forests– in recent tree mortality events occurring in Quercus petraea (Matt.) Liebl. stands across large areas of western Romania. We analyze how stand structure (tree size, competition) and climatic factors (drought, temperature and precipitation) drive tree radial growth patterns in neighboring standing dead and living trees. We analyzed basal area increments (BAI) trends, past management legacies and climate- and drought-growth relationships during the 20th century to distinguish the roles and interactions on recent warming-induced dieback. We observed that temperature rises and changes in atmospheric water demand during growing season let to increasing drought stress during the late 20th century affecting both managed and unmanaged forests. Dead trees from old-growth and managed forests showed lower growth than living trees after dieback onset. In both forests, dead and living trees displayed divergent growth patterns after dry 1980s, indicating that dieback was triggered by severe extreme conditions. Dead trees from managed stands experienced significant stronger growth reductions after 1980s though they experienced less tree-to-tree competition than dead trees in old-growth forest. High stand density negatively drove growth and enhanced climate sensitivity in old-growth stands. Competition acted synergistically with climate warming and drought causing tree mortality regardless of the management legacies in of Q. petraea forests. Our retrospective assessment of growth rates in relation with climate and structure changes offers valuable information for further forest conservation and management decisions of Q. petraea forests. These findings highlight the importance of past uses legacies driving recent forest dieback in temperate oak forests, making them more vulnerable under forecasted climate-warming related droughts in central Europe.

Оценка эффективности рубок ухода в сухих сосняках Казахского мелкосопочника

According to the current Forest Code of the Republic of Kazakhstan, in the forest plantations of specially protected natural areas, the main forestry measures aimed at the conservation and enhancement of the protective functions of forests are intermediate felling, that is, thinning. Pinus sylvestris L. is one of the main forest-forming species in Kazakhstan. Increasing the stability of forest plantations and the possibility of forming recreationally attractive landscapes through thinning is an urgent problem for plantations growing in arid conditions. The research purpose is to analyze the effectiveness of thinning of high intensity in dry pine forests of the Kazakh Uplands for a 70-year period, using dendrochronological methods. It was proved that in dense pine forests growing in dry conditions, there is an increase in radial growth in each subsequent 10-year period after thinning. It was found with statistical certainty that the greatest response to changes caused by thinning of high intensity was observed between 1949 and 1960. Analysis of indexed tree-ring chronologies using regional curves (indexed average tree-ring chronologies of pine) using a cubic spline function revealed differences in pine radial growth indices from the site where thinning of high intensity were carried out and from the control site (time periods: 1947–1955, 1961–1970, 1981–1990, and 1995–2015). The data obtained show a positive effect of thinning of high intensity on the radial growth of trees in the considered pine forests. In dense pine forests of dry growing conditions of the Kazakh Uplands it is recommended to make 1 or 2 cuttings with thinning intensity of 25–35 % at the age of 20–25 and 40–50, respectively, with subsequent increment thinning.

High-altitude tree growth responses to climate change across the Hindu Kush Himalaya

AbstractAimsRapid warming at high altitudes may lead to a higher sensitivity in tree growth to temperature. The key factors constraining tree radial growth and to what extent regional tree growth has suffered from climatic changes are unclear.MethodsTree-ring width data were collected from 73 sites across the Hindu Kush Himalaya (HKH), including three dominant genera (Abies, Juniperus and Picea) at high altitudes over 3000 m. Dynamic time warping was introduced to develop subregional chronologies by considering the synchrony of annual tree growth among different sites. We quantified the contribution of the climate variables, and analyzed the spatiotemporal variation of the growth–climate relationship.Important FindingsThe site chronologies were grouped into three clusters, corresponding to the three distinct bioclimatic zones, i.e. the western HKH, central-eastern HKH and southeastern Tibetan Plateau (TP). Tree growth was positively correlated to winter and spring precipitation in the drier western HKH, and to winter temperature and spring precipitation in the humid southeastern TP. Tree growth was markedly constrained by the minimum temperature, especially in winter, with its importance increasing from the west toward the east. As shown by moving correlation analysis, the signal of winter temperature in tree growth was weakened in the western and central-eastern HKH, while it was enhanced in the southeastern TP following rapid warming since the 1980s. Our results highlight that continuous warming may cause forest recession due to warming-induced moisture deficit in the western HKH, but forest expansion in the southeastern TP.

Radial growth of Pinus sylvestris in the East Ural Radioactive Trace (EURT): Climate and ionizing radiation

The East Ural Radioactive Trace (EURT) resulted from the explosion at the Mayak Production Association in September 1957 (the Kyshtym accident). The majority of the Scots pine stands near the epicenter of the accident died in 1958–1959. Currently, we have found several small pine-birch stands with pine trees that are over 70 years old. Cores (96 samples) were analyzed employing dendrochronological methods to detect the effects of climate and ionizing radiation on the annual growth of trees. Tree-ring chronologies were developed for two control and three impact sites with different levels of radioactive contamination. In the first years after the accident, acute radiation exposure significantly reduced radial growth in trees in two contaminated sites, where initial contamination levels of 90Sr measured 3.7–26.6 MBq m−2; synchrony of the chronologies from the contaminated and control sites were disturbed. In the subsequent period, all chronologies have been highly synchronous. The Scots pine has revealed decreasing radial growth due to limitations in the amount of available moisture. The results suggest that both climate and ionizing radiation have limited the radial growth of pine trees. The effect of acute radiation contamination on the radial growth of trees was comparable to the effect of droughts – the main extreme climatic event in this region.

Radial growth of Larix sibirica was more sensitive to climate at low than high altitudes in the Altai Mountains, China

Considering climate change, there is no consensus on the impact of global warming on forest growth. Therefore, it is crucial to investigate the effects of climate change on tree radial growth. However, in arid eastern central Asia, the large-spatial scale radial growth-climate relationship remains poorly understood. In this study, we collected radial growth data from 13 Siberian larch (Larix sibirica) plots along an altitudinal gradient (1100–2200 m a.s.l.) in the Altai Mountains, China. Principal component analysis (PCA), bootstrapped correlation, and linear regression were conducted to detect and quantify the effects of climatic factors on radial growth and to reveal the spatial variation in the radial growth-climate relationship. Results showed that chronologies were clustered into low- and high-altitude groups. In the low-altitude group, the self-calibrating Palmer Drought Severity Index (scPDSI) in current April showed a strong positive impact on radial growth, while the previous September mean temperature and diurnal temperature range in previous July both significantly negatively affected radial growth. These three climate variables explained 63% of radial growth variance. Among them, the scPDSI, with a relative importance of 63%, was the primary limiting factor. In the high-altitude group, the current January maximum temperature and previous July precipitation were significantly negatively correlated with radial growth, while the June minimum temperature was significantly positively correlated with radial growth. These three climatic factors together contributed to 38% variance in radial growth. Furthermore, the plot-specific relationship between climate variables and radial growth varied significantly with different altitudes. As the climate becomes wetting and warming in the Altai Mountains, our result suggests that the radial growth might benefit in an altitude between 1800 and 1900 m a.s.l., where temperature and precipitation were both positively correlated with radial growth. These findings will provide a basis for sustainable forest management under global climate change.

Spatial and temporal differences in the response of Larix sibirica to climate change in the central Altai Mountains

Because climate change impacts different ecosystems and geographies in myriad ways, it is important to study how forests in different regions respond to changing climatic conditions. In northwestern China, the transition in recent decades from a warm and dry climate to a warm and wet climate is well documented. However, the response of forests to this transition is poorly understood. To better understand these changes, we collected tree-ring samples of Larix sibirica from 12 plots on the southern slope of the central Altai Mountains and established site-specific tree-ring width residual chronologies. Through hierarchical cluster analysis, we could divide the sites into two levels: high-elevation level (HEL, >1900 m) and low-elevation level (LEL, <1900 m). We then used the first principal component of each level as the regional residual chronology and performed a moving correlation analysis with climate factors (monthly temperature, monthly precipitation, scPDSI) over the period 1957–2016. We found that changes in HEL trees are mainly related to temperature: from 1957–1967, growth was positively correlated with the temperature in July of the previous year. From the 1960s to present, growth was negatively correlated with the temperature in April of current year. Since the 1980s, however, temperature in current year January has been the most important factor limiting growth. The relationship between LEL tree growth and climate has been stable throughout the period of observation, and is significantly and positively correlated with moisture. We predict that the radial growth of HEL trees will be inhibited by continued warming, whereas LEL trees will benefit from improved moisture conditions in the future. Our findings help to provide a more comprehensive understanding of the effects of climate change on forest ecosystems, and provide a scientific reference for forest management and carbon sequestration and emission mitigation policies.

Legacy of forest composition and changes over the long-term on tree radial growth

The forests of North America have undergone important changes since European settlement, particularly in terms of stand composition and associated changes in soil properties. While soil nutrients availability is known to influence forest productivity, the causes and consequences of its variation through time remain poorly understood. This study investigates the effects of long-term changes in forest composition and soil properties on the radial growth of sugar maple (Acer saccharum Marsh.) and balsam fir (Abies balsamea (L.) Mill.), two important species of northeastern North America’s forests. Using data from 130 plots measured in 1930 and in 2012–2014 and a mixed-effects modelling approach, we studied the links between radial growth, soil nutrients availability, current stand composition, and shifts in vegetation. The radial growth of balsam fir was found to vary with soil available nitrogen and present-day relative basal area of yellow birch in the stand, while that of sugar maple was found to be invariant to soil characteristics, but proportional to present-day spruce (Picea spp.) relative basal area. However, no direct effects of vegetation change on radial growth were detected. Our results suggest that prior stand composition had no influence on radial growth of both studied species, yet vegetation change could indirectly influence balsam fir growth through an improvement of litter quality with an increase in the abundance of yellow birch (Betula alleghaniensis Britton). Moreover, despite clear differences between the studied species, we conclude that maintaining a certain proportion of compositional diversity may enhance radial growth of both balsam fir and sugar maple.