Higher climate warming sensitivity of Siberian larch in small than large forest islands in the fragmented Mongolian forest steppe

Forest fragmentation has been found to affect biodiversity and ecosystem functioning in multiple ways. We asked whether forest size and isolation in fragmented woodlands influences the climate warming sensitivity of tree growth in the southern boreal forest of the Mongolian Larix sibirica forest steppe, a naturally fragmented woodland embedded in grassland, which is highly affected by warming, drought, and increasing anthropogenic forest destruction in recent time. We examined the influence of stand size and stand isolation on the growth performance of larch in forests of four different size classes located in a woodland‐dominated forest‐steppe area and small forest patches in a grassland‐dominated area. We found increasing climate sensitivity and decreasing first‐order autocorrelation of annual stemwood increment with decreasing stand size. Stemwood increment increased with previous year's June and August precipitation in the three smallest forest size classes, but not in the largest forests. In the grassland‐dominated area, the tree growth dependence on summer rainfall was highest. Missing ring frequency has strongly increased since the 1970s in small, but not in large forests. In the grassland‐dominated area, the increase was much greater than in the forest‐dominated landscape. Forest regeneration decreased with decreasing stand size and was scarce or absent in the smallest forests. Our results suggest that the larch trees in small and isolated forest patches are far more susceptible to climate warming than in large continuous forests pointing to a grim future for the forests in this strongly warming region of the boreal forest that is also under high land use pressure.     

Increased Summer Temperatures Reduce the Growth and Regeneration of Larix sibirica in Southern Boreal Forests of Eastern Kazakhstan

The larch forests at the southern limit of the Siberian boreal forest in Central Asia have repeatedly experienced strong recent growth declines attributed to decreasing summer precipitation in the course of climate warming. Here, we present evidence from the southernmost Larix sibirica forests in eastern Kazakhstan that these declines are primarily caused by a decrease in effective moisture due to increasing summer temperatures, despite constant annual, and summer precipitation. Tree-ring chronologies (>800 trees) showed a reduction by 50–80% in mean ring width and an increase in the frequency of missing rings since the 1970s. Climate-response analysis revealed a stronger (negative) effect of summer temperature (in particular of the previous year’s June and July temperature) on radial growth than summer precipitation (positive effect). It is assumed that a rise in the atmospheric vapor pressure deficit, which typically increases with temperature, is negatively affecting tree water status and radial growth, either directly or indirectly through reduced soil moisture. Larch rejuvenation ceased in the 1950s, which is partly explained by increasing topsoil desiccation in a warmer climate and a high drought susceptibility of larch germination, as was demonstrated by a germination experiment with variable soil moisture levels. The lack of regeneration and the reduced annual stem increment suggest that sustainable forest management aiming at timber harvesting is no longer feasible in these southern boreal forests. Progressive climate warming is likely to cause a future northward shift of the southern limit of the boreal forest.     

Response of tree-ring width to climate warming and selective logging in larch forests of the Mongolian Altai

Aims The Mongolian Altai is an old settlement area, which is populated by pastoral nomads since 2000–3000 years. Forests in this region (at ca. 2300 m a.s.l.) are highly fragmented and border on steppe and alpine grasslands, which are used for mobile livestock husbandry. The climate in Central Asia is warming to levels clearly above the global average, which affects the vegetation. Furthermore, the transition from planned to market economy and the decollectivization of livestock 20 years ago has strongly changed land use practices in Mongolia, especially resulting in an increase in recent logging activities. We were interested in the question how climate warming and selective logging influence the annual stem growth and the stand structure.Methods The impact of climate and land use by the pastoral nomads on the annual stem increment of more than 1800 trees of Siberian larch (Larix sibirica) was analyzed. Different groups of trees with divergent growth trends depending on the social position and stand history were identified by non-metric multidimensional scaling and analysis of similarities. Long-term trends in the annual stem increment were analyzed by establishing separate regional growth curves for trees of different age classes.Important findings Instrumental climate data substantiate an increase of temperature by 2.1°C since 1940 at constant precipitation. Trees benefit from the increased temperatures. Climate–response analysis revealed that radial stem increment was promoted by the temperature in early summer, but also high precipitation in spring and in the year before tree-ring formation. Forest dynamics is also strongly influenced by anthropogenic activities. In addition to the natural forest dynamics, logging resulted in divergent growth trends within given age classes and habitats (forest interior and forest edge); overall, 22 groups of trees with different characteristics in the annual radial stem increment were identified. A tree-ring series-based reconstruction of logging intensity since 1935 suggests that moderate selective logging occurred throughout the study period. However, selective logging was strongly intensified after 1990 as the result of the breakdown of the Communist regime in Mongolia and the transition from centrally planned to market economy. Because tree stump densities showed that the ratio of felled to live trees was 2:1 in the interior or even 0.9:1 at the edges of the forests and most logging occurred during the past 20 years, it must be concluded that the forests of the Mongolian Altai are presently exploited far beyond the level of sustainability. A close correlation of the ratio of felled to live trees with the density of summer camps of pastoral nomads in the vicinity suggests that trees are primarily felled by the local population.     

Recent drought stress leads to growth reductions in Larix sibirica in the western Khentey,Mongolia

Trends in air temperature and precipitation in the forest‐steppe ecotone of the western Khentey, northern Mongolia were studied and related to stem increment and shoot water relations in Mongolia's most common tree species, Siberian larch (Larix sibirica). The area has been subject to a significant increase of summer temperature and a decrease of summer precipitation during the last 47 years. Tree‐ring width series from >400 larch trees show a strongly decreasing annual increment since the 1940s. The onset of this decrease is independent of the age of the trees and, therefore, can be attributed to the increasing aridity in the 20th century. Simultaneously to the declining annual increment, regeneration of Siberian larch decreased as well; today regeneration is virtually lacking in the larch forests on mountain slopes of the western Khentey. Measurements of shoot water potentials during the growing season exhibited daily minimum water potentials close to the point of zero turgor for extended periods. The drought stress indicated by these results is in line with the current low annual increment. Trees in the forest interior were more severely stressed and grow more slowly than trees at the forest line to steppe. This is attributable to the recent increase in aridity, as the stand density and probably also the trees themselves in the forest interior are adapted to moister conditions, whereas the trees at the forest edge have always been exposed to a more extreme microclimate. The progressing increase in aridity during the 21st century that is predicted for the western Khentey, suggests a future decline of larch forests. A widespread increase of aridity predicted for most parts of the Mongolian forest belt, suggests even a supra‐regional decline of larch.     

Rapid warming induces the contrasting growth of Yezo spruce (Picea jezoensis var. microsperma) at two elevation gradient sites of northeast China

Accurately assessing the impact of climate changes on tree growth or forest productivity is vital to better understand global carbon cycles. Here, we carried out dendroclimatological research on Yezo spruce (Picea jezoensis var. microsperma) along an elevation gradient in two sites to investigate the effect of rapid warming on spruce growth in northeast China. Results indicated that trees at two low-elevation sites had significantly wider ring widths and higher basal area increment (BAI) compared with high-elevation sites. Ring widths and BAI of Yezo spruce at low elevations showed a clear growth increase during the 1940s–1970s followed by a significant decline after 1980. However, trees at high elevations showed a relatively stable growth during the 1940s–1970s followed by a significant increase after 1980. Rapid warming after 1980 increased the radial growth of Yezo spruce at high-elevation sites, but reduced tree growth at low-elevation sites. Winter precipitation and growing season temperature were positively correlated with radial growth of Yezo spruce at high elevations, but negatively correlated with tree growth at low elevations. A clear pattern of growth and growth-climate relationship changed in 1980. The temperature threshold for determining the impact of climate on Yezo spruce could change with latitude or site. Difference in drought caused by warming may be the main reason for the opposite response of tree growing at different altitudes in northeast China. The mechanism of rapid warming driving contrasting growth at different elevations should also be investigated in other tree species in NE Asia. In the context of future climate warming, our findings are of great significance for tree growth in assessing forest dynamics and carbon cycling.     

Stand basal area and solar radiation amplify white spruce climate sensitivity in interior Alaska: Evidence from carbon isotopes and tree rings

The negative growth response of North American boreal forest trees to warm summers is well documented and the constraint of competition on tree growth widely reported, but the potential interaction between climate and competition in the boreal forest is not well studied. Because competition may amplify or mute tree climate‐growth responses, understanding the role current forest structure plays in tree growth responses to climate is critical in assessing and managing future forest productivity in a warming climate. Using white spruce tree ring and carbon isotope data from a long‐term vegetation monitoring program in Denali National Park and Preserve, we investigated the hypotheses that (a) competition and site moisture characteristics mediate white spruce radial growth response to climate and (b) moisture limitation is the mechanism for reduced growth. We further examined the impact of large reproductive events (mast years) on white spruce radial growth and stomatal regulation. We found that competition and site moisture characteristics mediated white spruce climate‐growth response. The negative radial growth response to warm and dry early‐ to mid‐summer and dry late summer conditions intensified in high competition stands and in areas receiving high potential solar radiation. Discrimination against ¹³C was reduced in warm, dry summers and further diminished on south‐facing hillslopes and in high competition stands, but was unaffected by climate in open floodplain stands, supporting the hypothesis that competition for moisture limits growth. Finally, during mast years, we found a shift in current year's carbon resources from radial growth to reproduction, reduced ¹³C discrimination, and increased intrinsic water‐use efficiency. Our findings highlight the importance of temporally variable and confounded factors, such as forest structure and climate, on the observed climate‐growth response of white spruce. Thus, white spruce growth trends and productivity in a warming climate will likely depend on landscape position and current forest structure.     

Recent decline of high altitude coniferous growth due to thermo-hydraulic constrains: evidence from the Miyaluo Forest Reserve,Western Sichuan Plateau of China

Tree growth decline has been reported in many places around the globe under the context of increasingly warming climate, and strengthening drought intensity is detected to be the primary factor for such decline, particularly in northern forest sites, as well as arid and semi-arid areas. Yet, the forest growth decline in high altitude, high mountain sites certainly merits investigation. Here, we reported faxon fir (Abies fargesii var. faxoniana) forest growth decline (slope = -0.64) at the tree line (4150 m above sea level) in Miyaluo Forest Reserve (MFR) at the Western Sichuan Plateau, southwestern China since 2000. We investigated the cause of tree growth decline by applying dendrochronological approaches. We took tree-ring samples from fir trees at the tree line and developed tree-ring width (TRW) chronology. The tree growth – climate relationship analysis showed that maximum temperature (Tmax) was the primary factor limiting the radial growth of fir trees in the investigated area. The moving correlation analysis indicated the strengthening positive influence of Tmax, spring precipitation, and cloud cover during winter and monsoon period on radial growth since 2000s. Our results have shown that both thermal and hydraulic constrains accounted for the radial growth decline of fir trees at the tree line of MFR in the western Sichuan Plateau.     

Long‐term forest resilience to climate change indicated by mortality,regeneration, and growth in semiarid southern Siberia

Several studies have documented that regional climate warming and the resulting increase in drought stress have triggered increased tree mortality in semiarid forests with unavoidable impacts on regional and global carbon sequestration. Although climate warming is projected to continue into the future, studies examining long‐term resilience of semiarid forests against climate change are limited. In this study, long‐term forest resilience was defined as the capacity of forest recruitment to compensate for losses from mortality. We observed an obvious change in long‐term forest resilience along a local aridity gradient by reconstructing tree growth trend and disturbance history and investigating postdisturbance regeneration in semiarid forests in southern Siberia. In our study, with increased severity of local aridity, forests became vulnerable to drought stress, and regeneration first accelerated and then ceased. Radial growth of trees during 1900–2012 was also relatively stable on the moderately arid site. Furthermore, we found that smaller forest patches always have relatively weaker resilience under the same climatic conditions. Our results imply a relatively higher resilience in arid timberline forest patches than in continuous forests; however, further climate warming and increased drought could possibly cause the disappearance of small forest patches around the arid tree line. This study sheds light on climate change adaptation and provides insight into managing vulnerable semiarid forests.     

Quercus robur survival at the rear edge in steppe: Dendrochronological evidence

Climate change is altering forest ecosystems worldwide, particularly in steppe landscapes, where the rare tree communities are challenged with steadily increasing droughts. In the steppe of Eastern Europe, amid dry conditions, Quercus robur occupies mostly riverine habitats and ravines. Here we study the climate sensitivity and drought vulnerability of a Q. robur population located at the rear edge of the species range, in the steppe of Ukraine. The population occupies two adjacent but clearly contrasting in their microclimatic conditions sites: a river floodplain and a steep-sloping river bank. We develop tree earlywood, latewood, and total ring width site-level chronologies and evaluate their relationship with regional climate variables and the local river's water level using response function analysis. We find that trees growing in the floodplain and at the steep river bank have exhibited slightly different growth patterns. The trees at the flooded site have benefited from water proximity, which facilitated their earlywood growth. These trees have responded positively to the current May and previous September precipitation and previous and current May temperatures. At the non-flooded site, the trees have experienced warm temperatures and the lack of precipitation in June. The extreme drought episodes have triggered a decrease in the latewood and total ring width in trees from both sites. We established that oak growth in the floodplain had been depressed by an unknown stressor around 1900, therefore limiting our ability to identify the more beneficial steppe habitat for Q. robur conclusively. Nevertheless, our results provide a dendrochronological evidence of Q. robur survival in a dry steppe environment and lend new insights into local microclimatic factors contributing to it.     

Tree species growth response to climate warming varies by forest canopy position in boreal and temperate forests

Reports of forest sensitivity to climate change are based largely on the study of overstory trees, which contribute significantly to forest growth and wood supply. However, juveniles in the understory are also critical to predict future forest dynamics and demographics, but their sensitivity to climate remains less known. In this study, we applied boosted regression tree analysis to compare the sensitivity of understory and overstory trees for the 10 most common tree species in eastern North America using growth information from an unprecedented network of nearly 1.5 million tree records from 20,174 widely distributed, permanent sample plots across Canada and the United States. Fitted models were then used to project the near-term (2041–2070) growth for each canopy and tree species. We observed an overall positive effect of warming on tree growth for both canopies and most species, leading to an average of 7.8%–12.2% projected growth gains with climate change under RCP 4.5 and 8.5. The magnitude of these gains peaked in colder, northern areas for both canopies, while growth declines are projected for overstory trees in warmer, southern regions. Relative to overstory trees, understory tree growth was less positively affected by warming in northern regions, while displaying more positive responses in southern areas, likely driven by the buffering effect of the canopy from warming and climate extremes. Observed differences in climatic sensitivity between canopy positions underscore the importance of accounting for differential growth responses to climate between forest strata in future studies to improve ecological forecasts. Furthermore, latitudinal variation in the differential sensitivity of forest strata to climate reported here may help refine our comprehension of species range shift and changes in suitable habitat under climate change.     

A multi-scale dendroclimatological analysis of four common species in the southern Canadian boreal forest

A comprehensive assessment of the tree growth/climate relationship was undertaken to better understand the potential impacts of climate change on the growth dynamics of four widespread and common boreal tree species, namely jack pine (Pinus banksiana), black spruce (Picea mariana), eastern larch (Larix laricina), and trembling aspen (Populus tremuloides), located at the southern limits of the Canadian boreal forest. Over intra-annual time scales, results show that precipitation is likely the main driver of stem radius change (∆R), with jack pine radius exhibiting the most consistent positive relationship. Precipitation had a stronger relationship with stem radius variation in black spruce and eastern larch during periods when volumetric water content (VWC) in the root zone was below average, pointing to the likelihood that certain species rely more heavily on available moisture in the uppermost layers of the soil column to replenish stem water, especially during extended dry periods. Warm air temperatures had an immediate negative impact on stem water content due to transpiration. This was most marked during periods of reduced moisture availability in the root zone, when trees are more susceptible to net water volume loss. During periods when moisture was not limiting, a positive relationship between lagged air temperature and ∆R was detected. Warm air temperatures may therefore play an important role in stimulating radial growth when moisture requirements are met. At annual temporal resolution, the growth/climate relationship changed over the lifetime of our study species. Over the last several decades, the relationship between precipitation and annual radial tree growth has weakened, while positive relationships between spring and summer air temperature and annual radial tree growth have emerged, likely signaling a decrease in moisture limitations, and a positive response to spring warming. Our findings reveal that boreal forest tree species may benefit from spring and summer warming over the near term, providing there is sufficient moisture to support growth. Over the long term, rates of evapotranspiration are expected to overshadow gains in moisture related to an increase in precipitation. Under these circumstances, we are likely to see reduced growth rates and an increasingly negative response of boreal tree species growth to warm air temperatures.     

Gypsy moth-induced growth decline of Larix sibirica in a forest-steppe ecotone

Effects of a gypsy moth attack on the productivity of Larix sibirica on tree-ring width were analyzed in a case study of a mountain site in the western Khentey in the northern Mongolian forest-steppe ecotone. A major aim of the study was to assess whether reduced productivity by gypsy moth herbivory could contribute to fluctuations of the forest edge to the steppe in larch-dominated woodlands. In the year of the infestation, larch trees at the forest edge lost 90% of their needles and latewood formation was strongly reduced. However, earlywood formation was widely completed before the gypsy moth attack and, therefore, total tree-ring width was not below the average of the five years prior to infestation. In the two years following the gypsy moth invasion, annual stem increment was strongly reduced. Trees growing 30–100 m inside the forest showed a much weaker response of tree-ring widths to the gypsy moth infestation consistent with significantly higher defoliation at forest edge than in the forest interior. Old trees exhibited a stronger growth decline than middle-aged trees, indicating higher infestation of dominant, exposed trees, which are thought to be better accessible to the wind-dispersed gypsy moth larvae hatching in the early growing season on the steppe. Under the current climate, occasional growth reductions are thought to be of little effect on the performance of L. sibirica, as fast-growing competitors of other tree species, which are not or hardly affected by gypsy moth, are absent.     

Connections between climatic variables and the growth and needle dynamics of Scots pine (Pinus sylvestris L.) in Estonia and Lapland

The relationships between climatic variables and Scots pine (Pinus sylvestris L.) growth and needle dynamics were studied in three stands in Estonia and in four stands located near the northern timberline in Lapland. The trees sampled in Estonia had low correlations with the analysed climatic variables (air temperature, precipitation and indices of atmospheric circulation). Moreover, the weak cross-correlation of the time-series of the Estonian sample trees indicated that Scots pine is affected mainly by local factors in that region. In Lapland, however, height increment and needle production correlated strongly among trees within a stand (mean r=0.45 and 0.46, respectively) and between stands (r=0.32 and 0.37). Radial increment also showed a high inter-correlation among the trees within a stand in Lapland (r=0.45). Both height increment and needle production were strongly influenced by the temperature regime of the previous summer in Lapland (mean r=0.64 and 0.64, respectively). Radial increment was correlated with the mean July temperature of the current year (mean r=0.29). The correlations between the indices of atmospheric circulation and tree attributes were weak, while the strongest correlation was between the Ponta Delgada NAO index (PD–NAO) and height increment and needle production in Lapland. Height increment, needle production and radial increment have increased since the 1990s in the trees growing in Lapland. This may indicate a positive effect of climate warming on tree growth in Lapland. In Estonia, where climatic conditions do not limit tree growth, the climate warming seems not to directly influence the growth and needle dynamics of Scots pine.     

祁连山东部青海云杉死亡和存活个体径向生长对气候变化的响应差异

研究建立了祁连山东部青海云杉(Picea crassifolia)存活个体与死亡个体的树轮宽度年表和断面生长增量(BAI)序列,对青海云杉存活个体与死亡个体的径向生长特征及其对气候要素的响应关系进行了比较,尝试厘清青海云杉径向生长与气候要素之间的响应机制。结果表明:青海云杉死亡个体的径向生长速率明显低于存活个体(P ＜ 0.001),尤其是在20世纪80年代快速升温后,这一差距进一步加大,青海云杉死亡率也大幅增加。在1951-1986年期间,青海云杉径向生长主要受生长季前(上一年9月到当年2月)干湿状况的影响;而在1987-2020年期间,青海云杉径向生长则更多受生长季(当年6-8月)干湿状况的影响。并且与活树相比,死树对区域干湿状况表现出更高的敏感性,对气温的负响应也更强。随着气候变暖加剧,高温及其带来的干旱胁迫已成为区域内限制青海云杉径向生长的主要气候因素,青海云杉的死亡率可能会继续升高。     

Warming-Induced Decline of Picea crassifolia Growth in the Qilian Mountains in Recent Decades

Warming-induced drought has widely affected forest dynamics in most places of the northern hemisphere. In this study, we assessed how climate warming has affected Picea crassifolia (Qinghai spruce) forests using tree growth-climate relationships and the normalized difference vegetation index (NDVI) along the Qilian Mountains, northeastern Tibet Plateau (the main range of Picea crassifolia). Based on the analysis on trees radial growth data from the upper tree line and the regional NDVI data, we identified a pervasive growth decline in recent decades, most likely caused by warming-induced droughts. The drought stress on Picea crassifolia radial growth were expanding from northeast to southwest and the favorable moisture conditions for tree growth were retreating along the identical direction in the study area over the last half century. Compared to the historical drought stress on tree radial growth in the 1920s, recent warming-induced droughts display a longer-lasting stress with a broader spatial distribution on regional forest growth. If the recent warming continues without the effective moisture increasing, then a notable challenge is developed for Picea crassifolia in the Qilian Mountains. Elaborate forest management is necessary to counteract the future risk of climate change effects in this region.     

Contrasting climate-growth relationship between Larix gmelinii and Pinus sylvestris var. mongolica along a latitudinal gradient in Daxing’an Mountains,China

The Daxing’an Mountains is one of the areas with the most serious climate warming in northern China. Dahurian larch (Larix gmelinii) and Mongolian Scots pine (Pinus sylvestris var. mongolica) are two major coniferous species in boreal forests of the region. Their growth-climate relationship is crucial for understanding the effects of climate change on boreal forest ecosystems. To examine and compare the changes of climate-growth relationship between larch and pine, a total of 418 tree-ring cores of the two species were collected at six sites in the Daxing’an Mountains, and the tree-ring chronologies were developed. The results showed that water availability (Palmer Drought Severity Index, PDSI) played a key role in the stable growth of larch and pine. The temperature and precipitation in January, June-August are important factors affecting the radial growth of the two coniferous species along the latitude gradient. The correlation coefficients of growth and the seasonal temperature and precipitation of larch and pine showed a completely opposite trend with the increase of latitude. In summer and autumn, the correlation coefficients between larch growth and seasonal mean temperature decreased first and then increased with the increase of latitude, while that of pine, on the contrary, increased first and then decreased. In winter, spring and autumn, the correlation coefficients between larch growth and seasonal total precipitation decreased first and then increased with the increase of latitude, while that of pine was opposite. However, the correlation coefficients between larch and pine growth and PDSI showed the same trend with the increase of latitude, decreasing at first and then increasing. Before and after rapid warming (around 1980), the correlation coefficients between larch and pine growth and PDSI showed a completely opposite change. Our findings emphasize that the growth-climate relationships of Dahurian larch and Mongolian Scotts pine shows an opposite trend with latitude, which means that the two species may exhibit a completely opposite response with climate change along the latitude gradient.     

Influence of moss and lichen spatial mosaics on radial growth of Gmelin larch (Central Evenkia)

The results of research studies on the influence of high moss-and-lichen mosaicity typical for the northern taiga in Central Evenkia on the dynamics of radial growth of Gmelin larch trees (Larix gmelinii (Rupr.) Rupr.) are reported. Three sample plots were laid out in the larch ledum–cowberry–green moss forest on the northwest facing slope (8°–10°) in close proximity to each other; they differed in the proportions of area occupied by the principal components of a moss–lichen cover (Aulacomnium turgidum, Pleurozium schreberi, Cladonia rangiferina, and Hylocomium splendens), spatial distribution, and thermal properties of seasonally thawed soil layers. The worst thermal soil properties under the current climatic conditions have determined the comparatively low radial growth of trees in the sampling area, reduced response to air-temperature variations in the period of preseason cambium activity and at the beginning of the radial growth period, and heightened the response to this factor at the end of a season.     

Observed and projected impacts of climate on radial growth of three endangered conifers in northern Mexico indicate high vulnerability of drought-sensitive species from mesic habitats

A decline in productivity and radial growth in conifer forests from mesic areas has been associated with increased drought stress induced by climate warming. Nevertheless, studies showing how vulnerable tree species will be in response to forecasted warming conditions are scarce in such mesic habitats. Here we address this issue by analyzing how growth responds to drought and to observed and projected climate conditions in a conifer forest from northern Mexico, which is a hotspot of conifer diversity. We quantify the trends in radial growth (quantified as basal area increment, BAI) of three species (Abies durangensis, Picea chihuahuana, Cupressus lusitanica) using dendrochronology and a process-based model of tree growth. Growth decreased in A. durangensis and P. chihuahuana from the late 1980s onwards in response to warmer and drier conditions, whereas C. lusitanica growth showed very low sensitivity to precipitation and increased as temperature did. Winter-spring dry conditions adversely affected the growth of A. durangensis and P. chihuahuana. Our modeling approach anticipates growth reductions and an increase in the vulnerability of A. durangensis and the endangered P. chihuahuana against the warmer and more arid conditions predicted after the 2050s. Future warmer and drier climatic conditions could reduce the productivity and lead to growth decline of these mesic conifer forests triggering dieback episodes in highly drought-sensitive species as A. durangensis and P. chihuahuana.     

寒温带森林白桦径向生长的海拔差异及其气候响应——以奥克里堆山为例

气候变化深刻影响森林生态系统的结构和功能。在全球升温背景下,不同环境中不同树种的生长模式及其气候响应决定着生态系统的发展和稳定。本研究基于大兴安岭地区奥克里堆山的白桦年轮宽度数据,采用树木年代学方法,分析(兴安)落叶松森林中先锋树种白桦的生长-气候响应与升温和海拔变化的关联。结果表明: 气候变暖使具有明显海拔环境差异的白桦产生生长分异。较低海拔(1050 m)区域的白桦生长显著增加,而在相对偏高海拔(1250 m)区域的生长变化不大;在具有明显快速升温变化(1980年)之前,各海拔区域白桦生长的冬季(上年10月至当年2月)低温胁迫均达到显著水平;在快速升温阶段(1981—2010年),白桦生长的冬季低温胁迫降低,生长季(5—7月)温度成为偏高海拔处白桦生长的主要限定性因子;在水热条件较为调和的低海拔区域,白桦生长加快。研究区内白桦的分布总体上随着气候的持续变暖逐渐向高海拔区域扩散。     

Species-specific growth-climate responses of Dahurian larch (Larix gmelinii) and Mongolian pine (Pinus sylvestris var. mongolica) in the Greater Khingan Range,northeast China

Responses of tree growth to climate are usually spatially heterogeneous. Besides regionally varying external environments, species specificity is a crucial factor in determining said spatial heterogeneity. A better understanding of this species specificity would improve our estimations of the warming effects on forests. In this study, we selected two widely-distributed boreal conifers, Dahurian larch (Larix gmelinii) and Mongolian pine (Pinus sylvestris var. mongolica), to compare their growth-climate responses, including long-term growth-climate correlations and short-term growth resilience to drought. We sampled 160 trees and 481 tree-ring cores from the two species in two pure and two mixed forests, located in the Greater Khingan Range, northeast China. We found that Dahurian larch was generally positively correlated with spring temperature and negatively correlated with summer temperature. In contrast, Mongolian pine was more sensitive to summer moisture. Our results suggest that the main climatic limitations were low spring temperatures for Dahurian larch and summer moisture deficits for Mongolian pine. Dahurian larch represented higher growth resistance to drought, while Mongolia pine represented higher recovery. Based on this, we inferred that Dahurian larch was more vulnerable to extreme droughts, while Mongolian pine was more vulnerable to frequent droughts. We also demonstrated the effects of forest type on growth-climate responses. The negative effects of summer temperatures on Mongolian pine seemed to be more significant in mixed forests. As warming continued, Mongolian pine in this area would suffer severer moisture deficits, especially when coexisting with Dahurian larch. Our results suggest that Dahurian larch gained an advantage in the competition with Mongolian pine during high moisture stress. Driven by the warming trends, the species specificity in growth response would ultimately promote the separation of the two species in distribution. This study will help improve our estimations of the warming effects on forests and develop more species-targeted forest management practices.