Rapid warming accelerates tree growth decline in semi‐arid forests of Inner Asia

Forests around the world are subject to risk of high rates of tree growth decline and increased tree mortality from combinations of climate warming and drought, notably in semi‐arid settings. Here, we assess how climate warming has affected tree growth in one of the world's most extensive zones of semi‐arid forests, in Inner Asia, a region where lack of data limits our understanding of how climate change may impact forests. We show that pervasive tree growth declines since 1994 in Inner Asia have been confined to semi‐arid forests, where growing season water stress has been rising due to warming‐induced increases in atmospheric moisture demand. A causal link between increasing drought and declining growth at semi‐arid sites is corroborated by correlation analyses comparing annual climate data to records of tree‐ring widths. These ring‐width records tend to be substantially more sensitive to drought variability at semi‐arid sites than at semi‐humid sites. Fire occurrence and insect/pathogen attacks have increased in tandem with the most recent (2007–2009) documented episode of tree mortality. If warming in Inner Asia continues, further increases in forest stress and tree mortality could be expected, potentially driving the eventual regional loss of current semi‐arid forests.     

Drought timing and local climate determine the sensitivity of eastern temperate forests to drought

Projected changes in temperature and drought regime are likely to reduce carbon (C) storage in forests, thereby amplifying rates of climate change. While such reductions are often presumed to be greatest in semi‐arid forests that experience widespread tree mortality, the consequences of drought may also be important in temperate mesic forests of Eastern North America (ENA) if tree growth is significantly curtailed by drought. Investigations of the environmental conditions that determine drought sensitivity are critically needed to accurately predict ecosystem feedbacks to climate change. We matched site factors with the growth responses to drought of 10,753 trees across mesic forests of ENA, representing 24 species and 346 stands, to determine the broad‐scale drivers of drought sensitivity for the dominant trees in ENA. Here we show that two factors—the timing of drought, and the atmospheric demand for water (i.e., local potential evapotranspiration; PET)—are stronger drivers of drought sensitivity than soil and stand characteristics. Drought‐induced reductions in tree growth were greatest when the droughts occurred during early‐season peaks in radial growth, especially for trees growing in the warmest, driest regions (i.e., highest PET). Further, mean species trait values (rooting depth and ψ₅₀) were poor predictors of drought sensitivity, as intraspecific variation in sensitivity was equal to or greater than interspecific variation in 17 of 24 species. From a general circulation model ensemble, we find that future increases in early‐season PET may exacerbate these effects, and potentially offset gains in C uptake and storage in ENA owing to other global change factors.     

Recent climatic drying leads to age‐independent growth reductions of white spruce stands in western Canada

Since 2001, climatic conditions have been notably drier than normal across large areas of the western Canadian interior, leading to widespread impacts on the forests of this region. This poses a major concern for the future, given climate change projections for continued warming and drying. We conducted tree‐ring analysis in 75 pure stands of white spruce (Picea glauca) across Alberta and west‐central Saskatchewan to examine the effects of recent climatic drying on the growth of this important boreal tree species. Allometric equations were used to calculate annual growth in aboveground tree biomass (G_BM) from ring width measurements. Results showed an increasing trend in G_BM from the 1960s to the 1990s, followed by a sharp decline during the severe drought of 2001–2002. Of the 75 stands, only 18 recovered sufficiently to cause an increase in mean G_BM from the predrought decade of 1991–2000 to the subsequent decade of 2001–2010. The remaining 57 stands exhibited a decline in mean G_BM between these decades. Climatic drying was a major cause of the growth decline, as shown by the significant stand‐level relationship between percentage change in decadal mean G_BM and the change in decadal mean values of a climate moisture index from 1991–2000 to 2001–2010. Subsequent analyses of boreal stands sampled across Alberta during 2015 revealed that white spruce growth had declined even further as drought conditions intensified during 2014–2015. Overall, there was a 38% decrease in mean G_BM between 1997 and 2015, but surprisingly, the percentage decrease was not significantly different for young, productive stands compared with older, less productive stands. Thus, stand ageing cannot explain the observed decline in white spruce growth during the past quarter century, suggesting that these forests are at risk if the trend towards more frequent, severe drought continues in the region.     

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.     

Relationships between individual‐tree mortality and water‐balance variables indicate positive trends in water stress‐induced tree mortality across North America

Accounting for water stress‐induced tree mortality in forest productivity models remains a challenge due to uncertainty in stress tolerance of tree populations. In this study, logistic regression models were developed to assess species‐specific relationships between probability of mortality (P_m) and drought, drawing on 8.1 million observations of change in vital status (m) of individual trees across North America. Drought was defined by standardized (relative) values of soil water content (W_s,z) and reference evapotranspiration (ET_r,z) at each field plot. The models additionally tested for interactions between the water‐balance variables, aridity class of the site (AC), and estimated tree height (h). Considering drought improved model performance in 95 (80) per cent of the 64 tested species during calibration (cross‐validation). On average, sensitivity to relative drought increased with site AC (i.e. aridity). Interaction between water‐balance variables and estimated tree height indicated that drought sensitivity commonly decreased during early height development and increased during late height development, which may reflect expansion of the root system and decreasing whole‐plant, leaf‐specific hydraulic conductance, respectively. Across North America, predictions suggested that changes in the water balance caused mortality to increase from 1.1% yr^?1 in 1951 to 2.0% yr^?1 in 2014 (a net change of 0.9 ± 0.3% yr^?1). Interannual variation in mortality also increased, driven by increasingly severe droughts in 1988, 1998, 2006, 2007 and 2012. With strong confidence, this study indicates that water stress is a common cause of tree mortality. With weak‐to‐moderate confidence, this study strengthens previous claims attributing positive trends in mortality to increasing levels of water stress. This ‘learn‐as‐we‐go’ approach – defined by sampling rare drought events as they continue to intensify – will help to constrain the hydraulic limits of dominant tree species and the viability of boreal and temperate forest biomes under continued climate change.     

Temperature‐induced water stress in high‐latitude forests in response to natural and anthropogenic warming

The Arctic is particularly sensitive to climate change, but the independent effects of increasing atmospheric CO₂ concentration (pCO₂) and temperature on high‐latitude forests are poorly understood. Here, we present a new, annually resolved record of stable carbon isotope (δ¹³C) data determined from Larix cajanderi tree cores collected from far northeastern Siberia in order to investigate the physiological response of these trees to regional warming. The tree‐ring record, which extends from 1912 through 1961 (50 years), targets early twentieth‐century warming (ETCW), a natural warming event in the 1920s to 1940s that was limited to Northern hemisphere high latitudes. Our data show that net carbon isotope fractionation (Δ¹³C), decreased by 1.7‰ across the ETCW, which is consistent with increased water stress in response to climate warming and dryer soils. To investigate whether this signal is present across the northern boreal forest, we compiled published carbon isotope data from 14 high‐latitude sites within Europe, Asia, and North America. The resulting dataset covered the entire twentieth century and spanned both natural ETCW and anthropogenic Late Twentieth‐Century Warming (~0.7 °C per decade). After correcting for a ~1‰ increase in Δ¹³C in response to twentieth century pCO₂ rise, a significant negative relationship (r = ?0.53, P < 0.0001) between the average, annual Δ¹³C values and regional annual temperature anomalies is observed, suggesting a strong control of temperature on the Δ¹³C value of trees growing at high latitudes. We calculate a 17% increase in intrinsic water‐use efficiency within these forests across the twentieth century, of which approximately half is attributed to a decrease in stomatal conductance in order to conserve water in response to drying conditions, with the other half being attributed to increasing pCO₂. We conclude that annual tree‐ring records from northern high‐latitude forests record the effects of climate warming and pCO₂ rise across the twentieth century.     

Contrasting resistance and resilience to extreme drought and late spring frost in five major European tree species

Extreme climate events (ECEs) such as severe droughts, heat waves, and late spring frosts are rare but exert a paramount role in shaping tree species distributions. The frequency of such ECEs is expected to increase with climate warming, threatening the sustainability of temperate forests. Here, we analyzed 2,844 tree‐ring width series of five dominant European tree species from 104 Swiss sites ranging from 400 to 2,200 m a.s.l. for the period 1930–2016. We found that (a) the broadleaved oak and beech are sensitive to late frosts that strongly reduce current year growth; however, tree growth is highly resilient and fully recovers within 2 years; (b) radial growth of the conifers larch and spruce is strongly and enduringly reduced by spring droughts—these species are the least resistant and resilient to droughts; (c) oak, silver fir, and to a lower extent beech, show higher resistance and resilience to spring droughts and seem therefore better adapted to the future climate. Our results allow a robust comparison of the tree growth responses to drought and spring frost across large climatic gradients and provide striking evidence that the growth of some of the most abundant and economically important European tree species will be increasingly limited by climate warming. These results could serve for supporting species selection to maintain the sustainability of forest ecosystem services under the expected increase in ECEs.     

Integrating ecophysiology and forest landscape models to improve projections of drought effects under climate change

Fundamental drivers of ecosystem processes such as temperature and precipitation are rapidly changing and creating novel environmental conditions. Forest landscape models (FLM) are used by managers and policy‐makers to make projections of future ecosystem dynamics under alternative management or policy options, but the links between the fundamental drivers and projected responses are weak and indirect, limiting their reliability for projecting the impacts of climate change. We developed and tested a relatively mechanistic method to simulate the effects of changing precipitation on species competition within the LANDIS‐II FLM. Using data from a field precipitation manipulation experiment in a piñon pine (Pinus edulis) and juniper (Juniperus monosperma) ecosystem in New Mexico (USA), we calibrated our model to measurements from ambient control plots and tested predictions under the drought and irrigation treatments against empirical measurements. The model successfully predicted behavior of physiological variables under the treatments. Discrepancies between model output and empirical data occurred when the monthly time step of the model failed to capture the short‐term dynamics of the ecosystem as recorded by instantaneous field measurements. We applied the model to heuristically assess the effect of alternative climate scenarios on the piñon–juniper ecosystem and found that warmer and drier climate reduced productivity and increased the risk of drought‐induced mortality, especially for piñon. We concluded that the direct links between fundamental drivers and growth rates in our model hold great promise to improve our understanding of ecosystem processes under climate change and improve management decisions because of its greater reliance on first principles.     

Moisture‐driven shift in the climate sensitivity of white spruce xylem anatomical traits is coupled to large‐scale oscillation patterns across northern treeline in northwest North America

Tree growth at northern treelines is generally temperature‐limited due to cold and short growing seasons. However, temperature‐induced drought stress was repeatedly reported for certain regions of the boreal forest in northwestern North America, provoked by a significant increase in temperature and possibly reinforced by a regime shift of the pacific decadal oscillation (PDO). The aim of this study is to better understand physiological growth reactions of white spruce, a dominant species of the North American boreal forest, to PDO regime shifts using quantitative wood anatomy and traditional tree‐ring width (TRW) analysis. We investigated white spruce growth at latitudinal treeline across a >1,000 km gradient in northwestern North America. Functionally important xylem anatomical traits (lumen area, cell‐wall thickness, cell number) and TRW were correlated with the drought‐sensitive standardized precipitation–evapotranspiration index of the growing season. Correlations were computed separately for complete phases of the PDO in the 20th century, representing alternating warm/dry (1925–1946), cool/wet (1947–1976) and again warm/dry (1977–1998) climate regimes. Xylem anatomical traits revealed water‐limiting conditions in both warm/dry PDO regimes, while no or spatially contrasting associations were found for the cool/wet regime, indicating a moisture‐driven shift in growth‐limiting factors between PDO periods. TRW reflected only the last shift of 1976/1977, suggesting different climate thresholds and a higher sensitivity to moisture availability of xylem anatomical traits compared to TRW. This high sensitivity of xylem anatomical traits permits to identify first signs of moisture‐driven growth in treeline white spruce at an early stage, suggesting quantitative wood anatomy being a powerful tool to study climate change effects in the northwestern North American treeline ecotone. Projected temperature increase might challenge growth performance of white spruce as a key component of the North American boreal forest biome in the future, when drier conditions are likely to occur with higher frequency and intensity.     

Stand structure modulates the long-term vulnerability of Pinus halepensis to climatic drought in a semiarid Mediterranean ecosystem

We investigated whether stand structure modulates the long-term physiological performance and growth of Pinus halepensis Mill. in a semiarid Mediterranean ecosystem. Tree radial growth and carbon and oxygen stable isotope composition of latewood (δ(13)C(LW) and δ(18)O(LW), respectively) from 1967 to 2007 were measured in P. halepensis trees from two sharply contrasting stand types: open woodlands with widely scattered trees versus dense afforested stands. In both stand types, tree radial growth, δ(13)C(LW) and δ(18)O(LW) were strongly correlated with annual rainfall, thus indicating that tree performance in this semiarid environment is largely determined by inter-annual changes in water availability. However, trees in dense afforested stands showed consistently higher δ(18)O(LW) and similar δ(13)C(LW) values compared with those in neighbouring open woodlands, indicating lower stomatal conductance and photosynthesis rates in the former, but little difference in water use efficiency between stand types. Trees in dense afforested stands were more water stressed and showed lower radial growth, overall suggesting greater vulnerability to drought and climate aridification compared with trees in open woodlands. In this semiarid ecosystem, the negative impacts of intense inter-tree competition for water on P. halepensis performance clearly outweigh potential benefits derived from enhanced infiltration and reduced run-off losses in dense afforested stands.     

Tree growth variation in the tropical forest: understanding effects of temperature,rainfall and CO2

Tropical forest responses to climatic variability have important consequences for global carbon cycling, but are poorly understood. As empirical, correlative studies cannot disentangle the interactive effects of climatic variables on tree growth, we used a tree growth model (IBTREE) to unravel the climate effects on different physiological pathways and in turn on stem growth variation. We parameterized the model for canopy trees of Toona ciliata (Meliaceae) from a Thai monsoon forest and compared predicted and measured variation from a tree‐ring study over a 30‐year period. We used historical climatic variation of minimum and maximum day temperature, precipitation and carbon dioxide (CO₂) in different combinations to estimate the contribution of each climate factor in explaining the inter‐annual variation in stem growth. Running the model with only variation in maximum temperature and rainfall yielded stem growth patterns that explained almost 70% of the observed inter‐annual variation in stem growth. Our results show that maximum temperature had a strong negative effect on the stem growth by increasing respiration, reducing stomatal conductance and thus mitigating a higher transpiration demand, and – to a lesser extent – by directly reducing photosynthesis. Although stem growth was rather weakly sensitive to rain, stem growth variation responded strongly and positively to rainfall variation owing to the strong inter‐annual fluctuations in rainfall. Minimum temperature and atmospheric CO₂ concentration did not significantly contribute to explaining the inter‐annual variation in stem growth. Our innovative approach – combining a simulation model with historical data on tree‐ring growth and climate – allowed disentangling the effects of strongly correlated climate variables on growth through different physiological pathways. Similar studies on different species and in different forest types are needed to further improve our understanding of the sensitivity of tropical tree growth to climatic variability and change.