Seawater exposure causes hydraulic damage in dying Sitka-spruce trees

Sea-level rise is one of the most critical challenges facing coastal ecosystems under climate change. Observations of elevated tree mortality in global coastal forests are increasing, but important knowledge gaps persist concerning the mechanism of salinity stress-induced nonhalophytic tree mortality. We monitored progressive mortality and associated gas exchange and hydraulic shifts in Sitka-spruce (Picea sitchensis) trees located within a salinity gradient under an ecosystem-scale change of seawater exposure in Washington State, USA. Percentage of live foliated crown (PLFC) decreased and tree mortality increased with increasing soil salinity during the study period. A strong reduction in gas exchange and xylem hydraulic conductivity (K_s) occurred during tree death, with an increase in the percentage loss of conductivity (PLC) and turgor loss point (π_tlp). Hydraulic and osmotic shifts reflected that hydraulic function declined from seawater exposure, and dying trees were unable to support osmotic adjustment. Constrained gas exchange was strongly related to hydraulic damage at both stem and leaf levels. Significant correlations between foliar sodium (Na⁺) concentration and gas exchange and key hydraulic parameters (K_s, PLC, and π_tlp) suggest that cellular injury related to the toxic effects of ion accumulation impacted the physiology of these dying trees. This study provides evidence of toxic effects on the cellular function that manifests in all aspects of plant functioning, leading to unfavourable osmotic and hydraulic conditions.

Hydraulic and osmotic shifts during tree death under seawater exposure are related to the toxic effects of ion accumulation on the maintenance of cellular function.     

干旱胁迫下新疆杨树冠不同高度叶片水分状况与非结构性碳动态北大核心CSCD

水力结构调整及非结构性碳动态对于认知干旱胁迫下植物生存前景和死亡风险至关重要。该研究以新疆杨(Populus bolleana Lauche.)为对象,通过干旱处理分析树冠不同高度叶片水力性状、光合生理特性、不同功能器官间的可溶性糖、淀粉及非结构性碳水化合物(NSC)含量变化,以甄别树冠不同高度叶片对干旱胁迫的生理响应和适应差异。结果表明:(1)在干旱胁迫下,树冠上部的叶水势、叶含水量、枝条含水量普遍低于下部,饱和膨压渗透势及膨压损失点水势在不同树冠高度间差异不显著;(2)干旱处理组植株净光合速率随树冠高度的增加而降低,叶绿素SPAD值在树冠的上部显著低于下部,而水分利用效率在树冠的上部却高于下部;(3)干旱处理组比叶面积在各树冠高度下显著低于对照组,而胡伯尔值在中部及上部高于对照组但差异不显著;(4)干旱处理组,树冠上部叶片淀粉含量显著高于下部,枝条可溶性糖及NSC含量在树冠上部显著高于下部,韧皮部的可溶性糖、淀粉及NSC含量在不同树冠高度间无显著差异,细根的可溶性糖、淀粉及NSC含量在对照组与干旱处理组间的差异不显著。研究发现,干旱处理下,树冠高度的增加会加剧新疆杨枝叶的干旱胁迫,致使树冠上部枝条木质部发生栓塞的风险大于下部,并导致NSC在不同器官间的分配和组分产生差异,但新疆杨植株可通过水分利用效率和形态上的适应性调整减缓干旱胁迫。     

Water stress, shoot growth and storage of non-structural carbohydrates along a tree height gradient in a tall conifer

We analysed concentrations of starch, sucrose, glucose and fructose in upper branch wood, foliage and trunk sapwood of Douglas-fir trees in height classes ranging from ~2 to ~57 m. Mean concentrations of non-structural carbohydrates (NSC) for all tissues were highest in the tallest height class and lowest in the lowest height class, and height-related trends in NSC were most pronounced in branches. Throughout a 17-month sampling period, mean values of branch NSC from the 57 m trees ranged between 30 and 377% greater than the 2 m trees. Branch NSC was inversely correlated with midday shoot water potential (Ψ(l)), shoot osmotic potential at full turgor (Ψ) and shoot extension. Temporal fluctuation in branch NSC was inversely correlated with height, and positively correlated with midday Ψ(l) , Ψ and shoot extension. The positive correlation between height and storage of NSC, and the negative correlation between NSC storage and shoot extension provide evidence that size-related growth decline in trees is not strongly associated with constraints on photosynthesis. The negative correlation between height and fluctuation in NSC suggests that mobilization of photosynthate in taller trees is constrained by some factor such as reductions in turgor-driven cell expansion or constraints on phloem transport.     

Carbohydrate dynamics and mortality in a piñon‐juniper woodland under three future precipitation scenarios

Drought‐induced forest mortality is an increasing global problem with wide‐ranging consequences, yet mortality mechanisms remain poorly understood. Depletion of non‐structural carbohydrate (NSC) stores has been implicated as an important mechanism in drought‐induced mortality, but experimental field tests are rare. We used an ecosystem‐scale precipitation manipulation experiment to evaluate leaf and twig NSC dynamics of two co‐occurring conifers that differ in patterns of stomatal regulation of water loss and recent mortality: the relatively desiccation‐avoiding piñon pine (Pinus edulis) and the relatively desiccation‐tolerant one‐seed juniper (Juniperus monosperma). Piñon pine experienced 72% mortality after 13–25 months of experimental drought and juniper experienced 20% mortality after 32–47 months. Juniper maintained three times more NSC in the foliage than twigs, and converted NSC to glucose and fructose under drought, consistent with osmoregulation requirements to maintain higher stomatal conductance during drought than piñon. Despite these species differences, experimental drought caused decreased leaf starch content in dying trees of both species (P < 0.001). Average dry‐season leaf starch content was also a good predictor of drought‐survival time for both species (R² = 0.93). These results, along with observations of drought‐induced reductions to photosynthesis and growth, support carbon limitation as an important process during mortality of these two conifer species.     

虫害叶损失造成的树木非结构性碳减少与树木生长、死亡的关系研究进展

大规模虫害爆发可造成区域森林死亡, 近年的气候变化进一步增加了虫害的频度和危害程度。森林和林地植物死亡会导致植被生产力降低, 改变生态系统结构和功能, 使森林由一个净的碳汇转变为一个碳源。因此, 加深虫害对树木危害机制的认识有重要意义。虫害造成的叶损失(虫害叶损失)降低树木光合作用能力, 增加非结构性碳(NSC)消耗, 使得树木体内碳储备降低, NSC降低到一定程度会导致树木因碳饥饿而死亡。外部环境和树木自身的补偿性机制也会对这个过程产生正或负的影响。在近年气候变化背景下, 树木死亡在全球尺度上增多, 重新激起了人们对碳饥饿的重视, 碳饥饿被视为解释树木死亡的主要生理机制之一。该文介绍了碳饥饿的定义, 综述了虫害叶损失减少树木NSC储备与树木生长、死亡的关系, 以及树木虫害和叶损失与气候变化之间的关系, 并对今后的研究进行了展望。     

The relationship between the reduction of nonstructural carbohydrate induced by defoliator and the growth and mortality of trees

Large scale herbivorous insect outbreaks can cause death of regional forests, and the events are expected to be exacerbated with climate change. Mortality of forest and woodland plants would cause a series of serious consequences, such as decrease in vegetation production, shifts in ecosystem structure and function, and transformation of forest function from a net carbon sink into a net carbon source. There is thus a need to better understand the impact of insects on trees. Defoliation by insect pests mainly reduces photosynthesis (source decrease) and increases carbon consumption (sink increase), and hence causes reduction of nonstructural carbohydrate (NSC). When the reduction in NSC reaches to a certain level, trees would die of carbon starvation. External environment and internal compensatory mechanisms can also positively or negatively influence the process of tree death. At present, the research of carbon starvation is a hotspot because the increase of tree mortality globally with climate change, and carbon starvation is considered as one of the dominating physiological mechanisms for explaining tree death. In this study, we reviewed the definition of carbon starvation, and the relationships between the reduction of NSC induced by defoliation and the growth and death of trees, and the relationships among insect outbreaks, leaf loss and climate change. We also presented the potential directions of future studies on insect-caused defoliation and tree mortality.     

Long‐term response of temperate canopy trees to removal of browsing from an invasive arboreal herbivore in New Zealand

Defoliation of forest tree canopies by herbivores and other agents, leading to tree mortality and reduced productivity, threatens the ecological stability of forests globally. This study shows that long‐term control of a mammalian arboreal folivore (brushtail possums; Trichosurus vulpecula Phalangeridae) reduces crown dieback and increases foliage cover in browsing‐damaged canopy trees. We monitored indices of possum density, possum browsing, tree foliage cover and crown dieback for 20 years following initiation of possum control in 1994 that repeatedly reduced possum densities to near zero every 5–6 years and kept the population below 35% of pre‐control levels over the entire period. Observable possum browsing was recorded on 20–49% of individuals of three palatable tree species at the time of first control. Those percentages fell to zero after control and never exceeded 2–10% for individual species over the next 19 years. We recorded significant increases in foliage cover attributable to recovery from defoliation by possums for all three species during the first 10 years. Large increases in foliage cover occurred on individuals that were heavily browsed in 1994 (mean increases: 36–89%), but mean population increases were modest (3–19%) because only 10–19% of trees were initially heavily browsed. Twenty‐year mortality rates were similar for plants with, or without, initial possum browsing, indicating no residual impact of pre‐control browsing on tree mortality. Times for full recovery of crown foliage cover varied from 10 years for the youngest trees and faster growing species to more than 20 years for mature individuals of the slowest growing species.     

Causes of vertical stratification in the density of Cameraria hamadryadella

1. The density of Cameraria hamadryadella, a leaf-mining moth, is vertically stratified within the crown of oak trees. It occurs at higher densities on foliage in the lower crown. 2. Oviposition preference tests indicate that females display no preference to oviposit on foliage from the lower tree crown over foliage from the upper tree crown. 3. Experiments in which potted trees were placed at various heights indicate that foliage nearest ground level receives more oviposition, and that the higher rates of oviposition on foliage near the ground is not caused by differences in quality between foliage from low or high in the tree crown. 4. Host-plant- and natural-enemy-mediated juvenile mortality and the abundance of parasitoids did not differ between the upper and lower crown. 5. Vertical differences in the timing of leaf production within the tree crown are unlikely to account for the observed gradient in the abundance of C. hamadryadella. Furthermore, the mean date of leaf fall does not differ among heights within the tree crown. 6. It is argued that microclimatic gradients and interspecific competition are also unlikely to account for the observed gradients in the density of C. hamadryadella within the tree crown. 7. Because of the absence of effects of other potentially causal factors, the most likely explanation for the gradient in density of C. hamadryadella is a lack of movement by females into the upper tree crown from overwintering, emergence, or resting sites located in the lower tree crown, combined with egg depletion by females as they move from the lower to the upper tree crown. 8. It is suggested that the lack of movement and egg depletion hypothesis should serve as the null hypothesis in studies of vertical stratification of tree-feeding insects. In the absence of evidence of an effect of foliage quality, natural enemies, plant phenology, interspecific competition, or microclimate, the movement and egg depletion hypothesis is the most reasonable.     

Precision and accuracy of tree-ring-based death dates of mountain pines in the Swiss National Park

Key message

Mountain pines in the Swiss National Park show evidence of partial cambial mortality, which affects the precision of tree-ring-based death dates, followed by lagged crown mortality.

Abstract

The time of tree death is commonly reconstructed by dating the outermost ring of tree-ring series. However, due to the occurrence of partial cambial mortality, the date of the outermost tree ring may vary between different locations on the tree stem. Furthermore, a tree may continue to live following the formation of the most recent tree ring. In this study, we quantified precision and accuracy of tree-ring-based death dates from 229 dead mountain pines (Pinus montana) from a 28 km² study area in the Swiss National Park. For almost two-thirds of the trees, a maximum difference of just 0–4 years between the dates of cambial mortality from three increment cores was observed, however, for a few trees the difference reached 30–65 years. Higher maximum differences between the dates of cambial mortality are expected for trees on steep slopes, for old trees or for trees that died a long time ago. For 84 % of dead mountain pines, which were sampled in a permanent sample plot with 2-year remeasurement intervals, the difference between the date of observed crown mortality and the death date determined from three cores was 0–5 years. Sampling two or just one core per tree decreases the accuracy of tree-ring-based death dates. Based on the findings of our study, we recommend a prior assessment of the precision and accuracy of tree-ring-based death dates for any dendroecological study dealing with the reconstruction of tree mortality.     

Defoliation increases risk of carbon starvation in root systems of mature aspen

Large trees are noted to decline and die over several years after defoliation or extended periods of drought. The underlying mechanisms of this decline are thought to be driven by moisture limitations to photosynthesis, but alternative considerations also suggest carbon storage limitations as a driver. This research assesses the non-structural carbohydrate reserves in crowns and roots tissues during and after defoliation by insects in trembling aspen. We monitored the non-structural carbon reserves of nine tall mature aspen forest stands over 8 years, including two defoliation events. We report on the carbohydrate dynamics in root and crown tissues during and after defoliation. Following defoliation, branch reserves recovered to levels of undefoliated control trees within the same season, while roots took up to 2 years to recover. We argue that in large trees, tissues closest to the foliage are the first sinks to access C in the phloem stream, while roots, which are more distal and separated by a long bole, will only receive adequate supplies of C when the other more proximate sinks are sated. These results support the hypothesis that in times of limited carbon assimilation, root tissues in mature trees experience the longest reduction in carbon reserves, which likely plays a critical role in tree decline and mortality.     

Modeling the vertical foliage distribution of an individual <Emphasis Type="Italic">Castanopsis cuspidata</Emphasis> (Thunb.) Schottky,a dominant broad-leaved tree in Japanese warm-temperate forest

The vertical foliage distribution of Castanopsis cuspidata (Thunb.) Schottky was examined in trees of various sizes to clarify its variation in relation to tree size and the light environment in a stand. As indices of these parameters, we analyzed crown social position (CSP: percent of stand height) and specific leaf area (SLA). The vertical foliage distribution of trees was expressed by a Weibull function. The variation in the vertical foliage distribution of C. cuspidata could be categorized into three types using crown social position and light environment. In the first type, leaves were concentrated to the top 20% of the tree; such trees are canopy trees that can receive full sunlight. The second type had a large relative crown depth and an asymmetric distribution with the maximum foliage located near the top of the tree; such trees are suppressed trees whose crowns do not receive sufficient light. The third type had a large relative crown depth and a symmetric distribution; such trees occur in high light environments, although their crowns are in the understory layer. The differences in the vertical foliage distribution are related to the strategies used to capture light. Multiple regression analysis showed that CSP and SLA at the top layer of the tree explained successive changes in the vertical foliage distribution. These results will contribute to scaling-up the vertical foliage distribution to the community level in pure stands of C. cuspidata using an individual-based model.     

白桦和紫椴树干非结构性碳水化合物的空间变异

以中国东北温带森林两个散孔材树种白桦和紫椴为对象,研究落叶后树干木质部中非结构性碳水化合物(NSC)浓度的空间变异.结果表明: 两种树种的可溶性糖与淀粉的总和(TNC)与可溶性糖浓度均随树干径向深度增加而缓慢下降,淀粉的径向变化不明显,即使在树干径向深处仍存有大量的NSC.两种树种树干的TNC、可溶性糖和淀粉浓度从根颈到胸高降低,之后逐渐升高,最大值出现的高度因树种和TNC组分而异.两种树种树干糖淀粉比值的纵向变化趋势为：白桦随树干升高而增大,紫椴则随之减小.树干NSC储量估算的误差主要来源于NSC浓度的纵向变化,其次是径向变化.喜光树种白桦的树干TNC浓度(1.0%干质量)显著低于耐阴树种紫椴(4.3%干质量),可能与其生活史对策差异有关.采用考虑了树干NSC纵向和径向变化的取样方法,可以有效地降低树木或林分水平上NSC储量估算的不确定性.
     

Large seasonal fluctuations in whole-tree carbohydrate reserves: is storage more dynamic in boreal ecosystems?

Background and AimsCarbon reserves are a critical source of energy and substrates that allow trees to cope with periods of minimal carbon gain and/or high carbon demands, conditions which are prevalent in high-latitude forests. However, we have a poor understanding of carbon reserve dynamics at the whole-tree level in mature boreal trees. We therefore sought to quantify the seasonal changes in whole-tree and organ-level carbon reserve pools in mature boreal Betula papyrifera.MethodsNon-structural carbohydrate (NSC; soluble sugars and starch) tissue concentrations were measured at key phenological stages throughout a calendar year in the roots, stem (inner bark and xylem), branches and leaves, and scaled up to estimate changes in organ and whole-tree NSC pool sizes. Fine root and stem growth were also measured to compare the timing of growth processes with changes in NSC pools.Key ResultsThe whole-tree NSC pool increased from its spring minimum to its maximum at bud set, producing an average seasonal fluctuation of 0.96 kg per tree. This fluctuation represents a 72 % change in the whole-tree NSC pool, which greatly exceeds the relative change reported for more temperate conspecifics. At the organ level, branches accounted for roughly 48–60 % of the whole-tree NSC pool throughout the year, and their seasonal fluctuation was four to eight times greater than that observed in the stemwood, coarse roots and inner bark.ConclusionsBranches in boreal B. papyrifera were the largest and most dynamic storage pool, suggesting that storage changes at the branch level largely drive whole-tree storage dynamics in these trees. The greater whole-tree seasonal NSC fluctuation in boreal vs. temperate B. papyrifera may result from (1) higher soluble sugar concentration requirements in branches for frost protection, and/or (2) a larger reliance on reserves to fuel new leaf and shoot growth in the spring.     

Fast replenishment of initial carbon stores after defoliation by the pine processionary moth and its relationship to the re-growth ability of trees

Defoliation by herbivores may alter the source:sink balance of trees leading to transient decreases in carbon (C) stores. When C stores are replenished concurrently with re-growth both processes may compete, store formation proceeding at the expenses of growth. However, the interactions between both processes are not fully understood. We investigated the effects of defoliation by the pine processionary moth (PPM, Thaumetopoea pityocampa Dennis and Schiff.) on the non-structural carbohydrate (NSC) and nitrogen (N) stores and the growth of Pinus nigra Arnold trees. Short-term effects were evaluated immediately after a PPM outbreak and at the end of the first growing season in trees suffering a range of defoliation damage. Long-term effects were explored by a 17-year-long PPM defoliation experiment, with 11?years of repeated defoliation treatments followed by 6?years of recovery. Defoliation by PPM was followed by transient NSC decreases, but trees were able to exceed initial NSC pools and compensate growth in just one growing season. Such recovery was linked to increased foliage N. Repeated severe defoliations decreased growth and survival of trees in the long-term, but trees increased starch allocation to stems. Defoliation led to an accumulation of C storage compounds in P. nigra trees irrespective of their ability to re-grow. In trees included in the short-term experiment, the accumulation of stores proceeded concurrently with re-growth. However, the repeated severe defoliations included in our long-term experiment impaired the growth of trees, surplus C being accumulated as stores. These results indicate that, growth declines in pines defoliated by PPM are not due to C (source) limitation but may respond to the reduced sink strength of growing meristems due to defoliation, and thus, a decrease in C allocation to growth.     

秦岭东段不同海拔栓皮栎粗根非结构性碳水化合物含量的季节动态

掌握树木根部碳存储规律对于准确估算碳在地上器官与地下器官间的分配非常必要。本研究以栓皮栎(Quercus variabilis Blume)为对象,在2016年5月—2017年6月,通过周期性采样方法(共计采样14次),测定了高、低海拔(970和650 m)栓皮栎粗根非结构性碳水化合物(non-structural carbohydrates,NSC)及其组分(可溶性糖和淀粉)含量的年内动态变化。结果表明:除高海拔淀粉外,栓皮栎粗根NSC及其组分含量均随季节变化差异显著(P<0.05)。粗根NSC含量呈现生长季初期(3月)下降,非生长季(2月)达到最高值的变化趋势;栓皮栎粗根NSC组成以淀粉为主,高、低海拔淀粉含量占比分别为61%和71%,这可能与栓皮栎适应区域环境特征有关。不同海拔间,栓皮栎粗根NSC及其组分含量的差异出现在生长季初期(3月,P<0.05)。高海拔(10.26%)栓皮栎粗根NSC含量小于低海拔(13.96%)。栓皮栎粗根NSC含量存在明显的季节波动,粗根在生长季末及非生长季积累的NSC对下一年树木生长启动非常重要,研究结果有助于理解树木地下器官对树木生长的碳供应机制。     

秦岭东段栓皮栎叶片非结构性碳水化合物含量的季节动态

研究树木叶片非结构性碳水化合物(Nonstructural carbohydrate,NSC)组分的季节变化是掌握树木碳代谢规律的基础,也有利于判断以往研究仅凭生长季单次或几次(5次)的取样方法是否存在一定局限性。以秦岭东段栓皮栎(Quercus variabilis Blume)优势群落为研究对象,在其分布的海拔上下限(650 m和970 m),于2016年5月至2017年5月,通过月尺度周期性取样(共计9次),测定栓皮栎叶片NSC及其组分含量,并观测同期叶片物候变化。结果显示:(1)栓皮栎叶片NSC及其组分季节变化差异显著(P0.05),可溶性糖、淀粉和NSC变异系数分别为20.99%、52.28%和25.96%;(2)整体而言,栓皮栎叶片NSC最小值在展叶初期(3月末—4月初,5%左右),最大值在展叶末期(5月上旬,12%左右),之后NSC呈持续下降趋势。不同海拔NSC极值出现时间略有不同,叶片物候可能是影响年内极值的主要原因。(3)栓皮栎叶片NSC组成以可溶性糖为主(65%),这可能是树种在暖温带所采取的生长策略。(4)海拔对栓皮栎叶片NSC及其组分影响差异不显著,低海拔栓皮栎叶片NSC及其组分含量略大于高海拔。研究结果,栓皮栎叶片NSC含量存在明显的季节波动,适当加大NSC采样频率对于正确理解树木碳代谢十分必要。     

岳桦幼苗光合特性和非结构性碳水化合物积累对干旱胁迫的响应

以长白山林线树种岳桦为对象,利用生长控制试验进行干旱处理,研究干旱对岳桦幼苗光合特性及非结构性碳水化合物(NSC)积累的影响.结果表明:干旱显著降低了岳桦幼苗的净光合速率和气孔导度,提高了其水分利用效率;干旱显著增加了岳桦幼苗叶、皮、干和根中的可溶性糖和总NSC的含量,但显著降低了淀粉含量;随着干旱的持续,叶片的气孔导...     

Global patterns of mobile carbon stores in trees at the high‐elevation tree line

Aim Across all latitudes, high‐elevation tree lines represent a drastic change in the dominant plant life‐form, from upright trees to low‐stature alpine plants. Although associated with low temperatures, the physiological mechanisms controlling this boundary are still not clear. The growth‐limitation hypothesis assumes a direct low‐temperature restriction of tissue formation at otherwise sufficient photoassimilation. In order to test this hypothesis, we present a global synthesis of previously published and new data on tree carbon supply status at high‐elevation tree lines. Location Global; 13 regions between 68° N and 45° S. Methods Late‐season concentrations of non‐structural carbohydrates (NSC) in foliage and branch wood were measured at three elevations across the tree line ecotones, from upper tall forests (timber line) to the edges of aborescent tree growth (tree line). Year‐round records of ?10 cm soil temperatures were taken at the tree line. Results Despite large differences in elevation and season length, the mean growing season temperature at the tree line was similar (approximately 6.6 °C) between all sites. NSC concentrations were not depleted at any of the elevation gradients between timber line and tree line, indicating no shortage of C at the uppermost tree stands. Tested across all sites, NSC concentrations in fact significantly increased with elevation. On average, NSC increased by 18% in leaves and 26% in branch wood from the lowest to the highest stands, primarily due to higher starch concentrations rather than free sugars. Hence, these responses do not reflect osmotic adjustments to lower temperatures at the end of the growing season. Main conclusions This global data set contributes to a mechanistic understanding of tree line formation based on biological principles across climatic zones and tree genera. No evidence of C shortage was found at the high‐elevation tree line in either seasonal and non‐seasonal regions. The increasing trend of NSC concentrations with elevation is in line with the growth‐limitation hypothesis.     

Patterns in nonstructural carbohydrate contents at the tree organ level in response to drought duration

Nonstructural carbohydrates (NSCs) facilitate the adaptation of trees to drought stress. There have been a large number of studies exploring NSC changes in individual plant species and individual organ under drought and showed different trends; however, an understanding of the universal pattern of the plant NSCs responses to drought, particularly to drought duration, is still lacking. Here, we compiled data from 47 experimental studies on 52 tree species and conducted a meta‐analysis to evaluate the responses of soluble sugars, starch, and TNSC (total nonstructural carbohydrates including both soluble sugars and starch) concentrations in different tree organs (leaf, stem, and root) to drought intensity and duration. We found that starch in all organs decreased and soluble sugars in leaf increased with prolonged experiment time, and the changes in soluble sugars in all organs were stronger under severe drought than under slight‐to‐moderate drought. Under slight‐to‐moderate drought, the NSC content of each organ varied with time, while with the extension of the drought duration, the NSCs gradually approached the control value (no drought stress); this trend remained in the late drought, which means that trees activated physiological regulation processes to increase carbon storage and reduce the risks of carbon starvation. In contrast, long‐term severe drought could lead to a net loss of carbohydrates, especially in the root, implying that prolonged severe drought could lead to NSC depletion in the whole plant. As prolonged drought duration has occurred in and is projected for many regions, this paper could shed light into studies on how trees respond and adapt extending drought duration through nonstructural carbon production, transportation, and reallocation.     

Canopy tree mortality depends on the proportion of crown exposed to sunlight,but this effect varies with species' wood density

Understanding what drives changes in tree mortality as well as the covariates influencing trees' response is a research priority to predict forest responses to global change. Here, we combined drone photogrammetry and ground-based data to assess the influence of crown exposure to light (relative to total crown area), growth deviations (relative to conspecifics), tree size, and species' wood density (as a surrogate for light-demanding and shade-tolerant life-history strategies) on the mortality of 984 canopy trees in an Amazon terra firme forest. Trees with lower wood density were less prone to die when their proportion of crown was more exposed to sunlight, but this relationship with relative crown exposure weakened and slightly reversed as wood density increased. Trees growing less than their species average had higher mortality, especially when the species' wood density decreased. The role of wood density in determining the survival of canopy trees under varying light conditions indicates differential responses of light-demanding versus shade-tolerant species. Our results highlight the importance of accounting for life-history strategies, via plant functional types, in vegetation dynamic models aiming to predict forest demography under a rapidly changing climate. Abstract in Spanish is available with online material.