Drought events influence nutrient canopy exchanges and green leaf partitioning during senescence in a deciduous forest

The increase in the frequency and intensity of drought events expected in the coming decades in Western Europe may disturb forest biogeochemical cycles and create nutrient deficiencies in trees.One possible origin of nutrient deficiency is the disturbance of the partitioning of the green leaf pool during the leaf senescence period between resorption,foliar leaching and senesced leaves.However,the effects of drought events on this partitioning and the consequences for the maintenance of tree nutrition are poorly documented.An experiment in a beech forest in Meuse(France)was conducted to assess the effect of drought events on nutrient canopy exchanges and on the partitioning of the green leaf pool during the leaf senescence period.The aim was to identify potential nutritional consequences of droughts for trees.Monitoring nutrient dynamics,including resorption,chemistry of green and senesced leaves,foliar absorption and leaching in mature beech stands from 2012 to 2019 allowed us to compare the nutrient exchanges for three nondry and three dry years(i.e.,with an intense drought event during the growing season).During dry years,we observed a decrease by almost a third of the potassium(K)partitioning to resorption(i.e.resorption efficiency),thus reducing the K reserve in trees for the next growing season.This result suggests that with the increased drought frequency and intensity expected for the coming decades,there will be a risk of potassium deficiency in trees,as already observed in a rainfall exclusion experiment on the same study site.Reduced foliar leaching and higher parititioning to the senesced leaves for K and phosphorus(P)were also observed.In addition,a slight increase in nitrogen(N)resorption efficiency occurred during dry years which is more likely to improve tree nutrition.The calcium(Ca)negative resorption decreased,with no apparent consequence in our study site.Our results show that nutrient exchanges in the canopy and the partitioning of the green leaf pool can be modified by drought events,and may have consequences on tree nutrition.

Interannual dynamics of stemwood nonstructural carbohydrates in temperate forest trees surrounding drought

Interactions between water and carbon dynamics underlie drought-related tree mortality.While whole-tree water relations have been shown to play a key role in the response to and recovery from drought,the role of nonstructural carbohydrates(NSC) and how their storage and allocation changes surrounding drought events deserves further attention and is critical for understanding tree survival.Here,we quantified in situ NSC responses of temperate forest trees to the 2016 drought in the northeastern United States.Sugar and starch concentrations were measured in the stemwood of five tree species from 2014 to 2019,which allowed us to monitor NSCs in relation to climatic conditions before,during,and after the natural drought.We found that immediately following the drought,measured stemwood NSC concentrations decreased.However,NSC concentrations rebounded quickly within three years.Notably,trees allocated proportionally more to starch than to sugars following the 2016 drought.In winter 2017,starch comprised 45% of total stemwood stores,whereas starch made up only 1-2% in other years.Further,we modeled and assessed the climatic drivers of total NSC concentrations in the stem.Variation in total NSC concentrations was significantly predicted by the previous year’s temperature,precipitation,and standardized precipitation-evapotranspiration index(SPEI),with stemwood concentrations decreasing following hotter,drier periods and increasing following cooler,wetter periods.Overall,our work provides insight into the climatic drivers of NSC storage and highlights the important role that a tree’s carbon economy may play in its response and recovery to environmental stress.

Annual growth of Fagus orientalis is limited by spring drought conditions in Iran’s Golestan Province

Due to the lack of a uniform and accurate defi-nition of‘drought’,several indicators have been introduced based on different variables and methods,and the efficiency of each of these is determined according to their relationship with drought.The relationship between two drought indices,SPI(standardized precipitation index)and SPEI(standard-ized precipitation-evapotranspiration index)in different sea-sons was investigated using annual rings of 15 tree samples to determine the effect of drought on the growth of oriental beech(Fagus orientalis Lipsky)in the Hyrcanian forests of northern Iran.The different evapotranspiration calcula-tion methods were evaluated on SPEI efficiency based on Hargreaves-Samani,Thornthwaite,and Penman-Monteith methods using the step-by-step M5 decision tree regression method.The results show that SPEI based on the Penman-Monteith in a three-month time scale(spring)had similar temporal changes and a better relationship with annual tree rings(R^(2)=0.81)at a 0.05 significant level.Abrupt change and a decreasing trend in the time series of annual tree rings are similar to the variation in the SPEI based on the Penman-Monteith method.Factors affecting evapotranspiration,temperature,wind speed,and sunshine hours(used in the Penman-Monteith method),increased but precipitation decreased.Using non-linear modeling methods,SPEI based on Penman-Monteith best illustrated climate changes affecting tree growth.

Old Pinus massoniana forests benefit more from recent rapid warming in humid subtropical areas of central-southern China

Trees progress through various growth stages,each marked by specific responses and adaptation strate-gies to environmental conditions.Despite the importance of age-related growth responses on overall forest health and management policies,limited knowledge exists regarding age-related effects on dendroclimatic relationships in key subtropical tree species.In this study,we employed a den-drochronological method to examine the impact of rapid warming on growth dynamics and climatic sensitivity of young(40–60 years)and old(100–180 years)Pinus mas-soniana forests across six sites in central-southern China.The normalized log basal area increment of trees in both age groups increased significantly following rapid warming in 1984.Trees in young forests further showed a distinct growth decline during a prolonged severe drought(2004–2013),whereas those in old forests maintained growth increases.Tree growth was more strongly influenced by temperature than by moisture,particularly in old forests.Spring tem-peratures strongly and positively impacted the growth of old trees but had a weaker effect on young ones.Old forests had a significantly lower resistance to extreme drought but faster recovery compared to young forests.The“divergence problem”was more pronounced in younger forests due to their heightened sensitivity to warming-induced drought and heat stress.With ongoing warming,young forests also may initially experience a growth decline due to their heightened sensitivity to winter drought.Our findings underscore the importance of considering age-dependent changes in forest/tree growth response to warming in subtropical forest man-agement,particularly in the context of achieving“Carbon Peak&Carbon Neutrality”goals in China.

Drought and Spatiotemporal Variability of Forest Fires Across Mexico

Understanding the spatiotemporal links between drought and forest fire occurrence is crucial for improving decision-making in fire management under current and future climatic conditions. We quantified forest fire activity in Mexico using georeferenced fire records for the period of 2005–2015 and examined its spatial and temporal relationships with a multiscalar drought index, the Standardized Precipitation-Evapotranspiration Index(SPEI). A total of 47975 fire counts were recorded in the 11-year long study period, with the peak in fire frequency occurring in 2011. We identified four fire clusters, i.e., regions where there is a high density of fire records in Mexico using the Getis-Ord G spatial statistic. Then, we examined fire frequency data in the clustered regions and assessed how fire activity related to the SPEI for the entire study period and also for the year 2011. Associations between the SPEI and fire frequency varied across Mexico and fire-SPEI relationships also varied across the months of major fire occurrence and related SPEI temporal scales. In particular, in the two fire clusters located in northern Mexico(Chihuahua, northern Baja California), drier conditions over the previous 5 months triggered fire occurrence. In contrast, we did not observe a significant relationship between drought severity and fire frequency in the central Mexico cluster, which exhibited the highest fire frequency. We also found moderate fire-drought associations in the cluster situated in the tropical southern Chiapas where agriculture activities are the main causes of forest fire occurrence. These results are useful for improving our understanding of the spatiotemporal patterns of fire occurrence as related to drought severity in megadiverse countries hosting many forest types as Mexico.

Multi-year throughfall reduction enhanced the growth and non-structural carbohydrate storage of roots at the expenses of above-ground growth in a warm-temperate natural oak forest

The more frequent occurrence and severer drought events resulting from climate change are increasingly affecting the physiological performance of trees and ecosystem carbon sequestration in many regions of the world.However,our understanding of the mechanisms underlying the responses and adaption of forest trees to prolonged and multi-year drought is still limited.To address this problem,we conducted a long-term manipulative throughfall reduction(TFR,reduction of natural throughfall by 50%–70%during growing seasons)experiment in a natural oriental white oak(Quercus aliena var.acuteserrata Maxim.)forest under warm-temperate climate.After seven years of continuous TFR treatment,the aboveground growth in Q.aliena var.acuteserrata started to decline.Compared with the control plots,trees in the TFR treatment significantly reduced growth increments of stems(14.2%)and leaf area index(6.8%).The rate of net photosynthesis appeared to be more susceptible to changes in soil water in trees subjected to the TFR than in the control.The TFR-treated trees allocated significantly more photosynthates to belowground,leading to enhanced growth and nonstructural carbohydrates(NSC)storage in roots.The 7-year continuous TFR treatment increased the biomass,the production and the NSC concentration in the fine roots by 53.6%,153.6%and 9.6%,respectively.There were clear trade-offs between the aboveground growth and the fine root biomass and NSC storage in Q.aliena var.acuteserrata trees in response to the multi-year TFR treatment.A negative correlation between the fine root NSC concentration and soil water suggested a strategy of preferential C storage over growth when soil water became deficient;the stored NSC during water limitation would then help promote root growth when drought stress is released.Our findings demonstrate the warm-temperate oak forest adopted a more conservative NSC use strategy in response to long-term drought stress,with enhanced root growth and NSC storage at the expenses of above-ground growth to mitigate climate changeinduced drought.

Tree sapling vitality and recovery following the unprecedented 2018 drought in central Europe

Background:Ongoing climate change is anticipated to increase the frequency and intensity of drought events,thereby affecting forest recovery dynamics and elevating tree mortality.The drought of 2018,with its exceptional intensity and duration,had a significant adverse impact on tree species throughout Central Europe.However,our understanding of the resistance to and recovery of young trees from drought stress remains limited.Here,we examined the recovery patterns of native deciduous tree sapling species following the 2018 drought,and explored the impact of soil depth,understory vegetation,and litter cover on this recovery.Methods:A total of 1,149 saplings of seven deciduous tree species were monitored in the understory of old-growth forests in Northern Bavaria,Central Germany.The vitality of the saplings was recorded from 2018 to 2021 on 170 plots.Results:Fagus sylvatica was the most drought-resistant species,followed by Betula pendula,Acer pseudoplatanus,Quercus spp.,Corylus avellana,Carpinus betulus,and Sorbus aucuparia.Although the drought conditions persisted one year later,all species recovered significantly from the 2018 drought,albeit with a slight decrease in vitality by 2021.In 2018,the drought exhibited a more pronounced adverse effect on saplings in deciduous forests compared to mixed and coniferous forests.Conversely,sapling recovery in coniferous and mixed forests exceeded that observed in deciduous forests in 2019.The pivotal factors influencing sapling resilience to drought were forest types,soil depth,and understory vegetation,whereas litter and forest canopy cover had a negative impact.Conclusion:Long-term responses of tree species to drought can be best discerned through continuous health monitoring.These findings demonstrate the natural regeneration potential of deciduous species in the context of climate change.Selective tree species planting,soil management practices,and promoting understory diversity should be considered when implementing adaptive management strategies to enhance forest resilience to drought events.

The ecological scale mediates whether trees experience drought legacies in radial growth

The impact of lag effects produced by disturbances on primary production has been a major concern among ecologists during the last decade.Sudden and extreme climatic events are imposing drastic reductions in radial growth of trees as evidenced in tree-rings series Dendrochronological samples are obtained at tree level but analyzed at an aggregated scale(i.e.,mean chronologies),although aggregating tree-ring chronology on a regional scale may reduce the possibility of studying the variability of individual tree response to drought,by amplifying the average population response.Here,we conducted experimental research in which 370 trees of 5 species were analyzed to assess the potential statistical and scaling issues that may occur when using regressionbased methods to analyze ecosystem responses to disturbances.Drought legacy effects were quantified using individual and aggregated scales.Then,lag effects were validated using confidence and prediction intervals to identify values falling outside the certainty of the climate-growth model Individual scale legacy effects contrasted with confidence intervals were commonly distributed across species but were scarce when compared with prediction intervals.The analysis of aggregated scale legacies detected significant growth reductions when validated using prediction intervals;however,individual scale legacy lag effects were not detected.This finding directly contrasts the results obtained when using an aggregated scale.Our results provide empirical evidence on how aggregating ecological data to infer processes that emerge from an individual scale can lead to distorted conclusions.We therefore encourage the use of individual based statistical and ecological procedures to analyze tree rings as a means of further understanding the ecosystem responses to disturbances.

基于无人机多光谱与热红外数据的冬小麦土壤水分反演

引入植被覆盖度会在一定程度上提高土壤水分反演模型的精度,但大多数研究均基于归一化植被指数NDVI估算植被覆盖度,未深入研究基于其他植被指数估算植被覆盖度对模型的影响。为此,以河南省驻马店市西平县人和乡冬小麦部分种植区域为实验区,基于分辨率高、机动性强的无人机平台搭载多光谱与热红外成像仪开展冬小麦覆盖地表的土壤水分反演研究,探究引入不同植被覆盖度参数后模型精度的变化,并弥补基于卫星遥感影像的土壤水分监测分辨率低、时效性差的不足。基于随机森林算法,将温度植被干旱指数TVDI、垂直干旱指数PDI两种干旱指数分别与7种植被指数估算的植被覆盖度参数耦合搭建土壤水分反演模型,并根据最优模型的反演结果对实验区的土壤水分空间分布情况进行分析。同时,建立耦合TVDI与PDI指数、不引入植被覆盖度的土壤水分反演模型TP模型为对照组。结果表明:在0~10 cm和>10~20 cm深度时,TP模型的决定系数R^(2)分别为0.606、0.670,均方根误差RMSE分别为0.045、0.041。7种引入植被覆盖度的模型精度较TP模型精度均有一定程度的提升,其中最优模型TPOSAVI的R^(2)较TP模型分别提高0.143、0.158,RMSE分别降低0.7百分点、0.8百分点。基于干旱指数引入植被覆盖度能够提高模型精度,且不同植被覆盖度参数对模型精度的提升程度有差异。

氮添加和干旱对亚热带4种幼树生长的影响

为探究氮沉降和干旱交互作用对亚热带森林植物生长的影响,研究了亚热带4典型树种马尾松(Pinus massoniana)、杉木(Cunninghamia lanceolata)、海南红豆(Ormosia pinnata)和木荷(Schima superba)幼树生长对不同氮添加量和干旱程度的响应。结果表明,干旱和氮添加处理2.5 a对不同树种生长的影响不同。氮添加显著促进了对照和轻度干旱下木荷与杉木的总生物量、株高和基径生长,抑制了轻度干旱下马尾松的根茎生物量和基径生长,对重度干旱下4种幼树的生长均没有显著影响。氮添加和干旱处理降低了海南红豆、马尾松和杉木的株高和总生物量,且随干旱程度的加重抑制作用更明显。不施氮和干旱对木荷总生物量和株高无显著影响;氮添加和重度干旱显著降低木荷的基径和总生物量。干旱显著增加杉木根冠比,但对其他树种根冠比无显著影响。干旱和氮添加对非豆科植物(木荷)生长的促进作用均比豆科植物(海南红豆)明显,这表明氮沉降可能加剧极端干旱对亚热带部分树种生长的抑制作用。

不同程度干旱胁迫和复水处理对刺槐苗木生理指标的影响

在北京市大东流苗圃(北方国家级林木种苗示范基地),采用盆栽控水法,以1年生‘豫刺槐1号’(Robinia pseudoacacia‘Yucihuai 1’)苗木为研究对象;试验设置充足供水(对照)、轻度干旱胁迫(60%田间持水量)、中度干旱胁迫(50%田间持水量)、重度干旱胁迫(40%田间持水量)4个处理,自苗木土壤含水量下降到预定值并维持一定处理时间(干旱10、30、50 d),测定苗木水分状况、光合参数、生理生化等指标;不同程度干旱胁迫处理50 d之后进行复水处理,复水20 d后测定苗木相关生理指标;采用单因素方差分析法(One-way ANOVA)、新复极差法(Duncan),分析干旱胁迫、复水处理,对刺槐苗木叶水势、光合参数、非结构性碳水化合物质量分数及酶活性等生理指标的影响。结果表明:干旱导致刺槐苗木叶水势不同程度下降,4种处理苗木叶水势由大到小依次为对照、轻度干旱、中度干旱、重度干旱。干旱导致刺槐苗木叶绿素质量分数呈现先增加后降低的趋势,重度干旱的反应更敏感,复水后逐渐恢复。轻度干旱和重度干旱导致刺槐苗木光合速率、气孔导度等光合生理指标明显下降,重度干旱时2个指标降到最低,而中度干旱时刺槐苗木光合速率未出现明显下降。干旱导致刺槐苗木叶和根可溶性糖质量分数显著增加、淀粉质量分数下降,重度干旱的影响大于中度干旱、轻度干旱处理。干旱处理后期,轻度干旱、中度干旱处理时,茎中淀粉和非结构性碳水化合物迅速积累,其中淀粉的积累在干旱50 d时达到最大值,茎中淀粉和非结构性碳水化合物分别比充足供水(对照)增加了69%、31%。复水后,非结构性碳水化合物用于器官的修复和重建,各处理茎淀粉质量分数、非结构性碳水化合物质量分数均下降。重度干旱在短期内迅速积累更多的游离脯氨酸、可溶性蛋白,同时根抗坏血酸过氧化物酶、过氧化氢酶、过氧化物酶活性也显著高于其他处理,以应对干旱,但在干旱10~30 d时根和叶的游离脯氨酸质量分数、叶可溶性蛋白质量分数、根和叶过氧化氢酶活性及根的过氧化物酶活性显著下降;而中度干旱,根和茎的游离脯氨酸质量分数、可溶性蛋白质量分数在干旱胁迫30~50 d时达到最大值,根超氧化物岐化酶活性始终维持在较高水平,根和叶过氧化氢酶活性均呈先升高后下降的趋势。综合研究结果表明:轻度干旱时,刺槐苗木会选择关闭气孔减少水分的丧失,同时增加非结构性碳水化合物的储存和积累,以应对干旱;而中度干旱时,刺槐苗木通过增加碳吸收进行碳积累,同时提高抗氧化酶活性和渗透调节物质质量分数,维持细胞的膨压和代谢,这是刺槐苗木应对干旱的防御机制;重度干旱时,刺槐苗木严格关闭气孔以减少水分的丧失,维持基本的生理活动时间为主。干旱后期,非结构性碳水化合物由发挥渗透调节作用逐渐为增加淀粉的积累,并将非结构性碳水化合物逐渐向根和茎转移以用于复水后的修复和重建,但当土壤含水量低于土壤田间最大持水量的40%时会对刺槐苗木生理代谢产生严重的影响,复水后难以迅速恢复。

土壤水分数据融合及其在旱涝灾害多维度评估中的应用

获取高精度的土壤相对湿度对开展土壤墒情和旱涝精细化监测评估和预报预警有重要意义。该研究基于2020–2023年4–11月中国气象局陆面数据同化系统(China Meteorological Administration Land Data Assimilation System,CLDAS)逐日土壤相对湿度、全国土壤水分自动站逐小时土壤相对湿度以及土地利用类型、土壤属性、地理信息等数据,采用随机森林和支持向量机模型构建土壤水分自动站观测和CLDAS反演的土壤相对湿度动态融合订正模型,基于融合的土壤相对湿度构建土壤旱涝强度-面积-时间多维度评估指数,开展多维度旱涝监测评估。结果表明:1)采用随机森林模型融合后,0~10、0~20、0~50 cm土壤相对湿度与观测的土壤相对湿度的决定系数分别为0.79、0.81、0.80,相对均方根误差分别为13.81%、11.40%、9.50%,优于支持向量机模型。2)全国土壤缺墒日数百分率呈东南至西北增加趋势,内蒙古中西部、西北地区大部普遍在70%、甚至80%以上,内蒙古东南部、华北中北部、西南地区中西部为50%~70%,中东部大部在40%以下;土壤过湿日数百分率呈东南至西北减小趋势,华南东部和南部、西南地区南部、东北地区东北部多数在50%以上。3)基于融合土壤相对湿度数据构建的土壤缺墒、土壤过湿、墒情指数以及旱涝面积、持续时间指数,明显提升了2022年长江流域高温干旱、2023年台风“杜苏芮”和“卡努”等典型灾害性天气过程动态评估的定量化、精细化水平。土壤湿度融合数据及其旱涝评估指数可有效助力旱涝灾害多维度精细化定量评估,为防灾减灾提供重要支撑。

基于随机森林的西北地区综合干旱监测模型研究

西北地区干旱灾害频繁,精准监测西北地区干旱情况对农业生产和经济发展具有重要意义。以2001年6月—2020年5月的气象站点数据和综合反映降水、植被、土壤等环境因素的多源遥感数据集为数据源,利用随机森林算法构建综合干旱监测模型,并研究模型在西北地区的适用性。结果表明:该模型输出的综合干旱指数(CDI)与气象综合干旱指数(MCI)各月份干旱等级一致率在75%以上;CDI与气象站点的标准化降水指数(SPI-3)和标准化土壤湿度指数(SSI-1)的相关系数分别为0.318~0.726和0.173~0.433,均达到极显著水平;CDI能准确反映西北地区典型干旱实例旱情的发展情况。研究表明,随机森林方法可用于西北地区综合干旱监测,具有一定的应用价值。

陕北黄土高原人工混交林和纯林对干旱的适应性差异

为揭示渭北黄土高原混交林和纯林径向生长对气候变化的响应及应对极端干旱能力的差异,以黄土高原典型丘陵区延安市安塞县油松、刺槐混交林和纯林为研究对象;依据帕默尔干旱指数(PDSI)选择2005-2009年和2019年两次极端干旱事件,使用156根树轮样芯,分别建立了4个林分的树轮宽度年表并量化了树木的抵抗力和恢复力。结果表明,在极端干旱事件发生年和气候适宜年,油松混交林年轮宽度均显著高于纯林,但刺槐纯林的年轮宽度显著高于刺槐混交林;气候响应方面,PDSI对油松纯林和混交林径向生长的促进作用强于刺槐;刺槐混交林对干旱的抵抗力显著高于刺槐纯林和油松林,四种林分的恢复力没有显著差异。本研究结果表明,混交营林模式显著提高了刺槐对干旱的适应能力,营造刺槐混交林将有助于提高黄土高原地区人工林对日益干旱环境的适应性。

各种属性森林对干旱胁迫的响应研究进展

全球气候变化的背景下,干旱事件的发生频率、强度和持续时间不断增加,增加了森林生态系统面临的风险,探讨森林对干旱胁迫响应的规律与特征是生态学领域研究的热点。以干旱的定量表达为切入点,总结和归纳了评估森林对干旱响应的常见指标、评估方法和应用案例,特别是梳理了各种属性(如林龄、冠层高度等)的森林对干旱胁迫响应的差异性。基于当前研究进展和问题,提出在未来研究中,亟待发展多尺度综合解析各种属性森林对干旱胁迫响应差异的驱动机制研究;各种属性森林生态系统稳态转换临界点的探测;森林对干旱的响应规律在森林管理和模型优化的实践应用。

基于海表温度和随机森林的珠江流域干旱预报模型研究

海表温度(Sea Surface Temperature,SST)是干旱预报的主要因子之一,传统预报模型主要采用固定海域SST(如ENSO),未从全球大范围角度搜寻可利用的SST信号。通过回归分析筛选全球具有预报意义的SST区域,结合随机森林算法构建了一种气象干旱预报新模型,以珠江流域为例进行应用检验。结果表明:①该模型可有效预报干旱的时空变化规律,且预见期越长,预报精度相应下降;②非汛期干旱预报准确率高于汛期,沿海地区比内陆地区有更好的预报效果;③珠江流域干旱发生可能与典型的气候波动有关,如厄尔尼诺南方振荡和北大西洋振荡。

降雨与土地覆盖对中国典型流域水文干旱的影响分析

水文干旱严重制约了我国社会经济的发展。在气候变化和人类活动的影响下,水文干旱频繁发生,导致流域水资源短缺问题更加突出,因此,需从多因素角度分析不同因素对水文干旱的影响,从而为流域抗旱提供参考。为此,选取中国用水矛盾突出,旱灾较为严重的典型流域——湘江和渭河流域为研究区域,探究降雨特征与土地覆盖的变化对两地水文干旱的影响。首先,通过Pettitt突变检验法与降雨-径流双累积曲线法确定两个流域径流突变点,根据标准化径流指数识别了径流突变点前后湘江、渭河流域的水文干旱事件,最后基于随机森林与线性回归方法对比分析两地不同时期内降雨特征与土地覆盖的变化对水文干旱的影响。结果表明:湘江流域径流在1982-2015年间未发生突变,降雨特征是影响湘江流域干旱的主要因素,贡献率为53.4%,其中降雨量的贡献率为33.25%,比降雨集中度的贡献率高出13.1%,而土地覆盖的变化对湘江流域水文干旱影响不显著,其对水文干旱的贡献率仅为36.6%。渭河流域径流在1993年及2003年发生突变,不同时段内流域水文干旱的主要影响因素各异:1982-1993年间,影响当地水文干旱的主导因素为降雨量,其贡献率为34.4%;降雨特征与土地覆盖对干旱的贡献率在1994-2003年间持平;而在2004-2015年间,森林覆盖率的变化成为影响流域干旱主要因素,其贡献率为40%。基于研究结果,水利管理部门可根据当地流域的气候条件、土地覆盖类型,有针对性地制定防旱措施。

长江流域不同时间尺度下农业干旱恢复时间的量化及其影响因素分析

为了减少由农业干旱引起的相关损失,基于总初级生产力(GPP)制定了干旱恢复标准,并以此量化了不同时间尺度下的农业干旱恢复时间.通过随机森林算法分析了农业干旱恢复时间的影响因素,明确了不同时间尺度下影响农业干旱恢复时间的最主要因素.结果表明,长江流域在1、6、12、24个月时间尺度下的平均干旱恢复时间分别为5.7、3.9、3.8和3.0个月,随着时间尺度的增加干旱恢复时间会逐渐降低.从空间分布状况来看,干旱恢复时间较长的地区主要分布于长江流域的西部和东南部地区.对影响因素的分析结果表明,在短时间尺度下气象变量是影响干旱恢复时间长短的最主要因素,但对较长的时间尺度而言,干旱恢复时间的长短则更多受干旱发生季节的影响.

黄河中游不同地貌条件下植被干旱时空特征及影响因素

利用归一化植被指数、地表温度和降水数据构建温度植被降水干旱指数(TVPDI)分析黄河中游2000—2021年植被干旱时空特征及影响因素,论证了黄河中游退耕还林还草工程实施与区域植被干旱特征间的关系。结果表明:①黄河中游2000—2021年多年平均TVPDI值呈不显著上升趋势,TVPDI多年均值为0.708,对应干旱等级为轻旱。②黄河中游2000—2021年TVPDI有较强的空间分异性,呈现“东北、西南部地区旱情较轻,西北、东南部地区旱情较重”的空间分布格局;从各地貌分区来看,黄土高塬沟壑区旱情较轻,风沙区与河谷平原区旱情较重。③黄河中游TVPDI空间分异的主要影响因子为气温、蒸散发和降雨量,且与TVPDI存在显著线性关系。黄河中游2000—2021年植被覆盖度与蒸散发量均呈增加趋势,土壤水分呈下降趋势。

Recent tree growth decline unprecedented over the last four centuries in a Tibetan juniper forest

Forest structure and function are subject to risks of growth declines from intensified drought and frequent extreme events related to climate warming.Knowledge of tree growth declines will help anticipate future responses of forests to climate change.In this study,we investigated tree growth declines over the last four centuries in a juniper forest on the eastern Tibetan Plateau.By analyzing the radial growth trajectories of individual trees,we identified two events of intense growth decline,one in 1817–1830 and the other in 1969–1999 over the past four centuries.The intensity of the recent decline was unprecedented in the period under study.Ring-width chronology showed a positive correlation with self-calibrating Palmer Drought Severity Indices and a negative correlation with mean monthly temperatures in May and June.The recent intensified growth decline may have been due to temperatureinduced frequent droughts in the study area.Our findings suggest that trees in this juniper forest may face a higher risk of growth decline and even mortality under continued climate warming.