青藏高原东北部祁连圆柏径向生长对不同类型干旱的响应

为研究祁连圆柏径向生长对不同时期(生长季前2—4月和生长季5—7月)气候因子的响应及面对不同类型(高温、缺水、高温+缺水)干旱事件的弹性(抵抗力和恢复力)变化,利用青藏高原东北部17个采样点的祁连圆柏树轮宽度资料,分析径向生长与不同时期气候因子的相关性,探究高低海拔祁连圆柏面对各类干旱事件的弹性差异。结果表明:祁连圆柏径向生长与干旱指数呈显著正相关,与生长季温度呈负相关(P<0.1)。祁连圆柏面对不同时期干旱事件的弹性存在显著差异,生长季前发生的干旱事件中,低海拔圆柏的抵抗力比高海拔增高2.3%,恢复力降低25.1%;生长季干旱事件中,低海拔圆柏的抵抗力比高海拔降低23.7%,恢复力增高107.1%。祁连圆柏面对缺水型干旱时恢复力更强,均值达到1.68,而面对高温型干旱时祁连圆柏的抵抗力更强,均值达到1.43。未来我国西部高山祁连圆柏,尤其是处于低海拔区的,其径向生长受到全球变暖造成的极端干旱事件的影响会更加显著。     

亚热带人工和天然马尾松、杉木林生长对干旱的生态弹性差异

选取福建中西部地区相似气候条件下马尾松和杉木的天然林和人工林进行研究,利用年轮宽度、年轮宽度指数和断面积增量重建了4种林型共109株松树20年(1993—2012年)的年生长量,计算其对连续两次极端干旱事件(2003—2004年和2011年)的抵抗力、恢复力和弹性指数,分析人工林和天然林在抵抗力和弹性方面的差异。结果表明:马尾松和杉木对水分的需求在时间上存在差异,这解释了其对2003—2004年干旱事件的响应不一致。干旱压力极大地降低了马尾松和杉木的生长,但树木生长并未表现出干旱遗留效应。受干旱强度的影响,4种林型径向生长对2003—2004年干旱的响应强于2011年。干旱事件后马尾松比杉木具有更强的恢复能力;天然林比人工林对干旱的敏感性更高,同时弹性也更大。杉木人工林更容易受到频发的极端干旱事件的影响,在人工林抚育管理中应选择抗旱能力较强的遗传种源,以应对气候变暖导致的干旱频发。     

黄土高原植被日光诱导叶绿素荧光对气象干旱的响应

随着气候变化的加剧,干旱的频率、持续时间以及发生范围都越来越严重,探索植被光合对干旱的响应以及气象因子对植被光合的影响对于人们如何应对干旱具有重要意义。基于遥感的日光诱导叶绿素荧光(SIF)具有对干旱条件下区域植被光合作用进行早期监测和准确评估的潜力。本研究基于星载SIF和标准化降水蒸散发指数(SPEI)研究了黄土高原地区2001—2017年生长季内(4—10月)植被光合作用对干旱的响应关系及其受气象因子的影响程度。结果表明: 黄土高原地区植被生长季内SIF与SPEI呈显著正相关关系的区域占比为87.8%,其中,半干旱地区植被光合对干旱的响应较敏感,半湿润地区敏感性较低。不同类型植被光合对干旱的响应存在差异,草地对干旱响应的敏感性最高,响应最强的SPEI时间尺度为3～4个月;林地的敏感性最低,SPEI时间尺度为3～10个月。气象因子与SIF存在显著的相关关系,其中,温度和降雨是影响黄土高原植被光合的重要影响因子,光合有效辐射的影响模式与温度相似。黄土高原地区生长季内不同的气候和植被类型条件下,植被光合所受干旱及各气象因素的影响存在较大差异。     

2001-2018年中国-老挝交通走廊核心区植被稳定性对极端干旱的响应

中国-老挝交通走廊核心区干旱事件的频繁发生威胁着该区域生态系统的稳定性。基于Palmer干旱指数（PDSI）和增强型植被指数（EVI）数据量化了中老交通走廊核心区不同植被类型的稳定性（年均EVI与其标准差之比）及其对干旱的抵抗力（干旱期间植被结构和功能保持其原始水平的能力）和恢复力（植被恢复到干旱前水平的速度）。结果表明：（1）2001-2018年间,中老交通走廊核心区共发生5次极端干旱事件,出现极端干旱的年份有2005年、2010年、2015年、2016年和2017年,以上年份极端干旱面积占总面积的比例分别为13.37%、47.46%、10.41%、12.00%和3.05%;（2）不同植被类型对极端干旱的响应不同,中老交通走廊核心区内常绿阔叶林的稳定性显著高于其他植被类型,和其他植被类型相比,常绿阔叶林虽然暴露在干旱环境中的时间更长,但其具有更高的稳定性,在维持中老交通走廊核心区的生态系统稳定性上发挥着重要作用;（3）常绿阔叶林和木本稀树草原对极端干旱的抵抗力和恢复力显著高于混交林、草地和农田,研究区内各植被类型对极端干旱的抵抗力与温度和降水呈显著正相关关系。     

华北地区2001-2014年植被变化对SPEI气象干旱指数多尺度的响应

近年降水量的减少以及全球气候变暖的影响导致我国华北区域干旱程度加剧,影响植被生长状况,使得区域生态环境恶化。基于华北地区2001-2014年的TRMM及MODIS数据,以归一化植被指数NDVI、净初级生产力NPP、植被状态指数VCI作为植被状况表征指数,以标准化降水蒸散指数SPEI作为气象干旱表征指数,对华北地区近年的气象干旱及植被状况时空变化进行评价,并分析植被对干旱的多尺度响应。结果表明：（1）华北地区干旱在西南部地区呈明显加重趋势,东北部地区干旱状况有所好转;针对不同时间尺度的SPEI表示干旱的变化趋势,得出月份尺度干旱呈现干湿交替特征,选取SPEI时间尺度越长,干旱化趋势越明显;（2）NDVI与NPP所反馈的植被长势空间分布略有差异,总体而言华北地区植被状况大部分地区呈好转趋势,但研究区中部部分地区及部分沿海地区植被状况转差;（3）植被状况指数与SPEI指数在大部分地区呈正相关,NPP与SPEI的相关性强于NDVI与SPEI的相关性,且相关程度在草原地区及中高海拔地区最高,林地对干旱的敏感度最弱;各植被类型在植被生长季的多数月份对SPEI-3响应最明显,且在夏季相关程度最高,夏季及其前期的季尺度干旱更易影响植被生长状况,SPEI-12对植被的影响主要表现为影响植被生长季初期的植被状态。     

植被对不同时间尺度干旱事件的响应特征及成因分析

利用标准化降雨蒸散发指数(SPEI)与归一化植被指数(NDVI)系统研究了中国不同区域、不同土地利用类型的植被对不同时间尺度干旱事件的响应特征,并对成因做出探讨。研究表明:(1)全国大部分区域NDVI与SPEI呈显著正相关,表明中国大部分区域植被生物量变化受干旱时空特征的影响。而沿北纬30度附近的长江流域区域,尤其是长江流域东南部、珠江流域下游等降水相对丰沛区域;黑龙江东北部及长白山地区、四川西部等高寒区域,NDVI与SPEI表现出弱相关性,受干旱影响较小;(2)多年平均水平衡是影响植被对干旱响应的关键因素,土壤水分变化是植被活力与生物量变化的关键影响因子。多年平均日照时数较长的区域,植被变化受干旱影响较大。从对干旱影响敏感性程度来讲,越是水量丰沛的区域,植被受干旱的影响越小,其中,草地对干旱的影响最为敏感,其次为灌木与森林。     

黄河上游甘蒙柽柳生长对极端旱涝的响应

青藏高原黄河上游河岸带是典型的生态脆弱区, 然而近年来气候变暖加剧了该地极端旱涝事件的频繁发生, 高原河岸带生态脆弱区植被是否能够应对极端旱涝事件的干扰成为流域生态环境管理工作所关注的重点问题。为了研究黄河上游河岸林中主要树种对极端旱涝的响应, 该研究选取青海省同德县和兴海县3处河岸林中的47株甘蒙柽柳(Tamarix austromongolica), 分别从树干面向邻近山体一侧及与之垂直的一侧分别获取1根树轮样本, 分析其历史生长。通过对比两个方向上的生长速率判断甘蒙柽柳是否受到地质灾害影响从而将其划分为受伤组和对照组, 分析两组甘蒙柽柳在过去63年中径流极值年的抵抗力状况及两个方向的生长差异。研究发现, 甘蒙柽柳对干旱和洪涝均有着很强的抵抗力, 河岸带多样化的水分来源有助于甘蒙柽柳在极端干旱环境中较好地生长; 但洪涝伴随泥石流等地质灾害的频发使甘蒙柽柳面向山体侧面受到严重的生长抑制, 表现出显著的方向性差异, 从而影响甘蒙柽柳的形态。较长的创伤恢复期带来的遗留效应可能造成甘蒙柽柳对外界干扰的较高敏感性。研究黄河上游甘蒙柽柳生长对极端旱涝的响应, 将有助于评估生态脆弱区生态弹性过程, 同时为高原河岸带生态建设和恢复提供科学依据。     

树木年内径向生长对干旱事件的响应——以贺兰山油松为例

在贺兰山苏峪口国家森林公园,利用径向生长测量仪监测2017和2018年2个生长季内、2个海拔(2010和2330 m)油松的径向生长,研究树木径向生长对干旱的响应。结果表明: 2018年6月的干旱事件使得油松径向生长速率减慢,生长量减小;而7—8月的降水使油松的径向生长重新激活。2018年油松的径向生长主要发生在6—8月,相比2017年延长一个月。油松径向生长与气候因子的响应关系在不同海拔间没有明显的差异。生长季早期干旱对树木径向生长有抑制作用,生长季中后期降水对树木径向生长具有促进作用。该区的气候重建工作中应当充分考虑8月的气候要素对树轮宽度的影响。     

闽东南地区马尾松古树对气候变化和虫灾的生态弹性

马尾松是我国南方地区广泛分布的先锋造林树种。在全球变暖、气候干旱化和虫灾频发的趋势下,研究马尾松对环境干扰的生态弹性对森林管理有重要意义。本文对福建省仙游县百松村的马尾松古树进行树木年轮样品采集,建立区域首个马尾松树轮宽度标准年表(1865—2014年)。结果表明: 当年7—9月低相对湿度和5—9月极端高温是树木生长的主要限制因素。根据树轮极端窄年确定1869、1889、1986、1991和1993是极端事件年。时序叠加分析发现,极端事件发生前两年的持续低值加剧了极端事件的影响。干旱年份更容易引发虫灾。1889年是受虫灾影响最严重的年份,1986和1991年受到虫灾和干旱气候的双重影响,其余极端年主要受干旱气候的影响。树木对虫灾的抵抗力弱于对干旱事件的抵抗力;除1991年外,树木对虫灾的相对弹性力高于对干旱事件的相对弹性力。1889年的相对弹性力最高,1991年受到连续极端事件的影响,相对弹性力最低。2000年以来研究区干旱化趋势加强,马尾松古树遭受干旱和虫灾的干扰加强,部分树木死亡。     

2001—2020年川西北高原归一化植被指数演变特征及其对极端气候的响应

川西北高原是典型的生态气候敏感区,其植被状况与气候变化密切相关。本研究基于2001—2020年MODIS-NDVI数据集和气象数据,采用最大值合成、地理探测器模型、线性趋势分析、相关分析等方法,研究川西北高原生长季归一化植被指数(NDVI)的变化趋势及其对气候因子的响应机制。结果表明: 研究期间,川西北高原植被覆盖度整体状况良好,86.8%的区域植被稳定,12.6%的区域NDVI呈弱持续性上升趋势,0.6%的区域NDVI呈下降趋势,全区生态环境呈稳中向好的发展趋势。研究区植被覆盖度空间差异大,总体呈由西南向东北上升的趋势,并有显著的立体变化。海拔1350 m以下,NDVI随海拔升高而上升;海拔1350～3650 m,NDVI无显著变化;海拔3650～5900 m,NDVI随海拔升高而下降,在4750～5900 m快速下降;海拔5900 m以上,几乎无植被。川西北高原的NDVI受多种自然因子交互作用影响,热量因子(月最高气温极大值、月最低气温极小值、植物生长期、年均温、生长期均温)是主导气候因子,除月最高气温极大值外,其余温度因子对NDVI均以正贡献为主。NDVI对气温指数的响应高于降水指数。在气候变暖背景下,极端气温暖指数对川西北高原植被生长尤其是高海拔地区植被生长及改善以促进作用为主。     

Drought footprint on European ecosystems between 1999 and 2010 assessed by remotely sensed vegetation phenology and productivity

Drought affects more people than any other natural disaster but there is little understanding of how ecosystems react to droughts. This study jointly analyzed spatio‐temporal changes of drought patterns with vegetation phenology and productivity changes between 1999 and 2010 in major European bioclimatic zones. The Standardized Precipitation and Evapotranspiration Index (SPEI) was used as drought indicator whereas changes in growing season length and vegetation productivity were assessed using remote sensing time‐series of Normalized Difference Vegetation Index (NDVI). Drought spatio‐temporal variability was analyzed using a Principal Component Analysis, leading to the identification of four major drought events between 1999 and 2010 in Europe. Correspondence Analysis showed that at the continental scale the productivity and phenology reacted differently to the identified drought events depending on ecosystem and land cover. Northern and Mediterranean ecosystems proved to be more resilient to droughts in terms of vegetation phenology and productivity developments. Western Atlantic regions and Eastern Europe showed strong agglomerations of decreased productivity and shorter vegetation growing season length, indicating that these ecosystems did not buffer the effects of drought well. In a climate change perspective, increase in drought frequency or intensity may result in larger impacts over these ecosystems, thus management and adaptation strategies should be strengthened in these areas of concerns.     

Differentiating drought legacy effects on vegetation growth over the temperate Northern Hemisphere

In view of future changes in climate, it is important to better understand how different plant functional groups (PFGs) respond to warmer and drier conditions, particularly in temperate regions where an increase in both the frequency and severity of drought is expected. The patterns and mechanisms of immediate and delayed impacts of extreme drought on vegetation growth remain poorly quantified. Using satellite measurements of vegetation greenness, in‐situ tree‐ring records, eddy‐covariance CO₂ and water flux measurements, and meta‐analyses of source water of plant use among PFGs, we show that drought legacy effects on vegetation growth differ markedly between forests, shrubs and grass across diverse bioclimatic conditions over the temperate Northern Hemisphere. Deep?rooted forests exhibit a drought legacy response with reduced growth during up to 4 years after an extreme drought, whereas shrubs and grass have drought legacy effects of approximately 2 years and 1 year, respectively. Statistical analyses partly attribute the differences in drought legacy effects among PFGs to plant eco‐hydrological properties (related to traits), including plant water use and hydraulic responses. These results can be used to improve the representation of drought response of different PFGs in land surface models, and assess their biogeochemical and biophysical feedbacks in response to a warmer and drier climate.     

Drought timing influences the legacy of tree growth recovery

Whether and how the timing of extreme events affects the direction and magnitude of legacy effects on tree growth is poorly understood. In this study, we use a global database of Ring‐Width Index (RWI) from 2,500 sites to examine the impact and legacy effects (the departure of observed RWI from expected RWI) of extreme drought events during 1948–2008, with a particular focus on the influence of drought timing. We assessed the recovery of stem radial growth in the years following severe drought events with separate groupings designed to characterize the timing of the drought. We found that legacies from extreme droughts during the dry season (DS droughts) lasted longer and had larger impacts in each of the 3 years post drought than those from extreme droughts during the wet season (WS droughts). At the global scale, the average integrated legacy from DS droughts (0.18) was about nine times that from WS droughts (0.02). Site‐level comparisons also suggest stronger negative impacts or weaker positive impacts of DS droughts on tree growth than WS droughts. Our results, therefore, highlight that the timing of drought is a crucial factor determining drought impacts on tree recovery. Further increases in baseline aridity could therefore exacerbate the impact of punctuated droughts on terrestrial ecosystems.     

陕西省植被覆盖时空变化及其对极端气候的响应

基于2001—2018年MODIS NDVI数据,从生态分区视角分析陕西省归一化植被指数(NDVI)的时空变化特征,并结合该地区31个气象站点日值数据,探讨NDVI对极端气温和极端降水指数的响应特征。结果表明：(1)陕西省及其各生态区的NDVI变化均显著上升,整体呈南高北低的分布特点,其中秦巴山地落叶与阔叶林生态区(IV)NDVI值最高为0.86,陕北北部典型草原生态区(I)NDVI值最低为0.38。(2)年际尺度上,陕西省NDVI与极端气温暖极值(暖夜日数)和极端降水指数总体呈显著正相关(P<0.05),在陕西省北部NDVI变化主要受极端降水的影响,南部则对极端气温的敏感度更高。(3)多年月尺度上,各生态区NDVI对极端气温冷极值(最低气温、日最低气温的极低值和日最高气温的极低值)和极端气温暖极值(最高气温、日最低气温的极高值和日最高气温的极高值)存在明显的滞后性,滞后时间多为3个月;与极端降水指数(单日最大降水量和连续5日最大降水量)的滞后时间为2个月,说明陕西省内NDVI对极端气候的响应具有显著的滞后效应。     

西南山地流域NDVI变化特征及降水敏感性——以贵州沅江流域为例

归一化植被指数(Normalized Difference Vegetation Index,NDVI)是反映植被生长的重要指标,探究其时空变化特征,对生态保护、退耕还林和灾害防治具有重要意义。选取2000—2015年贵州沅江流域典型年份NDVI,采用重心分析、标准差椭圆、地理加权回归等方法,从干旱视角探究流域NDVI变化和降水敏感性。得出以下结论:(1)2000—2015年贵州沅江流域NDVI总体处于上升趋势,植被正在改善,期间西部植被改善最为明显,退耕还林还草取得较好成果。(2)2000—2015年贵州沅江流域干旱年与正常年NDVI空间变化趋势不一致,干旱程度对植被影响具有区域差异。(3)贵州沅江流域NDVI与降水呈负相关,空间敏感性自西向东递增,降水对流域西部喀斯特山区NDVI影响更明显。(4)2000—2015年贵州沅江流域NDVI时段变化与降水主要为负相关,降水越多NDVI时段变化敏感性越弱,退耕还林还草工程应重点放在负敏感区。研究结果可为退耕还林还草和干旱灾害防治提供科学参考。     

Historical and event‐based bioclimatic suitability predicts regional forest vulnerability to compound effects of severe drought and bark beetle infestation

Vulnerability to climate change, and particularly to climate extreme events, is expected to vary across species ranges. Thus, we need tools to standardize the variability in regional climatic legacy and extreme climate across populations and species. Extreme climate events (e.g., droughts) can erode populations close to the limits of species' climatic tolerance. Populations in climatic‐core locations may also become vulnerable because they have developed a greater demand for resources (i.e., water) that cannot be enough satisfied during the periods of scarcity. These mechanisms can become exacerbated in tree populations when combined with antagonistic biotic interactions, such as insect infestation. We used climatic suitability indices derived from Species Distribution Models (SDMs) to standardize the climatic conditions experienced across Pinus edulis populations in southwestern North America, during a historical period (1972–2000) and during an extreme event (2001–2007), when the compound effect of hot drought and bark beetle infestation caused widespread die‐off and mortality. Pinus edulis climatic suitability diminished dramatically during the die‐off period, with remarkable variation between years. P. edulis die‐off occurred mainly not just in sites that experienced lower climatic suitability during the drought but also where climatic suitability was higher during the historical period. The combined effect of historically high climatic suitability and a marked decrease in the climatic suitability during the drought best explained the range‐wide mortality. Lagged effects of climatic suitability loss in previous years and co‐occurrence of Juniperus monosperma also explained P. edulis die‐off in particular years. Overall, the study shows that past climatic legacy, likely determining acclimation, together with competitive interactions plays a major role in responses to extreme drought. It also provides a new approach to standardize the magnitude of climatic variability across populations using SDMs, improving our capacity to predict population's or species' vulnerability to climatic change.     

念青唐古拉山南坡高寒草甸生产力对温度和降水变化的敏感性及其海拔分异

目前人们仍不清楚不同海拔高寒草地植被生长对气候变化的敏感性差异及其与最适宜海拔分布中心的关系。利用西藏当雄县念青唐古拉山南坡7个海拔梯度固定样地的高山嵩草草甸地上净初级生产力(ANPP)观测数据(2009—2013),建立了ANPP与同期遥感植被指数(MODIS NDVI)的线性回归方程。基于长时间序列的NDVI数据,利用建立的回归方程估算了研究区2000—2013年的ANPP。结合沿海拔梯度的HOBO气象站数据(2006—2013)及当雄县气象站数据(2000—2013),分析了2000—2013年该地区高寒草甸ANPP对降水和温度变化的敏感性及其随海拔的变化规律。结果表明:(1)多年平均ANPP随海拔的变化均表现为先增加后降低的单峰分布格局,最大值出现在海拔4893—4942 m,说明在海拔梯度上存在一个最适宜高寒草甸植被生长的分布中心;(2)ANPP与生长季降水量(GSP)呈正相关关系,与生长季平均气温(GST)呈负相关关系,其相关斜率的绝对值(指示ANPP的降水敏感性和温度敏感性大小)与ANPP的海拔格局具有相反的变化趋势,即在最适宜高寒草甸植被生长的海拔分布中心附近,ANPP对降水和温度变化的敏感性最低,而在远离该分布中心的较高和较低海拔,ANPP对降水和温度变化的敏感性则相对较大。研究明确了高寒草甸ANPP对降水和温度变化的敏感性随海拔的分异性及其与高寒草甸最适宜海拔分布中心的关系,这有助于理解沿海拔梯度不同水热组合环境下高寒生态系统对未来气候变化的响应模式。     

The impact of drought spells on forests depends on site conditions: The case of 2017 summer heat wave in southern Europe

A major component of climate change is an increase in temperature and precipitation variability. Over the last few decades, an increase in the frequency of extremely warm temperatures and drought severity has been observed across Europe. These warmer and drier conditions may reduce productivity and trigger compositional shifts in forest communities. However, we still lack a robust, biogeographical characterization of the negative impacts of climate extremes, such as droughts on forests. In this context, we investigated the impact of the 2017 summer drought on European forests. The normalized difference vegetation index (NDVI) was used as a proxy of forest productivity and was related to the standardized precipitation evapotranspiration index, which accounts for the temperature effects of the climate water balance. The spatial pattern of NDVI reduction in 2017 was largely driven by the extremely warm summer for parts of the central and eastern Mediterranean Basin (Italian and Balkan Peninsulas). The vulnerability to the 2017 summer drought was heterogeneously distributed over Europe, and topographic factors buffered some of the negative impacts. Mediterranean forests dominated by oak species were the most negatively impacted, whereas Pinus pinaster was the most resilient species. The impact of drought on the NDVI decreased at high elevations and mainly on east and north‐east facing slopes. We illustrate how an adequate characterization of the coupling between climate conditions and forest productivity (NDVI) allows the determination of the most vulnerable areas to drought. This approach could be widely used for other extreme climate events and when considering other spatially resolved proxies of forest growth and health.     

Effects of decadal experimental drought and climate extremes on vegetation growth in Mediterranean forests and shrublands

Increased drought combined with extreme episodes of heatwaves is triggering severe impacts on vegetation growth, particularly for plant communities in arid and semiarid ecosystems. Although there is an abundance of short‐term field drought experiments in natural ecosystems, remaining knowledge gaps limit the understanding and prediction of vegetation growth to ongoing and future climate scenarios. Here, we assessed the impacts of long‐term (1999–2016) experimental drought (ca. ?30% rainfall) on the vegetation growth of a Mediterranean high (H) and low (L)‐canopy forests and an early‐successional shrubland, as indicated by above‐ground biomass increment (ABI) and standing density, respectively. We found habitat context (impact of historical climate change, soil depth and successional status) of the study sites significantly affected the magnitude of climate impacts; there were synergistic effects of experimental drought and meteorological drought (Standardised Precipitation–Evapotranspiration Index, SPEI) as well as extreme dry years on vegetation growth. Long‐term experimental drought decreased the ABI for the two forest canopy types and the standing density for the shrubland. Water availabilities in winter–spring (SPEIs) were positively correlated with the ABI and standing density. Moreover, experimental drought decreased the vegetation growth in extreme dry years for the shrubland. We propose that future work not only study the vegetation dynamics with physiological, phenological and demographical changes in long‐term processes and across climate gradients, but also should explore the changes of multiple functions simultaneously (e.g. multifunctionality) under long‐term processes and extremes. This type of analysis of long‐term data is essential to understand and predict biodiversity loss, composition shifts, declines in ecosystem function and carbon budgets at temporal and spatial scales, to enable policy makers to design and implement strategies for the maintenance of sustainable ecosystem function under future climate change scenarios.