植被年际变化对蒸散发影响的模拟研究

利用通用陆面模式(CLM3.0)及其植被动力学模式(DGVM)研究植被覆盖度(FC)和叶面积指数(LAI)的年际变化对全球蒸散发的影响。设计两套实验方案,其植被的FC和LAI的气候态相同,但一套实验中植被的FC和LAI有年际变化,而对照实验中则没有。结果表明:(1)在草、灌木、树占优势的地区植被FC年际变化依次减小;LAI年际变化较大的地区集中在草和灌木覆盖的地区,在落叶林地区,春秋两季植被LAI的年际变化也较大。(2)全球树占优势的大部分地区,植被的年际变化使得年平均蒸散发和地表蒸发增加、冠层蒸发和蒸腾减少;而在灌木和草覆盖区,变化则大致相反。(3)低纬度地区蒸散发季节循环变化比较明显,而北半球中纬度地区,蒸散发变化明显区随着纬度增加而在时间上向后推延。(4)FC和LAI年际变化较大时,蒸散发及地表蒸发降低,而蒸腾增加;这些差异随FC和LAI年际变化的增加而增加。单点分析进一步表明植被年际变化不仅改变蒸散发的多年平均值,同时改变其分量间的相对比例。     

全球变暖对太行山植被生产力及土壤水分的影响

将相同的自然植被用Lysimeter从高海拔下移至低海拔,温度升高2℃,同时设置平均降水,增加10％降水,增加20％降水,减少10％降水和减少20％降水5个降水处理,模拟全球变暖带来的温度升高和降水变化对植被生产力和土壤水分的影响,两年的野外实验表明,温度升高造成生态适应性差的野古草（Arundinelia hirta)生产力显著下降,致使整个测试群落生产力降低,低海拔实验点生产力显著低于高海拔实验点,温度升高对铁杆（Artemisia sacrorum)和黄背草（Themeda japonica)的影响较小,太行山区的植被对降水的变化反映,降水增加使植被的生产力水平显著提高,其中降水增加20％的处理生产力比平均降水处理的生产力增加22％,增加降水处理的土壤含水量显著高于平均降水和降水减少的实验处理,由于植被的减少,温度升高的实验点从第二年开始土壤水分较高。     

1981-2013华北平原气候时空变化及其对植被覆盖度的影响

基于1981—2013年华北平原气象数据,对华北平原近30a的气候时空变化趋势与突变情况进行分析,并结合GIMMSNDVI(1981—2006年)和MODIS-NDVI(2000—2013年)遥感数据,探讨气候突变影响下,华北平原1981—2013年植被覆盖度的空间分布和变化特征。结合生态学分区,从不同时空尺度出发,分析华北平原不同生态分区内气候因子与植被覆盖度在年代际、年际和月变化的相关关系。结果发现:(1)1981—2013年,华北平原气温整体呈现显著上升趋势(0.20℃/10a,P0.01),春季气温的升高、15℃等温线控制范围的扩大和年均温0℃等值线在华北平原的消失,是区域平均气温升高的诱因。华北平原降水整体呈现显著减少趋势(-1.75mm/10a,P0.05),其中秋季降水量减少过快,400—600mm降水等值线控制范围的扩大、600—800mm和800—1000mm降水等值线的范围的缩小,共同造成区域降水量的减少。四季气候倾向率的特征变化敏感区域主要位于北纬35°—39°之间。1991—1994年为华北平原气候的突变时期。(2)华北平原植被覆盖度总体呈现上升的趋势,呈增加趋势的面积占总面积的55%。人类活动不仅加速了区域植被覆盖度的降低,也加剧了降低速率的变快。(3)总体上,研究区月尺度植被覆盖度与气候因子的相关性高于年尺度的值。植被覆盖度与年降水量的偏相关性高于其与年均温的偏相关性。年均温对农业生态区和森林生态区的植被覆盖度的影响更大,草原生态区的植被覆盖度对年降水量的依赖性更强。在月尺度上降水量对植被的影响具有时滞效应。气温对草原生态区具有时滞效应,降水量对农业生态区具有时滞效应。(4)华北平原干热化的气候突变降低了植被覆盖度的增加趋势。从植被覆盖度出发,草原生态区对气候突变的响应最明显;从变化速率角度出发,农田生态区的响应最明显。就整体而言,人类活动的影响力还在持续增强,且呈现出在退化区的作用力高于改善区的趋势。气候突变后,出现了人类活动在植被覆盖度的改善区的相对作用力高于退化区的变化。     

黄土高原近10年植被覆盖的动态变化及驱动力

基于Timesat的非对称高斯函数(AG)拟合法重建MODIS-NDVI数据,利用像元二分模型估算了黄土高原近10年的植被覆盖度(VC),并分析了年植被覆盖度的变化趋势和其与降水温度的相关性。研究结果表明:黄土高原植被覆盖度总体上呈现东南高西北低、由东南向西北递减的特征。其中森林生态系统平均覆盖度最高,灌木、草地生态系统次之,荒漠生态系统最低,空间差异明显。2010年森林生态系统植被覆盖度达到81.6%,主要包括太行山、吕梁山和秦岭地区。暖温带森林区植被组成以落叶阔叶林为主,覆盖度常年较高,为80%以上。西北部温带草原区,植被覆盖度达到38.8%。温带草地主要依水分梯度,由东南到西北分布有以旱生性多年生草本植物为主的典型草原,植被覆盖度呈现相应的递减趋势。黄土高原总面积78.6%的地区年植被覆盖度呈增加趋势;而占总面积19.4%的地区年植被覆盖度呈下降趋势。在空间分布上,植被覆盖度显著增加的区域主要分布在榆林至延安周边地区和秦岭一带;植被覆盖度显著减少区域沿兰州至银川呈条带状分布。     

陕西省退耕还林植被覆盖度与湿润指数的变化关系

使用MODIS-NDVI数据和气象站点资料,通过GIS遥感技术和数理统计等方法,分析了陕西省退耕还林后(2000—2012年)植被覆盖度与湿润指数的时空变化规律及两者变化的关系。结果表明,陕西省植被覆盖度和湿润指数都呈现由南向北递减的分布规律并且有明显的季节变化特征。2000—2012年,陕西省植被覆盖度在波动中呈现大幅增加的趋势,陕北地区增加最为显著,生态环境得到明显改善,然而部分城市周边地区植被有退化的迹象。2000—2012年湿润指数年际变化波动较大,有上升的趋势,陕南地区增加显著。空间分布上随着植被覆盖度的增加湿润指数呈指数变化趋势,相关性与植被覆盖度面积取值范围有关,范围取值越大相关系数越高。植被覆盖度的年际变化受到气候和人为因素影响,陕南地区植被覆盖度与湿润指数的相关性较显著,而受到人为影响比较明显的陕北、关中地区相关性不显著。     

黄土高原植被-降水关系的临界现象及其在植被建设中的意义

以退耕还林还草为主要内容的大规模的生态环境建设正在我国兴起,如何进行科学的植被配置规划,是一个急待解决的理论和实践问题。以自然植被初级生产力(NPP)表征潜在的即最大可能的植被条件,并与现在的森林覆盖率分布以及植被构成特征进行了比较。通过大量实测资料的分析,查明了降水条件对森林覆盖率和NPP的影响及其临界条件。研究表明,以NPP表征的天然植被特征值在黄土高原地区的分布具有显著的地带性。NPP与年平均降水量的关系中存在着两个临界点,分别代表着植被类型的变化,Pm=250mm可视为荒漠与草原植被之间的临界点,而Pm=480mm则为草原(森林草原)与森林(落叶阔叶林)之间的临界点。尽管次生的人工植被受人为因素的控制,由于存在着自然稀疏化过程,将使人工林地最终达到某种与水分承载能力相一致的平衡状态,自然条件如年降水仍然是决定其覆盖度的重要因素。在森林覆盖率与年平均降水量的关系中表现出一个临界点。当Pm<480mm时,森林覆盖率很低,且随年降水的增大而增大的速率很慢;当Pm>480mm时,森林覆盖率随年降水量而急剧增大。人为建造的水土保持植被,是一个自然的人工生态系统,也是一个自组织系统,它通过自我调节而趋向于达到某种平衡,建立一定的群落结构甚至于趋向于某种顶极群落,从而实现乔木、灌木、草本植物组成的最优组合。从这一原理出发,提出了一个基于实测资料进行统计分析的方法,来确定使乔木面积在乔木、灌木、草本总面积中的最优或较优的比例,得出了具体的数据。     

砒砂岩区植被覆盖度环境驱动因子量化分析——基于地理探测器

砒砂岩区地形破碎,生态环境恶劣,降水量少且以暴雨为主,研究该区植被覆盖变化及环境驱动因子作用机制对区域植被建设具有重要的理论意义。基于1999—2018年的NDVI数据分析了砒砂岩区近20年植被覆盖度时空变化特征,利用了地理探测器方法量化分析了不同环境因子对植被覆盖度的影响。结果表明：1)近20年砒砂岩区平均植被覆盖度为42.3%,时间尺度上1999—2018年区域植被覆盖度呈增加趋势,平均上升幅度为0.086/10 a,空间尺度上植被覆盖度呈现从东南向西北递减的空间分布特征;2)近20年区域植被覆盖整体得到改善要比退化的区域面积大,45.5%的区域面积植被覆盖度极显著增加,主要分布在砒砂岩区东部区域,该区植被覆盖度未来变化趋势将以持续性改善为主,但仍有约41.6%的植被将由改善向退化方向变化;3)降水、土壤水分和气温是影响砒砂岩区植被覆盖空间分布的主导环境因子,且降水同其他环境因子的交互作用对植被覆盖影响最大。     

内蒙古生态系统质量空间特征及其驱动力

植被作为生态系统的重要组成部分,联结着大气、水分和土壤等自然过程,其变化将直接影响该区域气候水文和土壤等状况,是区域生态系统质量变化的重要指示器。植被状况的好坏,主要通过生物量和植被覆盖度因子来表示。内蒙古自治区是我国北方生态环境问题十分严重的省份,弄清当前区域生态系统质量状况与变化及其近10年来变化的驱动因素,对分析与制定区域生态环境保护决策具有十分重要的意义。基于2000—2010年生物量和植被覆盖度,并结合地区植被区划数据,对内蒙古植被生态系统质量状况进行分析,并评估其与气候(降水、温度),人类活动(交通密度、农业发展、生态恢复工程)的相关关系,在此基础上探讨了气候和人类活动对近年来内蒙古生态系统质量变化的影响。结果表明:(1)内蒙古生态系统质量状况整体偏低,其中森林生态系统平均质量最高,灌丛、草原生态系统次之。空间分布呈明显的经度地带性,由东向西,质量逐渐降低。2000—2010年内蒙古生态系统质量总体上呈现缓慢增长趋势,但局部地区生态系统质量仍存在恶化,其中在107°E以东的草原和森林区域,生态系统质量变化十分剧烈。(2)近10年来内蒙古生态系统质量的变化与气候和人类活动的关系非常密切,其与降水、GDP1、化肥施用量、天保工程和退耕还草工程呈现明显的正相关。而与温度、道路密度和京津风沙治理工程呈现明显的负相关。其中,生态保护工程实施区域内和区域外的相关性存在显著的差异性。随着内蒙古社会经济的快速发展,人类活动对生态系统质量的影响逐渐加强,但降水仍是该地区生态系统质量的主要影响因子。(3)在内蒙古生态系统质量变化典型区域内,质量的增长主要是由于降水的增加、温度的降低、农业的发展、退耕还草工程的作用和交通发展的放缓。质量的降低则是因为降水的减少、温度的增加、农业发展缓慢和交通发展的加快所致。     

全球变化的中国气体—植被分类研究

区域潜在蒸散具有作为植被－气候相关分析与分类的综合气候指标的功能。根据区域潜在蒸散对气候－植被分类的热量与水分指标进行了初步探讨,并对中国气候－植被分类进行了初步的定量研究。根据该模型对中国陆地生态系统对全球变化的反应进行了探讨,结果表明我国自然植被在气温增加２℃或４℃、降水增加２０％时,森林和草原的面积都有所减少,且随着温度的升高而减少,沙漠化趋势增强。特别是青藏高原地区对全球气候变化非常敏感,     

延安北部丘陵沟壑区植被指数变化及其与气候的关系

利用GIMMS和SPOT两种归一化植被指数(NDVI)数据和气候资料,分析延安北部丘陵沟壑区1982—2007年植被覆盖的历史演变及其与气候因子的关系。结果表明:(1)延安北部丘陵沟壑区植被覆盖状况26a来尽管有波动起伏,但是整体在持续转好,年平均NDVI增加了14.2%。夏季的NDVI值最高、波动起伏最大,其次是秋季,春、秋季的NDVI年际变化具有明显的上升趋势。各季NDVI与年NDVI均有相关关系,春、秋季NDVI与年NDVI相关显著。NDVI年内变化曲线为单峰型,春季NDVI缓慢增加,秋季NDVI降低速度比较快。(2)年平均NDVI与年温度相关不明显,夏、秋、冬三季NDVI与同期温度相关也不明显,只有春季平均NDVI与该季温度相关显著。3—4月份温度对植被的影响呈正相关,温度越高,返青生长越快;初夏6—7月份,温度对植被生长有滞后影响,前期温度与后期NDVI为负相关。降水量是引起NDVI年际波动的影响因子之一,年降水量与当年7月和9月份NDVI相关,决定了一年植被最为旺盛时的好坏。月降水量对NDVI影响具有滞后性,上年9月份降水影响翌年4—6月份的NDVI,6月和7月份NDVI受当月和前期降水影响。(3)1999年以来,延安北部丘陵沟壑区植被覆盖快速上升,除与降水增多有关外,非气候因素中生态保护和环境建设等人为措施,如植树造林、封山禁牧等封育措施是导致植被显著增加的重要原因。     

Sub-saharan desertification and productivity are linked to hemispheric climate variability

Vegetation productivity and desertification in sub‐Saharan Africa may be influenced by global climate variability attributable to the North Atlantic Oscillation (NAO) and El Niño Southern Oscillation (ENSO). Combined and individual effects of the NAO and ENSO indices revealed that 75% of the interannual variation in the area of Sahara Desert was accounted for by the combined effects, with most variance attributable to the NAO. Effects were shown in the latitudinal variation on the 200 mm isocline, which was influenced mostly by the NAO. The combined indices explained much of the interannual variability in vegetation productivity in the Sahelian zone and southern Africa, implying that both the NAO and ENSO may be useful for monitoring effects of global climate change in sub‐Saharan Africa.     

Adapting a patch model to simulate the sensitivity of Central-Canadian boreal ecosystems to climate variability

Aim To investigate effects of within-season and interannual climate variability on the behaviour of boreal forest ecosystems as simulated by the FORSKA2 patch model. Location Eleven climate station locations distributed along a transect across the boreal zone of central Canada. Methods FORSKA2′s water balance submodel was modified to enable it to behave more realistically under a varying climate. Long-term actual monthly time-series of temperature and precipitation data were detrended and used to drive the modified model. Long-term monthly averages of the same detrended data were used to drive the unmodified model. Results Modifications created significant improvements when simulating species composition at sites in boreal Canada. Simulated forest biomass values were slightly higher than those obtained from the unmodified model using averaged climate records, but resembled the observed distribution of vegetation more closely. Main conclusions Modified FORSKA2 suggests that boreal forest composition and distribution may be more sensitive to changes in monthly rainfall data than to changes in temperature. Climate variability affects seasonal water balances and should be considered when using patch models to forecast vegetation dynamics during and following a period of climate transition. The modified model provided improved representation of the latitudinal trend in spatially averaged biomass density in this region.     

Mean conditions predict salt marsh plant community diversity and stability better than environmental variability

Environmental variability and the frequency of extreme events are predicted to increase in future climate scenarios; however, the role of fluctuations in shaping community composition, diversity and stability is not well understood. Identifying current patterns of association between measures of community stability and climatic means and variability will help elucidate the ways in which altered variability and mean conditions may change communities in the future. Salt marshes provide essential ecosystem services and are increasingly threatened by sea‐level rise, land‐use change, eutrophication and predator loss, yet the effects of temporal environmental variation on salt marshes remain unknown. We synthesized long‐term plant community monitoring data from 11 sites on both coasts of the United States. We used an information‐theoretic approach and linear models to determine the associations among long‐term mean conditions, interannual environmental variability, and plant community stability and diversity. We found that salt marsh community stability and diversity were more strongly related to long‐term means of temperature and precipitation than to interannual variation. Warm and wet environments had fewer species and less turnover among years. Our results suggest that communities in cool, dry environments may be more resilient to climate warming due to greater species richness and turnover. Mean conditions are sufficient to predict contemporary patterns of salt marsh plant community dynamics, but environmental variability may have stronger impacts as it increases with climate change.     

毛竹林总初级生产力年际变化及其驱动因素——以安吉县为例

森林生态系统碳通量的年际变化及其驱动因素分析是了解森林碳收支动态变化以及预测未来气候变化对森林碳收支影响的重要理论基础,对评估森林应对气候变化的贡献具有重要意义。结合MODIS叶面积指数(LAI)和归一化植被指数(NDVI)产品、MERRA气象数据和通量塔观测数据,采用光能利用率模型模拟2004—2011年安吉县毛竹林生态系统总初级生产力(GPP)空间分布,并分析GPP年际变化及其驱动因素。结果表明:(1)小年毛竹林GPP稍高于大年GPP;(2)2004—2011年安吉县毛竹林年日均GPP呈下降趋势,东部、西部和整个安吉县毛竹林年日均GPP变化速率分别为-0.064、-0.033和-0.045g C m-2W-1,年均温度持续下降是主要驱动因素;(3)LAI年际变化是GPP年际变化的主要驱动因素,主要原因是毛竹林大小年交替规律引起了有效LAI年际间差异;(4)西部GPP年际变化幅度大于东部,环境和生物因素对GPP年际变化的作用方向决定了毛竹林GPP年际变化的幅度。     

The effect of past changes in inter‐annual temperature variability on tree distribution limits

Aim The northern limits of temperate broadleaved species in Fennoscanndia are controlled by their requirements for summer warmth for successful regeneration and growth as well as by the detrimental effects of winter cold on plant tissue. However, occurrences of meteorological conditions with detrimental effects on individual species are rare events rather than a reflection of average conditions. We explore the effect of changes in inter‐annual temperature variability on the abundances of the tree species Tilia cordata, Quercus robur and Ulmus glabra near their distribution limits using a process‐based model of ecosystem dynamics. Location A site in central Sweden and a site in southern Finland were used as examples for the ecotone between boreal and temperate forests in Fennoscandia. The Finnish site was selected because of the availability of varve‐thickness data. Methods The dynamic vegetation model LPJ‐GUESS was run with four scenarios of inter‐annual temperature forcing for the last 10,000 years. In one scenario the variability in the thickness of summer and winter varves from the annually laminated lake in Finland was used as a proxy for past inter‐annual temperature variability. Two scenarios were devised to explore systematically the effect of stepwise changes in the variance and shape parameter of a probability distribution. All variability scenarios were run both with and without the long‐term trend in Holocene temperature change predicted by an atmospheric general circulation model. Results Directional changes in inter‐annual temperature variability have significant effects on simulated tree distribution limits through time. Variations in inter‐annual temperature variability alone are shown to alter vegetation composition by magnitudes similar to the magnitude of changes driven by variation in mean temperatures. Main conclusions The varve data indicate that inter‐annual climate variability has changed in the past. The model results show that past changes in species abundance can be explained by changes in the inter‐annual variability of climate parameters as well as by mean climate. Because inter‐annual climatic variability is predicted to change in the future, this component of climate change should be taken into account both when making projections of future plant distributions and when interpreting vegetation history.     

Interannual variability in global soil respiration, 1980–94

We used a climate‐driven regression model to develop spatially resolved estimates of soil‐CO₂ emissions from the terrestrial land surface for each month from January 1980 to December 1994, to evaluate the effects of interannual variations in climate on global soil‐to‐atmosphere CO₂ fluxes. The mean annual global soil‐CO₂ flux over this 15‐y period was estimated to be 80.4 (range 79.3–81.8) Pg C. Monthly variations in global soil‐CO₂ emissions followed closely the mean temperature cycle of the Northern Hemisphere. Globally, soil‐CO₂ emissions reached their minima in February and peaked in July and August. Tropical and subtropical evergreen broad‐leaved forests contributed more soil‐derived CO₂ to the atmosphere than did any other vegetation type (～30% of the total) and exhibited a biannual cycle in their emissions. Soil‐CO₂ emissions in other biomes exhibited a single annual cycle that paralleled the seasonal temperature cycle. Interannual variability in estimated global soil‐CO₂ production is substantially less than is variability in net carbon uptake by plants (i.e., net primary productivity). Thus, soils appear to buffer atmospheric CO₂ concentrations against far more dramatic seasonal and interannual differences in plant growth. Within seasonally dry biomes (savannas, bushlands and deserts), interannual variability in soil‐CO₂ emissions correlated significantly with interannual differences in precipitation. At the global scale, however, annual soil‐CO₂ fluxes correlated with mean annual temperature, with a slope of 3.3 Pg C y^?1 per °C. Although the distribution of precipitation influences seasonal and spatial patterns of soil‐CO₂ emissions, global warming is likely to stimulate CO₂ emissions from soils.     

Responses of biomass allocation across two vegetation types to climate fluctuations in the northern Qinghai–Tibet Plateau

The Qinghai–Tibet Plateau (QTP) is particularly sensitive to global climate change, especially to elevated temperatures, when compared with other ecosystems. However, few studies use long‐term field measurements to explore the interannual variations in plant biomass under climate fluctuations. Here, we examine the interannual variations of plant biomass within two vegetation types (alpine meadow and alpine shrub) during 2008–2017 and their relationships with climate variables. The following results were obtained. The aboveground biomass (AGB) and belowground biomass (BGB) response differently to climate fluctuations, the AGB in KPM was dominated by mean annual precipitation (MAP), whereas the AGB in PFS was controlled by mean annual air temperature (MAT). However, the BGB of both KPM and PFS was only weakly affected by climate variables, suggesting that the BGB in alpine ecosystems may remain as a stable carbon stock even under future global climate change. Furthermore, the AGB in PFS was significantly higher than KPM, while the BGB and R/S in KPM were significantly higher than PFS, reflecting the KPM be more likely to allocate more photosynthates to roots. Interestingly, the proportion of 0–10 cm root biomass increased in KPM and PFS, whereas the other proportions both decreased, reflecting a shift in biomass toward the surface layer. Our results could provide a new sight for the prediction how alpine ecosystem response to future climate change.     

A Multilevel Analysis of the Impact of Land Use on Interannual Land-Cover Change in East Africa

Abstract The aim of this study was to characterize the short-term land-cover change processes that were detected in Eastern Africa, based on a set of change metrics that allow for the quantification of interannual changes in vegetation productivity, changes in vegetation phenology and a combination of both. We tested to what extent land use, fire activity and livestock grazing modified the vegetation response to short-term rainfall variability in East Africa and how this is reflected in change metrics derived from MODerate Imaging Spectrometer (MODIS) time series of remote sensing data. We used a hierarchical approach to disentangle the contribution of human activities and climate variability to the patterns of short-term vegetation change in East Africa at different levels of organization. Our results clearly show that land use significantly influences the vegetation response to rainfall variability as measured by time series of MODIS data. Areas with different types of land use react in a different way to interannual climate variability, leading to different values of the change indices depending on the land use type. The impact of land use is more reflected in interannual variability of vegetation productivity and overall change in the vegetation, whereas changes in phenology are mainly driven by climate variability and affect most vegetation types in similar ways. Our multilevel approach led to improved models and clearly demonstrated that climate influence plays at a different scale than land use, fire and herbivore grazing. It helped us to understand dynamics within and between biomes in the study area and investigate the relative importance of different factors influencing short-term variability in change indices at different scales.     

Hydrologic variability in dryland regions: impacts on ecosystem dynamics and food security

Research on ecosystem and societal response to global environmental change typically considers the effects of shifts in mean climate conditions. There is, however, some evidence of ongoing changes also in the variance of hydrologic and climate fluctuations. A relatively high interannual variability is a distinctive feature of the hydrologic regime of dryland regions, particularly at the desert margins. Hydrologic variability has an important impact on ecosystem dynamics, food security and societal reliance on ecosystem services in water-limited environments. Here, we investigate some of the current patterns of hydrologic variability in drylands around the world and review the major effects of hydrologic fluctuations on ecosystem resilience, maintenance of biodiversity and food security. We show that random hydrologic fluctuations may enhance the resilience of dryland ecosystems by obliterating bistable deterministic behaviours and threshold-like responses to external drivers. Moreover, by increasing biodiversity and the associated ecosystem redundancy, hydrologic variability can indirectly enhance post-disturbance recovery, i.e. ecosystem resilience.