Spatial variation in abundance of the junin virus hosts in endemic and nonendemic Argentine haemorrhagic fever zones

Abstract Argentine haemorrhagic fever (AHF) is caused by Junin (JUN) virus, which is hosted by the drylands vesper mouse (Calomys musculinus). In this work we monitored population abundance of C. musculinus and rodent assemblages for 3 years in and outside the AHF endemic zones (northern Buenos Aires, southern Córdoba and Santa Fe Provinces, Argentina). The study area was divided into endemic and nonendemic zones. In the endemic zone epidemic sites were recognized, characterized by recent emergence and maintenance of AHF cases, and also historical sites, characterized by decreased incidence or disappearance of AHF human cases. In the nonendemic zone AHF has never been recognized. Although differences were statistically significant only during some periods, population abundance of C. musculinus was usually lower in the nonendemic sites. The pattern and magnitude of seasonal fluctuations in C. musculinus populations were also distinct in the nonendemic sites as compared to endemic sites. The relative abundance of C. musculinus in rodent assemblage was lower in nonendemic sites than in the endemic sites. The lower population densities and dampened seasonal dynamics may be at least partly responsible for the absence of AHF cases in the nonendemic zone. It is suggested that the balance between intra and interspecific interactions might be the cause of the pattern of incidence and prevalence of pathogens in the host species.     

陕北长城沿线风沙区植被指数变化及其与气候的关系

陕北长城沿线风沙区位于毛乌素沙漠东南部边沿,属毛乌素沙地向东南移动的最活跃地段,生态环境十分脆弱。使用1981～2003年23a长时间序列的NOAA/AHRR NDVI数据、气候资料,分析了陕北长城沿线风沙区植被覆盖的历史演变及其与气候因子的关系。结果表明:(1)陕北长城沿线风沙区植被覆盖状况23a来尽管有波动起伏,但是整体在持续转好,年平均NDVI增加了10.62%。低覆盖率植被面积在减少,高覆盖率植被面积在增加。夏季的NDVI值最高、波动起伏最大,其次是秋季;春、夏、秋三季的NDVI具有明显的上升趋势,季平均NDVI年增长率夏季最大,秋季次之;夏、秋季NDVI与年NDVI具有很高的相关性,这两个季节的植被状况基本决定了全年的植被分布状况。NDVI年变化曲线为单峰型,春季NDVI缓慢增加,秋季NDVI降低速度比较快。(2)年平均NDVI与温度的年际变化相关不明显,各季节NDVI与温度相关也不明显。近年来长城沿线风沙区的年降水量没有明显增加,而年平均NDVI线性增加趋势显著,降水量是引起NDVI年际波动的主要因子,非气候因素是年平均NDVI线性增加的主要原因。降水量与NDVI存在着明显的年相关和隔季相关。年降水量与年NDVI的相关,冬季降水量与春季NDVI的相关,春季降水量与夏季NDVI的相关,夏季降水量与秋季NDVI的相关性都非常高。(3)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、草原围栏封育等是导致植被显著增加的重要原因。     

Beyond Precipitation: Physiographic Gradients Dictate the Relative Importance of Environmental Drivers on Savanna Vegetation

Background

Understanding the drivers of large-scale vegetation change is critical to managing landscapes and key to predicting how projected climate and land use changes will affect regional vegetation patterns. This study aimed to improve our understanding of the role, magnitude and spatial distribution of the key environmental factors driving vegetation change in southern African savanna, and how they vary across physiographic gradients.

Methodology/Principal Findings

We applied Dynamic Factor Analysis (DFA), a multivariate times series dimension reduction technique to ten years of monthly remote sensing data (MODIS-derived normalized difference vegetation index, NDVI) and a suite of environmental covariates: precipitation, mean and maximum temperature, soil moisture, relative humidity, fire and potential evapotranspiration. Monthly NDVI was described by cyclic seasonal variation with distinct spatiotemporal patterns in different physiographic regions. Results support existing work emphasizing the importance of precipitation, soil moisture and fire on NDVI, but also reveal overlooked effects of temperature and evapotranspiration, particularly in regions with higher mean annual precipitation. Critically, spatial distributions of the weights of environmental covariates point to a transition in the importance of precipitation and soil moisture (strongest in grass-dominated regions with precipitation<750 mm) to fire, potential evapotranspiration, and temperature (strongest in tree-dominated regions with precipitation>950 mm).

Conclusions/Significance

We quantified the combined spatiotemporal effects of an available suite of environmental drivers on NDVI across a large and diverse savanna region. The analysis supports known drivers of savanna vegetation but also uncovers important roles of temperature and evapotranspiration. Results highlight the utility of applying the DFA approach to remote sensing products for regional analyses of landscape change in the context of global environmental change. With the dramatic increase in global change research, this methodology augurs well for further development and application of spatially explicit time series modeling to studies at the intersection of ecology and remote sensing.     

Agricultural Legacies in the Great Basin Alter Vegetation Cover, Composition, and Response to Precipitation

The land-use history of an ecosystem influences current structure and possibly response to modern disturbances and stresses. In semiarid systems the nature of land-use legacies is poorly understood, confounding efforts to establish sustainable management approaches. We compare previously cultivated and non-cultivated lands in Owens Valley, California, where cultivation once extended to 34% of the valley floor but was largely discontinued by 1940, to measure the influence of past disturbance on modern vegetation. We combined historic maps of cultivated and non-cultivated land with an extensive vegetation survey, historic aerial photographs, and satellite measurements of vegetation response to precipitation variability to examine the importance of land-use history in determining the sensitivity of vegetation to annual variations in precipitation. Remote sensing analysis showed that total plant cover on previously cultivated lands was lower and fluctuations in cover were marginally more dependent on precipitation compared with plant cover on non-cultivated lands. We then compared modern plant assemblages within cultivated and non-cultivated land to determine if compositional differences could explain the current patterns of vegetation cover. We found lower species richness on previously cultivated parcels, and higher frequency and cover of perennial grasses on non-cultivated lands. Therefore, we showed differences in land-cover patterns, isolated a mechanism that could account for the differences (species differences), and developed a method for remotely analyzing land regions that have experienced historic anthropogenic disturbance.     

Variation in species composition and vegetation structure of succulent scrub on Tenerife in relation to environmental variation

Abstract. On Tenerife, the occurrence of environmental gradients over short distances provides a unique opportunity to investigate the relationship between vegetation and environmental factors. In the semi‐arid coastal region of Tenerife, floristic composition, species richness and vegetation structure of perennial plants have been studied in 67 locations covering the existing precipitation gradient. On the island as a whole, variation in species composition could be best explained by mean annual precipitation; at coastal sites, substrate age and soil characteristics also played a significant role. On the other hand, substrate chemistry and the type of eruptive material explained little of the floristic variation. Stand biomass was strongly correlated with mean annual precipitation and was, on the youngest lava flows studied, also affected by substrate age. The native stem succulent species made up the bulk of total biomass along the whole precipitation gradient. Disturbed and undisturbed sites differed significantly in stand biomass and cover. Species richness was correlated with precipitation and substrate age. Distribution of plant functional types was also related to the precipitation gradient. The relative abundance of hemicryptophytes and shrubs with non‐hairy leaves increased with increasing precipitation whereas the ratio of shrubs with hairy/non‐hairy leaves and succulent plants decreased. Some alien plants were quite frequent at disturbed sites but, on the whole, they contributed little to the species spectrum and to the stand biomass. Undisturbed sites remained almost free of introduced species not considering annuals.     

宁夏近20年来植被覆盖度及其与气温降水的关系

利用1981—2004年的植被指数(NDVI)资料,结合实地植被调查资料,将宁夏植被类型划分4个区域,确定了各地植被指数与覆盖度的关系,分析了宁夏各区域植被覆盖度的年际变化、季节变化、植被覆盖度的面积变化及其与气温和降水量的关系。结果表明:24年来宁夏贺兰山与贺兰山东麓的年植被覆盖度呈下降趋势,中部干旱带植被覆盖度呈上升趋势;近4年来,贺兰山植被状况整体变好;贺兰山东麓植被状况整体变差;中部干旱带的植被状况夏季变差,秋季变好;南部山区的植被覆盖度整体变好。宁夏春夏季降水量是影响植被覆盖度的关键性因子,气温对植被覆盖度的影响不显著。     

Consistent response of vegetation dynamics to recent climate change in tropical mountain regions

Global climate change has emerged as a major driver of ecosystem change. Here, we present evidence for globally consistent responses in vegetation dynamics to recent climate change in the world's mountain ecosystems located in the pan‐tropical belt (30°N–30°S). We analyzed decadal‐scale trends and seasonal cycles of vegetation greenness using monthly time series of satellite greenness (Normalized Difference Vegetation Index) and climate data for the period 1982–2006 for 47 mountain protected areas in five biodiversity hotspots. The time series of annual maximum NDVI for each of five continental regions shows mild greening trends followed by reversal to stronger browning trends around the mid‐1990s. During the same period we found increasing trends in temperature but only marginal change in precipitation. The amplitude of the annual greenness cycle increased with time, and was strongly associated with the observed increase in temperature amplitude. We applied dynamic models with time‐dependent regression parameters to study the time evolution of NDVI–climate relationships. We found that the relationship between vegetation greenness and temperature weakened over time or was negative. Such loss of positive temperature sensitivity has been documented in other regions as a response to temperature‐induced moisture stress. We also used dynamic models to extract the trends in vegetation greenness that remain after accounting for the effects of temperature and precipitation. We found residual browning and greening trends in all regions, which indicate that factors other than temperature and precipitation also influence vegetation dynamics. Browning rates became progressively weaker with increase in elevation as indicated by quantile regression models. Tropical mountain vegetation is considered sensitive to climatic changes, so these consistent vegetation responses across widespread regions indicate persistent global‐scale effects of climate warming and associated moisture stresses.     

黄河流域植被时空变化及其对气候要素的响应

在气候变化和人类活动的双重作用下,黄河流域生态环境不断发生变化。探讨植被生长动态对于实施生态保护政策至关重要。利用Advanced Very High Resolution Radiometer(AVHRR) Leaf Area Index(LAI)遥感资料,结合气候要素数据,分析1981—2017年黄河流域植被覆盖的时空分布特征,探讨气候要素对其变化的影响及贡献率。研究结果表明：(1)时序上,黄河流域植被覆盖呈显著增长趋势,夏季植被覆盖的增长幅度和年际波动最大,冬季植被覆盖呈缓慢平稳增长,波动最小。(2)空间上,植被覆盖显著提高的区域占整个区域的52.1%,主要分布在中东部平原;显著降低的区域占4%,主要分布在北部和西部高原山地;生态脆弱的区域植被覆盖率大多有不同程度的提高,但生态环境良好的部分区域植被覆盖率降低。(3)时序上,黄河流域植被覆盖与气温具有显著的正相关关系。春夏冬三季的植被覆盖与气温呈显著正相关,与降水呈不显著关系;秋季的植被覆盖与气温和降水量均呈显著正相关;春秋冬三季的植被覆盖与太阳辐射呈不显著负相关,夏季的植被覆盖与太阳辐射呈不显著正相关。春夏秋冬四季的气温对植被覆...     

Delayed autumn phenology in the Northern Hemisphere is related to change in both climate and spring phenology

The timing of the end of the vegetation growing season (EOS) plays a key role in terrestrial ecosystem carbon and nutrient cycles. Autumn phenology is, however, still poorly understood, and previous studies generally focused on few species or were very limited in scale. In this study, we applied four methods to extract EOS dates from NDVI records between 1982 and 2011 for the Northern Hemisphere, and determined the temporal correlations between EOS and environmental factors (i.e., temperature, precipitation and insolation), as well as the correlation between spring and autumn phenology, using partial correlation analyses. Overall, we observed a trend toward later EOS in ~70% of the pixels in Northern Hemisphere, with a mean rate of 0.18 ± 0.38 days yr^?1. Warming preseason temperature was positively associated with the rate of EOS in most of our study area, except for arid/semi‐arid regions, where the precipitation sum played a dominant positive role. Interestingly, increased preseason insolation sum might also lead to a later date of EOS. In addition to the climatic effects on EOS, we found an influence of spring vegetation green‐up dates on EOS, albeit biome dependent. Our study, therefore, suggests that both environmental factors and spring phenology should be included in the modeling of EOS to improve the predictions of autumn phenology as well as our understanding of the global carbon and nutrient balances.     

Growing seasons of Nordic mountain birch in northernmost Europe as indicated by long-term field studies and analyses of satellite images

The phenophases first greening (bud burst) and yellowing of Nordic mountain birch (Betula pubescens ssp.tortuosa, also called B. p. ssp. czerepanovii) were observed at three sites on the Kola Peninsula in northernmost Europe during the period 1964–2003, and at two sites in the trans-boundary Pasvik-Enare region during 1994–2003. The field observations were compared with satellite images based on the GIMMS-NDVI dataset covering 1982–2002 at the start and end of the growing season. A trend for a delay of first greening was observed at only one of the sites (Kandalaksha) over the 40 year period. This fits well with the delayed onset of the growing season for that site based on satellite images. No significant changes in time of greening at the other sites were found with either field observations or satellite analyses throughout the study period. These results differ from the earlier spring generally observed in other parts of Europe in recent decades. In the coldest regions of Europe, e.g. in northern high mountains and the northernmost continental areas, increased precipitation associated with the generally positive North Atlantic Oscillation in the last few decades has often fallen as snow. Increased snow may delay the time of onset of the growing season, although increased temperature generally causes earlier spring phenophases. Autumn yellowing of birch leaves tends towards an earlier date at all sites. Due to both later birch greening and earlier yellowing at the Kandalaksha site, the growing season there has also become significantly shorter during the years observed. The sites showing the most advanced yellowing in the field throughout the study period fit well with areas showing an earlier end of the growing season from satellite images covering 1982–2002. The earlier yellowing is highly correlated with a trend at the sites in autumn for earlier decreasing air temperature over the study period, indicating that this environmental factor is important also for autumn phenophases.     

藏北高原典型植被样区物候变化及其对气候变化的响应

植被物候作为陆地生态系统对气候变化的响应和反馈的重要指示,已成为区域或全球生态环境领域研究的热点。基于非对称高斯拟合方法重建了2001—2010年MODIS EVI时间序列影像,利用动态阈值法提取藏北高原植被覆盖2001—2010年每年关键物候参数。选取研究区内东部高寒灌丛草甸、中部高寒草甸及西部高寒草原和高寒荒漠4种典型植被类型,并结合附近的4个气象台站气候资料,分析典型植被物候在近10a对关键气候因子的响应特征。研究结果表明:(1)4种不同典型植被的物候特征(EVImax降低、返青期延后和生长季长度缩短)均表现出高寒灌丛草甸→高寒草甸→高寒草原→高寒荒漠草原的过渡;(2)藏北高原近10a的年平均气温及春、夏、冬三个季度的平均气温均呈显著升高的趋势,升温幅度在0.8—3.9℃/10a,降水减少趋势不显著,在这种水热条件下典型植被均表现出返青提前(7.2—15.5d/10a)、生长季延长(8.4—19.2d/10a)的趋势,而枯黄出现时间为年际间自然波动;(3)高寒灌丛草甸EVImax主要受春季降水量和气温影响,且降水的影响程度大于气温;对高寒草甸植被而言,春、夏季的气温和降水均有较大的影响;而高寒草原和高寒荒漠草原主要受夏季平均气温和降水量影响;(4)高寒灌丛草甸的返青时间主要受前一年秋季降水量的影响,相关系数达-0.579;而高寒草甸、高寒草原和高寒荒漠草原主要受春季平均气温影响,高寒荒漠草原的特征最为明显(r=-0.559)。     

Environmental factors covary with plant diversity–productivity relationships among Chinese grassland sites

Aim Our objective was to document the general relationship between plant species richness (SR) and above‐ground net primary productivity (ANPP) at different spatial scales and the environmental influence on this relationship. Location Temperate and alpine grasslands of China. Methods We investigated SR and ANPP at 321 field sites (1355 plots) across the widely distributed temperate and alpine grasslands of China. Ordinary least squares (OLS) regressions were used to test SR–ANPP relationships among site means. Plot‐level data of SR and ANPP were analysed with general linear models (GLMs) and the correlation between SR and ANPP was decomposed into covariance components to test the influence of climatic variables, region, vegetation type and remaining variation among sites on SR, ANPP and their relationship. Results We found positive linear relationships between SR and ANPP among sites in both the alpine and temperate grassland regions and in different grassland vegetation types of these biomes. Environmental gradients such as growing‐season precipitation affected both SR and ANPP in parallel. However, after removing the among‐site environmental variation, residual SR and ANPP were no longer correlated at the pooled within‐site level. Main conclusions The positive SR–ANPP relationship across large‐scale environmental gradients among sites was most likely the result of climatic variables influencing SR and ANPP in parallel. Our results suggest that in China's natural grasslands there is no direct relationship between SR and ANPP, presumably because the pool of available species for local community assembly is large, in contrast to experiments where species pools are artificially reduced.     

Modeling Dynamic Introduction of Chikungunya Virus in the United States

Chikungunya is a mosquito-borne viral infection of humans that previously was confined to regions in central Africa. However, during this century, the virus has shown surprising potential for geographic expansion as it invaded other countries including more temperate regions. With no vaccine and no specific treatment, the main control strategy for Chikungunya remains preventive control of mosquito populations. In consideration for the risk of Chikungunya introduction to the US, we developed a model for disease introduction based on virus introduction by one individual. Our study combines a climate-based mosquito population dynamics stochastic model with an epidemiological model to identify temporal windows that have epidemic risk. We ran this model with temperature data from different locations to study the geographic sensitivity of epidemic potential. We found that in locations with marked seasonal variation in temperature there also was a season of epidemic risk matching the period of the year in which mosquito populations survive and grow. In these locations controlling mosquito population sizes might be an efficient strategy. But, in other locations where the temperature supports mosquito development all year the epidemic risk is high and (practically) constant. In these locations, mosquito population control alone might not be an efficient disease control strategy and other approaches should be implemented to complement it. Our results strongly suggest that, in the event of an introduction and establishment of Chikungunya in the US, endemic and epidemic regions would emerge initially, primarily defined by environmental factors controlling annual mosquito population cycles. These regions should be identified to plan different intervention measures. In addition, reducing vector: human ratios can lower the probability and magnitude of outbreaks for regions with strong seasonal temperature patterns. This is the first model to consider Chikungunya risk in the US and can be applied to other vector borne diseases.     

Cholera Outbreak in Senegal in 2005: Was Climate a Factor?

Cholera is an acute diarrheal illness caused by Vibrio cholerae and occurs as widespread epidemics in Africa. In 2005, there were 31,719 cholera cases, with 458 deaths in the Republic of Senegal. We retrospectively investigated the climate origin of the devastating floods in mid-August 2005, in the Dakar Region of Senegal and the subsequent outbreak of cholera along with the pattern of cholera outbreaks in three other regions of that country. We compared rainfall patterns between 2002 and 2005 and the relationship between the sea surface temperature (SST) gradient in the tropical Atlantic Ocean and precipitation over Senegal for 2005. Results showed a specific pattern of rainfall throughout the Dakar region during August, 2005, and the associated rainfall anomaly coincided with an exacerbation of the cholera epidemic. Comparison of rainfall and epidemiological patterns revealed that the temporal dynamics of precipitation, which was abrupt and heavy, was presumably the determining factor. Analysis of the SST gradient showed that the Atlantic Ocean SST variability in 2005 differed from that of 2002 to 2004, a result of a prominent Atlantic meridional mode. The influence of this intense precipitation on cholera transmission over a densely populated and crowded region was detectable for both Dakar and Thiès, Senegal. Thus, high resolution rainfall forecasts at subseasonal time scales should provide a way forward for an early warning system in Africa for cholera and, thereby, trigger epidemic preparedness. Clearly, attention must be paid to both natural and human induced environmental factors to devise appropriate action to prevent cholera and other waterborne disease epidemics in the region.     

Environmental determinants of the small mammal assemblage in an agroecosystem of central Argentina: The role of Calomys musculinus

Agricultural intensification in the central region of Argentina has been accompanied by rodent assemblage changes, involving an increase in Calomys species densities. In particular, Calomys musculinus, the main reservoir of Junín virus (etiological agent of the Argentine Haemorrhagic Fever, AHF) selects field borders over crop-fields. Borders represent relatively-stable habitats within agroecosystems, otherwise highly human modified landscapes. In this article, we assessed the effect of environmental variables on rodent species occurrence and assemblage in the field borders of a pampean agroecosystem. Small rodent occurrence was examined for variation according to vegetation species composition, structure and productivity, using canonical ordination techniques. In a total of 72 crop-field borders of Córdoba province, in spring, summer and autumn (2005-2006), we captured 1041 rodents of 8 different species. Akodon azarae, Calomys venustus and C. musculinus were the dominant species of the assemblage, with the first two associated one another, while C. musculinus tended to be related to different environmental variables. We showed that rodent species associations were mediated by resources that changed seasonally, such as vegetation cover. Also, the strength and “sign” of the association between species seemed to be a consequence of different habitat preferences and activity patterns. Finally, the intra-annual population cycle seemed to influence the relation between species and habitat structure and, possibly, inter-specific competition for resources. Therefore, we suggest that less generalist species of the rodent assemblage, well adapted to linear habitats, may limit C. musculinus occurrence throughout the agroecosystem by habitat and spatial competition. Any action aimed to control AHF, and therefore C. musculinus population density, should exclude negative effects on coexisting species.     

Haplotype diversity of the mitochondrial DNA D-loop region in Calomys musculinus (Rodentia,Muridae) detected by PCR-RFLP

In order to contribute to knowledge of colonization patterns in the rodent Calomys musculinus, a natural reservoir of the virus producing Argentine hemorrhagic fever (AHF), we studied the haplotype diversity of the mitochondrial DNA D-loop region in five natural populations from central Argentina. Digestion with eight restriction enzymes (RsaI, MseI, Tsp509I, AluI, AciI, HaeIII, NlaIII, and AseI) revealed polymorphism in the 1300 bp fragment amplified by PCR. Twenty different composite haplotypes were detected. Hierarchical analyses indicated that almost all variation (94%) is contained within local populations. Haplotypes 1 and 2, shared by all populations, were the most frequent. Nonsignificant genetic differentiation was found among populations of the endemic and nonendemic areas of AHF: All locations sampled presented exclusive haplotypes in spite of their geographic proximity, which would support previous observations indicating restricted gene flow among C. musculinus populations.     

青藏高原植被生长对PDO响应的季节分异

利用卫星遥感观测的区域尺度归一化植被指数(NDVI)和格点气候数据,借助Spearman相关分析及基于多变量回归分析的结构方程模型,研究了1982—2015年青藏高原植被生长季节变化对太平洋10年际涛动(PDO)的响应格局及机理过程.结果表明:青藏高原生长季(4—10月)平均NDVI与PDO指数存在显著的负相关关系,但是PDO与不同季节NDVI之间的关系呈现出明显的季节分异,具体表现为PDO与秋季NDVI的负相关关系强于夏季,且冬季PDO显著影响次年青藏高原夏季植被生长.另外,PDO对青藏高原植被生长的调控过程在季节间存在明显分异,夏季表现为PDO对温度和降水的共同调控,而秋季则以对温度调控为主.     

Quantifying non‐breeding season occupancy patterns and the timing and drivers of autumn migration for a migratory songbird using Doppler radar

For seasonal migrants, non‐breeding regions can play different roles in the ecology of their annual cycles: as stopover habitat, overwintering habitat, or as a combination in which some individuals stop‐over and others over‐winter. Such functional variations can lead to variation in occupancy dynamics and migration phenology to these different regions. In this study, we used data from archived Doppler weather surveillance radar to compare site‐occupancy and movement dynamics of a migratory songbird (tree swallow Tachycineta bicolor) between two non‐breeding areas: southeastern Louisiana and central peninsular Florida, USA. Specifically, in each area, we 1) quantified long‐term (1996–2012) non‐breeding season occupancy dynamics, 2) quantified variation in timing of autumn migration, and 3) tested which climate variables along their respective flyways were best correlated with variation in dates of arrival. Additionally, we cross‐validated the dynamics from archived radar with data from eBird, a large‐scale citizen science database that provides an independent measure of avian occupancy. We found strong and significant correlations between radar‐estimated and eBird‐estimated occupancy dynamics in both Louisiana and Florida. Long‐term Louisiana occupancy dynamics conformed to our hypothesis that this region acts as a combined stopover and overwintering region whereas Florida occupancy dynamics were akin to a traditional winter region. Arrival to Louisiana during the study period was much earlier and took place over a much shorter arrival window than did arrival to Florida, which showed much more gradual arrival over the course of several months. At both sites, annual variation in mean arrival date was best explained by the amount of precipitation along the lower portions of their respective migration flyways.     

欧亚大陆不同生态区植被生长对降水响应的季节变化规律

欧亚大陆是"一带一路"战略规划的核心区域,了解欧亚大陆植被生长对降水的响应机制是该战略顺利实施的科学与生态基础。采用1982—2015年的归一化植被指数NDVI(Normalized Difference Vegetation Index)和全球气象格点数据,通过计算偏相关系数,排除温度及日照辐射的共同作用,分析植被对降水的响应关系(R_(NDVI-Prep))在不同季节、不同生态区的分异特征。在夏季,R_(NDVI-Prep)为显著正相关的地区广泛地分布在35°—60°N之间的欧亚大陆干旱、半干旱地区以及南亚次大陆西北和南部地区;在春季和秋季,R_(NDVI-Prep)为显著正相关的地区分布则相对于夏季显得更为集中,某些显著相关的区域的边界和现有的生态区划边界有很好的一致性。使用17年为一个周期的时间滑动窗口,分析不同季节上R_(NDVI-Prep)从1982至2015的变化规律,发现夏季植被呈现对降水依赖的地区所处纬度要高于春、秋两季。中亚干旱、半干旱生态区的植被在春、夏两季对降水依赖较强;南亚次大陆西北部地区的干旱、半干旱及东南亚的雨林地区在夏、秋两季对降水依赖较强,且秋季的分布最为集中。以生态区为视角,对欧亚大陆的植被与降水的关系有了更为深入的了解,可以为不同区域应对和适应气候变化提供决策依据。     

Spatiotemporal Patterns of Production Can Be Used to Detect State Change Across an Arid Landscape

Methods to detect and quantify shifts in the state of ecosystems are increasingly important as global change drivers push more systems toward thresholds of change. Temporal relationships between precipitation and aboveground net primary production (ANPP) have been studied extensively in arid and semiarid ecosystems, but rarely has spatial variation in these relationships been investigated at a landscape scale, and rarely has such information been viewed as a resource for mapping the distribution of different ecological states. We examined the broad-scale effects of a shift from grassland to shrubland states on spatiotemporal patterns of remotely sensed ANPP proxies in the northern Chihuahuan Desert. We found that the normalized difference vegetation index (NDVI), when averaged across an eight-year period, did not vary significantly between these states, despite changes in ecosystem attributes likely to influence water availability to plants. In contrast, temporal relationships between precipitation and time-integrated NDVI (NDVI-I) modeled on a per-pixel basis were sensitive to spatial variation in shrub canopy cover, a key attribute differentiating ecological states in the region. The slope of the relationship between annual NDVI-I and 2-year cumulative precipitation was negatively related to, and accounted for 71% of variation in, shrub canopy cover estimated at validation sites using high spatial resolution satellite imagery. These results suggest that remote sensing studies of temporal precipitation–NDVI relationships may be useful for deriving shrub canopy cover estimates in the region, as well as for mapping other ecological state changes characterized by shifts in long-term ANPP, plant functional type dominance, or both.