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)。

Phenological variation of typical vegetation types in northern Tibet and its response to climate changes

Vegetation phenology is closely related to seasonal dynamics of the lower atmosphere and,therefore,is thus considered as an important indicator that influences the feedback mechanisms of global climate variation to the terrestrial ecosystem.Detecting vegetation phenological metrics using time series remote sensing dataset at the regional or global scale has become an active research topic in geography and ecology.This paper presents a dynamic thresholding method to extract the vegetation phenological metrics(greenup date,growing season length,maximum EVI and wilting date) of typical grassland,including alpine shrub meadow in the eastern region,alpine meadow in the central region,alpine steppe and alpine desert steppe in the western region of northern Tibetan Plateau,using a time-series TERRA/MODIS EVI dataset from 2001 to 2010 which was reconstructed using the asymmetric Guassian function fitting method.Furthermore,the relationship between inter-annual variation of phenological metrics and climatic factors was analyzed based on the measured data of weather stations close to the sampling areas,the results showed that:(1) The ordinal relationship of these typical vegetation samples in phenological metrics was in the sequence of alpine shrub meadow,alpine meadow,alpine steppe,and alpine desert steppe(for greenup date and the wilting date,the sequence respectively referred to be earlier and later).(2) The annual,spring,summer and winter average temperature of northern Tibetan Plateau all showed significant ascending trend,the increased rate was between 0.8℃/10a and 3.9℃/10a.While the decreased trend of precipitation was not apparent.Due to the warming and drier climate,the greenup date of four types of typical vegetation was advanced(approximately 7.2—15.5d/10a earlier) and the growing season length became longer(approximately 8.4—19.2d/10a),whereas the wilting date of four types of vegetation cover did not show apparent advanced or lingering trend;(3) The maximum EVI of different typical vegetation cover types was affected by different climatic factors.As follows,alpine shrub meadow was mainly affected by spring average temperature and spring precipitation,and precipitation had more important influence on phenological variation than temperature;Alpine meadow was affected by both spring and summer average temperature and precipitation;Alpine steppe and alpine desert steppe were primarily influenced by summer average temperature and precipitation,while for alpine steppe vegetation the inhibition of summer average temperature was more intense than the promotion effect of summer precipitation,among four types of vegetation cover,alpine desert steppe had the closest relationship with summer precipitation(the correlation coefficient,-0.752);(4) As for greenup date,alpine shrub meadow was mainly affected by autumn precipitation of the previous year,the correlation coefficient was closed to-0.579;however,other three types of vegetation cover were mainly influenced by spring average temperature,and the character was the most obvious for desert steppe(the correlation coefficient,-0.559).That was due to that high temperature in spring expedited snow melting and tundra soil thawing which are indispensable for vegetation greenup,especially for high-altitude regions in northern and western Tibet.In addition,the autumn precipitation and winter average temperature in the previous year also had some impact on the vegetation greenup of alpine steppe and alpine desert steppe.