枯落物对高寒草地植被指数和返青期遥感估算结果的影响

The Qinghai-Tibetan Plateau is a sensitive and ecologically fragile zone of climate change and it response very quickly to both climate change and human activities. In the past decades, the Qinghai-Tibetan Plateau has experienced significant climate warming, and this warming trend will continue. How does the green-up date of alpine grasslands on the Qinghai-Tibetan Plateau respond to climate warming has drew a lot of attention, without unified conclusion. It is worthy to note that during the period of regreening, standing litter is widespread on the Qinghai-Tibetan Plateau. Meanwhile, along with climate warming, the vegetation activity on the Qinghai-Tibetan Plateau has increased universally, as a result, the content of standing litter increased during the non-growing season and the regreening period. Besides, the change of meteorological factors and grazing policies on the Qinghai-Tibetan Plateau are both beneficial to the accumulation of standing litter. The spatial and temporal variations of standing litter is bound to have a significant impact on the procedures of vegetation index calculation and green-up date estimation, which are both based on remote sensing data, namely NDVI. Therefore, the quantitative evaluations of the spatial and temporal variations of standing litter and its influences on the remote sensing estimation results of NDVI and green-up date of alpine grasslands, on one hand, is very important to improve the estimate precision of the remote sensing vegetation index products and reduce the uncertainties of remote sensing estimation results of green-up date of alpine grasslands; on the other hand, can also provide a basis data for ecological simulation in this region.

青藏高原是气候变化的敏感区和生态脆弱带，对气候变化和人为扰动十分敏感。高寒草地生态系统是一个在全球范围内都非常重要的植物群落。过去几十年来，青藏高原经历了显著的气候变暖，并且这种增温趋势还将持续。高寒草地的返青期将如何响应气候变暖受到了极大的关注，但却没有统一的结论。需要注意的是，返青时节，枯落物在青藏高原上普遍存在。同时，随着气候变暖，青藏高原的植被活动普遍增强，非生长季和返青期的枯落物含量将增加。同时，青藏高原地区气象因素的变化和放牧政策，均有利于枯落物的积累。枯落物的时空变化必然会对基于遥感数据计算和估算得到的植被指数和返青期产生重大影响。因此，定量评估高寒草地枯落物的时空变化特征及其对NDVI和返青期遥感估算结果的影响，对于提高遥感植被指数产品的估算精度，减少返青期遥感估算结果中存在的不确定性非常重要，同时也可以为这一地区的生态模拟研究提供基础资料。

青藏高原是气候变化的敏感区和生态脆弱带，对气候变化和人为扰动十分敏感。高寒草地生态系统是一个在全球范围内都非常重要的植物群落。过去几十年来，青藏高原经历了显著的气候变暖，并且这种增温趋势还将持续。高寒草地将如何响应气候变暖受到了极大的关注（尤其是返青期），但却没有统一的结论。需要注意的是，返青时节，枯落物在青藏高原上普遍存在且往往与绿色植被共存。同时，当前的气候因素、生物因素和政策因素，均有利于枯落物的积累。枯落物及其时空变化必然会对植被参数（尤其是NDVI、植被盖度等）产生显著影响，从而影响相关植被参数（返青期等）的遥感估算结果。本项目旨在定量评估这一影响，研究结果对于提高遥感植被参数数据集的估算精度，减少返青期遥感估算结果中存在的不确定性非常重要，也可为这一地区的生态模拟研究提供基础资料。通过开展样方尺度的剪草实验和样地尺度的无人机航拍实验，得到的初步结论为：1）随着枯落物量的减少，绿色植被盖度（GFVC）增大而枯落物植被盖度（LFVC）减小；2）随着GFVC/LFVC的增加/减少，NDVI迅速增大，R2分别为0.54和0.42（488）；3）随枯落物含量的减少，NDVI的增幅约为0.03/g；4）在所有枯落物指数中，DFI的性能最优，CAI和NDSVI次之，其他指数的效果较差；5）枯落物信息提取的无人机航拍测试结果显示，精灵4系列的效果最佳。