College of Forestry, Oregon State University, Corvallis, OR, 97331, USA.
Glob Chang Biol. 2014 Feb;20(2):418-28. doi: 10.1111/gcb.12365. Epub 2013 Dec 19.
The Mongolian Steppe is one of the largest remaining grassland ecosystems. Recent studies have reported widespread decline of vegetation across the steppe and about 70% of this ecosystem is now considered degraded. Among the scientific community there has been an active debate about whether the observed degradation is related to climate, or over-grazing, or both. Here, we employ a new atmospheric correction and cloud screening algorithm (MAIAC) to investigate trends in satellite observed vegetation phenology. We relate these trends to changes in climate and domestic animal populations. A series of harmonic functions is fitted to Moderate Resolution Imaging Spectroradiometer (MODIS) observed phenological curves to quantify seasonal and inter-annual changes in vegetation. Our results show a widespread decline (of about 12% on average) in MODIS observed normalized difference vegetation index (NDVI) across the country but particularly in the transition zone between grassland and the Gobi desert, where recent decline was as much as 40% below the 2002 mean NDVI. While we found considerable regional differences in the causes of landscape degradation, about 80% of the decline in NDVI could be attributed to increase in livestock. Changes in precipitation were able to explain about 30% of degradation across the country as a whole but up to 50% in areas with denser vegetation cover (P < 0.05). Temperature changes, while significant, played only a minor role (r(2) = 0.10, P < 0.05). Our results suggest that the cumulative effect of overgrazing is a primary contributor to the degradation of the Mongolian steppe and is at least partially responsible for desertification reported in previous studies.
蒙古草原是世界上最大的草原生态系统之一。最近的研究报告称，草原的植被已经广泛减少，大约 70%的生态系统已经退化。在科学界，人们一直在积极争论观察到的退化是与气候有关，还是与过度放牧有关，或者两者都有关。在这里，我们采用了一种新的大气校正和云筛选算法（MAIAC）来研究卫星观测到的植被物候学的变化趋势。我们将这些趋势与气候和家畜种群的变化联系起来。我们采用一系列调和函数来拟合中分辨率成像光谱仪（MODIS）观测到的物候曲线，以量化植被的季节性和年际变化。研究结果表明，全国范围内 MODIS 观测到的归一化差异植被指数（NDVI）普遍下降（平均下降约 12%），但在草原和戈壁沙漠之间的过渡带尤为明显，最近的下降幅度比 2002 年平均 NDVI 低 40%。虽然我们发现景观退化的原因存在相当大的区域差异，但约 80%的 NDVI 下降可以归因于牲畜的增加。降水的变化可以解释全国范围内约 30%的退化，但在植被覆盖率较高的地区（P < 0.05），解释程度高达 50%。虽然温度变化显著，但只起次要作用（r(2) = 0.10，P < 0.05）。我们的研究结果表明，过度放牧的累积效应是蒙古草原退化的主要原因，至少是先前研究报告中荒漠化的部分原因。
