西藏当雄县高寒草地NDVI的海拔分异特征及其指示

通过提取西藏当雄县Landsat ETM影像NDVI以及DEM的海拔要素,在坡面的小尺度水平上分析了NDVI的海拔格局,并利用沿海拔梯度的草地地上生物量实测数据进行验证,进一步在全县尺度上分析了当雄NDVI >0.2 的区域内草地NDVI 的海拔分异特征.主要结果如下:①在坡面尺度上,随海拔的增加,不同计算方式的NDVI均表现为先增加后降低的单峰格局,与沿海拔梯度实测的地上生物量格局一致,NDVI最大值与围栏外生物量最大值均出现在4 950 m左右;②在全县尺度上,当雄草地NDVI随海拔的变化略为复杂:在4 300 m以上,NDVI随海拔的增加表现为先升高后降低,最高值出现在4 700 m左右;在4 300 m以下植被可能以湿地草甸为主,形成另一个NDVI高值区,但其分布面积仅为总草场面积的10%左右.上述结果表明,海拔是影响当雄县NDVI 空间分布的重要地形因子,NDVI 的海拔格局体现了较大尺度上草地植被对不同水热组合环境的适应特征,这对于在气候变化背景下实施高寒地区生态保护工程以及制定合理的草场资源配置方案具有重要指导意义.

Altitudinal Differentiation of Alpine Grassland NDVI and Its Indications in Damxung County,Tibet

It is still unclear if the altitudinal pattern of aboveground biomass detected from small-scale transect resembles those at larger-scale landscapes such as county-level, and if the threshold vary among different scales. Based on NDVI values extracted from Landsat ETM images and the terrain factors detected from DEM, we first assessed the altitudinal patterns of NDVI (a surrogate of aboveground biomass) and validated it by aboveground biomass measurements sampled along a transect of grassland at a south slope in Damxung County, Tibet. We then analyzed the altitudinal pattern of NDVI of the whole county where NDVI >0.2. The results are as follows: 1) at the slope scale, the NDVI values calculated by different methods all showed a unimodal pattern that increased first and then decreased with the increase of altitude, with the maximum value appeared at the height of about 4950 m, and then decreased when altitude continually rose, which displays the same pattern as the investigated aboveground biomass with the peaked at 4950 m (313 g/m²); 2) at the whole county scale, the average NDVI showed a bimodal pattern: one peak appeared around 4700 m and the other below 4300 m. For the area where the altitude was above 4300 m and which accounted for more than 85% of the total area), the variation of NDVI with rising altitude also presented a unimodal pattern. For the peak below 4300 m, the high levels of NDVI came from wet meadows or wetlands which only accounted for a small part of total grassland area (≈10%). In conclusion, similar altitudinal patterns of NDVI at different space scales indicate that it is the elevation that primarily controls the spatial pattern of NDVI in Damxung, Tibet, and the altitudinal patterns of NDVI reflect the adaptations of grassland vegetation to varied combinations of temperature and precipitation. This is important for the ecological conservation at alpine regions and the protection and utilization of grassland resources reasonably under climate change scenarios.