地面测量与ASTER影像综合计算植被盖度

选择我国北方温带典型草原作为研究对象,在研究区内分别针对植被盖度是高、中、低的地区随机选取49块样地,在每块样地内采用嵌套的方式作样方,样方尺度有60、30、15m3个尺度,嵌套的方式是60m的样方嵌套30m的样方,30m的样方嵌套15m的样方。15m的样方内作3个1m的样方,30m的样方内作5个1m的样方,60m的样方内作10个1m的样方。对于每个1m的样方,采用将数码相机架设在离草本层2m高的位置,垂直对样方进行拍摄,并记录影像的方式估计植被盖度。基于以上地面实测数据以及ASTER遥感数据,建立植被盖度经验模型。模型建立的步骤主要包括以下内容:(1)利用植被盖度信息提取模型计算野外实测的各张数码相片的植被盖度值,在15、30m以及60m3个样方尺度上计算各样方的植被盖度;(2)计算ASTER数据的各种植被指数(RVI,NDVI,NDGI);(3)将地面样方与相应的ASTER影像像元在不同的尺度上相匹配。(4)基于某一样方尺度,计算与地面样方相对应区域内的ASTER影像像元植被指数的中值。(5)基于该样方尺度,建立地面实测植被盖度值与ASTER数据植被指数值之间的回归模型;(6)对回归模型进行显著性检验。在建立经验模型的过程中,研究植被指数(NDVI、NDGI以及RVI)与植被盖度的相关性以及地面样方尺度对经验模型的影响。此外,还将经验模型与目前广泛使用的亚像元分解模型作比较,检验亚像元分解模型在中国北方温带典型草原的适用性。结果表明,(1)对于中国北方典型草原区而言,利用植被指数NDVI监测草地的植被盖度是比较适宜的,它优于植被指数RVI,也比植被指数NDGI的效果好。(2)地面样方尺度的选择对于植被盖度经验模型的建立有很大的影响。就中国北方典型草原区来看,在地面作大样方,取其中值的方法在一定程度上可以克服由于地面的测量和遥感测量之间发生空间错配而产生的影响,有利于提高植被指数与植被盖度之间相关系数的显著水平。(3)60m×60m的样方尺度,基于NDVI的植被盖度经验模型相比,利用亚像元分解模型估测中国北方温带典型草原植被盖度的精度低于前者。

基于数字相机、ASTER和MODIS影像综合测量植被盖度

选择我国北方温带典型草原作为研究对象,基于Bottom-up方法,采用地表实测和多尺度遥感综合测量的方法,建立基于地表实测与多尺度遥感数据综合测量的两阶段植被盖度经验模型。此外,还将该模型与常用的亚像元分解模型相比较,结果表明:1)两阶段经验模型可以较好地实现将地面数据扩展到中尺度空间范围,从而完成数据空间尺度的转换,提高大区域草地植被盖度的测量精度;2)MODIS遥感影像数据,结合地面数据和ASTER遥感影像数据可以较好地在区域范围内对北方典型草原的植被盖度进行估测;3)目前常用的亚像元分解模型,应用于中空间分辨率的MODIS影像,估测北方温度典型草原植被盖度的精度不够理想。

Multifunctionality and Thresholds of Alpine Grassland on the Tibetan Plateau

Ecosystems can simultaneously provide multiple functions and services. Knowledge on the combinations of such multi-dimensional functions is critical for accurately assessing the carrying capacity and implementing sustainable management. However, accurately quantify the multifunctionality of ecosystems remains challenging due to the dependence and close association among individual functions. Here, we quantified spatial patterns in the multifunctionality of alpine grassland on the Tibetan Plateau by integrating four important individual functions based on data collected from a field survey and remote sensing NDVI. After mapping the spatial pattern of multifunctionality, we extracted multifunctionality values across four types of grassland along the northern Tibet Plateau transect. Effects of climate and grazing intensity on the multifunctionality were differentiated. Our results showed that the highest values of multifunctionality occurred in the alpine meadow. Low values of multifunctionality were comparable in different types of grassland. Annual precipitation explained the large variation of multifunctionality across the different types of grassland in the transect, which showed a significantly positive effect on the multifunctionality. Grazing intensity further explained the rest of the variation in the multifunctionality(residuals), which showed a shift from neutral or positive to negative effects on multifunctionality across the different types of grassland. The consistently rapid declines of belowground biomass, SOC, and species richness resulted in the collapse of the multifunctionality as bare ground cover amounted to 75%, which corresponded to a multifunctionality value of 0.233. Our results are the first to show the spatial pattern of grassland multifunctionality. The rapid decline of the multifunctionality suggests that a collapse in the multifunctionality can occur after the vegetation cover decreases to 25%, which is also accompanied by rapid losses of species and other individual functions. Our results are expected to provide evidence and direction for the sustainable development of alpine grassland and restoration management.