藏北草地地上生物量及遥感监测模型研究

草地退化已成为藏北地区面临的主要生态环境问题。为了定量监测草地生物量和退化草地的生物量动态变化,利用2004 年8—9 月藏北地区草地地上生物量最大时期的地面实测数据,分析了其地上生物量大小和空间分布特征,在此基础上,结合同期的Terra MODIS植被指数数据,建立了草地地上生物量的遥感监测和估算模型。主要结论如下:①由于受高寒气候、土壤、水分等环境因素的限制,8—9 月藏北地区平均草地地上生物量较小,为96.88g/m²,其中绿色鲜草的比重在80%以上;不同区域不同草地类型地上生物量差异很大,范围在37.10~589.12 g/m²,平均而言,高寒沼泽化草甸的地上生物量最大,达356.84 g/m²,其次是温性草原(64.48 g/m²)和高寒草甸(61.61 g/m²),高寒草原草地最低,为48.87 g/m²;②基于MODIS NDVI的合成、生长型、指数函数、逻辑斯谛等4 个模型是估算藏北草地地上生物量的最优模型;③生物量的空间分布呈东南向西北减少态势,东南部部分地段在100 g/m²以上,西北部则在20 g/m²以下。

Aboveground Biomass in the North Tibet and Estimate Model Using Remote Sensing Data

Estimation of aboveground biomass (AGB) is necessary for studying productivity, carbon cycles, nutrient allocation, and fuel accumulation in terrestrial ecosystems. Remote sensing techniques allow scientists to examine properties and processes of ecosystems and their inter-annual variability at multiple scales since satellite observations can be obtained over large areas of interest with high re-visiting frequencies. In this study, AGB and its spatial distribution in the North Tibet are analyzed using in-situ measurements during August to September in 2004, and the regression models are established using in-situ AGB data and corresponding Terra MODIS NDVI and EVI. The results show that due to constraints of alpine climate, soil and moisture conditions, the average AGB in the North Tibet is very low, being 96.88 g/m². Of the total AGB, the dry matter content of fresh grass (hereafter referred to as fresh AGB) is above 80% and the dry matter content of dead material is below 20% . AGB raries greatly at different sampling sites and for different grassland types, ranging from 37.1-589.12 g/m². On an average, the largest AGB occurs in swampy meadow, being 356.84 g/m², followed by temperate steppe (64.48 g/m²) and alpine meadow (61.61 g/m²); the lowest value occurs in alpine steppe (48.87 g/m²). The optimum regression models for the North Tibet by using MODIS NDVI and EVI are compound, growth, exponent and power models, which can be used to estimate AGB and fresh AGB in the study area and four models have same performance. The efficiency of regression models based on MODIS NDVI or EVI for fresh AGB is better than for total AGB, which reflects unique spectral response of green plant. At spatial level, both AGB and fresh AGB decrease from the southeast to the northwest in the study area, reaching above 100 g/m² in some part of southeastern region while below 20 g/m² in northwestern region.