Abstract:

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.

Key words: climate change, alpine meadow, NDVI, grassland biomass, unimodal pattern