Seasonal dynamics of aboveground biomass of typical grassland typeson the Tibetan Plateau

A study of four major grassland types (alpine meadow, alpine steppe, temperate steppe and alpine swamp meadow) in central Tibetan Plateau was carried out in 2004. And the seasonal variations of abovegraund biomass (AGB) and growth characteristics were analyzed. Results showed that the highest AGB occurred in fenced swamp meadow with annual mean AGB of 384.45 gm-2, which was 6 times of temperate steppe and 12~14 times of alpine steppe and alpine meadow, and it was also obviously higher than unfenced alpine swamp meadow with 73%. Compared with free choice grazing, enclosure obviously increased AGB and was one of the most effective approaches to improve degraded grassland. The production of temperate steppe was higher than alpine meadow and alpine steppe, and the grassland AGB near urban areas was greater than areas away from urban region, and urbanization was an effective way to reduce the intensity of grassland use and to restore grassland productivity. As semiarid temperate climate zone, soil moisture was the main factor to constrain grassland vegetation growth in Tibet. Absolute growth rate (AGR) and relative growth rate (RGR) of four grassland types were positive before reaching to apex of the vegetation growth and negative after apex. Due to differences in hydrothermal conditions and their seasonal variations results in different fastest growing period of AGB for four grassland types.