Productivity In The Qinghai-Tibet Plateau During The Past 34 Years And Analysis Of Its Local Degradation Characteristics

Grassland is the main vegetation of the Qinghai-Tibet Plateau,which plays a vital role in water conservation,soil and water conservation,energy flow and so on.Due to its unique geographical location and extreme climatic characteristics,the grassland on the Qinghai-Tibet Plateau is quite sensitive to climate change and human activities.Once the grassland in this region is degraded,it will be difficult to recover.In this paper,the Carnegie-Ames-Stanford Approach（CASA）model was used to estimate the net primary productivity（NPP）of grassland on Qinghai-Tibet Plateau from 1982 to 2015,and the method of piecewise linear regression was used to study its variation.At the same time,using the design concept of block experiment and degradation gradient experiment,this study selected 3 grasslands with different degradation degrees caused by overgrazing（3 degradation gradients）and compared the characteristics of vegetation height,coverage,biomass and soil physical and chemical properties in severely,moderately,mildly and undegraded grasslands.It was found that:（1）The grassland NPP of Qinghai-Tibet Plateau estimated based on CASA model has shown an overall upward trend in the past 34 years(mean 1.0 g·m^-2·yr^-1,regression analysis slope),and only a few showed a significant downward trend（365/21724 grid cells,about 1.7%）.（2）Piecewise linear regression analysis showed that the rising trend of NPP of vegetation on Qinghai-Tibet Plateau from 1982 to 2015 was significantly inhibited around 1996（that is,the average value of turning points was 1996）.The main performance is that the turning points in the central part of Qinghai-Tibet Plateau are concentrated around 1993,while the growth rate of grassland NPP in the northern part is relatively lagging behind（latter than 2000）.（3）The results of the degradation gradient transect survey showed that the species richness of the mildly degraded grassland was the highest,and the plant species richness and Shannon index decreased significantly with the increase of degradation.（4）Soil organic carbon content in the surface layer（0-10 cm）decreased with the deepening of degradation gradient.It had no significant difference in total nitrogen content among different degradation gradients.With the deepening of degradation gradients,soil nitrate nitrogen and ammonium nitrogen,soil total P and available P all showed a trend of first decreasing and then increasing.The total potassium content showed no regular trend with the degree of degradation,while the available potassium content first increased and then decreased with the degree of degradation.The change of soil p H was not significant in all degradation gradients.（5）With the aggravation of degradation,the biomass of grasses decreased significantly,while that of miscellaneous grasses increased significantly,and the proportion of aboveground biomass decreased significantly.To sum up,the grassland vegetation activities on Qinghai-Tibet Plateau,characterized by grassland net primary productivity,are generally improving on the whole region,but local degradation occurs to a certain extent,nevertheless,NPP growth slows down around 1996.Grassland degradation mainly shows the degradation of grassland plant biomass,the main trend of biomass distribution from aboveground to underground and the community dominated gradually changes from grasses to forbs.