Analysis On The Evolution And Driving Force Of Grasslands Aboveground Biomass Carbon In Northwestern China Under Changing Environment

Grassland is the main vegetation type in Northwestern China,and it plays an important role in carbon cycle.Northwestern China is an arid and semi-arid region with fragile ecological environment and extreme sensitivity to climate change.In recent decades,the climate in Northwestern China has changed from warm-dry to warm-wet,and the influence of human activities has been increasing.Therefore,the quantifying the impacts of climate change and human activities on grassland carbon stocks is one of the hot scientific issues.The aboveground biomass carbon?ABC?is an important parameter for grassland ecosystem function.Under the background of changing environment,the research on grassland ABC is helpful to evaluate the role of grassland ecosystem in global terrestrial carbon cycle and the relative contribution of different driving factors to grassland ecosystem in Northwestern China.In this paper,based on the data of ABC from 1993 to 2012 obtained by satellite and LPJ?Lund-Potsdam-Jena?model and corresponding meteorological data,the spatial and temporal distribution of grassland carbon storage was studied in Northwestern China.Moreover,the influence of human activities and climate change on the change of grassland ABC was separated using the residual analysis method.The main conclusions are as follows:?1?According to the spatial distribution characteristics,the distribution of grassland ABC in Northwestern China presented spatial heterogeneity,and it decreased from northwest to southeast as a whole.The average value of grassland ABC by remote sensing was 53.62 g C·m^-2.The average annual carbon density of the grassland ABC simulated by LPJ model was 68.85 g C·m^-2,which was higher than the value estimated by remote sensing.According to the characteristics of temporal change,the grassland ABC had been increasing in the past 20 years.Among the six grassland types,the fluctuation ranges of the ABC were different,and the ABC of desert steppe,typical steppe,meadow steppe,temperate meadow and alpine meadow showed no significant change trend.However,the ABC in the alpine steppe increased significantly.?2?On the whole,the grassland ABC was positively to the changes of precipitation whereas it was negative to the temperature in Northwestern China.There were differences in the response of different grassland types to climate change.Only alpine steppe and alpine meadow had positive correlation between ABC and temperature,and desert steppe and typical steppe had significant positive correlation between arid grassland ecosystem and precipitation.When the relative temperature changes to 2?,-1?,1?,2?,the ABC of desert steppe,typical steppe,meadow steppe and temperate meadow decreases with the increase of temperature,while that the ABC of alpine steppe and alpine meadow increases with the increase of temperature.When the relative changes of precipitation were-20%,-10%,10%and 20%,the ABC of all grassland types increased with the relative changes of precipitation.?3?The cumulative curve of temperature,precipitation and grassland ABC showed consistent growth changes before 2000,while the annual cumulative curve of grassland ABC was generally higher than that of temperature and precipitation after 2000.The difference between the two factors indicated that human activities had a positive effect on the grassland ABC in Northwestern China.In general,grassland area had been deteriorating continuously in the past 20 years,and the trend of grassland degradation was much slower after 2000 in Northwestern China.From 1990 to 2000,grassland area in Northwestern China decreased by 16,783 km²,and from 2000 to 2010,grassland area in Northwestern China decreased by 8,408 km².Since 2000,a series of ecological projects had played a very positive role in land use change.Through afforestation and grass planting in accordance with local conditions,vegetation greening trends are increasing all the time in Northwestern China.?4?By constructing a calculation and evaluation method for the relative effects of climate change and human activities under various circumstances in the grassland change area,the residual analysis method was used to quantitatively analyze the impact of human activities and climate change on the grassland ABC in Northwestern China.During the period from 1993 to 2012,there was a slight increase in the grassland ABC.Due to the restoration of grassland vegetation,the grassland ABC increased by 49.22 Tg C in Northwestern China.Due to grassland degradation,the ABC decreased by 32.51 Tg C.All in all,from 1993 to 2012,the ABC increased by 16.71 Tg C in Northwestern China.?5?Quantitative analysis of the relative role of different driving factors in the change process of grassland ABC in Northwestern China in different periods from 1993 to 2000 and from 2001 to2012.The results showed that during 1993-2000,the change trend of the grassland ABC was mainly degradation in Northwestern China,and the grassland ABC decreased by 15.86 Tg C.From2001 to 2012,the change trend of grassland ABC was mainly recovery in Northwestern China,and the grassland ABC increased by 31.57 Tg C.During 1993-2000,due to unreasonable grazing and other human activities,there was a trend of degradation of the grassland ABC.With the implementation of ecological engineering after 2000,the grassland ABC increased from 2001 to2012.