Responses Of Ecosystem Carbon Cycle To Drought In Southwest China

Drought disaster has become increasingly prominent in China under the effect of climate warming and human activities.Drought,as one of the most important stress factors affecting the carbon sink function of terrestrial ecosystems,will not only weaken the carbon sink function of terrestrial ecosystems at regional and continental scales,but also make it become carbon source.As an important indicator of carbon revenue and expenditure of ecosystem,net ecosystem productivity(NEP)can quantitatively describe the carbon source/sink function of terrestrial ecosystem.It can be calculated by net primary productivity deducting soil heterotrophic respiration.Therefore,it is of great practical significance to study the variation characteristics of NEP in terrestrial ecosystems under the influence of drought.Which is helpful for in-depth understanding of ecosystem carbon cycle process,regional carbon sink management,rational allocation of water resources.As an important ecological barrier and main agricultural production area in China,the Southwest china has suffered serious ecological problems such as soil erosion,land desertification and vegetation degradation,and drought events occurred frequently in recent years under the influence of human activities and climate change.Therefore,based on the SPEI index,the frequency,intensity,influence range and period of drought events in the study area were analyzed.The effectiveness of solar-induced chlorophyll fluorescence(SIF)and normalized difference vegetation index(NDVI)in the ecological stress monitoring of regional drought has also been further studied.Then,The net ecosystem productivity(NEP)were calculated based on Carnegie-Ames-Stanford Approach(CASA)terrestrial ecosystem model and soil heterotrophic respiration model,and it was used as the index to study the carbon cycle of terrestrial ecosystems.On this basis,the response of carbon cycle of different vegetation ecosystems to two types of drought events(long-term/short-term)in southwest China was quantitatively analyzed,and the impact of drought events on carbon sinks of typical vegetation ecosystems under different climatic zones and the difference of vegetation resilience after drought were further evaluated.The main conclusions are as follows:(1)The SPEI indexes at different time scales in Southwest China showed a slight increase trend from 2000 to 2018;Seasonal,drought trend of spring and summer is obvious,autumn and winter showed a trend of wetting;Spatially,the frequency and intensity of drought in eastern Yunnan and central Sichuan are more serious;The influence range of drought in the study area showed a slow downward trend in the past 19 years(the downward trend rate was-0.004/10a);The SPEI index in southwest China has two periodic changes in time scales of 4-5 a and 2-3 a;Compared with NDVI,the negative SIF anomaly caused by drought is more significant.(2)From 2000 to 2018,the overall vegetation NEP in Southwest China showed a fluctuating increase trend;The annual variation of NPP,RH and NEP roughly showed a mono-peak curve;Spatially,the carbon sink in some regions of Guangxi Zhuang Autonomous Region has an obvious increase trend;Seasonally,the average NEP values ranked as summer(105.35 g C·m-2)>autumn(39.61 g C·m-2)>spring(24.62 g C·m-2)>winter(-3.10 g C·m-2);The averaged NEP values of different vegetation types ranked as broadleaf forest>Croplands>meadow>grassland>Needleleaf forest>Shrublands;The carbon sink capacity of the ecosystem is the highest when the regional altitude is less than 200 m;On the monthly scale,the NEP values of forests and farmlands in Southwest China are greatly affected by temperature,and precipitation has a great influence on the NEP of grassland ecosystem;At the annual scale,the NEP of regional vegetation mainly positively correlated with solar radiation,and the spatial distribution of the correlation between temperature and precipitation has strong heterogeneity.(3)The SPEI index was positively correlated with vegetation NEP in most areas of southwest China;SIF,NDVI and vegetation NEP showed a significant linear correlation(P<0.01).The linear fitting R2 of NEP and SIF in other vegetation ecosystems were higher than NDVI except needleleaf forest and meadow ecosystems;Shrublands,croplands ecosystems have the longest recovery period(6 months)and meadow have the shortest recovery period(3 months);The carbon sink function of grassland is mainly affected by long-term drought stress,while meadow are susceptible to short-term drought;The needleleaf forest,shrublands,grassland in the plateau climate have strong carbon sink capacity.however,drought will lead to the delay of growth period of needleleaf forest and shrublands in subtropical climate area for about 2 months.