Differences In The Response Of Vegetation To Extreme Drought And Long-term Drying In Southwest China

In the context of rapid global warming,the research on the response of vegetation to climate change is a key issue.On the one hand,the vegetation has a wide greening trend on the global scale,namely,the vegetation greenness index has an interannual increasing trend.On the other hand,drought and extreme events have a serious inhibition effect on vegetation growth.Therefore,it is necessary to analyze the response of vegetation to climate change from the perspectives of short-term extreme events and long-term climate trends.Southwest China is located in a humid region with abundant natural resources,but its ecosystem is sensitive to climate fluctuation and human activities.In particular,there have been serious drought events in this area,which caused a more serious impact on vegetation.In addition,climate change in this region also has an obvious trend.Therefore,this study selected five provinces and cities in Southwest China as the study area to study the response of vegetation in this region to short-term extreme events and long-term climate change,especially whether extreme drought events are enough to affect the long-term change trend of vegetation.Temperature and precipitation data were obtained from grid data set of China Meteorological Administration(CMA)based on station observation interpolation,and standard precipitation evapotranspiration index(SPEI)was calculated,one,six,and twelve month scale SPEI indices were selected in this study.Vegetation index Leaf Area Index(LAI)and Enhanced Vegetation Index(EVI)were selected to study vegetation status.The study period was 1982-2016,and the extreme drought period was 2009-2010.The interannual,growing season and seasonal variation characteristics and spatial distribution characteristics of climate in Southwest China were calculated and analyzed.By selecting severe drought events in Southwest China from September 2009 to May 2010 for typical analysis,the spatial and temporal variation characteristics of values and anomalies during LAI and EVI severe drought in Southwest China were analyzed to clarify the characteristics of vegetation response to short-term drought events.Further,the long-term change trend of vegetation was studied based on LAI and EVI data.Finally,the correlation between LAI and meteorological factors and multi-scale SPEI was analyzed to study the influence degree of long-term climate change on LAI in Southwest China.The research results obtained in this paper show that:(1)During the study period,Southwest China was dry on the whole,especially in the border region of Yunnan,Guizhou and Sichuan provinces.The SPEI01,SPEI06 and SPEI12indexes in these regions were all negative,indicating drought state.Since 1982,the temperature increased significantly in Southwest China,with a temperature growth rate of0.032°C yr^-1.The temperature increased most in spring,followed by autumn,and the temperature increased least in summer.On the contrary,the annual precipitation in Southwest China decreased during the study period,with a decreasing rate of-1.028 mm yr^-1(p<0.1).The drought indices of SPEI01,SPEI06 and SPEI12 all showed a downward trend on the whole,with the annual decreasing rates of-0.006 yr^-1,-0.011 yr^-1 and-0.015 yr^-1,respectively,and all passed the significance test of 0.1,indicating that Southwest China was in a state of long-term drying.In terms of the spatial climate change trend,the temperature in Southwest China is on the rise,and the rate of increase is increasing from east to west,while the precipitation is decreasing from southeast to northwest.During the study period,SPEI01,SPEI06,and SPEI12 decreased in most areas of Southwest China,especially Sichuan,Chongqing,and Yunnan,indicating that Southwest China became drier during the study period.The drought index increased slightly only in the eastern part of Guizhou and the southern part of Guangxi,indicating that a small part of the region became wetter,but this did not affect the long-term drying trend of the whole Southwest China.(2)During the period of extreme drought in Southwest China(September 2009 to May2010),the SPIE01 value in Southwest China reached the lowest value of-1.605 in February2010,which was the most severe drought in Southwest China and reached the extreme drought level.Vegetation in the same period did not suffer the most serious damage and LAI value did not reach the lowest value.It shows that in extreme drought events,vegetation was inhibited by drought,but did not cause devastating damage to vegetation.The time change curve of EVI shows that in extreme drought events,vegetation is indeed affected by drought,and the growth trend of vegetation temporarily stagnates.But as the drought continued,the vegetation continued to grow unimpeded and the Browning trend did not last long.In February 2010,the whole region of Southwest China was very dry,and even a severe drought occurred at the junction of Yunnan,Guizhou and Guangxi provinces.However,LAI anomaly in that month was not the minimum value,and the LAI anomaly values in Yunnan,Guizhou and Guangxi were all positive.So the response of vegetation change to drought is out of sync and is likely to have been influenced by last month's drought.During the most severe drought,the vegetation was affected by the drought,and there was a downward trend in vegetation change,but the downward trend did not last for long.(3)Based on the comprehensive analysis of the response of vegetation to short-term extreme drought events and long-term climate drying trend in Southwest China,we found that under the influence of extreme drought events,vegetation was temporarily damaged,but the extreme drought events had little impact on vegetation in this region.However,the long-term greening of vegetation in Southwest China was not affected by short-term drought events.Therefore,the response of vegetation to drying events at different time scales is quite different.During the study period,LAI and EVI values in Southwest China increased,showing a significant upward trend on the whole,with rates of 1.034×10^-2 m² m^-2 yr^-1 and 3.451×10^-3yr^-1,respectively,indicating a significant greening in Southwest China.The long-term greening trend of vegetation was not affected by extreme drought events.During the study period,LAI and EVI increased in the whole territory of Southwest China,but the whole region turned green.The correlation coefficient between LAI and various meteorological factors and SPEI was calculated.During the study period,the correlation coefficient between vegetation and temperature in Southwest China was the largest and the correlation was very significant positive.The correlation coefficient between annual average LAI and annual average temperature in Southwest China is 0.71(p<0.1),indicating that temperature increase is conducive to vegetation growth in Southwest China,so vegetation growth in Southwest China is mainly affected by temperature.