Effects Of Drought And Heat Wave On Vegetation Greenness And Productivity In The Yangtze River Basin

Terrestrial ecosystems absorb atmospheric CO₂ through plant photosynthesis and store about25%of CO₂emissions from human activities every year.It is not only the main carbon sink of the earth system,but also an important way to slow down the rate of climate warming.As a typical extreme climate event,drought and heat wave may produce serious water and heat stress,resulting in the weakening of photosynthetic carbon absorption of vegetation and even large-scale extinction,and then change the biophysical and chemical processes such as surface energy balance and terrestrial carbon sink,produce positive feedback on climate warming and aggravate the rate of climate warming.However,the destructive power of extreme climates such as drought and heat wave to the structure and function of terrestrial ecosystem depends on the variability of extreme climates on the one hand and the adaptability or tolerance of ecosystem on the other hand.Therefore,different regions or types of vegetation may have different response processes to drought and heat wave,but at present,this response and process have not been fully revealed.Taking the Yangtze River Basin as the research object,based on the long-term historical observation climate data,the hydrological data simulated by Noah land surface model,the leaf area index（LAI）retrieved by multi-source satellite remote sensing and the vegetation productivity（GPP）simulated by photosynthetic model,this study analyzes the basic characteristics of climate and hydrological changes in the Yangtze River Basin,and resolves the temporal and spatial changes and causes of drought and heat wave in the basin,and explores the effects and possible mechanistic processes of drought and heat wave on LAI and GPP of basin’s vegetation.The main conclusions are as follows:（1）The Standard Precipitation Evapotranspiration Index（SPEI）was selected to characterize the dry and wet changes of the basin.The results showed that there was no significant change trend of SPEI,but there was a large interdecadal variation;In general,SPEI showed an increasing trend before 1997/1998,from drought to humidity,then showed a decreasing trend,and then from humidity to drought.The defined Cumulative Heat Wave Index（CHWI）increased significantly at the watershed scale and all vegetation types,indicating that the heat wave intensity in the whole region increased significantly.Based on EOF analysis,the temporal and spatial variation of SPEI and CHWI in the Yangtze River Basin is analyzed,and the synthetic analysis is used to reveal the atmospheric anomalies corresponding to the spatial main modes of SPEI and CHWI in the Yangtze River Basin.Through stepwise regression,the relationship between the changes of different modes of SPEI and CHWI and large-scale climate anomalies is established.The results reveal that the large-scale climate anomalies lead to the changes of water vapor input in different regions of the Yangtze River Basin by causing the anomalies of the location,intensity and area of the Western Pacific subtropical high,and then it causes the change of spatial pattern of drought and heat wave.（2）The LAI and GPP of vegetation in the Yangtze River Basin have increased significantly since 1980s,indicating that the vegetation in the Yangtze River Basin has become"green"as a whole,the productivity has increased and the ecology has improved as a whole.On the whole,LAI and GPP are not sensitive to the variation of SPEI and CHWI,which shows that there are few areas and vegetation types that reach a significant correlation level.However,through 9-year sliding window regression analysis to test the changes of vegetation LAI and GPP sensitivity to SPEI and CHWI(LAI^SPEI/LAI^CHWI and GPP^SPEI/GPP^CHWI),it is found that different types of vegetation LAI^SPEI/LAI^CHWI and GPP^SPEI/GPP^CHWI have changed greatly（both positive and negative effects）since 1980s,resulting in no significant impact of SPEI and CHWI on LAI and GPP variation in a long time scale,It masks the effects of drought and heat wave on vegetation growth.Specifically,LAI^SPEI and GPP^SPEI in most forest ecosystems experienced a significant reduction process,indicating that the limiting effect of drought on the increase of LAI and GPP was weakened;The LAI^SPEI and GPP^SPEI of savanna and dryland showed a significant increasing trend,which had changed from negative correlation to positive correlation,which meant that the limiting effect of drought on the increase of vegetation LAI and GPP was increasing.Except for grassland LAI^CHWI and GPP^CHWI,LAI^CHWI and GPP^CHWI of other types of vegetation have experienced the transformation process of first increasing and then decreasing.（3）The regulation effects of temperature and precipitation on the changes of LAI^SPEI and LAI^CHWI of vegetation are tested.The results show that except that LAI^SPEIand LAI^CHWI of grassland（GRA）types continue to increase with the increase of temperature,LAI^SPEI and LAI^CHWIof other types of vegetation show a response curve of first increasing and then decreasing,indicating that heating within a certain temperature range is conducive to enhancing the sensitivity of vegetation to drought（negative effect enhancement）and heat wave（positive effect weakening）,However,exceeding a certain temperature will reduce the sensitivity of drought（positive effect enhancement）and heat wave（negative effect enhancement）;With the increase of precipitation,except that LAI^SPEI and LAI^CHWI of deciduous broad-leaved forest decrease with the increase of precipitation,LAI^SPEI and LAI^CHWI of other types of vegetation increase with the increase of precipitation,indicating that the increase of precipitation enhances the negative effect of drought and weakens the negative effect of heat wave.（4）A three-dimensional Vine Copula model based on temperature stress factor,water stress factor and GPP in the Yangtze River Basin is constructed to analyze the conditional probability of GPP damage under different dry and heat conditions.The results show that for most areas of the Yangtze River Basin,the impact of temperature stress on GPP is generally greater than that of water stress,but different vegetation types have different responses to these two stresses.Shrubland and grassland vegetation are more vulnerable in low temperature areas,while sparse forest land and farmland ecosystem are more vulnerable in high temperature areas;In terms of spatial distribution,the GPP damage probability of the source area of the basin and the Yunnan Guizhou Plateau in the southwest of the basin are higher than the other area of the basin,which needs the according governments to paid more attention to;With the deepening of heat wave,the probability of vegetation GPP damage increases,and about 5%of the area is seriously affected（the damage probability increases from 10%to 20%-50%）.Similarly,with the deepening of drought,the probability of GPP damage increases from 10%to more than 20%in about 20%of areas（the largest area rises to 80%）.The study reveals the change process of the overall improvement of vegetation（LAI and productivity increase）under the background of general warming in the Yangtze River Basin.By changing the hydrological cycle process（ET,Runoff,SM）and the resulting cooling effect,the risk of vegetation carbon absorption reduction caused by drought and heat wave is reduced.Therefore,the implementation of ecological protection projects,maintaining ecosystems,especially forest ecosystems,maintaining high vegetation coverage and reducing the damage of human activities to ecosystems are the key to improve the resistance of ecosystems to drought and heat waves,maintain the stability and development of ecosystem service functions,and increase the carbon sink capacity of the Yangtze River basin and even China’s terrestrial ecosystems.