Response Of China's Regional Vegetation To Climate Change And Its Simulation Of Regional Climate Feedback

Vegetation is an important part of the terrestrial ecosystem,thus investigations of the vegetation-climate system over East Asia will help us to understand the future climate changes in this region.However,current results over East Asia are mainly based on regional climate models with static vegetation,which are lacking in vegetation-climate interactions.As a result,a series of numerical experiments,which is based on CLM-CNDV(Community Land Surface Model including representations of carbon-nitrogen dynamics and dynamic vegetation)and RCM-CLM-CNDV(CLM-CNDV coupled with Regional Climate Model Version 4),are conducted to investigate the future changes of vegetation-climate system in China during the1.5°C and 2°C warming periods,the impacts of dynamic vegetation model on regional climate simulations,the uncertainty of vegetation changes to the regional climate changes across China and the impacts of vegetation feedback on future climate changes.The main conclusions are summarized as follows:(1)The potential natural vegetation in China responses significantly to the 1.5°C and 2°C warming,and there will be more vegetation coverage and more vegetation primary productivity compared to the baseline period.The results show that the surface climate is projected to be warmer and wetter,and the proportion of bare soil will decrease gradually,meanwhile,the vegetation will be transformed into types that can survive in the warmer climate.For example,in Tibetan Plateau and Northeast China,the boreal vegetation decreases,and the temperate vegetation increases,while temperate deciduous vegetation changes into temperate evergreen vegetation in North China and Southeast China.Most areas are affected by rising temperatures and CO₂ fertilization,thus GPP will increase except for Northeast China.In contrast,the GPP of boreal forest is strongly affected by annual mean temperature in Northeast China.When temperature increases,the simulated GPP decreases significantly.Although GPP changes in most regions are not significantly affected by the temperature,the overall GPP and NPP of potential natural vegetation in China will increase at 1.5?and 2?warming,and the NPP of the potential natural vegetation ecosystem in China will increase by 13.54%and 22.12%compared to the baseline period.Compared with the 1.5°C warming period,vegetation at the2°C warming will continue to be more active and absorb more CO₂ from the atmosphere.(2)The coupling of vegetation dynamics will cause great effects in simulating regional climate,and there are regional and seasonal differences in the effects of vegetation dynamics on climate.In Northeast China and Tibetan Plateau,the wintertime net surface radiation increases as a result of lower surface albedo caused by more trees,less ground and less snow cover.At the same time,the evapotranspiration(ET)is constrained by water availability,resulting in a significant increase in sensible heat flux and near surface air temperature.However,the summertime albedo effect is weakened by the enhanced ET effect,resulting in an insignificant increase in near surface air temperature.In Southeast China,the increase of vegetation coverage leads to the increase of local ET,which means the acceleration of water cycle and an increase in cloud cover,resulting in more long wave radiation absorbed by the surface from the atmosphere.The feedback between precipitation,cloud cover and radiation make the surface temperature rise slightly.At the same time,due to the competition and compensation among soil evaporation,vegetation evaporation and vegetation transpiration,the change of ET is small,so the precipitation differences are quite small compared to the temperature differences.(3)In the RCP8.5 emissions scenario,the vegetation is becoming denser in most part of China,but there will be great uncertainty in Northeast China as a result of climate projection uncertainties from GCM and vegetation dynamics.Although the climates inherited from the driven GCM show great uncertainty,the differences of simulated vegetation in most areas are small,while the uncertainty of simulated PFT(Plant Functional Type)and future PFT changes in Northeast China are great.The main reason is that the temperature in Northeast China is close to the threshold for the growth and maintenance of some temperate and boreal trees:higher historical temperature in summer will increase the threat of heat stress on boreal trees,while lower future temperature in winter will limit the establishment of temperate trees,at the same time,higher future temperature in summer will cause higher heat stress,which may cause the increase of Arctic C3 grass and the decrease of either boreal or temperate trees.Therefore,the varieties in temperature will have significant impacts on vegetation in Northeast China.In spite of the differences in PFT changes among different simulations,the multi-model ensemble shows that the future LAI(Leaf Area Index)will decrease in Northeast China and increase in other regions under RCP8.5.(4)It is found that there are nonnegligible impacts of vegetation feedback on regional climate change.At the same time,as a result of the seasonality of vegetation feedback impacts on the daytime maximum temperature(T_max)and nighttime minimum temperature(T_min)across China,there are asymmetric changes in extreme climate events.For the Tibetan Plateau,the albedo effect in winter increases greatly due to the vegetation coverage changes,thus T_maxincreases significantly while T_min changes slightly.However,the albedo effects are offset by the evaporative cooling and cloud effect in the summer,resulting in little changes in summertime temperature.For the same reason,the reduction of vegetation coverage in Northeast China results in a significant decrease in wintertime T_max.In North China,only T_mindecreases a little due to the different partitioning in net radiation between day and night.While the T_max and T_min are both alleviated because it is warm and humid in Southeast China thus the evaporative cooling and cloud effect can offset the increase in net surface radiation caused by the reduction in albedo even in the winter.These asymmetric changes in temperatures will further lead to different extreme events changes.It is also found that the impacts of vegetation feedback are increasing with the RCP scenarios because vegetation changes with climate changes.