Responses Of Vegetation Spring Phenology To Preseason Drought And Warming In The North China Plain

The impact of global climate change on terrestrial ecosystems has been a hot topic in ecology and environmental science.Global-change-type drought,a combination of drought and warmer temperatures,is projected to have severe effects on vegetation growth and the global carbon cycle.Vegetation phenology is highly sensitive to climate change,and is an important biological index to understand the response of vegetation growth to climate change.Climate warming has significantly advanced the start of spring phenology of vegetation in the Northern Hemisphere.Such climate-driven changes in vegetation phenology can have a significant impact on the structure and function of terrestrial ecosystems by regulating the exchange of carbon,water and energy.However,the differences in the effects of preseason drought and warming on spring phenology of different vegetation types are still unclear.It is of great significance to investigate the response of spring vegetation phenology to preseason drought and warming for predicting future vegetation phenology changes under future global-change-type drought.Here,we extracted the start of growing season(SOS)from GIMMS NDVI data using Spline-midpoint,HANTS-Maximum,and Timesat-SG methods in the North China Plain over the period 1982-2015,and then used linear regression to estimate the temporal trends of SOS at each pixel.Based on SPEI,we investigated the effects of preseason drought on SOS,and quantified the differences in drought response of forest,grassland,wheat,and other crops.Then,based on the results of the influence range of preseason minimum temperature(Tmin),maximum temperature(Tmax)and mean temperature(Tmean)on the SOS,multiple linear regression was used to compare the differences in temperature sensitivity of spring phenology of forest,grassland,wheat,and other crops.Lastly,in order to discuss the coupling effects of preseason drought and warming on spring phenology,the study years were divided into high temperature and drought,high temperature and no drought,low temperature and drought,and low temperature and no drought at each pixel,and the SOS differences under different climate conditions were compared.The main results are as follows:(1)There were great differences in the magnitude of the SOS advanced trend among different vegetation types in the North China Plain.In general,results showed a trend of advanced SOS in 81.7%of pixels in the North China Plain from 1982 to 2015,with an average rate of-0.5 days/yr.At the biome level,the spring phenology of crops displayed a more rapid advancement than natural vegetation.Wheat SOS showed the largest advanced trend,followed by other crops,forests and grassland.(2)The SOS was mainly negatively correlated with preseason SPEI in the North China Plain,indicating that frequent preseason droughts could delay the onset of spring phenology.The response of spring phenology to preseason drought was different among different vegetation types in the North China Plain.Generally,the effect of preseason drought on SOS of natural vegetation is greater than that of crops.SOS of wheat is the most resistant to preseason drought,followed by other crops,forests and grasslands.(3)In the North China Plain,the effect of preseason Tmin on SOS was greater than that of Tmax and Tmean.The sensitivity of SOS to Tmin was significantly different among different biomes in North China Plain.In general,SOS of crops is more sensitive to preseason temperature than that of natural vegetation.Multiple regression analysis showed that a 1? increase in Tmin would advance the onset of spring phenology by 10.5,7.6,2.9,2.1 days for wheat,other crops,forests,and grasslands,respectively.(4)The SOS in high temperature and drought years was significantly later than that in high temperature and no drought years in the North China Plain,but it was significantly earlier than that in low temperature years.Considering the coupling effects of preseason drought and warming on spring phenology,the future continuous warming could trigger earlier spring phenology,but frequent droughts may slow down this trend.(5)The response of spring vegetation phenology to preseason drought and its temperature sensitivity were nonlinear along humidity gradient in the North China Plain.This nonlinear relationship indicated that the existence of a threshold beyond which improved humidity conditions would no longer contribute to vegetation's resistance to climate change.In summary,there are differences in SOS advanced trend among different vegetation types in the North China Plain.The SOS of grassland has the least resistance to drought,and increased drought may have a wider range of effects on grassland spring phenology.Wheat is the most sensitive biome to preseason warming,which highlights the need to improve the heat tolerance of wheat varieties for food security under future climate change.Under the future global-change-type drought,continued warming will trigger earlier spring foliage,but increased frequency and intensity of droughts may slow this trend.This study provides a scientific basis for the specific and non-linear responses of vegetation to climate change in phenological models,and is of great significance for monitoring terrestrial carbon dynamics and ecosystem management.