| Understanding the impact of urbanization on vegetation can predict the effect of global environmental and climate changes in the future,since urban environmental changes are considered to be the precursors of future global changes.In the context of rapid environmental change,the interactions among various driving factors change gradually over time,which have a significant impact on vegetation growth.Previous studies were usually limited to discussing the influence of interaction changes between two or several factors on vegetation growth.Therefore,under the background of global change,the long-term effects and interactions of multiple global change drivers and the adaptation strategies of plants to them remain unclear.Given the circumstances this study quantify the impact of urbanization on vegetation growth,which were subdivided into direct(the replacement of vegetation with impervious surfaces)and indirect impact(urban environment),and analyze the change of impact over time,it contributes to a further understanding of the response of vegetation growth to environmental change and provides an opportunity to predict the dynamics of altering climate and environmental impacts in the future.Using Changsha and its nine administrative districts with different average urban intensity as a case study,this paper comprehensively explore how the effects of urbanization on vegetation growth varied over years.The main conclusions are as follows:(1)The trend of between EVI along urban intensity gradient in Changsha and its nine administrative regions from 2000 to 2017 statistically conforms to cubic regressions fitting for all years(p<0.001).Generally,the EVI decreased with urbanization intensity,and the observed EVI values were mostly higher than the zero-impact lines(urbanization has no indirect effect on vegetation growth),indicating that urbanization enhanced vegetation growth.This growth enhancement can offset the partial loss of vegetation productivity resulting from the replacement of vegetation with impervious surfaces.However,regional differences are also shown with different urbanization levels,such as the background EVI value,the proportion above the zero-impact lines,and the scattering of EVI observations on the intensity gradient increases with the decrease of urban area.(2)EVI enhancement caused by indirect impact increased along urban intensity,and the trend accelerates over years as shown by the increase of the slope from 0.09 in 2000 to 0.17 in 2017.This indicate that between EVI enhancement caused by the indirect effect of urbanization and urban intensity was not stable across years but rather demonstrated a clear pattern of temporal evolution.(3)The offsetting effect of indirect(urban environment)positive effects on direct(the replacement of vegetation by impervious surfaces)negative effects became stronger over time.The offset values increased with urban intensity gradient and reached a peak when urban intensity was in 0.8-0.9,which showed that the proportion of indirect influence reached maximum in the total impact of urbanization on vegetation growth.The maximum offset values have been increasing linearly over year from 28.1%(2000)to 43.6%(2017)indicating that indirect effects on the total impact of urbanization on vegetation growth are increasing in proportion.(4)The annual average EVI values of the core area(urban intensity in 0.99-1)in the nine regions all gradually converges towards 0.22(stabilizing)over years,implying that the promotion of vegetation growth by urbanization becomes saturated once EVI in the urban core areas exceeds the threshold. |
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