Divergent Drivers of Declining Urban Vegetation Productivity and Transpiration During Heatwaves

Abstract Heatwave events, characterized by high vapor pressure deficit (VPD) and low soil moisture (SM), considerably disrupt the regional carbon and water cycle. However, research pertaining to the impact of heatwaves on vegetation productivity (VP u ) and vegetation transpiration (VT u ), along with their underlying drivers, particularly in urban areas, remains limited. This study investigates the response of VP u and VT u to heatwave events across 895 global cities from 1990 to 2022. The analysis reveals a notable upward trend in the average heatwave frequency, intensity, and duration across the global cities. Heatwave events demonstrate a detrimental impact on VP u and VT u , resulting in an average decrease of 28% and 26%, respectively, during heatwave occurrences. The attribution analysis reveals divergent driving factors for the decline in VP u and VT u during heatwaves. The decrease in VP u is primarily influenced by SM, contributing 60% to the downward trend of VP u during heatwaves. Notably, VP u displays a sharp downward trend when SM falls below 0.38 m 3 /m 3 . In contrast, the primary driver of VT u decline is VPD, contributing more than 66% to the downward trend of VT u during heatwave events. VT u exhibits a significant downward trend when VPD exceeds 1.35 kPa. The results of this study show the important effects of increasingly frequent heatwaves on vegetation transpiration and productivity, and that can be used as quantitative factor to be evaluated when investigating policy measures for the resilience of urban areas.