Differentiated influences of atmospheric dryness on urban plant cooling effect between temperate and tropical/subtropical zones

Atmospheric vapor pressure deficit (VPD) is a critical variable related to water flux that may either promote or limit plant evapotranspiration under different conditions, consequentially, influencing the effect of urban vegetation on mitigating urban heat island. However, how VPD influence the cooling effect of urban vegetation in different climate zones, have not been examined to date. Here, we combined field observations of 3634 samples from the global published literature and satellite data to investigate the spatial-temporal impacts of VPD on vegetation cooling and reveal the pathways of “atmospheric dryness-evapotranspiration-climate effect”. Our results showed that the cooling effect was generally remarkable in urban areas in the humid season of the tropical/subtropical zones and in the warm season of temperate zones. In the tropical/subtropical zones, the cooling effect was enhanced with increasing VPD across the range of low VPD, but was obviously constrained by high atmospheric dryness when VPD exceeded 9.43 hPa. In contrast, the cooling effects of urban vegetation were gradually strengthened as VPD increased in temperate zones. Notably, tropical and subtropical urban vegetation began to exert a great cooling effect when evapotranspiration per unit leaf area was >31.93 mm/month. Our findings suggest that stronger constraints of atmospheric dryness on cooling effect in the tropical/subtropical zones versus the temperate zone. These findings provide important insights for understanding the patterns of urban vegetation response to atmospheric dryness. They can also assist managers and decision-makers in optimizing the design of urban green space in different geographic locations.