Impact of early heat anomalies on urban tree cooling efficiency: Evidence from spring heatwave events in India

Abstract

India’s heat waves have recently led to huge losses to the natural ecosystem and human society, and are projected to occur earlier and more frequently in the future. Frequent heat waves around the world in recent years highlight the emerging need for heat mitigation in sustainable urban development. Urban greening has been widely recognized as an effective nature-based solution to mitigate thermal stress. However, knowledge of how recent early heat anomalies affect the growth of urban trees and their cooling efficiency (CE) remains unclear. To inform this issue, here we compared the trees’ growth status and CE in 2022 spring heatwave period to that of the same period in 2019–2021 in New Delhi and Haryana, India as a natural experiment, to investigate the relationship between early heat anomalies and urban trees’ CE. Results showed that (1) the 2022 warm spring increased the greenness (reflected by Normalized Difference Vegetation Index-NDVI) of urban trees and advanced urban trees’ growing stage, but more warming effects of urban trees with higher coverage were observed, which is related to the water use. (2) Interannual pattern of the urban trees’ CE in 2022 was different from that of 2019–2021, which is attributed to the changing meteorological variables, i.e., air temperature, precipitation and wind speed from global model Climate Forecast System (CFS), especially the air temperature, and the biophysical responses of trees, using evapotranspiration (ET) as a proxy. We advocate planting more trees in areas with low tree cover percentage, equitable water usage within cities, and more effective thermal mitigation measures are expected to play a sustainable and significant role at the forefront of climate change’s worst impacts. Meanwhile, local trait database and field observation data are essential for the selection of urban trees adaptable and climatically useful in extreme heat events in Indian cities.