Phenology acts as a primary control of urban vegetation cooling and warming: A synthetic analysis of global site observations

Urban vegetation can influence local air temperatures through its biophysical effects on surface energy balance. These effects produce gradients (Delta T-a) between air temperature of vegetation spaces (T-veg) and air temperature of open spaces (T-open) (Delta T-a = T-veg - T-open), hereafter referred to as vegetation cooling (negative values of Delta T-a) and warming (positive values of Delta T-a), respectively. But vegetation cooling or warming highly depends on background climate of urban areas as well as on vegetation states. Field observations are usually restricted to one or few cities, setting limitations to a general understanding. In this study, a synthetic analysis of 3634 point-scale in-situ observations from 77 global sites in 35 cites was conducted using the bootstrap sampling and hierarchical partitioning methods. Results show that vegetation cooling is generally stronger during the daytime periods, in warm seasons, at low latitude zones, for forest lands and at leaf growth stage, while vegetation warming usually occurs in the opposite contexts. Urban vegetation begins to exert considerable cooling effects when the daily mean background air temperature (BAT) is >10.0 degrees C, but on average has a slight warming effect when BAT is <10.0 degrees C. Besides, vegetation cooling increases sharply when evapotranspiration is >61.7 mm/month or when area of urban vegetation is >35.2 ha. Plant growth stages (i.e., canopy leaf growth, senescence and dormancy stages) (37.6 +/- 0.11%), a vegetation phenology proxy, acts as the primary biotic factor, while seasonality (23.0 +/- 0.11%) and latitude (11.4 +/- 0.07%) that control the background climate are two most important abiotic contributors. Our findings suggest approximate thresholds for distinguishing vegetation cooling/warming effects and provide helpful information for future urban greenspace planning aimed at mitigating local climate warming.