Seasonal and meteorological effects on the cooling magnitude of trees in subtropical climate

The cooling effect of trees is one of the most important ecosystem services offered by natural components in cities. However, the large variations in cooling magnitudes reported in different studies call for deeper understanding of the underlying mechanisms. This study investigated the seasonal and meteorological effects on the cooling magnitude of trees in the humid subtropical climate. The meteorological conditions at four peri-urban woodland sites and a rooftop control site were continuously monitored for one year. The annual mean (+SD) cooling magnitudes were 0.9 +/- 0.5, 2.5 +/- 1.4 and 1.6 +/- 0.8 degrees C in air temperature (Ta), physiological equivalent temperature (PET) and universal thermal climate index (UTCI) respectively with notable seasonal and diurnal variations. The daily total incoming shortwave radiation (Sin) explained 24.7, 39.2 and 35.7% of the variability in cooling magnitudes in Ta, PET and UTCI respectively. For every 1 MJ/m(2) increase in Sin in a day, the daily mean cooling magnitude increased by 0.03, 0.16 and 0.08 degrees C in Ta, PET and UTCI respectively. The cooling magnitude measured in Ta could increase by 0.05 degrees C for every 1 degrees C rise in background Ta. The monthly mean interception of S_in was generally over 80% with an annual mean of 82.3%, which allowed the cooling benefit of trees to extend to the transitional season. Future studies are suggested to conduct a continuous measurement for at least 24 h under sunny and cloudy conditions in both hot and cold seasons with a ground-level control site to capture a more complete picture of the fluctuations in cooling magnitude.