Do shade trees change our exposure to radiation more than they change the surrounding air temperature?

Details

Microclimate regulation is one of vegetation’s most important services directly perceived by urban residents, and many studies have sought to evaluate and map it at different spatial scales. Most investigations have focused on just one parameter: the reduction of air temperature. However, thermal sensation, health and well-being also depend on air humidity, wind velocity, and the three-dimensional short- and long-wave radiation environment. This makes it necessary to assess separately how urban vegetation modifies the different thermal components.

The paper reports initial results from a series of microclimate investigations in Szeged, South-East Hungary. In a comparative observational field study, researchers made systematic on-site measurements with a pair of special human-biometeorological stations on 20 clear summer days in 2015 to investigate the small-scale climate-regulation potential of individual urban trees. They selected five healthy, mature trees without additional shading from other trees or artificial objects. They separately compared median air temperature, relative humidity, and short- and long-wave radiation components from the upper and lower hemispheres, measured beneath the tree canopies and in the sun.

All five tree specimens had significantly greater impacts on radiation-budget components, while changes in air temperature and humidity were rather small. Differences between species appeared small during the warmest hours of the day and may be attributable to tree dimensions and canopy characteristics.

The authors recommend that ecosystem-service indicators use integrated human-biometeorological indices that account for all meteorological parameters that substantially influence human thermal comfort.