Role of street trees in mitigating effects of heat and drought at highly sealed urban sites

Vegetation and trees in particular play a key role in mitigating thermal load in urban areas. The goals of this study were to analyse species-specific microclimatic effects and to identify the role of leaf-area-density, transpiration, and stomatal conductance for cooling effects at urban sites. Air temperature, relative humidity and surface temperature were used to study the effects of six tree species with similar age structure in a residential area with comparable site conditions. Transpiration, stomatal conductance and leaf-area density were measured during the summer months in 2013. Air temperatures and relative humidity were monitored for four trees per street in hourly intervals, and surface temperatures were recorded in the afternoon for three hot days on the shaded and unshaded asphalt surfaces beneath the trees. Species differ significantly in their ability to reduce air and surface temperatures, as well as to increase relative humidity. Trees showing both a high leaf-area density and a high rate of transpiration are more effective in cooling the air temperatures. Differences in the surface temperatures of the tree shaded areas are more pronounced compared to the air temperatures. Thus, the surface temperatures of the full sun exposed areas are up to 15.2 K warmer than that in the tree shaded areas. The results provide information to decrease thermal load in urban areas for future tree planting by choosing species with high cooling potential. The highest cooling potential was measured for Corylus colurna and Tilia cordata 'Greenspire' and the lowest for Ulmus x hollandica 'Lobel'. (C) 2015 Elsevier B.V. All rights reserved.