Solar Permeability of Different Tree Species in Szeged, Hungary

The heat stress modification capacity of urban trees is widely acknowledged and makes these natural landscape elements very important in the field of climate conscious urban planning. Many studies proved that shading, i.e. the reduction of direct solar radiation is the most effective way to moderate summer heat stress under Central European climatic conditions. The investigation aims at determining the transmissivity of four tree species that occur frequently in Hungarian cities: Sophora japonica, Tilia cordata, Celtis occidentalis and Aesculus hippocastanum. In order to accomplish that, a systematic radiation measurement campaign was carried out in the South-Hungarian city of Szeged, from early summer (foliated condition of trees) to late autumn (nearly leafless condition). Short-wave radiation from the upper hemisphere was measured with Kipp & Zonen pyranometers under carefully selected tree specimens (transmitted radiation), as well as on a roof station free from sky obstruction (actual value of global radiation). The calculated transmissivity values varied greatly with the seasonal status of the canopy (the median value of transmissivity increased from 0.03 to 0.47 in case of A. hippocastanum), and we found considerable inter-species differences too, evidencing that solar permeability depends on the amount of leaves, leaf density and other tree crown-related characteristics.