Tree canopy characteristics influence human heat stress reduction: comparative case study for two sites in different climates

Increasing tree canopies is one of the effective measures to reduce heat at different spatial scales in cities. From a human-biometeorological perspective, tree canopies cool the trunk space below by reducing solar radiation, thus providing shade and lowering the net radiation overall. They also reduce the air temperature above them through transpiration, but this process also increases the water vapor pressure, which slightly counteracts a lowering of human heat stress. However, these two effects mainly affect the layer above the tree canopies. Therefore, they are less likely to promote the lowering of outdoor human heat stress at the pedestrian level below the tree canopies.

As a valuable benefit for the enhancing of human thermal comfort in urban areas, the cooling potential of tree canopies depends on their dimension, shape and leaf density. Even if trees have comparable physical states, they may influence the micrometeorological variables that control local human thermal comfort differently in various climate zones. In this context, the study shows the human-biometeorologically significant cooling potential of street trees at two exemplary selected urban sites in different climate zones. According to the Köppen and Geiger climate classification, the Jeju site (N 33˚ 49'00'', E 126˚ 50'00''), Republic of Korea, is in the Cfa climate zone, whereas the Stuttgart site (N 48˚ 46'38'', E 9˚ 10'30''), Germany, is in the Cfb climate zone.

Based on the validated version of the ENVI-met v5.0.2 software, systematic simulations were conducted on typical summer days to show the effect of various tree canopy characteristics, which refer to two tree dimensions, two values of the leaf area index (LAI), and three shapes of tree crowns (ellipsoid, triangle, and inversed triangle), on the level of outdoor human thermal comfort at both sites.

In the simulation results for sunny conditions, it is noticeable that tree canopies in the shape of ellipsoids exhibit the highest reduction in mean radiant temperature (T_mrt), which is considered a key factor in human thermal comfort. In Jeju it varies between 6 and 25 K and in Stuttgart between 11 and 27 K. The remarkable reduction of T_mrt leads to a maximum cooling potential of the physiological equivalent temperature (PET), as quantitative measure for human thermal comfort, of 12 K in Jeju and 14 K in Stuttgart. Assuming that the PET classification applies to both climate zones, the result is that the level of PET classification at both locations in different climate zones decreases from “very hot” to “warm”. 

This research forms the basis for complementary studies on the human-biometeorologically significant cooling effect of tree canopies with various characteristics that extend to other different climate zones.