Do urban tree hydraulics limit their transpirational cooling? A comparison between temperate and hot arid climates

Evaporative cooling due to transpiration of urban trees in two contrasting climates is the subject of this study. Transpiration was studied experimentally on local tree species at ‘tree lab’ sites in Munich, Germany (temperate climate) and in Beer Sheva, Israel (hot arid climate), within various settings (park, street, square) with natural and sealed surface conditions. The comparison was based on linear relationship of midday canopy resistance to atmospheric vapor pressure deficit (VPD), where the slope is proportional to tree hydraulic conductance and midday stem water potential. Sap flux densities in all trees were similar but crown projected area (CPA) was larger in Beer Sheva, resulting in higher hydraulic resistance that limits transpirational cooling (regression slopes of 0.25–0.44 in Beer Sheva vs. 0.10–0.18 in Munich). The contribution of the transpirational cooling, studied as the proportion of latent heat to total available energy was about 40% less significant at noon hours in summer in Beer Sheva than in Munich. Results suggested that shading should be the main consideration in planting local trees in hot dry climates. The linear relationship of midday canopy resistance to VPD is proposed as an operative tool for comparing evaporative cooling between local trees in different climatic regions.