Urban trees are important for improving human thermal comfort on hot days. It is convenient for analysis of tree cooling to differentiate between that resulting from shading and that from dissipation of energy as latent heat, i.e. evaporative cooling due to transpiration.
Trees’ hydraulic systems are limited and in hot climates evaporative demand regularly exceeds the hydraulic capacity and/or the ability of the tree to draw water from the soil. Thus, we often find that mid-day transpiration, as measured with sap flow sensors, reaches a plateau because leaf conductance is reduced as leaf water potential exceeds a certain threshold. The height of the plateau may change with soil water availability or the relative distribution of leaves, stems and roots. But for a given tree the height of the plateau, or the average mid-day transpiration may be more-or-less constant for many days.
This constant value or plateau implies that the relationship between canopy resistance (Rs) and vapor pressure deficit (VPD) will be linear and we have shown that the slope of that relationship is proportional to hydraulic conductance of the tree and the mid-day stem water potential. These relationships will be reviewed.
In a current study on the potential of trees to cool urban climates we analyzed the shading vs. evaporative cooling potential of urban trees in semi-arid Beer Sheva, Israel and in temperate Munich, Germany. Tree transpiration measured with sap flow sensors was normalized to crown projection area in order to compare to other energy fluxes. Slopes of the relationship between Rs and VPD were much higher in summer in Beer Sheva than in Munich. However, for horticultural species in Israel, e.g. citrus, avocado, persimmon and nectarine, slopes were closer to those in Munich. Irrigation recommendations for trees in urban gardens are significantly lower than for horticultural orchards. This suggests that the urban trees that we measured were not well watered and that their water status was poorer than for horticultural trees. Another possibility is that their leaf area index was lower. Therefore water use was less than might be expected for horticultural trees or those measured in temperate Germany, where rainfall is plentiful.
Conversion of water use to latent energy equivalents shows that the urban trees in Israel dissipate less energy as latent heat than in Munich. Considering that the overall energy load is greater in Beer Sheva, this means that the latent heat contribution to tree cooling is very low and of less significance than in Munich. Thus in Israel urban tree cooling is mainly from shading and not from latent heat dissipation.
