Urban tree transpiration over turf and asphalt surfaces

We used a two-layer canopy model to study transpiration of tree species as affected by energy-balance properties of a vegetated and paved surface. During several dawn-to-dusk studies, tree transpiration, stomatal conductance, leaf temperature (T(l)), and several microclimate variables, were measured over turf and an asphalt surface. Cumulative transpiration was estimated from a leaf energy-balance equation applied to a tree crown apportioned between sunlit and shaded layers. Afternoon asphalt surface temperatures (T(s)) were 20-25°C higher than turf T(s) in all studies. Air-temperature differences between sites were minimal due to the size and proximity of the two surfaces that resulted in mixing of air. Trees over asphalt had consistently higher T(l) than those over turf, apparently due to interception of the greater upwards long-wave radiation flux from higher T(s). In one study flowering pear over asphalt in a humid environment had higher T(l) resulting in one-third more total water loss compared to trees over turf. In other studies, however, water loss of green ash and Norway maple over asphalt in an arid environment was either equal to or less than that over turf. Less water loss was due to higher T(l) over asphalt causing prolonged stomatal closure. Model manipulation indicated that tree water loss over asphalt will depend on the degree of stomatal closure resulting from how interception of increased energy-fluxes and ambient humidity affect leaf-to-air vapor pressure differences.