URBAN TREE TRANSPIRATION OVER TURF AND ASPHALT SURFACES

We used a two-layer canopy model to study transpiration of tree specie s as affected by energy-balance properties of a vegetated and paved su rface. During several dawn-to-dusk studies, tree transpiration, stomat al conductance, leaf temperature (T-l), and several microclimate varia bles, were measured over turf and an asphalt surface. Cumulative trans piration was estimated from a leaf energy-balance equation applied to a tree crown apportioned between sunlit and shaded layers. Afternoon a sphalt surface temperatures (T-s) were 20-25 degrees C higher than tur f T-s in all studies. Air-temperature differences between sites were m inimal 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 upward s long-wave radiation Bur from higher T-s. In one study flowering pear over asphalt in a humid environment had higher T-l resulting in one-t hird more total water loss compared to trees over turf. In other studi es, 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. L ess water loss was due to higher T-l over asphalt causing prolonged st omatal 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 affec t leaf-to-air vapor pressure differences. (C) 1997 Elsevier Science Lt d.