SURFACE ENERGY BALANCE AFFECTS GAS EXCHANGE AND
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We investigated surface energy balance for a pine bark mulch and a turf surface and how each surface influenced gas exchange and growth of recently transplanted trees. Over a three year period, several morning-to-evening studies of stomatal conductance and leaf temperature were measured on leaves of Norway maple (Acer platanoides ‘Emerald Queen’ L.) and little leaf linden (Tilia cordata ‘Greenspire’ Marsh) trees growing over each surface. Tree water loss was estimated using a leaf energy-balance equation applied to the tree crown apportioned between sunlit and shaded layers. Crop coefficients were also estimated. Throughout each day, microclimate variables were measured over each surface with a permanent weather station. Tree growth data were collected at the end of each growing season. Heat flux data revealed that a greater portion of incoming radiation was prevented from entering the soil below mulch than below turf. Due to this insulating effect, and the lack of transpirational cooling, mulch surface temperature was greater than turf surface temperature throughout the majority of each study day. Because leaves over mulch intercepted more upward longwave radiation, trees over mulch generally had greater leaf temperatures and leaf-to-air vapor difference than trees over turf. Greater leaf-to-air vapor pressure difference reduced stomatal conductance for trees over mulch compared to trees over turf. Depending upon incoming global radiation, estimated tree water loss varied between surfaces. On sunny days, intercepted longwave radiation flux from pine bark mulch increased the leaf-to-air vapor pressure difference such that stomata in leaves over much closed earlier in the day, and for longer periods than stomata in leaves over turf. Therefore, water loss was greater for trees over turf than for trees over mulch. However, on partly cloudy days, higher relative humidity and reduced longwave radiation flux from the mulch lowered leaf-to-air vapor pressure difference for trees over mulch. Because trees over mulch had greater leaf-to-air vapor pressure difference, on partly cloudy days trees over mulch had greater water loss than trees over turf despite reduced stomatal conductance. Growth data indicates that trees over turf had greater shoot elongation and leaf area than trees over mulch. These data suggest that gas exchange and growth of recently transplanted trees may be reduced if planted over non-vegetated, urban surfaces such as pine bark mulch. Supporting pdf: Surface Energy Balance Affects Gas Exchange and Growth of Two Transplanted Landscape Tree Species |
