We investigated water loss of shade trees over turf and asphalt in an arid and humid climate for Russian olive and silver maple. Total daily tree water loss, and dawn-to-dusk stomatal conductance (gs) and leaf temperature (Tl), as well as air temperature, surface temperature, and vapor pressure deficit, were measured in Logan, Utah, and Stillwater, Okla., in early and mid-summer. Midday air temperatures in mid-summer were similar at both locations, 30 to 35 °C. Comparable vapor pressure deficits (VPD, kPa) were much higher in Logan, 3.5–5.0, than Stillwater, 2–2.5. Differences in humidity and air temperature between asphalt and turf were negligible at both Stillwater and Logan. Midday surface temperatures for asphalt and turf averaged 34 and 50 °C, respectively, in Logan, but were 10 °C higher for both surfaces in Oklahoma. The effect of higher longwave radiation from hotter surfaces on stomatal conductance and water loss of trees over asphalt in Stillwater was not consistently different for either species from those over turf. However, at Logan, Tl of trees over asphalt were consistently 2 to 4 °C higher and gs was 10% to 20% lower than those over turf. Stomatal closure for trees over asphalt resulted in water loss that was the same or slightly lower as trees over turf. The effect of paved surfaces on tree water loss appears to be more pronounced in an arid than a humid climate. The combined effect of higher VPD in an arid region and greater longwave radiation from hotter paved surfaces induces stomatal closure that limits water loss, and likely photosynthesis. By contrast, in humid regions, increased tree radiation interception over asphalt does not appear to trigger stomatal closure due to lower VPD.
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