Transpiration from a multi-species deciduous forest as estimated by xylem sap flow techniques

Thermal dissipation probes inserted into hydro-active sapwood were used to measure rates of xylem sap flow for six major hardwood species growing in an upland oak forest of east TN. Species-specific estimates of sap flow were combined with total sapwood area for trees of the forest overstory and understory, and daily rates of stand transpiration were derived. A seasonal analysis of sap flow for nine chestnut oak (Quercus prinus L.) trees measured in 1996 showed that radiation, vapor pressure deficit, and fractional leaf area index (LAI) were sufficient to describe rates of daily transpiration. Application of an empirical model to climatic data collected in 1997 and maximum daily rates of sap flow for white oak (Quercus alba L.), northern red oak (Quercus rubra L.), black gum (Nyssa sylvatica Marsh.), red maple (Acer rubrum L.), and yellow-poplar (Liriodendron tulipifera L.) indicated that stand transpiration peaked at 2.2 mm day^-1 in mid-May prior to canopy closure. Total transpiration during the season was 267 mm; 221 mm from overstory trees and 46 mm from understory saplings. Transpiration from the overstory was dominated by red maple (59 mm) and black gum (49 mm). Chestnut oak, which accounted for 27% of the stand basal area, contributed only 35 mm or 16% to total overstory transpiration. The relative contribution of each species to stand transpiration was driven largely by sapwood area per unit ground area and to a lesser extent by species-specific differences in daily water use. Such information should prove useful in exploring the impact of harvest operations on site water balance and in understanding the ecological basis for how species composition affects forest water use.