Leaf Water Relations and Sapflow in Eastern Cottonwood (Populus deltoides Bartr.) Trees Planted for Phytoremediation of a Groundwater Pollutant

Plants that remediate groundwater pollutants may offer a feasible alternative to the traditional and more expensive practices. Because its success depends on water use, this approach requires a complete understanding of species-specific transpiration patterns. The objectives of this study were (1) to quantify tree and stand-level transpiration in two age classes (whips and 1-year-old seedlings) of eastern cottonwoods (Populus deltoides Bartr.), and (2) to determine climatic and physiological driving variables at the Carswell Air Force Base in central Texas, USA. Trichloroethylene (TCE) was detected in shallow (2 to 3 m) groundwater in the early 1980s. Cottonwood whips and 1-year-old potted seedlings were planted in two separate 0.15-ha plantations in spring 1996. Sapflow gauges determined sapflow on 14 to 16 trees in May, June, July, August, and October 1997. Without adjusting for differences in tree size, sapflow rates were greater for 1-year-old trees than whips (peak values were 0.75 and 0.53 kg hr^-1 tree^-1, respectively). When adjusted for tree size, the pattern reversed, with whips having significantly greater sapflow rates than 1-year-old trees (peak values were 0.053 and 0.045 kg cm^-2 hr^-1, respectively). Temporal variation (diurnal and seasonal) in sapflow rates was principally related to VPD, solar radiation, and leaf conductance. Extrapolating to the stand and across the growing season, the plantations transpired ~25 cm of water. Early attainment of high levels of transpiration indicates that the stands will transpire considerably more water as leaf area and root exploitation increases with stand development.