Comparison of transpiration rates among saltcedar, cottonwood and willow trees by sap flow and canopy temperature methods

Transpiration (Et), measured by stem sap flow gauges, and canopy and air temperature differential (Tc−Ta) of Populus fremontii (cottonwood), Salix gooddingii (willow) and Tamarix ramosissima (saltcedar) were compared to determine if remotely sensed canopy temperatures could be used to estimate Et or water stress in these trees in desert riparian zones of the United States and Mexico. Controlled experiments were conducted in which containerized plants were placed closely together and allowed to grow into a single, dense canopy over a summer in a desert climate. At the end of the growth period, two canopies of each species were measured for Et and Tc−Ta over 11 days, first under unstressed conditions then under water or salt stress. Et and Tc−Ta were significantly (P<0.05) correlated for all species. Correlation coefficients improved when a radiation term was included in the equation predicting Et from Tc−Ta. During the non-stress part of the experiment, canopies of all three species had similar rates of Et, but saltcedar maintained higher Et rates and lower rsv than the native trees on the stress treatments. For each species, models were developed, using both meteorological data and a canopy, energy-balance equation, to predict daily Et and stomatal resistance (rsv); these models had standard errors of 15–22% when compared with measured Et over the unstressed portion of the experiment.