Nocturnal transpiration in riparian Tamarix thickets authenticated by sap flux, eddy covariance and leaf gas exchange measurements

Tamarix chinensis Lour., which is common throughout the southwestern USA, is a phreatophytic riparian tree capable of high water use. We investigated temporal congruence between daily total evapotranspiration (E_sf) estimated from stem sap flux (J_s) measurements (E_sf) and eddy covariance (E_cv), both seasonally and immediately following rain events, and used measurements of leaf-level gas exchange, stem water content and diurnal changes in leaf water potential to track drivers of transpiration. In one study, conducted near the end of the growing season in a pure T. chinensis stand adjacent to the Rio Grande River in central New Mexico, nighttime E_sf as a proportion of daily E_sf increased with water availability to a peak of 36.6%. High nighttime E_sf was associated with underestimates of nighttime E_cv. A second study, conducted in west Texas, beside the Pecos River, investigated the relationships between nighttime J_s and stem tissue rehydration, on the one hand, and nighttime E, on the other hand. Leaf gas exchange measurements and stomatal impressions suggested that nighttime J_s was primarily attributed to concurrent transpiration, although there were small overnight changes in stem water content. Both vapor pressure deficit and soil water availability were positively related to nighttime J_s, especially following rainfall events. Thus, both studies indicate that T. chinensis can transpire large amounts at night, a fact that must be considered when attempting to quantify E either by eddy covariance or sap flux methods.