Nocturnal transpiration in riparian Tamarix thickets authenticated by sap flux, eddy covariance and leaf gas exchange measurements
Georgianne W. Moore (1, 2), James R. Cleverly (3) and M. Keith Owens (4)
1. Department of Ecosystem Science & Management, 2126 TAMU, College Station, TX 77843, USA / 2. Corresponding author () / 3. Department of Biology, University of New Mexico, Albuquerque, NM 87154, USA / 4. Texas Agricultural Experiment Station, 1619 Garner Field Road, Uvalde, TX 78801, USA / Received February 28, 2007; accepted July 30, 2007; published online February 1, 2008
Summary
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) estimated from stem sap flux (Js) measurements (Esf) and eddy covariance (Ecv), 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 Esf as a proportion of daily Esf increased with water availability to a peak of 36.6%. High nighttime Esf was associated with underestimates of nighttime Ecv. A second study, conducted in west Texas, beside the Pecos River, investigated the relationships between nighttime Js 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 Js 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 Js, 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.
