Effect of gender on sap‐flux‐scaled transpiration in a dominant riparian tree species: Box elder (Acer negundo)

Abstract

Acer negundo is a dioecious riparian tree species with a spatial segregation of the sexes along soil moisture gradients. Females are typically more common in wet sites along streams (typically F/M ≈ 1.6), whereas males are more common in drier sites away from streams (typically F/M ≈ 0.6). Spatial segregation between sexes may develop because of the higher reproductive cost in females compared to males. If so, female Acer negundo trees would be under stronger selection to maximize resource uptake, and would therefore likely occur at greater frequencies in high resources sites (i.e., along streamsides), and increase rates of resource acquisition (i.e., water and nutrients). The spatial segregation of the sexes leads to the hypothesis that male and female individuals have varying influence on ecosystem evapotranspiration. To address this, stem sap flux was measured on mature streamside (≤1 m from stream channel) and nonstreamside (>1 m from stream channel) male and female Acer negundo trees occurring in Red Butte Canyon near Salt Lake City, Utah, during the 2004 growing season. Despite having similar predawn and midday water potentials, sap flux density was 76% higher in streamside female trees than in males (P < 0.0001), while sap flux density was 19% greater in nonstreamside female trees compared to males (P < 0.0001). Mean daily sap flux density of all A. negundo populations was highly correlated with mean daily vapor pressure deficit (P < 0.0001), and was moderately correlated with mean daily photosynthetic active radiation (P = 0.0263). At the watershed scale, nonstreamside male and female A. negundo trees contributed 20 and 21% respectively to the estimated 1.7 mm d−1 transpiration flux from dominant riparian vegetation away from streamsides (estimated from scaled sap flux measurements of all dominant riparian tree species in Red Butte Canyon). Male and female A. negundo trees contributed 31 and 46% respectively of the estimated 8.0 mm d−1 transpiration flux from dominant riparian vegetation adjacent to the stream channel. Results from this investigation show that the population structure of dioecious riparian trees has direct consequences on ecosystem ET, particularly along stream margins. Shifts in population structure therefore, may have profound impacts on several ecohydrological processes including stream discharge, biogeochemical cycling, and ecosystem productivity.