Quantifying environmental controls on sap flow in Great Basin tree species and their possible significance for mountain groundwater recharge under anthropogenic climate change

Transpiration of trees in mountain recharge zones may significantly limit the input of water to groundwater systems in Great Basin mountain ranges. This removal of vadose zone soil water eventually constrains the bioavailability of water for both agriculture and human consumption. The objective of this study is to develop a quantitative understanding of the response of sap flow to key modulating environmental factors. Quantifying the interaction between these factors will determine which have greatest influence on transpiration rates and how rates may be affected by shifts in these factors under anthropogenic climate change. Data recorded from mature trees growing at a montane site (2,730 m) on the western slope of the Snake Range in eastern Nevada measured during a precipitation-free month (July 2011) indicated that photosynthetically active radiation (PAR), and covarying air temperature and vapor pressure deficit (VPD), most strongly modulated diurnal patterns of sap flow in all species studied (Abies concolor, Pinus flexilis, Pseudotsuga menziesii and Cercocarpus ledifolius). Daily sap flow sums were also closely correlated with daily PAR sums. Declines in topsoil (0-30 cm) volumetric water content during the month by 5-10% appeared to have no effect on sap flow. This suggests that soil water potentials remained in a range that still enabled trees to extract water easily. Although species differed from each other in actual sap fluxes, temporal responses to variability in these control factors were similar among species. Anticipated higher VPDs under projected climate scenarios may further increase transpirational water losses.