Five-year time series of sap-flux-scaled transpiration reveals inter-annual variation between tree water use and precipitation in a pinyon and juniper woodland

Woody plant encroachment into dryland grasslands and savannas is an ongoing phenomenon across the globe. Yet there remains no unifying theory of how encroachment impacts ecohydrological processes across broad temporal scales, particularly at locations with large inter-annual variability in precipitation. We addressed this knowledge gap by quantifying water use of single-leaf pinyon (Pinus monopylla) and Utah juniper (Juniperus osteosperma) in the Great Basin of Nevada, USA using five consecutive years of continuously measured sap-flux-scaled transpiration. Three major results emerged. First, pinyon was more sensitive to declines in shallow soil moisture relative to juniper such that sap flux density (Js) decoupled from vapor pressure deficit (VPD) at higher soil moisture conditions than juniper. However, above the soil moisture breakpoint, Js in pinyon was generally less constrained by VPD than juniper. Second, despite species differences in sensitivity to shallow soil moisture and VPD, annual whole-tree water use was similar in magnitude for both species. Nevertheless, stand-level water use of juniper was an order of magnitude less than pinyon, due to the lower basal area density of juniper. Finally, total annual tree transpiration in relation to annual precipitation was negatively correlated with precipitation amount (r2=0.98), ranging from 46% during the wettest year to 63% during the driest year. These data, representing the longest continuously measured time-series of sap-flux-scaled transpiration in a pinyon and juniper woodland in the Great Basin, reveal that impacts of woody plant encroachment on the water balance of dryland grasslands and savannas will likely increase in hotter and drier climates.