Ecophysiological comparison within a riparian area between two co-occurring woody species: juniper (encroached) and willow (naturally occurring)

Juniper expansion across the western U.S. has the potential to alter ecosystem ecohydrology in riparian areas, but many uncertainties remain in understanding the ecohydrological changes that occur when one woody plant replaces another. In this study, we measured the ecophysiological differences between two woody riparian species: willow (naturally occurring) and juniper (encroaching) to examine if junipers are competing with willow for the same water resources. To determine the ecohydrological differences between these species, we used stable isotope analysis of oxygen and hydrogen to trace the different sources of water (e.g. deep or shallow soil water, stream water, ground water), transpiration rates to quantify total water lost per tree, and water potential to determine seasonal water use patterns of these species. We found both species had similar patterns of water use throughout the growing season, but juniper transpired more throughout the year; however, this increase in transpiration occurred during spring, when snowmelt was contributing heavily to streamflow.

Willow, on the other hand had higher transpiration rates during the late season, when late season, or base-flow, was low. Our data also suggest that juniper and willow are accessing a similar pool of deeper soil water. Collectively, these results demonstrate that while juniper are transpiring more than willow, this extra water loss occurs when soil moisture levels are high and streamflow is at peak flow. These results also provide important insights into water-use associated with juniper encroachment that can be used by managers to evaluate if, when, and where junipers should be removed along a riparian area.