Water use strategies between two co-occurring woody species in a riparian area: Naturally occurring willow, Salix exigua, and expanding juniper, Juniperus scopulorum, in central Montana

Juniper expansion across the western United States has the potential to alter watershed hydrology, especially within riparian areas. Given the uncertainties in the ecohydrological response to the expansion, this study focused on examining the water use strategies between two woody species co-occurring in a riparian area in south central Montana—Salix exigua (sandbar willows) and Juniperus scopulorum (rocky mountain junipers)—in order to address three questions: (1) Are junipers and willows using the same soil moisture pool that contributes to streamflow? (2) Are junipers transpiring more water than willows on a per tree or per sapwood area basis? (3) Are the seasonal transpiration rates between junipers and willows different? To determine the differences in water use strategies between willows and junipers, we used stable isotope analyses to trace different sources of water, water potential to determine seasonal water stress patterns and transpiration rates to quantify water loss. Our isotopic analyses suggest that junipers and willows in the riparian area were not directly using stream water but relied on different pools of soil water at different times of the year: shallow soil water in spring when soils were wet and deeper soil water in late summer. We also found that junipers transpired more than willows during the spring and late fall, but that both species had similar transpiration rates during periods of low streamflow. However, higher juniper transpiration rates in spring and late fall can potentially lead to soil moisture deficits if winter snowpack is low, suggesting that the additional water loss through transpiration by junipers may be mitigated under wet winters but exacerbated under dry winters.