Juniper encroachment into riparian areas

JUNIPER VERSUS WILLOW: WHICH SPECIES USES MORE WATER?

(Collaboration with Nathan Korb, Nature Conservancy and funded by Turner Enterprises and the Montana Water Center)

The expansion of Juniperus spp. across the western U.S. has lasting ecological and economical impacts. One main concern of juniper expansion is the alternation of ecosystem hydrology. Because junipers have deep roots, they can often access a pool of water unavailable to shallow rooted species. As junipers deplete this deep soil water, less water might be available to recharges streams and groundwater. Although some studies have documented an increase in groundwater and spring recharge after juniper removal at the watershed scale, these studies have primarily focused on invasion of junipers in areas once dominated by non-woody species. Less well understood is how conversion from one woody species to another (in this case willow to juniper) influences hydrology. However, in order to begin assessing the impacts of this species transition on watershed hydrology, we must first quantify total water use and seasonal patterns of water use between juniper and willow. In this proposed research, we will address four main questions:

1) Are junipers growing in riparian areas using water from streams?

2) Are junipers using more water than other riparian woody plants, such as willow?

3) Are the seasonal patterns of water use between juniper and willow different?

4) After juniper removal, do willow have access to more water (i.e. do they transpire more)?

We are using stable isotope analysis of oxygen and hydrogen to trace the different sources of water that willow and juniper are using (e.g. deep or shallow soil water, stream water, and ground water). We are also measuring transpiration rates in both species throughout the growing season. These transpiration rates along with measurements of tree sapwood area will then be used to quantify total water loss per tree per year. We have established two plots for these field-based measurements: one control plot and one juniper removal plot. After one year of field measurements, we will remove juniper from the riparian areas in the latter plot and then measure willow responses to juniper removal. We can use these results to compare differences in total water use in willow with and without juniper, as well as assess if willow change their water source after juniper are removed. These results can be used by managers to evaluate if, when, and where junipers should be removed along a riparian area.