University of New Mexico Albuquerque, New Mexico, United States
Abstract: As wildfires increase in size and severity, large areas of forest are undergoing substantial increases in shrub cover. While shrub species are common in early successional forest communities, it has long been thought that shrubs may hinder the success of co-occurring tree species through competition for resources. In forests where water is the limiting resource, the effect of shrubs on tree water-use may determine how trees respond to increasing precipitation variability with climate change. Here we analyzed the δ2H and δ18O values of xylem water for two conifer species (Abies concolor and Pinus jeffreyi) and two shrub species (Arctostaphylos patula and Ceanothus cordulatus) commonly found in the Sierra Nevada to determine how shrub cover affects the water source dynamics of nearby conifers. Additionally, we used tree-ring and carbon isotope analysis to calculate basal area increment, Δ13C and iWUE to assess how shrub cover influenced tree growth and water-use efficiency from 2016-2021, a period of high precipitation variability. We hypothesized that trees growing near shrubs would use deeper water sources and have higher growth rates and lower intrinsic water-use efficiency than trees growing in the absence of shrubs.
We found that growing near shrubs did not have a significant effect on the water source dynamics of white fir or Jeffrey pine, with similar source water contributions calculated for both shrub absent and shrub dominant growing environments. However, we did find that water source dynamics varied by species with Jeffrey pine using shallower soil water than both shrub species throughout the growing season. White fir, however, exhibited a shift in δ2H and δ18O values during the growing season, suggesting it uses more deep soil water in early summer and more shallow soil water in late summer. While we did not find an effect of shrub cover on water source dynamics, we did find a positive effect of shrub cover on tree growth and intrinsic water-use efficiency, suggesting that shrubs may increase tree photosynthetic rates and growth. While it has long been postulated that shrubs will hinder the survival of co-occurring conifer trees through competition for soil moisture, our results suggest that growing near shrubs does not affect the water-uptake patterns or water stress of Sierra Nevada conifers. As wildfire activity increases the abundance of shrub cover in conifer forests, we may see an increasing coexistence of shrubs and conifers in post-disturbance landscapes.