In temperate montane regions of the western U.S., forest tree growth is often tightly linked to snowpack accumulation. However, because both the timing and character of seasonal western climate patterns are expected to change considerably over coming decades, tree dependency on different sources of moisture may also shift, benefitting some trees while challenging others. In mountain ecosystems, source water use can difficult to characterize both spatially and temporally because of the high degree of variability in landscape characteristics. The central aim of this study was to quantify the importance of winter precipitation as a water source and how its importance is controlled by landscape characteristics. To address this aim, we asked three questions: 1) Where along the soil depth profile are trees sourcing water from? 2) What proportion of that soil water used is derived from winter precipitation? 3) How do landscape characteristics such as elevation and hillslope topography influence the importance of winter precipitation as a water source for trees?
Results/Conclusions
In a conifer forest of the Northern Rockies, we tracked tree water use in the dominant conifer species for three years (2013-2015) using stable water isotopes while simultaneously quantifying and tracking precipitation inputs to soil moisture across a vertical soil profile. We found that prevailing soil moisture conditions drive variation in effective rooting depth that in turn determines which seasonal precipitation inputs are available to support tree growth and function. In general, trees at lower elevations relied more on winter precipitation sourced from deep soils while trees at higher elevations used more summer rains from near-surface soil layers. We then coupled water use with in-situ dendrometer measurements to link source water use with woody biomass production. We found that snowmelt accounted for 81.1% of woody biomass production in trees growing at lower elevation sites, compared to 72.4% in trees growing in higher elevations. As both the timing of seasons and phase of precipitation (rain vs. snow) are likely to change considerably across much of the west, such patterns in tree water use are likely to play a role in determining the evolution of forest composition and structure in a warming climate.