Difference in Water Balance Between Aspen and Conifer Communities: The Fate of Spring Snow Melt in a Northern Rocky Mountain Watershed

Differences in water balance between mature deciduous quaking aspen (Populus tremuloides) and evergreen conifer (e.g. Abies concolor, Abies lasiocarpa, Picea engelmannii and Pseudotsuga menziesii) forest communities occur as a result of complex physical and biological interactions. To better understand the potential effects of aspen conversion to evergreen conifer on watershed water yield in the northern Intermountain West, USA, we assessed several water balance transfer mechanisms, including snow accumulation, ablation, evaporation/ sublimation, soil moisture recharge, and transpiration, to determine how major differences in water balance occur in adjacent mature aspen and conifer forests during spring snowmelt. Measurements of fall sap flux were indexed to determine the annual transpiration activity period for aspen and conifer communities. Soil moisture was monitored to determine the timing of soil moisture recharge during winter snow accumulation, and field observations of snow water equivalent (SWE) and snowpack sublimation were obtained. We found SWE was the factor most different between aspen and conifer stands, with conifer stands averaging 34% less SWE at peak snowpack than adjacent aspen stands. We conclude peak SWE is likely the most important factor affecting watershed water yield between aspen and conifer stands, but that differences in soil moisture accumulation could further enhance this difference.