Differential snowpack accumulation and evapotranspiration in aspen and conifer communities: Implications for water yield and function

Early succession aspen and late succession conifer forests have different architecture and physiology affecting hydrologic transfer processes. An evaluation of water pools and fluxes was used to determine differences in the hydrologic cycles of aspen (Populus tremuloides) and conifer species including: white fir (Abies concolor), Douglas-fir (Pseudotsuga menziesii) and Engelmann spruce (Picea engelmannii). In 2005 and 2006, hydrologic measurements of snow water accumulation (SWE), snow ablation (melt), soil water content, snowpack sublimation, and evapotranspiration were measured in adjacent aspen and conifer stands. Snow accumulation was 34 to 44% higher in aspen. Snow ablation rates were faster in aspen (21 mm d^-1) compared to the conifer (11 mm d^-1). When changes in soil moisture (due to over winter snowmelt) were combined with peak snow accumulation in 2006, aspen had greater potential (42-83%) for runoff and groundwater recharge. Snowpack sublimation during the ablation period was not different between meadow, aspen, and conifer sites and comprised <5% of snowpack losses. Extended conifer transpiration in spring and fall did not contribute to large differences in snowmelt water yield (<28 mm y^-1).  Summertime ET rates were higher aspen (3.6 mm d^-1) than in conifer (2.7 mm d^-1); net transpiration largely reflected soil column porosity. This study shows that the largest differences in annual water yield between aspen and conifer stands result from differences in snow accumulation and net summertime ET.  Although snow accumulation was greater in aspen it was partly offset by greater net annual ET losses in aspen.