Snow depth impacts on photosynthesis, recruitment, and mortality at the Great Basin Desert – Sierra Nevada ecotone

Background/Question/Methods

Snowfall is the dominant hydrologic input for high elevations and latitudes of the western United States. Many climate models envision changes in California’s Sierra Nevada snow pack characteristics, which would impact the hydrology and ecology of the plant communities of this range and adjacent arid lands. Snow pack will be affected via later snowfall in autumn, reduced snowpack depth, changes in snow water equivalents, earlier snowmelt, rain-on-snow events, and an upward migration of the snowline. How will these altered snow characteristics affect patterns and processes at the ecotone between montane coniferous forest and high desert shrub land? To address this question, experiments utilize large-scale, long-term (>50 year old) snow fences to manipulate snow depth and melt timing at a desert-montane ecotone in eastern California, USA. A combination of long-term snow course data, in situ instrumentation, plant community surveys, annual ring cores, and instantaneous gas exchange and water potential measurements were used to couple physical and biological processes at daily, monthly, annual, and decadal scales. 

Results/Conclusions

At this site, snow pack depth on April 1 (from 1928 – 2012) averaged 1344 mm, with an interannual CV of 48%. Evapotranspiration (ET) in this arid location averaged 3.66 mm day^-1 (CV = 60%). Winter ET was about 37 mol m^-2 d^-1 of water loss from the snow pack between January 1 and May 1; sublimation was 10% of ET for the same period. Soil moisture pulses were shorter in duration and lower in magnitude in low- than medium- or high-snowfall years. Snow fences doubled equilibrium drift snow depth and increased spring soil moisture content. Despite considerable interannual variation in snow depth and total precipitation, early-June plant water potential stayed relatively constant over eight consecutive years (CV ~20%), but photosynthesis was more variable (CV ~40%). Over the long-term, changes in snow depth and melt timing have impacted growth of only three plant species. Recruitment of shrub seedlings is essentially nil on increased-depth plots. Adult mortality was higher on increased- and decreased- compared to ambient-depth plots. Results indicate complex interactions between snow depth, soil water, and ecological processes at multiple scales. Some plant processes were resilient to large interannual variations in snow depth. By contrast, the species assemblage may not be resilient to future snow depth variation. Thus it is unclear whether this ecotone will be stable in the face of anticipated changes in future snow characteristics.