The goal of this project is to examine the variation in ecosystem processes due to exposure to long-term climate change. Climate change is simulated by altered snow depth (both increases and decreases) that is the result of 50-yr old snow fences near Mammoth Lakes, CA. Snow fences cause snow to build up in drifts immediately downwind of fences, and to decrease in depth further downwind. The aim of the snow fences we work with is to keep US Hwy 395 clear of snow during the ski season at Mammoth Mountain Ski Area. Because the fences cause both increases and decreases in snow depth, they provide us with simulations of two opposing climate change scenarios (wetter winters and drier winters) of the future.

We started this particular work in 2003, when we first obtained measurements of snow depth in winter, and soil moisture content in spring and summer due to altered snow depth caused by the fences. During summer 2005, the SCAP Team made measurements of soil moisture, leaf-level photosynthesis, plant water potential, stomatal conductance, transpiration, Leaf Area Index, and Normalized Difference Vegetation Index. They also weighed the contents of “litter traps” and “decomposition bags” (constructed by SCAP 2004 team members), that will help us quantify nutrient cycling on the different snow depth treatment zones. Finally, the SCAPers helped harvest, dry, and process biomass measurements for shrub growth in the different snow depth treatments. Overall, this was a tremendous amount of work for the second full year of this project. It was way beyond all my expectations.

Snow depth was manipulated using eight long-term snow fences near Mammoth Lakes, Mono County, California, USA. Snow depth, soil moisture content, water potential, photosynthetic gas exchange, and leaf area index changes were measured for Artemisia tridentata and Purshia tridentata in response to increased and decreased snow depth in the spring and summer following typical (2003 – 2004) and El Niño (2004 – 2005) snowfall years. Snow depth on increased-depth (“+snow”) plots was about twice that of ambient-depth plots in both years, and about 2.2 times of that for decreased-depth (“-snow”) plots. Soil water content on + snow plots was double that on ambient and – snow plots in the El Niño year, and approximately 5% greater following the typical snowfall year. Photosynthetic gas exchange differed across snow depth treatments for both species in June 2004 (following the typical snowfall winter of 2003 – 2004). Following the El Niño winter, plant water potential did not differ across snow depth treatments until early September, and photosynthetic CO₂ assimilation increased from May to September for all treatments. We conclude that there is a threshold level of sensitivity of soil and plant water relations and photosynthesis to snow depth such that plant physiological responses to microscale differences in snow depth are reduced in high-snowfall El Niño years, whereas snow depth effects are more pronounced in typical-snowfall years.

Our results were in contrast to our SCAP findings from 2004 in which we made the same measurements. In 2004 there were significant differences in plant water potential and photosynthesis across the different snow depth treatments. In 2005, there were no significant differences. The difference between the two years is due to the heavy snowfall of the El Nino winter 2004-2005, compared to the normal snowfall year of 2003-2004. In 2004 the different snow depths translated into different soil and plant moisture levels, and this lead in turn to varying amounts of photosynthesis. In 2005, the excessive snowfall overwhelmed this response. In the larger context of climate change, this is an incredibly important finding, as it suggests that there is a threshold level of snowfall above which the ecosystem is unresponsive in terms of vegetation growth. Currently, we have little knowledge about how the frequency of El Nino vents may change in the future, but there could be dramatic changes in ecosystem functioning, wildlife habitat quality, and quality and quantity of water for downstream stakeholders. This latter point is especially important given that our site is at the headwaters of the Owens River, which has been extensively engineered and withdrawn for the water needs of Los Angeles.