Fens As Ecohydrologic Gauges of Climate Change in California

The purpose of this study was to demonstrate the use of montane groundwater-fed peatlands called fens as gauges for changes in groundwater recharge through time. Due to their consistent groundwater flows, fens remain saturated during the growing season, thus serving as ecological refugia for a range of sensitive biota including the federally threatened Yosemite toad (Anaxyrus canorus). In this project, five fens in the Sierra Nevada and two in the southern Cascade Mountains were studied over a 50-80 year period using historic aerial photography and GIS analysis. Fen areas were delineated using vegetation distributions and site wetness as visual keys to fen areal extent. In addition, a climate analysis of precipitation, snowpack, evapotranspiration, and mean minimum monthly temperature (Tmin) was carried out for the years 1950 - 2010 using a distributed parameter water-balance model called the Basin Characterization Model. Over the study period, all five Sierra fens decreased by 10-15% in delineated area, but little change occurred in the Cascade fens. The climate analysis revealed major changes through time in Tmin and the frequency of June months with zero snowpack (snowpack longevity) at the five Sierra sites. At the Cascade sites, increases in Tmin occurred more recently and decreases in snowpack longevity were more subtle. The different behavior of fens in the Cascades versus the Sierra Nevada suggests that underlying geology, particularly hydraulic conductivity, rock porosity, and size of groundwater reservoirs, may be an important buffer to change. These results show that fens in the Sierra Nevada are highly responsive ecohydrologic gauges that can be used to track reductions in groundwater recharge due to climate change. Further studies using isotopic and/or geochemical approaches are needed to quantify the changes in groundwater recharge that were identified with fens. Because fens provide critical habitat for sensitive species, major changes in hydrology would likely lead to impacts on wildlife populations. The establishment of fen monitoring networks could be a highly effective method by which to identify and track both hydrologic and ecologic impacts from climate change in California and potentially other parts of the arid West.