DECLINES IN THE SPATIAL EXTENT AND VEGETATION HEALTH OF FENS IN THE SIERRA NEVADA: SAGEHEN CREEK BASIN, CA

Watersheds in the mountain west are rapidly changing due to warming temperatures, declining snowpacks, and shifting precipitation patterns. Uncertainty remains as to how these headwater catchments, which support downstream communities and agriculture, may be impacted by these climatic changes. In our study, we focus on fens as indicator ecosystems responding to the changing climatic conditions. Fens are groundwater supported, peat-forming wetlands and are important biodiversity hotspots that regulate nutrient cycling and can mitigate some harmful effects of surface runoff (e.g., erosion). Due to their reliance on a perennial groundwater supply, the health of fens is directly tied to groundwater recharge, and they are observed to contract as recharge decreases (Drexler et al., 2013). Therefore, by monitoring the stability of fens (i.e., their changing geographic extent), we gain insight into the health of the underlying groundwater system under changing climate conditions. The focus of our work is the Sagehen Creek Basin (SCB), an experimental catchment in the north-central Sierra Nevada hosting multiple fen complexes (n>5). The SCB is situated at an elevation between 1820-2660 meters and is experiencing dramatic warming, reductions in annual snowpack, and declines in stream baseflow. This collection of observations suggests reductions in groundwater recharge and subsequent impacts on the fen complexes.

In our study, we use methods adopted from Drexler et al. (2013) to quantify changes in the spatial extent of three fen complexes using National Agricultural Imagery Program orthoimagery from 2005-2022. Our preliminary analysis finds decreases in the spatial extent of Kiln Fen (-11.4%), Mason Fen (-4.5%), and S1 Fen (-5.2%) that stems from both contraction of the historical fen perimeters and from encroachment and development of new saplings/tree stands. We also find that periods of major drought (e.g., 2011-2016) coincide with periods of dramatic aerial declines of fens. Our ongoing work is utilizing Google Earth Engine to analyze patterns of vegetation indices (e.g., NDVI, SAVI, NDMI) over the same temporal window as the orthoimagery analysis and will provide further insights into the relationship between the changing climate, groundwater recharge, and ecosystem health.