Nitrogen Pollution and Exotics

Around the world, serpentine-derived soils characterized by relatively low ratios of calcium to magnesium (Ca:Mg) and low N support edaphically isolated communities of unique plant and animal species. Serpentine grasslands found in and near the San Francisco Bay Area are considered a “hotspot” for native and endemic biodiversity in California. They support a highly diverse mixture of native forbs and grasses specialized to the unique soil properties found here. Several federally endangered plant and animal species occur only in these serpentine grasslands, most notably the Bay checkerspot butterfly.

Until the 1980s, serpentine grassland ecosystems were threatened mainly by development and protected by reserve creation. However, N fertilization due to increasing fossil fuel emissions in the expanding Bay Area, along with changing weather patterns and the gradual evolution of serpentine-tolerant invaders, are thought to underlie rapid, recent invasion of this grassland by exotic annual grasses that are progressively eliminating rare and endemic serpentine species (Weiss 1999). Documenting the patterns and cumulative effects of N deposition in this ecosystem can direct policy and management actions to mitigate the role of N inputs in transforming this last oasis of California’s grassland biodiversity.

The goal of this project is to document and understand the history of accumulation of anthropogenic N on serpentine grassland ecosystems, through isotopic reconstruction of N inputs in (1) tree rings and (2) archived plant specimens; (3) current rates of atmospheric N deposition into serpentine grassland, in areas both downwind (high N deposition) and upwind (low N deposition) of the San Francisco Bay Area; and (4) implications of gaseous and dry atmospheric N inputs for plant uptake and the relative performances of native and exotic plant species in these threatened ecosystems. This work addresses both current and future consequences of increased N at scales ranging from individual plants to the landscape level. more details…

Project participants: Dena Vallano, Paul Selmants and Erika Zavaleta
Thanks to: California Academy of Sciences and UC-Berkeley Jepson for herberia samples.