A centurial signature of anthropogenic nitrogen and carbon in California serpentine ecosystems

Increasing anthropogenic emissions of nitrogen (N) and carbon (C) are major threats to ecosystems globally. Although atmospheric N deposition is likely affecting N cycling and community composition in California's serpentine ecosystems, a historical record of N inputs to vegetation has yet to be reconstructed for these nutrient-limited biodiversity hotspots. For leather oak (Quercus durata var. durata), a foundational, serpentine-endemic species, we investigated leaf N and C isotopic composition (δ¹⁵N and δ¹³C) and leaf %N of herbarium and modern leaf samples collected from 1899 to 2009 from serpentine ecosystems in two study areas in California: Santa Clara County, and Lake and Napa Counties combined. We also evaluated tree ring growth over a similar time period in long-lived leather oak individuals. Leaf δ¹⁵N and δ¹³C values decreased over time in both study areas, likely reflecting changes in the regional and local atmospheric N and C pools caused by human perturbation. However, leaf %N values and stem growth did not change over time with increasing N deposition, indicating that increasing atmospheric N deposition and CO₂ concentration may not translate to increased N uptake or productivity in plants with conservative growth strategies, even in ecosystems thought to be N-limited. In serpentine systems, this could competitively advantage nitrophilic invasive annual grasses and accelerate trends toward native species loss. While the rates of decline in leaf δ¹⁵N values were similar between study areas, rates of decline in leaf δ¹³C values were steeper in Santa Clara County, possibly reflecting its more urban environment. Herbarium samples combined with tree ring data can provide a valuable opportunity to explore the historical record of human-induced changes in N and C cycling and their biotic impacts.