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

Abstract 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 (δ 15 N and δ 13 C) 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 δ 15 N and δ 13 C 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 CO 2 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 δ 15 N values were similar between study areas, rates of decline in leaf δ 13 C 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.