Post-fire succession of plants and soils in chaparral shrublands: the role of ephemeral herbs and mammalian herbivores
Understanding the drivers of ecological succession requires linking ecological interactions and biodiversity to ecosystem soil processes. California’s widespread chaparral shrublands provide an ideal setting to investigate mechanisms linking plant and soil dynamics during succession. Periodic crown-fire in chaparral temporarily removes all living shrub cover, deposits mineral N on soils, and allows functionally diverse herbaceous communities to dominate the landscape for 3-5 years. In many areas, chaparral stands are bordered by non-native grasslands and fire can facilitate invasion of chaparral by grasses. Herbivorous small mammals in chaparral have been previously demonstrated to impact the composition of the chaparral understory and consume non-native grasses at the grassland-chaparral border. Herbivory may also impact soil N and C cycling through changes to plant community composition and direct additions of herbivore feces and urine. The composition of post-fire herbaceous communities, whether influenced by herbivory or another factor, may itself impact soil nutrient cycling. In particular, N-fixing and non-N-fixing herbs can have functionally distinct effects on ecological and ecosystem processes, influencing herbivore feeding preferences, competitive or facilitation relationships between plants, and litter decomposition rates.
In this dissertation, I conducted three field experiments in Northern California chaparral. In Chapter 1, I established an herbivore-exclosure experiment at chaparral-grassland borders to investigate how mammalian herbivores impact invasive grasses and general understory herb composition, as well as how this effect was mediated by wildfire. In Chapters 2 and 3, respectively, I manipulated herbivore abundance and the biomass of N-fixing and non-N-fixing herbs to investigate how these factors influence soil C and N cycling in post-fire chaparral during the first two years after fire.
Mammalian herbivory reduced herbaceous abundance post-fire, particularly non-native grasses, suggesting herbivores limit grassland invasion into chaparral. However, herbivore exclosures did not measurably impact N and C cycling during the first two post-fire seasons. Removing N-fixing and non-N-fixings herbs dramatically influenced soil N and C dynamics in the same system. Two years after fire, plots with all herbs removed had significantly lower soil N and C than any treatment with herbs. Post-fire herbs do appear to enrich soil in C and N, which may benefit recovering shrubs even after herbaceous communities die back.