Using fire risk and species loss to set critical loads for nitrogen deposition in southern California shrublands

Abstract

Southern California deserts and coastal sage scrub (CSS) are undergoing vegetation-type conversion to exotic annual grassland, especially in regions downwind of urban areas that receive high nitrogen (N), primarily as dry deposition. To determine critical loads (CLs) of N that cause negative impacts, we measured plant and soil responses along N deposition gradients, fertilized vegetation at different N levels, and used biomass production output from the DayCent model. Nitrogen deposition gradients were identified from the CMAQ model and compared with measured N deposition values. Coastal sage scrub receives N deposition as high as 30 kg ha^− 1 year^− 1, while the desert has levels up to 16 kg ha^− 1 year^− 1. These ecosystems are subject to increases in exotic species production, loss of native species diversity, and increased fire risk at relatively low CLs. For instance, a gradient survey in CSS showed that exotic grass cover increased and native plant species richness declined by almost 50 % above 10 kg N ha^− 1 year^− 1. Fertilization studies in desert creosote bush scrub showed a significant increase in exotic species biomass with 5 kg N ha^− 1 year^− 1 in a wet year, and biomass output from DayCent modelling indicated an increased fire risk from exotic grasses with 1 t per ha production during years with moderate to high precipitation at 2.2–8.8 kg N ha^− 1 year^− 1. The difference in CL between desert and CSS is related to the different criteria used (diversity loss in CSS, productivity and fire risk in desert), as well as responsiveness of native vs. exotic plant species to N and the degree to which precipitation and soil N limits plant growth in the two vegetation types.

Citation

Allen, E.B.; Rao, L.E.; Tonnesen, G.; Johnson, R.F.; Fenn, M.E.; Bytnerowicz, A. 2014. Using fire risk and species loss to set critical loads for nitrogen deposition in southern California shrublands. In: Sutton, M.A. et al., eds. Nitrogen Deposition, Critical Loads and Biodiversity. Netherlands: Springer: 319-327. Chapter 34.