Vegetation responses to anthropogenic nitrogen deposition and nitrogen fertilization in chaparral and coastal sage scrub of southern California

Mediterranean-type ecosystems of southern California are exposed to atmospheric nitrogen deposition with some areas receiving 20-45 kg N ha-1 annually. Although nitrogen is a limiting nutrient to plant growth, most nitrogen deposition in southern California occurs as dry deposition when plants are physiologically inactive due to seasonal drought. Due to this inactivity, it is unclear whether or not plants can utilize the additional nitrogen. The effects of nitrogen fertilization on aboveground plant production were tested in coastal sage scrub and chaparral ecosystems, and the effects of nitrogen deposition on post-fire recovery were observed in chaparral ecosystems. I found that experimentally added nitrogen did not result in increased production in either coastal sage scrub or chaparral sites. The vegetation did absorb added nitrogen but did not allocate aboveground production. Chaparral ecosystems did not respond to nitrogen deposition with increased rates of total or shrub biomass production post-fire but did show low density of large shrubs at high nitrogen deposition sites and high density of small shrubs at low nitrogen deposition sites. Since vegetation absorbs nitrogen but does not allocate it to increased production, it is important to discover the fate of the nitrogen because nitrogen deposition is projected to increase in southern California in future years. Key words: disturbance, Mediterranean-type ecosystems, semi-arid ecosystems, Adenostoma fasciculatum, Ceanothus, air pollution