title: Combined effects of precipitation and nitrogen deposition on native and invasive winter annual production in California deserts.
abstract: Primary production in deserts is limited by soil moisture and N availability, and thus is likely to be influenced by both anthropogenic N deposition and precipitation regimes altered as a consequence of climate change. Invasive annual grasses are particularly responsive to increases in N and water availabilities, which may result in competition with native forb communities. Additionally, conditions favoring increased invasive grass production in arid and semi-arid regions can increase fire risk, negatively impacting woody vegetation that is not adapted to fire. We conducted a seeded garden experiment and a 5-year field fertilization experiment to investigate how winter annual production is altered by increasing N supply under a range of water availabilities. The greatest production of invasive grasses and native forbs in the garden experiment occurred under the highest soil N (inorganic N after fertilization = 2.99 g m(-2)) and highest watering regime, indicating these species are limited by both water and N. A classification and regression tree (CART) analysis on the multi-year field fertilization study showed that winter annual biomass was primarily limited by November-December precipitation. Biomass exceeded the threshold capable of carrying fire when inorganic soil N availability was at least 3.2 g m(-2) in piñon-juniper woodland. Due to water limitation in creosote bush scrub, biomass exceeded the fire threshold only under very wet conditions regardless of soil N status. The CART analyses also revealed that percent cover of invasive grasses and native forbs is primarily dependent on the timing and amount of precipitation and secondarily dependent on soil N and site-specific characteristics. In total, our results indicate that areas of high N deposition will be susceptible to grass invasion, particularly in wet years, potentially reducing native species cover and increasing the risk of fire.
authors.affiliation: Center for Conservation Biology, University of California, Riverside, CA 92521, USA. lrao@arb.ca.gov
authors.forename: Leela E
authors.initials: LE
authors.lastname: Rao
authors.forename: Edith B
authors.initials: EB
authors.lastname: Allen
mesh_terms: D000383:Agriculture; D018533:Biomass; D002140:California; D057231:Climate Change; D003889:Desert Climate; D005306:Fertilization; D005390:Fires; D005843:Geography; D009584:Nitrogen / Q000378:metabolism*; D063245:Plant Development; D010944:Plants / Q000145:classification; D006109:Poaceae / Q000254:growth & development; D011891:Rain; D012044:Regression Analysis; D012621:Seasons; D012987:Soil / Q000032:analysis; D013997:Time Factors; D014867:Water / Q000032:analysis*
chemical_list: D012987:Soil; D014867:Water; D009584:Nitrogen