Declines in air quality at Joshua Tree National Park and adjacent desert have been occurring over the past three decades. During the same time period, invasive grasses (Schismus barbatus, Bromus madritensis) have become more productive and fires more frequent in an area with historically infrequent fire. To test the hypothesis that elevated N may be related to invasive species, N fertilizer experiments were done at four sites in the Park with N deposition levels of 3 to 12 kg ha-1 yr-1. Ammonium nitrate was added at levels of 5, 15, and 30 kg N/ha between 2003-2006. During the 02/03 growing season, there was no response of the exotic vegetation to N; in 03/04 only the plots with 30 kg/ha responded with increased exotic grass growth; in 04/05 the plots with 5 kg/ha also responded; and there was no response to any level of N in 05/06. These responses were related to precipitation. The highest precipitation year, 04/05, had 5 times more precipitation than the lowest, 05/06. Soil N concentrations ranged from 2 to 23 mg N/kg soil along the N deposition gradient. Fertilized soils with vegetation responses had 23 mg N/kg, suggesting that exotic vegetation is already responding to elevated N deposition. During wet years such as 04/05 the vegetation responded to even lower soil N concentrations. These high precipitation years will drive the spread of invasive species and their seedbanks even under relatively low levels of N input of 5 kg ha-1 yr-1.
