Thunderstorms, SUVs, and ATVs: What do they mean for Mojave Desert perennials?

Altered precipitation, nitrogen deposition and soil disturbance are important global changes in the Mojave Desert. At the Mojave Global Change Facility, we applied seven treatments each year since 2001: 25 mm of water in June, July and August, 10 or 40 kg nitrogen (N) ha^-1 yr^-1, monsoon and each N level combined, or physically disturbing the soil by trampling. We examined the growth response of the C₄ grass Pleuraphis rigida to these treatments in 2004, 2005 and 2006, which had average, wet and average years of precipitation. We determined leaf number, stem length, and number of flowers produced by Pleuraphis.  Early in the season, monsoons decreased leaf number and stem length in 2004 and 2005; however, the late season grass immediately responded to the enhanced summer monsoon in 2004 but not in 2005. N deposition increased leaf number and stem length in 2004 but decreased them in 2005. Plots with both monsoon and N treatments mimicked the monsoon plots. Disturbance negatively affected leaf number and stem length early in the season in 2004 and 2005, but these effects neutralized later in the season. No parameters responded to treatments applied in 2006. Additionally, no treatments had strong effects on flower production.  Reponses to N addition and N removal via disturbing the soil biological crust probably affected the free-living N fixers of Pleuraphis. These results contrast with other functional species investigated and suggest that variable growth responses to global change will likely affect community composition in the Mojave Desert.