Many global changes are predicted to affect the Mojave Desert, including changes in precipitation, nitrogen, and disturbance. Previous results examining growth responses of a C3 drought-deciduous shrub and a C4 grass suggested that plant functional types respond differently to these global change factors. We investigated the effects of increased summer monsoons, nitrogen deposition, and soil disturbance on the C3 evergreen shrub, Larrea tridentata. Experimental manipulations occurred at the Mojave Desert Research Facility in southern Nevada by irrigating to simulate summer monsoons, adding Ca(NO3)2 in 10 or 40 kg N ha-1 yr-1 for nitrogen deposition, and creating soil disturbance by trampling the surface. Soil disturbance not only creates physical disturbance but also reduced nitrogen inputs by disrupting biological soil crust. Treatments examined in this study included control, monsoon, 10N, 40N, monsoon + 10N, monsoon + 40N, and disturbance. Relative growth rate (RGR), leaf production, branch production, and flower production were measured on a monthly basis in 2004, 2005 and 2006, which were average, above average, and average years of precipitation. Additionally, impacts of herbivory were measured at the leaf- and branch-level during 2005 and 2006 after observing large amounts of L. tridentata biomass removed by lagomorphs snipping off entire branches.
Results/Conclusions
Monsoons affected RGR, branches and leaves positively in 2004 and affected RGR and branches negatively in 2005. Impacts of water varied, depending upon parameter and season, during 2006. Nitrogen addition increased branches and leaves in 2005 but did not affect RGR in any year. Monsoons and nitrogen interacted to increase branches and leaves in 2005 and 2006. Surprisingly, monsoons decreased RGR during 2005, but only when applied without nitrogen. Disturbance decreased branches and RGR in 2005 but had no effect on leaves. No clear patterns emerged concerning treatment effects on fruits or leaf herbivory. Branch herbivory peaked across all treatments in the fall of 2005 and 2006. Additionally, monsoon-treated plants had higher amounts of biomass removed during the fall than non-monsoon plants. Interactive effects of monsoons and nitrogen were also found for branch-level herbivory. Nitrogen had no effect on biomass removal without monsoons; however, biomass removal significantly increased with addition of both monsoons and nitrogen. Variable responses to treatments among years suggest it may be difficult to predict growth responses to global change in an unpredictable system, such as the Mojave Desert. Furthermore, changes in growth are likely to have cascading effects into higher trophic levels.
