Effects of enhanced summer monsoons, nitrogen deposition and soil disturbance on Larrea tridentata productivity and subsequent herbivory in the Mojave Desert

Abstract

Important global change factors in the Mojave Desert include altered precipitation, nitrogen deposition and soil disturbance. We examined the effects of enhanced summer monsoons, nitrogen deposition, and decreased nitrogen inputs via soil disturbance on Larrea tridentata productivity, reproduction, and herbivory. Larrea growth increased with summer monsoons in the dry years of 2004 and 2006, while Larrea growth decreased with summer monsoons in the wet year of 2005. Contrary to predictions, nitrogen addition only increased branch production in the summer of 2005. Combined treatments of monsoons and nitrogen did not result in enhanced growth or reproduction. Disturbance, which was intended to decrease N inputs from biological soil crust, decreased stem elongation in 2005. No treatments affected Larrea reproduction or insect herbivory. Substantial branch-level herbivory by mammals was observed, which significantly increased in the fall of 2005 and 2006, particularly with enhanced monsoons. While nitrogen addition alone had no effect, the addition of water and nitrogen significantly increased branch removal. Our results suggest that increased summer monsoons and nitrogen deposition may result in only small growth increases by Larrea in the Mojave Desert. However, any biomass gains are likely to be lost due to extensive mammalian herbivory with water and nitrogen additions.