UC San Diego

Exotic annual plant species are an increasing problem across Southern California deserts where they can alter ecosystem processes and deter native plant establishment. The southwestern US is expected to experience increasingly variable rainfall as a result of climate change, and larger rainfall events could facilitate invasion by increasing soil moisture because exotic species often benefit over native species under high resource conditions. Exotic species may also benefit from plant- soil feedbacks, where their dense shallow roots may increase soil organic matter and soil water holding capacity, creating a positive feedback to further invasion. Alternatively, fine root inputs could create negative feedbacks to exotic plant growth if they stimulate soil microbial nutrient immobilization. I tested these hypotheses in a greenhouse experiment using native and exotic desert species grown individually and in competition under varying watering frequency (frequent versus infrequent) and root additions (high versus low). Early in the growing season soil moisture was highest in infrequent watering and root addition treatments. Plant growth was higher when watered infrequently, but exotic species growth declined more than native species growth with root additions, suggesting that the positive effect of increased soil moisture was overwhelmed by other factors. Mechanistically, adding roots lowered inorganic nitrogen availability but microbial biomass nitrogen was increased when infrequently watered. This suggests increasing soil moisture and adding roots can stimulate soil microbial nitrogen immobilization resulting in diminished plant growth. Specific species had variable responses to the treatments and this suggests potential management strategies to combat problematic desert invaders