The spread of invasive non-native plant species threatens native plants in most California ecosystems. Brassica tournefortii (Sahara mustard) is a highly invasive non-native annual herb that is spreading rapidly in arid ecosystems of southern California, particularly in the Mojave Desert. B. tournefortii displaces native annual species by forming dense, persistent populations. Although many factors could influence invasion success, resource availability is likely to play a role. Resource variability in California, particularly in soil nitrogen and water, may be linked with the expansion of B. tournefortii in California’s arid ecosystems, which historically are relatively resistant to invasions. We conducted a greenhouse experiment to explore the possible interactive effects of water and nitrogen availability on growth, allocation, and reproduction of B. tournefortii. We applied two fully crossed factors to B. tournefortii: two levels of water availability (high or low) and three levels of nitrogen availability (high, medium, or low). The nitrogen addition treatments span the range of recorded levels of nitrogen in the Mojave Desert. The growth and reproductive variables were measured by destructively sampling plants three times throughout the experiment.
Higher water and nitrogen availability resulted in increased total dry biomass and higher average fruit and seed count. Higher water availability increased the rate of transition across phenological stages, while higher nitrogen availability increased the proportion of plants in reaching each developmental stage. Although both water and nitrogen impacted growth and reproduction, we found no evidence of interactions between water and nitrogen availability. Previous studies have shown that higher water availability can lead to greater expansion of B. tournefortii in favorable conditions, but this study demonstrates that nitrogen may have a stronger influence on productivity and reproduction. Future studies on how nitrogen availability influences the competitive impacts of B. tournefortii on native species will add to our understanding of this invasion, and may aid in the management of existing populations in the arid regions of California.