Terrestrial plant communities are increasingly threatened by multiple drivers of global environmental change, including anthropogenic nitrogen (N) deposition, climate change, and the invasion of nonnative plant species. A growing body of evidence suggests that both drought and elevated N deposition may favor nonnative annual species over native perennial coastal sage scrub species in southern California, potentially contributing to increased invasion and vegetation-type conversion. We sought to understand the role of plant functional traits, and tradeoffs among functional traits, in mediating these community-level dynamics. We grew mixes of native and invasive plant species in plant community mesocosms in the greenhouse under altered N and water availability in a full-factorial design. We measured multiple above- and belowground plant traits throughout the experiment, and compared trait responses across species, community types (native, invasive or mixed) and resource treatments.
We observed differences in growth and functional traits between native and invasive species, as well as differences in tradeoffs between traits experienced by these species. Overall, invasive species grew more rapidly and accumulated significantly greater biomass than native species, particularly under high resource availability. Invasives also greatly suppressed the growth of natives in mesocosms where these species were grown together. Natives experienced a tradeoff between growth rate and water-use efficiency (WUE) both within and across species, where more rapid growth resulted in reduced WUE, while invasives maintained both high rates of growth and high WUE. Invasives also exhibited lower root to shoot ratios and nonmycorrhizal root fungal infection than natives. Together, these results suggest that seedlings of native perennial species in this system are subject to physiological constraints that are weaker or not present in co-occurring invasives. These superior trait combinations may contribute to successful invasion of these nonnative annuals, posing a major challenge for the reestablishment and restoration of native coastal sage scrub species.