Nutrient enrichment and plant interactions influence native and exotic plant communities, but they were unaffected by climate warming

Background/Question/Methods

Anthropogenic factors including climate warming and nutrient enrichment may influence invasion success and alter community composition patterns. Bioclimatic models focus on exotic species’ range expansion and contraction but overlook interactions between anthropogenic factors as well as their impact on native species. It is critical to experimentally evaluate effects of multiple anthropogenic factors on plant invasions and their impact on native plants. We asked, 1) Do nutrients and climate warming reduce native species’ performance? 2) Are impacts smaller on exotic species that occur at higher temperatures and nutrient levels in their native range? 3) How do nutrients and warming alter plant community composition patterns? We conducted a 3×2×3 factorial outdoor mesocosm experiment with four replicates. We established mesocoms in 454L tanks by adding 15cm of soil/sand mixture, municipal water (4 day hydraulic residence time), and 20 native snails. Mesocosms were randomly assigned to treatments: 1) plant addition: [native (4 species), exotic (3 species), or mix (7 species)], 2) nutrients: [ambient or high (6mg/L N)], and 3) warming: [ambient, low (+1°C), or high (+2°C) manipulated using solar water heaters]. We conducted monthly temperature and visual plant surveys. After 11 months, we harvested, sorted by species, and oven dried aboveground plant mass.

Results/Conclusions

Neither native nor exotic plant mass was affected by the presence of the other group in low nutrient conditions. However, increases in native plant mass with nutrients were significantly smaller when exotic plants were present. The same pattern existed for exotic plant mass (natives only reduced exotic mass with increasing nutrients). Communities with only exotic or native plants had higher mass on average than mixed communities (underyielding). These results suggest that native and exotic plants compete with each other and reduce each other’s growth. The increases in exotic plant mass with nutrient addition were significantly larger than native plant mass in single origin communities (27-fold vs. 4-fold) and in mixed communities (16-fold vs. 3-fold). These results suggest that exotic plants are more able to take advantage of high resource conditions and that nutrient enriched ecosystems are more vulnerable to plant invasions. Warming did not significantly affect native or exotic plant mass but exotic plant mass tended to decrease (P=0.11) while native plant mass tended to increase with warming (P=0.26) suggesting that any effect of warming would reduce probability of plant invasions. Overall, our results indicated that nutrient enrichment is the critical anthropogenic factor that influences invasions in this freshwater ecosystem.