Invasive plant species can crucially influence native plant communities and ecosystem processes. However, what determines the magnitude and direction of invasive plant effects on native species and ecosystems? More specifically, how does the type of mycorrhizal-association of resident species affect the invader success and impacts? We established mesocosms planted with five 1-2 year old saplings of ectomycorrhizal (ECM) or arbuscular mycorrhizal (AM) tree species, or both mycorrhizal types (Mixed). Following tree establishment we added 200 seeds of the invasive understory plant Microstegium vimineum to half of the mesocosms and left the other mesocosms as “uninvaded” controls. We hypothesized that the negative effects of Microstegium on tree growth would be greatest in AM mesocosms as Microstegium and AM trees both compete for the same inorganic nutrients. In each mesocosm, we measured tree growth (monthly), photosynthetic activity of two dominant trees (four times), and if present, Microstegium peak biomass. Soil cores and resin bags (mixed beads) were used to measure nutrient availability in each pot over the course of the experiment.
Results/Conclusions
Microstegium productivity was significantly (p = 0.001) inhibited in the AM mesocosms, as we harvested a 46 % lower Microstegium biomass in AM than in ECM or Mixed mesocosms. Despite lower Microstegium biomass, we observed lower photosynthetic activities (by 7 ± 3 %) of AM trees in invaded than uninvaded mesocosms. However, although there was more Microstegium biomass in ECM than in AM mesocosms, ECM trees had up to 16 ± 6 % greater photosynthesis in presence of Microstegium. In general, the presence of Microstegium reduced N mineralization by 50 % and nitrification activities by 14 % in the invaded mesocosms. Regardless of the presence of Microstegium, we measured by 71 % and by 44 % significantly (p < 0.05) lower N mineralization and nitrification activities in AM than in ECM or Mixed mesocosms. Related to lower nitrification activities, less NO3- was released in the AM (0.07 ± 0.03 µg/g wet resin/day) than in the ECM (2.17 ± 0.6 µg/g wet resin/day) mesocosms, suggesting a stronger nitrogen competition between plants and microbes in AM mesocosms. Microstegium had stronger negative effects on tree performance in the AM than ECM dominated mesocosms, indicated by reduced tree photosynthesis and N cycling, due to increased nutrient competition. Thus, a high similarity in traits between invasive and native plant species can lead to an increase in nutrient competition and intensify negative invasive plant effects.