Effects of plant invasions on ecosystem processes: Linking above- and below-ground resource-use strategies of native and invasive species in eastern U.S. forests

Despite the increasing number of non-native invasive species worldwide and their potential impacts on ecosystems, the mechanisms that invaders alter ecosystem nutrient processes remain elusive. Invaders are often more productive than native species which suggests invaders may have different above- and below-ground resource-use strategies that can profoundly alter ecosystem processes. Here I investigated above- and below-ground plant traits and soil properties associated with resource-use strategies and soil nitrogen (N) dynamics for multiple native and non-native forest understory species in the Eastern U.S. to better understand invader impacts on ecosystem processes. In the first study, performed in a common garden, I examined the linkage between above- and below-ground resource-use strategies for native and invasive species that allow invaders to be more productive than co-occurring natives. Results showed that, despite invaders losing a significant amount of N from litter, they had greater root production and specific root length associated with a greater soil nutrient uptake capacity than natives. In the second study, I examined whether the different tissue traits are associated with litter decomposition rate and if invaders can increase nutrient cycling through faster litter decomposition than natives. Results revealed no differences in leaf and root decomposition rates between native and non-native forest understory woody species, suggesting that litter decomposition rate is not a process that invasive species affect with regard to soil nutrient processes in the Eastern U.S. forests. Finally, I investigated invader impacts on soil N processes in a monoculture experiment. After two growing seasons, invaders had greater above- and below-ground productivity. Invaders facilitated N cycling via greater litter N input into the soil that increased soil N availability, and had greater fine root production and SRL that increased plant N uptake. Although the greater aboveground production of invaders reduced soil temperature and moisture, which can reduce soil microbial activity, the stimulatory effects of a greater flow of litter N to the soil appeared to overwhelm any negative effects that invaders had on the soil microclimate. Taken together, my results suggest that invaders have different above-and below-ground resource-use strategies and invaders' greater productivity is one of the major drivers that can significantly change ecosystem processes.