Plant invasion affects biodiversity by altering community composition and functional structure. Today, many habitats are co-invaded, but we have limited understanding of how co-occurring invaders influence ecosystem processes. Plant community alterations cascade to change soil microbial communities as well as the ecosystem processes that soil communities mediate, such as soil carbon (C) storage, which is critical because soils store at least three times more C than the atmosphere or living plant material. Decomposition, an ecosystem process that determines C release from the soil, is influenced at the local level by changes in plant and litter functional traits. Thus, a better understanding of shifts in plant community composition can help predict changes in ecosystem functions. We hypothesize that the overall ecosystem impact of co-occurring invaders should be driven by functional differences between co-occurring invasives and natives, where the largest impacts are expected when there is little functional overlap between invaders and natives. Using a suite of lab microcosm decomposition studies, we test these predictions with 26 common native and nonnative plant species to southern New England deciduous forests.
Results/Conclusions
There was wide variation between species in litter functional traits predicted to be important for decomposition (specific leaf area [SLA], total phosphorus, percent nitrogen [N], percent C, acid detergent lignin), but on average invasives had higher total phosphorus, percent N, and SLA compared to natives. Native and invasive plants also varied in their total decomposition rates, as measured by carbon mineralization rates, and species traits explained three-quarters of the variation in carbon mineralization and percent litter mass loss between all 26 species (carbon mineralization: R2 adjusted=0.79, F13,142 = 44.72, p < 0.0001; percent litter mass loss: R2 adjusted=0.78, F15,140 = 37.95, p < 0.0001). Using these functional traits, we measured native and invasive plant functional richness to assess differences in “trait space” between native and invasives. Native and nonnative plants had similar functional richness (native=11.8, invasive=12.7), but occupied different areas of trait space indicating that nonnative plants are altering trait space in co-invaded ecosystems. We demonstrate that this “trait space” framework can be useful for making predictions of when invasives should have the greatest impact on decomposition processes.