Emergence of nutrient-cycling feedbacks related to plant size and invasion success in a wetland community–ecosystem model

Invasive plants in wetlands may alter community composition through complex interactions related to elevated N inflows, plant size, litter production, and ecosystem N retention and recycling. To investigate these interactions, we constructed an individual-based model, Mondrian, that integrates individual growth and clonal reproduction, nutrient competitive interactions among species, and ecosystem processes. We conducted in silico experiments, parameterized for Great Lakes coastal marshes, where invaders that differed only in size attempted to invade native communities across a range of N inflows. Small invaders were able to persist only at low N inflow and never dominated. Large invaders were not able to reproduce clonally at low N inflow but they successfully coexisted with natives at intermediate N inflow and dominated at high N inflow, excluding natives in some cases. In both native and invaded communities, a positive feedback in plant-detritus N cycling emerged, amplifying ecosystem N cycling to nearly 2× the range of N inflows. The largest invaders augmented this N-cycling feedback over the native community by up to 23%, increasing with greater N inflow, driving community NPP higher than the native community by 33% and litter mass higher by 35%. In communities dominated by the largest invader, wetland N retention was increased but species diversity decreased. Results demonstrate that a single trait difference, plant size, simultaneously allows natives to resist invasion at low N inflows and allows invaders to dominate at high N inflows, partly through augmenting ecosystem N-cycling feedbacks.