Propagule pressure (PP) influences the ability of introduced plants to invade new locations. In herbaceous wetlands, where the majority of plants are clonal, differences in clonal architecture can further influence invasion outcomes. The effect of clonal trait variation on invasion success likely depends on nitrogen (N) loading, due to the link between clonal structure and a plant’s effectiveness in competing for available nutrients. We investigated the interaction of PP and clonal architecture across a N gradient using MONDRIAN, an individual-based community-ecosystem model that simulates growth and competition among individual ramets. Using parameterizations for Phragmites australis, we explored the success or failure of invader colonization into a bareground environment (no interspecific competition) or a 3-species native community at four levels of PP (1, 3, 10, 30 rhizomes/m2) across a N loading gradient (0.86 to 30 g N m-2yr-1). In addition, we either turned off the invader’s ability to branch beyond a single chain of daughter rhizomes or varied terminal and lateral branching probabilities between 0 (no branching) and 1 (always branching). We compared these scenarios to determine if and how the invasion success of P. australis is determined by the interaction between PP and clonal architecture.
Results/Conclusions
Phragmites australis was unable to persist at low N loading at any PP in either bareground or invasion scenarios. At 9 to 30 g N m-2yr-1, when added to bareground with clonal branching ON, P. australis was able to colonize and reach the same steady state NPP across all PP levels at a given N loading rate. This suggests that in the absence of interspecific competition, PP doesn’t influence colonization success. However, in invasion scenarios, P. australis was not able to successfully invade at low PP but was somewhat successful at higher PP (maximum of 25% of the community biomass total at 9 g N m-2yr-1). At higher N loading levels, all PP levels resulted in complete native suppression by the end of the 45-year simulation, though invasion rate varied by PP. At the lowest PP it took 9 years to fully suppress the natives, compared to 4 years at the highest PP. Interestingly, without clonal branching, P. australis wasn’t able to colonize as successfully or reach comparable NPP as when clonal branching was ON. Our results suggest that invasion success is a complex outcome influenced by both external factors (N loading, PP, and competition) and internal factors (clonal branching architecture).