Physiological trait variation in plant invaders influences invasion success and C cycling across a hydrology and nitrogen gradient in a simulated wetland ecosystem

Background/Question/Methods

Wetlands provide numerous ecosystem services, are one of the most invaded habitats worldwide, and store C disproportionally to their area.  It is thus important to understand how variation in key plant traits influences invasion success and ecosystem C cycling.  MONDRIAN is an individual based model, simulating growth and competition for nutrients among individual ramets, as well as clonal connections and horizontal expansion.  Simulated ecosystem N cycling both drives, and is driven by, plant growth, litter production, and biogeochemistry of litter and sediment organic matter.  We conducted a modeling experiment to test the effects of variation in key physiological traits (tissue C/N ratios, maximum relative growth rate, N resorption, and litter lability) of Phragmites australis and Typha xgluaca during invasions of a 3-species native community along a N and hydrology gradient.  We examined whether trait variation interacted with hydrology and/or N loading to affect invader success and C storage in living biomass, litter, muck, and mineral SOM, along with the sum of all pools (ecosystem C).  The gradient of N loading ranged from oligotrophic to eutrophic and wetland sediments were either always anaerobic (flooded), always aerobic, or varied throughout the year.     

Results/Conclusions

Variation in physiological traits had similar effects on invasion success and C storage for P. australis and T. x glauca.  Under all hydrologic regimes, varying tissue C/N ratios or N resorption did not affect invasion success at high or low N loading, but at intermediate N loading (9 g N m^-2 yr^-1) increasing these traits increased invasion success, thereby shifting the steepness of the invasion threshold along the N gradient.  In contrast to invasion success, which was only sensitive to variation in physiological traits at intermediate N loading, ecosystem C pool response to trait variation occurred at all the N levels when invasion was successful, though the degree of response depended on hydrology. In aerobic conditions increasing tissue C/N ratios and N resorption did not affect C storage, whereas in anaerobic conditions the majority of C pools increased at high N loading.  Increasing litter lability significantly decreased C in both above and belowground litter pools at high N loading under all hydrologic regimes.  Altering maximum relative growth rate surprisingly did not have a consistent effect on any C pool.  These simulations demonstrate how plant traits of the most successful invasive wetland plants in North America can influence invasion success and wetland C storage.