Ecologists have long been interested in the relationships among species functional traits, community composition, and ecosystem function. We performed in silico experiments with a simulation model, Mondrian, applied in coastal Great Lakes wetlands with clonal plants Eleocharis smallii, Schoenoplectus acutus, and Juncus balticus. Mondrian is a spatially-explicit, individual-based model that simulates competition for N and light, while including clonal transfers of C and N within genets. It incorporates four levels of organization: Individual plant physiology and morphology including clonal branching, population processes including reproduction and mortality, community composition and dynamics, and ecosystem processes. The latter include complete C and N cycles with live biomass and detrital pools, simulate the main biogeochemical processes in wetlands and the effects of climate and water level. To gain insight into the ecosystem N cycle as a dynamic phenomenon arising from the interplay of plant traits, environmental conditions, and biogeochemical processes, we simulated constant annual conditions over a 50 year period without disturbance. In sets of model runs, we examined the effects on ecosystem N cycling of a range of species functional traits under a range of constant environmental conditions including N loading, climate, and water level.
Results/Conclusions
The ecosystem N cycle functioned as an attractor on decadal time scales. Starting with low N storage in detritus (2.5 g N/m2) and an inflow of 10 g N/m2y, the ecosystem retained N as it built detritus (to 27 g N/m2), eventually supporting a community N uptake of 10.7 g N/m2y. Alternatively, starting with high N storage in detritus (48 g N/m2) the wetland lost N until it reached the same equilibrium point. Alteration of species traits or biogeochemical parameters altered the attractor values of N fluxes. Decreasing N resorption (a functional trait) in J. balticus caused, for the same N loading, a higher equilibrium in detrital N storage (28 g N/m2) and greater community N uptake (11.8 g N/m2y). The ecosystem-level altered N cycling fluxes fed back to produce altered species-level outcomes. While decreasing resorption in J. balticus caused a rise in N availability for all species, it harmed J. balticus in competition, lowering it from 81% to 18% of the community. An altered environmental condition such as water level had an effect that changed over the long term, as ecosystem N cycling and plant community compositions both changed and adjusted to one another on decadal time scales.