Nitrogen and phosphorus availability, ecosystem processes and plant community dynamics in boreal wetland meadows
The effects of nitrogen (N) and phosphorus (P) availability on above-ground primary production and plant community dynamics in wetland "beaver" meadows were examined in Voyageurs National Park in northern Minnesota, USA. Five meadows were floristically distinguished as either "sedge-dominated" or "grass-dominated" and were sampled to describe patterns of above-ground primary production, seasonal depth to the water table, N and P availability, and in-situ N mineralization. In two sedge meadows, production was higher, N and P were more available during the non-growing season, and water tables were closer to the soil surface than in three grass meadows. Across all meadows, productivity correlated positively with available N and P, but not with N mineralization, which was insufficient to meet the requirements of the plant communities, supporting the existence of external, presumable hydrologic, sources of N to the meadows.Based on these and other observations, two hypotheses regarding N and P limitation to production were developed: (H1) lower production in the grass meadows is due to a greater nutrient limitation in the grass meadows, and (H2) wet-sedge meadows are proportionately less P limited than the grass meadows. A third hypothesis (H3) was developed to test a theory that plant species function with all resources equally limiting. An experiment varying ratios and amounts of N and P was conducted in all meadows. Biomass response after one year reflected ambient hydrologic and fertility conditions; production in the wetter, more nutrient-rich sedge meadows was not nutrient limited, while production in the drier, relatively less rich grass meadows was nutrient limited. Thus, while hydrologic and fertility conditions tended to co-vary, differences in production are attributed differences in fertility, supporting H1. As stated, H2 assumed that biomass in the sedge meadows would respond to nutrient additions, which did not occur. The pattern of response in the grass meadows supported a Leibig model of nutrient limitation, leading to a rejection of H3.Nutrient enhancements may alter plant community dynamics according to two theoretical predictions: 1) plant species will segregate along gradients of resource supply if their resource requirements differ (Tilman 1982) and, 2) species richness will decrease if production increases in competitive environments (Grime 1979). Two hypotheses (H4, H5) were developed to test these theories. After two years, botanical convergence with treatment was not indicated in any meadow, leading to a rejection of H4. High N and P caused a reduction in species richness in both community types but because of the lack of biomass response in the sedge meadows, mechanisms supporting changes in richness independent of changes in productivity are suggested, and H5 is rejected. Thus, while the ecosystem property of production (in the grass meadows) was affected by N and P additions, dynamics of the dominant plant species appeared to be relatively unaffected.
- Forestry