ERDC/EL TR-04-14, "A Simulation Model on the Competition for Light of Meadow-forming and Canopy-forming Aquatic Macrophytes at High and Low Nutrient Availability"

A simulation model has been developed that focuses on the ability of two competing submersed macrophytes, meadow-forming and canopy-forming, to maintain their biomass under different environmental conditions. Vallisneria americana (American wildcelery) serves as the example for meadow-forming plants and Stuckenia pectinata (until recently known as Potamogeton pectinatus or sago pondweed) for canopy-forming plants. The model can be used to predict changes in species composition of submersed vegetation as a result of changes in the availability of resources in shallow freshwater bodies. In the model, the two plant species compete for light and exhibit different species-specific relationships between plant tissue nitrogen (N):phosphorus (P) ratio and plant biomass production. The latter species-specific relationships have not been determined in V. americana and P. pectinatus, and, therefore, for calibration of the model, the specific relationships between plant tissue N:P ratio and reduction in plant biomass production of Zannichellia palustris and Elodea canadensis were used. The latter species have habitat preferences similar to those of V. americana and P. pectinatus. Competition for light proved to be a far more important determinant of species composition than the availabilities of N and P in the sediment. Intraspecific competition for light did not occur in V. americana in a temperate climate, but it was observed at densities > 8-9 plants m in a more southern climate. It occurred in P. pectinatus at plant densities >4-5 plants m. Coexistence of both species occurred only at V. americana:P. pectinatus plant density ratios of 28:2 to 26:4 plants m in the absence of N and P limitation of growth, irrespective of climate (temperate and more southern climates tested). At density ratios higher than 28:2, V. americana excludes P. pectinatus, and at density ratios lower than 26:4, P. pectinatus excludes V. americana. The density ratio range at which coexistence was possible increased with water turbidity between extinction coefficients of 0.43 and 2.00 m. Light interception by epiphytes at a level of 25 percent of observed maxima in the Upper Mississippi River allowed coexistence in clear water but prevented it in turbid water in a more southern climate. Under N limiting conditions for both species, P. pectinatus displaced V. americana, but under P limiting conditions for P. pectinatus, V. americana won the competition. Coexistence was expanded by fertilization with both N and P. These results indicate that P. pectinatus has a high potential of replacing V. americana when allowed to colonize gaps in dense V. americana stands. N limiting conditions strengthen and P limiting conditions weaken the competitive advantage of P. pectinatus relative to that of V. americana, while raised N and P availabilities enhance the potential for coexistence of both species. These notions can be used as a basis for management of submersed macrophytes. It is recommended to verify/determine the species-specific relationships between plant tissue N:P ratio and plant biomass production of V. americana and P. pectinatus and validate the model coexistence results by comparison with outcomes from plant competition experiments. DISCLAIMER: The contents of this report are not to be used for advertising, publication, or promotional purposes. Citation of trade names does not constitute an official endorsement or approval of the use of such commercial products. All product names and trademarks cited are the property of their respective owners. The findings of this report are not to be construed as an official Department of the Army position unless so designated by other authorized documents.