Competitive Interactions

A competition study conducted in Germany compared the biomass production and cover of bush grass (Calamagrostis epigejos), a tall rhizomatous grass, Canada goldenrod (Solidago canadensis), a tall rhizomatous forb, and common tansy. They were grown for five years in monocultures and mixtures on three soils representing a productivity gradient; top soil, “ruderal soil” (a mixture of top soil and subsoil), and sand. When growing in mixtures, average plant cover of common tansy was greater than either of the other species in all soil types in all years with a few exceptions. The exceptions were; the high productivity topsoil in the fifth year when slugs selectively defoliated the common tansy, and when growing in combination with Canada goldenrod in the mid-productivity ruderal soil in each of the five years. However, replacement diagrams suggest that common tansy and bush grass were equally competitive in sand and ruderal soil but bush grass was more competitive in the topsoil where common tansy was selectively defoliated by slugs. Similarly, slugs reduced common tansy’s ability to compete with goldenrod in the high productivity topsoil. However, on the ruderal soil, Canada goldenrod was more competitive than common tansy and the reverse was the case on the low productivity sand. There was evidence of niche separation between common tansy and the other species only on the ruderal soil, and when common tansy grew with Canada goldenrod on topsoil.

The results suggest common tansy was a strong competitor on low productivity soils where it was better at acquiring nutrients than the other species. This could explain why common tansy populations grow in near-monocultures on sandy soils of riparian areas. On the high productivity experimental soil, common tansy was more productive than the other species, but was also susceptible to predation, presumably because of high nitrogen content in the leaves. This suggests selectively defoliating common tansy growing on productive soils will shift competition to favor rhizomatous grasses and forbs. On less productive soils, management targeting shoots and rhizomes may be needed to shift competition.

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Last Modified: 02/25/2008