The dynamics of designed plant communities of rosette forming forbs for use in supra-urban drainage swales

This study looked at the medium term (5 years) dynamics of a sown, designed community of uncompetitive rosette forbs in wet, seasonally anaerobic soils. Twenty-one species of wet grassland habitats (15 non-native and 6 native) were sown in 2004 in a split plot experiment, and then subjected to management treatments (cutting and irrigation) to test the response of individual species and the community as a whole. Density of sown plants declined across the experiment due to self-thinning, but this decline was not uniform, and resulted in three response groups, persisters, decliners and intermediates. These patterns of decline were not related to whether the species were native or non-native but rather reflected the fitness of individual species in relation to the specific conditions prevailing in the experiment. Three of the five persistent species were non-native. Neither irrigation nor cutting time (September or November) affected persistence. The main effect of the management treatments was to increase (in the case of irrigation plus November cut treatments) overall standing sown biomass and the biomass of individual plants. These larger plants flowered more effectively and produced more seeds. Contrary to our initial assumptions, despite the moderately productive soil, communities of rosette forbs were very slow to invade with taller, leafier stem species. The ratio of invading graminoid biomass to sown species biomass was lowest on the irrigated, November cut treatments, suggesting that longer term persistence of many species is likely to be maximised on wetter soils when cut later in the year