How tussocks structure sedge meadow vegetation

Abstract

Carex stricta dominates sedge meadows in southern Wisconsin, USA. In contrast with invasive species that dominate as monotypes, C. stricta supports a diversity of co-occurring species by forming tussocks. Concerns about diversity loss and the potential to restore species-rich tussocks led us to ask how tussocks foster high species richness and affect composition. Our study of 181 tussocks at three sites showed that tussocks structure sedge meadow vegetation by adding both surface area and micro-habitats. Tussocks, which average 15–25 cm tall, increase surface area of an otherwise flat plot by 40%. Species richness was positively correlated with tussock size, but the patterns differed among sites. At a site where water was deep, tussock height was the best predictor of richness; at two other sites, surface area and circumference were the best predictors. Micro-habitats differed in conditions and species composition: tussock tops were high in light, variable in temperature, and high in litter cover. Tussock sides had less litter but more moss; off-tussock areas tended to be aquatic. Of the 56 taxa found, 34 showed significant affinities for one or more micro-habitats. Of these, four species occurred primarily on tops, several extended from tops to lower levels, seven aquatic taxa preferred areas off-tussocks, and one species specialized in tussock sides. Over the 2004 growing season, species richness increased in an additive process, from 35 species in May to 56 in September 2004. Cover of litter and moss increased and bare surface decreased with season, but only seven of the 34 taxa with micro-site preferences expanded their distributions as the September drawdown exposed the off-tussock micro-habitat. We conclude that tussocks enhance species richness in three ways, by increasing surface area, by providing multiple micro-habitats, and by undergoing seasonal changes in composition. Our detailed data on plant-diversity support by large tussocks form a benchmark for tussock meadow conservation, as well as a target for restoration of degraded meadows.