Plant diversity in sedge meadows: Effects of groundwater and fire.

Abstract

To explain diversity and coexistence in fire-prone herbaceous communities, most studies examine prairies and concentrate on the interactions of adult plants. Here I present the first study of diversity in sedge meadow wetlands, which are uncommon perennial-dominated communities that harbor many rare species. I focus on the influence of seedling regeneration as well as adult plant vegetative regrowth. In an initial descriptive survey of seven sedge meadows, I found that out of 34 different variables, mean groundwater conductivity and mean water table height were the best predictors of species density in sedge meadows. Although aboveground productivity and groundwater pH are often important influences on diversity in other wetlands, neither were significant predictors of species density for these sedge meadows. Because litter biomass in sedge meadows is similar to amounts in tallgrass prairie, where fire strongly influences diversity, I evaluated fire in sedge meadows with five different spring burns. Seedling emergence increased after each of the fires; most of these seedlings were perennial forbs. The clearly dominant species in these communities, Carex stricta, showed no increased germination after fire. Vegetative regrowth of all plants increased after fire as measured by standing biomass and shoot density. Although C. stricta, an early season C-3 sedge, did increase vegetatively after fire, no clear pattern of differential vegetative regrowth was identified for any plant group. Therefore, if fire has a defining influence on diversity of sedge meadows, it is likely this influence will result from differential regeneration among seedlings and not from differential regrowth by adults. I directly tested three potential mechanisms of fire-induced vegetation change: nutrient addition from ash, litter reduction, and shifts in competitive relationships. In experimental gradients, the reduction of litter increased both standing biomass and seedling emergence, but neither of these responses was affected by adding ash. Results from a two year removal experiment suggest that competitive interactions are different in unburned and burned areas, but it is unclear whether these differences affect overall community structure.