Individual study: Effects of water regime and competition on the establishment of lake sedge Carex lacustris in restored wetlands, University of Minnesota Landscape Arboretum, Minnesota, USA
Budelsky R.A. & Galatowitsch S.M. (2000) Effects of water regime and competition on the establishment of a native sedge in restored wetlands. Journal of Applied Ecology, 37
Restoration of mid-continental North American depressional wetlands (e.g. pothole prairies) typically involves re-flooding previoulsy drained basins. There have been several thousand such restoration attempts over the past 10 years. Deliberate revegetation is rarely undertaken as it is assumed that native vegetation will eventually establish naturally. Sedges Carex spp., which occur commonly on the periphery of native wetlands and adjacent saturated meadows, do not however readily re-establish, and it is unlikely that Carex will do so without deliberate reintroduction.
In this study the environmental constraints that might limit the establishment of the native lake sedge Carex lacustris, in re-flooded wetland basins was investigated.
Study site: The study was undertaken at the University of Minnesota Landscape Arboretum in Chanhassen, Carver County, Minnesota, USA (44°51'45' N, 93°36'00' W).
The experimental basins were in the shallow depression of a former wetland (approximately 0.8 ha) that had been divided into four square 0.2 ha research basins. Re-profiling in summer 1994 removed all vegetation and reshaped each basin to produce four, shallow sided slopes (20:1 gradient) and a small flat rectangular bottom. Basins were flooded in the autumn and winter prior to prevent weed growth. The basins each had a water-control mechanism to allow water-level manipulation. The soil was similar to that within frequently ploughed agricultural areas.
Treatments: To evaluate the effect of water availability (and depth) on lake sedge survival and growth, four elevational treatments (four parallel rows) were established along the east-facing slope of each of three of the basins. The rows were identified by their height according to average water level: +22.5 cm, +7.5 cm, −7.5 cm and −22.5 cm. Treatments within each row consisted of four combinations of two planting densities and two competition levels.
The number and size of treatment plots were constrained by the width of the slope on in each basin. Sedges were planted in bare soil plots in late May 1995, at either a high (45 plants = 9 plants/m²) or low (10 plants = 2 plants/m²) density. Plots were either weeded (no competition) or not weeded (allowing natural recolonization of weeds from the seed bank = competition treatment).
A different water-level fluctuation regime was established in each of the three basins to assess the effect of water-level changes on plant establishment.
Monitoring: In each plot, species abundance was scored monthly using a Braun-Blanquet cover/abundance scale. Above-ground biomass was collected during the last 2 weeks of August 1995, 1996 and 1997 from sample strips 83 cm wide (approximately one-third of the original 2.5 m plot width) and 2 m long (the height of the plot). Biomass was sorted by species and oven dried. Soil seed bank samples were collected and analysed for species diversity, but the data were inconclusive.
Sedges in plots were measured monthly (in a sampling strip 50 cm x 2 m in the first growing season, and 25 cm x 2 m in the second and third growing seasons) for: survival, above-ground tiller number/m² and tiller height.
The response of immature lake sedge to water-level fluctuation in the first growing season did not indicate the outcome of mature stand development by the end of three growing seasons. Initial survival and growth were lowest in the falling water basin, but stem density, height and biomass were greatest under this regime by the third growing season.
Interspecific competition was most intense at upper elevations, particularly in the rising water basin. Although sedge growth was limited by increasing water depth in the absence of competition from weeds, growth was uniformly low across all elevations in the presence of competition.
Conclusions: Results suggest that lake sedge can produce dense stands under a primarily annual weed community within two to three growing seasons, but that reed canary-grass Phalaris arundinacea (a tall-growing, invasive species) can prevent successful sedge establishment. Control of water-levels and weed competition was most crucial during the first growing season to ensure successful sedge stand establishment. The potential for growth as well as mortality is greatest during this first year. Competition from weeds can be a significant constraint to the successful establishment of lake sedge and tackling this problem should therefore be addressed when developing restoration plans.
Note: If using or referring to this published study, please read and quote the original paper. The original paper can be viewed at: http://www.blackwell-synergy.com/doi/full/10.1046/j.1365-2664.2000.00540.x