Individual study: Establishment of upright sedge Carex stricta seedlings in experimental wetlands with implications for restoration, University of Minnesota Landscape Arboretum, Minnesota, USA
Budelsky R.A. & Galatowitsch S.M. (2004) Establishment of Carex stricta Lam. seedlings in experimental wetlands with implications for restoration. Plant Ecology, 175, 91-105
In mid-continental North America, t he loss of sedge Carex dominated prairie wet meadows due to drainage has been extensive. Restoration of such depressional wetlands typically involves re-flooding previously drained basins. There have been several thousand such restoration attempts over the past decade or so. Deliberate revegetation is rarely undertaken as it has been assumed that native vegetation will eventually establish naturally. Sedges Carex spp., which occur commonly on the periphery of native prairie wetlands and adjacent wet meadows, do not however readily re-establish, and it seems unlikely that they will do so without deliberate reintroduction. The lack of natural recruitment at sites where wetland restorations have been attempted, led to this study which investigated effects of different water level regimes and weeding on the establishment of the native, tussock-forming, upright sedge Carex stricta.
Study site: Study site: The study was undertaken over three growing seasons (May 1995 to September 1997) at the University of Minnesota Landscape Arboretum in Chanhassen (44º51'45' N, 93º36'00' W), Carver County, Minnesota, USA.
Experimental design: The experimental basins were in the shallow depression of a former wetland that had been divided into four square (0.2 ha) research basins. Re-profiling in summer 1994 removed all vegetation and produced four, gently sloping sides (20:1 gradient) and a small flat rectangular bottom in each. Basins were flooded through autumn and winter prior to the experiment to prevent weed growth.
To evaluate the effect of water availability and depth on 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 had average water levels of: +22.5 cm, +7.5 cm, −7.5 cm and −22.5 cm. Within each row, four 2.5 m x 2 m plots were established separated within the row by 25 to 75 cm depending upon the remaining available slope width. Space within rows was limited by the presence of a parallel experiment on lake sedge Carex lacustris (for a summary see: www.conservationevidence.com/ViewEntry.asp?ID=658).
Treatments: Within each row were applied combinations of two C.stricta planting densities (high density - 45 plants = 9 plants/mÂ²; or low density - 10 plants = 2 plants/mÂ²) and two levels of 'weed colonizer' (i.e. plots were either weeded, or not weeded). A total of 1,320 sedge seedlings were planted in bare soil in May 1995. A different water-level regime was established in each basin to assess the effect on seedling establishment:
i) static - soil surface exposed in the upper two rows and inundated in the lower two for the duration of the study (max. inundation 30 cm);
ii) rising water regime - water level set at the lower edge of the lowest row at the beginning of the growing season (mid-May). Over the growing season, the water level was raised at a rate of 5 vertical cm/2 weeks, until plots in the upper row were inundated (mid-October);
iii) falling water regime - water level at the top of the uppermost row at the beginning of the growing season. Water was allowed to evaporate or was drained at a rate of 5 cm/2 weeks until the soil in the lowest row was exposed by the end of the growing season. This pattern is most similar to that observed in native seasonal wetlands.
Vegetation monitoring: Seedling response was assessed monthly for survival, above-ground tiller number, flowering stem number and plant height. Biomass assessments were also made.
Mortality: Seedling mortality was highest in the first growing season and was clearly influenced by water regime. Of the 440 sedges planted in each regime, 22% died in the falling, 15% died in the static, but only 2% died in the rising water regime. In the second year, mortality was lower but was still similarly influenced by water regime. By the third season mortality was low (1% overall) and was not significantly influenced by any treatment.
Tillering: Tillering rate was the only growth measurement significantly influenced by water regime in the first year, when it was correlated with water regime between May and June, and June and August. Growth between May and June was greatest in the rising (3.23), followed by the static (2.56), and then the falling (2.12) water regime. In contrast there was a reversal between June and August: falling (3.04), static (2.35) and rising (1.76) water regimes. Thus sedge growth was greatest early in the season in unsubmerged plots in the rising water level regime and growth declined as water level rose. Similarly, growth increased as water levels dropped as the season progressed in the falling water regime.
Biomass: Biomass was influenced by water regime only in the second growing season; average above-ground dry weight per plant at the end of the growing season: falling (64.2 g), rising (47.2 g ) and static (36.0 g) water regimes.
Non-sedge colonizers: C.stricta seedling and juvenile growth was slowed by non-sedge colonizers during the first two growing seasons, but by the third sedge plants could out-grow all annual and perennial weeds, except the tall, invasive perennial, canary reed-grass Phalaris arundinacea.
Conclusions: The authors consider that, given the rapid growth of C.stricta plants once established, this indicates that planting seedlings is a successful method for reintroducing this sedge into wetland restoration sites. Comparison with other studies performed under similar conditions suggests that planting of seedlings is a better method of establishment of this species than transplanting rhizomes.
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