Individual study: Recruitment and survival of dominant emergents and mud-flat annuals in response to artificial drawdowns at Delta Marsh, Lake Manitoba, Manitoba, Canada
Welling C.H., Pederson R.L. & Van der Valk A.G. (1988) Temporal patterns in recruitment from the seed bank during drawdowns in a prairie wetland. Journal of Applied Ecology, 25, 999-1007
Establishment of emergent plants is the main purpose of drawdowns in wetlands managed as waterfowl breeding habitat in North America. Artificial drawdowns (temporary drainage) are one way of controlling composition and structure of wetland vegetation, when typically most recruitment comes from the soil seed bank. The objectives of the Canadian experimental study described here were to: i) describe patterns of seedling recruitment; ii) assess the impact on recruitment of environmental conditions during drawdowns; and iii) determine whether there is any significant difference in the vegetation produced by drawdowns lasting for 1 or 2 years.
Study area: The study was undertaken at the 15,000 ha Delta Marsh (a series of interconnected bays) on the south shore of Lake Manitoba, in Manitoba, Canada (50°1l' N, 98º19' W). The dominant emergents are common reed Phragmites australis, whitetop or river grass Scolochloa festucacea, reedmace Typha spp., and bulrush Scirpus lacustris. This study was done within the Marsh Ecology Research Program (MERP) complex, consisting of 10 contiguous marshes, each 5-6 ha, surrounded by borrow (drainage) ditches. Water depth can be maintained within 2-5 cm of a specified level by electric pumps.
Experimental design: Prior to drawdown, all 10 marshes were flooded for 2 years to a depth 1 m greater than normal to simulate periodic floods that occurred before water levels were stabilized in Lake Manitoba. This flooding killed most emergent vegetation, thus creating conditions favourable for its re-establishment from the seed bank during drawdown. Drainage of the experimental marshes was undertaken in May and completed by 1 June. Water levels were maintained at a level 30 cm below normal during the drawdowns.
In 1983, eight marshes were randomly selected and drained and kept drawndown for 2 years; the remaining two were drained in 1984, and kept drawndown for 1 year. For comparisons of vegetation between treatments to be valid, seed banks had to be similar. To test this, data on the seed density of the dominant emergents and mud-flat annuals in the seed bank of each marsh were collected prior to drainage.
Seed bank sampling: In each marsh, 25 sampling sites were chosen in 1979 in five elevation zones. In June 1980, prior to the flooding, one 30 x 30 x 5 cm block of substrate was collected at each site. Samples were sieved with 1 x 1 cm mesh to remove coarse material and were kept from June to the end of September in a greenhouse. As seedlings emerged they were identified, counted and removed. It was presumed that for a given species, seed density in the seed bank was at least proportional to seedling density in the greenhouse.
Recruitment during drawdown: In 10, 2 x 2 m plots in each marsh in different elevation zones. Plots higher than 35 cm above the normal water level where stands of P. australis survived the flooding and plots more than 25 cm below the normal level which had little or no emergent vegetation and a depauperate seed bank prior to flooding were excluded from this analysis. Of the remaining plots, 65 were in 2-year drawdown marshes and 19 in the 1-year treatment. During the first half of June, July and August 1983 and 1984, and also in September 1984, seedling densities were estimated in quadrats. In 1984, second-year shoots produced by year-old plants could be distinguished by their greater size from new seedlings.
Re-establishment of emergents from rhizomes was not investigated. S.lacustris, S.festucacea and P.australis did not survive flooding at the lower elevations analysed, but a few isolated shoots of Typha spp. were present in some plots.
Soil variables: In each plot each month the following was recorded: Soil temperatures; soil moisture (% oven dry weight) and conductivity (from measurements made on 300 g of soil taken from the upper 8 cm of the substrate).
Seasonal patterns of seedling recruitment: Within a season, most seedlings were recruited during June when soil moisture was high, temperature was moderate, and conductivity was low. After June, recruitment generally declined during the remainder of the season while conductivity increased and moisture decreased.
Differences between drawdown treatments: Differences between drawdown treatments in first-year recruitment of certain species appear to be due to differences between years in soil moisture and temperature, not differences in densities of seeds in the soil prior to drawdowns.
Far fewer emergents but more mud-flat annuals were recruited during the second year of the 2-year drawdown than during the first. Considerable mortality occurred during the second year in seedlings of emergent species established in the first year of the 2-year drawdown.
Conclusions: A 1-year drawdown appears sufficient if the purpose of management is to establish maximum numbers of seedlings of emergent species, as recruitment of emergents occurred primarily during the first year of the 2-year drawdown treatment and considerable mortality of first-year emergent seedlings occurred during the second year of drawdown.
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