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Seed Production of Mixed Prairie in Grasslands National Park, Saskatchewan

Clark, G.T. 1997. M.Sc. Thesis, Dept. Crop Science and Plant Ecology, University of Saskatchewan, Saskatoon, SK


Native grassland is a potential source of seed for revegetation and restoration of ecosystems. Dynamics of community seed production within and between growing seasons has not been studied in the northern mixed prairie. The objectives of this research were (1) to determine seasonal trends in seed production, and (2) to assess the effects of stand thinning by physical and chemical disturbance on seed production. Study sites were located in Grasslands National Park in southwest Saskatchewan. Ripe seed was harvested from three community types corresponding with upland, slope, and lowland topographic positions at eight dates over the growing season in 1994 and 1995. Maximum standing crop seed yield (SCSY) was greatest in the slope (44 kg/ha), followed by the lowland (20 kg/ha) and upland (6.6 kg/ha) at mid-season 1994. SCSY of all three communities was low in 1995 (maximum = 4.3 kg/ha). Maximum standing crop seed density (SCSD) was greatest in the lowland (4251 seeds/m2), which peaked in mid-June 1994, and the slope (1730 seeds/m2) which peaked in August 1994. SCSD of the upland peaked late September, 1995 (1345 seeds/m2). The drop in seed yield and density in the slope and lowland in 1995 compared to 1994 likely was due to precipitation patterns during the growing season that did not correspond to the phenology of dominant seed producing species. Seed yield from perennial grasses was greatest in the slope and lowland community in 1994. Seed density of perennial grasses was greatest at mid-season of 1994 in the slope and lowland, and in mid season of 1995 in the upland. Seed yield of perennial forbs did not differ between years, however, seed density increased later in the season in all communities. Seed yield and density of annual species was greatest in the lowland community at mid-June. Seed production within a species varied between years. In the upland community seed yield and density was dominated by perennial forbs, in the slope by perennial grasses and forbs, and the lowland by annuals and Agropyron smithii. Cumulative seed production over the season in upland, slope and lowland communities was 13, 51, and 33 kg/ha in 1994 and 10, 6, and 4 kg/ha in 1995, respectively. The cumulative seed density was 1189, 3090, and 6489 seeds/m2 in 1994, and 2867, 1303, and 3306 seeds/m2 in 1995 in the upland, slope, and lowland communities, respectively. Seed number per unit weight increased in 1995 in the three communities. The number of species producing seed was greatest in the slope, and least in the lowland. Similarity of species producing seed in a community through the season was greater in 1994 than in 1995. The similarity between seed yield and density and basal cover of species in the community averaged less than 40%. Seed production was compared between undisturbed, physically disturbed (loosening soil with spade cuts), and chemically disturbed (spraying strips with glyphosate) grassland in the upland, slope and lowland communities. Seed production was not increased by physical disturbance in any community. Chemical disturbance was effective for increasing seed production in the upland only, from 607 to 7761 seeds/m2. Increased seed production was due primarily to production from Artemisia frigida. Wide variation in the time at which seeds ripen, large numbers of species, and the range of different plant morphologies make it difficult to harvest the diversity of seed from mixed prairie using conventional agronomic techniques. Seed stripping or hay harvests should be investigated. Seed production can be increased in upland communities by artificial stand thinning, but the effect may be greatest for weedy species that naturally respond well to disturbance. In general, seed yields are most strongly controlled by precipitation.