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
Abstract
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.
