Colorado Front Range (USA) mountain grasslands typically comprise 25-50% warm-season plants, and the remainder cool-season species. Some long-term large mammal exclosures have nearly pure stands of cool-season species indicating that grazing may promote a good mix of warm- and cool-season species. This study, undertaken at 2,800 m altitude in theRoosevelt National Forest,assessed effects of single season and rotation harvesting on cool- and warm-season grasses.
The study was undertaken in a mountain bunchgrass community (ungrazed by livestock for 6 years; large mammal grazed mostly elk Cervus elaphus in winter) dominated by cool-season Parry oatgrass Danthonia parryi (48% of grass/sedge cover) and warm-season slimstem muhly Muhlenbergia filiculmis (39% cover; warm-season grasses made up 40% of total grass/sedge cover). Sedges Carex spp. contributed 0.5% cover.
Treatments were early (June 15) partial harvest of cool-season grasses and late (July 15) partial harvest of warm-season grasses. The first phase was initiated in 1986 and evaluated in 1988, and repeated in during 1987-1989.
There were the 40 treatments (8 schedules x 5 harvest intensities i.e. an estimated 10, 30, 50, 70 or 90% of the weight of species being clipped in l m² circularplots mimicking different grazing intensities) with 15 replicates of each. Forbs were pulled from plots. As a measure of grass response, biomass in plots were clipped in mid-August and separated into cool- and warm-season species. Samples were dried and weighed.
Warm-season grasses were greatly reduced by repeated July harvest, slightly reduced by July harvest in alternate years, and increased slightly in June harvest plots of cool-season grasses. Warm-season grass weight declined from about 9 g/m² with strong (70-90% clipping intensity) early harvest treatments to about 6 g/m² with strong late harvest treatments.
Dominant cool-season grasses responded less to repeated early harvests than did the less abundant warm-season grasses to repeated late harvests. Cool-season grass weight increased from about 18 g/m² (strong early harvest treatments), to about 24 g/m² (strong late harvest treatments).
The author states that the results strongly suggest that early or rotation grazing is necessary to achieve a good balance of warm-season and cool-season grasses in a mountain bunchgrass community.
If using or referring to this published study, please read and quote the original paper, this can be viewed at: https://www.uair.arizona.edu/holdings/journal/issue?r=http://jrm.library.arizona.edu/Volume44/Number4/