In the Northern Rocky Mountain foothills of northwest USA, the perennial forb, spotted knapweed Centaurea maculosa (native to Eurasia) is invasive, replacing native perennial grasses (e.g. Idaho fescue Festuca idahoensis). This study undertaken near Bozeman city (111°4’ N, 45°36’ W; 1,570 m, Montana), tested whether three summers of sheep grazing would reduce spotted knapweed without detrimentally impacting the native grassland community.
In 1991, three 10 x 10 m grazing plots were enclosed with electric fencing within a 3-ha pasture (horse-grazed prior to study). Before sheep grazing commenced, standing crop in mid-June averaged 1,580 kg/ha, dominated by Idaho fescue (41%) and spotted knapweed (25%). In the summers of 1991-1993, each plot was grazed by five Targhee ewes for: 5-7 days in mid-June, 2-6 days in July and 1-2 days in early September.
Four 10 m transects (with 0.5-m² quadrats at l m intervals) were established within and outside each plot. The following were recorded prior to grazing in June 1991, and subsequently in June 1992 and 1993, and June 1994 (9 months after grazing ceased): densities of spotted knapweed (seedlings, rosettes and mature plants), Idaho fescue and arrowleaf balsamroot Balsamorhiza sagittata; Kentucky bluegrass Poa pratensis frequency; bare ground and litter cover.
Basal area, number of flower stems, flower stem and leaf height of knapweed and Idaho fescue plants were measured in 1994. Soil seed bank samples were collected in summer 1991 and 1994, from which viable knapweed seed densities were estimated.
Spotted knapweed: In 1994, grazed areas had lower seedling, rosette and mature plant densities (3, 10 and 65/m² respectively) than ungrazed areas (11, 79 and 115/m² respectively; note: values approximate, read of original graphs). Basal area was greater in grazed (8.2 cm²) than ungrazed areas (4.0 cm²). There were fewer seeds in soil samples in grazed (12 seeds/m²) than ungrazed (49 seeds/m²) areas. Presence of few seedlings after 1991 (both treatments) may have been due to herbivory by seedhead fly Urophora spp. (first observed in 1992).
Other plants: Idaho fescue plant density increased in grazed areas from about 20% in 1991 to 31% in 1994; leaves and flower stems were 38% and 17% shorter, respectively, than in ungrazed areas. Kentucky bluegrass frequency(approx. 61% in 1991) by 1994 was about 90% in grazed and 69% in ungrazed areas.
Arrowleaf balsamroot frequency varied from 7-8% in grazed areas and 12-15% in ungrazed areas from 1991 to 1994. Grazing did not affect litter cover (59-70% in all years). Bare soil increased from 2.2 to 5.6% cover in grazed areas, and decreased from 4 to 1% in ungrazed areas.
In conclusion, the three summers of sheep grazing negatively impacted spotted knapweed whilst minimally affecting the native grass community.