Extirpation or coexistence? Management of a persistent introduced grass in a prairie restoration

Published source details Wilson S.D. & Pärtel M. (2003) Extirpation or coexistence? Management of a persistent introduced grass in a prairie restoration. Restoration Ecology, 11, 410-416.
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Summary

Persistence of non-native perennial grasses is a problem in North American prairie restorations. Crested wheatgrass Agropyron cristatum (native to Europe), for example has been sown on 6-10 million ha of the Great Plains since the 1930s, and abandoned fields planted with this species have ower plant diversity than fields of similar age colonized by native grasses. To investigate the effects of herbicide and cutting on crested wheatgrass control, a 50-year-old wheatgrass stand was seeded with native grasses and treated with herbicide annually for seven years, with some areas clipped in the final three.

Study site: The study was undertaken in Grasslands National Park (Saskatchewan, Canada) in a field cultivated until the late 1940s when sown with crested wheatgrass. The surrounding vegetation was prairie dominated by blue grama Bouteloua gracilis and needle-and-thread Stipa comata and S.densa.

Treatments: Herbicide or no herbicide and native grasses sown or not, were randomly applied in a factorial design to each of five replicate plots (3 × 10 m). Glyphosate was sprayed in early May 1994, when wheatgrass had begun to grow but native plants were dormant; it was reduced by about 50% that autumn. In 1995-1998, glyphosate was re-applied (wick applicator) in: May, June and September 1995; May and June 1996, May 1997; and May and June 1998. The wick was about 10 cm above ground and applied glyphosate to the relatively tall wheatgrass but not to the shorter native species. In 1999 and 2000, herbicide was applied using a hand-held wick in May and June.

Seed sowing: Seeds of native grasses were broadcast in May 1994 into tilled plots. The seed mix was dominated by blue grama (99%; 23.4 kg seed/ha). Other natives e.g. needle-and-thread grass and June grass Koeleria cristata were incorporated.

Clipping: In 1998 when it was clear that glyphosate acheived only partial wheatgrass control, two subplots (1 × 1 m) were established per plot (0ne unclipped control, the second clipped to 1 cm each May, June, July and August 1998-2000).

Monitoring: Species cover per plot was measured each August (1994-1997) in three quadrats (0.5 × 1 m). Clipping was examined in 1999 and 2000 by establishing three subplots (1 x 1 m) not treated with herbicide. Seed production was estimated by counting seed heads (1998-2000) in quadrats; soil samples were taken to investigate the seed bank.

Blue gama cover was significantly higher in herbicide plots and seed addition plots but was unaffected by clipping. Seed addition increased its cover to about 10% in non-herbicide plots and 30% in herbicide plots. Wheatgrass cover (in August 2000) also decreased significantly with increasing blue gama cover.

Total cover of native species in 2000 was significantly higher in herbicide and clipped plots but was unaffected by seed addition. Clipping increased native cover in plots without herbicide but had little effect in plots receiving herbicide. The most abundant species (all natives) in herbicide plots were spike moss (21%), pasture sage Artemisia frigida (18%), blue gama (15%), and pussytoes Antennaria neglecta (8%).

Wheatgrass seed head density varied little between control and herbicide plots in 1998 but was three times higher in plots receiving herbicide (42 heads/m²) than controls (12/m²). Seed head density was significantly reduced by herbicide in 1999 (control, 70/m²; herbicide, 2/m²) and was unaffected in 2000 (control, 1.4/m²; herbicide, 1.8/m²). The density of seedlings emerging from the seed bank did not vary significantly among plots.

Herbicide and clipping significantly reduced crested wheatgrass cover. Clipping produced an immediate and consistent decrease (up to 90%), whereas herbicide control effectiveneess varied among years. Wheatgrass decreased significantly with increasing cover of seeded blue grama. No treatment had any effect on wheatgrass seed bank. Even in the most effective treatments, wheatgrass persisted, albeit at low (approximately 5%) cover. Blue grama increased significantly in plots with seed addition and glyphosate and after 7 years, cover being similar to that of the surrounding prairie. Eradication of crested wheatgrass in such grasslands might not be realistic, but relatively simple management allowed native species to become more dominant.

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