Study

The effects of grazing exclusion on regeneration of semi-desert rangeland vegetation and soils, east central Idaho, USA

  • Published source details Yeo J.J. (2005) Effects of grazing exclusion on rangeland vegetation and soils, east central Idaho. Western North American Naturalist, 65, 91-102

Summary

In Idaho, large areas of semi-arid habitats have been grazed by livestock, mainly free-ranging cattle, since European settlement. In the past, over-grazing was a problem and today many semi-arid areas are degraded as a consequence. In order to try and restore some of the former plant communities, an investigation was undertaken as to the beneficial (or otherwise) effects of excluding livestock and other large vertebrate grazers by erecting fences to prevent grazing. Two habitats characterised by distinct plant communities, Sagebrush Steppe and Shadscale Rangelands, were investigated.

The study was undertaken in east central Idaho, USA. Nineteen exclosures on Sagebrush Steppe (a habitat characterised by wormwood Artemesia spp. and grasses) and Shadscale Rangelands (characterised by shadscale saltbush Atriplex confertifolia and other salt-tolerant desert species) varying in age from 18 to 38 years, were sampled for plant species richness, plant composition, indicators of soil erosion, ground cover, vegetative cover, and herb-low shrub layer screening cover. Features within the exclosures were compared with adjacent sites of the same size that were open to grazing by livestock and wildlife.

Species richness typically was slightly greater inside exclosures compared with areas outside them (on average about two more species inside exclosures) but the difference was not statistically significant. Similarity of plant community composition between exclosures and adjacent grazed sites ranged from 45% to 82%. Evidences for soil movement, soil pedestals and soil flow patterns were all more pronounced outside exclosures even though many sites were on flat to gentle slopes (median slope 12%).

Meta-analysis of the 19 livestock exclosure sites indicated that grazing exclusion resulted in less bare ground compared with adjacent grazed sites. The effect of grazing exclusion on soil surface cryptogams (lower plants reproducing by spores) was significant, with generally greater cover inside exclosures. Cryptogam cover differences between grazed sites and exclosures tended to increase with the number of years of grazing exclusion.

Bluebunch wheatgrass Pseudoroegneria spicata, a principal livestock forage, averaged greater basal cover inside exclosures than outside on 4 of 10 sites where it occurred; no exclosure sites had greater P.spicata cover in adjacent open-to-grazing sites. Grazing exclusion also resulted in greater cover in the herb-low shrub layer (0-0.5 m height).

Sandberg bluegrass Poa secunda (a short grass that initiates growth early in the spring and is not important livestock forage), averaged greater basal cover outside exclosures on 5 of 15 sites where it occurred.

Conclusions: These results indicate that despite improved livestock grazing management over the past half century, the now less-intensive grazing regimes can still limit the regeneration of native plant communities in sagebrush steppe ecosystems, and that their regeneration can be improved with livestock exclusion in the absence of other disturbances. However, a few exclosure sites were similar for the measured parameters to equivalent areas outside, suggesting that these sites were ecologically stable and that exclusion of livestock was not sufficient to move succession toward more pristine conditions, at least within the time periods studied.


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