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Accueil du site → Doctorat → États-Unis → 1995 → Long-term vegetation changes at eight exclosures in the arid semi-desert of southwestern Wyoming

University of Wyoming (1995)

Long-term vegetation changes at eight exclosures in the arid semi-desert of southwestern Wyoming

Johnson-Barnard, Julia

Titre : Long-term vegetation changes at eight exclosures in the arid semi-desert of southwestern Wyoming

Auteur : Johnson-Barnard, Julia

Université de soutenance : University of Wyoming

Grade : Doctor of Philosophy (PhD) 1995

Résumé
Exclosures are an important tool for determining changes in vegetation composition with time caused by grazing pressure, in that they account for yearly variation in precipitation and long-term climatic changes. Eight exclosures on rangelands in southwestern Wyoming were evaluated for vegetation changes due to grazing between 1959 and 1994. Three of these exclosures had been sprayed once with 2,4-D at the time of establishment, which allowed examination of the long-term effects of herbicide use, as well as grazing, on the vegetation. Seven different techniques were used to measure differences in vegetation characteristics inside and outside exclosures. Only differences in shrub height existed inside versus outside across all exclosures ; shrub height was significantly greater inside exclosures. Few shrubs were present in areas treated with 2,4-D, even 30-35 years after spraying. Patterns in other vegetation characteristics (cover, density, and above-ground standing crop of shrubs, grasses, and forbs) were site- and species-specific, appearing to respond to environmental factors (probably precipitation) as well as grazing and herbicide treatments. The level of shrub utilization by domestic and wild ungulates is an important evaluation parameter for determining grazing effects on the shrub component of vegetation, but available methods for determining shrub utilization have not been very successful. This study presents a technique to evaluate utilization by measuring twig length on dominant species of rangeland shrubs. The method may be a good indicator of loss of vegetative and reproductive structures, but does not successfully measure use by ungulates specifically, and must be tested under a greater variety of management situations before it can be widely recommended. Soil erosion is one of the most serious problems facing range managers in the western United States. This study describes a method for determining the degree of erosion in semi-arid rangelands that is relatively simple, inexpensive, and potentially suitable for relative comparisons, but does not hold much promise for detecting real changes in erosion and deposition with time. Its implementation and the results of a permanent transect study at four of the exclosure sites are discussed.

Mots clés : Ecology, grazing, erosion, Biological sciences, Range management

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