Individual study: Recovery of arid grasslands following long-term removal of livestock in San Simon Valley, Arizona, USA
Valone T. J., Meyer M., Brown J. H. & Chews R. M. (2002) Timescale of perennial grass recovery in desertified arid grasslands following livestock removal. Conservation Biology, 16, 995-1002
In parts of Arizona, intensive livestock grazing has resulted in degradation of natural arid grasslands; perennial grass cover has decreased whilst woody shrubs have increased. Livestock removal as a measure to restore grasslands has had little or no impact on the composition of these degraded habitats for at least up to 20 years following removal. Researchers have suggested that, due to alterations in soil properties and specific requirements of perennial grass species, development to a stable-state shrubland is difficult to reverse. A longer-time scale may be necessary in order for the effects of reduced grazing pressure to be noticeable. To assess the response of perennial grasses to long-term removal of grazing pressure, a study was conducted on two arid grassland sites following the removal of livestock after 20 and 36 years in Arizona, USA.
Study site: The two study sites were located in southern San Simon Valley. Grazing of domestic livestock began in the early 1900s and resulted in the conversion of grass-dominated habitat to a dense cover of unpalatable shrubs.
Site 1 (20 ha) had livestock excluded in 1977 (i.e. 20 years since livestock removal). Site 2 (9.3 ha) 6 km distant, and had livestock removed grazing in 1958 (i.e. 39 years prviously). Both sites were fenced and grazing continued outside these areas; they had similar climatic and soil conditions and, prior livestock removal, similar grazing intensities.
Monitoring: Between 17 July and 21 August 1997, perennial grass basal cover and shrub canopy cover was estimated (line-intercept method) along fixed transects 25 m in length. Pairs of transects were set up perpendicular to the exclusion fence, one inside and one outside at 5 m from the fence. Vegetation was recorded at 10 cm intervals over 24 transects (12 pairs) at Site 1, with 25 m between each pair of transects, and 30 transects at Site 2, with 10 m between each pair of transects. Between 19-21 August 1997, annual plant abundance was recorded along the same transects within 0.25 m² quadrats, placed at 6 m intervals.
There were no differences in perennial plant community structure (Fig.1, attached) or annual plant abundance and diversity, between the grazed and ungrazed areas after 20 years of livestock removal. However, Site 2 showed, overall, significant differences in species composition of perennial shrubs and grasses to that of the adjacent grazed site (Fig. 1). Higher cover of the perennial grasses (black grama Bouteloua eriopoda and Aristida spp.) were found in ungrazed areas, while higher cover of the shrub, broom snakeweed Gutierrezia sarothrae and fluffgrass Tridens pulchellus were noted in the grazed areas. Total species abundance and diversity of annuals did not differ significantly in the ungrazed for 39 years and grazed sites.
Conclusions: The site ungrazed for 20 years showed no differences in vegetation cover, inside or outside the ungrazed area. The dramatic difference in grass cover between the grazed and ungrazed areas of Site 2 (39 years of grazing exclusion), indicates that recovery of the arid grassland took longer than 20 years. This is supported by previous studies of these areas, in which little change in vegetation was observed at Site 2 after 19 years of grazing removal. These fragile grasslands require long time periods (> 20 years) to recover from the effects of intense livestock grazing.
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