Fire and grazing are major influences on prairie plant communities. Evidence suggests that their removal in semi-arid areas may cause succession toward more mesic (moist) grassland with litter accumulation and loss in productivity. In this study, large mammal exclosures erected in 1927 in a semi-arid Mixed Prairie community at the Agriculture and AgriFood Canada substation in Alberta (49°07', 110°28'; Canada) were examined to assess the effects of long-term exclusion of large grazers on vegetation and soils.
Fifteen exclosures were examined (11 on Chernozemic soil, 4 on Solonetzic soil). Outside the exclosures the area was livestock grazed, average stocking rates ranged around 0.5 animal unit moths (AUM)/ha (in 1931) and from 1992 to1997 from 0.25 to 0.67 AUM/ha (mostly late summer to December grazed)
Soil variables were recorded during surveys (August 1993) and from analyses of soil samples (September 1997 and 1998) inside and outside the exclosures.
Species cover were recorded within 20, 20 x 50 cm quadrats placed along a 10-m transect, inside and outside each exclosure (4-27 July 1996 and 16 June-20 July 1997). Standing biomass and litter were harvested at ground level in quadrats after plant cover was estimated.
Most results suggested that protection from grazing and associated disturbance had no effect or infact increased production (contrary to previous beliefs). Soil type influenced herbage production within exclosures; there were greater yields on the Chernozemic soil (annual net primary production grazed 92 g/m²; ungrazed 129 g/m²) but no effect on the Solonetzic soil (grazed 78 g/m²; ungrazed 79 g/m²).
Within exclosures, average standing litter was much higher than outside (Chernozemic soil: grazed 28 g/m², ungrazed 94 g/m²; Solonetzic soil: grazed 26 g/m², ungrazed 53 g/m²), which seemed to benefit Chernozemic soil quality. Within exclosures there was a tendency of reduced native species diversity on the Chernozemic soil, but evenness and richness were not affected. Cover of some species increased in exclosures (primarily Pascopyrum smithii and Tragopogon dubius) whilst others decreased (e.g. Bouteloua gracilis and Poa sandbergii).
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