Individual study: The effects of grazing intensity on cover of plant species on moorland in north-east Scotland
Welch D. (1984) Studies in the grazing of heather moorland in north-east Scotland. III. Floristics. Journal of Applied Ecology, 21, 209-225
It is known that monocotyledonous communities (dominated by grasses) replace heather Calluna vulgaris and other ericaceous dwarf shrubs under heavy grazing, and many authors claim that grazing in the British uplands, principally by free-ranging domestic sheep Ovis aries has impoverished the flora and caused the spread of unpalatable graminoids.
In this study, botanical composition was recorded at 32 moorland sites over periods of 4-11 years. Changes vascular plant cover and in species number, were related to grazing intensity (measured by dung deposition), Calluna trend and soil type. Although not a conservation management intervention experiment per se, this study gives important insights on appropriate grazing intensities if the management aim is to avoid loss of desirable plant communities through overgrazing.
Study sites: In north-east Scotland, 32 sites were chosen which were representative of different soil types, altitudes and grazing regimes.
Plant monitoring: Plant cover was estimated by point-quadrat analysis. Standing crops were measured by harvesting with hand-shears, with the biomass of certain species and species-groups estimated.
Grazing intensity: Assessments of trends in plant community composition were related to grazing intensity which was estimated through dung plot sampling. Analyses were made every second summer, half the sites starting in 1969 and half in 1970. Several sites were again recorded in 1974, to check that the observed trends did not depend on weather sequences. Some sites were abandoned after a third analysis and others in 1976. Harvesting then ceased, and at the remaining sites herbivore usage was assessed only periodically, from amounts of dung present rather than accumulation every 3 weeks. By 1980 just twelve sites were being monitored, further losses occurring through burning and ploughing.
Certain plant species benefited from particular levels of grazing.
Light grazing favoured ericoids (e.g. bell heather Erica cinerea, cross-leaved heath E.tetralix) and lichens (e.g. Cladonia impexa and Parmelia physodes).
Heavy grazing favoured graminoids e.g. (common bent Agrostis (capillaris) tenuis, sweet vernal Anthoxanthum odoratum, sheep’s fescue Festuca ovina, smooth meadow-grass Poa pratensis) and forbs (e.g. sheep’s sorrel Rumex acetosella and white clover Trifolium repens).
Bryophytes were inconsistent in reaction, e.g. the common moss Hypnum cupressiforme benefited from light grazing, whilst nodding thread-moss Pohlia nutans and several mosss Rhytidiadelphus spp. were more abundant on heavily grazed moorland.
Mat-grass Nardus stricta and bilberry Vaccinium myrtillus were significantly favoured by intermediate levels of grazing, but further species showed a similar response.
Species number and species hit per point were greater at base-rich sites than base-poor, reflecting the more-species rich nature of base-rich sites. Species diversity increased when graminoids-forbs replaced Calluna and also when Calluna increased in cover substantially, mainly due to the appearance of additional lichen species. The number of species in unit biomass was greater in graminoids-forbs than other plant groups.
Conclusions: Different grazing intensities benefited particular plant species or species groups. Light grazing favoured ericaceous shrubs and lichens. Heavy grazing reduced the dwarf shrub community and favoured graminoids and forbs. Unsurprisingly more species were present on base-rich soils.
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