Individual study: Responses of purple moor-grass Molinia caerulea swards and associated vegetation to controlled grazing by cattle in the Cleish and Bell Hills, southern Scotland
Grant S.A., Torvell L., Common T.G., Sim E.M. & Small J.L. (1996) Controlled grazing studies on Molinia grassland: Effects of different seasonal patterns and levels of defoliation on Molinia growth and responses of swards to controlled grazing by cattle. Journal of Applied Ecology, 33, 1267-1280
Purple moor-grass Molinia caerulea is a tussock-forming perennial grass typically found on wet, acidic peaty soils. Around 10% (600,000 ha) of Britain’s uplands are dominated by Molinia and were it occurs with heather Calluna vulgaris, it may be encouraged by burning, a traditional management method of heather moorland. Where grasses already dominate (as opposed to Calluna), frequent burning favours Molinia at the expense of other grass species. Experiments were carried out at two hill pastures sites in southern Scotland to investigate the long-term responses of purple moor-grass to controlled cattle grazing, and the effect of cutting regime on floristic competition.
Study sites: The experiments were conducted at two hill pastures sites: a 0.75 ha area (c.230 m altitude) in the Cleish Hills (National Grid ref: NT 083936), Fife; and a 3.6 ha area (c.255-280 m altitude) at Bell Hill, the Sourhope Field Research Station (National Grid ref: NT 537975) in Roxburghshire (450 m altitude).
Treatments: Plots (2.4 ha and 1.2 ha at Ben Hill; 0.37 ha each at Cleish) were stocked with cattle during the Molinia growing season so as the cattle consumed either 33% or 66% of Molinia leaf production. Small exclosure areas (3 x 3 m) were established as no-grazing controls.
Measurements: Molinia leaf length was assessed twice weekly in the growing season by measuring 100 leaves/plot. At Cleish, leaf growth was measured on protected tillers in early July, and then June-August in 1988, and at both sites in may-August 1990. The plots were unstocked in 1991 and measurements made at the end of July to determine effects of grazing on leaf growth per tiller.
Biomass was measured at the end of grazing in 1990, the last grazing year. Samples were analysed for total water soluble carbohydrates (TWSC), starch and N, P and K content.
Floristic composition: Floristic composition was recorded annually each June from 1985 to 1991 using a point quadrat.
Leaf growth and biomass: The The plots grazed by cattle for 6 years compared treatments where . rates of Molinia leaf extension were reduced at the higher level of use (66% of the herbage removed by grazing). In comparison with ungrazed controls during the final year of treatment only, Molinia and other grass biomass data showed that Molinia was grazed to a much greater extent than the other grass species present.
After 6 years of grazing, Molinia biomass at 33% utilization was reduced by 46-65% compared with ungrazed exclosures, and at 66% utilization by 86%.
Carbohydrate and nutrient content: Basal internode size was greatly reduced in the grazed plots compared with the ungrazed exclosures, with effects on tiller base size being more important than variation in concentrations in determining amounts of starch, total water soluble carbohydrates, N, P and K on a per tiller basis.
Floristic diversity: Diversity was increased on grazed compared with ungrazed areas. At 33% utilization, Molinia cover appeared to be leveling off at around 60-65% after 3-5 years. At 66% utilization a continued downward trend was evident. Exclusion of grazing led to a reduced cover of sheep’s fescue Festuca ovina only where conditions were favourable for high yields of taller grass species.
Conclusions: The results indicate that to retain Molinia as a grazing resource in the areas where the trials were undertaken, annual utilization should not exceed 33% of annual Molinia growth. Conversely if a reduction in Molinia and an increase in floristic diversity is the desired objective, grazing intensity should be increased.
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