Grazing Intensity, Aspect, and Slope Effects on Limestone Grassland Structure

Three treatments were used to evaluate the effect of grazing intensity (ca 30% and 50% herbage removal), aspect (north and south), and slope (< 10% and 10%–30%) on plant community structure of mountain grasslands in the Basque Country (Spain). Plant species richness was not significantly affected by grazing intensity, aspect, or slope. Although plant species composition was similar (Sorensen's similarity index = 0.87) between both grazing intensities, species frequency and cover were affected by grazing intensity. Festuca rubra L. and Agrostis capillaris L. were the most common species under both grazing pressures. Moderate grazing intensity (50% herbage removal) plots contained a greater number of plant species with a frequency of more than 50%. The lowest cover for F. rubra corresponded to low grazing intensity, north aspects, and steeper slopes. The lowest cover for A. capillaris was found under low grazing intensity (30% herbage removal) and steeper slopes. Danthonia decumbens (L.) P. C., Potentilla erecta (L.) Räuschal, and Trifolium repens L. were significantly affected by aspect and grazing intensity. Low grazing intensity on sites with northern aspects and steep slopes favored Agrostis curtisii Kerguélen, a species with a low nutritional value. A. capillaris, A. curtisii, P. erecta, and T. repens were sensitive to soil properties and aspect. Nitrogen and K soil concentrations were significantly higher in areas with low grazing intensity, most likely due to greater dead herbage accumulation. Significant (P < 0.05) correlations between plant species and soil pH or P concentration were found in areas with low grazing intensity. Reduction in grazing intensity together with the effect of slope and northern aspect has resulted in changes in plant community structure, leading to increases in forages with lower nutritional value.