Responses of Mediterranean Grassland Plants to Grazing and Protection

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Abstract
(1) Differences in plants species'' abundances in response to cattle grazing and protection were measured across fences at fifteen sites in productive semi-natural mediterranean grasslands. The results were tested for the consistency of species'' responses to grazing intensity and for their association with plant attributes expected to be involved in the grazing response. (2) Of the seventy-three most common species, forty-nine showed responses that were largely consistent over sites, being either significantly more abundant on the more protected side, significantly more abundant on the more grazed side, or not significantly different. The remaining twenty-four species were inconsistent, being significantly more abundant on the protected side in some sites and on the grazed side in others. (3) Of the inconsistent species, fourteen showed a pattern of responses over sites that could be explained by a unimodal response to grazing intensity, with an optimum at intensities between the extremes of complete protection and very heavy continuous grazing represented in study. (4) Species response to grazing was not significantly associated with family, which inferred chemical palatability, nor with spininess. (5) Perennial species with long growing seasons were somewhat more frequent among protection increasers, and their total cover greater in protected grassland. (6) Grazing response was strongly and significantly associated with plant growth form: protection increasers were mostly tall erect plants; grazing increasers mostly small, prostrate or rosette plants; and species with intermediate response mostly erect plants of medium height. (7) Ungrazed grassland was dominated (60-80% cover) by tall perennials and tall, annual grasses. Underlight to moderate grazing their cover decreased to the benefit of annuals of a wide range of growth forms and families; of those, under heavy grazing, small and prostrate annuals, rosette crucifers and thistles remained abundant. (8) The observed responses to grazing from none to light and moderate intensity can be explained best by the opening of establishment gaps in the closed sward of foliage and mulch maintained by the dominants. At moderate to heavy grazing the vertical differential defoliation gradient imposed by cattle, particularly early in the growing season, becomes the major mechanism of grassland change.