INTERACTIVE EFFECTS OF FIRE AND GRAZING ON STRUCTURE AND DIVERSITY OFMEDITERRANEAN GRASSLANDS

The separate and combined effects of fire and cattle grazing on struct ure and diversity of productive Mediterranean grasslands in northern I srael were examined within a set of climatically and edaphically simil ar sites. Cover and height of green and dry plants in winter, and spec ies richness and diversity in spring, were measured in paired transect s on both sides of cattle fences, and on both sides of boundaries of b oth incidental and experimentally lit fires. Early in the first growin g season after a fire, plant cover as well as height of green plants w ere reduced, compared to unburnt grassland. These structural effects o f fire were similar to the effects of grazing, but they were greater i n ungrazed than in grazed grasslands, indicating a fire-grazing intera ction. The effects of fire were considerably attenuated in the second growing season after the fire. Species richness and diversity tended t o be higher in grazed than in adjacent ungrazed grasslands. Richness c onsistently increased after a fire only in grazed grasslands with a st rong perennial component. In ungrazed grasslands, and in predominantly annual grasslands, fires reduced species richness and diversity at le ast as often as they increased it. Fire and grazing should be regarded as two agents with distinct and interactive effects on the community, rather than as two alternative mechanisms of a general disturbance fa ctor.