Effects of grazing,topography, and precipitation on the structure of a semiarid grassland

Structural aspects of the shortgrass steppe plant community, functional groups, and species populations were examined in response to long-term heavy grazing and exclosure from grazing, contiguous wet or dry years, and an environmental gradient of topography. Of the three factors, relatively greater differences in community similarity were observed between catena positions, particularly on the ungrazed treatments. Grazing was intermediate between catena position and short-term weather in shaping plant community structure. Grazed treatments and ridgetops had a less variable species composition through fluctuations in weather.An increase with grazing of the dominant, heavily grazed species was observed. Basal cover and density of total species was also greater on grazed sites. The more uniform grazing lawn structure of the grazed plant communities had an influence on segregation of plant populations along topographical gradients. Segregation was less on grazed catenas, but diversity and the abundance of introduced and opportunistic-colonizer species was also less.Although the shortgrass steppe community was relatively invariant, less abundant species were dynamic and interactions occurred with respect to grazing, weather, and catena position. The effects of grazing may be mitigated by favorable growing seasons but magnified in unfavorable years in populations that are adapted to favorable sites. Grazing can be considered a disturbance at the level of the individual but it may or may not be a disturbance at the level of the population, and it is not a disturbance at the level of the community in this particular grassland.