Competition and the Local Distribution of the Grass Stipa Neomexicana

Within a grassland in southern Arizona, Stipa neomexicana occurs only on dry ridge crests with low total grass cover, while total grass cover is greater below the ridge crests in moister, low—lying areas. I hypothesized that Stipa neomexicana was limited to these dry ridges by competitive exclusion. This hypothesis was tested by removal experiments conducted at three positions along the topographic gradient. The responses of Stipa were compared with those of Aristida glauca and other neighboring species. The predictions made under the hypothesis were confirmed; competitors limited seedling establishment, seeding survival, flower production, and the growth of mature plants. The response to the removal of competitors increased from the ridge crest to the downslope positions. Competition depressed estimated finite rates of population increase for Stipa neomexicana, particularly on the lower slope. Thus, increasing competition from neighboring grasses along the topographic gradient was responsible for restricting Stipa neomexicana to the unfavorable ridge crest sites.