Competition in desert winter annuals: Effects of spatial and temporal variation.

Abstract

Removal experiments were conducted to determine particular spatial and temporal conditions that can influence competitive interactions in several desert winter annual species. During the 1987-88 season, variation in the magnitude of competition at three habitats along a topographic gradient was demonstrated in two co-occurring species of winter annuals, Plantago patagonica and Pectocarya recurvata. Density effects on the survival and reproductive success of either species were weakest at the slope. However, the habitats where the two species experienced the most intense competition differed. Plantago was most affected by competition at the wash while Pectocarya was most affected at the base of the hill. The most striking pattern observed was that, for both species, the habitat with the highest reproductive success for plants that were not experiencing competition tended to be the worst habitat for plants in competition. A comparison of results from two experiments performed on Plantago patagonica during two growing seasons showed that competition occurred despite large seasonal differences in weather and plant performance. When wet and dry conditions of different year types were simulated by artificial irrigation during a dry season, competition was still detected in both rainfed and irrigated plants regardless of the marked differences in plant size as a result of the irrigation treatment. A neighborhood density roughly equivalent to 8 plants/dm² appeared sufficient to create competitive conditions for Plantago. Effects of competition were consistently manifested in reduced plant growth and fecundity. There was no evidence for density-dependent seedling mortality (self-thinning) even with seedling densities as high as 48 plants/dm². In two pairs of species tested, Plantago patagonica-Schismus barbatus and Plantago patagonica-Pectorcarya recurvata, there was no statistically detectable effect of neighbor species identity on target plants of Plantago and Pectocarya suggesting the possible equivalence of competitive effects in these species of desert winter annuals.