Competition in Two Species of Desert Annuals Along a Topographic Gradient

Abstract

Details on the nature of density—dependent processes in desert annuals and their interaction with density—independent factors are critical to an understanding of adaptation to variable environments and the role of environmental variation in the maintenance of species coexistence. In this study we examined intra— and interspecific competitive interactions along a topographic gradient for two co—occurring species of desert winter annuals, Plantago patagonica and Pectocarya recurvata. This investigation was carried out in a creosote flat in the Sonoran Desert during 1987—1988, a growing season with high plant densities and low rainfall. Three stations were established along the topographic gradient: one in the sandy rivulets of a tertiary wash or arroyo, one in the sandy clay soil at the base of a small hill, and the third on the southeast side of the gently sloping (9°) hill. The wash habitat had the greatest moisture availability (soil water potential) shortly after rainfall. Yet it dried out quickly, presumably due to greater evaporation and percolation, leaving the base of the hill with the greater average moisture availability. The slope had the lowest moisture availability. Competition experiments were set up at each station using a neighborhood design with varying density in either mono— or mixed cultures. Target plants were scored for survival, fecundity, and size. Plantago and Pectocarya were found to have equivalent competitive effects in all three habitats using either species as a target: the species identity of neighbors had no measurable effects so that competition coefficients appear to be equal. However, density—response curves had different slopes in the different habitats along the gradient: both species had flat density—response curves on the slope, while Plantago had its steepest curve in the wash and Pectocarya had its steepest curve at the base of the hill. While density—dependent processes were important determinants of success for these desert annuals in this low—rainfall year, the strength of the density response varied spatially. We argue that the habitat—density interactions that differ among species may promote their coexistence.