The ecology of Yucca (Agavaceae) over an environmental gradient in the Mohave Desert: distribution and interspecific interactions

Three species of Yucca are associated over an environmental gradient in the Mohave Desert of south-eastern California. Two species, Y. brevifolia and Y. schidigera, have an extensive elevational overlap and are distinctly different from each other in physiological characteristics that can affect their utilization of water. Y. brevifolia has lower leaf temperatures, lower internal water potentials and higher transpiration rates over the course of the day than does Y. schidigera. A third species, Y. baccata, is very similar in these physiological characteristics to Y. schidigera and replaces it abruptly at higher elevations. Competitive interactions, as measured by nearest neighbour analysis (Pielou, 1960), occur both intraspecifically and between all interspecific combinations. Intensity of interspecific competition is greatest between Y. schidigera and Y. baccata, the species most similar physiologically and which replace each other elevationally. It is hypothesized that co-occurrence of these three species is maintained by competitive displacement elevationally of the two most ecologically similar species and by differential utilization of soil moisture by the ecologically dissimilar pairs of species.