Yucca brevifolia (Engelm), commonly known as the Joshua tree, is endemic to the Mojave Desert. Its populations demarcate the limits of the Mojave at the transitions between it and the Great Basin and Sonoran deserts. At approximately 30,000 BP, paleological data shows its distribution as far south as the Ajo Mountains, 35 km north of the Mexico border. This is 225 km lower than the current day distribution. The elevational range was also 200-300 meters below present day distributions. Current subpopulations are scattered throughout the Mojave in discrete, disjunct stands. The factors influencing the reproductive and recruitment limitations are unknown. The dynamics of recruitment within stands at varying elevational and latitudinal sites could shed some light on what may be preventing a continuous population of Yucca brevifolia throughout the Mojave. An historical analysis of the location of stands using surveys from the early part of the twentieth century through current censuses, provides an understanding of the recent movement of subpopulations over elevational and latitudinal gradients.

Key words: distributions, Mojave desert, recruitment, Yucca brevifolia