Patterns of abundance and population structure of Pachycereus pringlei(Cactaceae), a columnar cactus of the Sonoran Desert

Understanding the mechanisms that determine the distribution and abundance of plants is a major problem in ecology. However, very few studies have explored the factors controlling the abundance of columnar cacti throughout their range of distribution. In this paper, we describe the density and size structure of 26 populations of Pachycereus pringlei throughout its distribution range in the Sonoran Desert. Major differences in abundance were detected between island and mainland and peninsular areas, with islands sustaining significantly larger densities than mainland and peninsular populations. Within peninsular populations, the abundance was negatively associated with latitude and positively associated with annual and seasonal rainfall. In contrast, the abundance in mainland populations showed neither latitudinal trend nor an association with rainfall. In peninsular populations, mean height and basal diameter of branched plants showed a negative association with population density whereas mainland populations showed no significant association. None of the populations exhibited a population structure that fitted the log-normal distribution expected for young, growing populations with constant recruitment. Insular, peninsular and␣mainland populations showed a population structure with an uneven size distribution typical of populations experiencing regeneration pulses.