DEMOGRAPHICS OF MORE THAN 12,000 INDIVIDUALS OF A KEYSTONE SPECIES IN THE NORTHERN SONORAN DESERT SINCE THE MID-1800S

We studied a cohort-based, long-lived species to determine whether favorable periods that promote regeneration are driven by local, regional, or global-scale factors. Limited demographic data exist for the keystone species Carnegiea gigantea over its range. We obtained a data set collected for 12,232 plants over an area of more than 11 km(2) at a restricted military zone, located far from any other studied population. We developed the establishment pattern for the species over the last 200 yr and compared population growth trends with those for other sites and with global-scale volcanism, which has been previously linked to the regeneration of the species. This population was significantly related to the two closer sites, suggesting a regional influence in regeneration and cohort establishment. Other eastern sites like ours also exhibited a regeneration peak in the 1920s. This population was not significantly related to volcanic eruptions, as has been documented elsewhere, though this may be due to subsequent mortality at the site, which would have effectively erased the record of establishment in past periods. Unlike past work, our study goes beyond a single population. We synthesized data from other known populations in order to produce, for the first time, a coherent picture of the patterns of regeneration over much of the species' range. Large-scale climatic (e. g., unusually rainy periods) and geologic (global-scale volcanism) events promote the establishment of cohorts during the same time periods historically, though to differing extents in different populations: hot, western populations have particularly large cohorts in response to volcanism, while more eastern sites may have observable cohorts during known wetter periods. Thus, these different large-scale phenomena appear to be of varying significance in a population on the basis of the population's location and associated environment. This provides insight into global climate change impacts on the species, whose keystone status will ultimately affect the success of and changes in the entire biome.