Cohort establishment on slopes: Growth rates, demographic patterns, and the relationship to volcanic eruptions

Little is known about keystone Carnegiea gigantea’s populations and the global-scale factors that influence its distribution and regeneration success. We sampled 300 plants on a north- and on a south-facing slope, determined age structure, and compared results to those of other studies in topographically flat areas using regression and ANOVA. We found that growth rates are about the same for the two slopes, but more young Carnegiea were documented on the south-facing than on the north-facing slope, likely due to microclimatic differences and susceptibilities. Fewer individuals were represented in the higher age classes than often found in topographically flat populations. We found that individuals are faster growing on slopes than on flats, likely due to differences in sediment size and make-up. Finally, a recent study linked worldwide volcanism with cohort establishment in topographically flat areas. We found that successful cohort establishment in both populations (north- and south-facing) was significantly higher during years with greater volcanic activity (and the years that were subsequently influenced by airborne materials ejected from the eruption), the first such observation for sloped populations. Volcanic eruptions occurring worldwide impact the regeneration success of this species, and in a variety of topographic settings, extending our previous knowledge of the link between global geologic events and climate change with the regeneration of this species locally (the first ever documented), which with further research will surely be extended to other species in other biomes worldwide.

► Plants grow faster on both north and south-facing slopes than in flat areas nearby.
► Young individuals have greater establishment in the south-facing population.
► Cohort establishment is higher during greater volcanic activity years (worldwide).