The distribution and population structure of Aloe pillansii in South Africa in relation to climate and elevation

Abstract

Aloe pillansii is a large succulent tree confined to the hyper arid region of the Richtersveld and southern Namibia. Because of its narrow distribution, its small population size and decline in adult plant numbers it is considered an endangered species, susceptible to the impacts of climate change. Using two databases compiled for the 1 202 known South African individuals of the species we identified 10 sub-populations, defined as clusters of individuals within 2 km of their nearest con-specific neighbour. Seven sub-populations were located relatively close to each other in the northern part of their range while three were located together 60 km further south than this. Using correlation and multivariate ordination analyses of the environmental variables associated with each sub-population, we show that the sites of the southern populations are higher, cooler and wetter than their more northern counterparts, which were themselves divided into warm, low-lying sites in the west, with a predominance of winter rain, and moderately high elevation sites with more summer rain in the east. An examination of the height class frequency distribution of all South African individuals shows a bi-modal distribution with more than 25% of the recorded individuals being less than 1 m in height. There is a significant decline in the 2–4 m height class, but an increase to 5 m and a steady decline thereafter to the largest height class of 12 m. The best explanation for this pattern suggests that there has been poor recruitment in the species prior to the last 20 years, although theft and herbivory of small individuals might also explain the absence of 2–4 m high individuals in the population. The height class frequency distributions of the various sub-populations differ significantly. The three southern sub-populations exhibit a uni-modal distribution with a low proportion of seedlings and juveniles (individuals <2 m high) in the population. Most of the northern sub-populations show a bi-modal distribution except for one very small population with no seedlings or juveniles, and one with an inverse J-shape distribution pattern. The number of seedlings and juveniles in a sub-population is significantly positively correlated with the number of adults. We suggest that the small sub-populations, particularly at relatively cool, high-elevation sites are the most vulnerable to extinction. The influences of these factors, particularly population size, on recruitment and mortality are important to understand if the impact of climate change on this species is to be mitigated by some form of intervention.