Long-term population changes in the giant quiver tree, Aloe pillansii in the Richtersveld, South Africa

With probably fewer than 3000 individuals alive in the biodiversity hotspot of the Succulent Karoo in southern Africa, populations of the endemic, Giant Quiver Tree, Aloe pillansii, are thought to be declining and thus threatened with extinction. Using repeat photography and field data we investigated the long-term changes in one population of A. pillansii at its type locality, the roughly 100 ha Cornell's Kop in the Richtersveld, South Africa. There are currently 75 individuals alive at this site. Of these, 44% are < 1 m in height (seedlings), 4% are 1-3 m (juveniles) and 52% are > 3 m (adults). An analysis of 14 repeat photographs shows that since 1937 an average of 1.4% of the plants > 3 m in height has died annually. At this rate all the remaining 39 plants on Cornell's Kop in this size class will be dead in 71 years. The relative paucity of plants in the 1-3 m size classes could be explained by several factors including plant theft, animal damage and unfavourable recruitment conditions during the first 80 years of the 20th century. Annual growth rates decrease as plants age. Individuals < 1 m in height grow at 42.5 mm yr(-1) while plants 1-3 m and those > 3 m grow at 31.0 and 16.4 mm yr(-1) respectively. At 8 m, the tallest plant on Cornell's Kop could be as old as 382 years and thus to maintain itself at this site, A. pillansii would only need to recruit relatively infrequently. The relatively high proportion of seedlings suggests that conditions have recently been favourable for recruitment at this site. Seedling ages, estimated from their heights, indicate that over 50% of the plants < 1 m in height germinated 5-10 years ago. This is consistent with local rainfall records which show that rainfall was consistently above the long-term annual average of 75 mm during this period. However, the loss of six seedlings from the population in the last 5 years, probably due to grazing or theft, suggests that without intervention this species will not survive on Cornell's Kop.