Long-term population changes in the Giant Quiver Tree, Aloe pillansii in the Richtersveld, South Africa

Abstract

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.

Population structure of an arborescent aloe (Aloe marlothii) in Botswana

“…The authors surmised that because of the loss of six seedlings from the population in the last 5 years, probably due to grazing or theft, this population will not survive unless there is intervention (Duncan et al, 2006). The significant correlations between a large number of plant characteristics (Table 2) indicates that measurement of a single basic attribute, such as height or leaf number, will be sufficient to gain information about the plant.…”

“…Long-term data sets have supported this size-age inference although growth rates generally decline as species reach a certain height (Pierson and Turner, 1998;Duncan et al, 2006) and the relationship improves when summer rainfall patterns are included (Drezner, 2003(Drezner, , 2006.…”

“…Succulent recruitment is episodic because of the rarity of suitable climatic events, that is, periods of sufficient precipitation combined with appropriate temperatures. These variations in climate strongly influence regeneration patterns in a variety of arborescent succulent species Nobel, 1979, 1982;Pierson and Turner, 1998;Titus, 2004;Drezner, 2006;Duncan et al, 2006). These large arid-zone succulents are relatively long-lived species, which allows plant populations to survive for many decades without recruitment before local extinction.…”

African aloe ecology: A review

“…In the arid Northern and Western Cape Karoo, A. dichotoma and A. pillansii are regarded as keystone species, where they provide food, nesting and shelter, as well as vantage points for raptors in an otherwise sparsely vegetated landscape Powell, 2005). Adult A. dichotoma and A. pillansii also contain large water reserves, which a number of species rely on during periods of extended drought (Duncan et al, 2006;Powell, 2005).…”

“…The tree aloes A. dichotoma and A. pillansii, in the xeric Northern Cape, South Africa and Namibia, have received much attention in recent years due to significant changes in population structure and numbers (Bolus et al, 2004;Duncan et al, 2005Duncan et al, , 2006Foden et al, 2007;Jack, 2012;Midgley et al, 1996Midgley et al, , 1997. In the decade prior to 2003, A. pillansii suffered a 50% reduction in population size (Powell et al, 2003) and there have also been numerous reports of widespread mortalities in A. dichotoma, apparently induced by drought (Foden et al, 2007).…”