Elucidating patterns in the population size structure and density of Aloe plicatilis, a tree aloe endemic to the Cape fynbos, South Africa

Abstract

Studies on plant population size structure provide important baseline information for monitoring and conservation. Traditionally, inverse J-shaped size class distributions (SCD) were considered indicative of ‘healthy’, stable plant populations; however, this may not hold true for long-lived, slow-growing species. This study assessed the population size structure of 19 populations of Aloe plicatilis, a Cape fynbos endemic tree aloe, and quantified population size and density. A strong positive linear relationship between stem diameter (SDr) and height was used to align SDr and height SCD bins, and to harmonize stage and size classes. Onset of reproduction occurs at ~15cm SDr and ~0.8m in height. Some A. plicatilis individuals appear to display a ‘bonsai effect’ whereby the growth of individuals in very rocky sites is suppressed. Stunted plants may be non-reproductive ‘suppressed juveniles’ or ‘reproductive dwarfs’. The short, shrub-like growth form of many individuals may also result from height restrictions imposed by persistent strong winds. Population size, extent and density ranged from 31 to >110 000 individuals, 0.05–103ha and 75–3000 plants/ha respectively. Bell-shaped SCDs were the most common (50% of populations). The SDr and height SCDs for all populations combined were also bell-shaped. Seven populations displayed irregular SCDs, but were similar in structure to the bell-shaped SCDs, both of which were attributed to an adult-persistence population survival strategy. This study challenges the suitability of the inverse J as the only SCD indicative of healthy, stable populations for long-lived, slow-growing species. The study provides baseline demographic data on A. plicatilis across its distribution for long-term monitoring, and provides insights useful for examining trends in the demographics of other long-lived, slow-growing species.