Proposed long-term monitoring protocol and applications for Aloidendron dichotomum populations

Abstract

The iconic succulent tree, Aloidendron dichotomum, is found throughout the arid and semi-arid parts of South Africa and Namibia. It has been suggested as a climate change indicator species with long-term monitoring of populations initiated by various institutions in order to improve understanding of its population dynamics and main climate drivers. This proposed monitoring protocol attempts to combine various methodologies already used in order to standardise on field data collection procedures. In order to illustrate the value of the data collected this paper compares baseline demographic data recorded for 12 populations across the Northern Cape (South Africa).Live and dead tree densities were highly variable across populations. Size class distribution (SCD) analysis using height classes and basal diameter classes resulted in different population curve types depending on the variable plotted emphasising the importance of variable choice.Comparison of the midpoint of the basal diameter classes and the natural logarithm of density of individuals per class indicated different linear regression slopes for the Kokerboomkloof population and eight other populations and in one instance, the Kokerboomkloof intercept value differed. In all populations, the calculated centroid value was smaller than the median indicating growing populations.The low occurrence of leaf fungi and scale as well as damage to leaves and stems, indicate that populations are generally healthy with regard to pests and disease. Percentage live and dead heads, dead canopy and dead trunks for surveyed trees were highly variable across populations.At present, the 12 surveyed South African A. dichotomum populations appear to be in good health and are recruiting. Our study highlights that the choice of population surveyed is of importance and that a wide range of populations in close proximity to one another and at different sites situated far apart have to be surveyed in order to produce an unbiased assessment of populations.