Population Dynamics of Marula, Sclerocarya birrea sub sp. caffra in Mwenezi, Zimbabwe

Abstract

This study, conducted in Ward 3, Mwenezi, determined the abundance, distribution and size structure of Marula, Sclerocarya birrea subsp. caffra. The relationship between incidence of debarked stems and size structure was investigated. The study also aimed at establishing whether the current S. birrea population will be able to sustain continued extraction. Results indicate that communities vary spatially and cannot, therefore, be extrapolated from one site to another. Marula stem densities varied from transect to transect, with a pooled density of 7 stems/ ha, but distance from processing centre to each of the belt transects could not be related to the distribution of Marula (r2 < 0.025, p>0.05). There were no significant differences, at transect level in total stem counts (F = 1.16, df = 10, p>0.05), number of debarked stems (F = 1.65, df = 10, p>0.05) and fruiting stems (F = 1.87, df = 10, p>0.05). Differences were only significant at size class level. The most debarked stems were those with basal diameters of 40.1 to 60 cm, which coincidentally, had the high fruiting incidence. The population size structure in the various transects differed significantly. Frequency of size classes in Belts 6 and 10 indicated that younger trees (saplings and seedlings) are relatively scarce and that mortality of seedlings and saplings may be sufficiently high to prevent recruitment. Inverse J-shaped distribution of Marula in Belts 2, 4, 5, 7 and 9, across the size classes however, indicates that the population is normal (recruitment into upper size classes is ccurring) with no previous record of overexploitation. Marula tree variables: height, basal diameter, basal area and canopy cover were positively correlated, and the relationships could be described by simple linear regression model. Marula population projections carried out using the Leslie matrices seem to show that there may be a gradual decrease in stem numbers on a per size class basis and the overall stem counts.