The spatial variation of the regeneration structure of riparian tree species in the Okavango Delta, Botswana

Abstract

Riparian woodlands in the Okavango Delta play a significant role in the hydrological cycle, directly and indirectly. Despite their ecological importance, very little is known about their regeneration status in the Okavango Delta. The aim of this study was to investigate the spatial variation in the regeneration structure of different riparian tree species in the Okavango Delta. The sampling sites were Seronga, Boro, Moremi Game reserve, Santawani and Vumbura. These species were Croton megalobotrys Müll. Arg, Philenoptera violacea (Klotzsch) Schrire, Senegalia nigrescens Oliv and Diospyros mespiliformis Hochst. ex A. Dc. The height of individual trees was measured in randomly selected plots of 20m × 50m. Individuals were assigned to 0-0.5m, 0.6-1m, 1.1-2m, 2.1-4m and >4m regeneration classes. A total of 75 plots were sampled at intervals between February 2012 and April 2013. Linear regression was used to infer the regeneration status of each species. One way ANOVA followed by Tukey post hoc analysis were used to compare the seedling density/ha and sapling density/ha of different species across different sites. Philenoptera violacea showed a similar regeneration structure of reverse J shaped regeneration across all the sites. D. mespiliformis showed a reverse J shaped regeneration structure in Vumbura, Moremi and Boro while in Seronga it showed a U shaped regeneration structure. Croton megalobotrys showed U shaped regeneration structure in Santawani and Boro, a J shaped pattern in Vumbura and Moremi while in Seronga it exhibited an irregular structure. S. nigrescens showed a reverse J shaped regeneration structure in Vumbura and  a J shaped structure in Moremi and Seronga. In Santawani it had a U shaped structure. Seedling and sapling density/ha for each species varied significantly (p<0.05) among some sites. However, the sapling density of D. mespiliformis did not differ significantly among the sites. These results indicate that there is variation in the regeneration structure of different species in different sites. This calls for further research on the environmental factors that influence the regeneration of the woodland species in the Okavango Delta.