Land-use impacts on the composition and diversity of the Baikiaea– Guibourtia–Pterocarpus woodlands of north-western Zimbabwe

Abstract

Variation in floristic composition of recovering Baikiaea–Guibourtia–Pterocarpus woodlands was studied in different development stages (from early regrowth to mature woodland) under different land-use systems (protected areas, timber harvesting, pole and firewood collection, and abandoned crop fields), in the Gwayi and Tsholotsho areas in north-western Zimbabwe. A total of 150 nested circular plots were sampled representatively in four different development stages related to the land-use systems. The DBH (stem diameter at 1.3 m above ground level) and tree height were recorded by species for all stems of tree species with DBH ≥ 15 cm in a 30-m-radius plot (0.283 ha) and for trees with DBH 5.0–14.9 cm in a 11.3-m-radius plot (0.04 ha) (both centred around the same midpoint). Stems with DBH < 5 cm were counted by species in an inner sub-plot of 5.65 m (0.01 ha) radius. Tree data (stem DBH ≥ 5 cm) and regeneration data (stem DBH < 5 cm) by stem counts per species per plot, were used separately to run TWINSPAN (TWo-way INdicator SPecies ANalysis) classifications of species assemblages. Importance values were calculated for all tree species per community. Shannon–Wiener diversity indices were calculated for each community and tested for differences using one-way ANOVA in SPSS version 21. Detrended correspondence analysis (DCA) implemented in the CANOCO ordination software was used to determine the extent of variation amongst the identified communities. The classification identified 12 tree communities and 13 regeneration communities, clustering plots from different land uses together. Baikiaea plurijuga was the most important tree species in all tree communities, except where Combretum collinum, C. apiculatum, Commiphora mossambicensis and Pterocarpus angolensis were dominant. Pterocarpus angolensis showed low importance in most communities, except for communities from undisturbed sites (mostly mature trees) and abandoned fields (mostly young trees). Baikiaea plurijuga was most important in most regeneration communities, except in communities dominated by Baphia massaiensis, C. collinum, C. apiculatum and P. angolensis. Species diversity differed significantly (p < 0.05) amongst tree communities. The DCA ordination showed little variation amongst the communities. The cumulative contribution of environmental factors explaining variation in species composition was 22.6% for tree communities and 26.1% for regeneration communities, suggesting that recovery from disturbance after different land uses may explain more of such variation.

Keywords: disturbance, regeneration, species diversity, woodland