Developing integrated silvicultural management systems for woodlands of northwestern Zimbabwe

Abstract

Silvicultural management systems based on natural disturbance-recovery processes are fundamental so as to integrate multiple-use practices in the natural tree resources inorder to address local needs and global concerns. The study aimed at developing a new understanding of the Baikiaea-Guibourtia-Pterocarpus woodland recovery dynamics when exposed to different disturbances such as single tree selection, harvesting of poles and firewood and cultivation. Four specific objectives were studied to examine different parts of this overall study: Disturbance impacts on the composition and diversity of the Baikiaea-Guibourtia-Pterocarpus woodlands of North-western Zimbabwe; Ecological drivers of floristic and structural composition of Baikiaea-Guibourtia-Pterocarpus woodland communities; Disturbance impacts on regeneration of key ecological and/or economic species in the Baikiaea-Guibourtia-Pterocarpus woodlands; and Age and growth rate determination using growth rings of selected Baikiaea-Guibourtia-Pterocarpus species. Variation in floristic composition of recovering Baikiaea-Guibourtia-Pterocarpus woodlands was studied in different development stages (from early re-growth to mature woodland). TWINSPAN (TWo-way INdicator SPecies ANalysis) was used for classification of plots from different utilization systems and different development stages into communities. One-Way ANOVA in SPSS Version 21 was used to test for differences in species diversity for the different communities. Detrended Correspondence Analysis (DCA) in the CANOCO ordination programme was used to determine the extent of variation amongst the identified communities. Correspondence analysis in SPSS Version 21 Statistical Package was performed to analyse grain status for nine tree communities. Stem diameter class profiles were analysed in STATISTICA version 7 to determine the ratio of regeneration to mature trees of individual canopy tree species across tree communities. . The research revealed that even if the sites provide a potential for woodland of a specific type to develop, plant community formation is mainly influenced by existing utilization systems. The intense disturbance factors of clearing for crop cultivation and to a lesser extent harvesting for poles and firewood are necessary disturbances for the regeneration and growth of the Baikiaea-Guibourtia-Pterocarpus woodlands. The grain status (relative similarity between the composition of canopy species in the regeneration and in the canopy of the same stand) and stem diameter distributions in the different communities suggested that the species require some more intense disturbances, such as with cultivation and harvesting for poles and firewood, to promote regeneration and growth to higher size classes. The study also revealed that disturbances that cause the opening of canopy and more light penetration on the forest floor are required to facilitate sprouting and growth of suppressed shoots in these woodlands. The study concludes that clearing for cultivation and harvesting for poles and firewood are important components to which the woodland ecosystem is adapted and important for the recovery of the Baikiaea-Guibourtia-Pterocarpus woodlands. It is recommended that woodland utilization and management should integrate cultivation and harvesting for poles and firewood into forest management. Also, a comprehensive program is necessary to monitor the levels of cultivation and the impacts on woodland recovery. Cutting cycles should be based on growth rate of the selected species. Timber species harvesting should go side by side with these disturbance factors so as to open up the canopy to allow maximum sunlight for the regeneration stock.