Long term impacts of crop cultivation and tree shading on Piliostigma thonningii (Schumach) Milne-Redhead population, growth and biomass production after charcoal production in central Zambia

Charcoal production in Sub-Saharan Africa is often perceived to have serious ecological and environmental effects even although it does not necessarily result in forest conversion to other land uses. Forest recovery through natural or assisted regeneration is particularly appealing for managing tropical dry forests following degradation by charcoal production because of the coppicing ability of the majority of tree species. However, the impacts of crop cultivation and tree enrichment planting on the regeneration of indigenous trees following charcoal production have rarely been investigated. The present study was conducted on adjacent cultivated and control subplots for over 20 years after charcoal production at a site in central Zambia, southern Africa, to assess the effects of short term crop cultivation, tree enrichment planting and tree shading on Piliostigma thonningii, a potential agroforestry species in sub-Saharan Africa. Seedling emergence, sapling survival and annual tree growth were monitored from 2000 to 2017 and aboveground wood biomass estimated using an allometric equation developed from trees cut at the site and the surrounding area. Seedling emergence from untreated seeds occurred over a 6-year period and tree shading significantly reduced seedling germination and survival. Saplings grew very slowly and none transitioned into a tree after 12 years of observations. Although crop cultivation had no significant effect on the population dynamics of P. thonningii, it reduced tree growth rates, biomass production and peak biomass accumulation. Peak aboveground wood biomass on post-cultivation blocks of 13.7 t ha⁻¹ represented only 61% of the 22.5 t ha⁻¹ on control blocks. Some of the major challenges in cultivating P. thonningii as an agroforestry species are seed dormancy and slow sapling growth and these need to be resolved if the species is to be widely cultivated.