Structure, composition and regeneration of miombo woodlands within harvested and unharvested areas

摘要

Production of tobacco by small holder farmers in Zimbabwe after the year 2000 land reform programme impacted woodlands due to the widespread use of fuel wood instead of coal for tobacco curing, but impacts have been rarely quantified. We assessed the effect of fuel wood harvesting for tobacco curing on structure, composition and diversity in harvested and unharvested miombo woodlands in the central watershed of Zimbabwe. Tree size class distributions provide demographic information on regeneration and recovery from disturbances. Vegetation structure and plant species composition were assessed in thirty 20 x 20 m plots in each site. Tree basal area, canopy cover and height were significantly higher in the less disturbed unharvested compared with harvested site. Species composition differed between harvested and unharvested sites, with higher diversity in the harvested compared with unharvested site. Trees had reverse-J-shaped size class distribution curves for both stem diameter and height at the harvested site and bimodal distributions with fairly large densities in larger size daises at the unharvested site. Recruitment and regeneration were higher in the harvested than unharvested site. Our findings show that harvesting fuel wood for tobacco curing has negative effects on tree structure, but not yet the diversity of the miombo woodland, however the latter will likely negatively impact tree species favoured for harvesting over the medium-long term. When species composition was compared using NMDS ordination, the harvested and unharvested plots were distinctly separated. Because of the rotational time for miombo woodlands of > 25 years, continuous harvesting of the woodland may permanently keep the trees small, failing to provide sufficient wood for tobacco curing. Therefore, there is need for the consideration of alternative or supplementary sources of energy such as solar, with a zero deforestation in the production chain of tobacco in the whole miombo eco-region.