Miombo woodland under threat: Consequences for tree diversity and carbon storage

Agriculture is expanding rapidly in the miombo woodlands of sub-Saharan Africa. Clear felling results in the loss of species and ecosystem services. The remaining woodland is used as a vital support system for the farming communities, and the impact of this utilisation on biodiversity and ecosystem service provision is not clear. Understanding these effects will aid the development of effective, sustainable land management strategies for multiple outcomes, including biodiversity conservation and resource utilisation. This study provides new data on miombo woodland tree species diversity, structure and carbon storage from a 8766 km2 landscape in south-western Tanzania, which is undergoing rapid conversion to tobacco cultivation. Human utilisation of the woodland was classified by ground surveys which recorded evidence of use (e.g. cut poles and timber, removal of bark and roots, access routes). Nine sites were surveyed and categorised into three groups: high, medium and low utilisation. To determine the effect of utilisation on the tree community stem density, diameter at breast height, tree species richness and carbon storage were recorded. In the low utilisation sites carbon storage was similar to that found in other miombo woodlands (28 t Ha−1), and the Shannon Wiener diversity score for tree species diversity was 3.44. However, in the high utilisation sites, tree species diversity (2.86) and carbon storage declined (14.6 t Ha−1). In areas of moderate utilisation diversity and carbon storage were maintained, but the structure of the woodland was affected, with a reduction of Class 1 (Diameter at Breast Height (DBH) < 10 cm) stems, demonstrating low recruitment which leads to a reduction in sustainability. Tree species richness and abundance demonstrated an intermediate disturbance effect in relation to utilisation, with highest levels at medium utilisation sites. Key miombo woodland species from the subfamily Caesalpinioideae in the two genera Brachystegia and Julbernardia were present in all sites, but the frequency of Brachystegia species declined by 60% from low to high utilisation. The IUCN near-threatened timber species Pterocarpus angolensis, highly protected in Tanzania, was harvested throughout the study site, and the majority of trees recorded were immature (DBH ⩽ 20 cm), suggesting that it is commercially extinct for the foreseeable future. These findings illustrate that in miombo woodlands with low to medium utilisation levels key miombo species are retained, and tree species diversity and carbon storage remains optimal. Sustainable land management plans need to regulate utilisation within miombo landscapes and retain areas of woodland. This will ensure their long term viability, and continue to support the 100 million people who are reliant on miombo woodlands for their goods and services