Carbon dynamics of African miombo woodlands: from the leaf to the landscape

Abstract

Africa’s carbon (C) cycle is one of the least well understood components of the global C cycle. Miombo woodlands are the most common woodland type in southern Africa, but despite their vast extent and importance in the biogeochemical cycles of Africa, their C dynamics are not well understood. This thesis addresses a set of science questions related to miombo woodland C dynamics that cover a range of scales, from the leaf to the landscape. The questions are related to seasonal controls on C uptake at the leaf level, to spatial distributions and scales of variation of C stocks in the landscape, and to the drivers and spatial patterns of deforestation and degradation at the regional scale. In miombo woodlands, the seasonality of productivity remains poorly understood, and it is unclear whether stomatal limitations or variations in leaf traits cause seasonal changes in productivity. I use data of leaf gas exchange and leaf traits collected in dry and wet seasons to assess the response of photosynthesis to seasonality. I found a large degree of inter-specific responses, where photosynthetic capacity was maintained between seasons in some tree species but not in others. This was linked to inter-specific stomatal regulation on leaf gas exchange, access to soil water and varied leaf traits, indicating differing timing of leaf development during the dry season. Differing timing of leaf flushing can create niche separation, facilitating the co-existence of miombo woodland tree species. I use data collected along a 5 km transect through miombo woodland to characterise the spatial distributions and scales of variation of C stocks in woody biomass and soils, and assess the links between them. I found that on the scale of a few meters, soil C stocks varied in relation to soil texture. At the kilometre scale, surface soil and woody C stocks were coupled, and varied in relation to topography. By understanding the scales of variation I was able to make recommendations for optimal sampling of C stocks in a miombo woodland landscape for improved C stock assessments. I developed and tested a simple spatial model of deforestation and degradation, using a rule-based approach, to produce risk maps of areas more likely to be affected by deforestation and degradation for a study site in central Mozambique. I found that my model was able to accurately predict the locality of high risk areas, and that roads were the major axis for forest biomass loss. Risk maps created from this method are useful for exploring the drivers of deforestation and degradation in a region dominated by miombo woodland, and for targeting policy and management efforts. Overall, this thesis has contributed significantly to our understanding of natural and human driven miombo woodland C dynamics over a range of scales, from the leaf to the landscape. In the final chapter, I discuss the implications of each chapter for our understanding of miombo woodland C dynamics, and suggest areas for further research.