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