Assessing the effects of land use/land cover changes on carbon storage of natural forest ecosystems in Southern Africa: a remote sensing approach

Abstract

The extensiveness of Africa’s natural forests with approximately 20% of the continent’s land cover, makes them an important source of livelihood to over 100 million people. The exclusion of many natural forest types from the United Nations Reducing Emissions from Deforestation and forest Degradation Plus schemes suggest that very little is known about their contribution to global carbon sequestration storage. Furthermore, there is often significant underestimation of forest total carbon storage as most studies have focussed mainly on monitoring, reporting and evaluating carbon stocks in aboveground live biomass. This is partly because the belowground carbon pool is influenced by several spatially heterogeneous abiotic and biotic factors which are themselves not well studied. This thesis uses multi-temporal Landsat remotely sensed images to map land use/land cover changes and aboveground forest carbon stocks for the periods 1986-1998, 1998-2001, 2001-2004, 2004-2011 and 2011-2016 in Luanshya, Zambia, and the periods 1992-1998, 1998-2001, 2001-2006, 2006-2010 and 2010-2016 in Bushbuckridge, South Africa. Aboveground forest carbon stocks were estimated from the regression model relating in situ estimates in 30 x 30 m plots to spectral reflectance measurements for different time periods. The study also used PCA to assess 11 edaphic factors associated with the spatial and downward vertical distribution of SOM content in three forest subsoil layers. The calculated rate of forest regeneration was almost twice lower than the rate of deforestation, and thus over 68% of natural forests are secondary growth forests. From the ecological survey on 126 analogous forest sample plots of different succession stages, it was apparent that forest disturbances result in the replacement of bigger individual trees by smaller trees, and subsequent increase in tree density and reduction in species diversity. These anthropogenic and natural phenomena explained the mean changes in aboveground forest carbon stocks which were 44.20 t ha-1 in 1986, 43.85 t ha-1 in 1998, 54.81 t ha-1 in 2001, 53.02 t ha-1 in 2004, 46.83 t ha-1 in 2011 and 36.67 t ha-1 in 2016 for Luanshya, and 4.67 t ha-1 in 1992, 9.52 t ha-1 in 1998, 40.84 t ha-1 in 2001, 34.15 t ha-1 in 2006, 41.92 t ha-1 in 2010 and 24.95 t ha-1 in 2016 for Bushbuckridge. Conversely, the effects of these changes were also observed in the complexity in the spatial and downward vertical distribution of soil organic matter content. Mean soil organic matter stock in the three soil depth of 0-10 cm, 10-20 cm and 20-30 cm were 7056 g m-3, 5427 g m-3 and 6135 g m-3 in Luanshya, and 5065 g m-3, 4784 g m-3 and 4733 g m-3 in Bushbuckridge, respectively. Principal components analysis revealed that the percentage of clay and silt correlated consistently with increased soil organic matter content, while bulk density and sand% impeded its accumulation. The baseline data and the empirical models can be used as counterfactual scenarios for monitoring the emerging forest carbon projects in southern Africa. The patterns and strength of relationships can be used as input or validation datasets for improving soil carbon models and for devising more efficient strategies for forest management.