What lies beneath : detecting sub-canopy changes in savanna woodlands using a three-dimensional classification method

QUESTION : Increasing population pressure, socio-economic development and associated natural resource use in savannas are resulting in large-scale land cover changes, which can be mapped using remote sensing. Is a three-dimensional (3D) woody vegetation structural classification applied to LiDAR (Light Detection and Ranging) data better than a 2D analysis to investigate change in fine-scale woody vegetation structure over 2 yrs in a protected area (PA) and a communal rangeland (CR)? LOCATION : Bushbuckridge Municipality and Sabi Sand Wildtuin, NE South Africa. METHODS : Airborne LiDAR data were collected over 3 300 ha in April 2008 and 2010. Individual tree canopies were identified using object-based image analysis and classified into four height classes: 1–3, 3–6, 6–10 and >10 m. Four structural metrics were calculated for 0.25-ha grid cells: canopy cover, number of canopy layers present, cohesion and number of height classes present. The relationship between top-of-canopy cover and sub-canopy cover was investigated using regression. Gains, losses and persistence (GLP) of cover at each height class and the four structural metrics were calculated. GLP of clusters of each structural metric (calculated using LISA – Local Indicators of Spatial Association – statistics) were used to assess the changes in clusters of eachmetric over time. RESULTS : Top-of-canopy cover was not a good predictor of sub-canopy cover. The number of canopy layers present and cohesion showed gains and losseswith persistence in canopy cover over time, necessitating the use of a 3D classification to detect fine-scale changes, especially in structurally heterogeneous savannas. Trees >3 min height showed recruitment and gains up to 2.2 times higher in the CR where they are likely to be protected for cultural reasons, but losses of up to 3.2-foldmore in the PA, possibly due to treefall caused by elephant and/or fire. CONCLUSION : Land use has affected sub-canopy structure in the adjacent sites, with the low intensity use CR showing higher structural diversity. A 3D classification approach was successful in detecting fine-scale, short-term changes between land uses, and can thus be used as amonitoring tool for savannawoody vegetation structure. Remove selectedThe Carnegie Airborne Observatory is made possible by the Avatar Alliance Foundation, Margaret A. Cargill Foundation, John D. and Catherine T. MacArthur Foundation, Grantham Foundation for the Protection of the Environment, W.M. Keck Foundation, Gordon and Betty Moore Foundation, Mary Anne Nyburg Baker and G. Leonard Baker Jr. and William R. Hearst III. Application of the CAO data in South Africa is made possible by the Andrew Mellon Foundation and the endowment of the Carnegie Institution for Science.http://onlinelibrary.wiley.com/journal/10.1111/(ISSN)1654-109X2016-07-31hb201

Unsustainable fuelwood extraction from South African savannas

Wood and charcoal supply the majority of sub-Saharan Africa’s rural energy needs. The long-term supply of fuelwood is in jeopardy given high consumption rates. Using airborne light detection and ranging (LiDAR), we mapped and investigated savanna aboveground biomass across contrasting land uses, ranging from densely populated communal areas to highly protected areas in the Lowveld savannas of South Africa. We combined the LiDAR observations with socio-economic data, biomass production rates and fuelwood consumption rates in a supply–demand model to predict future fuelwood availability. LiDAR-based biomass maps revealed disturbance gradients around settlements up to 1.5 km, corresponding to the maximum distance walked to collect fuelwood. At current levels of fuelwood consumption (67% of households use fuelwood exclusively, with a 2% annual reduction), we calculate that biomass in the study area will be exhausted within thirteen years. We also show that it will require a 15% annual reduction in consumption for eight years to a level of 20% of households using fuelwood before the reduction in biomass appears to stabilize to sustainable levels. The severity of dwindling fuelwood reserves in African savannas underscores the importance of providing affordable energy for rural economic development.The CSIR researchers were funded by the CSIR Strategic Research Panel and the Department of Science and Technology’s Earth Observation Unit. SUCSES study (Sustainability in Communal Socio-Ecological Systems) which provided data on fuelwood use in Justicia was funded by the South African National Research Foundation. The airborne campaign and analysis was funded by the Andrew Mellon Foundation.http://iopscience.iop.org/1748-9326am201