The social-ecological drivers across land-use intersects driving marula tree population dynamics in north-eastern South Africa

A range of social and economic factors impact tree species, such as Sclerocarya birrea subsp. caffra (marula), which provide livelihood sustenance for local communities in the savanna woodlands of southern Africa. As an ecologically important savanna species, valued both culturally and economically, it is imperative to understand if resource use is sustainable. Population stability can be understood by comparing tree density and size-class distribution (SCD) profiles across land-use types (homestead yards, fields and rangelands) in non-conservation savanna social-ecological systems. Marula tree population data were gathered in rangeland transects and randomly selected fields and yards from four human settlements in the Bushbuckridge municipality of north-eastern South Africa. Total density was lowest in fields (mean ± SE = 7.4 ± 0.7 trees/ha) and highest in homestead yards (mean ± SE = 25.7 ± 4.1 trees/ha). Social data revealed that elevated seedling and sapling population densities in yards is linked to discarded kernels from marula beer making. Total densities increased in yards over a 15-year period but declined in rangelands, with female densities remaining almost constant across land-use types over this period. This is an important finding as it is a strong indication of socially-mediated population structure changes, confirming that combined cultural and economic value can lead to examples of species conservation. In this case, the prioritization of female trees as the distinguished fruit producers. SCD revealed weak recruitment in fields and rangelands. Diminished regeneration, combined with overharvesting for fuelwood in rangelands and felling trees in yards, is likely to negatively impact population stability in the long-term. Understanding resource conservation and degradation in the context of important non-timber forest products (NTFPs) is a key knowledge challenge, with this study serving as an updated inventory benchmark for marula populations in the area. Important lessons learned here can be applied to other social-ecological contexts where a key natural resource is responsible for sustaining livelihoods.