How resilient are African woodlands to disturbance from shifting cultivation?

Large parts of sub-Saharan Africa are experiencing rapid land use and land cover change, driven largely by the expansion of small scale shifting cultivation. This practice creates complex mosaic landscapes with active agricultural fields and patches of mature woodland interspersed with remnant patches in various stages of re-growth. Our objective here was to examine the rate and extent to which carbon stocks in trees and soils recover after cultivation, and detail how this disturbance and re-growth affect patterns in tree species composition and diversity over 40 years of succession in a miombo woodland landscape in SE Tanzania. We sampled 67 areas, including plots previously cleared for cultivation, active fields and mature woodlands for reference purposes. Sites were further stratified by soil texture to test for associated effects. Tree carbon stocks accumulated at an average rate of 0.83 ± 0.10 tC ha-1 yr-1 with soil texture having no clear impact on accumulation rates. Bulk soil carbon stocks on both soil types appeared unaffected by both the initial land clearance, and the subsequent re-growth which resulted in no significant changes over time. Tree species diversity in re-growing plots developed rapidly and within around 10 years was equivalent to that of mature woodland. Many of species found in mature woodlands re-appeared relatively quickly after abandonment, although species composition is expected to take considerably longer to recover with at least 60-80 years required for the compositional similarity between re-growing and mature woodlands to reach levels similar to that among nearby mature woodlands. Through impacts on β-diversity, disturbance was also found to increase the total number of tree species present in the landscape, with many of the recorded species only found in re-growing woodlands. Our results are of relevance to carbon sequestration projects by helping to inform the potential future carbon and biodiversity benefits of restoring disturbed habitats (REDD+). At a time where the use of shifting cultivation is threatened by shifts to larger scale, commercial agriculture, we show that secondary woodland habitats can retain considerable biodiversity value and act as carbon sinks.