Environmental controls on carbon stock recovery rates in Afrotropical secondary forest

Tropical forests are rapidly changing due to land-use conversion, with major consequences for biodiversity and the terrestrial carbon balance. Secondary (regrowth) forests now cover a larger global area than primary forests, making their carbon accumulation central to the future strength of the terrestrial carbon sink. In sub-Saharan Africa, slash-and-burn agriculture remains the dominant disturbance and is expected to intensify as the human population triples by 2100. As a result, secondary forests are increasingly reshaping regional vegetation structure, yet biomass recovery rates, drivers, and uncertainties remain poorly constrained, limiting carbon stock estimates and vegetation model performance.

We compiled a new pan-African field dataset of aboveground carbon (AGC) stocks across Afrotropical secondary forest succession using 969 inventory plots from 31 sites in 12 countries. AGC recovery trajectories were modeled within a hierarchical Bayesian framework that propagates observational and process uncertainty. Median times to recover 90% of old-growth AGC (t₉₀) ranged from 36 to 91 years, with uncertainty intervals at some sites extending beyond a century. The early-successional AGC accumulation (first 20 years) varied widely, from 12 to 100 Mg C ha^-1. Mean annual precipitation emerged as a dominant control on AGC recovery, outperforming other climatic (temperature, photosynthetically active radiation, seasonality, and maximum climatic water deficit), soil (chemical and physical), and landscape predictors (forest cover and distance metrics). Absolute AGC accumulation during the first two decades was more predictable (R² = 0.56) than recovery relative to old-growth reference conditions (R² = 0.21), a pattern especially relevant for carbon sequestration assessments.

Overall, forest carbon recovery across the Afrotropics is relatively slow, heterogeneous, and often highly uncertain. By synthesizing extensive field data, this study provides empirical benchmarks for validating vegetation models and improving projections of the terrestrial carbon sink. However, the large uncertainties underscore the need to expand long-term forest inventories and reinforce the importance of conserving the remaining old-growth forests alongside the carbon sequestration potential of natural regeneration.