Afrotropical secondary forests exhibit fast diversity and functional recovery, but slow compositional and carbon recovery after shifting cultivation

Questions Human disturbance is increasingly affecting forest dynamics across the tropics. Forests can recover via natural secondary succession to pre-disturbance levels of biodiversity, species composition, and ecosystem carbon stocks. Central Africa will be subject to increasingly high shifting-cultivation pressure in the next decades, but succession trajectories of these ecosystem properties are still poorly known for the Congo basin. We addressed two questions: (1) how does taxonomic and functional composition and diversity shift during secondary succession; and (2) how fast do above-ground carbon stocks recover during secondary succession in tropical forests? Location Central Congo basin. Methods We conducted an inventory of trees (diameter at breast height [DBH] >= 10 cm), measured species traits and soil texture and carbon content in 18 plots, located along six secondary succession stages (i.e., from agricultural to old growth forest sites). We measured tree diameter, height for 20% of trees distributed across diameter classes, wood traits from all species, and leaf traits from species that contributed to 85% of the plot basal area. Results We showed that secondary forests recover relatively fast in terms of tree species diversity, alpha functional diversity, and fine-root carbon, with near-old-growth forest values after six decades past disturbance, while floristic composition exhibited slower recovery. Secondary forests only partially shifted from acquisitive to a conservative life history, with shifts in leaf traits being largely decoupled from wood traits. Only 43% of above-ground carbon recovered after 60 years of forest regrowth, potentially through a slow recovery of the large-sized tree stems that dominate carbon stocks of old-growth forests. Conclusions Our findings underline the capacity of Afrotropical forests to recover species and alpha functional diversity after clear-cutting through shifting cultivation. Simultaneously, old-growth forests harbor a particular floristic community and store a large quantity of carbon with much longer recovery trajectories, stressing the need for conservation of these forests in the Congo basin.

Automated summary

Human disturbance is increasingly impacting tropical forest dynamics, but forests have the capacity to recover species diversity and functional diversity relatively quickly through secondary succession. However, above-ground carbon stocks only partially recover after 60 years, with a slow recovery of large trees dominating carbon stocks in old-growth forests.