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Secondary forests constitute an increasingly important component of tropical forests worldwide. Although cycling of essential nutrients affects recovery trajectories of secondary forests, the effect of nutrient limitation on forest regrowth is poorly constrained. We used space-for-time chronosequences of secondary tropical forest succession to assess potential nutrient limitation along forest succession in central Africa. Using several approaches, we show that atmospheric phosphorus supply exceeds demand along forest succession, whereas forests rely on soil stocks to meet their base cation demands. Soil nutrient metrics indicate that available phosphorus increases along the succession, whereas available cations decrease. Fine root, foliar, and litter stoichiometry show that tissue calcium concentrations decline relative to those of nitrogen and phosphorus during succession. And finally, biological nitrogen fixation is downregulated along succession, with no more fixation taking place in older secondary forests, while the overall N cycle is shifting from conservative N cycling to a largely open N cycle along succession. Taken together, these observations suggest that calcium becomes an increasingly scarce resource in central African forests in later secondary succession. Furthermore, ecosystem cation – and calcium in specific- storage shifts from soil to woody biomass over succession, making it a vulnerable nutrient in the wake of land-use change scenarios that involve woody biomass export. Our results thus call for a broadened focus on elements other than nitrogen and phosphorus regarding tropical forest biogeochemical cycles and identify calcium as a scarce and potentially limiting nutrient in an increasingly disturbed and dynamic tropical forest landscape. The understanding of these underlying nutrient dynamics in secondary succession will be vital to come up with optimal management guidelines for restoring central African forests after disturbance.

Keywords:

secondary forests, nutrient limitation, chronosequences, biogeochemical cycles

Marijn Bauters

Presentation within symposium:

S-22 Tropical forest restoration: Role of soil biota-root symbioses (mycorrhizae and N-fixing bacteria)

Afrotropical forest succession: from nitrogen to calcium limitation

-Review-