Cations in Crisis: Limitation and Vulnerability of Secondary Forest Regrowth in dynamic Tropical Landscapes

Secondary forests will increasingly dominate tropical forest landscapes in the decades to come. Understanding how local biogeochemistry impacts regrowth trajectories is paramount, especially if this biogeochemistry is impacted by land-use change. In this study, we employ three lines of evidence from central African secondary forest succession sequences that suggest that cations might be a limiting and vulnerable resource to sustain secondary forest regrowth in the tropics. First – along succussion, our analysis reveals that atmospheric phosphorus supply exceeds demand during forest succession, while plant base cation demands are met predominantly through depletion of soil stocks. As such, soil nutrient metrics indicate an increase in available phosphorus along the succession, contrasting with a decrease in available cations.  Coincidentally, fine root, foliar, and litter stoichiometry collectively demonstrate a decline in tissue calcium concentrations relative to nitrogen and phosphorus during succession. These findings collectively suggest that calcium becomes a progressively scarce resource in central African forests during secondary succession. Second – also along succession, we show a substantial shift in ecosystem cation storage from soil to woody biomass over succession, while not for nitrogen or phosphorus,  rendering it a vulnerable nutrient in the context of land-use change scenarios involving woody biomass export. From independent data, we also show that with increasing repeated clearing, the total soil cation stocks are depleted, while we see no such consistent effect for nitrogen or phosphorus. Coincidentally, at a catchment scale, we see that cation losses increase as catchments get increasingly impacted by land-use change. Third – based on a pot experiment, tree growth seems limited first and foremost by cations, and then by nitrogen. This again reiterates that cations might be an important and overlooked limiting element for sustaining plant growth in highly depleted tropical soils. Collectively, this work calls for an expanded perspective on nutrient dynamics and highlights the vulnerability of cations in the face of changing land-use scenarios, with potentially important sustainability issues in the long-term – especially if the secondary regrowth potential is lowered.