Soil carbon storage is related to tree functional composition in naturally regenerating tropical forests

1. Regenerating tropical forests are increasingly important for their role in the global carbon cycle. Carbon stocks in aboveground biomass can recover to old-growth forest levels within 60-100 years. However more than half of all carbon in tropical forests is stored belowground, and our understanding of carbon storage in soils during tropical forest recovery is limited. 2. Importantly, soil carbon accumulation does not necessarily reflect patterns in aboveground biomass carbon accrual during secondary forest succession and factors related to past land-use, species composition, and soil characteristics may influence soil carbon storage during forest regrowth. 3. Using tree census data and a measure of tree community shade tolerance (species-specific light response values), we assessed the relationship between soil organic carbon stocks and tree functional groups during secondary succession along a chronosequence of 40–120-year-old naturally regenerating secondary forest and old-growth tropical forest stands in Panama. 4. Whereas previous studies found no evidence for increasing soil C storage with secondary forest age, we found a strong relationship between tree functional composition and soil carbon stocks at 0-10 cm depth, whereby carbon stocks increased with the relative influence of light-demanding tree species. Light demanding trees had higher leaf nitrogen but lower leaf density than shade-tolerant trees, suggesting that rapid decomposition of nutrient-rich plant material in forests with a higher proportion of light-demanding species results in greater accumulation of carbon in the surface layer of soils. 5. Synthesis. We propose that soil carbon storage in secondary tropical forests is more strongly linked to tree functional composition than forest age, and that the persistence of long-lived pioneer trees could enhance soil carbon storage as forests age. Considering shifts in tree functional groups could improve estimates of carbon sequestration potential for climate change mitigation by tropical forest regrowth