Functional groups, species and light interact with nutrient limitation during tropical rainforest sapling bottleneck

Potential variability in nutrient limitation among tree size classes, functional groups and species calls for an integrated community- and ecosystem-level perspective of lowland tropical rainforest nutrient limitation. In particular, canopy trees determine ecosystem nutrient conditions, but competitive success for nutrients and light during the sapling bottleneck determines canopy composition. We conducted an in situ multi-nutrient sapling fertilization experiment at La Selva Biological Station, Costa Rica, to determine how functional group identity, species identity and light availability can impact nutrient limitation of stem growth in three functional groups and nine species. Despite high soil fertility, we found nutrient-light limitation in two functional groups and four species. Unexpectedly, the nitrogen-fixing (“N₂ fixers”) and shade-tolerant functional groups were significantly nutrient limited, while the light-demanding functional group was not. This was partially explained by species-level variation in nutrient limitation within these functional groups, with only some species conforming to the prediction of stronger nutrient limitation in light demanders compared to shade-tolerants. Most surprisingly, we found strong nutrient limitation at low-light levels in the N₂ fixers (which were shade-tolerant), but not in the shade-tolerant non-fixers. We hypothesize that the N₂ fixers were actually nitrogen limited at low-light levels because of their nitrogen-rich leaves and the high carbon cost of their symbionts. This finding suggests a highly shade-tolerant, N₂ fixation strategy, in addition to the perception that N₂ fixation is mostly advantageous in high-light environments during early and gap succession. The shade-tolerant, N₂ fixation strategy may be part of a sapling and canopy tree feedback, where the canopy N₂ fixers enrich the soil N, enhancing growth of their shade-tolerant saplings relative to non-fixing competitors, enabling further canopy domination by shade-tolerant N₂ fixers, as seen at La Selva. Synthesis. The pervasiveness of functional group- and species-specific nutrient and light co-limitation in our saplings indicates that these interactions likely play an important role in the dynamics of lowland tropical rainforest nutrient limitation, potentially via other such sapling and canopy tree feedbacks as the one hypothesized.