Nitrogen fixer abundance has no effect on biomass recovery during tropical secondary forest succession

1. Nitrogen-fixing trees (N-2 fixers) provide new nitrogen critical for rapid biomass accumulation of tropical forests during early secondary succession, but it remains unclear how the abundance of N-2 fixers in the forest community affects the growth of non-fixers or the primary productivity of the whole forest. 2. On the one hand, N-2 fixers may enhance forest productivity by providing a facilitative effect through the provision of plant-available nitrogen to non-fixing trees. On the other hand, N-2 fixers may suppress the growth of non-fixers by growing faster and competing more vigorously for light and other resources. A third alternative is that the growth of N-2 fixers themselves accumulate biomass rapidly, while having a neutral effect on non-fixers, leading to an overall increase in forest biomass. 3. We examine these alternative hypotheses using 5-year tree census data from 88 plots in 44 seasonal tropical moist secondary forests (3-32 years old) across a human-modified landscape in central Panama. We examined whether N-2 fixers accumulated biomass more rapidly than non-fixers, and how relative biomass of N-2 fixers as a functional group and as individual species influenced the growth of non-fixer and whole stand primary productivity. 4. Surprisingly, we found no evidence for either a net competitive or a facilitative effect of N-2 fixers as a functional group or individual species on the biomass recovery in these young forests. N-2 fixers did not grow faster than non-fixers. Individual mortality rates were lower among N-2 fixers, but biomass losses due to mortality were similar between the two groups. Overall, we found no relationship between the relative abundance of N-2 fixers and stand primary productivity -during succession. 5. Synthesis. Nitrogen-fixing trees may be critical for reducing nitrogen limitation and accelerating biomass growth during tropical secondary forest succession, thereby impacting the global carbon cycle. However, our findings indicate that, in early successional seasonal tropical moist forests, nitrogen fixers provide neither a net competitive nor a facilitative effect on non-fixing trees or the whole forest stand, likely because tropical nitrogen fixers utilize facultative fixation and hence abundance poorly approximates the ecosystem function of fixation. Our results indicate that models should not simply scale symbiotic fixation and its effects from nitrogen-fixing tree abundance.