More than half of the world’s tropical forests are currently in some stage of recovery from human land use, and this regenerating biomass now represents the largest carbon-capturing potential on Earth. Understanding what controls rates of regeneration and carbon sequestration in these forests is of central concern for both conservation and climate-modelling efforts. Current evidence suggests that symbiotic nitrogen-fixing plants play a critical role in providing the nitrogen (N) that fuels biomass accumulation during tropical forest regrowth. Our understanding of the ecological consequences of these N inputs via symbiotic N fixation, however, remains poor. We examined the competitive dynamics between N-fixing and non-fixing trees during tropical forest development to help connect our understanding of N inputs from symbiotic fixation and biomass accumulation. Within a series of 8 tropical forest plots spanning ages from 10 years since abandonment to old-growth forest, we calculated growth, mortality, recruitment, and neighborhood crowding for each tree over a 17-year study period. We analyzed the relative influence of both N fixers and non-fixers on growth at the plot level and on the demographics of neighboring trees. We also assessed the influence of competing neighbors on the demographics of focal N-fixing and non-fixing trees.
Results/Conclusions
At the forest stand level, we found no relationship between the prevalence of N-fixing plants and tree growth for any forests in our study. At the individual tree level, the growth and mortality of N fixers was more strongly affected than non-fixers by neighboring competitors early in succession. A larger proportion of neighbor competition coming from N fixers also had a negative effect on the success of trees – both N fixers and non-fixers. Together, these results provide strong evidence that N-fixing plants may not serve the facilitative role in tropical biomass regeneration that might be expected given their N inputs into these systems. Despite strong theoretical and empirical evidence that symbiotic N fixers are an important source of N to regenerating tropical forests, our study demonstrates that more work is needed to link these N inputs to biomass regeneration and carbon sequestration during tropical forest development.