COS 27-9 - Nitrogen-fixing trees have stronger negative interactions with neighbors than do non-fixers in a tropical secondary forest

Tuesday, August 9, 2016: 4:20 PM
305, Ft Lauderdale Convention Center
Benton Taylor, Organismic and Evolutionary Biology, Harvard University, Cambridge, MA, Robin L. Chazdon, Ecology and Evolutionary Biology, University of Connecticut, Storrs, CT, Benedicte Bachelot, Department of Biology, Duke University, Durham, NC and Duncan Menge, Ecology, Evolution, and Environmental Biology, Columbia University, New York, NY
Background/Question/Methods

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.