Nitrogen-fixing trees inhibit growth of regenerating Costa Rican rainforests

Significance Regrowing tropical forests are critical for global biodiversity conservation and carbon capture. Nitrogen availability often controls how fast these forests can regrow. Because nitrogen-fixing plants are the primary source of new nitrogen into these forests, one might expect that more nitrogen fixers lead to faster forest regrowth. However, here we show that nitrogen fixers actually slow forest regrowth. Their competitive influence on neighboring trees outweighs any growth enhancement from their nitrogen inputs at this site. Our results call for a more critical evaluation of how nitrogen fixers influence the surrounding forest, especially given the large uncertainty in global climate projections that hinges on the role of nitrogen fixers during tropical forest regeneration. More than half of the world’s tropical forests are currently recovering from human land use, and this regenerating biomass now represents the largest carbon (C)-capturing potential on Earth. How quickly these forests regenerate is now a central concern for both conservation and global climate-modeling efforts. Symbiotic nitrogen-fixing trees are thought to provide much of the nitrogen (N) required to fuel tropical secondary regrowth and therefore to drive the rate of forest regeneration, yet we have a poor understanding of how these N fixers influence the trees around them. Do they promote forest growth, as expected if the new N they fix facilitates neighboring trees? Or do they suppress growth, as expected if competitive inhibition of their neighbors is strong? Using 17 consecutive years of data from tropical rainforest plots in Costa Rica that range from 10 y since abandonment to old-growth forest, we assessed how N fixers influenced the growth of forest stands and the demographic rates of neighboring trees. Surprisingly, we found no evidence that N fixers facilitate biomass regeneration in these forests. At the hectare scale, plots with more N-fixing trees grew slower. At the individual scale, N fixers inhibited their neighbors even more strongly than did nonfixing trees. These results provide strong evidence that N-fixing trees do not always serve the facilitative role to neighboring trees during tropical forest regeneration that is expected given their N inputs into these systems.

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