Phylogenetic Constraints Do Not Explain the Rarity of Nitrogen-Fixing Trees in Late-Successional Temperate ForestsReport as inadecuate




Phylogenetic Constraints Do Not Explain the Rarity of Nitrogen-Fixing Trees in Late-Successional Temperate Forests - Download this document for free, or read online. Document in PDF available to download.

Background

Symbiotic nitrogen N-fixing trees are rare in late-successional temperate forests, even though these forests are often N limited. Two hypotheses could explain this paradox. The ‘phylogenetic constraints hypothesis’ states that no late-successional tree taxa in temperate forests belong to clades that are predisposed to N fixation. Conversely, the ‘selective constraints hypothesis’ states that such taxa are present, but N-fixing symbioses would lower their fitness. Here we test the phylogenetic constraints hypothesis.

Methodology-Principal Findings

Using U.S. forest inventory data, we derived successional indices related to shade tolerance and stand age for N-fixing trees, non-fixing trees in the ‘potentially N-fixing clade’ smallest angiosperm clade that includes all N fixers, and non-fixing trees outside this clade. We then used phylogenetically independent contrasts PICs to test for associations between these successional indices and N fixation. Four results stand out from our analysis of U.S. trees. First, N fixers are less shade-tolerant than non-fixers both inside and outside of the potentially N-fixing clade. Second, N fixers tend to occur in younger stands in a given geographical region than non-fixers both inside and outside of the potentially N-fixing clade. Third, the potentially N-fixing clade contains numerous late-successional non-fixers. Fourth, although the N fixation trait is evolutionarily conserved, the successional traits are relatively labile.

Conclusions-Significance

These results suggest that selective constraints, not phylogenetic constraints, explain the rarity of late-successional N-fixing trees in temperate forests. Because N-fixing trees could overcome N limitation to net primary production if they were abundant, this study helps to understand the maintenance of N limitation in temperate forests, and therefore the capacity of this biome to sequester carbon.



Author: Duncan N. L. Menge , Jeanne L. DeNoyer, Jeremy W. Lichstein

Source: http://plos.srce.hr/



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