Friday, August 10, 2007 - 10:10 AM

OOS 49-7: Adaptive nitrogen fixation strategies with realistic tradeoffs allow persistent N limitation and N richness

Duncan N. L. Menge, Simon Levin, and Lars Hedin. Princeton University

Nitrogen (N) fixation lies at the heart of two unresolved and counterintuitive ecosystem patterns: 1) N-fixing trees are absent from mature temperate forests, even though most of these forests are N-limited; and 2) Tropical forests, which contain many potentially N-fixing trees, are N-rich, suggesting there is more fixation than might be needed. To elucidate physiological and/or ecological mechanisms that could drive these patterns we developed a simple, resource-based evolutionary model of N fixation. We assumed that a plant can be limited by N or some other resource (or both) and that the plant can acquire N from the soil or from fixation, then we allowed the trait of N fixation to evolve. In this basic model there are two possible evolutionary outcomes (continuously stable strategies), neither of which can explain the counterintuitive ecosystem patterns: 1) a population of non-fixers that is limited by the non-N resource and 2) a population of fixers that fix just enough N to be co-limited by N and the other resource. However, introducing tradeoffs between N fixation and the rates of mortality or resource uptake allows new evolutionary outcomes (either evolutionarily stable or continuously stable strategies) that match the observed empirical patterns: populations of chronically N-limited non-fixers or chronically non-N-limited fixers. We show analytically that the magnitudes of the tradeoffs needed to produce these patterns are well within the plausible ranges for real plants and ecosystems.

See more of OOS 49 - Controls over nitrogen fixation: A global perspective
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