COS 243-4 - Evolutionary history predicts nutrient strategy in tropical trees

Background/Question/Methods

Plant nutrient acquisition traits underlie important ecological processes and ecosystem functions. While plants can invest in a variety of traits to acquire nutrients, they are often lumped into functional groupings based on a single trait (such as the ability to fix nitrogen, N) without considering the potential effect of evolutionary history on trait expression. This is of special interest in tropical Fabaceae, which include N-fixers. These functional groupings (whether a species fixes N) are assumed to influence nutrient trait expression and are represented as such in earth system models. We are the first to explicitly test the effect of evolutionary history on nutrient traits across diverse clades and genera within Fabaceae. We also test the debated hypothesis that N-fixing species have more N to spend on root phosphatase enzymes. To do so, we grew 22 tropical Fabaceae species in a greenhouse for several months and measured key nutrient acquisition traits including N-fixation rate, root phosphatase activity, and mycorrhizal colonization, and aboveground traits like growth and carbon metabolism. We characterized trait relationships across species, measured phylogenetic signals in the traits, and controlled for phylogenetic conservatism in a model testing the effect of N-fixation rate on root phosphatase activity.

Results/Conclusions

Overall, we found that evolutionary history shapes nutrient acquisition traits in Fabaceae. There was evidence for phylogenetic conservatism in most nutrient acquisition traits. When accounting for evolutionary history, root phosphatase activity increased with N-fixation rate, providing strong evidence for a mechanistic relationship between these two traits. This relationship supports the popular assumption that species with more fixed N ‘trade’ it for root phosphatases to enhance phosphorus acquisition. We also found that non-fixing species tend to have less physiologically active roots compared to N-fixers, which highlights the importance of assessing these trait relationships in terms of both functional and phylogenetic differences. Our study is the first to show the effect of evolutionary history on these nutrient acquisition traits within a broad and diverse sampling of Fabaceae species. We stress the importance of considering the effect of evolutionary history in nutrient trait analyses in Fabaceae –especially when making assumptions about trait relationships that are represented in earth system models.