A meta-analysis of natural selection on plant functional traits

A common assumption in plant physiological ecology is that variation in functional traits reflects the adaptation of organisms to their abiotic environment. This assumption can be tested by estimating natural selection as the relationship between a functional trait and a fitness component (i.e. survival or reproduction) within a population. To understand how natural selection operates on plant functional traits, we compiled directional selection gradients (β), which estimate direct selection, and differentials (S), which estimate both direct and indirect selection, from studies conducted in manipulated and unmanipulated environments. We found that relative to manipulating biotic factors, manipulating abiotic factors had a ~5.7´ larger effect on β and a ~16´ larger effect on S, suggesting that functional traits primarily evolve in response to the abiotic environment. We found that the strength of selection on functional traits (β) did not vary with trait type, performance/fitness component, or measurement context (i.e. common garden vs. natural population). However, the direction of selection did differ between some trait types: β was positive for plant size traits, but negative for phenology traits. Lastly, we found that the absolute value of selection differentials (S) was ~2´ larger than the absolute value of selection gradients (β), indicating that there was indirect selection on functional traits. Overall, our meta-analysis illustrates that natural selection on plant functional traits is common, and that estimates of selection on these traits in experimentally manipulated environments can be used effectively to test hypotheses about the causes of adaptation.