Phenotypic flexibility and genetic adaptation along a gradient of secondary forest succession in the grass Danthonia spicata

Populations of Danthonia spicata were studied from sites of ages 0, 26, 32, 44, and 69 years after fire in the aspen–pine forests of northern lower Michigan. Along this gradient the environment changes from unshaded and dry to a shaded, moist pine and hardwood forest. Greenhouse treatments and transplant gardens were used to investigate the extent to which phenotypic flexibility and genetic adaptation were responsible for the persistence of D. spicata across this light and soil moisture gradient. With regard to phenotypic flexibility, we found that individual plants of D. spicata can grow and reproduce in light levels lower than those found at any site in the field. The populations were genetically distinct from each other but the differences were small relative to the range of phenotypic flexibility. Some populations and individuals performed better under high light conditions and all individuals performed equally poorly under low light conditions. A multivariate analysis suggests that genetic drift may have been more important than selection in differentiating the populations. Phenotypic flexibility was shown to be more important than genetic adaptation in explaining the persistence of D. spicata along the successional gradient.