Phenotypic Plasticity and Genetic Variation of Vaccinium macrocarpon, the American Cranberry. I. Reaction Norms of Clones from Central and Marginal Populations in a Common Garden

Vaccinium macrocarpon Aiton (Ericaceae) cranberry, a dwarf shrub and a typical taxon in temperate peat bogs, has its central distribution in cool temperate regions in eastern North America. Isolated southern marginal populations are distributed along the Appalachian corridor and on the North Carolina coastal plain. A common garden (Blacksburg, Virginia) was utilized to determine whether marginal cranberry clones exhibit greater phenotypic plasticity than central clones. Three central clones from Massachusetts (MA), Wisconsin (WI), and New York (NY) and three marginal clones from North Carolina (NC), Tennessee (TN), and West Virginia (WV) were tested. A suite of phenotypic traits was measured in response to edaphic variation in the common garden. An analysis of reaction norms took the form of an analysis of covariance to test for significant differences among clones and to estimate regression slopes (plasticity) when compared with environmental (nutrient) variation. There was no regional variation in phenotypic plasticity, but there was significant clonal differentiation for 77% of nonintercorrelated traits. However, in most cases the differences were seemingly random, with little biological importance. Hence little differentiation in relation to population origin was observed among clones. Matrix comparisons were performed using a Mantel test to check for pairwise correlations among the following matrices: geographic distances, trait means, plasticity, and molecular variation assessed by random amplified polymorphic DNA (RAPD) profiling. No correspondence was found among matrices. The recent postglacial distribution of cranberry may account for the absence of phenotypic and genetic heterogeneity.