Genetic relationships between three species of oaks as determined by common garden studies with populations from Michigan, Indiana and Wisconsin

Twelve populations of oaks of three species from Indiana, Michigan, and Wisconsin [USA] were tested under "common garden" in East Lansing, Michigan. The populations consisted of four Black oak (Quercus velutina), two of northern Scarlet oak (Quercus coccinea) formerly known as Hill oak (Quercus ellipsoidalis), and six populations of Red oak including both Quercus rubra and Quercus rubra v. borealis. Local genetic variation was found within eleven of the twelve populations. One population represented a hybrid swarm between Quercus rubra v. borealis and Quercus velutina. When species populations were compared using a Summation of Differences index, it was found that Black oak populations showed evidence of a genetic cline from Indiana to its northern range limits in Michigan. The two northern Scarlet oak populations from Wisconsin Rapids, Wisconsin and Grayling, Michigan were genetically very similar. A close relationship was found between the Grayling, Michigan Scarlet oak and the small acorn northern form of Black oak from nearby Indian River, Michigan. This was considered to be an example of introgression between these two species as also suggested by field studies in this part of Michigan.