Age Structure And Genetic Diversity Of Four Quercus Macrocarpa (Michx.) Populations In Fragmented Oak Savanna Along The Central Minnesota River Valley

Only 0.02% of historic oak savanna exists today. To provide management recommendations, we characterized age-class structure, measured cover of competing tree species and estimated genetic variation within and among four Quercus macrocarpa Michx (bur oak) populations in the Minnesota River Valley. At each site, few middle-aged bur oaks existed, most were 90-140 y. Quercus macrocarpa had the highest importance value (IV) in the older tree species cohort, but never the highest IV among younger trees suggesting that oak recruitment from 1910-1970 was very low. The four Q. macrocarpa populations and both age cohorts possessed moderate genetic diversity. Genetic distances between pairs of bur oak populations were small, but younger cohorts were more genetically differentiated than older cohorts suggesting that gene flow has declined in the past 100 y. Practices that reintroduce fire, reduce grazing and competition with mesic species, and restoration that introduces acorns from nearby populations have the potential to enhance both oak regeneration and genetic diversity.