Mixed-mesophytic hardwood tree species are replacing upland oaks in vast areas of the eastern US deciduous forest.  Some researchers have suggested that the leaf litter of mixed-mesophytic, oak replacement species renders forests less flammable where forest managers wish to restore a natural fire regime.  We examined fire behavior in sites containing three distinct species assemblages in northern Pennsylvania and performed chemical analyses on dried leaf litter from select oak and oak replacement tree species.  Sites dominated by maples and other mixed-mesophytic species burned cooler and for shorter duration than those dominated by oaks.  Laboratory analyses revealed that the litter of oak replacement species was generally lower in calorific value and higher in mineral ash content than that of oaks.  These results support a feedback theory that the flammability of oak litter favors the perpetuation of oaks over fire-sensitive species.  Incorporating this information into fuel and fire behavior models will assist forest managers in planning prescribed burning operations in areas where mixed mesophytic hardwood tree species are replacing oaks.