September 11, 2018, 1:00 PM central time
By: Dr. Lee Frelich, University of Minnesota Center for Forest Ecology, and Institute on the Environment
Abstract: The ability of fire-dependent oaks in the Midwest to persist on savannas or in mixed mesic forests with species like sugar maple depends on variability in timing of fires and physiography. Vegetation types from open grasslands to savannas, mixed oak forests and forests dominated by species like sugar maple can all occur on mesic sites. Both the average return time of fire and the variability in return time are important in structuring this grass-to-maple gradient, but the role of variability works differently in grasslands than in forests. On frequently burned grasslands, random variations in fire intervals can lead to longer than average intervals that allow oaks to become established. At the other end of the gradient, regular fires every few decades can keep maples from invading oak forests, and a longer than average interval of several decades between fires can allow maple to become established. Physiographic variability also interacts with fire, for example, on grasslands areas of exposed mineral soil, rocks or wetlands can create refuge locations for oaks that burn less often than most of the landscape. Restoring fire to mesophied forests formerly dominated by oaks, where fire has been excluded or suppressed for a long time, is also complex. A lot of fire--either higher intensity fires or a greater number of lower intensity fires--may be needed for sufficient cumulative effects to push the ecosystem to a state where oaks dominate. At the same time, a more humid understory created by maple, wetter climate in recent decades, and in some places European earthworm invasion, can make prescribed fires of sufficient intensity to be ecologically meaningful harder to carry out. On the other hand, removal of duff by the earthworms may create better seedbed conditions for oak in the absence of fire, and future increases in summer temperatures may make make forested sites drier, shifting the balance of competition from maple to oak, and making prescribed fires more effective at restoring oak.