Oak savanna is the most threatened ecosystem in Minnesota and fire, alone, is not restoring and preserving it. Our savanna restoration research started more than a half century ago in what had once been native savanna at Cedar Creek. It has shown that burning about 4 to 7 times per decade can help eliminate shrubs and non-savanna tree species and restore some prairie grassland species. However, our 50 years of research is also showing that these frequent and intense fires are preventing oaks from regenerating. Bison are now known to be a keystone species for restoring and preserving grasslands, but their roles in savanna ecosystems remain unknown. In grasslands, bison preferentially graze the dominant warm season grasses that would otherwise outcompete wildflowers, thereby promoting plant coexistence and enhancing plant diversity. Here we test whether bison grazing might promote the growth and survivorship of oak seedlings in burned savannas by reducing grass fuel for fires and by knocking back dominant grass competitors.
Bison graze during the summer and early fall seasons in a large enclosure (85 ha) that includes seven burn units that have historically received prescribed burning at frequencies ranging from nearly every year. Grazing exclosures were established, and oak seedlings were planted, to test effects of bison grazing on early oak growth and survivorship. With this project, we seek to improve restoration and management of oak savannas. In addition to considering how bison affect oak tree regeneration and plant diversity, we are also testing their effects on plant productivity aboveground and belowground.
Experiment Design
Within and adjacent to the bison enclosure we spatially randomized the locations of 42 plots, each 7 by 7 m, including 28 plots in 14 pairs inside the bison enclosure. Within the bison enclosure, one plot in each pair is surrounded by a livestock panel to exclude bison while the other plot is open to allow grazing by bison. In total, the bison enclosure includes seven burn units. The burning treatments in these savannas at Cedar Creek began in 1964 and treatments range from nearly annual burning to no burning, creating a gradient from open canopy grassland and savanna to closed canopy forest. There are two plot pairs in each of the seven burn units, with the locations of each plot pair randomized within the burn unit.
This project is funded by by Minnesota's Environment and Natural Resources Trust Fund