Much of the oak savanna that existed across North America has been lost due to conversion to agriculture and fire suppression. Restoration efforts to repair oak savanna structure and composition have been partially successful, however, recruitment of oak trees is often limited. Without enough new recruits, savannas will eventually lose their oaks and turn into open grasslands.
While fire was historically a natural part of oak savannas, and necessary for maintaining their open structure, intense fires often kill young oak seedlings. Adult oak trees can survive fires, but young trees are too small to escape them. Fires also promote dominant grasses in oak savanna, which can in turn intensify fires.
Historically, large herbivores, such as bison, were present in many savanna ecosystems but have been locally extirpated. Bison primarily consume dominant grasses in these systems, which may benefit young oaks in multiple ways: e.g., by reducing the competitive effects of dominant grasses on young oaks, and by decreasing fire intensity.
In the spring of 2018, we released a herd of bison at Cedar Creek Ecosystem Science Reserve to graze an 85-hectare oak savanna for the growing season. This oak savanna has also undergone a fire frequency manipulation experiment since 1964, with a gradient of fire frequencies ranging from unburned to nearly annual burning. We planted 600 oak seedlings into plots across the savanna in areas grazed or ungrazed by bison. Here we evaluate how fire and grazing by bison interact to determine the survival and growth of these planted oak seedlings.
Results/Conclusions
In areas of the savanna dominated by grasses, bison grazing reduced fire temperature by 125◦C on average (p=0.003). In contrast, bison grazing had no effect on fire temperature in areas dominated by shrubs. Oak seedling survival was higher in grazed plots than ungrazed plots (p=0.005). Of the trees that were top-killed, 60% re-sprouted after the fire. However, fire temperature did not differ between plots that had more or less re-sprouting seedlings(p=0.18). Re-sprouted seedlings had larger root collar diameters (p=0.02) in grazed plots than ungrazed plots but did not grow taller (p=0.5). These results suggest that bison grazing might help maintain oak savannas by allowing young oak seedlings to survive prescribed fires, at least in grass dominated areas. Future research will continue to monitor whether increased seedling survival and growth translates into more adult oak trees.