The changing role of fire in mediating the relationships among oaks, grasslands, mesic temperate forests, and boreal forests in the Lake States

Historically, oak forests and woodlands intergraded with southern boreal forest, temperate mesic forest, and grassland biomes, forming complex fire-mediated relationships in the Great Lakes region of Minnesota, Wisconsin, and Michigan, USA. Variability in fire recurrence intervals allowed oaks to mix with grasses or with mesic forest species in areas with high (2–10 yr) or moderate (several decades) fire frequencies, respectively. In the southern boreal forest, oak colonization was limited by cold climate. In recent decades former savannas have been largely converted to agricultural fields and the fate of oak remnants is controlled by human fire use. In mesic temperate forests, fire exclusion, wetter climate, and deer browsing have led to mesophication and increasing maple dominance. With ongoing warming, however, mesophication could reverse due to increased drought and fire frequency, and earthworm invasion, which enhances the understory environment for oak seedlings. Oaks are also likely to invade large tracts of southern boreal forest. However, deer grazing on oak seedlings will partially negate the positive influence of warming and fire. On balance, oaks have a more positive future outlook in the Lake States, as the climate becomes more favorable to oaks compared to temperate mesic and boreal forests.