Oak-hickory woodlands and fire

Walking around Big Oaks National Wildlife Refuge, you might notice several distinct habitats: open grasslands, marshes, lakes, creeks, shrubby areas filled with new saplings, as well as several types of forest or woodland. Forest is often technically defined as a stand of trees with greater than 60% canopy cover, and it predominates here at Big Oaks. Woodland has between 20 and 60% canopy cover, allowing more light to penetrate to the ground, often creating a rich herbaceous layer of vegetation on the ground, in contrast to the leaf litter sparsely punctured with green in most forests, especially after the season of spring ephemerals, driven by the light available temporarily before the canopy greens up.

You may notice certain patterns in the distribution of trees here, such as the growth of various oak and hickory species on south-facing slopes and high ridges, whereas beech and maple predominate on north-facing slopes and in ravines. Though the various genera often co-occur, this preference in distribution speaks to the mesophytic habits of beech and maple, which require moderate moisture in the soil continuously, as compared with xerophytic oaks and hickories, which can tolerate drought and dry soils better. Additionally, oaks and hickories, with their thick bark, are able to withstand fires as saplings much better, but they require more light to survive, while maples and especially beech can grow up in dense shade.

These contrasting characteristics not only affect the distribution of trees on a local scale as we have discussed- with oaks and hickories preferring high and dry sites- but also affect their geographic distribution in Indiana. Beech-maple forest predominates (or did predominate) on the deep, poorly drained, clay heavy soils of glacial till in central Indiana, whereas oak-hickory forests are more common in the rocky and hilly unglaciated Shawnee Hills and Mitchell Karst Plain in the south-central region of the state. Nevertheless, all these habitat types often occur close by each other.

Another trend in the distribution of these forest types is the gradual shift toward beech-maple forest and lack of oak and hickory regeneration across most of eastern North America. The causes are thought to be largely related to decades of fire suppression. Fire was a hugely important factor in maintaining oak-hickory forests and woodlands, due to its ability to kill saplings of competing species like red and sugar maple. This would prevent maples and beeches from growing up in the shade and eventually taking over, while oaks and hickories could regenerate where periodic gaps in the canopy occurred.

Fire suppression has allowed red maple, once relegated mostly to swamps, to become the most common tree species in the eastern U.S. It is thought that historic fire in our region was mostly the deliberate practice of Native Americans, who used it as a tool to maintain open oak-hickory woodlands which made travel easier and improved conditions for wild game, especially wild turkey. Additionally, the period of European colonization occurred within the Little Ice Age (ca. 1450-1850), which brought drier conditions to North America and perhaps increased fire frequency.

Although beech-maple forests are a natural component of the North American landscape and have been for a long time, their rapid increase in prevalence in many areas could have major effects on wildlife and the ecosystem. The canopies of maples create more shade than oaks, preventing herbaceous growth on the forest floor. The mast produced by oaks is an important food source for many animals, and the decline in oak numbers could put increased pressure on many species. As changes like these happen on a massive, continent-level scale, their effects could be correspondingly great.