Chestnut oak forests at the tops of West Meade hills contain some of the oldest trees in Davidson County, if not the oldest, with trees well over 250 years old. If we place value on conserving native Tennessee ecosystems, it is worthwhile to identify intact forest communities. Young forests that have been highly impacted by humans in the past tend not to support sensitive species such as ginseng and many salamanders, but instead often contain many non-native species.
Disturbances that wipe out stands of trees are often a natural part of forest cycles. Such events include fires, wind storms, and flooding. Where humans have settled, which is just about everywhere, we have added intensified disturbance regimes through logging and agriculture. Once a disturbance event has ended, forests will begin to regenerate following somewhat predictable stages of succession. The first generation of trees invading an open field tends to be dominated by short-lived, fast-growing, shade-intolerant species with small wind-dispersed seeds. Slow-growing, shade-tolerant species that have large, animal-dispersed seeds tend to be later-successional species.
On the rich, moist lower slopes of the hills of West Meade, you will often see stands of big tuliptrees, around 150 years old, with an understory of sugar maples and beeches. Tuliptrees have all the characteristics of early-successional species, while sugar maple and beech tend to be late successional species. When we see those forests, it suggests that 150 years ago, the land was deforested, and the tuliptrees invaded the field. Slowly, the sugar maples and beeches are growing up beneath the tuliptrees and will eventually replace them in the canopy. Sugar maple would be a dominant member of the "climax community" in these areas, as the species will continue to grow under its own canopy, replacing itself with each successive generation. On the dry hilltops, chestnut oak is likely the dominant climax canopy species, often mixed with a permanent understory of sassafrass waiting for a gap of light.
The type of disturbance event can have a big impact on how succession proceeds. Starting from an agricultural field, it could take 600 years or more to go through all of the stages of succession in some of our richer sites. But, if you cut down the chestnut oaks on the hilltop and then leave the forest alone, many of the trees will re-sprout from the roots, and the forest composition might return to its original state fairly quickly. Stump-sprouted trees can often be recognized because they may have multiple trunks coming from a single base, as opposed to seed-grown trees which typically only have one main trunk.
In old forests, trees will reach their natural life span and fall down. When a tree falls, it leaves a gap in the canopy for younger trees to fill in. As it falls, the root ball typically digs up a big scoop of earth, creating a mound of dirt next to a pit. The decomposing log on the forest floor provides nutrients and habitat for other plants and animals.
This suggests four more features to look for in old growth forests: gaps in the canopy, multiple-aged trees, pit-and-mound topography, and coarse woody debris. So, some might consider a forest "old growth" if it has many of those functional features, even if there is a history of logging in the forests' past.
Of course, one other feature of old growth forests is simply the age of the trees. The size of a tree often does not correspond directly with the tree age. Other features, such as knobby branches and characteristic bark patterning tend to appear in trees over 200 years old. While early successional species often live less than 200 years, late-successional species in our area can live over 500 years.
So, what do you consider an old growth forest? Since disturbances happen naturally, why does it matter whether a forest was disturbed in the past? Some of our forests clearly show evidence of past logging, but also have late successional trees over 100 years old and structural complexity. How do you value such a forest, compared to one with no history of logging, or one dominated by young early-successional trees?