Big trees are usually older than smaller trees. Although there is some logic to this statement, the premise is not necessarily true.
Most hardwood trees regenerate from a disturbance creating even-aged stands. These disturbances could be acts of nature such as tornados, windstorms, fires, or catastrophic pest outbreaks or human-caused disturbances such as timber harvesting. Like most agronomic crops, most trees grow best in full sunlight.
Different species of trees grow at different rates. Fast growers differentiate (spread their crown) above the slower growers, occupying growing space and most of the sunlight shading the slower growers. The shaded trees persist in the lower canopy. Crowns are sparse and growth is minimal. These trees are not vigorous and many succumb. They are the same age as the faster-growing trees having regenerated at the same time, but the overstory inhibits them.
Popular sentiment is that the small trees in the lower canopy when released will become the large trees of tomorrow. This assumption has been perpetuated in the diameter-limit harvests that have led to what we call high-grading today. The largest and the best trees are harvested leaving the smaller, inferior trees to perpetuate the next stand. In reality, the trees being released are the same age as those being cut. The small, released trees did not have a chance to prosper in competition with the faster-growing, overstory trees. These released trees are incapable of continued growth with their small spindly crowns.
Research on overtopped white oak trees on the Cumberland Plateau illustrates this point. Thirty-year growth data after a harvest of the largest trees shows that the remaining lower canopy trees were not an asset worth taking into the future. Height, diameter, and volume growth was minimal. A cycle of repeated high-grade harvests resulted in the development of a low-quality forest.
With an increment borer or by simply cutting a few trees and counting the rings, the ages of these smaller trees can be determined. If the trees are truly younger, a previous harvest or disturbance created wide enough openings to allow these trees to regenerate and grow without being totally inhibited by the larger, adjacent trees. The crowns of these younger trees should be thrifty (not spindly) with a string terminal leader and a balanced crown. These trees are capable of responding to a release treatment. You will need 40 to 60 well-distributed, younger trees per acre to have enough crop trees in the next stand. If you have these conditions, the overstory should be harvested, releasing these younger trees, taking care not to damage these smaller trees during the harvest operation.
However, most small diameter trees are as old as or older than the overstory trees. The small trees are the same age as the big trees or if enough "younger" trees are not present, then the stand should be regenerated. The regeneration sources will be from seed, sprouts, and advance reproduction (small seedlings or saplings already present). Good visual indicators that smaller trees are older and will not respond to release include flat-topped crowns (without a terminal leader), lopsided, spindly, and small crowns. However, cutting these trees will provide a strong stump sprout that will probably be a future crop tree. Sprouts, with their larger root systems, will usually have a growth advantage over seed or planted seedlings that must develop a root system before accelerated growth can occur.
The economic ramification of leaving smaller diameter trees with little capability of growing once released is substantial. Growth could be accumulating on a tree that is actively growing from a sprout (see cross-sectional disk photos). An older stem that is not responding to release and is usually of poor form is occupying growing space and is not adding value to the stand.
The belief that larger trees should be cut and that the smaller trees will be released to become future crop trees can be a serious error.
The lower canopy, smaller trees are usually just as old as the larger, overstory trees, they just did not have a chance to grow and prosper. Their weakened condition and advanced age are liabilities for the future.
Two white oaks trees of similar diameters from adjacent sites. The 60-year old tree grew in the shade of larger, overstory trees. The tree is old, and slow-growing that did not have an opportunity to grow in full sunlight. If left behind after a harvest, it will block the sunlight and slow down the growth of the next generation. The 11 year-old tree is a stump sprout from a clearcut. This tree was never shaded by larger trees and has already reached the diameter of the older tree.