Sustainable Tree Harvesting

Posted: December 15, 2016

Taking the time to understand forest development and how harvesting decisions can change a woodland for the next 100 years warrants careful consideration. Don’t fall prey to the idea that all you have to do is cut the big trees and the little ones will grow just as big and healthy.

Pennsylvania is blessed with beautiful and productive woodlands. If somehow we could go back 90 to 130 years, the forested hills would be bare, showing only the remnants of a forest – stumps and slash - as the state experienced the first exploitation of its wooded landscape. Then, trees were plentiful and the “mission” was to harvest them for myriad products – logs, fuel, chemicals, and charcoal. Big trees or little trees it did not matter; they all went to market.

The above history snippet sets the stage for understanding some rudiments of how forests develop. Following the near complete harvesting of Pennsylvania’s forests, trees quickly began to naturally reforest the naked hills – no one planted the forests we have today. The regenerating forests were not just one tree species; rather, they were a mix of oak, maple, pine, and hemlock, among many others, that in part reflected the species that had been there. Within each forest patch or woodlot, or across the larger forest, some species, which require more light, captured the prime growing space. Examples are black cherry, red oak, and yellow poplar. Those trees grew taller and larger in diameter more quickly than the red maple, sugar maple, beech, white oak, or hemlock. Within each species, some individuals grew faster than their neighbors, resulting in big trees and small trees of the same species.

Today, about 100 years after the last big harvest across the state, it is easy to think that bigger diameter trees are old and smaller diameter trees are young. However, if you followed the above short history, big trees are often just big and little trees are similarly just little and, most importantly, they are most likely of the same age. Simply, tree diameter is not indicative of age. At the risk of offense, think of friends or acquaintances who are the same age as you, are you the same diameter? Like all living things, trees need resources to grow – for them, it’s light, water, nutrients, and space. Unlike us, trees can’t move to get what they need. They are limited to those resources that they can attain in their spot, and they have to compete with other trees – some win and some lose and some just hang on hoping for the best – we can’t go by size alone to understand age and, by association, vigor.

Understanding how diameter varies within a forest by species is also fundamental to achieving sustainable forestry harvesting. If you have a woodlot, no doubt someone has approached you and asked if you would entertain a harvest. Or maybe you have gone looking for someone to cut trees for income. Without the understanding that a tree’s size is not strictly related to age, it would seem logical to cut the “big old trees” and leave the little young ones to grow larger. This type of harvest has many names and many of them sound right.

A select or selective harvest is one common name. What does it mean, though? What is the basis upon which trees are selected for harvest? Often, it is diameter – take the big ones and leave the little ones. This version of a select cut is also called a diameter-limit where all trees above some arbitrary diameter, commonly around 12 or 14 inches, which is the minimum size for a sawlog to make boards, are cut. Quite often low quality trees at or above the minimum diameter are often left. One final term used is high-grade, at least it sounds somewhat wrong. In this case trees are selected for removal based on their economic value, taking quality trees and leaving those with defects – take the best and leave the rest. In almost all cases, timber harvests implemented under these any of these descriptors are “wolves in sheep's clothing” or maybe more appropriately “the green lie.”

What defines forest health? Depending on a person’s level of understanding, forest health might simply be “green.” The select harvest finds favor in the green lie. The harvest is perceived as leaving plenty of green, the forest retains a leafy appearance, there are still lots of trees; there is the promise that those little trees will grow larger and the future holds the promise of another harvest, which it might. However, one thing is almost always certain, select cutting will result in slower growth, less tree species diversity, and less income in the long run.

The income point has several dimensions. First numerous studies have shown that the actual dollar value harvested from a forest subjected to select cutting will be less over time. Second, other values besides money will likely decline as well. For example, if oaks grow faster than maples before the first select harvest, fewer oaks will be there for the second harvest, and, likely, even fewer after the third harvest. Oaks, as most hunters know, are wonderful mast producers. Less mast means fewer wildlife that depend on these forest foods. The cycle may not always be apparent, but a careful observer will see change coming. The green lie is that little changes and the forest is still there. Looking to the future involves understanding the present. What has the select harvest done to the forest?

What is the alternative to perpetuating the green lie? Silviculture, which is analogous to agriculture, uses science to guide forest management through sustainable harvests. Our even-aged forests, those that started a 100 years ago and those that have been regenerated since then, offer many opportunities to improve the forest through careful thinning operations that remove competing trees and focus on retaining the best trees to stay in the woodlot based on species, quality, spacing, and objectives (e.g., economic, wildlife). Thinning operations focus on cutting the worst first.

Every entry into a woodlot should seek to retain tree species diversity for a healthy forest system and seed source, increase or at least retain average tree diameter (something hard to do when you just cut big trees), and improve average quality. Eventually most woodlots in Pennsylvania reach a point in their development where it is time to take the big trees; however, they have one more role and that is to ensure the next forest develops under them. Because a carefully managed forest has species diversity and over time light is let into the understory, these older big trees are there to ensure regeneration of a similar mixed species stand. If desirable, some of these big trees can be retained to add to the interesting tree size structure, which adds interest and leaves history for future generations to consider. No matter the type of harvest, it takes a skilled professional to understand age and species diversity, match the current forest with your goals as a woodland owner, and propose sound silvicultural practices that will improve your woods.

Some woodland owners chose select harvesting because they want to avoid clearcutting their property. Clearcutting seems to many people a harmful practice because the visual results are dramatic, but it is one way to regenerate tree species that require full sunlight as seedlings. It involves planning and assessing regeneration. Our forests often started with a clearcut; however, many things are different from the time when they started with a new crop of seedlings. We have different forest health issues, deer have changed understory conditions, and past management and insects and diseases have shifted species composition.

It takes nearly a century to grow a great crop of trees. Every decision affects forest values and quality. Taking the time to understand forest development and how harvesting decisions can change a woodland for the next 100 years warrants careful consideration. Don’t fall prey to the idea that all you have to do is cut the big trees and the little ones will grow just as big and healthy. It might work once or twice, but eventually future owners will have to deal with the consequences. Care well for your woodlands.

For more information, contact the Natural Resources Extension office and request Forest Stewardship Bulletin Number 7: Timber Harvesting – an Essential Management Tool.

Contact Information

James Finley, Ph.D.
  • Professor Emeritus of Forest Resources
Phone: 814-863-0402