- by Tom Worthley, UCONN Cooperative Extension
I know a fellow in Eastern Connecticut, a forest landowner like many of you. He has been told by a professional CT-Certified Forester that the mixed-oak stand on his property is “even-aged”. Red, white, scarlet and black oaks (among other incidental species) are all about the same age – ninety to one hundred years - and likely grew from sprouts following cutting for charcoal in the early 1900s.
“Pretty typical,” you say, because I’ve just described any number of stands that occur in any number of woodlots at any number of locations around the state.
But here’s the puzzler. Without knowing anything more about it, I’m prepared to go out on a limb (pun intended) and state with complete confidence that our friend’s oak stand has some old oak trees, young oak trees and middle aged oak trees in it.
“Now, how can that be,” you may ask, “if all the trees are the same age?”
Have you noticed, when walking in the woods, in even aged stands (80-100 years) where oaks are predominant, that some of the oaks appear less vigorous than others? Have you observed, especially in unmanaged stands like this, where no thinning has been performed and conditions are crowded, more than just the occasional standing-dead oak tree…and wondered why?
Part of the answer can be found in the same reason that an even-aged stand can have old trees, young trees and middle aged trees…
Size (diameter) has little if anything to do with it, of course. We can find very large and very small trees in the same stand that are the same age. Stand density and free-to-grow status determine the rate of diameter growth for a tree, not age necessarily. And besides, our friend has big trees and small trees and knows for a fact that they are all the same age. So what’s this about old and young?
He is apt to be even more confused when I tell him that, of his all-the-same-age large and small oak trees, some of the big ones are young trees and some of the small ones are old trees!
Had enough of this deceptive word-play? The answer lies in a characteristic of trees (or any other living organism) that I know as relative longevity. Different species of plants and animals have different natural life spans…some just naturally live longer than others…and this fact is true for trees as well. Nature, in its wisdom, granted the fast-growing shade-intolerant pioneer species a short life span, and those patient, slow growing species that have to survive in the shade of other trees were granted a longer natural life span. This adaptation permits the slower-growing, shade-tolerant species an opportunity to assume a dominant position in the canopy once the pioneer species have passed on. Many of us understand this as part of the process of succession.
In our friend’s oak stand a variety of oak species exist, and among them, at the age of 90 to 100 years, some of them are approaching the end of their natural life span and thus are “old” trees. The black and scarlet oaks may enjoy a life span of 120 years if conditions are good. Less in stressed or crowded stands, and perhaps a bit more if they have a dominant position in the canopy. Red oak at this age is right at the middle of its natural life span, and on a suitable site, with freedom to grow and barring a catastrophic disturbance, could continue to grow to 200 years of age or more. The young trees in our friend’s stand are the white oaks, which can live to be 400 years of age or more under the proper conditions, so from a relative longevity standpoint, these trees are just getting started.
Perhaps we might consider this to be Nature’s way of thinning…or selection harvesting…that trees with shorter life spans eventually drop out of the stand naturally. The growing spaces created then are utilized by longer-lived dominant trees or occupied by more shade-tolerant species (sugar maple, beech, hemlock) that eventually assume a more dominant position in the canopy.
The implications for management of our own forests are apparent. When faced with a decision to be made about thinning or harvesting, (and acknowledging that what is left behind to continue to grow is just as important as what is chosen for cutting) a strategy that mimics the natural process could be most beneficial. Many factors are considered when deciding whether to harvest or remove a tree from a stand, including tree health, form, condition, and proximity to other trees to name a few. Whether a tree is nearing the end of its natural life span may also be an important factor to consider.
If we were to plan a hypothetical commercial thinning operation to our hypothetical even-aged oak stand, its fair to assume that providing more space to trees that are already “old”, relatively, and naturally “slowing down”, may not produce the growth response we would like to see. Productivity, from a percent-volume standpoint, tends to drop off after a tree passes the mid-point of its life span, and approaches the end. In sawtimber stands, re-allocating or redistributing the growing potential of the site to trees that still have some growing time left, in this case the red and white oaks, will give us greater overall growth response for the effort. It may be tempting to cash in today on that 16-20 inch red oak, because we can get a better price for it than the black oak, but the real long-term value of our timber asset would be harmed. The unmanaged forest may be adding volume at a rate of 1-3% per year – about the same as bank deposits these days – and this growth is distributed among the high-value and low value trees. Research has repeatedly shown that dramatic increases in growth rate can be anticipated when healthy trees at the prime growing stages of their life are given more space. Released from competition, red oaks at this age on a good site are prime candidates for dramatic growth rate increases. Add to that the fact that board foot volume (thus value) increases with the square of the diameter, and that log grade (and value again) increases with size and you have a win-win-win long term investment strategy that can compete well with even the better investment vehicles on the market today! Since steady diameter growth produces volume exponentially, then the larger your trees grow, the more volume they add each year. An inch of diameter growth added over four or five years to a 20-inch tree is a heck of a lot more volume, and more valuable volume, than the same inch added to a 14-inch tree.
So it makes good long-term sense to favor those higher value sawtimber-sized trees that have some good growing time left, that will respond well to more space, and choose the ones to cut that are near the end of their natural life span when planning a commercial thinning operation. All other things being equal, I’ll choose to harvest the black oaks and hold onto the red and white oaks for awhile in my woodlot. It may mean less cash in my pocket today, but higher return in my “timber savings account” in the long run!
If some of these growth rate/geometric relationship/ financial analysis concepts are hard to grasp, then ask your CT Certified Forester. They should be able to help you with the idea of “economic maturity” of timber.
This article originally appeared in the December 2002 ECFLA/WDLT Newsletter.
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