Timber harvest provides an essential set of raw materials to society. Tree selection, timing, practices, and purpose behind harvesting are critical elements in forest sustainability. But, when is a forest ready for harvest?
Throughout history, the purpose of a timber harvest had focused almost exclusively on the manufacture of primary wood products such as sawlogs and pulpwood. Historically, if enough money could be earned to justify the harvest costs, then a forest was ready. However, over the past few decades, forest science and experience has taught us a lot about how to manage forests, not only for a sustained timber yield but also to optimize revenue over time and better provide for a wide range of environmental services.
Forest products aside, timber cutting has various ecological functions. Silvicultural terminology recognizes these functions by naming practices after them. Tree cutting designed to improve the quality of the stand is often called an “improvement cut” or “timber stand improvement.” Tree cutting intended to encourage regeneration is called a “regeneration cut”. All of these different kinds of cuts are harvests. There must be enough monetary value to interest a contractor. If that value isn’t there, then there is no practical way to get the job done.
Deciding when to harvest a stand often requires forestry expertise to not only administer a well-done harvest but to also produce the environmental services and desired conditions for the forest owner.
So, when is a forest ready for harvest? Well, it depends on upon the tree species and forest type. But measures such as age, stand density, tree health, and ownership needs are used regularly.
For stands where most of the trees are the same age, then age is often the main criterion. Forest types such as aspen, jack pine, and spruce-fir naturally occur as even-aged stands that were naturally regenerated through some sort of catastrophic disturbance, such as wildfire, major wind events, and cyclical attacks from native insects (e.g. spruce budworm). Once these stands reach a certain age, the risk of such catastrophic disturbance increases and the onset of wood-rotting fungi grows quickly. Harvesting these stands at a prescribed age, prior to natural catastrophe, captures healthy wood and regenerates the stand.
In the case of aspen, where age is the key factor, site conditions and tree health can modify the stand “readiness”. Around 50 years of age, or so, the trees begin to decline. That time period will be shorter on poorer sites, and longer if bigtooth aspen dominates, rather than quaking aspen. A clear-cut is used to regenerate the stand because aspen species are particularly sensitive to shade.
Clearcutting these stands at a certain age is called a “regeneration cut”. This is not merely a euphemism to disguise the practice of silviculturally-based clearcutting. It’s the ecological purpose of the harvest.
Red pine is managed differently. Trees can remain healthy and vigorous for well over a century. However, a red pine stand requires periodic thinning to maintain good growth and health. A stand is ready for a thinning (harvest) when stand density reaches certain thresholds or when the crown-to-height ratio drops to a certain point. Failure to thin on a regular basis leads to stand decline. Once crown ratios drop below 10-15 percent, achieving good growth is no longer possible.
Northern hardwood forest types (sugar maple, red maple, basswood, beech, hemlock, yellow birch, white pine) have yet another set of “readiness” measures. On better sites, a common management goal is to create a stand with a mix of age and size classes that reflect the potential of that particular stand. Management tries to build a size class structure.
“Readiness” is measured using stand density, tree species composition, and tree health. No two stands are identical. Seldom do two forest owners have the exact same idea of the desired future condition. Rarely do two foresters agree on all the specifics of which trees to keep and which trees to cut.
In these northern hardwood stands, various types of thinnings occur, always with an eye to the future. Many of the thinning’s are designed to “improve” the forest in ways desired by the forest owner. One owner may want to maximize the dollar value of high-quality sugar maple trees. Another owner may want to manage for a sugarbush. Yet another owner may wish to achieve more diversity among tree species and understory plants. When done properly, these “improvement cuts” will, indeed, deliver the intended outcomes. All of them require the cutting of trees.
At one point in northern hardwood management, a harvest will be needed to encourage regeneration. Like a clear-cut, this harvest is also termed a “regeneration” cut, as opposed to the “improvement cuts” previously employed. Different light conditions (stand density) and ground disturbance will favor various species in different ways, determining the future composition of the stand.
There are many permutations of several “standard” management systems. The application will vary with forest composition, site conditions, markets, owner goals, and many other factors.
Alternatively, any forest may be “ready” for harvest for reasons other than ecological measures such as age or stand density. Threats by certain insects and diseases call for “sanitation” cuts to remove those tree species before they are killed. Currently, common examples include the emerald ash borer (exotic), oak wilt (exotic), beech bark disease (exotic), jack pine budworm (native), and spruce budworm (native).
Wildfires and large blowdowns create an opportunity for “salvage” cuts, which are attempts to harvest recently killed trees before insects and fungi render the wood unmarketable. The dead and dying trees need to be harvested as soon as possible. It does not take long for insects and decay fungi to begin degrading the wood during the summer months. Much of the jack pine from was salvaged after the 21,000 acre 2012 Duck Lake fire in the Upper Peninsula.
There are a number of monkey wrenches that can be thrown into well-designed plans. Droughts, poorly-timed wet periods, late frosts, and exotic species are common examples. Deer browsing, however, is by far the greatest threat to forest regeneration, trees and other plants as well.
Of course, timber harvest can be done without regard to ecological impacts or future condition, and many times this is what a forest owner demands. When forest land is converted to non-forest land uses, foresters sometimes refer to the removal of trees as a “terminal harvest”. Land sales commonly involve over-harvesting to capture the dollar value of trees prior to the sale, leaving a damaged forest for the new owner. Some forest owners will turn to timber harvest during times of financial need to pay for things such as nursing home care or college expenses. Such needs may or may not be consistent with accepted forestry practices.
So, when is a forest ready for harvest? Well, that depends on a wide variety of circumstances and conditions. Sometimes the answer is fairly simple. Other times, the answer can be complex. In any case, Michigan State Univeristy Extension recommends hiring a forester to get the job done right. Assuming, that is, a forest owner is interested in sustainable forestry.