Learn the Processes Guiding Forest Development

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Typically, evenly aged woods are dominated by one or two shade-intolerant species that sprouted as a group, leading to fairly consistent forest development.
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Woods with shade-tolerant species consist of trees of varying ages and sizes. Forest development in this type of woods is generally uneven.
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“A Landowner’s Guide to Managing Your Woods,” by Ann Larkin Hansen, Mike Severson and Dennis L. Waterman, provides the aspiring forest steward with knowledge about forest development, timber harvesting and other essential information.

A Landowner’s Guide to Managing Your Woods (Storey Publishing, 2011), by Ann Larkin Hansen, Mike Severson and Dennis L. Waterman, will help you become an active and effective steward of your forest. Beginning with an explanation of the natural processes governing forest development, the authors present active steps you can take to guide your woodland toward a state of health and beauty and sustainably produce one of the world’s greatest resources — wood. The following excerpt from Chapter 2, “How Forests Grow,” explains how trees in dense forests manage to thrive and how forest development differs between woodlands with shade-tolerant trees and those with shade-intolerant trees.

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Tree Strategy

In the plant world’s race to reach sunlight, trees won by developing wood. Tree trunks raise tree leaves higher than those of any other plant, and the leaves spread out to suck up most of the sunlight before it gets to plants underneath. The more tightly packed the leaves are in the tree canopy, the denser the shade and the fewer plants that can survive below the tree on the forest floor.

But this creates a problem for the trees themselves. If they’ve shaded out everything beneath their leaves, how will their offspring get enough light to sprout and grow, so that when the big trees get old and die (as they all eventually do), there will be young trees to take their place?

Trees have solved this dilemma in different ways. Some species of trees, such as sugar maple, American beech, and western hemlock, are very efficient with photosynthesis. Like an energy-saving appliance in your house, these shade-tolerant species are able to use much less energy than their peers to do the same job. Shade-tolerant trees can photosynthesize enough even in the shade of their parents to power slow growth for many years, in some cases decades; they begin growing rapidly when the death or removal of the overstory trees bathes them in full sunlight. But if they never get full sunlight, even the most tolerant of species will eventually be irreversibly stunted. Shade-tolerant tree species are generally slow growing but have long lives. The trees in a forest dominated by shade-tolerant species will often be of all ages, since they are continuously reproducing and growing and dying.

Other species are not quite as efficient. For example, black ash can get by for quite a few years in partial shade, but again, it will eventually need a burst of sunlight to power growth to adult size. If these semitolerant trees don’t get sun soon enough in their life span, they lose the ability to grow and are doomed to die slowly or live a stunted existence. Trees that look young by size but old by their wrinkled bark are the ones that didn’t get enough light in time and have lost their chance to amount to much. But if a big tree overhead falls, the saplings still young enough to sprint for the sun leap up to fill the gap in the canopy. These semitolerant species are often faster growers than shade-tolerant trees, and they are also generally long-lived. The shade tolerance of some species of trees may vary by region; for example, white pine is considered to be semitolerant in Minnesota but relatively intolerant in New England. The tolerance of individual trees will vary by age and site condition, too. Trees tend to have greater shade tolerance when they are young and when site conditions are better. Trees reaching midlife or growing on poor sites are not as shade tolerant.

Shade-intolerant trees must wait until something destroys a whole swath of big trees, letting full sunlight hit the ground. Before people turned up on the scene, it was fires, windstorms, and disease outbreaks that made the bigger clearings that these species need to reproduce. With acres of big trees down or gone and full sunlight warming the ground, the shade-intolerant species, from aspens to longleaf pine, sprout in massive numbers from seeds, roots, or stumps and race to grow faster than all their siblings. It’s a race because, though there may be enough water, soil nutrients, and light for 60,000 seedling trees in an acre, there’s only enough for a few hundred adults. The saplings that grow fastest and straightest toward the light will win, and the rest will die as they’re shaded out. Shade-intolerant tree species are sprinters: they grow very fast, and many have short lives (for trees). In a forest dominated by these kinds of trees, all the trees will be pretty much the same age, since they all got their start at the same time.

Excerpted from A Landowner’s Guide to Managing Your Woods (c) Ann Larkin Hansen, Mike Severson, and Dennis L. Waterman, illustrations by (c) Steve Sanford. Used with permission from Storey Publishing, 2011. Buy this book from our store: A Landowner’s Guide to Managing Your Woods: How to Maintain a Small Acreage for Long-Term Health, Biodiversity, and High-Quality Timber Production.