While cutting trees on my woodlot for firewood with a friend a while back, he paused and asked “How do you decide which trees to cut and which to leave?” I had gone ahead and marked each of the trees that I wanted cut down, a few here and a few there, and he was curious about my decisions. “Well, I want to promote valuable, healthy trees, to leave it better than when I started,” I said. “And to restore species reduced in abundance. And to provide wildlife trees. And some I’ll leave as ‘legacy trees’.” I quickly realized just how many factors come into play when choosing which trees to cut and to leave.
Here are a few thoughts that crystallized as I thought about his question:
1. Favour old-forest species (long lived and shade tolerant)
Old forest species would naturally dominate most woodlots in the Maritimes. These include red spruce, sugar maple, hemlock, white pine, beech, white ash and yellow birch. Intensive logging and clearing for agriculture, however, have hugely reduced the abundances of these species, and have increased young forest species such as grey and white birch, pin cherry, poplar, balsam fir and tamarack. Not only are old forest species more economically valuable, but they have a better chance of surviving the changes to our forest that climate change will bring.
Old forest species often grow hidden among young forest species. They can very uncommon, perhaps only a few per acre, so it is necessary to look carefully to determine if any are present. When I find an old forest species, I favour them by cutting other trees away from them, ensuring they have room to grow.
|Species often reduced in abundance (favour these)||Species that are often over-abundant (cut these first)|
eastern white cedar
green (red) ash
aspen species (poplar)
pin and choke cherry
*These species form mature forest in certain habitats (high-elevation, or low fertility, or excessively wet, or very dry sites) but are generally over-abundant outside these areas.
2. Promote healthy, valuable trees
Assessing tree health starts with looking up. If you’re not tripping over your feet, you’re probably not looking up enough! The upper part of a tree (its crown) shows how well the tree is faring relative to neighbouring trees, and whether it is succumbing to the effects of insects or diseases. Assessing tree vigour can show which trees have potential to increase their growth and live long lives, and which are growing slowly and at risk of death or serious decline.
As shown in the illustrations below, the most obvious sign of tree decline is the death of small branches. For hardwood trees, this results in progressively less dense crowns and noticeable dead branches. Generally, the more leaf surface a tree has relative to its size, the better it can grow and sustain itself. The crown of a vigorous hardwood tree should be roughly two feet wide for every inch of trunk diameter.
For softwood trees, reduced vigour also results in less dense crowns, but is generally seen in crown length relative to the height of the tree. The live crown of softwood trees should cover at least 40% of the total height of the tree.
Other factors being equal, trees that are in poor health, and trees with poor form (forked tops, bark damage, crooked stems) are the ones to cut. Vigorous and well-formed trees are the ones to leave.
3. Leave an abundance of wildlife and legacy trees
Standing dead trees and trees with cavities or dens usually have low economic value, but have extremely high ecological value. Some 25% of all wildlife in the forest finds shelter in dead or dying trees. In addition, thousands of species of insects, fungi, bacteria, mosses, liverworts and lichens find nourishment in deadwood, gradually decomposing the wood as they feed on it. Gradually, deadwood is returned to the soil as nutrients and organic matter, feeding plants and building soil structure. As some folks say, deadwood is the life of the forest.
Legacy trees are large, healthy, dominant trees that are allowed to grow old and die. Alive, they provide structural diversity and a rain of genetically fit seed. When they die, they provide cavity nest sites while standing and a new source of large deadwood when they fall.