Illustration by Adelaide Tyrol
A guy down the road has been working in his woods for the last couple of years. He’s cleaning them up. And I mean cleaning. He cuts the underbrush. Takes out the dead trees, the downed logs, the dead branches.
Okay, I confess. The neatnik in me is envious. Part of me would like my 70 acres of woods to look like a park. But that’s the problem. A park is not a forest. And the forest is more than the trees. It’s an entire suite of complex systems, merging and interacting. An ecological orchestra in the woods.
Dead and dying wood, standing snags, rotting branches are more than Mother Nature’s litter. They’re an integral part of the forest symphony – what forestry types call “coarse woody debris,” or CWD for short.
Dead and dying wood, standing snags, rotting branches are more than Mother Nature’s litter. They’re an integral part of the forest symphony – what forestry types call “coarse woody debris,” or CWD for short.
“Without that debris our forests would not be what they are. Not just in terms of aesthetics, but nutrient capture and recycling and wildlife,” explained Andrew Whitman, a scientist with the Manomet Center for Conservation Sciences in Maine.
Snags provide nesting and resting spots for woodpeckers and other birds, squirrels and fishers. A rotting log on the forest floor is home to mosses, fungi, liverworts and lichens, salamanders and frogs, millipedes and centipedes, mice and voles. Piles of branches shelter snowshoe hares and hibernating bears.
Deadwood is critical to many species of fungi, which make their living breaking down the lignins and cellulose in wood and leaves. A dead tree will host a succession of species, Whitman said. “The fungi that use fresh dead logs are not the same as the ones that use logs that have been rotting for 50 years.”
Downed wood is also an important reservoir of bound-up carbon. “In older forests, 100 years or older, it can account for 22 tons per acre, or the weight of five cars. In younger forests it’s about 1 to 5 tons per acre or the weight of about one car. It’s not a trivial amount,” Whitman explained. That’s important because, in terms of climate change, carbon on the forest floor means less carbon in the atmosphere.
Carbon, nitrogen, calcium and the other elements that once made up a living tree are gradually released through the process of decay and then reused. It’s the forest fertilizing itself. In the meantime, those dead trees and branches littering the forest floor slow runoff in times of high rainfall.
Coarse woody debris also plays a crucial role in aquatic ecosystems, noted Whitman, providing nutrients to a stream. Downed wood also improves a stream’s “structure,” creating plunge pools, eddies, and places for fish to hide and stay cool.
In an intensively managed forest, the amount of coarse woody debris can decline over time. These days we often try to extract the last dime‘s worth of value from a harvest, chipping branches and tops for the biomass market. It’s not unusual for trees that once would have been left to die and then rot to get chipped instead.
“About 20% of bird species, 50% of mammal species, 44% of amphibian species, and 58% of reptile species in the Northeast use coarse woody debris for foraging, shelter, or other uses. And this is for the vertebrate species we know well,” said Whitman. “In an intensively managed forest, those species will struggle. While it looks great and it may be efficient in terms of fiber production, it can lead to a loss of biodiversity.”
The effects of that sort of harvesting may not show up for decades. But some studies indicate that over three rotations, nutrient levels in forest soils may start to drop if whole tree harvesting is used, he said. That would translate to slower tree growth and perhaps to a less complex forest that’s less able to cope with pests and diseases. And restoring a depleted forest isn’t economical, or easy, or fast. Dropping a few tons of 10-10-10 isn’t going to bring back those complex ecosystems or restore species that were lost. Only time would do that. Lots of time.
“Northern New England is fortunate to have its current levels of CWD,” said Whitman. “Similar forests in Europe have nowhere near the same amount of deadwood and so often lack species we still have. It is no wonder that tourists from Europe love to see our ‘wild” northern forests, where deadwood is a mark of wildness.”
Joe Rankin writes forestry articles from his home in central Maine
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What Is Forest Stand Structure and How Is It Measured? | Spring 2010 | Center for Northern Woodlands Education
Photo by Jim Block
Once upon a time, forest stand structure meant age structure. Areas of forests containing similar-aged trees were called even-aged stands, and areas with trees of multiple ages were considered uneven-aged stands. It was all quite straightforward. Stand structure was defined by how many trees were present in each age class within a given stand. It was beautiful. This view of stand structure saw tree diameters as a surrogate for something more difficult to measure: tree age. Foresters then set about trying to manage the number of trees in each diameter class according to some target age structure. Typically, each age class was supposed to occupy an equivalent proportion of the forested area in a way that was thought to ensure a regular output of wood over time. This was known as a “balanced age structure,” but of course it was really more a balance of tree diameters than tree ages. Remember, age and size are not the same; sometimes, small trees are surprisingly old, and many large trees are surprisingly young. Nevertheless, for years this was how foresters referred to and managed a stand’s structure.
While a lot of good came from this method, conventional management approaches have changed. Today, foresters are increasingly aware of, interested in, and asked to manage for more than just the sustained output of wood. We are asked to sustain the forest itself and its capacity to serve many additional functions, such as providing habitat for a diversity of organisms. Accordingly, our current definition of stand structure is far more complex. We now see structure as the physical form of a stand, with particular emphasis on what you might call the verticality of the woods – the extent to which both living and dead plants occupy horizontal layers from the ground to the tops of the tallest trees.
Picture the three-dimensionality of a forest stand, from the forest floor and the herb and shrub layers, into the understory, and through the canopy to the tree tops. See all of that space and the varying amounts of vegetation within it and then look from side to side to see its full horizontal extent. That’s stand structure. It is the vertical and horizontal arrangement of plants, dead and alive. Structural complexity is the combination of it all. Stands with more complex structures are thought to be more resilient and potentially even more productive. They assuredly provide valuable habitat for a greater diversity of plants and animals than do stands with less structural complexity.
But still, how to measure it? Structure is not like tree diameter, height, or even tree age – all of which can be readily measured. Instead, structure is a stand-wide feature, and there is no one measure or even a good index to quantify or express it, at least not yet in use. And so foresters measure a variety of stand attributes – tree diameter, trees per acre, basal area, live crown ratios – that each contribute to a stand’s overall structure but do not, individually, describe it completely. It is therefore insufficient, even meaningless, to simply add together our varied measures to produce some average quantification of stand structure. In this way, forest structure is like a good rock band: the whole is always much more than the sum of its parts. This is reflected in one of Webster’s definitions of the very word, structure: “organization of parts as dominated by the general character of the whole.”
The forester’s job then is to assimilate all of our various individual measures into an integrated, coherent sense of the whole. In particular, we seek an understanding of a stand’s full volume of growing space and the extent to which it is occupied. We often express this in generalities like patchy, dense, or multi-storied. It’s not mathematical, but it begins to paint a more telling picture of the stand and its growing space and opportunity.
Sure, we still try to optimize the growth of useable wood in managed stands, and we still use diameters to approximate age when appropriate. But our view of a stand’s structure has evolved to include vertical stratification and development through consideration of tree shapes, heights, spacing, and arrangement in addition to diameter and age. It’s far more difficult, but even more beautiful.
Michael Snyder is the Chittenden (Vermont) County Forester.
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