By JOE RANKIN
You're in a store, say, and you look around you at the other patrons. Can you tell who is healthy? For a few, maybe. For most, probably not.
Things are similar for a forest. Just look at the trees and it might be hard to tell whether a forest is healthy. If they aren’t turning brown, burned to a crisp or swarming with voracious beetles munching under the bark you might assume things are hunky dory.
Just as we once thought the absence of disease was a sign of health in a human, forest health was once seen as the absence of pests and diseases, mostly of those that would interfere with the production of timber.
That idea, call it the forest pathology view, was prominent well into the 20th century. But over the past few decades what a healthy forest is, and isn’t, has become more wholistic, says Kevin Smith, a senior plant physiologist and researcher with the U.S. Forest Service’s Northern Research Station in Durham, N.H.
These days forestry types don’t concentrate on the absence of disease to prove a forest is healthy. It’s recognized that there’s going to be a certain amount of disease and pests in any forest.
And why is it important that our forests are healthy?
Forests offer a plethora of benefits. Even if you live in the concrete jungle, you depend on healthy forests. They slow erosion, protect water quality, synthesize oxygen, store carbon, create jobs, provide us with materials for building everything from our houses to the furniture we fill them with, and offer homes for wildlife and places for people to hike, fish and hunt, notes the American Forest Foundation. The list is, as they say, endless.
So, what defines a healthy forest?
“What a healthy forest is is resilient. Resilient to what’s coming down the pike. It could be new pests or pathogens moving in, or climate change” that put new stresses on forests, he said.
“In the northeastern U.S, resilience means having a diversity of species and a diversity of sizes and ages of trees, so we don’t have all of our eggs in one basket. If there’s anything we’ve learned from forest pathology of the old days, it’s that monocultures are not a good idea. Diversity can help a forest be resilient to pests and pathogens we don’t even know about yet.”
Human analogy: the person who exercises, eats well, has a strong immune system and a positive outlook on life is one who has the most resilience.
Many people tend to look at a forest and see trees. But they are, literally, not seeing the forest for the trees. A healthy forest is much more. It is a complex, vibrant ecosystem. “Eco is Greek for house and system is a set of related parts and processes that are all functioning together,” said Smith.
A healthy forest will have soil alive with beneficial microbes and mycorrhizal fungi; the leaf litter will be flush with invertebrates like millipedes and pill bugs and salamanders and small reptiles. Small mammals like mice, voles and shrews will scuttle across the forest floor. A healthy Maine forest will have a suite of birds using every niche from forest floor to the topmost branches. There will be deer and moose; coyotes and bobcats; mink, porcupines, raccoons, weasels and other mammals.
One good indicator of a healthy forest, said Smith is an understory with a good population of shrubs like hobblebush and beaked hazelnut and woodland wildflowers like Canada mayflower, lady’s slipper, starflower, goldthread, and blue-bead lily. Or, in the case of the spruce-fir forest, bunchberry. “In properly resilient, healthy forests you can see them as far as you can see on the forest floor,” he said.
You might be tempted to look at a dead or dying tree sprouting conks and mushrooms and see those as a sign of forest illness. It’s not. Trees die, for various reasons. Fungi are a sign of a forest that’s efficiently recycling them into nutrients. And fungi do much more, said Smith, who has spent a lot of time researching these things. “They are agents for natural branch shedding. As the branches get shaded out, the fungi break the branch down and break it off so the tree can close around it. If you’re interested in production of clear wood for products you depend on them.”
The mycorrhizal fungi, which create an intricate web in forest soils, work with trees to help them take up must-have elements like phosphorous and nitrogen, and work also at breaking down organic material into nutrients available for the tree. The mushrooms you see after a rain are often the fruiting bodies of that network and, again, a good crop is a sign that the mycorrhizal network is in good shape.
Another health analogy: scientists now understand that people are ecosystems themselves, hosting billions of microbes and tiny creatures live on and in us and that work for our well-being. They call it our “microbiome” and say that each of us has a different suite of unseen helpers.
Wildflowers, invertebrates, birds, fungi, trees — all are “indicators of the large scale processes that are worked out on a very small scale, individual fungi, trees, roots that are providing for the fertility and continued success of the whole system. And that’s a wonderful thing,” said Smith.
You might be tempted to think that only a climax forest is truly healthy. The idea of tree species succeeding each other over the decades and centuries from, say, an abandoned field or burned over landscape to a forest of huge, ancient trees, doesn’t mean that those intermediate stages aren’t healthy. “The concept of a climax forest is ok, but it’s limited by forest disturbance, which is the nature of the forest, whether it’s ice storms or hurricanes or fires,” said Smith. “All of those will cause the clock to reset away from that climax.”
So, how can we help ensure a forest is healthy?
“Our challenge is to work within the natural system,” said Smith. That requires understanding the system’s parts and how they work together. And what can throw them out of balance. Out-of-balance might be seen as synonymous with unhealthy.
Deer, for instance, are a natural element of forests in the northeastern U.S. But in some areas deer are too numerous, hampering forest regeneration and adversely affecting forest health. Exotic pests and pathogens that gain a toehold, whether it’s barberry, Japanese stiltgrass, or the Asian longhorns beetle, can do the same by literally taking over a forest and shouldering aside native species. Recognizing these alien invaders, and keeping them from getting established, can keep forests healthy.
It helps to remember, Smith said, that all trees are under stress all the time. Kind of like people. For a human it might be a tough job or no job, a difficult relationship, too many bills. For a forest it might be too little water, too much water, fire, or pests.
“It’s important in your woodland management not to make more stress for the trees, whether they’re in a forest environment or an urban one, Smith said. Avoiding logging in early to mid-summer when growing trees are at their most vulnerable is one way to avoid adding to stress by damaging bark of trees to be left. Soil compaction is a big factor adding to tree stress in both urban and rural settings, he added.
Working with forests to help keep them healthy is important, as the American Forest Foundation notes, because forests in North America face a “toxic blend of natural and man-made threats” ranging from development and forest landscape fragmentation to introduced forest pests and pathogens to warming temperatures. People caused problems, most of them.
That being said, another thing we need to realize, is that people are part of the forest environment, and have been for tens of millennia. Our needs impact the forest, and forest health as well. “We need to take into account the diversity of the human communities that depend on forest much the same way we do the diversity of the other animals and plants that are part of the forest,” said Smith.
Joe Rankin is a writer on forestry and natural history who lives in Maine.