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Saturday
Jul282012

The fascinating process of tree decay

 

By Joe Rankin

Forests for Maine's Future writer

 

For trees, the forest is truly a jungle.

 

First there's the Olympic-scale intense competition for food and sunlight. Everyone straining for those life-giving photons, a silent stems-and-branches brawl. And it can go on for decades, centuries. In fact, it's never really over.

 

The Ghost Tree: decay in action (Photo: The Rankin File)And no struggle is free of death and injury. Ice storms, hurricane force winds. Animals gnaw at your trunk or rub bark down to sapwood. Insects punch holes in you and chew away your leaves; woodpeckers hammer away to get at the insects and larvae. And, swirling all around you, an unseen cloud of micro-organisms and fungal spores just waiting to pounce on a wound. Any wound.

 

Most of us have, at one time or another compared a forest of lofty trees to a cathedral of peace and tranquility. But that is an illusion, a cruel joke, says Kevin Smith, a forest pathologist and researcher at the U.S. Forest Service's Northern Research Station in New Hampshire.

 

“It's very competitive. And it's definitely life and death. Most trees that start as seedlings or suckers don’t make it. If you have a white pine stand of merchantable trees you know there were a zillion seedlings per acre a hundred or 150 years ago. Now there are a dozen or two standing trees. What happened from a zillion to a few dozen? Death and decay. That's part of natural forest development,” Smith said.

 

“The last thing a forest is is peaceful and tranquil. But it is in a dynamic equilibrium, in the sense that all pieces are working together, though some are in competition or conflict with each other. What we see when we're walking through the forest, a highly managed stand or a more wild stand, is the net interaction of all of those processes.”

 

Smith has been fascinated by tree decay since he was five years old and a hurricane stormed through his hometown in Pennsylvania and toppled a big maple in the yard. Workers sawed it up and he played among the chunks of wood until they carted it off. “I was really interested in the breakage patterns and the colors on the inside,” he Kevin Smith (Photo: USFS)remembers. “Some of the colors of freshly cut wood, especially with infection in it, can be very pretty. And that's pretty much what I get to work with now.”

 

We've all met those people who claim they've never been sick a day in their lives. Well, they're probably very young, or lying. Living organisms get sick, eventually they decay. Yes, there are very long-lived organisms. The bristlecone pines of the American west, for instance, are believed to be the oldest living things, just under 5,000 years. But, eventually, they too will die. No exceptions. It's called the Second Law of Thermodynamics: stated one way, all systems tend toward decay and death.

 

“It's difficult, if not impossible, to find a tree that has not been wounded in some way,” said William Ostrofsky, the forest pathologist with the Maine Forest Service. “In fact, if you want to extrapolate further, all trees have branches and all trees shed branches naturally. And you can consider that as a wound too because it's an avenue of dead material that goes to the core of the tree. And if that doesn't heal properly or quickly that can become a wound as well. So it's unavoidable that trees are going to become wounded, but their strategy is that with each year with that new annual ring they're building new material all the time and so they can keep ahead of a lot of these wounds, sometimes for decades.”

 

Like people, trees have to deal with injury and illness and infection. But there the similarity ends since they deal with it in a different way.

 

Basal wound in paper birch (Graphic from A First Look at Tree Decay, Smith and Shortle 1998 USFS)Humans have the ability to repair injuries, to replenish and replace damaged cells with new. Broken bones knit, cuts heal, new skin grows. In fact, science says that over a seven-year period all your cells are replaced. You are a new you. A tree walls off an injury and works to stop the spread of decay and infection with chemicals, then grows around it, confining the damage. It's a concept called compartmentalization, developed by Alex Shigo, the grand old man of tree decay science, who worked as a forest pathologist with the U.S. Forest Service for decades and was one of Smith's mentors.

 

Just look at a tree and you can sometimes see the effects of compartmentalization. The bark closures. The rotten wood. Sometimes you can't. Sawyers, woodworkers and woodturners are intimately acquainted with them. A woodturner can be working on a fine block of maple only to come across a long-buried decay flaw at 1,400 rpms. Sometimes the work will be salvageable, sometimes it will be enhanced, sometimes it will blow apart in your face. And there would have been no hint from the outside.

 

Trees vary in how good they are at containing injury and decay. Smith notes that osage orange, an especially tough tree of the midwest, and black locust “invest a lot in those chemicals to resist decay.” But, he notes again, “part of the beauty of nature is that eventually some fungus or a group of fungi will be able to break it down.” Everything rots, eventually.

 

Ostrofsky adds that some trees, like white birch and aspen, are especially poor compartmentalizers. Generally those are the trees that are the first to grow on disturbed land, those with “live fast, die young” embedded in their genetic code.

 

Others, especially the shade tolerant species like sugar maple, are much better at containing decay. “They put a lot of their life's energy into protection mechanisms and barrier zones and chemicals that limit the spread of decay,” said Ostrofsky. “That’s why we can tap sugar maples for syrup, because the tree is so well adapted to confining tree decay.”

 

But there's really no pattern, no rule. Beeches, for instance, are shade tolerant hardwoods like maples, but unlike maples “tend to be quite decay-susceptible. Decay fungi will move quite rapidly when they get into beech. So it doesn't fit that pattern. But it also has to do with beech's bark structure, which is different from any other trees,” Ostrofsky said.

 

When there's an injury – a wind-broken branch, bear claw scratches on a trunk, a top pruned back by a utility worker, or skin scraped off a trunk by a logger, the tree's enemies mobilize. The wood is first invaded by non-decay organisms, bacteria and fungi that can use the easily accessed exposed sugars, said Ostrofsky.

 

Branch stub in maple. (Graphic from A First Look at Tree Decay, Smith and Shortle 1998. USFS)“When they get done other fungi begin to move in because all the material that is left tends to be bound tightly in the cell wall and in order to get it those carbohydrates the fungi need to have special enzymes to do that. And it's the wood decay fungi that have those enzymes. The wood decay fungi don't compete very well, but they don't have to. They hold the trump card,” he added.

 

As decay proceeds it can involve many types of microorganisms. Some work at the same time, some follow one another. Some of the organisms are specific to particular species, others are generalists. Then bigger organisms, much bigger, join in.

 

“Usually there are several wood decay fungi and a whole host of little critters and big critters. Woodpeckers, other birds, other animals. And at the same time or even prior insects boring holes and clearing things out. That community of organisms are attracted to decaying wood,” said Smith.

 

A tree can often contain the damage. Or the invaders might slip past and overwhelm the tree's defenses. But even if the tree does manage to compartmentalize the damage and initial decay, there are always other injuries to come.

 

And, as the tree ages it becomes harder to fight the battle.

 

“Eventually the trees reach a point where they don't have the living mass to care for, to monitor, to protect all the tissues they're responsible for, so to speak. So they begin to decline because they just don't have the energy to take care of it. That tipping point is different for each species of tree and probably different for each individual depending on its life circumstances,” Ostrofsky explains.

 

Some trees go from injury to humus in a few years: gray birch or aspen, for instance. For others -- maples, osage orange, giant sequoias -- decay and decline can encompass many decades. An old maple, say a venerable giant of two centuries or more, can spend 60 or 80 years in decline – hollowing out, losing branches, its top popping off, shedding bark, then flaking off rotting sapwood. But even in that it is fulfilling an important role, providing homes for creatures as varied as raccoons and birds, centipedes and salamanders. It is a veritable ecosystem in itself.

 

For, while a tree's death and decay might seem a waste, it is anything but. The process itself ensures the continuity of the forest.

 

The end game. (Photo: The Rankin File)“A dead standing snag, or dead stems on the ground, well, we can call them dead stems, but in terms of the weight of living stuff there's more in the dead stuff than in the living stuff if you count all the fungi, bacteria and insects and birds and what else. There's more life associated with it than when it was a sound healthy living tree,” said Smith.

 

No one has yet quantified how much of this decaying matter should be retained in a forest during a logging operation, say, to ensure a healthy future forest, but researchers, including Smith's group at the NRS, are working on it.

 

“Decaying trees are an important part of the ecology,” Smith said. “I do understand the value of sound clear wood for wood products. Certainly we need to be producing that and growing trees to optimize that. But there is a definite ecological benefit in maintaining forests as communities, where there is wood production, but also the cycling of elements and energy which comes from having decaying snags and wood on the ground.”

 

The life and death struggle of a tree can be hard to watch, especially if it's a favorite old tire-swing shader, or a beautiful old street tree, or a historic old tree like Herbie the venerable Yarmouth elm that recently succumbed to Dutch elm disease despite decades of chemotherapy.

 

But, death begins with life. And, the process itself has fascination, because it is so complex, so intricate, so basic to biology, to existence.

 

“It's a long-term relationship between a tree and its wood decay fungi. It can begin with discrete event, usually a wound that occurs naturally or is caused by humans. That sets off a sequence of events that can go on for decades, with the tree and the wood decay organisms all interacting. And it's that interaction that fascinates me,” said Smith.

 

To Smith there's a beauty in the process of decay. He talks poetically about the green color he's seen on the margins of a wound in freshly cut red maple, a green that's more beautiful because it's so ephemeral: when exposed to oxygen it vanishes. Ostrofsky sees nobility in a stand of beeches that, while sorely beset by the deadly beech bark disease, still exuberantly leaf out in the spring.

 

Joe Rankin writes forestry articles, works his central Maine woodlot, turns bowls on his lathe and drives his wife crazy planting trees like northern catalpa, ginkgo, Kentucky coffeetree, and Korean stone pine in his expanding arboretum . . . uh . . .back yard.