[Note from the web master. The word heal may be used here to describe the closure of a wound. Dr. Shigo now calls it closure and not healing. That and other terms can be found at www.treedictionary.com for your use. John A. Keslick , Jr.]
HOW CAN YOU TELL?
How CAN YOU TELL, by looking at a living tree, what quality it has? One tree may be rotten at the core yet still may contain a large volume of high-quality wood. Another may be thoroughly sound from pith to bark yet may be so damaged by minute discolorations that it is worthless for high-quality products. How can you tell?
By dissecting living trees and studying the organisms that
infect them, we now know that discoloration and decay develop in certain
definite patterns. And the patterns of discoloration and decay within the
tree can be predicted from external signs.
Discoloration and decay are the most serious defects of northern hardwood trees. In speaking of defect, we must distinguish between injury and damage. Injury harms the tree: damage lowers the quality of the wood. For example, a disease like vascular wilt may kill the tree but do no damage to the wood. But an insect like the cambium miner may do very little harm to the tree yet do great damage to the wood.
The unseen damage done to a tree is important in the economics of forestry. Every operation in growing a tree, harvesting it, and converting it into products costs time and money. And after all the time and money have been spent on a tree, the product made from it may prove to be not worth the effort; and the tree might have been used more profitably for some other product that does not require high-quality wood.
The increased use of veneer offers an illustration. A veneer log brings top prices. But its actual value may not become apparent till it is put on a lathe and peeled. A log that looks very good, and sound to the core, may have minute streaks of discoloration scattered all through it, so that all the veneer it produces is badly streaked with defects. On the other hand, a log that has a rotten core surrounded by clear wood may produce the highest quality of veneer.
So it is not so important how much discoloration and decay a tree has, but where these defects are in a tree. The pattern of the discoloration and decay-that's the important thing.
STUDY METHODS & MATERIALS
This guide is based primarily on the findings from a series of studies made by the senior author in northern New England. Begun in 1959, these studies are being continued in efforts to clarify further our understanding of the discoloration and decay processes.
The northern hardwoods make up one of the major forest
types of northern New England. They are American beech (Fagus
grandifalia Ehrh.), paper birch (Betula papyrifera Marsh.), yellow
birch (B. alleghaniensis Britt.), sugar maple (Acer saccharum
Marsh.), red maple (A. rubrum L.), and white ash (Fraxinus americana
Most of the sample trees grew in the White Mountain National Forest in New Hampshire.
The resource of northern hardwoods is plentiful today, but it contains an overabundance of poor-quality trees. Foresters repeatedly ask two questions about the northern hardwoods: What can we do to assure future crops of high-quality trees? And how can we make the most of what we have now?
A new large-scale method of dissecting living trees was used in these studies, to get at the defects inside the tree. This might have seemed an obvious thing to try. Yet large-scale dissection had not been tried on trees-at least not in quite this way.
True, pathologists had studied decay in wood and had isolated and identified fungi. Studies had been made of the cut ends of logs. And wood defects had been studied on freshly cut boards at the sawmill. But logs taper, and many of them are not straight; and though the surfaces of lumber cut at a sawmill reveal discoloration and decay, they do not reveal the complete patterns.
The development of the gasoline-powered chain saw since World War II made possible this method of dissection. By using a portable chain saw, it was possible to go into the forest and work on living trees. One could fell a tree and cut it into bolts and disks at once and rather quickly-a tedious and discouraging job with hand saws. Moreover, it was now possible to begin at one end of a tree stem and slice it down through the middle, following the natural curve of the stem. And it was possible to cut at any spot and any angle, and to lay open any particular area of a tree for study.
After a tree had been cut open, the defects were systematically
mapped. Then, from specimens of wood taken into the laboratory, chips 1
x 0.3 cm. were cut with a gouge from clear wood, discolored wood, and decayed
wood, and from the zones between -six chips in each series, at intervals up and
down the tree stem. These chips were cultured on agar plates. Every
organism found was recorded, and most were identified.
Scope of the Studies
During these studies more than 3,000 trees were dissected, and some 100,000 isolations were made to identify organisms. More than 10,000 photographs-both black-and-white and color -were taken to record the patterns of discoloration and decay in the freshly dissected trees. From these, 100 color photographs have been selected to illustrate this guide.
As dissection of the living trees disclosed general patterns of discoloration and decay, a clear understanding about three aspects of the discoloration and decay processes emerged.
A Succession of Organisms
Decay in a tree had been thought to center about three simple events-a tree is injured: a fungus enters: decay begins. But our studies showed that a complex succession of events must take place before decay can begin. It works roughly like this-a tree is injured: the tree reacts: chemical changes take place in the wood: the wood discolors: bacteria and non-decay fungi become active: the wood discolors further: decay fungi infect: decay begins.
The process is irreversible. And it cannot be shortcut. Decay does not begin until all the other events in the succession have taken place. Decay does not begin until the wood has been discolored. And the succession may stop at any stage: the wood may become discolored but may not decay.
A Consistent Pattern
The discoloration and decay in a northern hardwood tree take a definite pattern. They form a column in the tree, related
to the location of the injury. This column can spread up and down inside the tree, but it seldom spreads outward. The diameter of the column of defect is no larger than the tree was at the time of injury. In effect, growth of the tree after the injury forms a pipe of healthy new wood, and the discoloration and decay are held within this pipe.
Discoloration not Heartwood
In some tree species, like black cherry and walnut, true heartwood forms as a result of chemical change that follows normal aging processes in the wood cells. But in northern hardwoods the darker wood is not true heartwood in this sense: it is a result of discoloration processes initiated by injury.
THE PROCESSES OF DISCOLORATION AND DECAY
The development of discoloration and decay in the wood of a living northern hardwood tree is a complex continuum of events that merge and overlap in time and place. Several columns of discoloration and decay may be present in various stages of development at the same time and place. It is difficult to say where one stage ends and another begins. But for the sake of convenience in describing the processes, we divide them into three broad stages.
The process begins with an injury to the tree. A
branch may die or break off; insects or birds or animals may attack the tree; a
fire may burn the base; or a logging machine may scrape it in passing.
Some cells are killed by the injury, and others may be injured to some degree.
The injured cells are exposed to the air. At this time gases and moisture
can pass out of the tree, and air and moisture can pass into the tree.
These changes start chemical processes in the wood cells about the wound.
Discoloration results. It may be due to the materials formed by the chemical processes, or to the darkening of cellular material
as a result of exposure to the air. Sometimes the discoloration may be
a bleaching rather than a darkening. Microorganisms are not involved.
These early discolorations do not alter the strength of the wood. And the process may stop right here. It depends on the severity of the wound and the vigor of the tree. But the discoloration may advance inward toward the pith, and around the tree.
Then in time a new growth ring forms. The first cells in this ring are different from the cells that are usually produced. They act as a barrier to the discoloration process. The discoloration seldom moves into the new cells. Instead, it moves up and down the stem within the pipe of barrier cells-but not outward into the new tissues.
The extent of discoloration depends on the vigor of the tree, the severity of the wound, and on time. Discoloration advances only as long as the wound is open. Thus the entire cylinder of wood present when the tree was wounded may not become discolored. Meanwhile the tree continues to form new growth rings that are free of discoloration.
Stage 2 begins when microorganisms infect. As soon as the tree is
wounded, many different organisms begin to grow on the wound surface. They
compete; and many do not survive. Of those that survive, only a few types
are able to begin growing into the wood through the wound-only those that can
thrive in the discolored wood.
These first microorganisms to invade the tree-the pioneers- are bacteria and fungi. The fungi are non-Hymenomycetes: they do not cause decay. These pioneers infect only the cells that have been altered by the chemical processes, so the new tissues formed after the injury remain free of infection.
The infected cells are further altered by the pioneer microorganisms. The discoloration may increase; the cells become more moist; pH (acidity) and mineral content of the cells rises; and certain parts of the cell walls may be eroded. The wood
affected at this stage is called wetwood, redheart, or blackheart, The process may stop at this stage.
Stage 3 begins when decay fungi (Hymenomycetes) become active and begin to
digest the cell walls. These fungi affect only those tissues that first
have been altered by chemical processes and then by the pioneer organisms.
The new growth of wood that continues to form remains free of infection.
The decay process continues as long as the wound remains open. Many species of microorganisms may interact until the wood is completely decomposed. The succession of organisms does not stop when the first decay fungus enters. It stops when the tissues are completely digested.
The advancing decay column is often separated from the new white wood by a band of discolored wood. In some species the margin of this band may be bleached. The pioneer organisms remain in this band. Later, as decay continues, the decay fungi slowly digest this discolored band; and only a hard black rim then separates the by-then hollow core from the healthy white wood. This rim forms first near the wound.
To this point the process may take 40 to 50 years. Healthy white blemish-free new wood will surround the hollow core, unless other wounds have been inflicted meanwhile.
We want to emphasize this point: the processes need not go through to completion. Healing of a wound, antagonisms among organisms, unfavorable environment, and other forces may cause the processes to abate in any stage.
Importance of Time
What we have described above is a continuum of events that follows a single wound at one period in the life of the tree. But a forest tree is apt to be injured a number of times. The same processes take place each time. And the discoloration and decay processes that follow a new wound are not affected by the effects of older wounds.
THE PATTERNS OF DISCOLORATION AND DECAY
A cross-section of an ideally healthy northern hardwood tree shows a pencil-thin cylinder of pith in the center of the stem, surrounded by unblemished white wood. But what you see in most trees is a core of darker wood, of varying diameter, surrounded by white wood. The pattern of the discolored wood will depend upon what has happened to the tree: when the branches died, how fast it healed its wounds; what logging wounds it had; and how much injury was done to it by insects, fire, birds, animals, or other agents.
The most typical pattern of discoloration is a column of
discolored wood extending up and down in the center of the tree. This
discolored wood is always associated with some injury to the tree, like a broken
branch or stem. Where decay occurs, it occurs within this column of
In northern hardwoods, this central column of darker wood is not true heartwood-though it is often called heartwood. It does not increase in diameter as the tree grows. The discoloration is due to changes in the wood brought about by processes that are begun by wounds or other injuries..
True heartwood does occur in some hardwood species such as black walnut, cherry, and the oaks. This darker wood is due to processes associated with normal aging of the tree. As the tree grows, the column of darker wood increases in diameter. The cylinder of true heartwood extends rather uniformly throughout the entire tree, from the base up into the branches.
Columns of discolored and decayed wood, caused by injury to the tree, can form within the column of true heartwood. These columns of discolored and decayed wood do not increase in size as the heartwood does. Of course in a tree like black walnut the difference between true heartwood and discolored wood is hard to distinguish.
The differences between true heartwood and discolored wood can be tabulated roughly this way:
Click here for comparison chart.
When a tree is injured at different times, multiple
columns of discolored wood develop. The multiple columns can be seen most easily
on the ends of logs, where they take on a concentric pattern or a cloud-like
pattern. Very careful dissections are necessary to trace each column to
If two injuries to branch stubs occur at about the same time, the two columns of defect may join to appear like one column.
But if wounds occur at different times, the processes that
result in discoloration and decay begin and develop independently for each
If a cylinder of discoloration is already present in the stem, a later wound does not affect it. The discoloration from the new wound forms around or beside the old column of discoloration. It may completely envelop the old column, like a pipe sliding down over a smaller pipe. Or it may develop alongside the old column in the shape of a crescent or half moon. It may be wedge- shaped in cross-section.
Major injuries to the tree-such as broken stems and branches -tend to discolor the entire core of the tree that was present at the time the injury occurred. Minor injuries such as small insect wounds may cause only localized streaks of discoloration-islands of defect within sound white wood.
Because each column of defect is separate in both time and cause, the discoloration and decay processes may be more advanced in one column than in another. Decay may form in one crescent-shaped column while other adjacent columns may be discolored but still sound. Sometimes columns of defect enclose areas of clear wood. For example, a tree may have a sound center surrounded by a ring of decayed wood.
When the wood begins to dry after the tree has been cut, checks often form between the boundaries of the defect columns. The checks often follow the growth rings-ring shake. When the defect column is a complete cylinder in cross-section, the shake may develop in a complete circle along the growth ring. Because the defect columns may be in different stages of the discoloration and decay process, the stresses that develop in drying are not uniform; so checking may occur in many different patterns.
For example, this is common in sugar maple when branch stub wounds and borer wounds occur in the same place on the stem. Honeycombing or tissue collapse may result in the moist discolored tissues. And because the beetles wound the wood in a diagonal pattern, crossing many growth rings, a jagged column of defect sometimes forms that checks in all directions when the wood dries.
Defects under Cankers
The defects that form under cankers are somewhat
different. A canker tends to produce a localized defect rather than a
column of discoloration and decay.
When a canker develops about a wound, the tree usually already has a central column of discoloration and decay. The defect from a canker usually forms beneath the canker, in the wood between the central column of discoloration and the bark surface area where the canker forms. The defect from a canker usually does not spread around the entire stem: it lies immediately beneath the canker.
Mineral Streak and Stain Wounds also begin the processes that result in the discolorations that are commonly called mineral streaks. The accumulations of salts in such discolorations may be due to the evaporation of liquids that are drawn to the exposed wound, as in a wick action. Because fungi are the agents most consistently associated with wounds does not mean that other organisms, especially bacteria, do not also alter the wood. And viruses can also cause discolorations; so they cannot be ruled out as agents inciting stain.
Above: Even though an entire central column may not be discolored by a small wound, the whole central column may take on a slightly darker hue.
The wounds caused by sapsuckers, fire, and other agents have some typical
patterns, but the course of discoloration and decay follows the same patterns
Sometimes, when a wound is small and not severe, the discoloration affects a small area rather than a complete new column; yet the new central column takes on a slightly darker hue -usually light pink.
EFFECTS ON DIFFERENT SPECIES
All the northern hardwood species do not react exactly the same way to injuries. Some notes about each of these species follow.
Of all the northern hardwoods, the most resistant to discoloration and decay is sugar maple. Yet many sugar maple trees are very defective. This is because some agents of injury seem to prefer sugar maple to the other species: for example, sap- suckers, cambium miners, squirrels, and-most important-the sugar maple borer. Discolorations associated with wounds of sugar maple are often called mineral streaks. Of the fungi that cause decay in sugar maple, the most important are Fomes connatus, F. igniarius, and Polyporus glomeratus.
In general, red maple is considered to be very susceptible to defect. Discoloration and decay advance much faster in red maple than they do in sugar maple. The central columns of discoloration are usually due to branch stubs, most of them between 4 and 10 feet up on the stem.
Sprout clumps are common in red maple, and they present some serious problems. Branch stubs on sprout stems of red maple cause more defect than the old parent stumps cause.
Many fungi cause decay in red maple, but the principal one is Polyporus glomeratus.
Discolorations and decays advance in this species fastest of all. Large low branch stubs - usually not well healed-are most common in yellow birch. In older trees, top breakage accounts for wide columns of discoloration. Yellow birch is one of the favorite feeding trees for sapsuckers. The most common fungi that cause decay in yellow birch are Poria obliqua, Pholiota species, and Fomes igniarius. F. igniarius var. laevigatus causes cankers on overmature trees.
Paper birch reacts like yellow birch in most ways, except that discoloration and decay do not advance so rapidly, and large low branch stubs are not so common. Advanced decay is like that in yellow birch. The cambium miner is more common on paper birch, and so is the ambrosia beetle. Paper birch is not a favorite feeding tree for sapsuckers; but they do attack it, and heavy attacks cause black bands on the stem. The same decay fungi that attack yellow birch also attack paper birch.
The beech bark disease is the most important disease of beech. It has decimated beech in many areas.
One special feature of beech is that its base is vigorous, and wounds near the base rarely cause much damage to the roots. The notable exceptions are the defects caused by Fomes applanatus and Armillaria mellea. Because of its vigorous base, a beech tree with a defective bole may stay alive for a long time.
Ants sometimes infest wounds on beech trees, and their activities increase the defect by keeping the wound open. These insects are rarely found on other northern hardwoods. Branches from dormant buds on the stem may cause small defects when they die.
In ash trees, most of the defect comes from the top downward. Poorly healed stubs in the crown, and broken tops, should be considered important in this species. The principal decay
fungus of ash is Fomes fraxinophilus, which affects the upper stem. Frost cracks are common on ash in some areas.
STUBS WITHIN THE STEM
How to Estimate Size, Angle, and Depth
The size and position of a branch stub buried in the wood
inside a tree can be estimated roughly from the scars on the bark. And
from this in turn you can estimate the size of the defect column and the amount
of clear wood outside it. Much of the information on this subject comes
from research done in Europe.
Two types of bark scars provide the clues. One, called a chinese beard, is like two drooping moustaches. The other, like a mouth under the moustaches, is the more or less round scar that marks where the stub was.
The chinese beard forms this way: as a branch grows on the tree stem, a layer of thick-walled cells forms in the axil above the branch. The bark becomes roughened over this ridge of thick-walled cells. In some species, like paper birch, the bark also turns dark here.
Above: Stub scar and chinese beard on a beech tree.
Above: How length and angle of stub is estimated from the stub scar and chinese beard.
This scar can be used to estimate both the length of the stub
inside the tree, and the angle at which the stub
joins the vertical axis of the tree center. From the center of the stub
scar, draw a line to the bottom end of the chinese beard. This equals the
length of the stub and shows the angle at which it lies.
The thickness of the stub can be estimated directly from the stub scar. Assume that the branch that broke off to form the stub was practically round. The height of the stub scar shows the size. If the stub scar is about 2 inches high, the stub is about 2 inches in diameter.
How far inside the tree the stub is buried can be estimated from the shape of the stub scar. As a tree adds new growth rings, the growth at anyone spot on the stem is outward, not upward. The height of the stub scar remains the same. But as the new growth is added, the bark is pushed outward, and the scar spreads at the sides, gradually changing in shape from a circle to an ellipse.
The scar's ratio of height to width indicates how far the stub extends from the pith center of the tree toward the bark; and this tells conversely how much new wood has formed over the stub.
For example, if the stub scar is round, it has a ratio of
1 : 1. This means that the end of the stub lies just at the bark surface
-an unhealed stub-and no new growth covers it. If the scar is twice as
wide as it is high, the ratio is 1 : 2. This means that the stub extends 1/2 the
distance between the pith center and the bark, and that the other 1/2 is white
wood. And if the ratio is 1 : 3, the stub extends 1/3 the distance between
pith center and bark. And so on.
This rough formula can be applied to all the northern hardwood trees. As time passes, use of this formula becomes more difficult, especially on the maples.
These photographs illustrate all the major defects of northern hardwoods, as we now recognize them, and the fruit bodies of the most important decay fungi.
FIGURE 1. - LOW BRANCH STUB.
This sugar maple (above) has a low stub, but the base is free of wounds. The long stub of hard wood shows that the stub has not been dead long, and that the discoloration processes have not had time to progress to an advanced stage.
Dissection (above) reveals the pattern of the discoloration. The wood at the base is clear. The column of discoloration from the large stub dwindles toward the base, but joins above with a wider column of discoloration from an older stub above, where the processes are more advanced. The wood formed after the stubs died remains free of discoloration. These discolored tissues are not heartwood.
FIGURE 2. - LOW BRANCH STUB DECAYED
This yellow birch (above) has a low stub, and the callus ridge and advanced decay show that the stub has been dead a long time. Roughened bark at the base indicates basal wounds and root wounds typical of injury caused by logging equipment.
Dissection (above) reveals discoloration and decay originating from both the basal wounds and the stubs above. The base is both discolored and decayed. The dark lines within the decay at the base indicate the limits of earlier columns of discoloration. The diameter of the widest column indicates the diameter of the tree when - the large branch died.
FIGURE 3. - HIGH BRANCH STUB, UNHEALED
This paper birch bolt (above) contains a large unhealed stub from high on the tree. The decayed stub indicates advanced discoloration and some decay. The dark drooping lines of the bark scar - chinese beard - indicate the angle and length of the stub inside the stem.
The discolored wood associated with the stub is redheart - very wet and dark, and - beginning to decay. The column of defect from the stub does not enter the older central columns of defect, but extends up and down alongside them. Multiple columns of defect like this are common.
FIGURE 4. - HIGH BRANCH STUB, HEALED
This healed stub wound on a paper birch illustrates how the position of the stub inside the tree can be estimated. The dark line of the Chinese beard shows the length and angle of the stub. The height of the stub scar tells the diameter of the stub.
Dissection shows the relationship. The length, angle, size, and depth of the stub are approximately as indicated by the external signs. Note that the stub healed before the last bit broke off, leaving a pinched-off piece; this affects the accuracy of the formula. In this bolt the discolored wood is as sound as the white wood; it is not moist (thus not redheart) , and no organisms have invaded to alter the wood.
FIGURE 5. - HIGH SMALL BRANCH STUB, HEALED
Full view of a small well- healed stub from high on a paper birch tree. The ellipse form of the stub scar (a 1 : 2 ratio) shows that the stub is buried about halfway between the tree center and the bark. Small, high, well-healed stubs like this indicate discolor- ation, but not decay.
Dissection reveals that the discolored wood in the stem is sound. The small stub was associated with a column of discoloration that formed after the branch died, when the tree was about 8 inches in diameter at this point. A central column of discoloration had formed earlier after branches died when the tree was about 4 inches in diameter. This is shown by the light-colored boundary streak in the discolored core. As other larger stubs died above, other columns of discoloration formed, enveloping the older and smaller columns.
FIGURE 6. - BRANCH STUBS IN SMALL STEMS
Dissection of these small red maple stems shows that discolorations begin early in the life of the tree. At this age, poorly healed stubs give invading organisms the advantage. A young tree that tends to heal branch wounds slowly is a poor risk for a crop tree.
FIGURE 7, - HYPOXYLON RUBIGINOSUM ON STUB
The red-brown patches on this large low red maple stub are fungus material that contains fruit bodies of Hypoxylon rubiginosum, a pioneer invader of wood. Its presence indicates that discoloration is advanced and decay is beginning. Trees that have several large low stubs like this usually have large central cores of defect. Large cracks may form under such stubs; and then the discoloration and decay processes go faster.
H. rubiginosum is one of the few non-Hymenomycetes that cause decay as well as discoloration. I t is one of the most aggressive fungi in the forest. It infects both living trees and slash.