An Expanded Concept
Alex L. Shigo
Plant Pathologist, U.S. Department of Agriculture, Forest Service, Northeastern Forest Experiment Station, Durham, New Hampshire.
This publication is the final one in a series on tree decay developed in cooperation with Harold G. Marx, Research Application Staff Assistant, U.S. Department of Agriculture, Forest Service, Washington, D.C.
The purpose of this publication is to clarify further the tree decay concept
that expands the classical concept to include the orderly response of the tree
to wounding and infection- compartmentalization-and the orderly infection of
wounds by many microorganisms-successions. The heartrot concept must be
abandoned because it deals only with decay- causing fungi and it states that
these fungi grow unrestricted through heartwood after infection of fresh wounds.
The heartrot concept emphasizes descriptions of decay- causing fungi and types
of decayed wood. It describes disordered wood and events that occurred in
the past. The expanded decay concept emphasizes the order of a
compartmented tree, the order of compartmentalization, and the order of
successions. Regulation of discoloration and decay depends on
understanding compartmentalization and successions.
Tree Decay Can Be Beneficial . . .
Recycling dead organic matter essential for life of new trees; providing food and shelter for wildlife and many other microorganisms; decreasing the potential for fire. (fig. 2)
Or Destructive . . .
Reducing the strength and value of trees and wood products; decreasing the attractiveness of trees. (fig. 3)
Decay Is a Natural Recycling Process . . .
Cell walls are digested; strength of wood is reduced. (fig. 4)
Decayed wood is the RESULT of the process. (fig. 5)
Tree decay involves interactions among trees, which are the tallest, greatest in mass, and longest-lived organisms ever to inhabit the earth, and microorganisms- (fig. 6) primarily bacteria and fungi-which are some of the smallest organisms on earth.
To Understand the Decay Process it is Necessary to Understand Size Relationships.
For example, if a large wood- inhabiting bacterium-almost 3 microns long:-were enlarged to the size of a 6-foot man, (fig. 7) the man enlarged proportionally would be over 700 miles tall! (fig. 8)
And a giant redwood enlarged to the same proportions would be over 40,000 miles tall, (fig 9) five times the diameter of the earth! We must keep these size relationships in mind when we consider ways to deal with the wood-inhabiting microorganisms.
Also, An Understanding of Gradations is Necessary . . .
Microscopic, wood-inhabiting organisms and the long-lived, gigantic trees interact intimately in a long and intense struggle for survival. This struggle starts with a wound and can end with total decomposition of the wood. Many biotic and abiotic factors are involved in the decay processes. It is difficult to determine where one event or process starts and another ends.
The events and processes overlap and mingle like the colors in a large spectrum or rainbow. Where does one color stop and another start? The natural process of decay is even more I complicated and it might be more accurately likened to a multidimensional spectrum that is constantly changing over time. When we get too close to some processes, they are changed because of our methods of study and measurements.
This is why we must consider the PATTERNS of events rather than specific
events. (fig. 10)
Then, what CAN and CANNOT be done about the decay processes? The more we learn about the processes the better are our chances for REGULATING them.
*Stop the processes. We cannot stop our ultimate death either, but that does not mean we cannot live a long, healthy, and productive life.
* Prevent decay-temporarily. * Decrease the rate. * Increase the rate. *Detect it.
* Predict its rate.
* Predict its ultimate configuration. *Minimize its volume.
How effectively we can do the above depends on how well we understand the decay processes. The decay processes are not so overwhelming that they defy regulation.
SURVIVAL of any organism depends on its ability to compete effectively with other organisms for space and an energy source. To survive, organisms must live long enough to complete a life cycle. They must compete for food and space under constantly changing environmental conditions. They must respond rapidly and effectively to injury caused by abiotic and biotic factors. The response must be such that it enables the organisms to continue to survive.
Click here for larger picture
1) Spores of fungi,
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The Major Reason Why Wood-inhabiting Microorganisms Survive is that They Become Established in Succession.
THE TREE DECAY PROCESSES START WITH A WOUND-a break in the protective bark that exposes the xylem. New space and nutrients become immediately available to a wide variety of organisms-bacteria. nondecay fungi, decay-causing fungi, algae, mosses, lichens, insects. slugs. spiders. and small animals. The competition is intense. Many organisms compete, but as time passes, fewer and fewer are successful. Environmental factors- rain, ice, snow, wind, heat, cold- affect their survival. And, while the wound surface battle rages, those living wood cells that are behind the wound are REACTING to the injury and infection. The normal physiological processes give way to new protective processes. Shifts in metabolism occur. Materials that are poisonous to some organisms are formed in the tree cells. In a sense, the tree begins to form a protective chemical shield around and immediately behind the wound. (fig. 11 above)
As time passes, fewer species of organisms survive on the wound
surface. The concentration of anyone group of organisms on the wound surface may fluctuate greatly if there are temperature extremes in the seasons. But now most of the action is inside the tree. A look into the tree after a year shows that a few microorganisms surmounted the chemical barriers formed by the tree. The microorganisms either used the protective materials in the barrier as nutrients or altered these materials in such a way that they were no longer toxic. The protective materials are mostly phenolic compounds in angiosperms and terpenes in gymnosperms. Oxidation and polymerization of these materials take place after wounding. Usually, but definitely not always, the microorganisms that are the first to infect are bacteria and nondecay fungi. In some cases, decay-causing fungi are first. The microorganisms that are the first to infect are called PIONEERS. Which microorganisms become the pioneers is affected greatly by many factors-time of year of wounding; type, position, and severity of wound. The pioneers in turn affect greatly the species of microorganisms that follow in the succession. And, the species that follow will affect greatly the rate and type of wood alteration. Successions are orderly, but complex. (fig. 12)
After 4 years fewer microorganisms are active behind the wound. Sporophores of decay fungi may begin to develop. The first few years after wounding are the most important for the tree and the microorganisms. Within this time, the rate and much of the extent or limits of the infection will be set. One group or species of organisms follows another until all the wood is decomposed-a succession. But all wounds do not follow such a pattern of infection to decomposition. Most of the time the tree is effective in blocking or limiting the infection. The wound may close. The final stages of the succession may not occur. But, after the tree dies, many OTHER groups of microorganisms will begin to digest the wood. And when this happens, another succession occurs. In summary MANY species of microorganisms are involved in the decay processes. The microorganisms become established in successions. (fig. 13)
1) Hyphae of Hymenomycetes,
6) Sporophores of Hymenomycetes.
Trees Survive Wounds and Infection Because They Are Highly Compartmented Plants That Compartmentalize the Injured and Infected Tissues.
Trees have evolved over a period of 200 to 400 million years while being
under the constant stress of wounding. Even with this stress, they still
have evolved to be the largest and longest lived organisms ever to inhabit the
earth. And yet trees have NO WOUND HEALING PROCESS- healing in a sense of
REPLACING or REPAIRING injured tissues. HEAL means to restore to a previous
healthy state. It is impossible to HEAL injured and infected xylem.
Trees have evolved as highly ordered, COMPARTMENTED plants, that instead of
healing, COMPARTMENTALIZE in an orderly way the injured and infected tissues.
A coded MODEL SYSTEM for explaining how a tree is compartmented and how it compartmentalizes infected and injured wood has been developed. It is called CODIT, an acronym for COMPARTMENTALIZATION OF DECAY IN TREES. Terms such as "walls" and "plugs" are used in the model only to help present a mental image of the compartments. These terms are not meant as technical- terms. (fig. 14)
Wall 1. After being wounded, the tree responds in a dynamic way
by plugging the vertical vascular system above and below the wound. The
conducting elements-vessels in angiosperms and tracheids in gymnosperms-are
plugged in various ways: tyloses, gum deposits, pit asperations, etc. The
plugged elements complete the transverse top and bottom walls of the
compartments. Wall 1 is the weakest wall.
Wall 2. The last cells to form in each growth ring make up the tangential walls of the compartments. These walls are CONTINUOUS around each growth ring-except where sheets of ray cells pass through. Wall 2 is the second weakest wall.
Wall 3. Sheets of ray cells make up the radial walls. They are DISCONTINUOUS walls because they vary greatly in length, thickness, and height. Walls 3 are the strongest walls in the tree at the time of wounding.
Wall 4. After a tree is wounded, the cambium begins to form a new protective wall. The wall is both an anatomical and a chemical wall. This wall separates the tissue present at the time of wounding from tissue that forms after. It is the strongest of the four walls. (fig. 15)
For additional information about CODIT see Agriculture Information Bulletin Number 405
A More Detailed Look Shows That. in a Diagrammatic Way. a Tree Is Made Up of Many Rooms or Compartments. (fig. 16)
In a sense, a tree is a multiple perennial plant. Every growth ring can be thought of as an individual tree. Every new "tree" envelopes all the older trees. (fig. 17)
Each growth ring is subdivided into compartments that have sheets of ray cells as radial walls (Walls 3) and the cells that are the last to form in each growth ring are the tangential walls (Walls 2). Within these walls there are fibers, vessels, and axial parenchyma in angiosperms, and longitudinal parenchyma and longitudinal tracheids in gymnosperms. The vessels and the pits between the longitudinal tracheids keep the tops and bottoms of the compartments partially open. This is essential to maintain the vertical flow of liquids. (fig. 18)
BUT AFTER WOUNDING, THE TREE REACTS.
The vertical conducting elements begin to plug in various ways. This completes the transverse walls (Walls 1). (fig. 19)
Another close look at Walls 2 and 3 gives the impression of subdivided three-dimensional wheels. (fig. 20)
There is great variation in the dimensions of Walls 3. (fig. 21)
To the invading microorganisms, Walls 3 present a maze of obstacles. Each sheet of ray cells in sapwood contains living parenchyma cells that present a chemical as well as an anatomical barrier. The chemicals in the living ray cells are altered after a tree is wounded. The altered chemicals may be poisonous to some microorganisms. (fig. 22)
Within the compartments are the elements-vessels, tracheids-that conduct liquids vertically. (fig. 23)
In angiosperms, one typical arrangement for the vessels is a uniform distribution of similar-sized vessels throughout each compartment. This is typical for diffuse-porous trees such as maples and birches. (fig. 24)
In other angiosperms, most of the larger diameter vessels may be clustered more towards the beginning of each growth ring or compartment, while much smaller diameter ones are found at the end of each growth ring or compartment. This ring- porous arrangement is typical for oak, cherry, and locust. Many trees have variations on these basic arrangements. (fig. 25)
In gymnosperms, the compartments are filled with longitudinal tracheids that are the vertical conducting elements. In conifers, there are also parenchyma cell arrangements that form resin ducts or canals in the rays. Their position is usually near the end of each growth ring or compartment. (Shown here in green.) There may also be radial resin ducts in the rays; however, none are shown in the drawing. (fig. 26)
After wounding has occurred, the conducting elements may be plugged by a wide variety of materials that come from living cells that surround the elements-often contact parenchyma in angiosperms. Or the pits between the tracheids may close. (fig. 27)
The Classical Concept of Tree Decay
ROBERT HARTIG developed the concept of tree decay almost a century ago. At that time, decay was well recognized as a serious economical problem. In tune with the theory of spontaneous generation, scientists believed that
Decay Caused Fungi.
Robert Hartig, in tune with the germ theory that emerged after 1845, said that
Fungi Caused Decay.
This simple reversal of two words set the stage for the decay concept and, in some ways, for the beginning of the science of FOREST PATHOLOGY.
The Classical Concept of Tree Decay has Three Major Parts:
1) WOUNDS started the processes (fig. 28)
2) DECAY FUNGI (Hymenomycetes) (fig. 29) infected the heartwood through fresh wounds.
(Robert Hartig showed that the sporophores on the wound surface (fig. 31) and the hyphae associated with the wood decay (fig. 29) were the same fungus.)
Figure 31 Phellinus tremulae on Populus tremuloides
3) DECAYED WOOD resulted. (fig. 30)
Decayed wood associated with Phellinus tremulae.