Shigo, A. L. Journal of Forestry 83: 668-673 (1985)

Have past forest practices predisposed our forest to stress?  A leading researcher on tree wounding presents his case.

    Acid rain, insects, and fungi are real problems that can kill trees.  Wounded forests and starving trees are also realities and part of the total picture.  Forest decline is a many-sided problem, yet recent attention has focused on only a few factors.  The blame for diebacks and declines can not be placed on well-publicized short-term agents.  Knowing how a tree or forest dies is as important as knowing the causal agent. It is time for the whole story.
    Trees suffer more than mechanical wounds to trunks and roots.  Injuries can also be caused by soil compaction, alteration of drainage patterns, disruption of niches for soil microorganisms, disruption of non woody and woody plant species composition.  The list goes on. 
    Starving trees face limitations other than water, oxygen and other chemical elements, and energy.  Trees may also starve because of space reduction.  As storage space for energy reserves in the tree decreases, so do reserves.  Trees can starve in the midst of plenty if storage space is reduced sufficiently. 
    Dying forests are nothing new, especially in the northeastern United States.  Yet the primary causes of birch dieback, ash dieback, and many other diebacks and declines have never been clearly identified.  Investigators debate the effects of stress and environmental factors that predispose trees to additional problems.  The words stress and predisposition occur frequently in the literature.  What do these words mean, and how do they relate to the mixed ingredients of current tree problems? 
    Voltaire said: "Define your terms and arguments will be less than a few minutes."  The glossary on page 673 con

Alex L. Shigo recently retired as chief scientist and project leader, Pioneering Project on Discoloration and Decay in Forest Trees, at the USDA Forest Service, Forestry Sciences  Laboratory, Durham, NH.

tains definitions based on energy relations-more from a view of physics than pathology.  It is doubtful that readers will agree totally with them.  These terms are commonly used but seldom defined.

Predisposition and Survival
    Some disease-causing agents are virulent and can invade rapidly.  Consider the fungi that cause chestnut blight and white pine blister rust.  But in most cases, disease-causing agents injure organisms that have been predisposed to disease.  Factors that predispose an organism are usually different from those that cause symptoms later.  Measuring how much an organism is predisposed presents difficulties. 
    When a disease-causing agent injures an organism that is predisposed to disease and another that is not, the difference between the organisms becomes obvious.  The intensity of the disease often depends on the degree of predisposition. 
    Predisposition results from stress-a reversible condition-which means that energy reserves are lowered, and the stage is set for "strain"-an irreversible condition.  Energy is required to fuel the biological machinery of the tree: to build cells, maintain living functions, reproduce, and defend the tree after injury and infection. 
    Survival of all living things depends on energy, space to grow, concentrations of water and chemical elements, temperature, time, and genetic capacity to resist stress and strain.  Because trees cannot move, all these survival factors are linked.  Trees either grow on suitable sites, adapt to unsuitable sites, or die.  Because survival factors are linked, any disruption in one affects the others.

How Trees Starve

    Trees can trap the energy from the sun, but they must have a place to store


Click here for Figure 1.

Click here for Figure 2.

it.  Tree structure is built up by compartmentation.  Trees are highly compartmented, perennial, woody plants that are usually large and single- stemmed.  The nonwoody parts-foliage, reproductive parts, absorbing roots-are shed after they age to a genetically programmed point. 
    Trees have a defense system based on arrangement of wood cells, corky bark, extractives in aged wood, and the low nitrogen content of wood.  As highly compartmented organisms, trees set boundaries to resist the spread of pathogens.  Rather than restore injured and infected cells, trees separate strained parts from healthy wood by forming barrier zones. This defense system, called compartmentalization, has long- term survival benefits. 
    Trees have static protection features and dynamic defense systems.  Groups of trees also have protection features, such as their gregarious nature, spatial associations with neighbors, and asynchronous timing of growth processes.  But little is known about group defense systems. 
    A tree remains alive after injury and infection because of its ability to set firm boundaries around affected tissues.  Like many natural processes, compartmentalization can be both beneficial and life threatening, depending on concentrations.  While forming barrier zones to isolate a pathogen, the tree also decreases space used for storing energy reserves.  As long as the volume of new energy-storing tissue formed after injury and infection is equal to or greater than the volume of tissues that are walled off, the tree can survive.  The tissues involved are crucial.  Branches may be walled off, and new branches will form at new locations, and older tissue in the center of the stem may be walled off without loss of energy-storing space (fig. 1). 
    Problems develop when energy-storing tissues in the most recently formed growth rings are walled off (fig. 2).  In-

Click here for Figure 3.

juries and infections that result in compartmentalization of cells where energy reserves are normally stored can seriously affect survival of the tree (fig. 3).  The tree still requires energy to build new cells, maintain biological machinery, reproduce, and defend itself.  When a tree cannot function, it dies. 
    Reduced energy reserves may delay or prevent reproduction in some trees.  In others, like the American elm, reproduction occurs before leaves are formed; high energy demands for reproduction are met first.  Much stored energy goes for defense.  Boundaries in wood, present at the time of infection, are called reaction zones, and are made up of phenol-based materials that act as carbohydrate sinks.  Once energy is used for defense, that energy can not be used again for other tree functions.  Energy reserves decrease as storage space shrinks.  Growth is slowed, reproduction is stalled in some trees, and yet defense is increased.  The defense system demands energy to wall off tissues as it sets new boundaries-barrier zones-about the infected wood. 
    A tree could live on a single growth ring provided that no new infections occurred.  The current growth ring usually does not store energy reserves until the end of the growth period.  If a new agent arrives that causes additional stress, the tree starts its final survival tactic; it begins to wall off more and more of itself as it walls off the injured and infected wood.  This response can still save the tree, if the tree becomes smaller in mass and maintains a reduced energy budget.  Tree starvation due to reduction of storage space, while not the sole cause of tree problems, is an important ingredient that has been overlooked. 
    I believe that many of our tree problems increased when man took machines into the forest.  What followed has been primarily a taking process.  Nature never knew a stump until the advent of axes and saws.  Since the late 1940s, use of powerful, light-weight chain saws and various tree-harvesting machines has proliferated in our forests.  Concomitantly, mechanical wounds to outer young-growth rings have increased greatly.  Root rots appeared as stumps and roots were left behind.  Where trees were pruned, branches were often flush cut to the joining stem.  This pruning, one of the most injurious treatments that can be inflicted on trees, became common practice. 
    Not only have protection features of individual trees been disrupted by wounds and root rots, but group protection features have been disrupted.  Stand composition has been abruptly changed; trees planted where they do not normally grow; drainage patterns changed by road construction; microorganisms affected by altered soil conditions; and forest soils compacted.  These actions have disrupted the connections among trees.  They have made the "community tree" smaller. 
    Changes came so fast that tree adaptation lagged.  Stress and predisposition have increased.  Many trees remain alive but in a weakened condition.  Now "new" destructive agents have arrived and stress is progressing to "strain?'  Yes, insects, fungi, and pollutants were always a part of the forest, and trees always died, but something must have been different in the virgin forests.  Today, many of the large organisms on earth are decreasing in numbers-blue whales, elephant&, coastal redwoods.  In the virgin forests, even though exposed to agents that could kill, old-growth trees somehow survived. 
    Consider the defects in trees from the old forests.  Most defects were central.  The heartrot concept really means central rot.  Most central defects are associated with branch death, which is normal, because all trees (except palms) have branches, and all trees lose some braches before maturity.  But a hollow center may actually be a survival feature for an aging tree.  This hollow pipe construction gives a tree greater resiliency.  Today, most defects are in the outer rings, where they not only reduce energy storage space but damage the quality of wood.

Though Trees

    Agents that cause stressed trees to become strained trees appear to be on the increase.  Yet some trees survive in spite of problems.  It is common to see


some trees still alive and healthy after many others are killed by some destructive agent.  High vigor due to genetic resistance is one explanation.  Some individuals respond to stress much better than others.  Experiments have demonstrated that individual trees of a species can compartmentalize more effectively and rapidly than others.  By quickly setting strong boundaries, they confine pathogens to smaller spaces and this maintains energy storage space (fig. 4).

Click here for Figure 4

Click here for Figure 5

Click here for Figure 6

    A strong compartmentalizing ability is shown by trees that wall off dead branches effectively (fig. 5).  These trees keep pathogens that are in the branch from spreading to the trunk.  The longer a tree can keep its trunk free of dead spots that do not store energy, the longer the tree will live.  The converse is also true; when every branch infection spreads to become a trunk infection, the tree will not live long.  This is common in some of our short-lived trees such as aspen (fig. 6) and gray birch, whose trunks quickly fill with dead spots that cannot store energy.  Many short-lived trees are short lived because they are poor boundary-setters.  Highgrading may well have reduced the number of strong compartmentalizers.  This is difficult to prove, because we do not know how many strong compartmentalizers there were in virgin forests.  Wounds are not the only starting points for energy problems.  Direct infections of leaves and nonwoody absorbing roots can lead to space and energy reduction.  When leaves are infected by disease or infested by insects, the energy-trapping system is affected directly.  When foliage infections

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spread to twigs, the problem is compounded.  Similarly, pathogens that spread from absorbing roots into support roots can also cause storage space and energy reductions.

Stress and Strain

    Space and energy reserves in trees are linked.  As space that contains living cells decreases, so do energy reserves.  As energy reserves decrease to the point where defense systems do not function effectively, the tree becomes more susceptible to strain.  Then any agent, abiotic or biotic, that causes additional stress can lead to strain and threaten the life of the tree.   Accumulation of many seemingly unimportant injuries often adds up to a serious condition for the tree, especially when injuries are repeated. 
    Experiments indicate that columns of discolored and decayed wood become significantly larger as artificially inflicted wounds penetrate closer to older, internal columns of defect.  Width of healthy, energy-storing wood affected the spread of microorganisms associated with new wounds. 
    Our actions of the last 200 years have stressed our forests.  Many of our current tree problems have developed over the years, and there are no fast and simple remedies for them.  Some sorting is needed-trees beyond help from trees that can be saved, sites that will produce healthy high-quality trees from unproductive sites, wild natural forests from timber-producing commercial forests, agents of long-term stress and predisposition from short-term killers.  We have taken too much too fast from our forests, and we still expect too much from them.

Lack of knowledge about trees is a major underriding problem that no-

Relieving Stress and Strain
   Foresters can relieve tree stress by adjusting forestry procedures in the following ways:
    Reduce logging damage to trees and soils (fig. 7 and caption).
    Avoid pruning cuts flush with the trunk (figs. 8 and 9 and captions).
    Plant trees resistant to the spread of decay in plantations and future forests (figure 10 and caption).

Click here for Figure 7

Click here for Figure 8

Click here for Figure 9


body wants to discuss.  Is there a course in tree biology given anywhere in the world?  And why do our textbooks perpetuate myths and half-truths, such as: Frost starts frost cracks; trees heal wounds; wound dressings stop rot; heartwood is a nonreactive tissue; branches should be pruned flush with the trunk? 
    Forests in trouble now are mainly those on tops of mountains and ridges, near mining and smelting areas, in forests where trees have been planted off site for centuries, and in forests that have been cut over repeatedly- wounded repeatedly.   These include trees in selected German forests; Jarrah dieback in Australia; forests in South Korea; and U.S. trees killed by gypsy moth and spruce budworm.  Many of these trees are growing with minimal energy reserves.  They are stressed, and any new destructive agent-insects, fungi, or pollution-can easily accelerate stress to strain to death.

Suggested Readings

Glossary: Terms by Shigo

Click here for Figure 10

                                                    NOVEMBER 1985  673