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HOW TREES SURVIVE

TREES' DEFENSES AGAINST INJURY AND DECAY

 
HOW TREES RESPOND TO INJURY AND DECAY

 

Most of us never really get over the idea that trees live forever. We all know of trees that have "always been there," and seem no bigger now than they did when we first saw them decades ago. It always comes as a shock when a gigantic oak just dies for no apparent reason, or blows over in a storm -- especially if its trunk turns out to be only a hollow shell. It almost seems as if the laws of nature have been suspended. 

But looking at it another way, trees might seem impossibly vulnerable, since they live for many years, rooted in place. But this weakness is the source of their strength: they have evolved ways to (1) prevent injury, (2) coexist with external and internal enemies, and (3) continue to grow despite harm that they can not prevent. 

A tree's principal defense is that it is highly compartmented at every level. 
Its basic structural unit is the cell, a box or tube with its living components surrounded by a cellulose shell; however, the interconnections between cells can be shut down. 
 

  • The cells of trunks, branches and roots are laid down in a succession of yearly layers.

  • The trunks and branches are also sectored by sheets of ray cells. 

  • The leaves, twigs, and branches are adapted to be shed when they cannot be maintained. 

  • An individual tree is actually one compartment within the species. 

    It is very important to remember that rot is permanent. Decay, once started, may be limited but it can't be cured. It may be be covered over and compartmentalized -- that is, isolated and sometimes shed by exploiting the tree's compartmented structure. But trees don't heal.

BASIC CONCEPTS OF PLANT PATHOLOGY 
  
Plant pathology, like many of the other life sciences, can be both fascinating and baffling. Many of its subjects can hardly be differentiated from each other, or even seen, without a microscope. Pathologists must often analyze, under sterile laboratory conditions, problems that occur in very un-sterile natural environments. 
  
Yet one outstanding forest pathologist, Dr. Alex Shigo, has made the principles of forest pathology accessible to anyone who will pause, look, listen, and think. The concepts he teaches focus on how trees deal with the fact that they can't run from their enemies. Fossil records show that trees' structure has changed little in millions of years; and we can see that trees of all types function the same way everywhere in the world. 
  
A few terms make the overall concepts easier to understand: 

  • Stress -- a condition that threatens to injure a tree, forcing it to live near the limits of its tolerance. Examples include drought, adverse site conditions, and the drain on a tree's energy reserves caused by wounds.

  • Strain -- actual, irreversible damage done to a tree, such as top dieback. 

  • Host -- the tree receiving the injury or disease. 

  • Pathogen -- an organism (fungus, bacterium, amoeba, nematode, virus, mycoplasma) that invades a plant causing injury or disease. 

  • Vector -- an organism or force (insect, bird, mammal, etc., wind, etc.) that carries a disease organism. Often disease transmission is an accident of some other process, such as when pine bark beetles carry and spread the spores of bluestain fungus. 

There are commonly very close relationships between the hosts (trees), the pathogens, and the vectors. In a well worked-out relationship, all the parts of the system live in an interconnected web over long periods of time. In less developed relationships, the pathogen kills the host. 
  
Many plant diseases depend on openings ("infection sites") through which the pathogens can enter the tree. These can be: natural openings, such as the pores on the undersides of leaves weak spots in trees' normal defenses. Examples are crotches of branches; crevices in the bark, where the bark cambium is near the surface; and sunken, undernourished bark areas below dead branches, dying branches, roots, or twigs wounds caused by animals, insects, or man.
  
Tree problems commonly build on each other -- a weakened tree is more attractive to insects, becomes diseased more easily, and has less chance of recovering from further injuries. To make matters worse, the effects of injuries are often delayed, and it is easy to mistake the actual cause of many problems. 
  
HOW TREES REACT TO INJURY: CODIT 
(Compartmentalization of Decay in Trees)
 
  
During the 1970s, Dr. Alex Shigo began publishing the results of his research as a pathologist with the U.S. Forest Service, deepening and elaborating on concepts he had learned earlier from his mentors. His model of "Compartmentalization of Decay in Trees" (usually referred to as "CODIT") explains much about how trees' defenses depend on their highly compartmented structure. Some boundaries in trees exist normally, before any injury occurs:
 

  • Leaf abscission zones (where leaves separate from twigs in autumn)

  • Twig abscission zones (where twigs easily separate from parent branches during periods of prolonged stress)

  • Layers of dense latewood ("Wall 2," below)

  • Sheets of ray cells ("Wall 3," below) arranged more or less like the membranes between sections of an orange; composed of ray cells, which serve as conductive links between the inner and outer wood tissues.

  • Branch protection zones (in true branches). (Note that when codominant stems die, large dead spots may develop under them on the stem. This is because they have no built-in protection zone, and conductive tissues are connected to about half of the tissues in the stem below the union.) 

  • Pith protection zones (in true branches) 

  • Root periderm zones 

    But other boundaries are formed in reaction to injury: 
     

  • the plugs in vertical conductive elements ("Wall 1," below), 

  • the modifications in the layer of wood laid down by the cambium zone 

  • immediately after a serious injury; this is referred to as the "barrier zone" ("Wall 4," below). 

  • Bark protection zones (reaction and barrier zones, as in wood) 
    Compartmentalization takes place by means of four "walls," which are actually modifications of existing or new growth: 

    "WALL 1":
    A reaction zone created as a tree plugs vertical cells above and below a wound. This "wall" forms the first line of defense, protecting the tree against the spread of decay up and down through the injured trunk or branch. 
     
    "WALL 2":
    A pre-existing defense consisting of the thick-walled wood cells that form late in each year's growth layer; these slow the spread of decay inward toward the center of the tree. 
     
    "WALL 3":
    Another pre-existing defense that is even tougher than Wall 2: vertical sheets of "ray cells," which run outward away from the center of the tree; these form walls that slow down the spread of decay around the tree. Wall 3 accounts for the roughly triangular areas of decay that can be seen on the log-ends in a stack of firewood. 

    "WALL 4":
    Tree's best defense is the "barrier zone," formed in response to injury; this is new wood containing strong natural fungicides, laid down just after the injury. The protective features of "Wall 4" include......
     

  • denser wood (simply harder to penetrate than normal wood)

  • altered arrangement of cells, with fewer conductive cells (fewer natural openings for fungi to exploit), but an increase in axial parenchyma (more energy storage in threatened areas)

  • a decrease in the amount of lignin in cells, but an increase in chemical deposits within cells and in cell walls. Thus, the wood cells offer less "food," and contain materials toxic to invading organisms.  

Since "walling-off" is a process of breaking connections between infected existing wood and newer wood, protective strength comes at a cost of mechanical strength. The tree's ability to limit the spread of decay is impressive, but whatever damage is done cannot be reversed or "healed," and the boundaries lack the strength of normally-formed wood; this is especially evident in the concentric "ring shakes" occasionally found in lumber. 

Injury and the advance of microorganisms in the wood also cause formation of discolored wood in the reaction and barrier zones. This is not the same as heartwood, and discolored sapwood can not be further altered to form heartwood. 

As true branches die, trunk tissues below them slow their growth rate, while those above continue at the normal rate, producing a more cylindrical trunk; this is why open-grown trees, which retain many large lower limbs, are more tapered than trees that shed lower limbs shaded out by neighboring trees in closed stands. 

The basic rules of 'CODIT' are simple: 

  • When a tree is injured, all wood present at the time of wounding may be decayed, depending on how well the tree's defenses work.

  • Wood formed after the original injury will not decay, unless the tree's defenses are broken down as a result of further injury or other stress. 

  • Don't confuse tree wounds with animal wounds. We actually "heal", replacing the injured tissues. Trees simply wall off decay, controlling its spread long enough so that new wood added to the outside of the tree can take over the functions of the wood rotted away.

  • All trees work basically alike -- broadleaf and narrowleaf, and trees that form heartwood as well as those that don't. 

  • The best way to deal with tree problems is by avoiding them. Learn to work with the tree's natural defenses.



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