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
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
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
CONCEPTS OF PLANT PATHOLOGY
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
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
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
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
(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
Leaf abscission zones (where leaves
separate from twigs in autumn)
Twig abscission zones (where twigs
easily separate from parent branches during periods of
Layers of dense latewood ("Wall 2,"
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
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
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,"
protection zones (reaction and barrier zones, as in
Compartmentalization takes place by means of four
"walls," which are actually modifications of existing or
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
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.
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
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
"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'
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
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.