By Michael Snyder
How Do Trees Heal Wounds on
Trunks and Branches?
Somehow trees put up with all manner of
injury and assault during their lives. They
have to: they are rooted in place and cannot
move to avoid injury. Whether it’s ice- or
wind-stripped branches or dings from the
lawnmower, trees are quite commonly beat
upon. Indeed, a mature, healthy forest tree
might easily have had a thousand wounds
– wounds that have the potential to expose
the inside of the stem (and thus the rest of
the tree) to bacteria and fungi, which can
lead to disease, decay, breakage, and death.
In order to survive, trees must overcome
their injuries. But technically they don’t
heal their wounds, at least not the way that human and animal
bodies repair, restore, or replace damaged cells or tissue. Trees
are built in layers of cells that are bound by rigid walls in a
modular, compartmented way. This structure dictates their
During each annual growth period, trees build their trunks
and branches outward from a layer of actively dividing cells.
Increments of new wood are added in a cone shape, enveloping
the previous year’s smaller, cone-shaped increment. Picture
stacked traffic pylons. Thus, trees grow ever upward and outward, in front of themselves, both in length and in girth.
When a cell is damaged, a tree cannot go back and fix or
replace it. But it can limit the damage from any given injury by
containing it and excommunicating it from the rest of the still-growing tree. The trick is in sealing, not healing. The focus is on
resisting the spread of damage – especially infections of bacteria
and fungi and the decay they cause – by isolating the wound and
then growing beyond it.
Trees close wounds in two separate processes that create
both chemical and physical boundaries around the damaged
cells. First, they produce what is sometimes called a reaction
zone, altering the chemistry of the existing wood surrounding
a wound and making it inhospitable to decay organisms. Then,
they build a barrier zone to compartmentalize the injured
tissue with new tissue called “callus” or “wound wood” growing
outward. If all goes according to plan, the callus growth covers
and seals the wound and allows new uncontaminated wood to
grow over and beyond it.
Unfettered by bark pressure, the responding callus cells on
the edges of a wound grow freely and form elongated rolls.
These are the “ribs” of new growth you see incrementally
enclosing wounds, such as on an increasingly less visible branch
pruning stub. This new growth separates the wood present
during the injury from the new wood formed after. The rate and
effectiveness of this response differs by tree species and health.
Both functions, the chemical and the physical, are necessary
but they occur somewhat independently of each other. Rapid
wound closure on the outside of a wound does not necessarily
indicate that the internal reaction zone has successfully thwarted
the spread of an infection.
Understanding the workings of wound response in trees
underscores another difference between the way the process
works in trees versus people. Whereas we may do well to
slather a cut with anti-microbial ointment and cover it with a
bandage, this is decidedly not helpful to a tree. Applying paint
or tar or other dressings and fillers – while a great temptation
to tree lovers everywhere – actually interferes with the normal
progression of a tree’s wound response and should be avoided.
Trees need to seal and close, and generally they do this much
better without additives.
It’s not a perfect system and decay abounds in all healthy
forests. But when trees are able to compartmentalize wounds
and contain them with new growth, infections remain localized
and do not spread to existing undamaged, uninfected wood.
Given just how prevalent wood rot is in trees – even otherwise
healthy, seemingly defect-free trees – it becomes clear how well
this tree injury defense system works. Wounds remain encased
and the trees simply grow around them.
Every wound ever suffered remains within a tree, but while
they may not heal, most trees do get closure.
Michael Snyder, a forester, is commissioner of the Vermont Department of Forests,
Parks, and Recreation.
Wounds will always remain within, but trees compartmentalize these injured areas to
prevent decay and allow new growth to continue outward.