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If you look at the individuals of one species, often they’re pretty similar looking.
An elephant is going to look very similar to a fellow elephant,
at least in terms of its general arrangement of limbs and stuff.
But individual trees can often have one or more growth features
that make them quite unique from their neighbors, especially as they get older.
Maybe it’s a large knotted hole where a branch used to be,
a big gnarled burl bulging out from the side,
or even a completely hollowed out trunk that makes a particular tree noteworthy.
And these unique features didn’t just grow as part of these trees by chance.
They’re actually the scars of past battles against infections
that raged on at the cellular level, sometimes for years on end.
When a tree gets wounded, say from a fire or an insect borrowing through the bark,
it’s not just that immediate damage the tree has to worry about.
Much like our own scrapes and cuts,
there’s also a risk of an infection getting into the wound and causing some serious issues.
The pathogens, or infectious agents, that you most often see with tree wood are fungi.
And these fungi can make their way into a tree through a crack or wound,
slowly decaying it from the inside out.
Luckily for the tree, it isn’t helpless against these infections.
Trees have developed incredibly successful protective measures
against the fungi that try to take them down.
Given that trees in all different environments are going to be encountering fungal foes,
it makes sense that both coniferous and deciduous trees
from around the world use these types of defenses.
That’s because plants have evolved a lot of built-in defenses,
given that they can’t exactly run away from their enemies.
Whether it’s a fir, spruce, pine, beech, oak, maple, or a plum tree,
you can bet it’s well prepared to face fungal invaders.
In response to an injury,
the tree can basically slam shut containment gates all around the wound.
The tissues surrounding the compromised area become calloused,
growing a thicker and denser woody boundary by reinforcing the cell walls in that area.
This process also produces a bunch of new cells to form an additional boundary
between the wound and surrounding tissues.
Those cells in turn grow tyloses,
outgrowths that essentially dam up the tissues.
And these tyloses are so tough against fungal decay,
they’re often left intact long after the rest of the tree has rotted away
-- if it loses the battle, that is.
This new growth compartmentalizes the infected area
and prevents the fungus from getting through the vulnerable xylem cells.
That’s the tissue that transports water –
and could otherwise give the pathogen a free ride throughout the tree.
By sacrificing the tissue that’s already in contact with the fungus
and cutting it off from healthy surrounding tissue,
the tree can pull off some significant damage control.
But the defensive measures don’t end there.
The cells that form this protective layer also produce fungicidal agents
that help keep unwanted invaders in check.
Along with the physical barrier, injured trees also produce
significantly more phenolic compounds.
These chemical compounds are found in most plant tissues,
and trees start pumping out even higher concentrations of them
in woody tissues that have recently become wounded.
Phenolic compounds are very diverse in what they can do for plants,
but in the case of those that get released after injury,
it appears their purpose is to directly fight off the offending fungi.
While the full extent of what is happening isn’t yet understood,
the compounds seem to be directly inhibiting the growth of the fungus,
stopping it in its tracks.
And that’s not all. The tree releases a whole tide of chemicals and proteins,
whose function we’re still unraveling.
Combine these defensive measures,
and you have a fungus that’s stuck in its tracks with nowhere to go.
Even though the tree-versus-fungus battle is happening at the cellular level,
the results manifest themselves in a number of ways.
Trees may develop thick growths and scars along their trunks and branches,
or have big knotted burls bulging out the sides of their branches or trunks.
Some trees, like European beech,
are prone to discoloration as a result of their battles against infections.
Even though the trees remain relatively healthy,
the resulting aesthetic effects are often frowned upon in the forestry industry
because it makes the wood less valuable.
These battles can even result in a nearly completely hollow tree trunk,
as some fungi species specialize in decaying heartwood, the inner core of a tree.
But as long as the growth tissues of the tree are protected from the fungal infection,
the tree can continue to thrive even if its core has rotted away.
Because heartwood is actually dead –
those interior cells die off as part of the natural, healthy growth of the tree.
But of course, no defense is perfect.
Some fungi can degrade the tree’s barriers to
keep eating away at the wood, eventually killing the tree.
And producing these physiological and chemical defenses is costly to the plant.
That’s why they concentrate their efforts only on wound sites,
instead of being distributed throughout the tree at all times.
It’s just not worth the energy to be in defense mode 24-7.
So the tree doesn’t always win this battle, and even if it does,
it may face some pretty dramatic physical consequences as a result.
That said, they certainly are very effective at limiting the damage done by fungal invaders.
If they weren’t, trees would probably be wiped off the planet pretty handily.
So the next time you see a gnarled, knobby tree,
commend it on its perseverance through some intense internal conflict.
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