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Hi Peter & All, Dec 19, 2006
That is a many sided question. There may be some anti-fungal agents
present in some bark. Some barks for example are rot resistant and used
for mulch or remain more or less intact for many years in forest litter.
But the primary barrier to invasion via bark is likely physical. The
outer bark is dead, as you supposed, and until the outer bark starts to
erode has as many annual rings as there are on the wood side. Only these
tissues are dense with almost no intercellular spaces (except in
lenticels), walls are suberized or thickened and firmly glued together, etc.
In contrast, a common mode of entry for wood-rotting fungi is
through wounds; broken tops, broken branches, torn bark, insect holes
and pruning cuts. This invasion usually (always ?) happens when the
trees are live and may eventually lead to death but it is not uncommon
to see e.g. large hollow but partly living hardwood trees.
In many trees there is a collar at the base of branches where the
wood of any one year of trunk and branch merge as a contorted bulge or
collar. When pruning is done with hand powered saws it is natural to
prune beyond this collar. But with chain saws, some are tempted to do a
'good' job and cut at the base of the collar rather than beyond it.
This increases the chance of fungal invasion because cutting at the base
of the collar exposes the wood of the trunk as much as a deep notch would.
An additional barrier to direct invasion of intact above-ground bark
is moisture. Even dead bark on a standing dead tree usually rots from
the inside out; the outside is dry too much of the time. The outside of
bark, of a similar dead tree lying on the ground such that the bark
remains moist will soon develop a flame of fungal hyphae.
I suspect that many fungal infections of heartwood are via broken
roots, especially roots near the surface, but do not know this for a fact.
Yours truly, Dave Webster, Kentville
Peter Payzant wrote:
> Here's a question for the plant biologists: A dead tree soon develops
> a garden of various kinds of fungi on its bark. This doesn't happen to
> living trees, as far as I know (although lichens, liverworts etc. do
> attach to them).
>
>
>
> The outer surface of the bark of a tree appears to be made up of dead
> tissue. How is the tree able to fight off infection by external fungi
> when it is more or less surrounded by dead material? Is it perhaps a
> matter of attacking the fungal "roots" when they penetrate the park to
> the living tissue?
>
>
>
> Peter Payzant
>
>
>
>
>
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