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black-throated green warb
James W. Wolford wrote:
> Also the burls on conifer trunks were caused by a rust fungus? (or
> perhaps a virus or bacterium??).
>
>
> Cheers from Jim in Wolfville, 542-9204
>
Hi Jim & All, May 20, 2008
I think the cause of these burls is unknown. In addition to large
sometimes frequent burls on trunks they also may be abundant on branches
and especially on roots. Root burls can be at least 20 times the
diameter of the root on which they develop.
Early stages of burl development are readily detected on small roots
& branches (<5 mm diam) as elongated (2-3 times as long as wide) low
swellings and I have never seen by eye or hand lens any discolored or
distorted nucleus that might have been a starting point.
Burls are very rare on fir (I can recall having seen only one small
one), absent on pine and hemlock (?). Above ground burls are usually on
white spruce and both white and red spruce have root burls. There seems
to be a soil effect, burls being most common on poorly drained soils,
less common on excessively drained soils and least common or absent (?)
on well drained soils with ample water holding capacity.
So I suspect some localized water inbalance that leads to localized
excessive cambial activity. Once initiated this localized growth
inbalance would tend to be self perpetuating due to the greater phloem
tissue area and length. The growth rings of burls, like the rings where
two trees are growing together to become one trunk, can be ~10 times
wider than the adjacent normal tissue.
Yt. Dave Webster, Kentville
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