Hello, sci.bio.botany

I subscribe to Nature and there is a article in the current issue
about paleontology of polar forests, and the mix between deciduous
and evergreen trees.

Apparently the mechanism by which the deciduous habit evolved is
currently viewed as a mystery. This surprised me, as in 1996 I
witnessed a meteorological phenomenon -- October's freak snowstorm --
which demonstrated the difficulty that dendritic-branching trees
have if snow comes and they have their leaves.

They simply can't support the weight and their limbs snap.

Evergreens don't have this problem since they deal with snow
by being shaped so that snow falls off of them.

The discussion in the article about carbon lost due to falling
leaves vs. carbon lost to shed needles seemed strange. Trees in
a forest concern themselves with getting the most light, which
they do by being taller than each other, as best they can.

If trees needed to engineer themselves to support leaves covered
with snow, their limbs would have to be much thicker. That is the
advantage of deciduousness, for a tree that is already committed to
having a branching structure, opposed to a central trunk and horizontal
limbs. By losing the leaves in the winter, and their associated
breakage risk, a deciduous tree can send its branches much higher.

A deciduous mutant has obvious competitive advantage in any snowy
region.

I'm sure that someone who is properly credentialed could publish
an article using a mathematical model to compare the maximum height
a non-deciduous branching tree could reach in a snowy area.

In polar regions, the advantage would be more pronounced, and the
pre-deciduous tree might have to be doing some other winterization
processes already. So I suppose from reading the article, maybe
the whole snow thing is common knowledge beneath the threshold
of mentionability (is it? someone please tell me) and what the
authors were suggesting is that the deciduous habit might have
first appeared in polar forests, where it might be considered
an absolute necessity, and then spread back from the poles as
a simple advantage.

Of course the other way works too, that deciduousness confers
a simple advantage (greater height with less wood due to mitigation
of snow breakage risk) that then allows colonization of even colder
zones.

Thanks for reading

David Nicol, unlettered autodidact, Kansas City