david wrote in message ...

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.

It is well to remember that deciduous trees in the polar regions
persisted during the geologic past when worldwide temperatures were
much warmer than present. Snow--ergo, cold snowy weather in
general--was not the major problem deciduous trees had to overcome.

Here is the abstract to the Nature article in question, by the way,
from http://www.nature.com/cgi-taf/DynaPa...e01737_fs.html
:

"Fossils demonstrate that deciduous forests covered the polar regions
for much of the past 250 million years when the climate was warm and
atmospheric CO2 high. But the evolutionary significance of their
deciduous character has remained a matter of conjecture for almost a
century. The leading hypothesis argues that it was an adaptation to
photoperiod, allowing the avoidance of carbon losses by respiration
from a canopy of leaves unable to photosynthesize in the darkness of
warm polar winters. Here we test this proposal with experiments using
'living fossil' tree species grown in a simulated polar climate with
and without CO2 enrichment. We show that the quantity of carbon lost
annually by shedding a deciduous canopy is significantly greater than
that lost by evergreen trees through wintertime respiration and leaf
litter production, irrespective of growth CO2 concentration. Scaling
up our experimental observations indicates that the greater expense of
being deciduous persists in mature forests, even up to latitudes of 83
°N, where the duration of the polar winter exceeds five months. We
therefore reject the carbon-loss hypothesis as an explanation for the
deciduous nature of polar forests."

Fossil Leaves And Seeds From West-Central Nevada
http://mywebpage.netscape.com/saline...iddlegate.html