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

Apparently the mechanism by which the deciduous habit evolved is currently
viewed as a mystery. BRBR

I think the deciduous habit is mainly due to freezing soil temperatures, hence
lack of water from the roots, rather than protection from snow load. In parts
of the tropics there is a wet and a dry season, and there also, the trees lose
their leaves in the dry season.
One of the commonest inhabitants of the boreal forest, or taiga, is the larch,
a deciduous conifer. it is true that the evergreen conifers are more suited to
snowy climates than broad leaved trees, but here is one conifer that plays it
safe.
For further details: A
HREF="http://www.radford.edu/~swoodwar/CLASSES/GEOG235/biomes/taiga/taiga.
html"Taiga/A
Iris,
Central NY, Zone 5a, Sunset Zone 40
"If we see light at the end of the tunnel, It's the light of the oncoming
train."
Robert Lowell (1917-1977)

Snow has nothing to do with the deciduous habit.

Nor is it at all a mystery why it occurs in plants.

As Iris says, it is a response to extended periods of drought.

Also most xerophytic plants are deciduous. The leaves are lost very quickly
during times of drought. Leaves should be viewed as being temporary
structures that are expendable to the plant and to be replaced with the new
growth. In some xerophytic plants, the leaves may be highly reduced in size
or modified into spines.


Iris Cohen wrote in message
...
Apparently the mechanism by which the deciduous habit evolved is
currently
viewed as a mystery. BRBR

I think the deciduous habit is mainly due to freezing soil temperatures,
hence
lack of water from the roots, rather than protection from snow load. In
parts
of the tropics there is a wet and a dry season, and there also, the trees
lose
their leaves in the dry season.
One of the commonest inhabitants of the boreal forest, or taiga, is the
larch,
a deciduous conifer. it is true that the evergreen conifers are more
suited to
snowy climates than broad leaved trees, but here is one conifer that plays
it
safe.
For further details: A
HREF="http://www.radford.edu/~swoodwar/CLASSES/GEOG235/biomes/taiga/taiga.
html"Taiga/A
Iris,
Central NY, Zone 5a, Sunset Zone 40
"If we see light at the end of the tunnel, It's the light of the oncoming
train."
Robert Lowell (1917-1977)

Iris Cohen schreef
http://www.radford.edu/~swoodwar/CLA...iga/taiga.html

+ + +
Note that this site appears a little out of date:
they use Sequoia gigantea instead of Sequoiadendron giganteum.
Also they have the age of "General Sherman" wrong.
PvR

david schreef
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.

+ + +
You are confusing deciduousness with the shape of trees?
+ + +

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

+ + +
I don't see where "mutant" comes in
+ + +

David Nicol, unlettered autodidact, Kansas City

+ + +
"unlettered"?
PvR

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

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