Subject: Re: why are some trees evergreen?

From: Kelly McGrew <[email-masked]>
Date: 29 Aug 1995 01:20:46 GMT

>>1. Why are some trees and bushes evergreen and why are some deciduous?

Because God made them that way.

Seriously, it has to do with the conditions under which they evolved, etc. I don't know that there is a hard and fast rule to use to segregate one from the other as far as the evolutionary process goes.  It is most likely an evoutionary adaptation to meet the changing climate.

>>2. What mechansim allows some trees to stay green all year?

"Mechanism?"  The evergreens don't 'drop' their leaves but the deciduous do.  A better question might be "Why do some trees loose thier leaves?"  In temperate climates these deciduous plants seem to have evolved a protective feature which drops the leaves when the temperature declines and the days shorten.  By doing so, the tree saves it's energy for the more productive time of the year.  Evergreens have evolved a short, needle-like leaf which is more resistent to freezing, thereby being able to photosynthesize throughout the year.  The needle-like leaf could also be an adaptation to prevent consumption of the leaves & stems by large herbivores.  Several cycads, some ferns, and some palms are poisonous, another example of protective adaptation.

3. Can evergreens be found in all climates?

Yes, except for the most extreme climates in the polar regions and in arid areas.

4. Do evergreens reproduce the same way that deciduous trees do?

Yes and no.  Which evergeens:  angiosperms or gymnosperms?   There are (true) flowering trees (angiosperms) which have different species that are either evergreen or deciduous; there are non-flowering trees (gymnosperms) which have different species that are either evergreen or deciduous.  Here is a table to provide examples:

                    Angiosperm              Gymnosperm
Deciduous    Maple, Oak, etc.         Tamarisk, tamarack or larch
Evergreen    Madrona, Chinquapin   Pine, Fir, Hemlock

The gymnosperms (naked seeds) are the oldest of the two .  They include cycads, conifers, and ginkoes, among others.  They evolved in the Devonian period and gave way to the flowering plants/trees in the late Cretaceous period.  The great conifer forests of the western U.S. are ancient relics of these gymnosperm forests, although several flowering plants have also found niches within the forests.

The forests of the eastern U.S. are mostly angiosperm (oak, maple, chestnut, etc.).  This may be due to the age of the landform compared to the much younger western U.S. landform.

You should also note that there are ferns (non-flowering, non-seed bearing plants) which are deciduous and others which are evergreen.  The sword fern common in the western U.S. is an example of an evergreen fern, while the lady fern is an example of a deciduous fern.  Mosses and liverworts are generally all evergreen, some even photosynthesizing under several inches of snow during the winter (at a slow rate though).

I know that Encyclopedia Britannica has a good section on both Angiosperms and Gymnosperms, you may want to look at it so you can 
offer it as a reference to the students.

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From: [email-masked]
Date: 28 Aug 1995 20:32:26 GMT

Please e-mail any answers to the following evergreen questions.  I am working on a problem for a 9th grade biology class.

Ann Del Vecchio, PhD
Senior Research Associate
Evaluation Assistance Center-West
New Mexico Highlands University
e-mail: [email-masked]

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From: [email-masked] (Marco_Bleeker)
Date: Wed, 30 Aug 1995 02:36:10

>1. Why are some trees and bushes evergreen and why are some deciduous?

Evergreen may best be considered as the "normal" or "initial" condition.  The 
seasonal shedding of leaves is an adaptation to draught. The water-household 
is very essential in plants. Leaves evaporate water, but when there is no 
water available to the roots, the plant will dry out. Shedding of the leaves 
in a dry  period is an (evolutionary) answer to this problem.

Cold, in a way, is exactly the same problem. When the water in the ground 
freezes, it becomes inaccessable for the plant, and it is in fact the same as 
draught.

Another answer to this problem is having very small leaves, like needles, that 
don't evaporate much.

>2. What mechansim allows some trees to stay green all year?
Wrong question. You should ask what the mechanism is to shed leaves. This is 
very complex. What would happen when you place a deciduous tree in a climate 
with no seasonal differences, an all-year-round optimal climate?  I think it 
would still shed it's leaves in October. It is probably a genetically 
programmed phenomenon. Temperature and draught help to determine the exact 
moment. It is not just that the leaves die and fall off, but it is an active 
process: first some of the nutricious substances of the leaves are retrieved 
>from  them (causing autumn colors), then a scar tissue is formed at a precise 
place at the leave base.

>3. Can evergreens be found in all climates?
Yes. There are also other ways to adapt to draught. Except of course on the 
North Pole and in the Sahara [the Date Palm is evergreen tho... but that's an 
oasis].

>4. Do evergreens reproduce the same way that deciduous trees do?
Evergreen is not the same as Coniferous. A Conifer like Larix is a deciduous 
tree (with spectacular aurumn colors). Reproduction has not much to do with 
the topic of evergreen/deciduous.

Hope this helpes, bye, Marco

<[email-masked]>------------<http://www.euronet.nl/users/mbleeker/>
>-------------<Plants, Programs, Primitive creatures>------------------<
<Marco Bleeker>------------<nl-en-de-fr>------------<Amsterdam, Holland>

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From: [email-masked]
Date: Tue, 29 Aug 1995 19:25:50 -0400

In article <[email-masked]>, Kelly McGrew
<[email-masked]> wrote:

> "Mechanism?"  The evergreens don't 'drop' their leaves but the deciduous
do.  >A better question might be "Why do some trees loose their leaves?" 
In >temperate climates these deciduous plants seem to have evolved a
protective >feature which drops the leaves when the temperature declines
and the days >shorten.  

Just to clarify a point that may be obvious to most: evergreens also lose
their leaves. They don't lose them en masse as desciduous plants do, but
rather they shed and replace them all year long.

Robert S. Mowbray
Continuing Student of Biology

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From: [email-masked] (Bob Kirk)
Date: 31 Aug 1995 22:47:09 -0500

At least for montane-alpine species, it would seem logical
that at some elevation (varying of course with latitude etc), 
the "growing season" simply becomes too short and the mean
temperature too low to allow a tree to grow a complete new
complement of leaves every year.

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From: [email-masked] (Jari Oksanen)
Date: Fri, 1 Sep 1995 10:13:19

>from them (causing autumn colors), then a scar tissue is formed at a precise 

Some old information that I cannot check just now since I don't have the book: 
Walter writes in his small and nice "Vegetation of the Earth" that some 
deciduous fruit trees planted in the tropics still shed their leaves, but in 
"wrong" month, and finally the synchronizing may collapse so that different 
branches shed their leaves independently. If there are always branches with 
leaves, the tree could be called "evergreen", since evergreens shed their 
leaves as well, but not all in one time. 

The shedding of leaves is genetically controlled, but it still needs some 
timing device. Here in the North it is for many species the day length. The 
triggering time varies with geographic areas, and this may cause problems in 
planting trees long outside their original area. I saw a funny example of 
confused timing when I spent a year in Canberra, Australia. The people down 
there had planted all kind of trees of the old continent. Birch was among 
them. In the North it is one of the first to get leaves in summer, and sheds 
them relatively late. However, the trees were probably fairly northern in 
origin and so the triggering day length was long, and so birch was the 
last with leaves and the first shedding them. 

A tiny information on day length: Here in Tromsoe it is 24hrs from mid May to 
the end of July. Now it 15hrs30min and it so short that birches are turning 
yellow, and the autumn storm will reap the leaves within some weeks.
--- Jari Oksanen, Tromssa, Ruija / Romsa, Norga / Tromsø, Norge