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Offline DoctorBeaver

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Question for Stuart about trees
« on: 18/06/2007 21:00:02 »
What is the advantage some trees get from having needles rather than conventional-type leaves?


 

Offline dentstudent

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Question for Stuart about trees
« Reply #1 on: 19/06/2007 08:22:07 »
(Caveat - This answer is a generalisation, and deals with temperate zone trees)

So - there are evergreen (coniferous) and broadleaved (deciduous) trees:

Coniferous = cone bearing
Deciduous = leaf dropping

The leaves of a deciduous tree are generally broad and flat. The leaves of a conifer are generally needles, and are thin and pointy. The leaf area is the primary area of photosynthesis in any tree.

Deciduous trees tend to drop their leaves annually at the end of the growing season during Autumn, whilst conifers retain theirs all through the winter. Conifers keep their needles for several years, though this changes with species and with a trees position in the forest.

There is a strong habitat difference between the two tree types deciduous trees tend to be at lower altitudes or in latitudes of shorter day maximum day length, but where the sun reaches a higher elevation. Conifers are found more commonly in more harsh environments, where the growing season is somewhat shorter, where there is an acutely angled solar radiation and water is potentially scarce due to soil depth, freezing conditions and so on. This is perhaps the key driver of the differences in the leaf morphology.

Deciduous trees require large quantities of water in order to grow, which in winter is less dependable.  They have become highly adapted to making use of spring and summer water, and putting on their growth during this period. As they grow, waste products are stored back in the leaves, which are then removed during leaf fall. You can see these products (tannins and so on) in the oranges and reds in the leaves as the green pigmented chlorophyll dissipates. Broadleaves also have a large storage system within the trunk for carbohydrates, ready for the following years new growth.

Conifers do not have the same level of carbohydrate storage capacity as boadleaved trees, and so need to make use of this "evergreen" system. Because the needles of coniferous trees are continuously available to the tree, the tree can take advantage of short growing periods without having to firstly grow its leaves. Therefore, the conifers can strongly out-compete broadleaves in the harsher terrains. Their needles also have a waxy coating, which helps their adaptability with reducing water-loss.

There are other reasons as to why there is a generalisation of broadleaves at lower altitudes and conifers at higher elevations.  For example, conifers can easily become water-stressed in hot temperatures due to their smaller root system. Deciduous trees can become frozen at higher elevations due to the internal cell structures of the wood. But, hopefully this answers why there are leaf differences between the two tree types.
 

Offline DoctorBeaver

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Question for Stuart about trees
« Reply #2 on: 19/06/2007 22:52:48 »
Thanks for that, Stuart; very informative & comprehensive answer Can I ask, though, has does the araucaria fit into that? I believe that particular tree comes from a hot climate yet has needles.
 

Offline dentstudent

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Question for Stuart about trees
« Reply #3 on: 20/06/2007 08:50:22 »
There are a few Araucaria, and the most common of these in the UK is the "Monkey Puzzle" tree (Araucaria araucana) which comes from Chile. My field is temperate forestry, so this won't be gospel, but shouldn't be too far from the truth! Araucaria inhabit the Andes range, where there is a great deal of snow and ice in the winter, so I suspect that the premise from above still holds, in that it is reducing water loss. There are however, many conifers that do inhabit hot and dry countries - Portugal has areas of Stone Pine for example, from which we get pine nuts. This adaptation to reduction of water loss is again the principle behind their competitiveness. Other species found here are Cork oak and Holme oak - which are both "evergreen" deciduous trees. Their leaves also have the reduced surface area and leatheryness associated with reducing water loss.

Does this help?
 

Offline DoctorBeaver

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Question for Stuart about trees
« Reply #4 on: 20/06/2007 13:59:21 »
Some araucaria grow in the West Indies
 

Offline dentstudent

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Question for Stuart about trees
« Reply #5 on: 20/06/2007 14:10:09 »
Are they native there? I'm not certain that they are....Araucaria also grow in the UK and many Mediterranean countries as ornamentals. This won't truly reflect their evolved morphological response to the environment.

As I mentioned earlier, I suspect it would be a water oriented morphology.
 

Offline DoctorBeaver

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Question for Stuart about trees
« Reply #6 on: 20/06/2007 14:59:40 »
Blame Google. I did a search for "araucaria" and "countries" and it came up with "Araucaria" and "Trinidad" as a match. However, when I read the article it was about Brazil  ???

BUT Araucaria cunninghamii comes from Australia & they were in the West Indies a while ago playing cricket. So, it stands to reason!  :P
 

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Question for Stuart about trees
« Reply #7 on: 20/06/2007 15:04:08 »
C'est logique!
 

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Question for Stuart about trees
« Reply #8 on: 20/06/2007 15:59:45 »
Indeedly-doodly, neighbour
 

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Question for Stuart about trees
« Reply #8 on: 20/06/2007 15:59:45 »

 

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