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Growing up, we were regularly taught in geography about the different seasons and how most trees would shed their leaves in autumn, while evergreen trees do not shed their leaves.

However, I live in India (at about 28° N), and I saw that many trees on our campus were shedding leaves in March instead. So is this shedding leaves in autumn a valid theory, or just a gross generalization?

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Cue an anthropological global warming rant from somebody... ;) –  Ardesco Apr 27 '11 at 7:58
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I think it's more likely this theory is only valid for Europe. –  apoorv020 Apr 27 '11 at 8:41
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Great example of the still prevailing Euro-centrism. (FYI, I don’t know the answer. I was taught that “of course” trees shed their leaves in autumn. But I live in Europe …) –  Konrad Rudolph Apr 27 '11 at 9:40
    
@Konrad it's also the commonly taught in the US –  Monkey Tuesday Apr 28 '11 at 0:10
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@Monkey Tuesday: It's also what happens around here, so it tends to be taught. The US is a very big and very powerful and rich country, and people in the US tend to disregard things that work differently in other countries. –  David Thornley Apr 28 '11 at 3:34

1 Answer 1

up vote 28 down vote accepted

The act of shedding leaves is a response to an environmental pressure (dehydration). In the northern hemisphere trees 'recall' the nitrogen and carbon present in their leaves in the fall in preparation for winter, to prevent dehydration of the tree during the dry period(s).

For your case in India, the 'dry season' is between March-May, and as a result the trees recall their C/N at that point.

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The winter preparation is only partly because the winter is dry - another factor is the water is frozen. The trees therefore not only shed leaves before the winter, but also make their wood dry to prevent the water being frozen, which would harm the wood. –  Suma Apr 27 '11 at 10:21
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Trees don't 'dry' their wood. They have ways of adapting to the formation of ice crystals: oleaeuropea.wordpress.com/2011/01/20/… –  Darwy Apr 27 '11 at 13:36
    
But what do deciduous trees do when the environment is constant throughout the year, like in the Amazon? AFAIK they would randomly drop leaves as this process is also used to remove toxins / waste by-products. –  Skizz Apr 27 '11 at 17:04
    
Even in the Amazon there is a relative 'dry season' lba.inpa.gov.br/conferencia/apresentacoes/apresentacoes/207.pdf The time window of shedding is dependent on the type of tree as indicated in the previous study, and refoliage is also species dependent. –  Darwy Apr 27 '11 at 17:30
    
@Darwy: Really great article. Still, it seems the trees (at least some of them) do reduce the amount of water contained, besides changing the sap (is that the right word?) composition. "These two strategies (less intracellular water to freeze and more concentrated solution within the cell) are the main ways most plants seem to survive really, really cold temperatures." Everyday experience agrees with this: when you cut trees when they are without leaves, the branches are lot drier, containing very little sap. –  Suma Apr 27 '11 at 18:26

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