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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?

Ambo100
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apoorv020
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    Cue an anthropological global warming rant from somebody... ;) – Ardesco Apr 27 '11 at 07:58
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    I think it's more likely this theory is only valid for Europe. – apoorv020 Apr 27 '11 at 08:41
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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 09:40
  • @Konrad it's also the commonly taught in the US – Monkey Tuesday Apr 28 '11 at 00:10
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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 03:34
  • @David sort of like Konrad said, This question is a great example of prevailing US-central mentality many Americans can have without ever even realizing it. Great question. – Monkey Tuesday Apr 28 '11 at 18:56

1 Answers1

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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.

Darwy
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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: http://oleaeuropea.wordpress.com/2011/01/20/how-do-trees-survive-the-cold/ – 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' http://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
  • @Suma the solution in the cell is more concentrated not because it is drying out (which infers a net loss of water from the tree) but because the water is drawn outside the cell wall (but is still retained in the tree), where it freezes. The water is not removed from the tree; it is relocated within the tree to areas which can withstand the freezing of the water. Even in winter dehydration is still an issue with trees, which is why they don't remove water nor reduce the amount of water they contain. They simply relocate it (much like they did for the C & N) – Darwy Apr 27 '11 at 22:15
  • @Darwy: I lack the theoretical background and I was unable to find any sources, but what you write agrees absolutely with what I see in coniferous trees, but not in leafy trees. 1) Coniferous trees need to be watered before winter, as dehydration is a serious issue for them, but this is not done with leafy trees 2) leafy trees wood (esp. thin branches) seems a lot drier when cutting it during winter. I can try to measure a density of freshly cut branches to compare, but for the results you will have to wait for the next winter. – Suma Apr 28 '11 at 08:01
  • Coniferous trees need to be watered because they do not lose their needles during the winter, which means they can still dehydrate via evaporation from the needles. Leafy trees drop their leaves, and thus do not have the same problem. – Darwy Apr 28 '11 at 18:30
  • I thought one factor was the prevalence of *storms* in the northern hemisphere winter. Shedding leaves means the trees are less likely to be blown down. – matt_black Nov 28 '12 at 00:25
  • fall time in europe is a combination of danger of freezing leaves, too cold to photosynthesize, and not dehydration, there is no danger of dehydration due to permafrost at the onset of leaf fall. – bandybabboon Sep 23 '15 at 06:53