Why Leaves Fall Off Trees

Discussion in 'Woody Plants' started by paxaran, Dec 1, 2006.

  1. paxaran

    paxaran Member

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    Hello,

    I am a novis here, but I cannot find a complete explanations as to why leave fall off for winter that include some observations that I have made in my life and which I think make perfect sense in addition to all the other common reasons they cycle every year. Perhaps someone here can help me? Thanks,

    Robert
     
  2. smivies

    smivies Active Member

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    Leaf drop, especially in a seasonal sense, occurs primarily because environmental conditions do not suit the continued expenditure of resources to maintain the leaves.

    If we look at a temperate climate, the plant's metabolic rate drops in the winter, light is reduced, freezing temperatures, drying winds, etc. For a species to maintain it's competitive advantage in a harsh climate, it made sense to evolve deciduous leaves. Paper thin leaves are 'cheap' to produce, produce carbohydrates for the best 8 months of the year, then fall. No winter damage, no heavy duty 'expensive' leaves all for the very lean winter months (in terms of metabolic rates).

    As one moves to warmer climates, it makes more sense to evolve evergreen leaves. Less winter damage & more carbohydrate production due to warmer conditions. As well, the leaves need not be as durable because of the warmer winters.

    Even evergreen broadleaf trees will drop leaves though. Two reasons I can think of...i) even leathery evergreen leaves have a usable lifespan, becoming more damaged as they age, and ii)as the plant grows, the older leaves are shaded by the newer leaves and no longer provide enough energy production to justify their retention.

    Climates with seasonal precipitation changes (monsoonal) rather than temperature changes have the same influence on leaf drop. It doesn't make evolutionary sense to retain leaves that cost the plant more to keep than to drop.

    The same principals apply to conifers but the needle is much better adapted at protecting against harsh conditions (dessication, freezing, etc.) than the broadleaf species are. Because of this, conifer needles are retained in climates where virtually all broadleaf species are deciduous.

    This is certainly not a comprehensive explanation but a short summary.

    Simon
     
  3. Michael F

    Michael F Paragon of Plants Forums Moderator 10 Years

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    Simple answer:

    Gravity

    :-)
     
  4. paxaran

    paxaran Member

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    Wow, thanks for that response. I agree with everything you said, but I think I have another reason to add and that reason explains why those trees like pines have some of the properties that they do. In high-school about 2 hours north of NYC, in my sophmore year, we had freak snow-storm very early in the season, around October 7 or something. Needless to say, all the leaves were still on the woody trees (Oak, Maple, etc.). So the snow landed on the leaves. What happened was amazing- hundred year-old mammoth trees were practically destroyed. Young trees on the same type were also affected. Branches up to a foot thick were snapped by the weight of the heavy snow like tooth-picks. By the end, it looked like a major hurricane had hit. So I came up with this hypothesis that ANOTHER reason that leave FALL of trees rather than just dye and say, NOT fall, is to reduce the organisms surface area. This would avoid things like that described above and also damage from strong winter winds- as we know, tree wood is signifigantly more brittle in the winter as most of the sap moves to the roots. This augments my theory (also, but retreating to below ground, the sap is less likely to freeze and thus damage cells due to crystalization other damage that can be accounted for from freezing. Now this brought me to my next theory by asking the question "why does this not affect pines". My answer is that pines, are flexible and I guess they have a sap that does not freeze (seems in my own experience to be almost tar-like.... ) and allows branches to flex. This combined with the tiny/ thin needles the point out and their overall shape (like a roof top) allow the pines to withstand the weight of snow and the power of winds and freezing.

    I have had these thoughts in my head for the past 15 years and would think about it and how beautifly simple and yet complex this reasoning is. I have tried to find this description many times before, but have never read anything that reflected my thoughts. So this is the first time I am writing them down and I am hoping that you or someone out there might be able to validate this for me. Is this common knowledge? If not, what do you think? Does this explanation ring true?

    Thanks again. I am just a novice interested in the subject.
    -Robert Turner
     
  5. paxaran

    paxaran Member

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    Oh, so just wanted to add in case I was not very clear- that leaves fall to avoid specifically the damage cause by heavy snow. The other stuff is just other augmenting ideas that are based on what little I do know about trees in this catagory.

    -Thanks
    Robert
     
  6. M. D. Vaden

    M. D. Vaden Active Member 10 Years

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    I don't know if I'd phrase it that way.

    Many trees happen to drop leaves before the snow, but I'm not sure if they "avoid" keeping leaves during snow.

    Many trees that loose leaves, form an abscission layer where the leaf will separate.

    There are two things I recall as being factors. One is the weather. The other is the length of the days shortening.

    With some plants, their stages of transition can be delayed by artificially extending day-length with lights.

    As soon as day-length is reduced, changes can begin to occur in the plants, or continue to develop.
     
  7. smivies

    smivies Active Member

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    Many deciduous trees do retain their dead leaves well into and through the winter. Several I can think of right away are, Q. robur, Q. acutissima, Q. imbricata, Castanea mollissima, Fagus grandifolia. They are not firmly attached mind you, and will break away if the area suffers through an ice storm or similar.

    Likely, the evolution of a deciduous habit in a harsh climate negated the need to develop extra strong or limber branches to handle the weight of snow and ice. Evergreen broadleaf trees in temperate climates have adapted to snow loads, such as the somewhat flexible Ilex (hollies).

    As for conifers, the species represented in northern areas with heavy snow loads and frequent ice storms have adapted the flexible branches you speak of. Conifers native to southern climates are not so lucky and remain relatively brittle in comparison.

    One has to determine what the major evolutionary force was and what was just the gravy? Did deciduous trees evolve the ability to drop leaves because of snow loads or because of the high overhead in maintaining leaves over a non-productive period?

    Simon
     
  8. Michael F

    Michael F Paragon of Plants Forums Moderator 10 Years

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    Probably the latter, since tropical/subtropical dry-season leaf shed likely has a far longer evolutionary history than cold-season leaf shed. But it may well vary between different genera; deciduousness has evolved on numerous occasions in numerous different families and genera.
     
  9. paxaran

    paxaran Member

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    Very interesting. You all are much more versed than I am, but I was going on my observations. The "chicken or the egg" point you made ("One has to determine what the major evolutionary force was and what was just the gravy? Did deciduous trees evolve the ability to drop leaves because of snow loads or because of the high overhead in maintaining leaves over a non-productive period?") is a problem with the snow theory. But if you corrolate the branch hardness/brittleness if trees that keep leaves vs those that don't and even bring in your (smivies) own point that southern conifers are not as lexible and can therefore not support high ice and snow loads. I also know of another plant that is in my back yard- I have no idea what it is called but I will take a picture (it has big, thick shiny green leaves but has white smudges on them almost like dripped paint)- that never loses its leave but that is so flexible that you could bend the thing to the ground and it would just pop back. The evolution of small leaves and flxibility (and it seems larger leaves and ultra flexibilty) that can stay on limbs in combination with the fact that trees with no flexibilty have to lose leaves I believe makes my point a bit more clear. Both types of trees had to adapt to snow in order to survive in temerate climates. One loses its leaves and the other keeps them but has braches that won't break under snow and ice.
     
  10. oscar

    oscar Active Member

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    I just thought i'd throw this in from Garden Plants of Japan.
    About Japanese cedars
    "The genus Cryptomeria consists of a single species, with distinct and geographically isolated varieties. Even in Japan there are natural differences between trees that grow on the sea of Japan side, which receives much snowfall, and the Pacific Ocean side. Trees growing wild in the area of heavy snowfall have downward pointing branches that allow snow to slide off, while trees growing in the area of less snowfall have horizontal branches".
     
  11. globalist1789

    globalist1789 Active Member

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    I don't see mentioned here that the dropping of leaves, regardless of species, also serves as a means of waste disposal. Metabolic waste (such as ammonia) accumulates in the leaves of all plants and discarded along with the leaf. This is why evergreens still need to shed.

    So, in short, shedding leaves is the way plants pee...

    M.
     
  12. paxaran

    paxaran Member

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    Well, I guess there are many reason why trees shed their leaves. I was mearly pointing out that another reason might be to avoid damage due to heavy snow. As I mentioned when I first started this thread, I witnessed a very early snowfall on trees still a month away from losing their leaves and the giant old trees were totally destroyed by the weight. They survived of course, but man, what a site.
     

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