Eli5: What causes leaves to fall off trees?

Submitted by ShookeSpear t3_ygmsja in explainlikeimfive

Obviously gravity, as well as the leaf dying as the tree moves into its dormant phase. This morning I was outside before the sun came over the horizon. It was pretty windless, and cold, and the Norway maple in my yard was holding its leafs. 10 minutes after the sun hit the tree, leafs started falling down very regularly, as if it was snowing.

My theory is that there is a small scale freeze/thaw cycle that encourages the leaf connection to weaken, causing the snowing effect I witnessed. I’d love to learn more!

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qwertyuiiop145 t1_iu9wlh4 wrote

When temperatures drop and the days shorten, trees detect that and produce a plant hormone called ethylene. Ethylene signals the leaves to stop growing, break down their chlorophyll and send nutrients back to the tree, then die. At the same time, a plant hormone called Abscisic acid (ABA) starts to form an abscission layer—a small area at the base of the leaf that’s designed to break easily and cleanly so that the leaf drops without breaking anything in the twig. Once the tree has retrieved any nutrients it can from the leaf and the abscission layer is complete, the leaf will drop without much force. A gust of wind or a change in temperature or a little rain will snap off the dead leaf on the breakable abscission layer.

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mamcita_becket t1_iubhyde wrote

Great reply thank you! My 3 year old asked me the other day and I didn't know the answer... can't quite say all that to him haha but at least now I know I guess!

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qwertyuiiop145 t1_iubvst7 wrote

Let me try a 3 year old version:

-Trees with big leaves go through cycles every year where they lose their leaves in the fall and grow new ones in the spring. Trees with skinny needles instead of big leaves don’t do this.

-The winter weather is too cold for the leaves, so the tree doesn’t need leaves in the winter

-The tree makes the leaves drop off because by the time winter is over, the leaves would be too damaged to use and the tree would need to make new leaves anyway.

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Practical_Chef497 t1_iuat3vq wrote

Is there an evolutionary advantage to leaves changing colors in the fall?

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qwertyuiiop145 t1_iub39vy wrote

Trees change color because they are taking back the green chlorophyll, leaving behind only the yellow/orange/red pigments behind. It’s part of the process of going dormant for winter.

It is expensive for trees to operate in the winter in temperate climates. There’s less sun to use, there’s less water available since trees can’t access water that’s frozen solid. To keep leaves working through the winter, trees would need to produce something to prevent freezing and continuously repair damage while getting a reduced benefit from the leaves. Keeping leaves is also a risk when there’s snow or freezing rain because the leaves catch the precipitation and that can cause the tree to break if the load becomes too heavy.

Pine trees and other evergreens get around this problem with a combination of different adaptations. The needle shaped leaves lose less water and catch less snow. The branches are arranged in a way that lets snow roll off more easily. The sap has components that serve as antifreeze. Combined, these let evergreens keep operating through the winter.

Deciduous trees instead maximize efficiency in the warmer months and go dormant through the winter. Deciduous tree leaves and branches maximize the amount of sun they get with big flat leaves and branches that spread far from the trunk and as high up as possible. This lets deciduous trees take full advantage of the sun’s energy when it’s strongest.

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Practical_Chef497 t1_iujtow6 wrote

Thank you for that; i understand the need to drop off but turn red and orange; why not turn from green to black then fall off?

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ShookeSpear OP t1_iuatzhc wrote

Hypothesis, the water present in the leaves would be extremely susceptible to the freeze thaw cycle, causing them to burst and explode during the winter. It would be much more difficult to repair that damage than it is for a tree to bud out new leaves.

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apple-masher t1_iu9nsbq wrote

At the base of each leaf stem (petiole) there is an area called an abscission zone. When it's time for the leaf to drop, enzymes are produced in that zone which digest and dissolve the stem, until it breaks.

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iDarick t1_iu9bbum wrote

Trees conserve themselves for the winter. Leaves store energy. You want all your beefy fat to go dormant and expect further survival. Trees consume power sources and take a nap after.

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na3than t1_iu9de9g wrote

That doesn't answer the question. What causes an attached leaf--one in which there is a bond between the leaf stem and the branch--to become an unattached leaf?

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iDarick t1_iu9dn10 wrote

On nature you drop off what's unnecessary for survival process. So the leaf do it's job and leaves.

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na3than t1_iu9dr3h wrote

That still doesn't answer the question. What causes an attached leaf--one in which there is a bond between the leaf stem and the branch--to become an unattached leaf?

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iDarick t1_iu9e0t0 wrote

The mechanism is called abscission - a layer of cells in the stalk of the leaf swell up and die, weakening the stalk so that the wind will dislodge the leaf. Before this happens, the tree draws resources out of the leaf (senescence), which causes the change in colour from green to browns and reds.

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iDarick t1_iu9e3ra wrote

Some folks have no chill huh

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iDarick t1_iu9e91q wrote

I thought that's ELI5, not biology sub mate, sorry for that.

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ShookeSpear OP t1_iu9foqt wrote

I suppose I chose the wrong sub then. I understand the conservation of energy and nutrients required by a tree for the dormant phase of its growing. I was looking for your very specific answer about abscission. So thank you for that!

My theory relating to the sudden heating of the petiole does make it seem like I have at least a 6 year old’s understanding of tree biology, so probably the wrong sub…

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iDarick t1_iu9pt3u wrote

u/ShookeSpear

Thank you for saying that, really, I'm a little autistic (diagnosed) and was a little upset/frustrated. To me I gave the answer my niece would easily grasp (please see no offense). I'm really trying to help people in every way because this is one of the few things that brings me true joy. And I'm at war right now. I need some joy.

Again, positive vibes only in the message I'm trying to convey.

I'm happy I helped you out and gave a prebuilt answer for you to use with kids :)

For some reason I can't reply to your comment

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ShookeSpear OP t1_iua3iab wrote

Hey friend, it’s all good. Tone and intention can be difficult to read via text. Just know you didn’t upset me, and I appreciate the answer you gave!

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