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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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