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That's a very interesting question. I found a link from Harvard that should explain it: https://harvardforest.fas.harvard.edu/leaves/biological

The red pigments are antioxidants that help protect the leaves from sunlight as the chlorophyll is drained. The bright colors can also attract animals to the less colorful fruits or deter insects from laying eggs on the leaves that could damage them. I never really thought about it before

My understanding is that part of it is that the colors are essentially part of the plant structure, and they are vibrant to us primates because the same colors show up on fruits when the clorophyll disappear from them. And fruit are important to us so we see the colors of clorophyll -depleted soft plant tissues as vibrant and pleasing.

>I’d always just assumed “that’s the colour of a maple leaf without the chlorophyll bits,” but I’m realizing now that that seems naive.

That's actually pretty much it. Leaves don't just have chlorophyll serving as an energy acceptor from sunlight but also other antioxidants in less concentration who accept energy from red and yellow, but also from green light unlike chlorophylls who accept red, yellow and violet light.

Here is the absorption spectra of chlorophyll A and B as well as Beta-Carotine. You'll notice that Beta-carotine absorpts quite a bit of green light.

Once it becomes fall, trees notice a change in the red light ratio during dusk and dawn and begin a process that is called "reverse clorophyll biosynthesis". Because chlorophyll is a very expensive molecule plants don't really degrade it. They save it from the leaves and suck it up into the wood, where it stays in a different form for the winter. That's why the leaves, now empty of chlorophyll, have this yellow and red color. Once the leaf loses the antioxidants as well, it becomes brown and dies. The chlorophyll will become active again in spring and will provide energy again in new leafs.

So the vibrant colors are not really a signal for anything. Just a change in light accepting molecules.

The pigments mainly serve their purpose during the mature part of the leaf cycle.

Having carotenoids (orange pigments) and anthocyanins (red pigments) on top of chlorophylls allow the leaf photosystems to absorb light in the 500-600 nanometers range (green light), which would be lost had it only be chlorophylls.

Once the mature part is over and the shorter, colder days of autumn set in, the pigments are metabolised for food / energy. The chlorophylls are metabolised first, revealing the other pigments that were already there.

That's interesting, I didn't know they pulled back the chlorophyll, thanks for the explanation!

Recently a few maples in our neighbourhood got infested with moths, one had all of its leaves eaten by mid-June. I thought it might kill the tree, but it popped out all of the empty leaf stems and simply regrew (smaller) the leaves. I always wondered what kind of energy expenditure it was for the tree to d o that, now it seems like more than I realized.

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Complete defoliation of a bonsai maple can be performed in late June to encourage a full second flush of smaller leaves in a beautiful deep red colour, the tree has more than enough stored energy to do this. It only takes around 4 weeks to get back to a full dense canopy. Not recommended every year as it does consume energy but you can do it every few years. I assume a full size maple can withstand a similar treatment, certain seems so according to your observations.

Is there a mechanism by which the pigments only express themselves after a certain time?

I ask because you see maples and other plants that go a vibrant flower red first go through a yellow phase. (You can see this when maples have a colour gradient from green through yellow to red in the canopy in early fall).

I don’t really understand how a leaf that always has a vibrant red pigment inside wouldn’t go straight to red from green.

(Pondering it: Something like a pigment that’s sensitive to Ph might make sense, if removing the chlorophyll changes the Ph balance in a gradual way.)

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Huh.

>>Not long ago the consensus was that autumn leaf colors were the result of the unmasking of the carotenoid or anthocyanin pigments, and could have no function. We now know that the anthocyanins are not un-masked, but are made.

According to this, these pigments are not there year-round at all.

I’m also wondering why the leaves which are shaded seem to go red last. One of my maples is turning red on the exposed (southeast to southwest) side first, with the leaves further to the north, shaded by the canopy, still green.

Yes, but it’s not a timed response as much as a temperature response (and admittedly, the cold always comes after the warmth, from the leaves point of view).

We know that carotenoids are present all year round and are seen in autumn because of the chlorophylls degradation. In contrast, anthocyanins are mostly produced in autumn, in response to cold stress. This could be because these pigments have a protecting effect against cold.

About carotenoid synthesis.

About anthocyanins and cold stress.

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We had a major spongy moth caterpillar problem this year, hitting mostly oak, but our crabapple and a few other trees as well. All refoliated. I had wondered about the metabolic costs and trade-off, thanks.

So, to summarize, there's a wide variety of different chemicals plants use for photosynthesis that all look different colors to us, and we just see the reds and yellows because plants are sucking up chlorophyl to store in wood for winter?

And then late leaves look brown because the leaves lose these other chemicals? Do they also store these other chemicals in their trunks over winter?

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Does have anything to do with maple syrup?

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Perhaps the evolutionary reason is on our side and not the plants. Like perhaps we evolved to pick up on those bright colors while our ocular system was evolving? Somebody smarter than me can word that in more official scientific terms. lol

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Maple sap runs in the spring, when the trees are pushing it quickly out towards the branches for new leaf growth.

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I had that same thought. I think fall colors and vibrant colors around sun set are the same. Both give calming and relaxing vibes so maybe our bodies evolved to have calming feelings around these things to prompt us to settle in. Sunset and fall are not good times to pack everything up and head on an adventure, it's a good time to pack it in for a bit.

To understand why trees are green is to understand light.

Trees are green obviously because of chlorophyll.

A leaf is green why? Because it has no use for green light because there is more energy in the rest of the suns spectrum .

As the leaf dies it no longer wastes energy of chlorophyll and withdraws it to save its resources for spring.

Death of anything including a leaf is something natural selection cannot evolve from so it’s just down to which molecules are still present and how they react to the light . Light/frequency is interlinked with all the elements and they either reflect or absorb this light accordingly so yes it’s random but perhaps this specific leaf has a more complex chemistry.

So to get to the point the yellow and red now means the leaf is now reflecting the high energy part of the light so it’s basically dead . Remembering that the Color of anything is only that way because it’s reflecting and nowt absorbing that Color .

Understanding light gives more answers to more fundamental questions than just the way trees use energy.

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Is there any noticeable effect on size, color, weight or any sort of composition of the tree when it pulls the chlorophyll back in?

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The colors actually aid in frost and UV protection. In the north as winter comes Carotenoids and anthocyanins will be produced at a higher rate, while reverse biosynthesis occurs revealing the deeper, reds, oranges, blues and purples. At least in cannabis, I can usually play with the final colors, by tweaking temps and also the pH on my feetigation, this can also change the sap and pistil color.

Size -> No

Color -> No

Weight -> Yes. It would lose multiple kg of weight because it gets rid of all it's leafs

No, that wasn't my question. Let me rephrase. You said chlorophyll is sucked up into the wood. I'm asking, after its sucked into the wood, if there's any change in the composition of the wood specifically. Especially in weight.

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Remember that colors really are in the eye of the beholder. The range of wavelengths and the number of visible colors vary from species to species. (And to a smaller degree: even from individual to individual.)

For example, what appears to be bright yellow to the human eye could be:

• pure light at a wavelength of ~580nm (e.g. the yellow of the rainbow),
• a mix of red at ~440nm and green at ~530nm (e.g. pure yellow on a computer screen),
• any mix of the former two options.

Therefore, it was not the trees that evolved to show exactly these colors, it's rather humans that evolved to see these colors in a specific way. To another species, the color of trees in fall might look quite unspectacular.

I heard the trees in the north eastern US are actually more vibrant than in other areas of the world. And one theory was that the color communicated to bugs not to eat them during this critical time when their energy production is dropping.

I forgot one exception. We have a white oak, and I've been told that they're wonderful trees to have. So I worked hard at keeping it clear of caterpillars, with some success. It partially refoliated where there was more damage, but kept on with the lightly damaged leaves. An interesting question in how much damage calls for refoliation.

The description of the origin of color from a physics and physiology perspective is helpful and accurate. I would urge caution about drawing firm conclusions from that. That does not mean that no plants evolved to reflect wavelengths that appear to animals as vivid colors. It seems to be the case for trees that the colors are a side effect of what they are really doing, but that is not a conclusion that applies to all plant colors and cannot be proven by the physics and physiology of color.