[♩INTRO].

In the northeastern US, you know it's autumn when deciduous trees start sporting stunning yellows, oranges, and reds. But in Europe, fiery orange and red leaves are less common, so autumn has a more yellow hue.

Now, we don't know exactly why this is. It's a question that has perplexed botanists for centuries. In fact, they're not even sure why red fall colors evolved in the first place.

But they have two main hypotheses for why only certain parts of the world get to see autumn's finest displays. Deciduous trees are those that drop their leaves in the fall to help them conserve energy and survive harsh winter weather. But before dropping the leaves, trees spend a few weeks re-absorbing nutrients from them.

It'd be a shame to let all the perfectly good stuff in those leaves go to waste. This includes breaking down chlorophyll, the pigment that makes leaves green. As the green disappears, you're left with yellow-to-orange xanthophylls.

Those yellow pigments were there all along, but you couldn't see them for the green. At the same time, however, some trees start making new pigments: specifically, orange, red, and purple anthocyanins. And we're not sure why.

If you're about to dump those leaves, why invest energy in turning them red? These pigments are more often seen in trees native to eastern North America and to parts of Asia and South America as well. To be clear, we're talking about genetic differences.

A given species will put out the same fall colors wherever it grows. We should also note that the number of native species with red leaves is actually similar across North America and Europe but the really vivid species are more abundant in North America. The mystery is why only some species turn red.

Hypothesis number 1 says it's all about photoprotection. That is, that red pigment is acting as a sort of sunscreen to protect the leaves from light damage. See, chlorophyll's normal job is to absorb photons of light.

It absorbs their energy to make food for the plant. As chlorophyll degrades, though, those energetic photons are left to bounce around inside leaf cells. Unchecked, they can damage the enzymes that are needed to recover precious nutrients.

Red pigments are especially good at absorbing stray photons and mitigating that damage. And they're even more effective at lower temperatures. So according to this hypothesis, red pigments help trees absorb more nutrients in the limited window before the leaves drop.

One 2019 study found that, compared to Europe, eastern North America has harsher light conditions in autumn and is more prone to cold snaps. The northeastern US also has a shorter growing season. So it could be that red pigments gave trees there an evolutionary advantage because it helped them squeeze out a few more nutrients from their leaves as days got colder, even as the light stayed harsh.

Europe has had fewer of these selective pressures, so leaves stayed yellow. Trouble is, the evidence for this is kind of mixed. On the one hand, trees do seem to intensify red pigment production in response to cold, sunny days.

But sunlight and temperature observations don't explain why you also see colorful displays in Japan, where the light is less intense but less so in the western US, even though the light there can be harsh. Plus, just because cold, sunny days trigger red leaves doesn't mean red leaves are there to deal with cold, sunny conditions. At least... that's what the proponents of hypothesis number two say.

According to this hypothesis, the red is actually a form of aposematic or warning coloration. This one builds from the idea that plants often use colors for signaling. Red leaves may be a kind of warning signal to insects that the leaves won't be around for long, or that the tree is prepared to defend itself.

Studies have found that some insects prefer green leaves, and will grow better when they eat them. And there's some evidence that trees that have evolved red leaves may be more resistant to pests year-round, though not all experts agree. The big question with this one is why New England trees would need to signal to bugs more than others.

Some think it's historical basically, there used to be red trees in Europe, but thanks to geographical barriers during past glacial periods, they went extinct. In North America, the mountain ranges run north to south. As ice sheets came down from the north, tree species were able to spread southward into warmer climates.

But trees in Europe couldn't do this. Their path to the south was blocked by the Alps, an east-west mountain range that was also covered with glaciers. So most of the trees with red leaves would have gone extinct.

At the same time and for the same reason, some insect pests may have also gone extinct, which meant European trees would have had less of a need for signaling. But some parts of the signaling hypothesis are hard to test experimentally. For example, if the bugs that drove trees to evolve red signaling are extinct, there's no way to test their color preferences.

Ultimately, we'll need more experimental data to know which hypothesis is right or if it's something else entirely. And they're not mutually exclusive. We might even find that both have their roles to play.

And it's worth pointing out that most of the research here has been done in the northern hemisphere. There could be more lessons to learn from looking at the rest of the world. Until we get more evidence, we'll just have to sit back and enjoy the show.

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