My planet has a star that peaks in the violet part of the spectrum. Its atmosphere has a purple-ish hue due to this. My planet has a 40.6° axial tilt, giving it frigid winters and hot summers. Now, the fleshy parts of plants, like leaves, are yellow, since the optimal pigment for absorbing violet light is Xanthophyll, which reflects yellow light. I want the trees on my planet to have blue bark with yellow leaves. What would be the optimal tree bark color on my planet, and is there anyway I could have blue trees?

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    $\begingroup$ What's wrong with just deciding to have trees on your planet have blue bark? Keep in mind that evolution doesn't find optimal solutions. $\endgroup$ – sphennings May 5 at 15:06
  • $\begingroup$ If it's already a planet with a different light spectrum, why not have it so that the eyes have evolved so it looks blue? $\endgroup$ – Trioxidane May 5 at 16:35
  • $\begingroup$ @Trioxidane very valid point. The very definition of "blue" becomes subjective, if the light spectrum is centered around a significantly different frequency. $\endgroup$ – PcMan May 5 at 17:59
  • $\begingroup$ By Blue, I mean that humans would perceive it as what we call "Blue," regardless of what intelligent life evolves on the planet. $\endgroup$ – asdf May 5 at 18:54
  • $\begingroup$ @sphennings has asked a good question. It's your world, there's nothing stopping you from having blue bark and moving on with your project. Are you specifically asking if it's scientifically plausible for blue bark to exist on your world? Then you'll need to provide much more detailed information about the star and your planet. But you should be aware that we know so little about what's scientifically plausible that any answer that said "no" would deserve skepticism. At best, all we could tell you is, "based on what we understand today...." Do you need a science-based answer? $\endgroup$ – JBH May 6 at 0:29

If the cellulose (or generically any substance used by the tree for making the bark) happens to be arranged in a periodic way, it might behave like a photonic crystal and therefore selectively reflect certain frequencies of the spectrum.

This is why certain feathers or bugs seem to have metallic reflexes. The same can happen for bark, in principle.

  • $\begingroup$ Not only blue but iridescent. Nice! +1 $\endgroup$ – Joe Bloggs May 6 at 13:55

So blue wood is really hard to do:

  • Copper Sulphate is a nice strong blue colour that dyes almost everything - it's also the active ingredient in those "root buster" products you put in your drains to kill tree roots getting into sewers. A literal tree poison

  • The blue / green wood you can buy at the hardware store is made by mixing Copper Chrome and Arsenic into the wood. These chemicals don't support growth - by design, it's a mould and termite protection.

enter image description here
(The blue is poison)

So I see two ideas:

  1. The tree is absorbing these chemicals from the ground and is able to do so as a natural form of protection against growth / insects / etc. I think this is a bit of a stretch - these chemicals kill trees as we know them, but these alien trees are not as we know them...

  2. Coat the bark with a a creeper / vine like plant that's predominantly blue.

Eg some variant of Wisteria blue vine

enter image description here

Or Climbing Hydrangeas:

enter image description here

  • $\begingroup$ Couldn't the tree adapt to utilize copper chrome and arsenic as a protection against mold and termites, even if only theoretically? $\endgroup$ – Alendyias May 5 at 15:22
  • $\begingroup$ @Alendyias Extremely unlikely. But I should probably include that idea. $\endgroup$ – Ash May 5 at 15:24
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    $\begingroup$ Bark is not wood; they are produced by different tissues. The color of the bark and the color of the wood are two different things. $\endgroup$ – AlexP May 5 at 15:47
  • $\begingroup$ Well then, these trees could always just lack bark. $\endgroup$ – asdf May 5 at 15:59
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    $\begingroup$ There are nickel accumulating trees with turquoise sap. No reason they could not have turquoise bark. bbc.com/news/…. $\endgroup$ – Willk May 5 at 17:33

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