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I recently started to wonder what a tree would be like on an alien world, and one thing that popped in mind for these trees was their color. The hosts star and environment would have a effect on said tree, such as its color.

So, let’s assume their is an earth-like planet-it has water, plants, animals, and other things that would be associated with earth. The only major difference would be the color of the tree. Its leaves and bark would be a light shade of red, and some are even capable of growing on rocks, like some trees on earth have.

What environmental factors would be required to form a red tree?

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    $\begingroup$ The color of the non-photosynthesizing parts of a plant you get as a freebie. They can pretty much be any color as long as biology can make the pigments (or, if structural coloration you can even skip that). But as for the photosynthesis, red isn't a particularly unlikely color at all. It would mean that the plants were reflecting red and absorbing other parts of the spectrum. It goes without saying that the star in that system would be different than our little yellow dwarf. In the long term if it had a star like ours, it will tend towards green chlorophylls. $\endgroup$ – John O Mar 27 at 20:59
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    $\begingroup$ @JohnO Why? Green plants throw away most of the light that falls on them. Taking the whole spectrum into account, chlorophyll is an incredibly inefficient antenna pigment. The only explanation I've ever heard of this being an actual benefit, rather than a random accident, is that it might protect plants from sunburn, because they don't need that much mid-range energy. But that justification goes away if the planet has a thicker atmosphere or a wider orbit around an identical star. $\endgroup$ – Logan R. Kearsley Mar 27 at 23:43
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    $\begingroup$ @JohnO All but 2. There's red algae, and purple sulfur bacteria (which actually range between purple, brown, and orange). I see no reason to believe that is anything other than a local minimum dictated by the fact that green photosynthesizers were the first to figure out how to crack water for electron acceptors. $\endgroup$ – Logan R. Kearsley Mar 28 at 2:22
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    $\begingroup$ Question: "Can trees be [pick any color]?" Answer: YES. Some examples of red (and almost every other color) in the image search. $\endgroup$ – Darrel Hoffman Mar 28 at 4:01
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    $\begingroup$ This question is bizarre; the conditions for red trees existing are found in my side yard where I have several. $\endgroup$ – Eric Lippert Mar 28 at 8:34
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As others have already noted, there are already trees on Earth that have red leaves. So, clearly, Earthlike environmental factors are sufficient to produce trees with red leaves.

But, you want red to be the default color, not just a thing that shows up sometimes. Well, there are a couple of whole other branches of the tree of life on Earth for which red is the default: red algae (rhodophytes) and purple bacteria. Red algae use the pigment phycoerythrin to absorb blue light, which allows them to get more energy under water, which strongly absorbs red wavelengths. Purple sulfur bacteria use bacteriochlorophyll a or b in conjunction with various carotenoids (the same compounds that make carrots orange and give plant leaves different colors in fall), which give them colors ranging between purple, red, brown, and orange. Plants evolved later than either of these groups, via endosymbiosis with green algae (which became chloroplasts)... which may have evolved to be green precisely to take advantage of the red parts of the spectrum which were under-utilized by red and purple photosynthesizers, and thus still available in shaded environments already dominated by those organisms.

Now, in our world, green algae, and by extension green plants, figured out oxygenic photosynthesis using over-abundant water as an electron acceptor, and both algae and plants figured out multicellularity which gave them a huge advantage over bacteria, and so green photosynthesizers have come to dominate our world. But that may very well have been largely a matter of pure chance. If red or purple photosynthesizers figured out oxygen photosynthesis first (which really should be easier for red photosynthesizers, since they'll have access to more energy in the yellow-green spectral range), and/or if red or purple photosynthesizers happen to undergo endosymbiosis to produce more complex and specialized eukaryotic organisms, then you could easily get a world that is otherwise identical to Earth in gross astrophysical and geochemical characteristics, but just happens to be dominated by red plant-analogues instead of green.

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    $\begingroup$ re: underutilized spectrum, apropos of nothing this reminds me of spectrum allocation agencies like the FCC. The red/purple incumbents had a monopoly, so green out-competed them on an unused frequency! $\endgroup$ – EdwardTeach Mar 30 at 20:43
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Urban planners are quite fond of decorative trees with reddish-purple leaves, mostly cultivars of Prunus cerasifera (cherry plum-tree).

Prunus pisardii

(Purple-leaved Prunus cerasifera var. pisardii. Photo by Arturo Reina Sánchez, available on Wikimedia under the CC-BY-SA-3.0 license.)

Those trees exist on Earth are quite common in European cities.

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    $\begingroup$ Also Japanese Maples. $\endgroup$ – Logan R. Kearsley Mar 27 at 22:03
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    $\begingroup$ This doesn't really answer the question ,"what environmental factors would be required to form a red tree?" other than to imply that they can evolve in some subset of earth-like conditions. $\endgroup$ – JBentley Mar 29 at 21:23
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    $\begingroup$ @JBentley I think the implication is that the answer to "what environmental factors..." is "nothing special". That picture shows red trees living just fine in the same conditions a green tree could. $\endgroup$ – Carl Kevinson Mar 30 at 20:29
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There is a Sherlock Holmes story called "the Copper Beeches", referring to the color of the trees and not what they are made of. A tree that I often saw at the Demuth Tobacco Shop at 114-116 East King Street, Lancaster, Pennsylvania was called a copper beech.

Fagus sylvatica purpurea is a variety called the copper beech or purple beech.

leaves purple, in many selections turning deep spinach green by mid-summer.

https://en.wikipedia.org/wiki/Fagus_sylvatica1

Here is a link to a photo of a copper beech in spring:

https://en.wikipedia.org/wiki/Fagus_sylvatica#/media/File:M%C3%B8lleparken_(maj_02).jpg2

Then there is the Summer Red Maple:

An outstanding shade tree with burgundy red new foliage that matures to dark purplish green; in autumn, older leaves turn yellow while younger ones turn orange or purple; a dense broad tree that provides welcome summer shade.

https://www.calloways.com/summer-red-red-maple/3

For red color from spring through fall, plant a "Bloodgood" Japanese maple (Acer palmatum "Bloodgood"). This 20-foot-tall tree thrives in partially to fully shaded sites in U.S. Department of Agriculture plant hardiness zones 6 to 8. Its palmate leaves emerge red in spring and deepen over the growing season. Another 20-foot tree, the tricolor dogwood (Cornus florida "Welchii") has red, pink and green foliage. This deciduous tree is hardy in USDA zones 5 to 8 and thrives in shady sites with acidic, moist soil. The tricolor dogwood blooms with pink-white spring flowers, followed by red fruits.

In spring, the "Royal Purple" smoke tree (Cotinus coggygria "Royal Purple") buds with red-purple leaves that keep their color through the growing season. In summer, red and purple blossoms add even more color. This 25-foot-tall tree is hardy in USDA zones 5 to 8, where it grows best in sunny exposures with well-draining soil. The "Newport" purple leaf plum (Prunus cerasifera "Newport") also has red-purple foliage. Reaching heights of 25 feet, this colorful tree also blooms with showy pink flowers in late winter, before the red-purple new growth appears in spring. Hardy in USDA zones 5 to 9, this plum tree grows well in moist soil.

For red spring leaves with a hint of copper, plant a bronze dracaena (Cordyline australis "Atropurpurea"). This evergreen has red-copper foliage that's offset by its fragrant, white spring flowers. Hardy in USDA zones 10 and 11, the bronze dracaena grows to 35 feet tall and tolerates drought, dry soil and alkaline pH. Also reaching heights of 35 feet, the "Rubylace" locust (Gleditsia triacanthos var. inermis "Rubylace") has red, purple and green leaves that turn yellow in fall. This spreading tree grows well in a variety of soil types, sun exposures and pH levels, but it doesn't tolerate salt spray. "Rubylace" is hardy in USDA zones 3 to 7.

The "Tricolor" European beech (Fagus sylvatica "Tricolor") has red, green and white leaves that darken to bronze in autumn. This deciduous tree grows to 45 feet tall and has a conical shape. It's hardy in USDA zones 5 to 7 and grows best in moist, acidic soil. Fill a large space in the landscape with a Chinese sweetgum (Liquidambar formosana). Reaching heights of 65 feet, this spreading tree has red-purple new growth that darkens to green, then turns red and yellow in fall. The Chinese sweetgum is hardy in USDA zones 7 to 9 and grows best in moist, acidic soil.

https://living.thebump.com/kind-tree-red-leaves-spring-6634.html4

So some Earth trees do have red leaves in seasons other than fall. To create a world where every species of tree everywhere has red leaves in every season you would have to replace chlorophyll with some other chemical for photosynthyesis; but if characters only visit one region on that world there might be only a few closely related species of trees there which all have red or reddish leaves in spring and/or summer, even if most trees in most regions of the planet have green leaves in spring or soummer.

..and some are even capable of growing on rocks..."

suggests that region of the planet might be rockier and have poorer soil than most regions of it.

As for bark, apparently a lot of tree species have red or reddish bark.

One of the most famous such trees is the coast redwood, Sequoia sempervirens:

The bark can be very thick, up to 1-foot (30 cm), and quite soft and fibrous, with a bright red-brown color when freshly exposed (hence the name redwood), weathering darker.

https://en.wikipedia.org/wiki/Sequoia_sempervirens5

And some trees have much redder bark - here is a link to a photo of a much redder tree trunk:

https://pacificnurseries.com/make-your-installation-a-success-with-uncommon-trees/6

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Just spitballing, but one idea could be parasitic plants, which are sometimes red in color. One such plant on Earth is the snowplant, which subsists not off of photosynthesis via sunlight + chlorophyll, but instead leverages microrhizzal fungi that are associated with other nearby plants. This doesn't completely match your idea though as these photosynthetic plants are certainly not 'trees' with large trunks / canopy structures.

More generally, chlorphyll is responsible for green plant color. So if you can come up with another pigment that reflects red light, you can have red trees!

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The manzanita tree in Arizona has a beautiful red bark: https://en.wikipedia.org/wiki/Manzanita

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The main reason why plants are mostly green on Earth is that it's the sweet spot of energy: The perfectioning of photosynthesis is an optimization between the energy that's gained, and the energy it costs to repair the damage it causes on the way in. Our sun gives plants a significant amount of energy, such that this optimization ends up rather on the low-production end. With a similar star, red would be more high-production, so create environmental factors that reduce the energy coming down on the leaf. For instance, you could increase the photodensity of outer atmosphere layers (or inner ones, but make sure to still have an air compound that can sustain life) or make fog and clouds significantly more common. (The reasons behind photosynthesis colors are a summary of what I personally find most compelling out of everything I've read on this site so far.)

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