Tree Shape on a Tidally Locked World

Question

I'm designing a tidally locked world around a red dwarf star. The habitable ring is an Earth analogue. The atmosphere and gravity are similar to Earth's, and we're going to pretend the wind is negligible.

Initially, I wanted to make concentric rings of Earth-like habitats: tropics near the day-side, temperate deciduous forests at moderate temperatures, and snowy, twilight forests near the night-side. However, since the star will always be in roughly the same place in the sky, I imagine trees will evolve differently.

• If the location of the sun is constant, will branches take a specific shape? (i.e. "solar panels" of leaves?)
• Since the angle of incoming light depends on latitude, will trees develop at different angles perpendicular to the sun's rays depending on location?
• Objects cast really long shadows at sunrise/sunset on Earth. Could dense forests even exist on a world where the sun is always low in the sky, or would the light be blocked?

These are my initial thoughts, and they really throw a wrench in my plans for Earth-like forests. I don't care about the biochemistry of fauna on my world just yet - I know the light emitted from a red dwarf star will lend itself to different photosynthesis. Right now, I only care about morphology.

What shape would trees have to take to survive on a tidally-locked world?

Answer 1

This is an interesting observation which I don't recall anyone making before!

A world where the sun appears to stand still in the sky (the planet is tide-locked and its orbit is circular) will indeed look very different from Earth's plants.

On nearly all terrestrial plants a leaf will remain green and healthy only if it gets enough light. A leaf that fails to get enough light will yellow and die. Evolution has driven this -- it makes perfect sense for a plan to lose unproductive foliage -- and evolution has also driven plants to have branching patterns and leaf patterns which are efficient at catching light and generate few unproductive leaves.

Because the sun moves through the sky and because there is significant diffuse light from the blue sky itself, Earth's trees tend to have fairly unpatterned branching and leafing patterns.

You can count on evolution on your tide-locked plant operating the same way: the leafing and branching habits of plants on that world will tend to minimize wasted growth. But evolution is based on the selection of random underlying events, so you can never count on evolution finding exactly the same result on another planet.

It's also worth remembering that other factors besides solar conversion efficiency are important, such as temperature (trees with resin-filled needles thrive in colder temperatures than broad-leafed trees), and parasite load (it seems likely that deciduous trees evolved leaf drop in part to get rid of parasites, since parasites are universal, it seems unlikely that large, energetically costly leaves will be favored).

Having said all that, it seems likely that trees will evolve to take maximum advantage of the stationary sun, so that leaf position will tend to be very ordered and regular and will be strongly oriented towards the sun, and that tree structure will closely correlate with the sun's elevation in the sky.

As far as the trees on the edge of light goes, they will not only be affected by shadows, but by the decreased sunlight due to scattering as it passes through all that atmosphere. (Think of a land dimmed by permanent near-sunset!)

I'd love to see the trees of such a world!

Answer 2

Trees on Earth grow upwards towards the sky, where the Sun is. The leaves of trees, where photosynthesis takes place, are concentrated near the upper parts of the trees, where there is less stuff between the leaves and the sky. Plants in general like to grow towards the light (that's what auxin is for).

I imagine that some of your trees will behave similarly. They will grow towards the light. Because the sun is perpetually on the horizon, this means these trees will grow sideways. The trunk will grow horizontally towards the sun. Roots will come out from the trunk and dig into the earth, structurally supporting the plant and providing nutrition. Branches with leaves will grow like those on Earth, but rotated by 90 degrees. The trees will climb up and over obstacles, including other trees, in their quest to have an unobstructed view of the sun and grow towards the light. Because they are not growing against gravity, these horizontal trees could potentially grow to be enormous, depending on the availability of water and nutrients.

The result would be a kind of horizontal forest. Roots would be everywhere, reminiscent of a mangrove at low tide. In some places the trees will be stacked two or three or more high. You'll get some branches with leaves all over the place, but the vast majority of the leaves will be on the sunward face of the forest. Such a forest could still be densely packed with wood, although not in the manner an Earthling would be used to.

An alternative morphology looks at the logic of ground cover on Earth. While Earth-trees gain sunlight by growing tall, Earth ground-cover gains sunlight by growing wide and covering a larger area.

So some plants on this planet will grow so as to form a large sheet perpendicular to the sun's rays. They will grow tall and high from the ground, so as to compete against all the other plants and get out of their shadows. They will spread their branches out wide, but not in all directions. The branches will be mostly constrained to a plane. You might get configurations which look like sea fans or hair combs or fern leaves. The trees will still need a sturdy trunk to provide structural support, otherwise wind could easily blow them over.

Such a forest would likely be sparsely inhabited by these trees. Each tree provides shade to the trees behind it. Some light would still get past, as trees do not completely block the light which strikes them, but there would be less light so fewer trees. One possible configuration for this forest would be rows of fan-like trees, with a lot of space between each row. Another possible configuration would be for the trees to simply be far apart but randomly distributed.

These two tree types are not mutually exclusive. It is possible for these two types of tree to coexist in the same forest, since they use different parts of the light. Some trees could even combine these two strategies.

The strong competition for light would lead to a few more features. A tree would want to ensure its offspring grows as far away from itself as possible, because it would not want its offspring to shade it or be shaded by it. As such, most large plants in this world would have some mechanism to send their seeds far away from where they are. It might be as simple as using fruit which gets eaten by animals and taken away, or it could involve something which gets carried away by the wind like a dandelion seed (although wind tends to blow night-wards on such planets, so wind is probably a poor vector). But the trees would not simply drop their seeds on the ground next to them (except for maybe some fan-trees, which could drop their seeds beside them, which would lead to the row-like forests).

Some trees might take to protecting their light more aggressively. Some Earth-trees drop chemicals around them which discourages other plants from growing beneath them. Some of your trees, particularly the horizontally-growing trees, might take a similar strategy where they either poison or attack any other plants they find. In forests dominated by such trees, the forests will have only sparse growth.

The forests will vary depending on how far they are from the terminator. As you go towards the sun and away from the terminator, the sun moves up in the sky and light becomes more abundant. Plants in such regions would be able to adopt more Earth-like configurations with less competition for light. However, such regions are also hotter and would have less liquid water, meaning we would get desert-like conditions which discourage the formation of forests.

If you go the other way, behind the terminator with the sun hiding behind the horizon, then we have a region in perpetual shade. The only source of light would be light which is reflected diffusely through the sky. You would likely get some similar growth patterns to the trees I mentioned above, because most of the light would still be coming from the sunwards direction. But when the light becomes too little for viable photosynthesis then you would get some very alien flora if you get any at all. Such flora are beyond the scope of this question.

Answer 3

Twilight forests would have layers like rain forests, but sideways

In the twilight ring of your planet, a band of only a few trees thick could block out most of the light coming in from the low angle. Behind them, plants would need alternate energy sources, such a parasitism or fungal-like feeding on decaying plant materials, much like the lower-light undergrowth in Earth rain forests.

If you were to walk into the forest from the sunward side, you might first encounter isolated shrubs growing on whatever slight hill they can find that catches a bit more light than the rest of the fertile-looking but empty ground. Next comes a band where thin plants grow quickly. They're limited in height and leaf density by the winds.

Behind the thin reed-like plants grow the tall trees that develop slower but grow tough trunks to withstand the wind. They effectively block most of the light and starve any similar trees behind them. Their seeds are huge balls that carry supplies for several meters of growth, enough to peek over the reeds before needing sunlight. In fact, the saplings do not develop any leaves at all, simply growing up like spears until they detect enough light or run out of nutrients.

With more and more light blocked, the trees in the back cannot grow anymore and weaken enough that vine-like parasitic plants can overcome their defenses and drain their remaining life, using it to sprout large, umbrella-like structures that serve for both reproduction and reaching the next trees. The vines' life ends inevitably with the death of the host trees, making way for the next fungal-based band.

The fungal band is wild, a myriad of different shapes and colors, all racing to grow as fast as they can off the remains of the trees and vines. The resources are limited, so this band too is doomed to die off, leaving a layer of mulch behind. The last survivors in this band are strange tree-shaped hybrids of plant and fungus, that feed on dead organic matter for their early growth and then sprout some leaves once the area sunward of them has been cleared.

A very few of these grow to immense size, being fed new organic matter by ant-like creatures that live inside the trunk, long after the nearby ground is exhausted.

The forests move

The bands of the twilight forest constantly move in a cycle as plants and fungus grow, age, die and provide food for the next cycle. An area with large trees will be a fungal forest in some years and an empty waste a few years later, until the winds bring new seeds and the cycle starts again.

The size of bands depends on local geography and the distance to the terminator. Generally, the closer to the terminator, the more dead space between bands, as the low angle of incoming light means more distance before light hits the ground again. Any hills or mountains create a more permanent contrast: sun-facing sides may have permanent trees (even of different species that can only live there), while the dark side would be home to fungi or even nothing at all, depending on how much organic matter is delivered by wind and water.

Answer 4

Trees on Earth have engaged in competition for light for well over a hundred million years and this has led to an arms race amongst trees to reach ever greater heights even though the sun moves across our sky. On a world where the source of light does not move and remains in a fixed position in the sky the evolutionary pressure to grow tall would be even greater.

Obviously there are many factors that limit trees' height apart from light, such as water and nutrient availability, but all other things being equal I would expect trees to have an much increased average height, and if conditions allowed to have a much greater height than they do on Earth.

Across all latitudes from the point closest to the sun to the boundary with the dark side of the planet the sun will have different apparent elevations from 90 to 0 degrees. So except for 90 degrees there would always be a preferred direction for the tree leading to asymmetrical tree growth especially at lower elevations, so the trees would tend to have a “facing”.

Trees might well grow much further out to the sides and much less to front and back leading to a flattened appearance like a sea fan. In this way the tree would be less likely to block its own light. Such trees might also grow aligned with all their long dimensions roughly aligned, but ground topology and other factors might interfere considerably in the alignment.

Answer 5

Most of the answers take this from a earth perspective, but they don't always go into what they would actually look like, so I'm going to suggest some very different possibilities.

Big leaves

Since there is little wind, the leaves are free to get as big as they want. I'm not a botanist, but I'd assume the size of leaves is somewhat related to how much wind an area gets. The more wind, the smaller the size of the leaves, since the wind would likely move the leaf out of alignment of the sun, getting less energy and being less productive.

Also, small leaves allow for gaps in coverage, in a world where the sun and wind moves things, that's not necessarily a bad thing, but when the sun is stationary and the leaves don't move much, gaps become much less productive.

I can imagine leaves being massively large, even to the point where it's only one gigantic leaf for a plant. Since this would be difficult to hold up with a single branch, I can see the plant producing multiple branches at various points across the underside of the leaf. Not only would this help in weight/balance management, but it would also provide multiple points for nutrients to flow. Even a plant understands distribution of work and avoiding a single point of failure.

These leaves become a canopy that soak up all the sun and work in competition of other plants.

They also serve as a repository for water. They effectively water themselves during times when rain isn't as prevalent. The leaves would likely have a way to drain themselves, if they get too much rain. This could be as simple as letting themselves rip then healing the tear, or it could be as complicated as sucking in more water when the strain on the branches gets to be too much, or it could be slits in the leaves that open and shut on command.

Big fruits

Because your plants grow large canopies that prevent most light from getting to the ground, the fruits become larger, so they can germinate and grow longer without the need of sunlight until they get to be a considerable size. These saplings may have adapted to pierce another plant's canopy leaf to get their own light and water. They might even cling to an existing plant, like a vine, to get tall enough to get to the sun, then expand their trunk which potentially chokes out the climbed plant.

More parasite/symbiotes

Plants may end up sprouting on other plants more, sucking their nutrients since they don't have the sunlight to produce their own energy. Depending on how close these plants are, the dependent plants may branch out and leech off multiple plants at the same time.

Growth Direction

BBeast has a good idea on how this would work, but the examples given are explicitly based on the idea of the sun being on the horizon, yet a significant portion of the globe wouldn't be that way. There's plenty of land where the sun would be considered "up", so the plants would point in whatever direction/angle they needed to in order to get more sunlight.

Also, older growths would tend to grow higher, so they don't have to compete with newer growth. There may be trees that snake along the ground, but even at the sunset ring, I'd imagine trees that are at around 45 or 60 degree angles, or trunks that have gnarly changes in direction depending on how their competition was during different growth periods.

I can definitely see these trunks shooting out roots at various points, but not just into the ground. They may also take root in other trees and plants, wherever they can gain a foothold to prevent falling over. Of course, this leads to the trees being leaned on needing to put out more roots for their own management. This could lead to a massive tangle of trunks and root systems that create a barrier to anything of any significant size, as literally everything would be woven into one big mass. Even old trees that died would potentially still be supported and supporting other plants. These dead trees may even sprout new plants that add to the mesh. Heck, living trees may end up having new growth coming from them.

Water plants

You'd see more plants like water lilies, where it's again a big leaf. Seaweed would likely be more horizontal than vertical. Algae would likely choke out some places, since there wouldn't be wind to create waves to move the surface around.

"Night" side

This would be a drastic change of scenery. Instead of massive trees and other plants, you'd see moss, mold, lichen, and other small, ground hugging growths. Because the daylight would have a fairly well defined cutoff point, you'd likely see as drastic a change in the foliage, too. It'd go from big stuff in massive forests to little stuff in just yards/meters. While there might be some truly large plants at the edge that have managed to come from this night side, they would be few and far between.

All the flora on this side would be relying on little to no light. Just a tiny bit from stars, some fraction from atmospheric refraction and clouds, and a small bit more from any satellites. Anything that can grow in Earth caves or generally underground without light would have an analogue here. There may even be large colonies of bacteria that grow openly.

Algae would likely dominate the water plant life, yet there probably wouldn't be much. Any water based plant live would be at the surface. Not enough light would penetrate to any depth for many plants to come from the bottom, except at the very shallow edges.

Fauna

Because of the major differences in plant life, there would be major differences in animal life. Animals would flourish on the bright side. Since there's no night and day, they would likely be just as varied as daytime animals here on earth. Some would be fast predators that rarely sleep, some would be pack animals that have guards during rest periods, some would be slow and armored, some would burrow into the ground, some would hide in the foliage, some would make nests, some would swing from the branches, some would fly, some would walk the ground, some would be large, some are small, and all would be geared towards what area of the world they generally lived on.

The dense meshes of the outer ring would likely be dominated by smaller animals that can quickly dart through the mess to avoid predators or follow their prey. Animals nearer the sun "pole" would tend to be larger, since the canopies could be directly overhead and prevent a large amount of competition, allowing for more room between plants.

Any fish would likely eat a lot of algae, except for the predators. Even predators might eat some plants, due to it being so available even deeper. Because the sun never moves, there would be more plant life in the water near the sun "pole", but as the bodies of water get to certain point, the angle of incident and refraction would cut off the sun before the sunset ring, so plant life in those locations would drop off sooner.

Answer 6

The short "horizontal" trees would be closer to the center of the sun side, while those farther away would be much taller. Here's why:

On Earth, the sun is almost never directly overhead. Even on the equator, it spends most of the day either rising or setting. This gives a distinct advantage to trees that are taller than their neighbors, as they get more of the light.

At the point on your planet directly under the sun, this does not apply. There is no advantage to being taller, so the trees there will not spend unnecessary energy to grow upward. Instead, they will prioritize spreading themselves over a larger surface area, close to the ground. Of course, when spread out along the ground there is also no reason to spend extra energy growing a thick, woody truck to support the weight. In other words, trees would not grow at all here and would be replaced by sprawling bushes. (For a more fantastical variant, an alternative might be giant lily pad type flora.)

As you move farther away from the center, and the sun gets lower in the sky, trees (also bushes, but we'll just call them all trees) would need to outgrow those closer to the sun side in order to get sufficient light. Keep in mind that, without sunlight directly overhead to shine between the taller trees, new growth will be difficult farther out.

The taller trees will likely have long, relatively narrow trunks with little to no foliage and lightweight, but sprawling canopies. They might also need to share a single root system (aka Clonal Colony) so new trees can be supported by fully grown trees until they are tall enough to reach the light and contribute to the system as a whole.

At some point, well before the "twilight band", the vertical distance needed to reach the light will become too large, and the great trees will stop. Beyond this point, there will still be plant life, even trees. These trees will be relatively small and will have dark, almost black foliage in order to absorb every bit of light that actually makes it past the great tree line.

Answer 7

Presuming trees managed to evolve on a Planet tidally locked to it's Star, and Presuming the Possibility of a 'Habitable ring' along the perimeter of the Terminus (Day / Night line), Variance could be expected in Leaf Structure according to the Amount of energy being recieved. Needle like leaves could be expected upon trees that persist in harsher environs (Strong Sunlight / High heat, Extreme Cold). Broader leaves could be expected within areas with persistent levels of low Light.

For a Tidally locked planet to be concievably habitable, The Persistent Direct Solar radiation on the daylight side must somehow be mitigated. A thick debree field / spherical ring system around the planet may be an acceptably believable reason.

As for pretending there's little to no wind, worlds with permanent day / night sides would experience fierce winds due to the temperature differences between the hemispheres. (Another reason why energy reaching the planet needs to be 'softened', PRIOR to passing throught the atmosphere).

A ring system of sufficient density and Albido could also scatter some light farther beyond the edge of the terminus into the night side of the planet, extending the area in which flora could survive - however, given the lower energy levels provided by a red dwarf, shrubs, bushes and trees would be unlikely to thrive in the conditions beyond the terminus. As mentioned in another post, Fungi, Lichens, algae Or conceptually similar forms of plant live would be vastly more viable for that environ.

Answer 8

Just like some earth plants when grown indoors. They tend to bend towards their constant-position light source (e.g. window).

https://www.google.com/search?q=plants+bending+towards+light&client=firefox-b-1-e&source=lnms&tbm=isch&sa=X&ved=2ahUKEwjTssSUv_TmAhXJAewKHSXIDTUQ_AUoAXoECAwQAw&biw=1432&bih=1186