how do i make trees as tall as possible?

i'm working on a sci-fi world set on the terminator of a tidally-locked planet. a big part of the worlds imagery is massive trees and fungi, this big dense rainforest that thrives on the eternally stormy, eternally twilight terminator line.

the only issue is... i don't want all these trees capped at ~400ft, like it is here on earth. how could i get trees taller than that? less gravity, more of a certain thing in the air, etc?

Less gravity is the main thing that you could do, though that may not be enough. The main thing constraining trees is that they have to be able to move that water up their entire height in a reasonable time frame, and though the process is poorly understood, there are presumably limits. (An upper limit of about 50 meters/160 feet per hour seems to be one.) A tree that's really big of course runs into the problem that the amount of nutrients it needs to be a good tree may be far too much to reasonably get from the ground. This, incidentally, is why almost all really large trees poison the ground around them, because they need all the nutrients they can get, and even that's usually not enough on its own to get them appropriate amounts of nutrients and water.

Redwoods get around this by being very efficient moisture capture machines even at their higher levels that have a deeply interconnected root network allowing them to share nutrients. (which allows even ones without chlorophyll to survive, they're called albino redwoods and they have yellow or white leaves.) Also they poison the ground around them.

Eucalyptuses get around it by being terrible vile things that grow some of the worst wood in existence that barely supports the tree itself much less anything else, fall over at the slightest provocation, and explode if they catch fire because they have so much sap. Also they poison the ground around them.

Anyway those are two strategies for becoming an enormous tree, that would most likely hold true on other planets.

Theoretically a slightly more carbon dioxide rich atmosphere could allow for easier growth. That's where most of the mass of plants come from. Though you'd need a way to keep that high level in the atmosphere, so that the trees don't sequester it all and stop growing tall and freeze instead.

You also will have to deal with photosynthesis's diminishing returns somehow. More leaves isn't any good when all those leaves are in the other leaves's shadows and they're not doing much of anything on their own. (like serving as a moisture capture system like on redwoods) Height too, has diminishing returns, because trunks don't photosynthesize or gather nutrients, and because of the square cube law, you need exponentially more trunk for every additional amount of tree no matter the gravity. You probably won't get any mile high trees or anything, that's just way too big, but you could probably say that a tree twice as tall as the tallest earth trees exists and have it seem plausible if plausible is what you're going for, though remember the tallest trees only become tall to compete with other tall trees. If they stand alone, a tree that isn't using its height as a moisture capture tool to trap fog or something (as redwoods do), is going to get wide, not tall.

Giant fungi though are a problem in and of themselves. The biggest real fungi (as tall as 9 meters or about 30 feet) were pretty boring looking (they were just vertical spikes) and went extinct pretty quickly after larger plants showed up, because the smaller ones could take advantage of the shade provided, while the big ones had to deal with the deadly sunlight. As the planet is tidally locked, this means that truly enormous fungi are even less likely to form because even around the inhabitable area, they'll be in constant light, and non photosynthetic things tend to avoid light when they can't take advantage of it. (sensibly, because sunlight is actively hostile to life and is best avoided.)

Also that better be a pretty bright star because otherwise the twilight will make photosynthesis less efficient and it's much more likely you'll get a rainforest with really really wide-canopied trees that aren't much bigger than earth trees vertically instead of tall ones.

A permanently stormy planet may also not be a great place for big trees, because they'd catch a lot of wind.

But yea, lays it out perfectly. Now the advantage of a tidally locked planet is that you'd likely need a thicker atmosphere (to spread out the heat and avoid a half-scorched half-frozen planet) and the permanent storm from the heat difference would also lead to a lot of moisture transport. Both would help a lot with allowing big trees to form.

One thought that occurs to me is to have the trees be naturally hollow, reducing the "amount of tree" that the local nutrients and water have to nourish. This hollow (or set of hollows) could further be used in water transport, perhaps—maybe the alien trees have a valve-system within the hollow(s) that allows them to shunt water upwards.

Of course, this would come at a cost to structural integrity, so perhaps the trees might develop a particularly tough "wood"—maybe even incorporating some mineralisation drawn from the soil.

As to the fungi... Well, perhaps they likewise mineralise their caps, but for a different purpose: here they might do so in order to fend off the sun, and then grow to their great size in order that their great caps shade their still-developing offspring.

Edited by ArsThaumaturgis on Mar 4th 2023 at 9:37:43 PM

Trunk mineralization could theoretically work to support a lighter tree, if it grew in proper structure, though that doesn't seem like something that would be likely to evolve on its own. Light structures are not unheard of in the plant kingdom obviously, but developing a mineralized trunk in conditions that would (if indeed this is a dense rainforest type thing) highly prioritize becoming tall fast, by slowing tree growth by increasing the absolute quantity of nutrients needed to grow would probably be selected against. The dirt would need to be very good to justify a mineralized but light trunk, because the tree would either have to evolve a trunk that's fairly hollow or a trunk that's mineralized, and both seem like they would be selected against in an environment that's going to prioritize height.

If the life was engineered, however, to some extent, or if a mass extinction changed the environment enough to justify that mineralized plants existed before but suddenly found themselves in an environment with a new tall tree niche, that could be explained. Though as said previously, the environment would have to be rich in minerals useful to incorporate into the tree.

Also a fungus using its own cap to shade its offspring sounds like in principle a poor recipe for a successful fungus. If it's that big already, it's probably already consumed all the nutrients worth growing on in the area which its cap shades, so its offspring would likely be sickly and weak and eventually wither, possibly before coming to maturity. Besides, its already in an environment where there's all these tall trees about, providing shade, and if there weren't shade, selective pressure would either make it smaller (to be safer from the sun by having less cells to kill and more options for not burning in the sun, like going under a rock or something) or go wide (for fungi that usually means large mycelia networks attached to a few small surface mushrooms which produce the spores.)

The giant fungi of the past filled their niche because they were around so long ago that shade, in a sense, hadn't been invented yet. (The tallest other thing on land was about the size of a dandelion.) It's thought that one way they protected themselves from the sun was by having lichen parts, which allowed them to make some use of the sunlight. (Though of course when shade was invented fungi very quickly moved to taking advantage of that, and the lichens went off and did their own thing).

Fungi can technically get very big today too, but they're not charismatic big fungi, they're mostly huge patches of fungus across a dead tree, or the fungus that causes dry rot. The biggest classical mushroom looking fungus is probably the Giant Puffball, which can grow to about the size of a halloween pumpkin under good circumstances, though very rarely they can grow to about the size of a person (they still weigh almost nothing though). They don't have much of a stem either. So triple that size of the thing that is best described as "a large pale or brown blobby thing" and you've got a plausibly large fungus under a super optimal environment. It's large enough to be fairly imposing. Imagine a giant marshmallow the size of a car, or maybe even an elephant under highly optimal on-world circumstances and you're about there. Though you're probably going to have to make do without a stem, stems really limit mushroom size.

Regarding the fungi, I might imagine that they hold a different biome to the trees; such that where on Earth you might go from savannah to forest, on the alien planet you go from fungi-forest to tree-forest.

Nutrients for the young are tricky, but might still work if they're being brought in from outside on a continual basis—perhaps a river system, or detritus blown in by the planetary storm.

Going by research, the ability to get water to the tree top is the main limiting factor for tree height. Wind is also bad as it can throw trees. It's important to remember though that the amount of leaves of a tree has nothing to do with its height - plenty of trees have a long bare trunk with a canopy on top. This is in fact one of the reasons why trees might become tall - the ability to rise above other trees and their shadows. I don't think that the trunk's energy requirements are a major consideration, however - a tree trunk is to a large degree dead tissue.

Also, a tidally locked planet is close enough to its star that the star's corona and its emissions (X-rays, hard UV radiation, and the stellar wind) can blow the atmosphere right off if its gravity is too low. So you can't have a planet with an atmosphere, tidally locked and low gravity except under uncommon circumstances.

Technically, you don't need the planet to be tidally locked with the star. You could have a planet tidally locked to a gas giant or accompanying planet surrounding a regular star. (Also OP does not specify what the planet is tidally locked to).

To the star. It was explicitly stated that the inhabited area is the terminator, which implies that it's locked to the star.

A strong magnetic field might get around that low gravity making it difficult to hold an atmosphere situation. I think.

Big thing to keep in mind is sap circulation. Sap needs to be pumped all the way up to the leaves and the roots need to deal with the pressure of that huge column of sap. Now, having 24 hour sunlight will help make those upper leaves cost effective to grow but a tree will want to mineralize the lower sections of it's trunk in order to compensate for the pressure. You might want to make up some fictional materials for the trunk to handle the pressure.

One project I saw had massive trees that got around the issue of supplying water to its top by having it collect water from surrounding clouds. However, the project pointed out that this could only be done in certain areas where the clouds were low enough to viably reach. In other regions, the trees were relatively shorter.

It's not a widely known fact, but magnetic fields do not protect an atmosphere from escaping even in the Solar System. They certainly wouldn't do anything against X-ray and UV radiation-driven escape; these are the same kind of radiation as sunlight and aren't influenced by magnetic fields.

The OP description specifies that the planet is tidally locked to the star, though.

It’s called cloud forest and fog drip. Sequoia sempervirens, the Coast Redwood and Sequoia Gigantium the Giant Sequoia reach the sizes they do owing to both. Coast redwood in particular rely heavily on fog drip which is plentiful in central to northern California and southern Oregon. To such a degree that a part of the same mountain without redwoods will be substantially drier hydrologically and climatologically than those with. That’s how plentiful of moisture those trees create out of the constant marine fog.

Similarly the sequoias of the Sierra Nevada rely upon almost cloud forest effects. They scrape moisture out of the clouds themselves as winter storms ascend the mountains. (And part of the reason why you’ll never find them on the desert side of the Sierras.)

Similar giant forests exist wherever there is ample cloud interaction and/or fog available on top of significant precipitation.

The redwoods examples though have a caveat. Like a lot of tall tree species in the Rocky Mountain West of North America, they have shallow but wide root systems to survive the often mountainous terrain. This makes them very susceptible to wind. There’s an old Native American saying “the forest holds each other up” that’s very true about not only redwoods but aspens, ponderosa pine, Douglas fir, and more. A singular example of any of those trees is easily felled by high winds. But have thousands to millions of them covering a valley or mountainside for acres and acres and acres and it proves all but impervious to windy weather. The trees are too top heavy otherwise. The interlocking root systems of those shallow roots provide the additional support they need to withstand the wind.

It of course also means that if just one of them fails and falls to high winds, you end up with an entire mountainside of blowdown as a result. They stand or fall together.