Filed under: science fiction, Speculation, Worldbuilding | Tags: science fiction
Up from the depths, sixty stories high…
This is just a little note, spun off again from a discussion on Antipope. The idea we were pitching around was whether a pelagic floating forest was possible on Earth. I explained there (and will here) why it would be difficult here. Still, I know of three floating forests in the science fiction literature: as a minor scene in Dan Simmons Hyperion, as the raft trees in Joan Slonczewski’s A Door Into Ocean, and last (or first, and arguably biggest) as the floatwood forest/trees in James Schmitz’s The Demon Breed. Oh, and there’s the slightly more realistic example of Prof. Mark McMenamin’s idea of a far future floating mangrove that kicked up in a long-ago issue of Discover. There is at least one other idea that’s not worth going into here (don’t open this link).
And yes, I like floating forests, and they were very much in my mind when I wrote that first Scion of the Zodiac so long ago. I really will have to rework that some day. Anyway, the point here is to go into why a floating forest might work biologically, but why it’s hard to get there from here, for reasons that aren’t obvious to a non-botanist (or even to most botanists–I’m a bit of a magpie).
I was curious, so I reread The Demon Breed after the discussion on Antipope, and I can now see the problems in the story I enjoyed so much before. The floatwood tree is probably a kilometer or miles across up to six hundred feet tall, built like an atoll with a central lagoon underlain by giant call them rhizomes. It’s a wonderfully exotic environment, so big that a hurricane doesn’t particularly shake it.
The first problem with it is the height. On Earth, the tallest theoretical height for a tree is between 400 and 426 feet (reference). It’s determined by a bunch of forces: the force of gravity, the ability of a tree to balance photosynthesis (sugar going in) with respiration (sugar going out), and capillary action, which is the force that sucks water from the roots up into the leaves. That water is held under tremendous tension inside the vessels of the tree, and if the column breaks and a sucks a gas bubble out of solution, that’s the end of water conducting nutrients up out of the roots. Redwoods get about as tall as it’s possible for any tree to get, and the ones that get that big tend to basically have their roots in creeks. To do this, a redwood is basically a column. In contrast, floatwood trees are twisting canopies six hundred feet tall. This is basically several times too big to exist and support itself through photosynthesis. Indeed, redwoods only pull off the trick because they live where it’s relatively cool and very wet. The cooler temperatures limit their respiration rates. In the tropics, it’s tricky for a tree to get that big, because it’s respiration rate is much higher.
Then there’s physics. The biggest wooden boat in the world is a lot smaller (reference), and the reason is that huge wooden boats aren’t particularly stable, any more than huge wooden buildings are. Wood’s not the strongest construction material to build a floating atoll out of.
I can go on, but Schmitz’s floatwood forest is a botanical godzilla, rising out of the ocean, but too big to exist. So sad, but a fun story nonetheless. Now, let’s go into what you need to build a floating forest.
First off, don’t make it so gynormous that it is hurricane proof. Better that it can be broken apart in those 50 foot waves and regenerate from the pieces.
Second, it needs to be light enough to float. Kelp does this quite nicely, but it doesn’t pop above the surface. Normally, trees are big so that they can compete successfully for light with smaller plants. In the deep ocean, the only reason to be a tree is to be a nutrient magnet. If the raft tree can be a home for birds and other amphibious pelagic species, their wastes can feed the tree. Coral reefs use a similar trick to grow so immense in nutrient-poor tropical seas. So the raft tree doesn’t need to be just a float, it needs to have light enough rhizomes to float trunks above water, plus various waste materials left by the critters it’s trying to attract. Something the density of balsa would work nicely, but then again, balsas are a conventionally built tree that don’t tolerate saltwater, so they’re we’re not going to see oceanic balsa trees anytime soon.
There are, in fact, floating aquatic plants: water hyancinth, wolfia, and friends (all fairly closely related). The ferns Salvinia and Azolla, and various rushes and grasses (such as papyrus. Oh, and sphagnum moss, which can make floating bogs wherein trees grow. These are all freshwater plants that grow in still or slow-moving water. Aside from sphagnum, all of them grow in nutrient-rich waters too, which allows them to cheat. Sphagnum does its own weird thing that sequesters nutrients with polyuranic acids, and make it really hard for any but specialist plants to grow in bogs. I’m pretty sure peat moss (sphagnum and friends) is not structurally stable enough to stand up to oceanic waves, even though it can hold small trees. So this is a dead end too.
Then there’s nutrient capture. If the rhizome floats are underwater, the plant doesn’t need to have a big root system for taking up water. Indeed, roots hanging down are basically fish food unless they’re covered by some protective symbiont. It’s better to have a poorly developed root system and take up water through rhizomes. Still, the plant will need adventitious roots inside the crap that it’s accumulated, so it needs a crap accumulation structure. Bromeliads do this best, but so do staghorn ferns and other epiphytes. In general, these are modified leaves that collect junk in their basket-like modified leaf bases, then send roots (or for bromeliads, modified hairs) into the resulting compost to get nutrients to grow. Again, none of these epiphytes are salt tolerant, so they’re not going into the ocean either.
There are salt tolerant plants in three places: deserts, beaches, and salt marshes. Deserts and beaches often have similar plants, because they’re very similar environments (this is true in California. In the tropics, it’s a different matter, and atolls are the kind of oceanic, nutrient-poor environments that might give rise to a floating plant, except that beach plants aren’t normally that aquatic.
Salt marsh plants are aquatic and salt tolerant. Their problem is that salt marshes have a lot of nutrients in them. Indeed, they often capture sediments upstream of where coral reefs are located, and they’re an integral part of the greater reef systems. While salt marsh plants could conceivably become pelagic floaters, the problem is that they’re going from a nutrient rich to a nutrient poor environment, and these take different adaptations.
You’re beginning to see the problem, I hope? A raft tree on Earth is a chimera, with the salt tolerance of a beach plant, the flotation capacity of a balsa or giant papyrus, the nutrient capturing capability of a bromeliad, and so on. No one plant lineage has all the traits that a raft tree would need to evolve from. To get there from here, you need to come up with a scenario wherein some plant acquires one trait after another, making it successful as a land-based or amphibious plant, before going on to become fully pelagic. It’s not impossible, but it is tricky and counterintuitive.
This doesn’t mean that such a plant couldn’t evolve on another planet. If it was, though, coastal swamps, rivers, and beaches would be covered with ecosystems that are much more complex than the coastal ecosystems we know on Earth. If, in the far future, plants were to take to the surface of the ocean, we’d expect the same complexification of coastal and wetland ecosystems as well.
There are a couple of points here. One is that botanical chimeras are just as chimerical as animals are. If you’re going to get grumpy about giving cats horns, you need to be grumpy about giving eudicot trees some structures from monocots and ferns. The second is that nothing happens in a vacuum, so if you want an alien biosphere that produces, say, floating forests, this is going to litter that biosphere with relatives that either aren’t floating or aren’t in forests. That’s something that all to seldom shows up in science fiction.
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Putting the life back in science fiction 
I think it is fair to say that there isn’t any SciFi worldbuilding that has a realistic ecology. They are invariably too simple, usually just to highlight some interesting flora or fauna for the story. Movies are perhaps even worse, with huge animal phenotypes that live in areas almost devoid of food, [The best example is the “snake” living in te asteroid in Star Wars: The Empire Strikes back. But there are many, many examples, that clearly don’t make any ecological sense.
Comment by alexandertolley October 17, 2016 @ 4:30 amDoes anything get easier if the floating forests are in freshwater lakes?
Comment by nancylebovitz October 17, 2016 @ 12:46 pmThey already exist. Black spruce grow on bogs in the US.
Comment by Heteromeles October 17, 2016 @ 3:40 pmIf you’re going to get grumpy about giving cats horns, you’d probably be mightily affronted by Ceratogaulus (https://en.wikipedia.org/wiki/Horned_gopher) and any number of other things that really exist(ed). Many successful animal lineages seem to have (metaphorically) crawled through tiny, twisty, counterintuitive channels in ecospace to attain an (eventually) winning combination of morphology and physiology. I say, if you can imagine a series of transitional forms that could survive (even by the most bizarre combination of lucky accidents) to arrive at the thing you want, go for it!
Comment by John Scanlon October 17, 2016 @ 3:17 pmI already know about Ceratogaulus and I don’t find them offensive. There’s a reason I picked cats.
Comment by Heteromeles October 17, 2016 @ 3:41 pmFirst off, if we’re arguing about possibilities that could evolve someday, it isn’t an issue that the different plants existing today which do part of what you want are not in position to go into the oceans and do the rest. We don’t have any ocean floating forests now, and circumstances could possibly arise in the distant future which would allow them to evolve the other pieces of the puzzle.
For that matter, it might someday be possible to do fancy genetic engineering to create what you want.
But put that aside. You have established that floating vertical tree trunks require various difficult adaptations. A heavy weight sticking out of the ocean requires something special. Etc. You didn’t mention the salt — to bring water to the top of the tree and evaporate it, you either bring salt with it and maybe let the salt crystals drop back into the ocean, or you must desalinate the water first.
But what is the purpose of having a tree trunk sticking into the air in the first place? People talk like tree trunks are there to get the light first, competing against other plants that also stick up into the air to get light. For that to be an issue in the ocean, you’d need enough other plants sticking into the air that they shade each other. How likely is that?
A second possibility is maybe your plant will need a sail. Without a sail it goes wherever the ocean currents take it, maybe around in a big circuit, sometimes father north and sometimes south. With a sail it can go in a consistent direction until it hits land. Ouch. Or possibly it can arrange to go in a small circle. That sounds better to me. If it’s a sail that’s needed the evolved sail might not look like a tree trunk. It might look more like a sail. And it might tend to be pretty low, because of the various disadvantages of growing high.
To survive as a big plant in an environment where most plants are very small? It might co-evolve with various fish etc. It makes little edible nubs for them, that supply them with some of the nutrients they need. To get the rest they must eat the parasites that have evolved to grow on the plant…. It could evolve a whole collection of defenses against all the parasites and herbivores that eat it. It *has to* evolve those defenses or it can’t survive. It doesn’t need to stop them completely, just slow them down enough for it to grow faster than they nibble at it.
Nutrient-poor environment. If it sequesters a whole lot of nutrients that aren’t replaced, that could be the limiting factor for its growth. It collects so many nutrients that algae can’t grow around it, and there’s no biome there except the plant and the things that eat it. If those nutrients are replaced then it can keep growing. Unlimited water. It can slowly accumulate unlimited carbon, and if it can spare sufficient energy it can get unlimited nitrogen from the air. It’s limited by whatever other element is in shortest supply. Whatever it could get from the deeper ocean, organisms that go there can bring back. Would anything go to the actual ocean bottom to bring back needed nutrients? that seems like a long way to go, but….
If it does travel a long circuit, it could eventually evolve to stockpile the things that are present now, that will be scarce later.
Whatever it needs and can’t get, limits its growth.
Something along these lines might be possible. In the long run I dunno. It sequesters the resources that everything else in the whole biome needs, and it produces a lot of energy and sequesters that. Every other organism benefits by finding ways to collect some of its substance. Eventually something evolves that can collect from it too fast and it winds up as tiny islands that split up grow faster than its taker can find it. Or it disappears entirely. Unless it can create a diverse enough variety of commensals that at least one of them usually stops the taker from getting too much out of hand.
I can sort of imagine it, but I haven’t imagined all the gotchas that might prevent it.
Comment by J Thomas October 17, 2016 @ 5:24 pmActually, there’s this nasty effect called “fetch” that suggests that probably trees won’t become sails. It’s how wind blowing ice flows across a lake can give them enough force to crush a dock, even though there’s a really low coefficient of friction between the wind and the ice floe. Unless an oceanic organism needs to go somewhere (like a portuguese man of war), there’s no particular reason to stick up at all. That’s why the biggest floating forests in the oceans are kelps rafts, which barely stick up at all.
Comment by Heteromeles October 17, 2016 @ 8:30 pmI imagined what you’d want a tree for if not to get first chance at the light, and a sail was all I came up with. A bunch of little low shrubs would probably be plenty. Why would the plant need to go somewhere? One possibility is that if it’s good at collecting minerals etc from nutrient-poor water, then traveling across that water might let it keep finding new water it hadn’t already drained of minerals.
It isn’t particularly compelling, but it’s all I got. There are lots of technical problems to solve for a tree in earth gravity floating in water, and I haven’t noticed anything that would be good at selecting that. So I’m kind of stuck.
Once you already have a floating island that’s maybe miles across, then it makes more sense. You get a big underwater structure that floats, that can support the weight of some shoots above the surface. You get lots of the energy from sunlight at or near the surface that supports this big underwater structure. I’m not clear yet why you need the big underwater structure supported by the photosynthesis on top, but maybe something will come up. Once it’s big, then things in the center can get more light by going high, and every plant in their shade needs to go higher, and you’re all set. Things will evolve to get as high as they practically can. But it already takes a fair amount of shade before you get a whole lot of selection.
I think you need to start with a giant floating raft that’s mostly underwater first, and maybe then you can start selecting for vertical growth.
Comment by J Thomas October 20, 2016 @ 12:22 amHello I know this is an old post, but consider a floating forest that develops in the dead zones that we create at the mouth of basically every river . It would have to kill or replace the alge, but either salt or fresh water is available and moving, the delta sand bars could act as temporary anchorage or nursery for young trees that could float freely along the coastline after growing a bit. Such a plant could develop naturally, but more likely would be developed by humans as an attempt to fix dead zones . It could be as thickly overgrown as a mangrove swamp without growing very tall at all.
Comment by ArtDecoHQ March 15, 2017 @ 5:28 am