Putting the life back in 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.