Five mosses for the lowest layer: five ways of being the same plant
The Long Watch has always had a name for mosses, and for most of its life it had nothing under that name. The floor of the world, the layer you would put a hand on, was the one nobody had built. In a single day it gained five of them, chosen to be as unlike one another as a single form allows.
Until then the northern forest floor had been getting by on a borrowed plant: a low creeper that belongs out on the cold steppe, spilled over the border and doing duty under the conifers. The real boreal ground layer is the most moss-dominated country there is. The hole in the forest and the hole in the plant list were the same hole.
Every layer above it had filled in by then. The canopy, the shrubs and the grasses grew up out of a single kind of grass, and the climates that once produced nothing at all were closed out in a day back in June. This gap had a different shape: a named form with no species under it.
A form, not a climate
We had sketched three mosses and built five, because counting species is the wrong way to grade a batch of them. Three mosses can easily turn out to be three versions of one moss. What we wanted was five ways of living: a carpet for the boreal forest floor, a cold wet mire moss that makes peat, a warm humid mat for rainforest, a shade mat for temperate woodland, and the odd one out, a living crust on arid ground.
Read across the five and what they actually share turns out to be narrow. They all want damp. They all put up with shade. None of them care much how rich the soil is. And all five travel as spores on the wind, where every other ground cover in the world drops its seed and lets gravity take it a metre or two. Everything else about them belongs to the place, not to the form.
The cold two, and the one that undoes them
The boreal carpet and the mire moss behave the way you expect a moss to behave. They grow slowly. They keep their dead material around for a long time. They give very little back to the ground beneath them. Both of those are simply what cold does to a plant, and we let the cold say so.
The mire moss takes it furthest. Its litter lies on the ground almost four times as long as the rainforest mat’s, and what it finally returns to the soil is less than any other plant in the world gives. Those are one mechanism rather than two settings to balance apart: when a place makes more plant matter each year than it can break down, the difference stays where it fell, and the word for that difference is peat. A bog holds on to what lands in it. The simulation has no notion of waterlogged, airless ground, so we reached the same behaviour sideways, by loosening the usual bond between how wet a place is and how fast things rot there.
Then the rainforest mat turns every one of those upside down inside the same form. It has the fastest litter turnover of any plant we have written, it hands its nutrients straight back to the soil instead of locking them up, and its year barely swings at all. Warm and constantly wet is the quickest place on earth for things to come apart; nothing accumulates on a rainforest floor. Two mosses, one form, opposite ends of the same behaviour.
The moss that does its living in winter
The crust was pencilled in as an optional extra, a sixth idea to build if there was room. It was promoted to full membership the moment it became clear it cost exactly what the others cost, and it went on to carry the sharpest thinking in the batch, because it inverts nearly everything its siblings do.
Every other plant in the world peaks in the warm half of the year and troughs in the cold. The crust runs the other way round. Through high summer it is air-dry and inert, growing at fifteen hundredths of its ordinary pace, where no other plant we have falls below seven tenths. Its winter runs at nine tenths, the shallowest cold season anything in this world has, against four tenths for the mire moss and a little over a third for the boreal carpet.
Decay inverts with it. In a cold place, winter is the pause: the two cold mosses rot at a twentieth of their summer rate once the ground freezes. In a hot desert the scarce thing is water rather than warmth, and water arrives in the cool season, so the bone-dry summer becomes the real pause. The crust breaks down twice as fast in winter as it does at the height of summer.
This is not a fifth moss. It is a different way of being one.
Its colour keeps the same inverted phase. Dry, it is a near-black grey-brown, and a patch of it reads as bare ground you would walk over without a second look. Wet it and it greens within minutes. Of the five it is the only one that wants full sun, and the moss living in the driest country in the world is also the one that weighs moisture least when it chooses where to settle. Where the mire moss locks nutrients away, the crust gives them back nearly three times as generously as the mire holds them: in dry country a crust is where the nitrogen comes from, and the world now says so.
The dryland plants that gave the desert a food web to stand on arrived a few weeks earlier. What that country still lacked was a living skin on the ground between them.
The mat we would not name for logs
The last row is a shade mat for temperate woodland, held under closed canopy on purpose so it would not read as another version of the meadow ground cover. Its spring is the fastest thing in the batch, half again its ordinary pace, standing in for the bright few weeks between the snow going and the leaves coming out.
A more tempting name was available. A fallen log is one of the stages a dying tree walks through here, and a real woodland moss would cover one. But a plant in this world is placed by the ground it stands on, and it cannot yet fasten itself to another plant’s rotting trunk. Naming this one for logs would have promised a relationship the world does not keep. So it claims only what it does: it likes shade, and it likes the floor of a wood.
What the counting said
We expected the woodland mat to be the most abundant of the five, reasoning that temperate forest turns up in every corner of the stretch of world we count in. It came in last. Forty-six plants, against a hundred and sixty-seven for the desert crust, a hundred and twenty-seven for the mire moss, a hundred and two for the rainforest mat, and seventy-six for the boreal carpet. Turning up everywhere and covering ground are two different measurements, and temperate forest turned out to be a thin transitional band between the extremes rather than the heart of any region. We wrote the mistake down in every place it had already spread to instead of deleting it.
There was a second surprise underneath that one. Adding a plant used to leave our counting patch untouched, because a cold or tropical species would land nowhere near it. Since the climates became regions of one continuous world, where you stand decides your climate, so all five mosses turned up inside the square, and five long-standing measurements of the world went out of date the moment they did.
The floor of that patch went from 2,544 living plants to 3,062, a gain of 518, almost none of them tall enough to reach your ankle. The roster went from twenty-five species to thirty. Nine kinds of plant are named in the design; after this day, eight of them have something in them, and the one still standing empty is mushrooms.
