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I've been experimenting with a fantasy world that is flat, with the sun fixed in place over the centre. The day-night cycle is provided by the sun dimming and then brightening again. Most of the rest of things are assumed to be 'as on Earth'.

The lack of sun motion, though, creates an environment that doesn't exist on Earth—large areas in permanent shadow, due to being behind hills and mountains. These places receive no direct sunlight—their main source of light is atmospheric glow—but they do receive a fair amount of light, and do experience a day-night cycle.

How would (macroscopic) life - plants especially, but other organisms as well - adapt to an environment like this?

Some plants and animals would focus on the borderlands.

While regular shade could exist anywhere, mountains and other large geology would give the deepest shade, enough to change the climate. The border areas would have the most balanced climates (breezes could normalize temperature differences to some extent). For animals, it allows for movement between shade and sun.

Many plants and animals would live in full sun.

Forests are, by definition, in full sun, because nothing is able to cast shade on them. Some forests are near mountains, but generally they are not as thick in the mountains.

Forests create their own shade. Plants and animals requiring more sun, grow or climb higher. Plants and animals benefiting from lower temperatures and lack of direct exposure to sun, stop growing or climbing (or climb down).

Forests change up the access to sun plants and animals get through death of large trees (either the leaves all die, which can also be seasonal, or the trees themselves fall over and expose new areas to sun) or from fire (which rejuvenates the ecosystem).

While forests aren't mountains, they are deep enough and thick enough to change microclimates.

Jungles are forests in more tropical and humid areas.

Deserts are low-growth landscapes in arid full-sun areas. They can be cold or hot, low or high altitude. There is plenty of plant and animal life but it does not get thick like a forest and the only shade is from a few individual plants (some of which can be very tall and/or store water for animals to use). Deserts in your world would be very similar to deserts on Earth, only any surrounding cliffs or mountains would have different amounts of shade/sun.

Plains would have similar ecosystems to Earth ones. Plants and animals would have a bit more movement during the day to adjust for the lack of shade changes from trees and small geological features.

Overall:

• Plants may develop more like vines or other types of flora with longer reach. So they can make use of local shade/sun differences.


• Many plants thrive in full sun so long as their roots are shaded (tomatoes, for example) and may evolve like a tomato to creep outward and upward so this happens.


• More plants may evolve with canopies, dense umbrella-like structures where the top gets plenty of sun but they create their own shade for their trunks and roots.


• Climbing animals will be common, along with nocturnal animals. Both are prevalent on real-life Earth, but there may be more of them in your world.


• Coldblooded animals will thrive in areas with local shade and sun (like they do currently in desert regions and elsewhere...using underground burrows and also landscape features like large rocks).

There are already environments on Earth that are similar to this, like the floors of rainforests.

I grew up close to some natural rainforest areas and 'shadow' is a reasonably good term for the light levels you get while walking through a dense rainforest. The canopy blocks out a great deal of the sun, which is why the trees in rainforests have already evolved to grow so tall; they need to compete with each other to get the most light out of the 'canopy' above the ground.

Some plants actually thrive in such an environment; you get a lot of ferns for instance that don't mind being in shaded environments and actually burn in full sun. Plants like 'elephant ears' evolve to have very large, broad leaves that take advantage of what sunlight they can get a hold of, and as such would burn in full sun because their leaves have such a large surface area.

And then you have mushrooms. These aren't animals, but they're not plants either and don't photosynthesise, so the presence of sunlight isn't a problem for them. What they do need though is plenty of organic material from which they can draw energy and material with which they grow.

Animal life is a bit more complicated because it generally relies on plants to survive. Most of the insects in rainforests for example thrive on the humus, or plant debris on the rainforest floor that starts to decompose and compost. Then, the larger animals live on them.

In your world, rainforests may be an option in some places where the shadow isn't particularly high, and trees can grow above it and do. In most places however, your plant biome will likely be fern prairies that contain additional broad leaved plants designed to maximise energy absorption. In such cases, there may well still be a humus layer and hence insects, but what you're unlikely to see would be fruits and flowers.

Why? Well, fruits represent a massive store of chemical energy (sugars and the like) that the plant invests in it's propagation. The trouble is, if energy is scarce, you simply don't reproduce like that because the energy required to produce fruit is a much larger percentage of the available energy, and hence the investment represents an impediment to survival.

So, any animals in such a biome are likely to be predators, starting with the consumption of the insects that work the humus layers in your fern prairie.