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What conditions would cause an extremely dense (dark) coniferous forest to grow?

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It is said that in dense rainforests, only 1% of sunlight (sometimes less) reaches the forest floor, which greatly restricts the types of plant and animal life that can survive there.

My (limited) experience of temperate and alpine forests seems to involve considerably more than 1% light at the forest floor. The only exception was an artificial plantation I once visited in Scotland, I believe of sitka spruce, where the close planting of the trees made the understorey so dark that it was difficult to see anything at all... at least while standing on the cleared pathway with my eyes accustomed to sunlight, peering into the gloom under the trees.

What conditions would be necessary to make spruce (or any coniferous species) naturally grow close together over a large area, in order to create a forest of near-total darkness in a mountain setting? What factors might limit the size of such a forest?

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Nature does this on its own, given enough time. Different tree species have evolved different strategies. Some require a lot of sun, grow fast, and make a lot of seeds before being overshadowed by taller trees. Others have evolved to tolerate shade, usually in return for being less competitive when there is a lot of light.

Here in the northeast US, the eastern hemlock (Tsuga canadensis) is one such tree. It is very shade-tolerant, and large trees produce a lot of shade. About the only tree that can grow underneath a bunch of hemlocks is another hemlock. In fact, hemlocks require shade. Like most trees, they do grow faster with more light, but don't utilize high light as well as fast-growing trees like birches.

As a result, hemlocks take a while to establish because they require a good canopy to compete. Once they take hold, they form exclusive stands with no other seedlings being able to grow. There are several hemlock groves in my town in north-central MA. These are still young enough around here that there are tall white pines (Pinus strobus) interspersed with the hemlocks. These pines are long-living, and left over from when the forest floor had more light. New pines aren't growing, but the old ones may still be around for many decades. They are taller than hemlock, so the hemlock don't shade the pines.

You aren't going to get "near total darkness", but hemlock groves do feel considerably darker than other forest types around here.

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