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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?