Forest Atlas Of The United States

Where Do Trees Grow
and Why?

Trees live in an amazing variety of places. They depend upon light, water, and nutrients to survive. These resources and the substrate necessary for establishment and growth vary substantially across the landscape. Tree species are adapted to different combinations of these requirements. Some species are generalists and are capable of surviving in a wide variety of resource environments while others are highly specialized and live in environments where certain resource limitations inhibit most trees from growing. The forests we see are the result of competition between species as they fight for survival in landscapes with even minor variations in light, water, and nutrients.

The borders of forests are often defined by the excess or limitation of one of these factors. The tree line visible in mountain landscapes is one example of limitations on tree establishment and growth: the increasing exposure and cold limits the ability of trees to grow further up the mountainside. Bogs provide another example: some tree species can grow in the hummocks of moss which keep their roots dry for part of the growing season, but the hollows are too damp, even for trees like black spruce and tamarack which are adapted to wet locations.

Certain tree species are adapted to similar conditions, and we can start to identify forest types, those combinations of trees which tend to be found together. Some forest types are relatively simple and involve only a few tree species living together. Other forest types are very complex and include a large number of tree species uniquely adapted to the resource environments where they are found.

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