A pygmy forest is a forest which, for pedological and geological reasons, contains only miniature trees. Pygmy forests may occur over various world locations with notable occurrences being noted in the literature of the: California coastal terraces and inner coastal mountains of Northern California; the Ross of Mull on the Isle of Mull in Scotland,[1] as example locations.
A variant of the pygmy forest is sometimes called elfin forest in parts of Southern California, Mexico, and Central America.
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In the case of coastal Pygmy forests in northern California and Oregon, the formation began with a series of marine terraces and should be viewed as part of that formation. A combination of uplift and changes in ocean level formed a system of terraces, resulting in an “ecological staircase,” with each terrace approximately 100,000 years older than the one below it and supporting a distinct association of soils, microbes, plants, and animals. A dune being pushed farther away from the coast by fluctuating sea levels solidified and slid under the one before it, raising the terraces. Pioneer plant communities colonized and took over the young terrace. The succession of plant communities that repeated on each terrace eventually formed a very specific podzol known as the Blacklock series,[3] which offers an inhospitable environment for species and greatly stunts further growth on the terrace. Part of this soil profile includes an underlying clay or iron hardpan. Each terrace is relatively level and many are footed by paleo-dunes. Drainage is poor at best on these stairs and plants sit in a bath of their own tannins and acids for much of the wet season. Plant communities on this terrace have reacted to limited root mobility and acidic soil by evolving stunted forms. Remnants of ecological staircases doubtless exist, however most have been destroyed for development or logging. Intact examples are found in these public lands:
Analyses of pygmy forest soils show low levels of macro— and micro—nutrients, and high levels of exchangeable aluminum, which limits the ability of plants to grow. Low pH conditions support formation of an iron hardpan, preventing the trees from setting deep roots and preventing internal drainage of soil water.
As a result, the pine trees in the area are rarely more than three or four feet high, in a sort of natural bonsai effect. Many of the tree trunks, though only an inch thick, contain 80 or more growth rings. Only yards away, but with younger soils, the same species of tree grows many dozens of feet high.
Stunted tree growth can also occur in some cases of highly alkaline soils such as the Stora Alvaret or Great Alvar formation on the island of Öland in Sweden. In that area there are certain extents of pygmy tree growth and also areas devoid of trees entirely with many associations of rare species, due to the unique soil chemistry.