pygmy forest

A pygmy forest is a forest which, for pedological and geological reasons, contains only miniature trees. Pygmy forests are usually associated with the coastal terraces and inner coastal mountains of Northern California. A variant of the pygmy forest is sometimes called elfin forest in parts of Southern California, Mexico and Central America.

The Jenny Pygmy Forest in Van Damme State Park, Mendocino County, California, for example, is an oligotrophic community caused by podzolized (nutrient-poor, highly acidic) soils. Underlying this relatively inhospitable soil is a clay hardpan. 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. The pygmy forest in this case formed on the oldest stable surface where soils are approximately 500,000 years old. Each terrace is relatively level, which prevents draining and allows rainwater to leach many of the nutrients away. Over time, this results in extremely high acidity. 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.

Several species of trees are found in the Mendocino and Sonoma County pygmy forests, notably the vulnerable species Mendocino Cypress. A pygmy forest of Sargent Cypress (Cupressus sargenti) can be found on the northwest slopes of Hood Mountain, near Santa Rosa in Sonoma County. In good soil these trees can grow to 50 or 60 feet high but in ultramafic soils such as serpentine they are much shorter.

Alkaline soils

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.

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