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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 at the Ross of Mull on the Isle of Mull in Scotland, as example locations.
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, 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.
Examples of high-terrace podzol pygmy forests include:
- Mendocino Pygmy Forest in Mendocino County, California, for example, is an oligotrophic community caused by podzolized (nutrient-poor, highly acidic) soils. The forest flora is dominated by dwarfed Bishop pine, Bolander pine (a variety of shore pine), and Mendocino cypress. The latter two species are endemics to the pygmy forest. Bishop pine occurs in both dwarfed and full size form, the latter being trees who's roots have broken through the hardpan layer into the more fertile soil beneath. This forest is found in several discontinuous areas, with significant portions on the following public lands:
- Jug Handle State Natural Reserve, where it is the feature of the highest portion of the Ecological Staircase Trail.
- Russian Gulch State Park.
- Jackson Demonstration State Forest includes a large pygmy forest area east of the towns of Mendocino and Casper.
- The Hans Jenny Pygmy Forest Reserve, comanaged by the University of California Natural Reserve System and The Nature Conservancy.
- The Van Damme State Park pygmy forest, designated a National Natural Landmark. A self-guided nature trail built entirely on an elevated walkway forms a short loop through the site.
- Salt Point State Park in Sonoma County, California has a much smaller pygmy forest and a fifth terrace prairie ranging from 900 to 1000 feet and formerly supporting elk populations. Like the Mendocino forest, the dominant trees are also Bishop pine, Bolander pine, and Mendocino cypress. Most of Salt Point is on the northernmost portion of the Salinian Block and the terrace staircase drops suddenly into the gulch that lies over the San Andreas fault.
- SFB Morse Botanical Reserve in the Del Monte Forest of the Monterey Peninsula is centered on the small Huckleberry Hill pygmy forest of Bishop pine and Gowen cypress, in the middle of a more extensive forest of native Monterey pine. Bishop pine, which tolerates extreme podzol conditions better than Monterey pine, is found in the most heavily podzolized areas, with a zone of Bishop pine/Monterey pine hybrids dominating the less heavily podzolized area.
Soil chemistry and effect on plant growth
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.
Other examples of California pygmy forests
- Hood Mountain — near Santa Rosa in Sonoma County. A pygmy cypress forest dominated by Sargent's Cypress (Cupressus sargentii) and Arctostaphylos species can be found on the northwest slopes.
- San Geronimo Ridge — just south of Whites Hill in Marin County.
- Henry Cowell Redwoods State Park — select pygmy forest locales.
- Cuesta Ridge pygmy cypress groves — in the southern Santa Lucia Mountains in San Luis Obispo County.
- Cuesta Ridge Botanical Special Interest Area — protects a Dwarf Sargent Cypress (Cupressus sargentii) pygmy cypress forest, operated by the Los Padres National Forest. Signed nature trails through the grove on the ridge.
- Dwarf Cupressus Preserve — managed by the Land Conservancy of San Luis Obispo County, another grove of the Cuesta pygmy cypress forest, on West Cuesta Ridge.
- El Moro Elfin Forest — 90 acres preserve of 'pygmy oaks' (Coast live oak (Quercus agrifolia ). Located on the southeastern shore of Estero Bay (Morro Bay system) in Los Osos, in coastal San Luis Obispo County. An oval wooden walkway loops around the forest, with viewing platforms.
- Mount Tamalpais grows a forest of small cypress trees (Cupressus pigmaea). The mountain’s serpentine soil stunts the growth of these trees, causing them to mature when only a few feet tall. To visit, take the Old Stage Road half a mile northeast of the Bootjack Picnic Area. 
- C.Michael Hogan, (2011) Sea of the Hebrides. Eds. P. Saundry & C.J.Cleveland. Encyclopedia of Earth. National Council for Science and the Environment. Washington DC.
- Pacific Southwest Region Viewing Area - Los Padres National Forest: Cuesta Ridge Botanical Special Interest Area . accessed 6.1.2012
- LCSLO-Land Conservancy of San Luis Obispo County — Land Conservation . accessed 6.1.2012
- California State Parks of San Luis Obispo Coast: El Moro Elfin Forest . accessed 5.25.2012
- Golden Gate National Parks Conservancy