A pygmy forest is a forest which, for geological reasons, contains only miniature trees.
The pygmy forest in Mendocino County, California, for example, is caused by varying levels of geological strata on its coastal terrace results in stunted tree growth because of its unusual flatness, which prevents draining and allows rainwater to leach many of the nutrients away. This creates extremely old soil structures (on the order of hundreds of thousands of years) with extremely high acidity, which limits the ability of plants to grow and creates an iron hardpan, preventing the trees from setting deep roots. 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 different soil conditions, the same species of tree grows many dozens of feet high.