Sato means "humanized or anthropogenic forest" and yama means "mountain." Sato yama is thus the name of a secondary forest managed according to a regeneration system that occupies the interstitial spaces in mountainous agricultural land (i.e., the narrow spaces between cultivated land). It is essentially a deciduous forest where farmers cut the trees (Castanopsis cuspidata, Quercus glanduliflora, Q. acutissima, Q. dentata, etc.) every 20 to 30 years for firewood. The stumps grow shoots that regenerate the tree layer, as happens in Mediterranean chestnut or oak forests. Organic fertilizer is made with undergrowth plants and leaf litter. In addition, these small woods have a recognized determining role in regulating water, flowing down slopes in sheets, which is very important in rice paddy landscapes.
Thus, the sato yama is an essential component of self-sufficient traditional agriculture, to the point that between 30-50% of the available space is reserved for this purpose. It is, however, a multiform and discontinuous space, usually located on very steep slopes with forest plots interspersed among equally small agricultural plots and small rural communities. This creates a mosaic-like landscape, enormously diversified, composed of a series of small interpenetrating or interlocking elements. The combination of rice paddies and small thickets or groves is very appropriate for wildlife. For example, migratory birds find both food and shelter there during their seasonal migration. At the same time, amphibians such as frogs (Rana japonica, R. ornativentris) and salamanders (Hynobius nebulosus nebulosus, Hynobius nebulosus tokyoensis) can reproduce in rice field waters and also settle in forest areas.
Elements of fauna and flora characteristic of previous glacial periods --more typical of colder areas than of the subtropical laurel forests that would potentially occupy such areas today--remain in these deciduous woods. One example of this are the spring ephemerals, which are herbaceous plants that cannot live in the shady undergrowth of evergreen broad-leaved forests, since they need plenty of light during their vegetative period. Thus, at the beginning of spring, when days become longer but leaves have not yet emerged on deciduous plants, herbaceous plants sprout and bloom, including Erythronium japonicum, Amana edulis, Anemone raddeana, Adonis amurensis, Corydalis decumbens, etc. When summer arrives and the green tree canopy closes in, spring ephemerals have already produced their seeds and become dormant.
During the most recent ice age, even the low areas of southwest Japan were covered with broad-leaved deciduous forests, such as beeches. Towards the end of the glacial period some 10,000 years ago, the climate became warmer and in the warmer regions of Japan these deciduous broad-leaved forests were gradually replaced by broad-leaved evergreen forests. Since seed dispersal of spring ephemerals is slower than that of dominant tree species in the evergreen broad-leaved forest, these plants had difficulty in remaining there. For example, the estimated dispersal speed of Erythonium japonicum is only 20-24 in (50-60 cm) per year, while evergreen oaks disperse their seeds at the rate of 98-131 ft (30-40 m) per year. For this reason, many spring ephemerals that used to live in southwest Japan were trapped by the progress of the broad-leaved evergreen forest during the glacial period and disappeared. The sato yama mitigated the effects of this process.
In fact, 4,500-5,000 years ago southwest Japan was already covered with a mosaic where evergreen forests alternated with deciduous forests. In those days, the Japanese population increased to a density of 8 inhabitants per square mi (3 per sq km), and started to practice tilling in their agricultural production. This type of traditional agriculture, based on clear-cutting and the use of fire in forest plots, produced secondary forests of deciduous trees, i.e. the sato yama, in areas that potentially would have developed broad-leaved perennial forests. Thus, it can be asserted that the Japanese farmer in a way masked the ecological environment.
Traditional tilling was practiced in a rotational cycle of 20-30 years, and converted the landscape into a mosaic of small plots in different tiers that often did not exceed 10,800-21,500 sq ft (1,000-2,000 sq m). It is interesting to note that this was also the surface area of clearings produced by the spontaneous fall of large trees in primary deciduous forests in eastern Europe, a natural phenomenon that also creates cyclical mosaics in the landscape. Even more interesting is the fact that this is also the size of the rice paddies dating from only half a century ago using traditional methods. Therefore these small plots are not the result of a whim, but of a certain natural logic--and especially difficulty. Both tilling agriculture and rice farming are highly labor intensive, so the same workload must still be invested today to keep a balanced mosaic-like landscape, sprinkled with rice paddies and sato yamas. This is the typical miniature landscape that characterizes Japanese rural areas.