Forests of Mt. Fuji
Just north of Mt. Fuji is the Aokigahara Jukai Forest or “Sea of Trees,” a forest largely untouched by human activities such as logging. It is barely a millennium old, and replaced an earlier landscape called the genshirin or “original forest,” patches of which are still preserved on the mountain.
A Silent Sea
The forest covers 30 square kilometers formed by lava flows from the Jogan Eruption of 864. This event reshaped the geography of the entire region. It dammed waterways and filled most of a vast, ancient lake known as the Senoumi, leaving only two isolated pockets that became Lake Saiko and Lake Shojiko. Viewed from high up, it is readily apparent that the area was formed by molten rock filling in the earlier landscape. The name “Sea of Trees” refers not only to the forest’s size but also to its sweeping flatness.
A thousand years after the rock surface formed, the soil in the forest is still thin and poor in nutrients. Igneous rock is highly porous, and only the hardiest plants can grow there, clinging to the mossy ground by exposed roots. Most of the trees are evergreen conifers: northern Japanese hemlock (kometsuga), Japanese larches (karamatsu), and cryptomeria (hinoki).
Ripples and ruptures in the lava bed give the area an otherworldly beauty, and igneous rock tends to dampen sound, making a walk in the forest a surprisingly silent experience. Rumors about malfunctioning compasses are not true: While cooled lava does have some magnetic properties, they are not powerful enough to affect a compass unless it is placed directly on the ground. Nevertheless, given the vast scale of the forest and lack of obvious landmarks, it is always best to keep on the trail.
The Genshirin: An Ancient Memory
The Jogan Eruption of 864 was a lateral one, with magma bursting from the side of the mountain rather than spilling over the peak. There are still patches of forest on the north side of Mt. Fuji that are thought to be examples of the forest in its ancient, stable state. These areas are called genshirin, or “original forest,” and their survival was attributed to their being located above the source of the eruption, or shielded from the lava by bluffs and outcroppings.
The differences between new and old forest can be subtle. One obvious giveaway is that in the old forests more than a few centimeters of soil can be felt underfoot, unlike the soil-poor igneous rock elsewhere. The species of trees differ as well. Although the genshirin are not uniform, compared to the new forest they usually contain more deciduous species, like the Japanese beech and oak (buna and mizunara), interspersed with evergreens like the Nikko fir (urajiromomi). In the colder genshirin areas further up the mountain, species of Japanese hemlock (tsuga) and silver fir (shirabiso) thrive.
There are many genshirin areas not far off the main trail, but as in the rest of the forest, one should be careful not to get lost. One relatively accessible patch of Japanese beeches can be found in the lee of Mt. Omuro, not far from the Motosu Fuketsu Wind Cave.
