○ The forest of Oirase
【Riparian trees & Beech Forest】
The forest of Oirase Stream is known as “Green Valley” and mainly consists of three tree species: Japanese horse chestnut (Aesculus turbinat ), Katsura tree (Cerciphyllum japonicum ), and Japanese wingnut (Pterocarya rhoifolia ). They are the dominant species in a riparian forest growing alongside a stream and they play a major role in the Japanese beech (Fagus crenata) zone of the Oirase forest.Japanese beech (Fagus crenata ) and oak (Quercus mongolica ) are prominent on a well-drained riverside and a river terrace that developed in a valley (a terrace-like area higher than the watercourse), and are a representative species of broad-leaved deciduous trees in northern Japan. Japanese beech (Fagus crenata ) grow in patches, or separate groups of many trees, creating the look of a forest quilt inside the ravine. In addition, they are widely distributed across the top half of the valley.
On the valley floor, the duration of sunshine is limited, so trees here grow straight and tall to get as much sunlight as possible. This is why Oirase’s tall trees are called “beanpoles.” Additionally, tree species in valleys make more and/or bigger leaves in order to absorb a sufficient amount of light during the few hours of sunshine, and this, in turn, makes trees grow large in diameter. To support such hefty bodies on the unstable soil in a valley, many tree species learn to grow their roots .
○ An Empire of Flowerless Plants
In the forest, all objects are covered with mosses, including rocks, tree trunks, fallen trees, stone walls, bridge-rails, etc. In addition, a sea of ferns spreads over the forest floor, lichens adhere to beech bark, and various fungi and slime molds appear on decomposing wood. These plants that grow without producing flowers, but with spores instead used to be called “flowerless plants” as a general term in natural history. Oirase is undoubtedly “an Empire of Flowerless Plants” in northern Japan. Being located at the southeast foot of the South Hakkoda Mountain Ranges, Oirase has a large quantity of snow in winter due to the seasonal wind (northwest wind). The melting snow is stored in the beech forest that spreads along the upper side of the valley and enables the valley floor to be a water reservoir that can supply water constantly. Yamase is a northeasterly wind that blows from an area of high atmospheric pressure developing in the North Pacific Ocean from the rainy season to mid-summer. Mists of this Yamase coming from the sea and along the river are blocked by a mountainous backbone—the Ou Mountain range—standing up like a folding screen inside the valley, which provides plenty of air moisture.
Additionally, Lake Towada, the water source of Oirase Stream, plays a role as a natural dam, as it contributes to securing a stable water level in the stream, which has helped the growth of aerial plants in this flourishing area. Large and small trees and grasses grow on every rock originating from the last volcanic eruption. Those green carpets covering the rocks make it possible for seeds to be carried away by wind, birds, and animals and begin growing in a new area. Additionally, it is also because of the power of fungi to return organic matter back to the soil, producing a new base. It is the flowerless plants, like mosses, that create “cradles” for seeds and seedlings. A world of wonder—the empire of flowerless plants—can be created when these three conditions meet: geographical features, weather, and vegetation.