One of the explanations can be found by digging in the ground. The thick clay layer that comprises the forest floor gives much nutrient to the plants. Black alder likes it best humid, and thrives therefore especially well in this clay, whose compactness prevents the water from disappearing down into the soil. The clay creates a continuous layer both out into the sea and in towards land. It was originally laid down in deep water, but later the land got uplifted, such that some of what once was seafloor is now dry land.
In some places the sea has inundated the black alder, which therefore stands like mangrove on a tropical beach with its roots on stilts. In the spring, there is a white carpet of wood anemone between the alder trees. Where the light from the sea gets reflected into the forest, the blossoms are tempted into flower. Also the green inlets along the shore are home to a unique vegetation. In the tidal zone between the fjord and sea the sediment is clay, and a rare species of marine eelgrass thrives. In the strand zone there are also small sandy meadows where brackish-mud rush, sea aster and hedge bindweed grow.
Black alder has a very important ecological function. It lives in symbiosis with a ray fungus (actually a bacteria) that grows in mounds on its roots. Ray fungus can take up nitrogen from the air and transform it into chemical bonds that the tree is able to use. Unlike the majority of other deciduous trees, black alder sheds its leaves while they are green. Much of the nitrogen that has collected in the leaves is released to the clayey soil when it is broken down in the autumn. In this way the black alder helps other plants meet their nutrient requirements.
The forest gives the impression of being old, with all the woodbind that winds its way up the trunks of the black alder. In fact, it is relatively young, less than 70 years old. The trees have grown quickly in this area that once was grazing land or sea meadow. There are still small remains of old, open grazing land, but the black alder is quickly invading. In the dense forest, in contrast, the tree's ability to reproduce is strongly reduced, and here there are scarcely any young trees at all. Instead, another deciduous tree is on the increase, the sycamore maple. A number of trees have come into the age of maturity, and many youngsters are now on their way up. If nature gets its way the imported sycamore maple will become a dominant tree species in Hystadmarkjo in the future
