Favourite woodland habitats for mosses
Not all woodlands contain the same species of mosses. For the vegetation growing in any one area is largely determined by theconditions. Beeches grow on a wide range of soil types from the shallow chalky soils of the Chilterns and the Weald to the acid sandy soils of Epping Forest or Burnham Beeches, and the moss population in these beech woods varies according to the soil types. For example, two mosses are common on the chalky soils of the beech hangers (wooded hillsides) so typical of east Hampshire. Ctenidium molluscum is restricted to the ground immediately around the bases of the tree trunks, and Hypnum cupressiforme grows on the bases of the trunks themselves. In the woodland of the Chiltern Hills, the soil is often deep and fertile and so a greater variety of plants grows in these woodlands. Frequently preventing mosses from growing at all, for at ground level there is little light. By contrast, the acid soil of the beech woods at Epping prevents the decomposition of the autumn leaves and here too, few plants grow in such deep leaf litter. One species, however. Thrives in these conditions-the great silvery cushions of the white fork moss (Leucobryum glaucum) form a beautiful feature of the woodland floor.
Oak woodlands, often regarded as the native forest of the British Isles, provide a greater variety of mosses than the beech woods. Atrichum undulatum and Thuidium tamariscinum grow in the damper spots. Together with several other slightly less common species. Thamnium alopecurum appears on chalky soils, while species of Polytrichum and Dicranum are characteristic of acid soils.
Pines and birches frequently grow together. Particularly on acid heathland soils in Scotland and other upland areas. The ground beneath the trees is frequently covered with heather and other shrubs and, if the cover is dense, little else will grow. If the covering is fairly sparse, mosses such as Hylocomium splendens and species of Rhytidiadelphus are found. In very wet spots. Sphagnum and Polytrichum may form small areas of Sphagnum bog.
Tree trunk mosses
The number and variety of habitats open to any one moss depends on the climate prevailing in the area. On a large scale, the climate is damper in the west of the British Isles than it is in the east, and so mosses tend to be larger and more common in the west where the conditions are more suitable.
On a tree itself, the distribution of mosses is determined by several factors. Mosses seem to grow more luxuriantly on the north side of a tree than on the south side, and the prevailing wind also affects moss growth, most mosses growing better on the lee side of the tree, out of the wind. A leaning tree trunk is an ideal moss habitat because the rainwater accumulates more readily on it than on a vertical tree trunk. However, once the upper surface of the trunk is covered by mosses, then the undersurface becomes quite dry. This is because the mosses themselves are perfectly structured to trap water between the spiralled leaves on their stems. As the mosses grow in cushions, these water-trapping stems are pressed closely together and the whole plant acts like a sponge. Indeed, in upland areas where the soil is thin and rocks protrude through the surface, mosses may provide an alternative medium for seed germination. It has been found that saxifrage seeds, for instance, germinate better in moss cushions than in soil because of the humus that is built up by the moss.
The water-trapping ability enables mosses to grow in some unlikely situations. Among sand dunes. Mosses such as Polytrichum piliferum play an important role in the succession from the shifting, unstable main dunes to the vegetation-covered stable fixed dunes further inland. These mosses colonize the sheltered zone behind the main dunes where they are protected to some extent from the wind. They help to stabilise the sand and trap the water that helps other plants to grow. Lichens may already be present, and grasses and other plants follow, until finally pasture land is formed.
Mosses also play a significant part in other successions of plants, notably in the colonization of rocks and screes and of burned ground. Without mosses and lichens, such areas would be colonized far more slowly and would remain harsh and unsuitable for any other plants to grow for much longer periods of time.