The Growth of Sphagnum: Some Effects of Environment

Some of the reasons for studying the growth of Sphagnum are summarized by Clymo (1970). Briefly, they are that the total mass of living plants is great, the physiology of Sphagnum is unusual, and the remains of Sphagnum form a large part of many peat deposits. This paper is concerned with two problems. First is the distribution of the plants on a large scale in relation to H+ and to Ca2+ concentration and supply. In the natural habitats of Sphagnum these factors are usually correlated, and both are correlated with shade, because the associated vascular plants grow larger (or are taxa with larger growth forms) where the water supply has high pH and Ca2+ concentration. For example, S. squarrosum* and S.fimbriatum may often be found in the shade of Betula spp. and Salix spp. in fen carr, in flushed zones in woodland, or in the central zone of valley mires, in situations where the water may be of pH 6-7 and Ca2+ concentration 1 m-equiv./l. At the other extreme, S. rubellum and S. fuscum growing on hummocks on raised or blanket bog may be in contact with water of pH just over 3, and of Ca2 + concentration less than 0-01 m-equiv./l. In these situations there may be some shading by dwarf shrubs, for example Calluna vulgaris, or no shade at all. (There is nearly always self-shading, by Sphagnum of the same taxon, but in such cases the response is by the individual plant, rather than by the taxon.) The second problem is the smaller scale distribution of the different species of Sphagnunm in the microhabitats of pool, lawn and hummock. Some species appear to be rather closely restricted: S. cuspidatum to pools for example. Others are usually found in one habitat but occasionally in others: S. fuscum and S. rubellum in drier situations (often hummocks). Yet others are found in a wider range of habitats; S. papillosum and S. imbricatum are examples. To throw light on these problems, experiments were made on the growth of several species of Sphagnum in relation to factorial combinations of H + and Ca2 + concentration, and to factorial combinations of water table and shading. Subsequent experiments were made on 'desiccation tolerance' and on rate of water loss in relation to water level.