Sphagnum Niche Diversification in Two Oligotrophic Mires in the Southern Alps of Italy

Abstract

Niche breadth and niche overlap values for water table depth, pH, conductivity, and base cation concentrations in mire water were calculated for nine Sphagnum species at two nutrient-poor mires in the southern Alps of Italy. Water table depth is more important than chemical status in controlling the distribution of Sphagnum species. The chemical status of mire water is best indicated by water pH. The Sphagnum species occupying the extremes of the water table gradient exhibit narrow niche breadth and a low niche overlap values. For these reasons they are best regarded as specialists. This is particularly evident for hollow species. The sharp segregation of hollow Sphagna along both the water table depth and the chemical gradient is thought to be determined by higher degree of competition taking place in hollow habitats. On the other hand, hummock species appear to be segregated by physiological tolerance rather than competition. The Sphagnum species occupying the middle of the moisture gradient exhibit broad niche breadth and overlap values, but most of them tend to segregate at least along one of these two gradients. Sphagnum species are important components of bogs and poor fens where they form a bottom layer characterized both by densely packed, individual plants and by several species growing in close contact (Ilomets 1988; Rydin 1986; Vitt & Slack 1975). For such reasons, competition for space and resources can be supposed to play an important role in those habitats (Li & Vitt 1995; Okland 1990b; Rydin 1986, 1993; Vitt & Slack 1984). Depth to the water table, peat and water chemistry, peat productivity, and light seem to be the main environmental factors affecting Sphagnum distribution on mires (Malmer 1986; Okland 1990a). In ombrotrophic mire water table depth and biotic interactions may play a major role in affecting the plant species distributional patterns (Karlin & Bliss 1984; Rydin 1993). Niche breadths and niche overlaps can be used to describe the relationships between organisms and their environment (Hutchinson 1978; Pianka 1986). All papers published dealing with Sphagnum niche breadth and overlap indices revealed a considerable niche differentiation along the main environmental factors (Gerdol 1995; Gignac 1992; Nordbakken 1996; Okland 1986, 1990b; van der Molen et al. 1994; Vitt & Slack 1984). Very few studies have been published about niche relationships of Sphagnum species in southern Europe (Gerdol 1995). Regional studies may be particularly useful since the response of plants may vary within their distributional area (Aletsee 1967). For these reasons, the present study was designed to: 1) quantify niche breadths and niche overlap of nine Sphagnum species with respect to mire surface water chemistry and water table depth, 2) determine the relative importance of each gradient in segregating Sphagnum species, and 3) describe the relationships among Sphagnum species.