INTERACTIONS BETWEEN SPHAGNUM MOSES AND FIELD LAYER VASCULAR PLANTS IN THE DEVELOPMENT OF PEAT-FORMING SYSTEMS

The interactions between field layer vascular plants and Sphagnum moss es in peat-forming systems are discussed in terms of differences in gr owth strategies, access to light, acquisition of mineral nutrients and water and the processes involved in the formation of the micro-topogr aphical structures characteristic for these systems. To keep pace with the vertical growth of Sphagnum, the co-occurring vascular plants req uire a growth strategy involving continuous movement of the growing po int and meristematic tissue upwards and a frequent formation of advent itious roots. The growth form and architecture of the vascular plants determine the occurrence and distribution of the structural units on a mire, the hummocks, lawns and hollows. Dwarf shrubs and other vascula r plants with an orthotropic growth pattern characterise hummocks, whe re they form a firm matrix which reinforces and supports the spongy bi omass of Sphagnum. In a similar way, clonal herbs stabilise the lawns because of the predominantly plagiotropic, or only weakly orthotropic, growth pattern of the rhizomes and coarse roots in the upper, oxic la yers. Extended periods of drought often may have deleterious effects o n the mosses but smaller impacts on the vascular plants because of the ir more efficient water conducting system. Different sources of minera l nutrients are used by Sphagnum (atmospheric deposition) and the vasc ular plants (mineralisation of the organic matter). The presence of Sp hagnum, therefore, reduces the supply of nutrient resources to the vas cular plants. Sphagnum thereby gains a competitive advantage. A high r ate of mineralisation would be beneficial to the vascular plants by in creasing their growth rates causing shading of the Sphagnum mosses and covering the moss layer by the above-ground litter fall. However, the slow decomposition of Sphagnum litter keeps the system in balance as it will reduce the nutrient supply to the vascular plants.