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 mosses in peat-forming systems are discussed in terms of differences in growth strategies, access to light, acquisition of mineral nutrients and water and the processes involved in the formation of the micro-topographical structures characteristic for these systems. To keep pace with the vertical growth of Sphagnum, the co-occurring vascular plants require a growth strategy involving continuous movement of the growing point and meristematic tissue upwards and a frequent formation of adventitious 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 vascular plants with an orthotropic growth pattern characterise hummocks, where they form a firm matrix which reinforces and supports the spongy biomass 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 layers. Extended periods of drought often may have deleterious effects on the mosses but smaller impacts on the vascular plants because of their more efficient water conducting system. Different sources of mineral nutrients are used by Sphagnum (atmospheric deposition) and the vascular plants (mineralisation of the organic matter). The presence of Sphagnum, therefore, reduces the supply of nutrient resources to the vascular plants. Sphagnum thereby gains a competitive advantage. A high rate of mineralisation would be beneficial to the vascular plants by increasing 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.