Ecological gradients, subdivisions and terminology of north‐west European mires

Summary 1 The historical development of mire ecology and terminology is reviewed in relation to evolving concepts and perceptions, and the diverse schools and traditions of vegetation and habitat description and research. 2 Most ecological and floristic variation within north‐west European mire vegetation is accounted for by three ecological gradients: the acid base‐poor vs. neutral, base‐ and bicarbonate‐rich gradient; the gradient in fertility related to availability of the limiting nutrient elements N and P; and the water level gradient. Effects of salinity and the spring–flush–fen gradients are of more local significance, usually easily recognized. Land use is an important additional factor. 3 The mineral‐soil‐water limit between ombrotrophic and minerotrophically influenced sites is not sharp, and cannot be related to consistent differences in either vegetation or water chemistry. It should be abandoned as a general main division within mires. 4 The most important natural division is between ‘bog’, with pH generally < 5.0, low Ca 2+ , and Cl – and SO 4 2– as the main inorganic anions, typically dominated by sphagna, ericoids and calcifuge Cyperaceae, and ‘fen’, with pH generally > 6.0, high Ca 2+ and HCO 3 – , vegetation rich in dicotyledonous herbs and ‘brown mosses’. This division is reflected in a bimodal distribution of pH. 5 The terms oligotrophic, mesotrophic and eutrophic should refer only to nutrient richness (fertility, mainly N and P), not to base richness (metallic cations and pH). 6 It is recommended that ‘mire’ should embrace both wetlands on peat and related communities on mineral soils, that ‘bog’ (unqualified) should encompass both ombrotrophic and weakly minerotrophic mires, including ‘bog woodland’, and that ‘fen’ should be restricted to base‐rich mires but include both herbaceous and wooded vegetation (‘fen carr’). 7 Definitions are given for a range of broadly defined categories that should cover most vegetation types commonly encountered. The relation of these to British national vegetation classification types and to major units in European phytosociology is outlined.