Mire induction, ecosystem dynamics and lateral extension on raised bogs in the southern coastal area of Finland

Abstract

The origins, development and lateral extension of the mire ecosystems of five raised bogs in Southern Finland representing different mire complex types were studied by means of 71 basal peat radiocarbon dates, stratigraphic analyses and detailed morphometric measurements and the results were compared with earlier theoretical (hydrodynamic) mire growth models. The purpose was to determine whether the development of a mire ecosystem is governed primarily by autogenous or allogenous processes and to assess what part is played in this by local environmental factors.

Three of the mire studied, Reksuo, Kontolanrahka and Maisaarensuo, are the result of the paludification of forest land and two, Munasuo and Punassuo, of the filling in of waterbodies by vegetation. Peat formation began around 8530, 7880 and 5860 B. P. respectively in the former cases and around 7190 and 4360 B.P. in the latter, precipitated by both internal and external factors. The later lateral extension of the mires has been regulated principally by the small-scale topography, for in each case the isochrones of peat age as determined from the radiocarbon dates coincide to a considerable degree with the contours of the mire bottom. The mire ecosystem proved to be a highly expansive one, so that in the absence of topographical barriers the mires have invaded the surrounding mineral soils at a mean rate of several meters a year. This lateral extension is seen to have taken place largely at a time when the mires were still entirely minerotrophic. The interval 6000-4000 B.P. would appear to have been a more peaceful period, probably as a consequence of drier climatic conditions than at present, and lateral extension is also observed to have slowed down significantly over the last 2000 years, during which time considerable vertical growth has taken place. The results suggest that extension of the mires in Southern Finland must have declined to a relatively minor level soon after reaching a state of equilibrium and to have diminished constantly from then onwards. This should be remembered when estimating the carbon balance of the mires of the Northern Hemisphere during the Holocene.

The results do not fully corroborate the theoretical models that emphasize the autogenous nature of the development of raised bogs, as (1) the mires were initiated over wide areas at one time, (2) lateral extension did not take place in the form of a steady process but rather went through faster and slower phases, (3) great variations have occurred in vertical peat growth even between different parts of the same mire, and (4) a marked scatter was to be seen in the diagrams of age vs. depth constructed for the dates of the basal peat layers. The mires have been closely bound to their physical environments throughout their history, and the present work reminds us that however important theoretical modelling may be for analysing natural processes, the abstractions that emerge from it are seldom of much practical significance when studying actual situations prevailing in nature.