Community and species responses to water level fluctuations with reference to soil layers in different habitats of mid-boreal mire complexes

Abstract

The present paper discusses water level fluctuations in different parts of boreal mire complexes (eleven localities), mainly aapa mire complexes, on the basis of measurements performed by means of shallow observation wells and a few deeper observation tubes (piezometers) in the coastal half of the southern aapa mire zone in Finland. The sites represented intact vegetation from 12 different habitat types (communities), which were divided a priori into habitats with a stable surface moisture status (stable habitats) and into habitats with an unstable surface moisture status (unstable habitats). In stable habitats water level fluctuations took place according to the acrotelm-catotelm model, but the unstable habitats clearly deviated from the general pattern: water level fluctuations in them were not at all concentrated to the surficial, porous peat layer. Direct gradient analysis was used for arranging the communities along the water level fluctuation gradient. Variability of the water table, using 80% amplitude of water table residence, was used for the arrangement. The gradient was split into three groups: (1) habitats with a slightly fluctuating water table, (2) habitats with a considerably fluctuating water table and (3) habitats with an extremely fluctuating water table. The last group nearly corresponded to aro wetlands, and represented a very special habitat type. Indirect gradient analysis (NMDS ordination) also revealed the water level fluctuation gradient along with the gradient of traditional water level categories. According to the results of direct and indirect gradient analysis, the water level fluctuation seems to be an independent and important vegetation gradient. In peatlands, it occurs alongside with the traditional gradient of water level categories reflecting the mean water table. The responses of species to the range of water level fluctuations seem to reflect their tolerance to disturbances and evidently to seasonal drought. Most Sphagnum species are absent or in poor condition in habitats with extremely fluctuating water table. Vascular plant species that experienced most extreme water level fluctuations (Carex nigra, Juncus filiformis) have earlier been regarded as disturbance indicators. In addition, the difference between the piezometric water level and simultaneously measured water table depth reached the highest values within the habitats of those species (i.e., within Polytrichum commune aro wetlands) showing the downward direction of water movement in sandy mineral soil.

Regional and Vegetation-Ecological Patterns in Northern Boreal Flark Fens of Finnish Lapland: Analysis from a Classic Material

“…The extremity appears in the pattern size (patches to small areas) and in the environment of the vegetation. The fen vegetation may be on a rock outcrop (Koitelainen Hill), on slopes (Riisitunturi Hill) or on flat, sandy substratum (Hirvisuo), but all the environments allow the loss of water from a thin peat layer, leading to repeated seasonal drought, evidently across the whole peat column (Laitinen et al 2008b). The latter circumstance is not characteristic of mires proper as defined by Tahvanainen (2005), where the water table resides continuously within the peat layer.…”

“…However, all of the extreme cases mentioned above may be viewed as belonging to a specific functional type of lawn mires with thin peat, namely the water-fluctuation lawn-fens (Laitinen 2008), whose best-known vegetation type Trichophoro cespitosi-Sphagnetum compacti occurs mainly in the subalpine belt of the boreal zone of northern Europe according to Dierssen (1995). A thin peat layer with high humification degree, caused by repeated aeration during drought periods, characterises the water-fluctuation lawn-fens; alongside the specific community composition, which appears to be partly structured by the seasonal drought tolerances of individual fen species (Havas 1961, Heikkilä & Lindholm 1988, Laitinen 2008, Laitinen et al 2008b.…”

Methane flux dynamics during mire succession

“…For CO 2 dynamics, the compositional change in vegetation was the driving factor (Leppälä et al 2008), whereas CH 4 dynamics appeared to be primarily driven by peat accumulation and the associated high water retentions capacity. At the early successional stages, the peat layer is shallow and the water table can vary greatly (Laitinen et al 2008), while at the later successional stages, the thicker peat layer maintains a higher and less variable water table.…”

A ground-penetrating radar study of the Vaidasoo bog (Estonia): no crater structure exists

“…De spite be ing formed of sand and gravel, glaciofluvial de pos its hin der sur face wa ter flow in the con sis tently slightly north wards tilted to pog raphy. A sim i lar ef fect was also ob served by Laitinen et al (2008) and The Proc tor and Red fern Group Ltd. (1984). As con stantly wet con di tions were pres ent, the growth of a peat-form ing flora and the ac cu mu la tion of a peat-started.…”