8.1.2 Mixotrophic mires 8.1.2.1 Mixotrophic grass mires (site type 3.1.2.1) Mixotrophic grass mires are situated on plains or slightly declining areas on the margins of raised bogs or in depressions with poor runoff. Part of the plants will still get necessary nutrients from groundwater, but several other plant species are forming hummocks and for their roots the groundwater is already inaccessible due to the thick peat layer: these plant get nutrients mainly from precipitation. In this way, mixotrophic grass mires represent an intermediate succession stage between minerotrophic and ombrotrophic mires. Soils are Eutri-Dystric Histosols. For the bush layer Salix rosmarinifolia, S. lapponum, S. myrtilloides and Betula humilis are typical. For the field layer beside the specific mesotrophic mire species (Trichophorum alpinum, Carex chordorrhiza, C. irrigua, C. pauciflora, Dryopteris cristata) species of both eutrophic and oligotrophic mires are characteristic. In the moss layer together with species of minerotrophic mires Sphagnum spp. have a considerable percentage (>10% coverage). Number and area. 1,601 areas on 33,904 hectares have been registered as mixotrophic grass mires. Of these areas, 1,292 (on 24,087 hectares) include only habitats of this site type, while in the marginal parts of other areas habitats of other site types are also represented (mainly mixotrophic quagmires, poor fens and/or mixotrophic bog forests). Additionally, 356 areas include mixotrophic quagmires as marginal habitats. The overall surface of open and treed mixotrophic quagmires in Estonia may be estimated to ca 35,000 hectares. This is three and a half times more than the estimate of Ilomets et al. (2010), according to which open mixotrophic grass mires equal to only ca 10,000 hectares (Table 6, row 1.4.1). Although in this table treed mixotrophic grass mires are included within wooded transitional mires on the row 1.4.2 (8,000 ha), it is clear that the respective estimate of the surface of mixotrophic grass fens (as well as mixotrophic forests) is underestimated to a great extent. Distribution. Mixotrophic grass mires are distributed unevenly all over Estonia (Fig. 16). Larger and more important areas are concentrated in western Estonia (district of West Estonian middle size and large mires) and easternmost Estonia (subdistricts a and c of the district of Central and Eastern Estonian large mires; after Allikvee, 1995). In western Estonia, large mixotrophic grass mires are found mainly in Läänemaa. In the southern part of this district, they are situated in Tuhu and Lihula mire complexes; the eastern part of Tuhu mire complex (Oidrema mire) is the largest representative of this site type (1,174 ha). In the northern part, large mixotrophic grass mires are situated in Läänemaa Suursoo and Orkjärve mire complexes, but also in Leidissoo and Marimetsa mires. In eastern Estonia, vast and relatively intact mixotrophic grass mires are located in the Emajõe Suursoo mire complex. Varnja, Surnusoo, Jõmmsoo, Pedaspää a.o. mires (larger units comprise 382–924 ha) are separated by the Emajõgi River and its tributaries. Some of these areas are in the succession stage between minerotrophic and mixotrophic mires. The most important mixotrophic grass mires in Alutaguse (northeastern Estonia) are found in Muraka, Valgesoo and Agusalu mire complexes. There they are somewhat smaller than in the two previous regions (the largest being Matkasoo, 547 ha), but their number and density are remarkable. Most areas of this habitat type in Puhatu mire complex have been degraded due to the neighbouring oil-shale quarries and alkaline air pollution from power stations (see Chapter 5.4), but some are still in good state. Totally degraded are mixotrophic grass mires of Kõrgesoo mire complex.
ESTONIAN MIRES: INVENTORY OF HABITATS
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