The biogeographical barrier of Transitional Estonia

A zone of landscapes full of forests and mires extends from Lahemaa to Latvia. The area is known as Transitional Estonia. On contemporary satellite photos Transitional Estonia can be well distinguished as a separate belt of forests. Its northern part, lying between the Harju Plain and the Pandivere Upland, is called Kõrvemaa; its southern part, on the western slope of the Sakala Upland, is called Soomaa.

Although the territory of Estonia is small, two geobotanical provinces — the Western-Baltic and Eastern-Baltic — are distinguished here, their boundary crossing the plains of Transitional Estonia. Big differences in the vegetation of the Western-Baltic and Eastern-Baltic provinces — especially in the vegetation of Estonia — are caused by the existence of such a barrier. This barrier, in its turn, has its origin in the geology of the region and its paleogeographical development. Large-scale paludification of the area of former ice-dammed lakes and sea transgressions have presented a natural barrier to plant migration since in the very distant past. On the other hand, such conditions have offered few opportunities for human habitation. Therefore the area has remained relatively untouched by human impact.

Transitional Estonia is typologically divided into two parts by Estonian raised bogs. The habitats of plant species characteristic of Eastern Estonia (Chamaedaphne calyculata) extend to this area on one side, and the species characteristic of Western Estonia (Myrica gale, Trichophorum caespitosum and others) reach it on the other side. A very distinct boundary separates the habitats of Atlantic species, (such as Taxus baccata, Hedera helix, Cladium mariscus) distributed in the western areas, and those of Pontic and Pontosarmatian elements of flora (Silene chlorantha, Gypsophila fastigiata, Anemone sylvestris, Saxifraga hirculus and others) distributed in eastern areas. Habitats of the above-mentioned species, with the exclusion of Myrica gale, do not spread across Transitional Estonia.

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