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Dengler, J; Boch, S: Forest-edge communities (Trifolio-Geranietea sanguinei) on the island of Saaremaa (Estonia): Phytosociology and biodiversity patterns. In: Dengler, J., Dolnik, C., Trepel, M. [Eds.]:Eds Flora, Vegetation und Naturschutz zwischen Schleswig-Holstein und Südamerika - Festschrift für Klaus Dierßen zum 60. Geburtstag. Flora, Vegetation, and Nature Conservation from Schleswig-Holstein to South America – Festschrift for Klaus Dierßen on Occasion of his 60th Birthday., Mitteilungen der Arbeitsgemeinschaft Geobotanik in Schleswig-Holstein und Hamburg, 65, 257-286 (2008)
Key words: classification; ecotone; hemiboreal zone; Knaution dipsacifoliae; northern Europe; site condi- tion; syntaxonomy; species density; tall-forb community
We studied the communities of the class Trifolio-Geranietea sanguinei on the Baltic island Saaremaa (Es- tonia, hemiboreal zone). These tall-forb communities of anhydromorphic, nutrient-poor sites usually grow as ecotones alongside forest edges. We documented the variety of community types occurring on the island by 41 relevés of 5 m2 plots, in which we recorded all vascular plants, bryophytes, and lichens, and meas- ured some soil parameters. According to the syntaxonomic analysis, the stands on Saaremaa belong to 12 communities, including 11 previously published associations and one unit with presumable association rank that has not yet been described formally. Of these communities, only one belongs to the acidophytic sub- class Melampyro pratensis-Holcenea mollis, while the others are equally divided between the mesophytic order Origanetalia vulgaris s.str. and the xerophytic order Antherico ramosi-Geranietalia sanguinei within the basiphytic subclass Trifolio-Geranienea sanguinei. Within the first order, we accept the alliances Knau- tion dipsacifoliae, which is nomenclaturally validated here, and Trifolion medii s.str.; within the second order we accept the alliances Galio veri-Geranion sanguinei and Geranion sanguinei s.str. For all commu- nities, we review the known distribution within the Nordic and Baltic region and provide a comprehensive overview of names that have previously been applied here. Due to the prevailing limestone bedrock, soil pH (H2O) of the plots was consistently high (6.0–7.8). The communities analysed showed high species densities throughout, with association means from 24 to 45 species on 5 m2. Compared to similar analyses of the same associations in NE Lower Saxony (n = 7) and NE Germany (n = 6), species densities of the Estonian stands were 1.2–2.2 times higher, with mean ratios of 1.82 relative to Lower Saxony and 1.60 relative to NE Germany. These differences were statistically significant in most cases and are in accordance with previous findings in various dry grassland types. As most of the causes suggested for dry grasslands do not hold for forest-edge communities, however, other explanations must be found. We consider the shorter vegetation period in the hemiboreal zone as a potential candidate, as this may reduce competition and thus enhance local coexistence of many species. Finally, we suggest further research that might help to explain the surprising biodiversity pattern we discovered.
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