Natural habitat and vegetation types of river gravel bars in the Caucasus Mountains, Georgia

摘要

River gravel-bar habitats are highly endangered. They are still well-preserved in the Caucasus, but developing conservation strategies is burdened by the lack of data from this region. We studied vegetation and habitat types on gravel bars of 22 rivers in Georgia, including successional stages from open early-successional herbaceous vegetation to scrub. We distinguished five vegetation types based on vegetation physiognomy and beta-flexible clustering of species composition, and described them as phytosociological vegetation units: Early-successional herbaceous vegetation at higher elevations was described as the new associationEpilobietum colchiciand that at lower elevations as thePetrorhagia saxifraga-Crepis foetidacommunity. The grassland dominated byCalamagrostis pseudophragmitesand scrub vegetation were assigned to the associations previously described from Central Europe (Tussilagini farfarae-Calamagrostietum pseudophragmitae,Salici purpureae-Myricarietum germanicaeandSalici incanae-Hippophaetum rhamnoidis). We established diagnostic plant species for each type using the fidelity calculation and related these types to environmental variables. We further compared them with the previously published data on gravel-bar vegetation from the Russian part of the Caucasus and with European systems of habitat classification. This study demonstrates that vegetation and habitat types occurring in Georgia correspond to those recognized earlier in Europe, and can be easily linked to the European systems of habitat classification. Unlike in other parts of Europe, these habitats are still well-preserved on rivers with natural hydrological dynamics, but they are threatened by plans of dam building and other river regulations. Our study provides baseline data for developing conservation strategies for the Caucasian gravel-bar habitats.