Forests of the Tertiary relict tree Zelkova carpinifolia in Colchis (South Caucasus) and their regional vegetation context

Zelkova (Ulmaceae) represents a relict woody genus comprising six extant species with a disjunctive distribution in eastern and southwestern Eurasia. Zelkova carpinifolia is a deciduous tree limited to the Caucasian Ecoregion and its surroundings. Most of its sites are located in the two essential Tertiary refugia of the Northern Hemisphere – Colchis (western Georgia, northeastern Turkey) and Hyrcania (northern Iran, southeastern Azerbaijan). In Georgia, Z. carpinifolia stands are recognized as a national priority habitat. However, data on their environmental conditions, structure and species composition were partial, sometimes contradicting and scattered in the literature. Our main goal was to describe Georgian Colchic Z. carpinifolia forests based a novel dataset of vegetation-plot records and to present them in the broader vegetation context using available phytosociological data. To address this issue we led a phytosociological survey of Z. carpinifolia forests in Georgian Colchis and obtained 35 vegetation-plot records supplemented by the original field data on slope inclination, aspect and soil pH. To explore Z. carpinifolia stands within the broader regional vegetation context, we combined our new dataset with relevés of similar vegetation types from northern Iran, northern Turkey and western Georgia extracted from databases and literature. We classified the vegetation datasets using classification algorithms (beta-flexible clustering and Modified TWINSPAN) and examined the resulting clusters with detrended correspondence analysis (DCA) in terms of diagnostic species and selected environmental variables. Colchic Z. carpinifolia forests exhibited a relatively uniform species composition and structure, with Carpinus orientalis and Quercus robur subsp. imeretina representing the most frequent canopy companions of Z. carpinifolia. The evergreen submediterranean species Ruscus aculeatus often dominated the understory, accompanied by forest generalists, while herbs of dry forests and evergreen species were also frequent. The average vascular plant richness was 24.7 species per 100 m2 . The investigated forests mainly inhabited soils of slightly acidic to subneutral reaction (average pH 6.2). A numerical comparison of Colchic and Hyrcanian Z. carpinifolia forests revealed significant differences: The Hyrcanian ones were much more mesophilous, with Hyrcanian endemics and forest mesophytes being diagnostic species. The underlying causes of this ecological discrepancy remain unclear. In the context of dry deciduous forests of the Euxinian Province, Colchic Z. carpinifolia forests were most similar to the Colchic-Caucasian association Campanulo alliariifoliae-Carpinetum orientalis described from western Georgia. However, Z. carpinifolia stands formed their own sharply delimited cluster, indicating deeper differences than just the canopy dominance. Based on the classification results, we described a new alliance Smilaco excelsae-Carpinion orientalis for the Colchic-Caucasian lowland and mid-mountain dry and xeromesophilous deciduous forests. A mixture of species characteristic of the Caucasus and Colchis is diagnostic for this alliance, including Campanula alliariifolia, Klasea quinquefolia, Polygonatum glaberrimum, Quercus robur subsp. imeretina, Vinca major subsp. hirsuta, Z. carpinifolia. Colchic Z. carpinifolia stands face numerous threats, especially overgrazing by cattle, invasions of alien species and infrastructure development.