Phylogeography and population genetics of the riparian relict tree Pterocarya fraxinifolia (Juglandaceae) in the South Caucasus

We aimed to (i) assess the extant genetic diversity of the riparian relict tree Pterocarya fraxinifolia across its current distribution range in the South Caucasus, including the past refugial areas Colchis and Hyrcan, and (ii) test if a separation of these areas is reflected in its phylogeographic history. Genetic diversity of natural populations was examined using nuclear microsatellite and plastid DNA markers. Spatial genetic structure was evaluated using Bayesian clustering methods and the reconstruction of plastid DNA networks. Divergence times of Colchic and Hyrcanian populations were estimated via divergence dating using a relaxed molecular clock. Allelic richness, private allelic richness, and expected heterozygosity were significantly higher in Hyrcan than in Colchis and the Greater Caucasus, and significant genetic differentiation was revealed between the two groups. Whereas only two plastid haplotypes were detected for the Colchic and Caucasian populations, the Hyrcanian populations displayed 11 different haplotypes. Significant isolation by distance was detected in Hyrcan. The most recent common ancestor of all P. fraxinifolia haplotypes was dated to a time well before a suggested glaciation period in the Caucasus during the late Pliocene (5.98 Ma [11.3–2.48 Ma HPD]). The widespread Colchic haplotype that also occurs along the southern slope of the Greater Caucasus and reaches south-eastern Azerbaijan has appeared more recently (0.24 Ma [1.41–0 Ma HPD]). This diversification pattern of Colchic haplotypes from ancient Hyrcanian haplotypes suggests a colonization of the region from south-east to north-west that predates the last glacial maximum (LGM). Natural populations of P. fraxinifolia show low-to-intermediate levels of genetic diversity and a significant decrease of diversity from Hyrcan to Colchis. However, the genetic differentiation between Colchic-Caucasian and Hyrcanian populations for nuclear markers suggests that independent gene pools existed in both areas at least since the LGM. Particular attention to conservation seems justified for the more diverse Hyrcanian populations.     

Vegetational patterns and distribution of relict taxa in humid temperate forests and wetlands of Georgia (Transcaucasia)

Small-scale vegetational patterns of Georgian lowland and montane forests and wetlands are documented by species per vegetation unit, function of species in particular vegetation units and vegetational sketches. The humid warm-temperate climate of western Georgia contributes to a different spatial pattern than known for most of Europe. One characteristic feature of western Georgian relict vegetation is the co-occurrence of 'Mediterranean', 'Sub-mediterranean', temperate, and Tertiary relict species, as well as of species normally confined to either zonal or azonal vegetation, in the same or closely related vegetation units. The extant distribution of Tertiary relict taxa strongly depends on how they are niching into changing environments. Niching strategies are assumed crucial for extinction or survival in times of rapid climatic/environmental changes. For Tertiary relict plant taxa in western Georgia we found four different niching strategies which also hold true of other northern hemispheric Tertiary relict plant taxa. Some amphibians and reptiles display similar distribution patterns and niching strategies as do relict plant taxa. A number of relict taxa in the warm humid regions of western Georgia occupy niches in swamp forests which might represent 'primeval' environments of species which are at present also elements of meso-Mediterranean vegetation and of thermophilous forest edges and hedges in Central Europe.