Christ's thorn formation (Paliureta spina-christi) in the Tbilisi environs (East Georgia, South Caucasus)

Abstract

Christ's thorn formation (Paliureta spina-christi) of Tbilisi environs is studied. This formation is one of the typical representatives of hemixerophilous shrubberies of shibliak type in the Caucasus region and one of the characteristic for vegetation cover of Tbilisi surroundings. Plant communities of Christ's thorn formation with different plots area are fragmentary spread almost all over the territory of Tbilisi environs from 400 to 800 (900) m above s.l.. Plant communities are developed on slopes and plane place with various exposure and inclination, mainly on the grey-cinnamonic and cinnamonic soils. The most xerophilous variants are developed on the erosive bare mother rocks as well. In Tbilisi environs the Christ-thorn's plant communities are either primary or secondary origin. Formation is characterized by rich typological and floristic composition. We identified 6 plant communities: (1) Paliureto-mixtofruticetum gramino-mixtoherbosum, (2) Paliureto-Rhmanetum gramino-mixtoherbosum, (3) Paliureto gramino-mixtoherbosum, (4) Paliureto bothriochlooso gramino-mixtoherbosum, (5) Paliuretum festuceto-bothriochloosum, (6) Paliureto-Astragaleto-Rhamnetum. For each separated plant communities the basic structural characteristics (general projective coverage, projective coverage, distribution and height of layers, sodding degree, dominant-edificator plants, characteristic species, number of species, moss cover, litter, species richness, spectrum of life forms), distribution area in the Tbilisi environs and main physical-geographical conditions (topography, altitude, exposure, inclination, soil type) are given. 190 species of vascular plants, which belong to 40 families and 132 genera, were recorded. In the floristic spectrum leading families are: 1. Poaceae – 27 species (14,2%), 2. Asteraceae – 25 species (13,2%), 3. Fabaceae – 17 species (8,9%), 4-5. Lamiaceae and Rosaceae – 15-15 species (7,9-7,9%), 6-8. Apiaceae, Caryophyllaceae and Rubiaceae – 8-8 species (4,2-4,2%), 9. Brassicaceae – 7 species (3,7%), 10-11. Caprifoliaceae and Rhamnaceae – 4-4 species (2,1-2,1%). The life form spectrum is as follows: hemicryptophytes (including biennials) – 91 (47,9%), therophytes – 58 species (30,5%), phanerophytes – 21 (11,1%), chamaephytes – 7 (3,7%), geophytes – 13 (6,8%). The article is attached full list of recorded plants.