Despite its small size, Georgia is ecologically very interesting. Located between the forests of northern Eurasia and the tropical deserts of Iraq and Iran, and incorporating Europe's highest mountains and a subtropical coastline, it has Europe's highest level of biodiversity and is a route for many migratory bird species. It is characterised by its complex interaction of west Asian, east European and purely local communities. There's a wide variety of plant communities, with examples of almost all the main habitat types found in Europe-and some of those in Asia; many are highly valuable in terms of biodiversity, including subalpine coniferous forests, meadows, wetlands, peat bogs and lakes; coniferous and beech forests; oak woodlands; caves and mountain gorges; unique Colchic forests with evergreen undergrowth; Mediterranean and sub-Mediterranean communities; steppe grasslands; arid light woodlands; and riparian shrub and forest vegetation along rivers such as the Alazani and Mtkvari.
Forest covers 2.7 million hectares (36.7% of Georgia's area), of which only 59,500ha is artificially planted; around 6% of the natural forest is virgin, and 40% has avoided serious human impact.
Overall, Georgia can be split into two main bio-geographical regions: firstly the Colchic and Caucasian districts, forest landscapes with plenty of autochthonous animals and plants, and others related to middle and eastern European species; and secondly the uplands of the Lesser Caucasus and the Mtkvari district, with species related in some places to Anatolia and the Middle East, and in others to the arid and semi-arid Turanian region, beyond the Caspian. Between these two main regions are mixed zones, notably the Borjomi Gorge and the Trialeti Ridge, as well as the southern slopes of the High Caucasus in eastern Georgia.
The Colchic (or Euxine) district covers most of western Georgia, between the Black Sea, the Meskhetian Mountains, the Surami Ridge and the High Caucasus; the climate is mild and humid, rarely freezing and with a metre or more of precipitation each year, and the characteristic landscape is subtropical forest with well-developed evergreen underwood consisting of many Tertiary relicts (such as Laurocerasus officinalis, Ilex aquifolium and Rhododendron ponticum).
The Caucasus district lies to the north at 2,000m and higher, with a severe climate and over a metre of precipitation per year. It harbours some of the most diverse and distinctive temperate coniferous and deciduous forests in Eurasia, ranging with altitude from subalpine beechwoods (half of the country's forested area), dark coniferous forests and crook-stem woods to subalpine, alpine and subnival plant communities and, above these, bare nival (ie: dominated by snow) landscapes. Its borders are fluid, with many Colchic elements in the west, and Turanian elements in the east; on the northern slopes there are many eastern European and boreal species.
The plateaux of the Lesser Caucasus are largely treeless grassland, either subalpine meadows or mountain steppes, as well as forest and semi-arid steppes. There's a severe continental climate, with annual precipitation between just 400mm and 800mm. The Mtkvari district covers much of Kartli and Kakheti, and is largely arid and semi-arid steppe, with xerophytic Turanian (or Armeno-Iranian) species predominating, and forested only along the banks of the Mtkvari. There's a warm continental climate, rarely dropping below -5°C, with under 400mm of precipitation per year.
The 'mixed' zones, at the borders of these main zones, are the most biologically fascinating regions of Georgia. There are three main mixed zones: firstly the northern slopes of the Trialeti Ridge, from the northwestern side of Tbilisi to the Borjomi Gorge, mostly dry deciduous mountain forests with a temperate climate and 400-800mm of precipitation per year - the fauna and flora are mostly Caucasian, with some Turanian and Colchic elements, and no great diversity; secondly the forests of eastern Georgia, which are relatively similar to the Trialeti forests, but with more Turanian elements - the climate is subtropical/mild, with 400-600mm of precipitation; and thirdly the smallest but most interesting is the Borjomi Gorge, which has a well-balanced range of elements from all over Georgia (although Turanian elements are scarce) with a mild temperate climate and 800- 1,200mm of precipitation per year. The gorge marks the divide between the humid west and the arid east, and between Mediterranean and Turanian fauna.
Endemic species comprise about 9% of Georgia's flora, a surprisingly high proportion for so small a country, with another 5% endemic to the Caucasus. The highest proportion (for instance 87% of western Georgian scree flora) is in certain mountain areas which were turned into islands when the seas rose around 15 million years ago, in the Miocene epoch; the surrounding areas have since dried out and gone their own way biologically, while the humid subtropical forests of the mountains have survived largely unchanged, and many species there now have their closest relatives in Anatolia and Europe. Indeed, the only relative of the Caucasian parsley frog (Pelodytes caucasicus) is P. punctatus in France and northern Spain, while the nearest relative of the Caucasian salamander (Mertensiella caucasica) and M. luschani (in Greece and southwest Turkey) is the gold-striped salamander (Chioglossa lusitanica) in Portugal and northwestern Spain. The population dynamics of the Caucasian salamander and rock lizards are particularly fascinating to scientists, with lots of more or less distinct species living together, some hybridising and some not. Some of the lizards also live in all-female colonies, reproducing by parthenogenesis (asexually). There's lots of interesting micro-evolutionary research to be done here, but the potential is almost unknown to foreign scientists. The most interesting areas are the Meskheti Ridge (from Batumi to Borjomi), Lagodekhi (in northern Kakheti) and also the lower-lying Colchic forests.