Caucasus

Caucasus sits on the land bridge between the Black Sea and the Caspian, with the Greater Caucasus range running west-to-east across the middle. To the north lie the steppe plains of southern Russia; to the south, the highlands of Armenia and the dry plateaux of eastern Azerbaijan that fall toward the Caspian. The full hotspot covers roughly 500,000 square kilometres, climbs to 5,642 metres at Mount Elbrus, and drops to around -28 metres at the Caspian shore.

Climate splits sharply across the divide. The western flank, facing the Black Sea, receives some of the heaviest rainfall in temperate Eurasia and supports the Colchis broadleaf rainforest — a relict of the Tertiary that survived the Pleistocene glaciations and was inscribed as a UNESCO World Heritage site in 2021. The eastern flank, in the Caspian rain shadow, turns dry and continental, with arid steppe and semi-desert in lowland Azerbaijan. Between the two extremes are mixed forests of beech (Fagus orientalis), Caucasian fir (Abies nordmanniana), and Oriental spruce (Picea orientalis), grading into subalpine meadow and high stony tundra.

The Caucasus is botanically dense out of proportion to its size. Around 6,500 vascular plant species grow here, of which roughly 1,600 — about a quarter — are found nowhere else. The understory holds many genera familiar from northern gardens: Galanthus (the snowdrop has its centre of diversity here), Cyclamen, Helleborus, Primula, and a wealth of Rhododendron, including the alpine Rhododendron caucasicum. The Colchic rainforest carries an unusually rich evergreen layer for a temperate forest — Rhododendron ponticum, Ilex colchica, Ruscus colchicus, Hedera colchica — survivors of warmer pre-glacial climates. The Hyrcanian forests on the Caspian's southern edge extend the same Tertiary relict story across the Azerbaijani border into northern Iran.