Hyrcanian zone: A green belt with distinct vegetation cover
The Hyrcanian vegetation zone, also called Caspian forest, is a green belt stretching over the northern slopes of the Alborz Mountains and covers the southern coasts of the Caspian Sea. This area stretches from Astara in the northwestern Iran to the Gorgan vicinity in the northeastern part of the country.
This area is approximately 800km long and 110km wide and has a total area of 1.85 million hectares comprising 15% of the total Iranian forests and 1.1% of the country’s area. The Alborz Mountains lying between the Caspian Sea and the Iranian Plateau has resulted in a climate causing a distinct vegetation cover. Hyrcanian forests stretch out from sea level up to an altitude of 2,800m and encompass different forest types thanks to their 80 woody species (trees and shrubs). The area is rich in hardwood species, but there are only four genera of endemic softwood (conifer) trees including yew, Greek juniper, oriental arbor-vitae and Italian cypress.
The primary function of the Hyrcanian forests, other than wood production, is supportive and environmental: They play a vital role in the conservation of soil and water resources and keep nature at balance on these susceptible steep mountain slopes. Rapid urbanization and industrialization, intensive grazing, over-utilization of forests for firewood production and farming in wooded areas are amongst the main causes of deforestation in this area.
The western and eastern parts of the Hyrcanian region have markedly different regimes. The higher amount of rainfall over the western Hyrcanian region during autumn is due to the location of this area at the head of northeasterly winds originating from the Siberian anticyclone or Polar front. These winds sweep the surface of the Caspian Sea and bring much moisture to the south Caspian region before becoming destabilized at the front zone with hot/dry continental air masses descending from the central Iranian high plateau.
The Hyrcanian zone is a humid zone in the north of Iran. The average annual rainfall ranges between 530mm in the east and 1,350mm in the west, reaching up to an occasional record of 2,000mm in the west.
The maximum annual rainfall is experienced during spring and late fall and winter. Relative humidity is also constantly high with an average value fluctuating from 74.6% in the east to 84.6% in the west, rarely dropping below 60% at the hottest hours.
Thus, the region can be considered as one of the world’s ever-wet areas. The rise in humidity at the highest temperatures results in the saturation of air and subsequent cloud formation in the afternoons especially on the northern slopes
According to climatic data from meteorological stations, the average annual temperature in the Hyrcanian region varied from 15°C in the west to 17.5°C in the east over the past decade. Temperature of the warmest month ranges from 28°C to 35°C while that of the coldest month is between 1.5°C and 4°C. Summer temperature ranges between 20°C and 30°C.
In general, the Hyrcanian climate is warm Mediterranean in the east and temperate and semi-temperate Mediterranean and occasionally temperate xeric in the central and western parts.
Forests of Alborz Mountains
The Alborz Mountains begin with the Azerbaijani frontier ranges (Caucasus) in the northwest and extend northeast, not far from the border with Turkmenistan.
This high mountain chain forms more or less an unbroken wall with elevations over 5,000m, and receives most of the precipitation from the Caspian and Black Seas.
Rainfall is evenly distributed over the year. The climate in the western part of the Caspian region is very humid with cold winters and without a dry period.
The climate at the oriental beech sites of Iran, which are the most important forest habitats, is very humid and cold in lowlands and midlands (up to 1,700m) and very humid and ultra-cold in highlands (up to 2,200m). The frost periods in midlands and highlands lasts three and five months, respectively.
The Asalem beech forests, the westernmost Hyrcanian forests, with an annual precipitation of 2,000mm, are the wettest beech forests of the country. Moving toward the east, the amount of precipitation rapidly drops, falling down to half in the Gorgan beech forests.
Moreover, the major part of the precipitation at Asalem occurs in the fall and summer whereas that of Gorgan occurs mostly in fall and winter.
The above is a lightly edited version of a chapter of ‘Forests of Iran: A Treasure from the Past, a Hope for the Future’ written by Khosro Sagheb Talebi, Toktam Sajedi and Mehdi Pourhashemi and published by Springer in 2013.