عنوان مقاله [English]
Iran is located in the North Temperate Zone from 25 to 40 degrees latitude and 44 to 63 degrees longitude, with a total area of approximately 1,650,000 Km2. A 50% of total lands area is covered with high mountain ranges. Elevations range from 26 meters below sea level on the shores of the Caspian Sea to 5860 meters above sea level at the pick of the Mt. Damavand. Drought or water deficiency is one of the most critical climatic factors in Iran. About 50% of Iran can be classified as arid or semi-arid zones. Climate parameters, particularly precipitation varies significantly in different parts of the country. There is not a good annual rainfall distribution in most regions of Iran, which limits the plant development and growth. Not only high temperature in southern, central and lowlands of Iran is a limiting factor, but also low temperature in northern, western and highlands is another limiting factor too. The Caspian region receives the largest part of the country’s precipitation while the central desert (Dasht-e-Lut) is faced with permanent drought. Forest ecological zones in Iran could be categorized as: a) North, Caspian forest, b) West, Zagros forest, c) North West, Arasbaran forest, d) South, Subtropical forest in Persian Gulf areas, and e) Central, Scattering forests. Some of the main tree species of Caspian forests: Fagus orientalis, Carpinus betulus, Acer velutinum, Quercus castaneifolia, Fraxinus excelsior. In this study, whole forest areas in southern part of Caspian Sea were monitored by study on vegetation map and visiting field. Three points representing major part of Hyrcanian Forests were selected from wet part in west to drier part in east. Four meteorological stations data used for investigation. Most of climatic factors including maximum, minimum and mean annual temperature; daily and annual precipitations were investigated. Mainly trends of mean annual temperature and annual precipitations were used for conclusion. It can clearly be concluded that during last half century climate in forest area in Caspian region become warmer. Precipitation trends especially in Anzali station that has highest precipitation records in Iran and also in Gorgan station have decreased. Precipitation trends in Rasht and Baboulsar stations have shown positive change. During last 49 years in Rasht station mean annual temperature increased about 1.28 °C and even its minimum temperature shows 2.45 °C increases. Increase mean annual temperature in Baboulsar station in last 54 yeas is about 1.44 °C and its minimum temperature shows 1.80 °C increases. Decrease of annual precipitation in Anzali station during last 54 years of records is about 409.4 mm and amount of decrease of precipitation for the period of last 53 years in Gorgan station is about 55.6 mm. Distribution of plants are directly depends upon temperature and precipitation conditions in each climatic zone. Generally with increasing about 100 meter of elevation amount of temperature will decrease by one degree of centigrade. In study zones especially in Gilan and Gorgan areas temperatures shown more than one degree increases and main species of vegetation cover moved upward about 100 meters. All data statistically were analyzed.