Ecology and late-Quaternary history of the Kurdo-Zagrosian oak forest near Lake Zeribar, western Iran

Abstract

The ecology and distribution of Quercus aegilops ssp. brantii and the associated Pistacia atlantica var. mutica and P. khinjuk are examined as a basis for explaining the Late Quaternary history of the forest in the Zagros mountains. The three species are tolerant of aridity and low temperatures but sensitive to heavy snowfall. In most of the area, summer rain does not occur. Forest expansion is generally limited by the inability of seedlings to survive the four-month summer drought. Pollen diagrams from Lake Zeribar, Kurdistan show the absence of trees during the last glacial period and the migration of forest into the region between 10 000 and 5 500 B.P. This has been interpreted as indicating aridity during the Pleistocene with gradually increasing precipitation in the late glacial and Holocene. However, the sensitivity of these species to snow and their tolerance of low overall precipitation implicate higher snowfall rather than lower precipitation as the cause of absence during the Pleistocene. The inability of seedlings to survive the present summer-dry conditions suggests that summer rainfall, and not higher total precipitation, was the factor that finally allowed migration. These conclusions are supported by independent evidence of increased winter precipitation during the pleniglacial and a period of summer rainfall but low annual precipitation during forest expansion.