VEGETATIONAL RESPONSE TO HOLOCENE CLIMATIC-CHANGE - POLLEN AND PALEOLIMNOLOGICAL DATA FROM THE MIDDLE ATLAS, MOROCCO

Pollen data from a radiocarbon-dated lake-sediment core from the Middl e Atlas of Morocco show that forests of evergreen and winter-deciduous oaks (Quercus rotundifolia, Q. canariensis), present from the start o f the Holocene, were invaded at 6200 BP by Cedrus atlantica in low num bers. Two thousand years later, after a short period characterized by higher deciduous oak frequency, Cedrus increased to its present abunda nce. These changes may have been in response to increasing effective m oisture as summer temperatures decreased with declining northern-hemis phere seasonality, under the influence of precessional forcing. Palaeo limnological data from the same core show that the lake level fell sha rply in five 200-400 yr-long arid intervals, but the pollen data show little or no evidence of a vegetational response. This suggests that s ummer (growing-season) rainfall remained adequate during the dry inter vals, whereas depletion of the groundwater aquifer was the result of r educed winter precipitation. Anthropogenic exploitation since c. 1300 BP has had a greater effect on the forest ecosystem than any of the Ho locene arid intervals.