Ecosystem responses to climate and disturbances in western central Mexico during the late Pleistocene and Holocene

Central Mexico is a complex area with late Quaternary volcanic activity, climatic diversity and a long history of human occupation. Pollen, microcharcoal and magnetic susceptibility analyses of two sedimentary sequences from Lake Zirahuen in western central Mexico spanning the last 17,000. years provide evidence of a highly dynamic environment. During the late glacial Pinus forest developed around the lake, indicating cold and dry conditions. During the last deglaciation climatic amelioration was recorded at 13.5. ka, driving a strong and rapid change in vegetation composition and increase in lake levels. From the latest Pleistocene to early Holocene a hiatus, probably related to an erosive event, is recognized in the northern sequence. The central sequence spans the entire Holocene and reveals three periods of important ecological changes during the early Holocene. A first episode between 9.5 and 9.0. ka with a decrease in pine forest seems to have been associated with summer insolation increases. A second peak of forest change at 8.2. ka and was probably associated with the cold oscillation documented in the North Atlantic. A third abrupt change was evident from 7.5 to 7.1. ka with an anomalous plant community related to wetter Holocene climates and possible to a volcanic event. An episode of dry conditions was recorded from 4.5 to 4.2. ka, which was related to an increase in ENSO activity. Human influence over the landscape was evident from 3.5. ka to the present. The Zirahuen records offer a complex history of a landscape characterized by short and long term vegetation changes associated with factors ranging from global climate to local disturbances.