CLIMATIC AND ANTROPOGENIC INFLUENCE IN THE VEGETATION OF EL BAJIO DURING THE HOLOCENE

Paleoecological studies allow us the recognition of patterns and process of evolution, adaptation, migration and extinction of species and ecosystems-Our study area is located in the maar La Alberca in Valle de Santiago, Guanajuato, Mexico, at 20° 23.35´ N-101°12.10´W, inside the biogeographical area known as El Bajio. Inside this crateric lake a core was extracted in the depocenter of the lake, getting a 6 meter core. The core was taken to the laboratory where the core was subsampled every 5 cm for pollen analyses.

72 pollen grains were identified, which were grouped according their ecological affinities into 5 vegetation types: temperate forest, tropical forest, scrubland, wetlands and cultivated. Our results indicated that agricultural activities were present inside the lake since 6000 yr BP. This agricultural activity was dominated by cabbage crops (Cucurbita spp. and Cynocysus) and was more ancient that the maize crop, which started in the lake later, by 3500 yr. B.P.

The agricultural activities caused during the late Holocene that the landscape dominated by the temperated forest of pine and oak during the middle Holocene, were transformed to a park like vegetation, to finally be transformed to a scrubland as the climate changed from a mesic to a dryer climate and the land use became more intensive. Prehispanic agricultural suddenly disappeared from the pollen record at 1200 yr. B.P coinciding with the mayan collapse.