Environmental history of the dry forest biome of Guerrero, Mexico, and human impact during the last c. 2700 years

Two lake sediment cores from Madre del Sur mountain range, Guerrero State, west-central Mexico were studied to examine the past dynamics of the dry forest biome. Pollen, spores of coprophilous fungi, cyanobacteria and lithological changes are presented. The 390-cm Tixtla core (17°30′N, 99°24′W, 1400 m altitude) represents the last 2700 cal. yr; the 340-cm Huitziltepec core (17°45′N, 99°28′W, 1430 m altitude) represents the last 2050 cal. yr. Pollen shows climate-dependent competition between dry deciduous forest (with Asteraceae, Chamaesyce-type, Bursera, Euphorbia, Myrica and Lysiloma-Pithecellobium-type as main components) and moister (mesophyllous) forest (with Pinus, Quercus, Alnus and Juglans). Human impact on the environment is apparent during the entire record, as evidenced by crop cultivation (Zea mays, Amaranthus, Capsicum, Chenopodium), coprophilous fungi (Podospora-type and Sporormiella-type), deforestation (at Tixtla in particular) and lake eutrophication, ie, cyanobacteria blooms. Environmental change at both sites is summarized as follows. Period 1 (2700-2450 cal. yr BP): abundant dry deciduous forest and climate conditions drier than today. Period 2 (2450-1950 cal. yr BP): little change in proportions of dry deciduous and mesophyllous forest but floral composition of the forests differed between sites. Human occupation was low. Period 3 (1950-1550 cal. yr BP): dry deciduous forest decreased while mesophyllous forest increased, indicating more humid climatic conditions. Populations expanded significantly around 1950 cal. yr BP. Period 4 (1550-1070 cal. yr BP): maximum expansion of mesophyllous forest. Climate was more humid than today. Pre-Hispanic populations at Tixtla reached high numbers leading to deforestation and eutrophication. Period 5 (1070-c. 750 cal. yr BP): shows abundant mesophyllous forest and lowest proportions of dry deciduous forest. Climate... [ABSTRACT FROM AUTHOR]