Holocene palaeoecological changes in a transitional climate zone of western-central Mexico: The pollen and non-pollen palynomorphs record

The Alberca de Tacámbaro (AT) is located at a transitional zone between tropical and temperate climates at the core of the North American Monsoon area. This location features a mosaic vegetation comprising pine-oak forests and tropical dry forests. Currently there is no information regarding the dynamics or evolution during the Holocene of these vegetation types. The palynological record of the last 9400 years BP preserved in the lacustrine sequence of the AT was used to document changes in the composition of terrestrial and aquatic communities, evenness, and rate of change. Pinus and Quercus forests dominated the terrestrial palynological record, but mesophytic forests were also recorded throughout the sequence. Four tropical dry forest taxa assemblage expansions were identified at 9200 to 8420 yr BP; 5900 to 5230 yr BP; 3100 to 2800 yr BP; and 2300 to 2000 yr BP. These phases correlated with times of reduced North American Monsoon intensity, suggesting that these communities thrived during drought periods. Variations in non-pollen palynomorphs were synchronic with the lithostratigraphic, geochemical data as well as with the terrestrial changes. During the Meghalayan, compositional shifts in the vegetation and increases in the herbaceous elements indicated human impact, while aquatic taxa, suggested eutrophic and warmer conditions in the lake. By examining the history of this vegetation mosaic, the data on temporal vegetation dynamics during the Holocene offered clues about the response to ongoing global warming and how climate change will likely shape plant communities in western-central Mexico.