Vegetation assemblages of central Mexico through the late Quaternary: modern analogs and compositional turnover

Question: When did vegetation assemblages that occupy modern central Mexico appear within the context of the late Quaternary?. Location: Central Mexico. Methods: We used modern and fossil pollen data sets from central Mexico to explore the evolution of regional vegetation through the late Quaternary. A total of 1397 samples (1357 fossil samples distributed in 17 records and 40 modern samples) were analysed through detrended correspondence analysis to evaluate the degree of analogy between modern and past vegetation assemblages, and regional vegetation compositional turnover through time. Results: There was a high degree of analogy between modern and glacial vegetation, whereas the early and mid-Holocene were characterized by a high frequency of no-modern-analogue pollen assemblages. Vegetation turnover during the glacial time was characterized by both pulses of abrupt change and episodes of stability. In contrast, vegetation turnover was high through most of the Holocene. Both vegetation turnover and past vegetation analogy to modern were not synchronous in altitudinal zones, except for the past 2000 yrs, when pollen assemblages suggest a high vegetation analogy to modern-day throughout the region. Conclusions: Human impacts on central Mexico regional vegetation were consistently evident during the last 2000 yrs, suggesting that biodiversity expressed in the modern vegetation assemblages incorporates a high anthropogenic imprint. High resemblance of glacial and modern landscapes suggests that the effect of human occupation on the landscape is similar to that exerted by glacial time environmental stress. However, loss of ecosystem connectivity under the modern pattern of landscape homogenization represents an unprecedented threat for regional biodiversity.