KEYNOTE: SOURCE OF MIGRANTS AND NO-MODERN-ANALOG COMMUNITIES: THE COMPLEXITIES OF THE MEXICAN LANDSCAPE

Vegetation responds to climate changes through migration, adaptation, or extinction. At centennial and millennial scales, these three mechanisms produce biota reassortments that are represented in the fossil record and can be translated in terms of climatic variables through the usage of transfer functions. However, during some periods of the past, the landscape has been dominated by communities that do not resemble any of the modern biotic associations. The identification of such assemblages is relevant as it provides means to evaluate the reliability of climatic reconstructions, given that they are based on realizations of modern climate-biota relationships. Thus, when no-modern-analog assemblages are present in the fossil record, the quantitative reconstruction of past climates is likely to be an extrapolation that should be interpreted cautiously. During the last ice age, no-modern-analog pollen assemblages were common in central Mexico and the Yucatan Peninsula. In both regions, these pollen associations prevailed mostly during massive ice discharges in the North Atlantic and the deglaciation, suggesting a common causative mechanism. Climates without modern analog and abrupt climate changes have traditionally been advocated as the main causes of the formation of these associations, and our data shows that these two factors played a significant role in the study area. However, no-modern-analog associations were more common in the highlands than in the lowlands, suggesting that source of migrants was probably another significant factor for configuring unique landscapes with respect to present. The temperature drop during the last glacial caused downslope migration of plant populations, creating three scenarios for no-modern analogs: i) diversity “crowding” in the lowlands, ii) diversity depletion in the highlands, and iii) the possibility of persisting communities in mid elevations. An opposite pattern is likely to emerge during times of increasing temperatures.