Palaeoecological evidence of pollen morphological changes: A climate change adaptation strategy?

Pollen records have been used to reconstruct changes in climate, based on the presence of taxa as proxies for environmental conditions. Pollen grains have morphological features that allow for the identification of different species, genera, and families of flowering plants. However, ecologically distinct species share similar pollen types that may make biogeographic and climate reconstructions problematic. Also, the responses of populations and species to climate changes adjusted through either plasticity or adaptation have been overlooked. Such adjustments are most probably the product of long-lasting in situ persistence in local refugia under favourable microclimates despite hostile regional climate. In southern South America three species of the dominant tree cover of the subgenus Nothofagus are present, two deciduous (N. antarctica and N. pumilio) and one evergreen (N. betuloides), identified in the pollen group Nothofagus dombeyi type. Here we present a pollen record from Punta Arenas (~53°S) that documents significant changes in pollen size that took place during the Late glacial and Early Holocene. The occurrence of smaller Nothofagus dombeyi type pollen grains during the Late glacial may indicate rapid local adaptation to colder and possibly wetter conditions. Our findings are supported by Ecological niche modelling which suggests limited availability of Nothofagus refugia during the Last Glacial Maximum. The local refugia may also have fostered interbreeding between congeneric species that led to unique morphological features that increased the fitness of the recipient pool. We postulate that such modifications were due to the potential transient hybridization between species that later diverged by backcrossing with each deciduous and evergreen taxa.