Southern-most Nothofagus trees enduring ice ages: Genetic evidence and ecological niche retrodiction reveal high latitude (54°S) glacial refugia

Abstract

Hypotheses of tree survival in refugia or long-distance postglacial migration can be disentangled combining genetic polymorphisms and ecological niche modeling. This is particularly relevant for ecologically distinct taxa which are undistinguishable by pollen morphology in fossil records. We hereby test the long-term persistence of the cold-hardy Nothofagus pumilio at high latitudes in southern South America during glacial cycles. Modern and past (LGM) ecological niche modeling (ENM) for N. pumilio was developed using current climate data and including 19 bioclimatic variables and topography. We collected fresh tissue for isozyme analysis from 14 locations within Tierra del Fuego selected on the basis of their location relative to LGM glacier extent: ice-free, ice-margin, and formerly glaciated areas. We resolved 11 isozyme loci and calculated within-population genetic diversity and among-population divergence parameters. Glacial effects on population structure were analyzed by UPGMA and Bayesian models. ENM yielded distinct areas in eastern and interior Tierra del Fuego exceeding > 0.3 probability values that would have been suitable for N. pumilio during the LGM. Populations within formerly glaciated areas hold lower genetic diversity and thus were affected by genetic drift during colonization from refugia. Our data provided no evidence for wide-ranging postglacial colonization from warmer northern locales and suggested local survival of Nothofagus forests at southern-most South America.