INTRODUCTION

There is growing interest in estimating rates of evolutionary change, motivated by the ongoing environmental change (Gingerich 2001; Stockwell et al. 2003; Carroll et al. 2007). Particular concerns have been raised about forest trees, which are thought to be less able to adapt to these rapid changes due to their long generation time (Reich & Oleksyn 2008). Other authors have suggested that large standing genetic variation in trees may enable rapid adaptive responses, at a pace matching that of ongoing climate change (Kremer 2007; Aitken et al. 2008). An elegant approach to get some insights on the evolutionary responses to global warming is to reconstruct past genetic changes and processes that occurred during the postglacial periods, when temperatures were steadily increasing (Petit et al. 2008). The timing and direction of spread of wind-pollinated trees following the last ice age can be reconstructed from their pollen remains in sediments (e.g., Cheddadi et al. 2005). Palynological data have now been compared to phylogeographic approaches based on range-wide surveys of genetic fingerprints of maternally inherited organelle genomes (Petit et al. 2002b for oaks, Magri et al. 2006 for beech, Cheddadi et al. 2006 for Scots pine). This combination has elucidated genetic and demographic processes associated with tree responses to environmental change. A second approach for predicting adaptive responses to environmental changes is based on theory and simulations (Bürger & Lynch 1995; Bürger & Krall 2004; Sato & Waxman 2008). These studies have been limited so far to single populations and have focused on the amount of environmental change that a population can tolerate, given its genetic and demographic properties.