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Tree taxa shifted latitude or elevation range in response to changes in Quaternary climate. Because many modern trees display adaptive differentiation in relation to latitude or elevation, it is likely that ancient trees were also so differentiated, with environmental sensitivities of populations throughout the range evolving in conjunction with migrations. Rapid climate changes challenge this process by imposing stronger selection and by distancing populations from environments to which they are adapted. The unprecedented rates of climate changes anticipated to occur in the future, coupled with land use changes that impede gene flow, can be expected to disrupt the interplay of adaptation and migration, likely affecting productivity and threatening the persistence of many species.
Modern plant taxa have persisted through a long period of variable climate, including glacial-interglacial cycles with large changes in temperature, precipitation, and CO2 concentration, over the past 2.5 million years. Rates of climate change varied widely: Regional temperature changes were as rapid as several degrees Celsius within a few decades or as slow as PC per millennium. The changes in species distribution evidenced by fossils provide a detailed record of plant responses to these changes. Hundreds of pollen diagrams, compiled in databases, provide regional and continental records of tree abundances as they changed through space and time (1-3). New pollen records supplemented by macrofossils (4) and DNA recovered from fossil pollen (5) provide increasing temporal and taxonomic detail. In and regions, where pollen-bearing sediments are less abundant, plant fragments preserved in middens made by packrats (Neotoma) and other rodents provide a spatially precise record of past species distributions (6). Changes in geographic distribution are so frequently documented in the fossil record that range shifts are seen as the expected plant response to future climate change (7).
Beyond changes in distribution, however, plants underwent genetic changes, adapting to changes in climate during the Quaternary. Yet adaptation at the population level is seldom considered in the literature describing Quaternary environments nor, with some notable exceptions (8-10), in discussions of vegetation response to anticipated global change.
Here we cite evidence of genetic adaptation to climate and argue that the interplay of adaptation and migration has been central to biotic response to climate change. Moreover, we discuss how rapid climate change challenges this...