Genetic divergence in forest trees: understanding the consequences of climate change

Abstract

Summary 1.Predicted climate change is heading in many respects into untested environmental conditions for trees and to the reshuffling of species distributions. We explore the consequences that these changes are likely to have on population differentiation of adaptive traits. Superimposed on the spatial redistribution of the species, will there be a redistribution of their genetic variation? 2. We base our predictions on a conceptual framework, whose elements are the extant differentiation, and the predicted divergent evolution of populations along purposely chosen altitudinal/latitudinal gradients. We consider simultaneously phenotypic and genetic divergence, but emphasize genetically driven population differentiation. We illustrate phenotypic and genetic patterns of variation with examples from well-studied northern and southern hemisphere tree genera Quercus and Eucalyptus. 3. Most phenotypic traits show very large in situ clinal variation with variation in altitude or latitude. Genetic clines detected in common gardens usually follow the observed in situ phenotypic clines, reflecting cogradient variation. Rare counter gradients have also been detected, where phenotypic and genetic clines exhibit opposing signs. These patterns suggest that plasticity and selection contributed in most cases synergistically to the extant differentiation. 4. We anticipate that microevolutionary processes will be different along environmental gradients. At the leading edge, availability of newly suitable habitats will trigger migration favouring genotypes equipped with colonists attributes. At the rear edges of the distribution, populations will be submitted to strong selective pressures favouring genotypes capable of withstanding drought and heat stress. Central populations will benefit from the plastic response of trees that will temporarily compensate for the maladaptation, until genetic adaptive variation will be restored by gene flow, mutation or recombination. 5. We make predictions about future differentiation along environmental gradients, by highlighting traits that are likely to diverge, the rate at which differentiation will take place, and the role of gene flow and hybridization. We envisage that parallel selection may maintain differentiation at extant levels, whereas divergent selection will promote substantial differentiation for traits facilitating adaptation to contrasting conditions along the environmental gradient. We anticipate that genetic divergence may occur very rapidly and will be enhanced by the multilocus architecture of most adaptive traits.

Climate adaptation and ecological restoration in eucalypts

“…internode length, leaf size, plant height) exhibit strong plasticity. Parallel trends at the genetic level suggest this plasticity is adaptive (Kremer et al 2014).…”

Examining plant physiological responses to climate change through an evolutionary lens

“…For instance, the budburst phenology of two Scottish birch (Betula) species may not evolve in pace with climate change without gene flow from populations with earlier phenologies (Billington and Pelham, 1991). Some plant species, including trees in the genera Quercus and Eucalyptus, display genetically based clinal variation across climatic gradients in physiological traits such as stomatal conductance and drought and frost tolerance (Marchin et al, 2008;Kremer et al, 2014). Under climate change, gene flow from central populations may benefit peripheral populations at the leading edge of the range by introducing alleles preadapted to warm conditions (Aitken and Whitlock, 2013;Kremer et al, 2014).…”

Constraints to and conservation implications for climate change adaptation in plants

“…In their review of life-history evolution in populations experiencing range-shifts, Phillips et al (2010) concluded that assortative mating as a result of climate change may result in an increased rate of evolution of lifehistory traits that promote dispersal distance and reproductive rate at the leading edge. Seed dispersal is a highly labile trait in plants and, as the best-dispersing individuals within a population should be highly represented at the leading edge of the new range, the chance of breeding between 'good dispersers' should be high, resulting in runaway natural selection for dispersal rate at the leading edge (Phillips et al 2010;Boeye et al 2012, Kremer et al 2014.…”