Genetic adaptation of aspen (Populus tremuloides) populations to spring risk environments: a novel remote sensing approach

This study investigates geographic patterns of genetic variation in aspen (Populus tremuloides Michaux.) spring phenology with the aim of understanding adaptation of populations to climatic risk environments and the practical application of guiding seed transfer. We use a classical common garden experiment to reveal genetic differences among populations from western Canada and Minnesota, and we present a novel method to seamlessly map heat-sum requirements from remotely sensed green-up dates. Both approaches reveal similar geographic patterns: we find low heat-sum requirements in northern and high-elevation aspen populations, allowing them to take full advantage of a short growing season. High heat-sum requirements were found in populations from the central boreal plains of Saskatchewan and Alberta, and populations from Minnesota exhibit moderately low heat-sum requirements for budbreak. Analysis of corresponding climate normal data shows that late budbreak is strongly associated with the driest winter and spring environments, which suggests selection pressures for late budbreak due to both frost and drought risks in early spring. We therefore caution against long-distance seed transfer of Minnesota provenances to the boreal plains of Alberta and Saskatchewan. Although such transfers have been shown to increase tree growth in short-term field tests, this planting material may be susceptible to exceptional spring droughts.