However, few studies have focused on local genetic differentiation in leaf-out phenology within a regional population in temperate trees. Hence, studies on genetic differentiations involved in local adaptation help in the evaluation of the adaptability of tree populations to environmental change. This is because gene flow between differentiated populations sustains high genetic diversity within populations, which facilitates adaptation to environmental changes. For tree populations with extensive gene flow, such local genetic differentiation can be regarded as possessing high adaptability to environmental variation in which focal traits are involved.
Similarly, if late frost regime varies locally according to landscape components, similar genetic differentiation of leaf-out phenology would be observed on a local scale within a metapopulation. For tree species that budburst in early spring, regional variation in late frost timing (the final day of late frost in spring) or late frost frequency often causes genetic differentiation along altitudinal or latitudinal gradient for budburst timing (day and temperature sum of budburst we defined “temperature sum of budburst” as accumulated temperature required for budburst): such species exhibit delayed budburst in regions where late frost occurs later compared to regions where late frost occurs early. Previous studies on the adaptive significance of leaf-out phenology for temperate tree species showed that natural selection imposed by late frost and competition influence the optimal day of budburst for each population, that is, trees that budburst before the optimal day suffer from late frost damage, whereas trees that budburst after the optimal day have disadvantages in competing for light under the same condition. Understanding the mechanisms underlying the adaptation of tree species to environmental seasonality is essential for predicting the effects of ongoing climate change on forest ecosystems. Changes in spring phenology are expected to degrade forest ecosystems by decreasing productivity and phenological mismatches in biological interactions. The timing at which leaf out occurs is becoming earlier in many temperate tree species because of climate warming. These results demonstrate that late frost is a key factor driving genetic differentiation of leaf-out phenology within a regional tree population. The genetic variations were significantly associated with variations in the last fatal frost day among the provenances, suggesting that natural selection due to late fatal frost causes genetic differentiation in the traits along the spatial gradient in late frost timing. For all provenances, the heritability for these traits were 0.7–0.8. Analyses of genetic variation showed significant genetic variations among provenances and families for both traits. Analyses of interpopulation variation showed that the day and the temperature sum of budburst positively correlated with the last fatal frost day in the habitats. For budburst timing, we analyzed interpopulation variations in habitats, genetic variations in a nursery, and the relationships between these variations and environmental conditions in the habitats. We studied the genetic differentiation in budburst timing among Fagus crenata populations along spatial gradients in late frost timing in the Hakkoda Mountains, northern Japan, by focusing on last fatal frost day and topography.
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