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PremiseBoreal and northern temperate forest trees possess finely tuned mechanisms of dormancy, which match bud phenology with local seasonality. After winter dormancy, the accumulation of chilling degree days (CDD) required for rest completion before the accumulation of growing degree days (GDD) during quiescence is an important step in the transition to spring bud flush. While bud flush timing is known to be genetically variable within species, few studies have investigated variation among genotypes from different climates in response to variable chilling duration. MethodsWe performed a controlled environment study using dormant cuttings from 10 genotypes ofPopulus balsamifera, representing a broad latitudinal gradient (43–58°N). We exposed cuttings to varying amounts of chilling (0–10 weeks) and monitored subsequent GDD to bud flush at a constant forcing temperature. ResultsChilling duration strongly accelerated bud flush timing, with increasing CDD resulting in fewer GDD to flush. Genotypic variation for bud flush was significant and stratified by latitude, with southern genotypes requiring more GDD to flush than northern genotypes. The latitudinal cline was pronounced under minimal chilling, whereas genotypic variation in GDD to bud flush converged as CDD increased. ConclusionsWe demonstrate that increased chilling lessens GDD to bud flush in a genotype‐specific manner. Our results emphasize that latitudinal clines in bud flush reflect a critical genotype‐by‐environment interaction, whereby differences in bud flush between southern vs. northern genotypes depend on chilling. Our results suggest selection has shaped chilling requirements and depth of rest as an adaptive strategy to avoid precocious flush in climates with midwinter warming. 

Abstract AimPatterns of genetic diversity within species’ ranges can reveal important insights into effects of past climate on species’ biogeography and current population dynamics. While numerous biogeographic hypotheses have been proposed to explain patterns of genetic diversity within species’ ranges, formal comparisons and rigorous statistical tests of these hypotheses remain rare. Here, we compared seven hypotheses for their abilities to describe the geographic pattern of two metrics of genetic diversity in balsam poplar (Populus balsamifera), a northern North American tree species. LocationNorth America. TaxonBalsam poplar (Populus balsamiferaL.). MethodsWe compared seven hypotheses, representing effects of past climate and current range position, for their ability to describe the geographic pattern of expected heterozygosity and per cent polymorphic loci across 85 populations of balsam poplar. We tested each hypothesis using spatial and non‐spatial least‐squares regression to assess the importance of spatial autocorrelation on model performance. ResultsWe found that both expected heterozygosity and per cent polymorphic loci could best be explained by the current range position and genetic structure of populations within the contemporary range. Genetic diversity showed a clear gradient of being highest near the geographic and climatic range centre and lowest near range edges. Hypotheses accounting for the effects of past climate (e.g. past climatic suitability, distance from the southern edge), in contrast, had comparatively little support. Model ranks were similar among spatial and non‐spatial models, but residuals of all non‐spatial models were significantly autocorrelated, violating the assumption of independence in least‐squares regression. Main conclusionsOur work adds strong support for the “Central‐Periphery Hypothesis” as providing a predictive framework for understanding the forces structuring genetic diversity across species’ ranges, and illustrates the value of applying a robust comparative model selection framework and accounting for spatial autocorrelation when comparing biogeographic models of genetic diversity. 

Summary Stomata regulate important physiological processes in plants and are often phenotyped by researchers in diverse fields of plant biology. Currently, there are no user‐friendly, fully automated methods to perform the task of identifying and counting stomata, and stomata density is generally estimated by manually counting stomata.We introduce StomataCounter, an automated stomata counting system using a deep convolutional neural network to identify stomata in a variety of different microscopic images. We use a human‐in‐the‐loop approach to train and refine a neural network on a taxonomically diverse collection of microscopic images.Our network achieves 98.1% identification accuracy onGinkgoscanning electron microscropy micrographs, and 94.2% transfer accuracy when tested on untrained species.To facilitate adoption of the method, we provide the method in a publicly available website athttp://www.stomata.science/. 

Uncovering the genes and molecular basis of phenotypic variation and adaptation is a major goal in conservation and evolutionary genetics; it also sets the basis for future operational breeding in commercial species, like forest trees. These taxa are characterized by their large size, growth habit and longevity, which hampers the use of reverse-genetic approaches (i.e. from gene function to phenotype) to pinpoint adaptive molecular variants. In this chapter, we summarize the basis of the forward-genetic approaches (i.e. from phenotype to gene function) currently used in forest trees. For each strategy, we provide a brief overview of the statistical approaches employed to identify candidate genes, and then highlight the main findings of landmark studies that provide evidence for adaptation in forest trees. Adaptive and commercial traits are generally well inherited in trees, although they are mostly affected by the variation of multiple genes, each one accounting for a small part of the phenotypic variance of each character. However, some individual and important genes involved in growth, phenology, drought resistance and cold hardiness have been identified; many of them showing evidence of selection across multiple taxa (sometimes including angiosperms and gymnosperms). Future challenges for detecting the signatures and understanding the molecular basis of adaptation in trees include more adequate and precise phenotype assessment in natural populations, and the inclusion of gene interactions and epigenetic variations in current models. The implications of these findings in conservation and breeding of forest trees are finally discussed.