Abstract
There is substantial interest in uncovering the genetic bases of the traits underlying adaptive responses in tree species, as this information is central to understanding how evolution proceeds in such systems and will aid conservation and industrial endeavors. Here, we synthesize evidence for local adaptation in trees by summarizing 129 common garden experiments across 20 genera of tree species that describe levels of heritability, differentiation of quantitative genetic variation (QST), and/or QST-FST comparisons. Given the abundant evidence for local adaptation, we discuss theoretical expectations for adaptive genetic architectures and contextualize progress in trees by synthesizing 52 genotype-phenotype association studies across ten genera. Our survey suggests that most tree traits generally exhibit considerably high heritability , that underlying genetic variation is often structured across populations and is significantly greater than FST in 69% of comparisons across the literature. Despite widespread evidence for local adaptation acting on abundant, heritable genetic variation, we find that single-locus associations explain only a small proportion of the phenotypic variation, often with small estimated per-locus effects . Together, these results suggest differential selection across populations often acts on tree phenotypes underlain by polygenic architectures consisting of numerous small to moderate effect loci. We close by addressing hurdles and promising alternatives to fully describing the underlying genetic architecture of quantitative traits in trees, remark upon the current state of tree genomics, and identify future directions for this field.