Gene flow, spatial structure, local adaptation, and assisted migration in trees

Gene flow is a process of transferring and exchanging genetic material among groups of organisms and is an important evolutionary factor that greatly affects genetic variation and differentiation in trees. Gene flow within and among populations of forest trees plays an important role in forest tree improvement, conservation genetics, and containment of genetically modified trees (GM trees). Although trees are a very diverse group of woody plants, they share many common life traits that affect gene flow, which we discuss in this review. Some unfavorable processes in current forest tree ecosystems, such as habitat loss and fragmentation, increased environmental stress due to global climate change, introgression from domesticated trees into their wild relatives, introduction of maladapted germplasm during reforestation, etc., may badly affect naturally established balance between gene flow, isolation, and local adaptation. Therefore, estimation of gene flow becomes increasingly important for monitoring these processes and developing the best possible strategy to manage and protect forest ecosystems. Here, we review what has been done in this area recently and the methods and approaches currently used to measure gene flow in forest tree populations.