Forest trees reinforce and even reorient their woody stems in response to gravitational and mechanical stresses. In angiosperm trees, this is achieved by the production of “tension wood,” which contains fiber cells with specialized secondary cell wall layers that can generate strong tensile forces. In this seminar, I will present experiments using the model forest tree, Populus, which describe the gravity sensing cells in the woody stem, the changes in auxin transport occurring during graviresponse, and key genetic and biochemical aspects of tension wood development. Gene co-expression networks for tension wood development will also be presented, including gene modules correlated with specific phenotypes and methods for identification of candidate regulatory genes. I will also present recent results from a new system for functional genomics in Populus based on perturbation of gene dosage relationships, which we are using to dissect heterosis, biomass and wood development traits.