In most tree-breeding programs worldwide, increasing the trees' growth rates and stem volumes and shortening their rotation times are important aims. Such trees would yield more biomass per unit area. Here we show that overexpressing a key regulatory gene in the biosynthesis of the plant hormone gibberellin (GA) in hybrid aspen (Populus tremula x P. tremuloides) improves growth rate and biomass. In addition, these transgenic trees have more numerous and longer xylem fibers than unmodified wild-type (wt) plants. Long fibers are desirable in the production of strong paper, but it has not as yet proved possible to influence this trait by traditional breeding techniques. We also show that GA has an antagonistic effect on root initiation, as the transgenic lines showed poorer rooting than the control plants when potted in soil. However, the negative effect on rooting efficiencies in the initial establishment of young plantlets in the growth chamber did not significantly affect root growth at later stages.