Important aims in nearly all tree-breeding programs around the world are to produce plants with increased growth rates and stem volumes, and shorter rotation times. Such trees would yield more biomass per area unit. Here the present inventors have shown that when over-expressing a key regulatory gene in the biosynthesis of the plant hormone gibberellin (GA) in hybrid aspen (Populus tremula x P. tremuloides), improvements in valuable traits such as growth rate and biomass are obtained. In addition, these trees also have longer xylem fibers than unmodified wild type plants. Long fibres are very desirable in the production of strong paper, but it has not (as yet) proved possible to influence this trait by traditional breeding techniques. A further advantage of the present invention is that it may reduce or eliminate the use of growth influencing chemicals in forestry.