Abstract: Nitrogen is one of the principal factors limiting vegetative production. The present invention has improved the nitrogen metabolism in Poplar by integrating a transgene constitutively expressing a pine glutamine synthetase into the plan genome. The resulting transgenic trees exhibit higher growth rates, protein and chlorophyll contents, and leaf area than equivalent untransformed trees. It is contemplated that this approach to nitrogen improvement will be equally successful for all woody perennials. Provided with the invention is an expression casette, a vector, and a method for increase glutamine synthetase activity in woody perennials, as well as transgenic woody perennials with enhanced nitrogen metabolism and accompanying phenotype.