We present a method for creating tree models with realistically curved branches, useful in the portrayal of natural scenes. Instead of attempting to replicate a tree?s final shape by observation, we obtain this shape as nature does — by considering the tree?s development in the context of its environment. The final shape of the branches results from their growth in length, girth, weight and rigidity under the influence of gravity and tropisms. Using the framework of L-systems, we extend Jirasek?s biomechanical simulation of a plant axis to correctly represent an entire tree. Our model also simulates the reaction wood which actively re-orients a leaning branch by differentiating the wood production in angular portions of the branch cross-section. To obtain realistic and controllable tree architectures, we regulate growth elements in the model using functions based on botanical findings. We create a multi-year simulation of tree growth under environmental influences, obtaining a realistic tree shape at every stage of its development.