Wolff, an 19th century Orthopedic surgeon postulated, and correctly, that bone growth reinforced itself in areas where stress was more predominant. Such an example would be the arch of the hip at the neck of the femur which has a thickened underside where compressive forces are high and the front of the neck where torque moment is high. You should know that the stress on the femoral neck in the stance phase of walking in a 180 pound person is roughly 1000 pounds, more on women than men due to a wider pelvis. Wolff's law breaks down in the elderly, since eventually they lack the capacity to grow bone at the same rate as its erosion, so a fracture of the hip at the peak of the arch is common. This is really a simple engineering principle the body has learned without any guidence from me or you. You might guess that Wolff did not write about trees that possess the same capacity that we do to reinforce areas of stress to avoid fracture. Wolff's law as pertinent to trees is thigmomorphogenesis. We both live a long time and these changes are slow and steady. If the mechanical feature for the tree, such as windy areas, and therefore exogenous stress, the thigmotropic response is morphogenesis of new wood on the tension side and less new wood on the compression side, known as TW and OW, ( opposite) leading to a plastic lean away from the wind known as negative thigmotropism. If there are windy cross-currents leading to torque as well, the new wood fibres are twisted but the whole is increased in strength. When ever one thinks he is different than a tree, the more it comes to pass that we are preserved from externals by similar mechanisms. By the way, one cannot will Wolff's law, nor thigmotropism or its morphogenesis. One can only say the chemistry of me and the tree are the Law of Life whether tension or compression is the stressor.