Modifying the elastomechanics of the stem and the crown needle mass distribution to affect the diameter increment distribution: A field experiment on 20-year old Abies grandis trees

In the spring of 2000, field experiments were begun on three 20-year old grand fir (Abies grandis) to influence the elasto-mechanical behavior of the stem, as well as the distribution of the assimilate crown production. The aim was to analyze, and then describe and model the expected resulting change in stem growth. Three stem sections of one fir were reinforced by rigidly attaching T-shaped steel bars to reduce bending stresses. Preliminary calculations with an elasto-mechanical tree and force model (that had been developed for a different tree, but was adapted by taking the size differences into account) gave first indications for the experimental design in regards to the necessary number of the bars and their dimensions required to guarantee that the stem rigidity would be highly increased. Furthermore, the simulations proposed no increased risk of stem breakage in the non-reinforced stem parts. The stability of the second tree was decreased by hanging sand bags with defined masses on the branches. Directly after loading, a significantly changed swaying behavior could be observed, which should cause correspondingly higher stress in the fibers. As for this load case, the simulated results prognosticate only a negligible increase in stress. The roughly-adapted model used, is seemingly invalid for this tree. In regards to the third tree, the bark at the base of all branches of the eastern half of the crown was removed to prevent any import of assimilates into the stem. The branches were not cut off because the mass distribution and thus, the mechanical behavior, of the tree was to be influenced as little as possible. The experiment will be concluded in the winter of 2003, after a four-year growth period. In addition to detailed stem analyses, the spatial crown structure with its needle and branch mass distribution, as well as the mechanical wood properties of the stem will be measured.