Long-term monitoring of Beech tree growth for quantifying relationships between the occurrence of tension wood, tree motion, and growth stresses

Bech trees is the most abundant broadleaf species in central Europe. The main wood quality problems affecting this species are red heartwood from one hand and growth stresses associated with tension wood on the other hand. Concerning the last point, recent research studies have been carried out during the FAIR project, “Growth stresses in beech”, and completed in 2001. In particular, they focused on the relationships between Beech silviculture and stress levels, but the results were mainly qualitative, indicating that low and high ratios of total tree height to diameter at breast height correspond respectively to low levels of stress. To obtain more quantitative information, a research project was developed to follow tree reaction after thinning, in terms of displacements of the trunk, spatial distribution of structural branches in the crown as well as modifications of the crown projection area. Measurements were performed yearly. This experiment was set up on the basis of the experience acquired from a previous study on poplar trees. That study was devoted to a detailed analysis of the relationships between the threedimensional shapes of the annual rings and the spatial distribution of tension wood (Badia 2003a). The trunk displacements were measured by means of a reflectorless total station used to measure Cartesian coordinates of small painted spots drawn on the tree bark. Furthermore, based on the results gained by Badia (2003b), we are developing a method able to quantify tension wood by locating automatically each spot on a disk surface. The aim of this long-term experiment is to follow the tree displacements through time, including crown extension. But the actual aim is to analyse, the joint spatial distributions of tension wood and radial increments on felled trees. Of course, at felling time, basic wood properties will be measured, as well as an indicator of growth stress at the periphery of the trunk by using a method such as the single drilled hole. The quantification of the internal stress will be the poorest investigated component in this experiment because of its inevitable sensitivity to the measurement process. Nevertheless, these data will be extremely relevant to adapt or improve a deterministic model assessing growth stresses from a Beech tree growth model. SimCoP (Ottorini 1991)is a distance dependent model recently adapted for even-aged beech stands. It is based on the three dimensions crown development to govern the tree growth and takes the competition between trees for aerial space into account. Then, it seems well adapted to include such a coupling between growth and stress processes, and to assess quantitatively the consequences of silvicultural practices on the development of tension wood, a major problem of Beech round-wood quality. The purpose of this paper is to present the different methodologies used in this experiment and to analyse the first results concerning the observed changes on studied trees, three years after the beginning of the experiment.