Mechanical stability and growth performance of trees

Trees’ mechanical stability is essential for the benefits provided by forests including wood and pulp products, specific ecosystems, recreation areas for people, and protection against natural hazards. Since tree growth depends on the environment in which the tree grows, it is possible to influence the mechanical stability of trees and forests. Different forest management strategies affect the tree stability and protection levels differently. It is therefore important to understand how tree growth properties influence tree mechanics and how the environment triggers the tree to grow in a mechanically more or less efficient way. Little information on such influence and triggering phenomena was, however available, to use directly in tree stability science and cause-effect analyses in forest management. Thus the present PhD thesis has the following aims: (1) to provide new allometric relationships for mechanical properties of the main structural elements of the tree, the root-soil system and the stem; (2) to investigate to what extent such properties are influenced by mechanical stress in nature; and (3) to assess the mechanical behavior of the whole tree subject to mechanical stress in nature originating from wind, snow, and/or rockfall. The approach taken was mainly experimental, with a focus on representative species and sites in the Alps. The results are presented in six papers (I-VI) which form the core of this work. In the first and the second papers, the focus is on the anchorage of the tree, provided by the root-soil system. The investigations include winching tests of conifers and subsequent examinations of the root-soil plate and roots. The five principle findings of the first study are: (1) the anchorage strength Ma was significantly reduced by root rot; (2) Ma did not differ between silver fir (Abies alba Mill.) and Norway spruce (Picea abies L. Karst.); (3) Ma could be well described by parameters of tree size and weight; (4) the shape of the root-soil plate could be well described by a depth-dependent taper model with an elliptical cross section; and (5) the root growth was preferential in the direction of frequent winds. The focus of the second paper is more on the mechanics of tree anchorage than in the first paper, with two main findings: (1) The resistive anchorage moment, with a maximum value of Ma, depends on the stem base inclination according to a function that could be well described by parameters of tree size and weight for the investigated Norway spruce; and (2) Ma for the Norway spruce on high elevation sites was lower than that growing on low elevation sites. Both studies show that the size of the root-soil plate, and hence size and extension of the roots, is difficult to predict with allometry. The third and fourth papers focus on the stem of the tree to obtain prediction models of mechanical properties, such as density and bending deformation and resistance, based on properties of radial growth, such as annual ring width, moisture content, and knottiness. In the third paper, this topic is explored in detail for one stem of Norway spruce. Small specimens sawn from different locations in the stem were used to determine the pattern of growth and mechnical properties within the stem, as well as relationships between growth and mechanical properties. The mechanical properties of the stem crosssection were then calculated by considering the annual wood layers as superimposed cylindrical shells with different mechanical properties predicted by the growth properties observed in the wood layers. The quality of the predicted cross-sectional mechanical properties was investigated in the fourth paper for stems of Norway spruce and silver fir. The predictions of stem deformation were precise, whereas those related to stem failure were approximate. In general, all mechanical properties improved with distance from the pith and from the tree top towards the base. The stems investigated can therefore be considered well designed for bending in nature, especially at a mature stage. The fifth paper is dedicated to the mechanical behavior of the entire tree. The results for the tree elements investigated in the papers I-IV are combined in the detailed analysis of the tree-rock interaction. A new experimental method was used to determine how Norway spruce reacts to and absorbs energy from an impacting rock. When hitting the tree at stem heights frequently observed in the field, the stem governed the tree’s reaction and energy absorption. The numerical determination of the reactions and relative energy absorption of different parts of the tree show their relative importance for providing protection against rockfall. The results contribute to further understanding of the tree-rock interaction process. They should therefore help make computer simulations more precise and improve management strategies for rockfall protection forests. The sixth paper is concerned with the mechanical performance and growth adaptation of the whole tree. The Norway spruce trees investigated in papers I-V grew in environments at different elevations (high and low) with different soils and competitive status. In paper VI their mechanical performance and growth adaptation to different combinations of natural load and tree state were analyzed. The three main findings were: (1) material and geometric non-linearities are important to