Influence of intra- and interspecific competition on timber quality of European beech
von Kirsten Höwler
Datum der mündl. Prüfung:2020-09-10
Betreuer:Prof. Dr. Christian Ammer
Gutachter:Prof. Dr. Christian Ammer
Gutachter:Prof. Dr. Holger Militz
EnglischThe timber quality of a single tree is considerably influenced by interactions with other individual trees. These competitive effects from neighbouring trees may be regulated through silvicultural treatments. Consequently, the competition a tree faces until the day of harvest is a strong driver for timber quality. This turns the regulation of competition into an integral part of silviculture. However, not only competition intensity determines quality. The species identity of neighbouring trees and forest mixture type can also influence quality related stem attributes such as branchiness or tree shape. Against the background of a forest management that is close to nature and adapted to climate change, the share of mixed forests and of deciduous trees has increased in the recent past and will continue to increase in the future. Therefore, it becomes important to understand the effects of different tree species mixtures and interactions on the quality of trees. Although mixed forest stands have been extensively investigated, research mostly focused on tree growth and productivity, or resistance and resilience under changing and uncertain climate conditions, but rather less on the effects of tree species mixing on timber quality. It is still unclear whether the numerous positive effects of mixed forests come at the expense of timber quality. Currently, pure coniferous forests are converted into mixed and deciduous forests and this will eventually lead to a changed availability of hardwood and softwood. Thus, deciduous timber will have to be used more intensely in the future. However, for that, information on deciduous timber quality in mixed forest stands is needed. In-situ measurements of timber quality have the potential to improve the economic yield of a stand, the sustainable utilisation of timber and timber products, and can further contribute to an optimal harvesting time. However, precise information on timber quality of deciduous trees, especially of standing trees, has often been lacking so far. In addition, measurements of quality attributes or the competitive situations of a tree have so far required high measuring efforts that were affected by significant errors in higher log sections. Through terrestrial laser scanning (TLS) it became possible to obtain a virtual three-dimensional (3D) representation of a tree and its direct neighbours. This enables a quantitative assessment of quality-related external stem characteristics of a tree in dependence of its neighbourhood. This thesis therefore aims to provide a quantification of both external and internal timber quality characteristics (e.g., bumps, branches, knots, discoloration) in order to investigate to what extent intra- and interspecific competitive situations affect these quality characteristics of European beech (Fagus sylvatica L.). Three different approaches were applied to analyse the effects of competition intensity and tree species identity of neighbouring trees on timber quality of 125 target European beech trees: TLS, a quality assessment on the standing tree by the local district foresters, and a quality assessment of the sawn timber (boards) after harvesting. In addition, the suitability of TLS for a quantitative assessment of external quality characteristics was examined. The relationship between external and internal quality characteristics was investigated by comparing the different approaches of quality assessment. Photographs of the sawn boards were used for a quantitative measurement of discolouration surface and knot surface as indicators of timber quality using the software Datinf® Measure. The external stem quality of European beech was significantly influenced by the degree of competition intensity. More precisely, two TLS-based measures of external stem quality that were newly developed in this work were influenced by the intensity of competition: With increasing competition intensity, the number of bark anomalies (BA) and stem non-circularity (SNC) decreased. Hence, external stem quality of European beech can be measured non-destructively, objectively, and quantitatively applying TLS. This makes TLS a valuable addition to a visual in-situ timber quality assessment. Furthermore, the externally visible quality features measured using TLS correlated to the internal timber quality. Similarly, the quality assessment of the local district foresters also correlated with the internal timber quality. Thus, external quality features help to predict the internal timber quality. This was exemplified, among other things, by the fact that it is possible to predict discoloration by the number of bark anomalies on the stem surface. Internal timber quality was also related to the competitive situation, with increasing competition improving internal timber quality. In pure beech stands, a tendency towards better quality with lower knot surfaces was observed compared to mixed stands of beech and spruce (Picea abies (L.) H. Karst.). In addition, a decrease in knot surface was found with increasing distance to the pith and lower values in the lower stem sections. These results suggest that the regulation of competitive levels through silvicultural treatments can improve timber quality and thus confirm empirical findings that indicate a positive relationship between competition intensity and timber quality. Although single effects of neighbourhood species identity could be identified, the overall species effect on timber quality was lower than the competitive effect resulting from size and distance of neighbouring trees. Lastly, this study provides a new methodology to assess external timber quality measures in the field objectively and non-destructively.
Keywords: competition intensity; Fagus sylvatica L.; mixed species forests; quality assessment; terrestrial laser scanning; timber quality