Evaluation of Softwood Timber Quality—A Case Study on Two Silvicultural Systems in Central Germany

Abstract

Norway spruce (Picea abies (L.) H.Karst) trees planted with high stem densities produce finely branched, solid logs but are vulnerable to extreme weather events, e.g., storms. Over the last decades spruce stands have been planted at lower stand densities, resulting in wider crowns, lower crown bases, and higher stand stability, but this might decrease the quality of coniferous timber due to an increased growing rate and wider annual rings. Therefore, in this case study we investigated the influence of different silvicultural treatments and stand densities on tree morphology and wood properties of 100 spruce trees up to sawn timber as the final product. Tree morphology was assessed using mobile laser scanning. Ring width analysis, wood density measurements, and the four-point bending strength test on visually graded boards were conducted to gain information on wood properties and product quality. In stands thinned from below, higher wood densities were observed due to smaller annual rings compared to stands that were thinned from above at equal annual ring widths. In addition, crown asymmetry and the height-to-diameter ratio were identified as proxies for wood density. Lastly, visually assessed quality differences between the forest stands were discerned on the examined boards.

Advancements in Wood Quality Assessment: Standing Tree Visual Evaluation—A Review

“…Additionally, slenderness, defined as the ratio of height to diameter, has been found to be positively correlated with the mechanical properties of sawn timber obtained from measured trees [27].…”

“…For instance, branch attributes showed a limited correlation with knottiness observed in sawmill logs [51,53]. Similarly, the knot parameter used in the strength grading of structural boards was only weakly correlated with the number of tree branches [27], partly because the knots detected in the logs (and boards) originated from branches no longer externally visible on the tree.…”

“…However, few studies have analyzed the correlation between branch attributes in standing trees and the characteristics of sawn timber after processing. Höwler et al [27] investigated the relationship between tree characteristics and the properties of the boards produced (performing visual strength grading and mechanical performance after destructive tests). They mainly focused on wood density, which showed no correlation with branch number, but they also noticed only a weak correlation between the number of branches bigger than 4 cm measured in the standing trees and the knot size used as a parameter in the visual strength grading.…”

Developing a Model for Curve-Fitting a Tree Stem’s Cross-Sectional Shape and Sapwood–Heartwood Transition in a Polar Diagram System Using Nonlinear Regression

“…Dendrochronology reads annual tree-ring variations to analyze a tree's archaeological and historical structures for reconstructing past climates and significantly contributes to environmental science development [22]. Tree-ring chronologies, which may contain dimensional variations and circular shape deviations, provide high-frequency signals [23] that record a tree's responses to the silvicultural system [24,25] and its habitat's environmental condition history (i.e., climate [26][27][28][29][30][31][32][33][34], pollutants [35,36], and water-heat stress [37]) during its growing periods yielding the xylem.…”