Abstract
Knot-related defects are the major cause of timber quality degradation, and diminishing this kind of defects is an important issue in forest management. For the purpose of clear-wood production, knot attributes and branch occlusion of Betula alnoides under natural pruning were investigated in a 14-year-old experimental plantation with five planting densities ranging from 500 to 3333 stems per hectare in southern China, and a total of 1325 occluded branches from 30 trees were sampled and dissected. The mean occluded branch diameter (OBD), radius of knots and branch insertion angle (IA) decreased significantly with increasing planting density. Planting with high stocking density significantly reduced the frequency of thick occluded branches (diameter ≥ 20 mm) while increasing the frequency of small ones (diameter < 10 mm). Branch occlusion time (OT) also tended to increase with decreasing planting density. The results of generalized linear mixed models showed that OBD was the major factor influencing OT, radius of dead portion of knot (RDP), total radius of knot (TRK) and IA. In addition, OT was positively correlated with RDP but negatively correlated with stem diameter growth rate during branch occlusion (SDGR). Silvicultural strategies with appropriate planting density for large-diameter clear-wood production of B. alnoides were discussed.
Highlights
The demand for high-quality timber has recently been increasing, which leads to a remarkable variation of timber prices depending on the quality [1]
A decreasing trend was found for the time of branch occlusion with increasing planting density (Table 3), and it varied from 2.3 years for 500 stem per hectare to 1.8 years for 3333 sph
Mäkinen’s [2] study with B. pendula Roth in which the difference of occlusion time was not statistically significant from 7.6 years under 5000 sph to 10 years under 400 sph. This discrepancy may be attributed to differences between both species in their growth characteristics and the climate conditions. This could be interpreted from the results of model building which showed that OT of B. alnoides was positively correlated with the diameter of occluded branches (OBD) and the radius of dead portion of knot (RDP), and negatively correlated with stem diameter growth rate at the knot position during branch occlusion (SDGR) (Equation (1) and Figure 6)
Summary
The demand for high-quality timber has recently been increasing, which leads to a remarkable variation of timber prices depending on the quality [1]. It is well known that knot-related defects in the stems greatly affect the manufacturing and joinery industries, and the amount and size of knot-related defects are crucial factors for log grading and timber quality [2,3,4]. The dead knot has no physical connection to the surrounding wood and interrupts the wood grain. Knot-related defects are mostly caused by the dead knot, which can lower the mechanical strength of timber, decrease the performance of veneered wood, and even make the logs unusable. Live knot is attached physically to surrounding wood and distorts the wood grain, which has less influence on the mechanical strength of timber. The distorted rings in the live knot sometimes can even increase the ornamental value of wood texture. Branch occlusion is the process of trees forming callus and clear wood over the dead branch stubs, which would become dead knots after occlusion [5]
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