Effect of stem guying on the incidence of resin pockets

摘要

Although resin pockets are a major cause of degrade for appearance grade timber, little is known about the environmental conditions that control the incidence of these defects. Water stress and mechanical bending stress due to tree sway in strong winds are thought to contribute to the formation of resin pockets, but this is based on anecdotal evidence from observations of resin pocket occurrence. Controlled experiments are required to better understand the factors leading to resin pocket formation. In this study data were analysed from an experiment on a dryland site, where 14-year-old Pinus radiata D. Don trees were guyed or left in an untreated unguyed condition from July 2006 to September 2008. Measurements of stem diameter were taken on all trees using dendrometer bands. At the end of the experiment in September 2008 resin pocket frequency was determined by cutting the lower 6m of each tree into 50mm sections. Each of these sections was then imaged and resin pockets were identified and allocated to a type, height and year of occurrence. Using these data, the aims of this research were to (i) determine how the reduction of tree sway through guying influences tree diameter increment and (ii) investigate the main and interactive effects of tree height, year of formation and guying on the incidence of both Type 1 and Type 2 resin pockets. Treatment divergence in cumulative diameter increment occurred 6 months after guying was applied and by the end of the experiment cumulative diameter increment of unguyed trees significantly (P =0.046) exceeded that of guyed trees by 34% (19.3 vs. 14.3mm). Differences in increment rate between treatments were significant on four of the monthly measurement intervals, with increment rate of unguyed trees exceeding that of guyed trees on all four occasions, by up to 184% (0.044 vs. 0.015mm dayp#). Differences in increment rate between treatments were most marked during periods of low rainfall. Compared to the unguyed control, guying significantly (P =0.049) reduced the incidence of Type 1 resin pockets by on average 54% (3.73 vs. 1.73 resin pockets yearp#). Although the incidence of Type 2 resin pockets was also reduced through guying by 45% (2.20 vs. 1.20 resin pockets yearp#) this reduction was not significant (P =0.28). Both tree height and year of occurrence had a highly significant (P <0.001) influence on frequency of both resin pocket types.