|In spring of 1999, on contract to the Fraser Lake Sawmills Division of West Fraser Mills, we began establishing a series of operational trials to assess the effectiveness of disk trenching site preparation treatments on a range of sites in the Nadina (formerly Lakes and Morice) and Vanderhoof Forest Districts. A total of 83 field trials were established between 1999 and 2001 in the SBSdk, SBSmc2 and ESSFmc subzones. Each installation consists of paired treated and untreated sample plots, accommodating 50?75 planted lodgepole pine or interior spruce seedlings per treatment. During the 2003 field season, third-year data were collected for trials established in 2001, and fifth-year data were collected for trials established in 1999. This report documents progress accomplished during 2003, and provides a summary of results to date. First growing season After the first growing season, disk-trenching showed a small but statistically significant negative impact on tree height and diameter in several site x species categories, and no significant difference in the rest. Effects on tree condition and survival were similarly small and mostly negative. Third growing season By the third year, average tree heights and diameters, and numbers of trees in good condition were generally slightly greater in disk-trenched plots than in the paired untreated plots, but results varied depending on tree species and soil moisture regime. Disk trenching had the most positive and statistically significant effects on lodgepole pine planted in very dry sites. Effects on interior spruce and pine on moist and wet sites were also generally positive, but to a lesser degree. Results for trees planted on mesic and slightly dry sites were highly variable. In most cases, differences between disk trenching and no treatment were relatively minor, with a few trials showing distinctly positive effects on tree condition and growth, and a few showing negative effects. Tree height and diameter growth generally responded favourably to disk trenching on sites with near-surface soil restricting layers. On mesic sites at year three, tree diameter showed a positive response to disk trenching more frequently than did tree height. On those trials with significant competing vegetation, approximately half as many trees on average were affected by competition in disk-trenched plots compared to untreated plots. The effects of disk trenching on the incidence of frost damage at three years were mixed. While trees in disk-trenched plots were less frequently and/or less severely affected than those in the paired untreated plot on some sites, they were more frequently and more severely affected on others. Disk trenching and planting in the berm generally appeared to reduce damage from frost on level, low-lying frost-prone sites. However, based on results from some of the trials, it appears that planting in the disk trench berm position may increase the frequency and severity of frost damage on gently sloping high elevation sites that are subject to radiation frosts. Fifth growing season With approximately one third of fifth-year data available, it appears that the trend observed between year 1 and year 3 is likely continuing, with increasingly positive, though still relatively small differences in height and diameter attributable to disk trenching on mesic sites. More conclusive interpretations will be possible with the completion of all fifth-year measurements.|
Anne Macadam and Rick Trowbridge.