Fungal Stain Development in Canadian Hardwood Logs
Dian-Qing Yang, M-C Bisson
In most sawmills around the world, trees are harvested into logs and stored in the forest or in a log yard for a period of time before being sawn into lumber. These logs may be attacked by various pigmented fungi, which results in staining of the wood. Since hardwood species are used to a great extent in furniture manufacturing and in the making of other valuable wood products, the reduction of wood staining in these species has a significant economic impact. This study investigated storage time limits in sawmill yards for the major Canadian hardwood species, with and without water sprinkling.
Sugar maple, yellow birch, and white birch trees were selected and felled in summer and winter and transported to two sawmills in Quebec, Canada. At each site, one group of logs was dry-stacked and another was stored under water sprinkling in the sawmill yard. Until October, inspections of dry-stacked logs were conducted every 2 weeks and the water-sprinkled logs were inspected every 4 weeks. The logs were cut into discs and wood moisture content was measured; fungal stain and check development were also examined. The results showed that 3 weeks after harvesting, fungal stain had not occured on summer-harvested sugar maple or yellow birch logs. Minor staining was measured in logs of both species after 5 weeks in storage. Significant staining was found after 7 weeks of storage and serious staining occurred after 9 weeks or more of storage. After 13 weeks in storage, most sapwood of the dry-stacked logs was stained. Log ends had more staining than internal sections. On exposed wood without bark protection, most fungal infections started within 5 weeks of felling. However, after 5 weeks of storage, when wood moisture content had decreased to a certain level, thus providing less protection, staining fungi began to attack bark-covered wood. The results of the second trial showed that winter-harvested logs were more resistant to fungal stain than summer-harvested logs. Moreover, those harvested in the winter did not sustain much staining the following spring. Until the end of June of the following year, only 1.9% wood of dry-stacked sugar maple logs, 2.2% wood of yellow birch logs, and 2% wood of white birch logs were stained. Stain development in the logs increased rapidly in July and August. White birch was most affected by stain, followed by yellow birch, and, finally, sugar maple. Water-sprinkled logs were well protected from stain. Check development on logs under the two storage methods showed that, in dry storage, sugar maple logs developed more physical checks than the yellow birch and white birch. Most checks formed on the ends rather than in internal sections of the logs. Water sprinkling limited check formation on all logs. Fewer checks developed in winter-harvested logs (site two) than in summer-harvested logs (site one).
Keywords: Wood stain, sugar maple, yellow birch, white birch, log storage, dry storage, water sprinkling