Nail withdrawal properties of beetle-transmitted blue-stain fungus in lodgepole pine dimensional lumber

Abstract

The mountain pine beetle has existed in North American forests for many years to some extent, yet it has reached levels of epidemic proportions is recent years. The recent damage in our forests has been growing at an exponential rate, and there is little that can be done to stop the momentum. The current outbreak has been attributed to two main causes--aggressive wildfire prevention practices in our forests; and warmer winters that no longer reach the lower temperatures which used to kill the beetles seasonally. These two factors have allowed some forests to generate mountain pine beetle infestation rates higher than 50 percent. While there is little that can be done to stop the beetle, the wood can be harvested for commercial purposes after the attack. When the mountain pine beetle enters a tree, a blue-stain fungus is subsequently inoculated into the sapwood. It is the fungus that ultimately kills the tree by preventing water translocation through the cells in the sapwood. A few studies have found that if the tree is harvested within a few years of the attack, the wood produced from these trees maintains most mechanical properties. However, the body of knowledge on blue-stained wood is still quite limited and many researchers agree more studies need to be conducted. Most wood grading rules do not degrade wood for the presence of blue-stain, which would lead one to believe that with the massive potential supply of blue-stained lumber in our forests, it would be consumed at a high rate. It has been found, however that blue-stained wood is failing to generate demand, due in part to negative consumer perception of the mechanical properties of the wood. This study was aimed to refute the negative perception of blue-stained wood by performing a comparative study of the nail withdrawal properties of blue-stained and clear lodgepole pine dimensional lumber. This study harvested 10 blue-stained and 10 clear lodgepole pine trees from Summit County, Colorado. The trees were then milled, planed and cut into small blocks. Each block was tested for moisture content, specific gravity, face withdrawal and cross section withdrawal. The study found that on average, the blue-stained samples had slightly lower withdrawal resistance when compared with clear wood, for cross section withdrawal. On the other hand, the blue-stained sample generated a slightly higher average resistance than the clear wood for face withdrawal, and that when controlled for moisture content; the blue-stained samples yielded approximately 40 lbs more resistance than the clear wood samples, or a 7% increase. The difference in means being so minimal, it was concluded that there was no statistical or practical difference in nail withdrawal properties between blue-stained or clear wood samples using the cross section withdrawal test. It was also concluded that presence of blue-stain in lodgepole pine dimensional lumber could increase face withdrawal resistance.