What's the pseudo-science behind cutting holly trees during the dead of winter when the sap is down and then getting them in a kiln pronto in order to reduce the moisture content?
Hey Jeff, it's not pseudo-science but actually well understood in the industry.
Of the domestic species that I am familiar with, holly is the most susceptible to stain from fungal activity
after the tree has died. Usually trees die when they are felled, but in some instances they will die while standing, which means that a standing dead, diseased or seriously pruned tree is already developing stain inside the lumber even though it has not yet been felled.
Maple and pine will also both stain quickly. The onset of stain, rapidity of spread through the wood, and extent of staining are directly linked to the temperatures occurring when the tree died (as well as when the log was stored), coupled with the chemical composition of the moisture contained within the tree (ie high sap content).
Stain from fungal activity develops while 1) temperatures in the wood are above 50 or so, 2), while the sap (sugar) content in the wood is high), and 3) when the logs sit for several weeks in conditions 1 and 2 before being milled. Usually the greatest amount of fungal stain takes place when the wood temperature is between 70 - 90 degrees. Once you get above 110F fungal stain does not seem to develop, nor does it seem to develop below 50F.
I've had pine logs felled in January that were still clear the following August. At the same time, I've had pine logs felled in July that were starting to stain 30 days later. Because the moisture content in trees is pretty much the same year round, the influence of sap on decay is considerable because the sap contains sugary and other substances that attract and foster microorganisms that cause decay and stain.
Wood dries very poorly in log form, because most of the moisture leaves the lumber through the face of the boards, not the ends. There are exceptions to this rule for certain porous species such as cedar, but in general terms the heard wood of logs will retain a high moisture content until they are milled. The sapwood will start drying out, but this causes problems due to shrinkage and cracking that develops in the sapwood portion of a log.
In general terms, for the minimal amount of stain in stain prone woods it is best to harvest in the middle of winter, mill within a few days after felling and get into the kiln immediately after milling. The greatest danger of staining in this scenario is during the initial phase of drying in the kiln cycle when the temperatures are lower. A typical kiln temperature for holly starts at 120F in order to quickly raise the chamber temp above the point where fungal stain will take place. Note that you have to go to 150F to kill the fungus, but it will be dormant above 110. What is notable here is that most hardwood species start the kiln drying process at a much lower temp (usually 90F in a DH kiln and 100 - 110 in a conventional kiln), which is right in the range where fungal activity can thrive.
So in short, for minimal fungal stain to develop in holly:
1. The tree needs to be alive and healthy until it is felled, as fungal stain can develop in diseased dying trees.
2. The tree needs to be felled when the daily high temperatures are below 50F, ie in the middle of winter.
3. The logs need to be milled within a few days of felling and while the temperatures are still below 50F, ie also in the middle of winter.
4. The lumber needs to be placed into a kiln immediately with a starting temp above 120F so as to prevent the onset or extension of fungal stain development.
Hope this helps.
Clearly there will be a significant investment required to harvest this tree (probably 2 - 4 thousand dollars). For me, the fact that the tree was partially removed earlier in the year elevates the risk level for stain to that which is above my comfort level. Had it been left healthy until being harvested in a January / February timeframe I might have a different opinion
Scott
