There have been some succesful legal challenges in regard to sales tax on forest products here in WI. As mentioned already, and same as some other states, firewood for home heating is not subject to WI sales tax, but the changes are in regard to forest products produced by the person who raises the trees. I think what i read was that a tree farmer in S WI sued because his farmer neighbors did not need to collect sales tax for products sold at the end of the driveway, and he felt his lumber should qualify as a farm product too.
If i am not mistaken he won this a while back and it has expanded. But what do i know.