If you own or lease forested property with the purpose of harvesting the timber on it, rather than holding it as an investment property, money you made on the sale of timber is income, and must be reported.
You may choose to cut the timber and sell it as logs, firewood or pulpwood, or you may raise Christmas trees for sale. The amount you receive from sales are ordinary farm income, while expenses related to cutting, hauling and other routine operations are expenses. This is true whether you harvest the timber yourself or allow others to do it.
Additional tax rules apply to the operation of a timber business, and you can learn more from IRS Publication 225, which includes a detailed explanation of how timber businesses are taxed.