Why is winter the best time to cut lumber?

The main reason lumber is primarily cut in the winter is that the ground is frozen, which allows access for heavy logging equipment with limited damage to the soil. Also, winter cut logs have tight bark, which is an advantage to protect against bug infestation for logs that sit a long time before processing, but with cold weather and since sap is not running, bugs are usually a minimal threat. Tight bark remaining on the edges of boards can be a design attribute for plank style lumber, however, usually the logs are debarked as a matter of course.