if you cut small sections out of the tree then you could be making the FC lose out on revenue. Typically the trees would be cut to certain lengths depending what the end purpose would be ie furniture, paper mill etc. Small bits that didn’t fit the criteria would get left in the trash piles.
PS wouldn’t reccommend it now. We had permission a few years ago about this time to collect some offcuts for firewood etc. Some local called the police and reported we were nicking loads of Christams trees, had helicopters and 4x4s come and find us!