From http://update.uo.com/oldupdates/10-10-97.shtmlFailure at lumberjacking ruins some of the wood.
I know it's an old patch, but it's the only one that refers to the skill aside from bug fixes. (Or the UOR patch for damage modification to axes)
Currently, with 0 lumberjacking (displayed is always > 0), one can hit a tree as many times as they want to and still get the maximum lumber that spawned on that tree. The only current benefit to being a gm lumberjack is that you can harvest more quickly as non-gms still have access to a whole node. There is no degradation of the lumber that's there. The reality is, with each failure it would remove a unit of spawn same as a harvesting success would.
Secondly, not something I can research, it seemed to me that there would be some trees that would spawn with somewhere in the area of 100-200 logs as well as some that spawned with just 10. The average was still 30-50 but there would be some crappy trees and some super trees. Currently it's almost always 40 logs with some variation. Seems like this is something that can be checked in the demo, just not something I have access to.