Lumberjacking

On Angel Island, lumberjacking adds a bonus modifier to damage while using 2 handed axes.

The bonus is 10% of the lumberjack skill up to 99.9, and 20% total for 100 skill.
+1% bonus for every 10 points of lumberjacking.
+10% bonus at Grandmaster.

So at GM Lumberjacking you get a 20% total bump in damage.

Training Lumberjacking

Lumberjacking can be trained from 0-GM by hiring a mercenary from the fighter pit in J'Helom. Buy 10 or 15 axes from the blacksmith and make a macro to heal your mercenary and to equip an axe if the one you're using breaks. It is recommended that you use a mercenary which is equipped with good armor and a shield for training against with such heavy weaponry.