ToME WikiOrvin: This would be available from the "m" menu if Axemastery>=20 and axe wielded. If you are adjacent to a tree square and adjacent to an empty "hard ground" square (basically, any type of solid ground), you can chop down the tree and use it to create a barrier on the "hard ground" square. The barrier would act like debris, but with a different symbol, and would be susceptible to fire, etc. like trees.
NeilStevens: Not really directly related to this idea, but just an aside: in ToME 3 the weapon types are written out from scratch, designed to be interesting and distinct from the ground up, so it becomes unnecessary to use extra abilities to make the specialists distinct from each other.
Orvin: Glad to hear that. Still, I think it might be interesting for axemasters to be able to create barriers (to LOS and movement) from trees.