1. Create birchwood lair.
2. Item → Set Data. Set Data1 as creature code, Data2 should be -1, Auxdata as creature attribute code.
3. If the lair is already on the ground, nudge it with a push to update settings. Otherwise, drop it on the ground and wait.

NOTE: If the creature count is at the set limit for the server map, spawns will not occur until creatures die and free up some space in the creature count.