When steam donkeys came into the woods the need for ox teams ceased. Initially the logs were dragged by the steam donkeys to the landing where they were loaded onto the logging cars (disconnects) to take them to the mill. Innovation in the logging came thick and fast and soon high-line logging was developed. This link gives a diagram of the wires and a brief description.
The next development was to rig the cables from a tall tree close to where the logs were being cut and carry them in the air to the landing to another tall tree (the tail tree). The tall tree needed close to where the logs were being cut was called the spar tree. The lower limbs of the tree had to be removed and the tree topped.
The diagram below shows the rigging between the spar tress and the lines going (on the left) to the steam donkey.
Topping is extremely dangerous because when the top falls the rest of the tree whips requiring the cutter to hang on very tightly. The pictures below show the spar tree being topped.
Climbing a Spar Tree
This article by Jerry (Gerald F.) Beranek, a local faller, is a superb piece on what it takes to climb a spar tree. Click the photo to read the whole article. A pdf version can be downloaded here