Spar tree in Vancouver, BC
It was a lot easier to drag a log over stumps, rocks, and other obstacles if the front end was in the air, and that's what a spar tree did. They'd find the tallest tree near the landing and send up a "high climber" to cut off the limbs and top. The high climber worked without safety equipment or a fall arrest system. Guy wires were used to stabilize the tree, and then pulleys were attached for the steel cable used in yarding.
Image courtesy of city of Vancouver archives
The spar tree didn't always have to be a tree. In many cases tall spars were built on-site using timber from the area. These structures looked like giant A-frames and were useful if a landing didn't have suitable trees for spars. Building the spars was arguably safer than high-climbing a tree, although people still had to climb the artificial spars during construction.