I prune to about 18 ft using a pole pruner. About 5 years ago, I took some branches off firs that I intend to build with. They have put on 3 inches of girth since then and the cut marks are closed or closing. When I harvest sometime in the next 3 years, my debarked logs will look a little nicer and the debarking process will be easier. Generally pruning is seen as a very long term strategy, but in an established stand, it can make sense to do it as little as 3 years before harvest. My decisions are often based on opening up views and making the terrain more walkable.
Another benefit to branch pruning a few years before harvest is that the next generation of little trees gets a little more light. Sometimes you can have some nice young ones coming up and plan on them filling the void and you come back a year later to find that they have been shaded out or that the tops have bent in search of light. I prune a little harder on the side where a promising replacement is reaching. At the same time, I use my loppers to clear the ground of other saplings that are competing for space with the chosen one. Competition that is cut is used as a mulch all around the replacement tree. The branches cut from the large tree also serve as mulch. Moisture is retained and ground cover is suppressed. Scotch broom is the perfect mulch since this member of the
pea family lays flat and provides a nitrogen
boost. When this is done right, the young tree grows very quickly after the large tree is cut. It quickly shades out broom, bracken and other undergrowth that can overwhelm saplings.
On difficult terrain it's important to choose a replacement that is positioned where it won't be harmed by the big tree being felled. Most of my good trees lie below the roadway and within 75 feet of the 1 km road. It's important for mine to fall uphill. They are tall enough to fall across the road, where the excavator and a choker strap can lift them to the level surface for easy processing.