While the doubled-rope technique (DdRT) is the preferred method for short ascents and canopy scouting, single rope technique (SRT) is the preferred choice for fast, energy-efficient ascents of 50 feet and higher.
The more traditional of the two approaches, SRT was introduced to tree climbing by climbers with mountaineering and caving experience.
For ascending the rope, cammed devices called ascenders are used. These devices are easily pushed up the rope and hold the climbers weight when pulled or loaded. For coming back down, or rappelling, a descender is used to manipulate friction and therefore the speed of a climbers descent.
Compared to the Doubled Rope Technique (DdRT), which is the most efficient way for moving around within a trees canopy, SRT does require more equipment and a higher degree of technical skill to engage and disengage.
To more easily change from SRT to DdRT while in the tree, its best to use a bowline with a Yosemite tie-off up to the supporting limb. Running the line over the supporting limb and tying it to the trunk is acceptable, but it increases tension on the limb and you will require help from the ground to change over.