Climbers are increasingly using ascenders to access trees, both as a substitute for Prusik loops used in footlocking a doubled rope and with the single rope technique. Manufacturers, however, have explicit limitations on use of ascenders, many of which are violated when used in tree climbing. Ascenders were tested on four arborist climbing ropes in a dynamic drop test; impact load and arrest distance were measured. Of 67 tests, arrest distance met the EN 12841-2006 Standard (≤2 m) only 10 times. Impact loads averaged more than five kN, adding a backup friction hitch to the ascender increased impact load to more than six kN. Climbers need to be made aware of the appropriate use of ascenders, and only use compatible ropes.