Cumulated acoustic emissions (CAEs) related to the total number of tracheids in Norway spruce (Picea abies) standard sections (13.3 mm longitudinal, 0.9 mm radial and 200 μm tangential) of different ages. Open circles, juvenile samples; closed squares, samples from the base of the crown; closed circles samples taken at 1 m from the ground. nE3, n × 10−3. ***, significant at the 0.001 level.