The response of good and poor aspen clones to thinning

The response of good and poor clones of trembling aspen (Populus tremuloide s Michx) to thinning was assessed 16 years after treatment. Prior to the th inning treatment, the clones had been assessed as either poor or good using a rating matrix that considered height, diameter, quality and vigour of th e clones. Results indicate that the 250 largest DBH stems . ha(-1) did not respond to thinning, irrespective of clone rating. The growth of these domi nant trees was unaffected by smaller competitors. Considering all trees, th e non-thinned (control) good clones were indistinguishable from the thinned good clones in terms of top height, basal area, quadratic mean DBH, volume -ha, and trees . ha(-1) 16 years after treatment. For the good clones, 16 y ears of self-thinning yielded the same result as a single manual thinning. Due to a slower rate of self-thinning, the non-thinned poor clones retained some of the small stems longer and thus had a higher basal area and volume than the thinned poor clones. Thinning did not increase the piece size of the dominant trees so there was no associated increase in value. Thinning good and poor clones of tremblin g aspen did not increase the standing volume or piece size. Therefore, thin ning is recommended only for good clones and only if it is profitable on it s own. The literature on the benefits of thinning of aspen is contradictory . This may be due, in pm to undocumented clonal differences.