Key message <p>Extrapolation of dynamic self-thinning lines in Canadian interior spruce forest stands intersected at a common point when quadratic mean diameter was used for average plant size but are the same line asymptotically when stand top height is used.</p> Abstract <p>In a forest stand undergoing steady and ongoing tree mortality, the self-thinning rule applies. This relates progressive stand stocking densities to average plant sizes with a power law. Often, quadratic mean diameter has been used as the measure of average size. It was found recently for a number of forest species, that when extrapolated in stands of differing site indices and initial stocking densities, such relationships intersect near a common point. This appeared to add a new concept to the self-thinning rule, but no explanation for it was immediately apparent. The present work applied an existing stand growth model for even-aged, Canadian interior spruce forests; these contain a mixture of two spruce species but are generally treated as monospecific, even-aged forests. It predicted the intersection at a common point of the self-thinning relationship for many stands. However, if average heights of tallest trees (top height) was used to represent average tree size at any age, the self-thinning relationship became a single relationship for different stands probably reflecting a balancing out with age of the competitive interactions between trees. It is clear that much further research is required to better understand how and why the self-thinning rule applies.</p>