Variation in Douglas-fir (Pseudotsuga menziesii var. menziesii) and red alder (Alnus rubra) stem taper across varying stand conditions in the Pacific Northwest

Tree taper has been of interest for over a century, yet questions remain regarding the effects of silvicultural treatments and forest health on recoverable volume. This work utilizes data from Douglas-fir ( Pseudotsuga menziesii (Mirb.)) ( n = 608) and red alder ( Alnus rubra (Bong.)) ( n = 495) trees to assess the influences of fertilization, pruning, thinning, regeneration origin, and defoliation caused by Swiss Needle Cast (SNC; Nothophaeocryptopus gaeumannii), on stem taper in the Pacific Northwest. The Kozak (2004; For. Chor. 80: 507–515) variable-exponent equation was used to test the addition of treatment and crown variables as the model is widely regarded for its flexibility in application. Using a mixed effects framework, results reveal that thinning of Douglas-fir can result in a 3.5% increase in upper stem diameter inside bark, while pruning may lead to a 4.1% decrease. SNC-induced defoliation of Douglas-fir reduced mean diameter above-breast height by 11.5%. Total volume of artificially regenerated red alder was 16% greater than naturally regenerated stems. Overall, thinning of healthy Douglas-fir and planting red alder may increase recoverable volume and C captured in long-term timber products in the region, and the inclusion of crown variables can increase the predictive power of taper estimates for some species.