Response of Douglas-fir stem profile to operational nitrogen fertilization in western Oregon

Nitrogen (N) fertilizer is a commonly applied silvicultural treatment intended to promote growth of intensively managed coast Douglas-fir [Pseudotsuga menziesii (Mirb.) Franco var. menziesii] plantations in the Pacific Northwest region of the US (PNW) and adjacent areas of Canada. Because responses have been geographically variable, fertilization trials were established in the Coast Range of northwestern Oregon to enhance general understanding of Douglas-fir growth response and test the economic viability of operational N applications in this subregion. Twenty-three operational stands comprising the trials were dominated by Douglas-fir, had an initial total age of 16-24 years, had been pre-commercially thinned 7-10 years prior to establishment of the fertilization trials, and started with 387-1260 trees ha-1. Nitrogen fertilizer was applied aerially as pelletized urea (46% N) at a rate of 224 kg N ha-1 to half of each stand during the dormant season of 2009-2010. In the dormant season of 2016-2017, seven growing seasons following fertilizer application, forty sample trees were felled in ten randomly selected stands to assess the influence of N-fertilization on stem form. A variable exponent taper model was developed using a nonlinear mixed-effects modelling approach. A tree-level random effect and a first-order continuous autoregressive [CAR(1)] error process were incorporated into the model to facilitate accurate tests of significance on model parameter estimates. The variable exponent taper model captured significant variation in stem form resulting from fertilization based on interaction with a crown variable, suggesting that the N-fertilization influence on stem form was mediated by initial crown length of trees with given diameter and height. Volume predictions from the resulting taper model were compared to those from the regional growth model ORGANON, which assumes no influence of fertilization on stem form. The regional growth model ORGANON significantly (p < 0.01) under-predicted fertilized tree volume by 3.6%. Results are being incorporated into individual-tree growth models to improve site-specific predictions of growth response. Improved understanding and modeling of mechanistic responses of foliage, growth, and stem form to fertilization may help discriminate between sites with high and low growth response potential. Mechanistic insights coupled with better site characterization should help improve the understanding of economic and environmental performance of fertilized Douglas-fir stands.