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.

氮 (N) 肥是一种常用的造林处理方法，旨在促进集约管理的海岸花旗松 [ Pseudotsuga menziesii (Mirb.) Franco var. 的生长。门泽西] 美国太平洋西北地区 (PNW) 和加拿大邻近地区的种植园。由于响应在地理上存在差异，因此在俄勒冈州西北部的海岸山脉开展了施肥试验，以增强对花旗松生长响应的总体了解，并测试该次区域施氮的经济可行性。包括试验在内的 23 个操作林以花旗松为主，初始总树龄为 16-24 岁，在施肥试验开始前 7-10 年进行了商业前间伐，开始于 387-1260树 ha ^-1。氮肥以颗粒状尿素 (46% N) 的形式以 224 kg N ha ^-1的比例空中施用在 2009-2010 年的休眠季节，每个展位的一半。在 2016-2017 休眠季节，施肥后的 7 个生长季节，在 10 个随机选择的林分中砍伐 40 棵样本树，以评估施氮对茎形态的影响。使用非线性混合效应建模方法开发了可变指数锥度模型。树级随机效应和一阶连续自回归 [CAR(1)] 误差过程被纳入模型，以促进对模型参数估计的显着性的准确测试。可变指数锥度模型捕获了受精导致的茎形状的显着变化，基于与冠变量的相互作用，表明施氮对茎形状的影响是由具有给定直径和高度的树木的初始树冠长度介导的。将所得锥度模型的体积预测与区域生长模型 ORGANON 的体积预测进行比较，该模型假设施肥对茎的形状没有影响。区域生长模型 ORGANON 显着 (p < 0.01) 低于预测的受精树体积 3.6%。结果正在被纳入个体树木生长模型，以改进生长响应的特定地点预测。提高对叶子、生长和茎形态对受精的机械反应的理解和建模可能有助于区分具有高和低生长响应潜力的位点。