Assessing nitrogen fertilizer response of coastal Douglas-fir in the Pacific Northwest using a paired-tree experimental design

Nitrogen is the nutrient most limiting growth of coastal Douglas-fir of the Pacific Northwest and, accordingly, urea fertilization has been applied through the region to improve Douglas-fir growth. While factors affecting fertilizer response have been studied for years, changing management techniques and improved genetics warrant new studies that investigate Douglas-fir fertilizer response. This study documents a paired-tree experimental design that tested two-year and four-year basal area, height, and volume response to 224 kg N ha−1 as urea in Douglas-fir plantations in the western Pacific Northwest. Significant fertilizer response for each installation was determined using a paired t-test for each measurement on 12-20 replicates of paired control and fertilized trees per installation. The close proximity of treatments in this study design was also tested as the effect of the number of surrounding fertilized plots on control tree growth using a mixed model. The greatest effect of fertilization was found in two-year basal area response where nearly half of the installations significantly responded to fertilization. Fewer installations responded significantly 2-4 and 0-4 years after fertilization. There was no effect of the number of adjacent fertilized plots on control tree growth. Fertilizer response in the paired-tree installations was similar to a recent single-tree fertilization study and was correlated to previous models of plot fertilizer response. These findings support the use of the paired-tree fertilization design to continue testing fertilizer response in plantation forestry.