Basal area growth, carbon isotope discrimination, and intrinsic water use efficiency after fertilization of Douglas-fir in the Oregon Coast Range

Abstract Many hectares of intensively managed Douglas-fir ( Pseudotsuga menziesii Mirb. Franco) stands in western North America are fertilized with nitrogen (N) to increase growth rates, but only about ⅔ of all stands respond. Understanding the mechanisms of response facilitates prioritization of stands for treatment. The primary objective of this study was to test the hypothesis that the short-term basal area growth response to a single application of 224 kg N ha −1 as urea was associated with reduced stable carbon isotope discrimination (Δ 13 C) and increased intrinsic water use efficiency ( i WUE) in a 20-yr-old plantation of Douglas-fir in the Oregon Coast Range, USA. Increment cores were measured to estimate earlywood, latewood, and total basal area increment over a time series from 1997 to 2015. Stable carbon isotope discrimination and i WUE were estimated using earlywood and latewood stable carbon isotope concentrations in tree-ring holocellulose starting seven years before fertilization in early 2009 and ending seven years after treatment. A highly significant (p  13 C and i WUE in earlywood and latewood. Fertilization significantly reduced earlywood Δ 13 C and increased earlywood i WUE in the first and second growing seasons after fertilization. Only a marginally significant fertilization effect was detected for latewood Δ 13 C and i WUE in the second growing season after treatment. Previous studies of N fertilization of Douglas-fir forests have reported consistently increased growth and i WUE on low productivity sites treated with relatively high fertilization rates. This study suggested that these responses can also be observed on highly productive sites despite their lower frequency and apparently shorter duration. Other key mechanisms driving growth responses appear less important than i WUE, including an increase in LAI and shift from belowground to aboveground carbon allocation.