Predicting Douglas Fir Growth and Response to Nitrogen Fertilization in Western Oregon

Abstract

The accurate prediction of which Douglas fir [Pseudotsuga menziesii (Mirb.) Franco] stands may or may not respond to N fertilization should improve the economics of the practice. Our objective was to determine the efficacy of various stand (site index, age, and relative density), climatic (total precipitation, average daily solar radiation), site (elevation, soil depth, and available water‐holding capacity), and soil‐test variables (anaerobically mineralized N, total N, organic matter, and C/N ratio) to predict relative and absolute response to a single application of 224 kg N ha−1 as urea. The core equation with stand variables accounted for 70% of residual variation in average annual volume growth. Predicting response of fertilized stands proved much less precise. The best core equation explained 37% of the residual variation for average percentage response in volume growth and explained less variation in absolute response in both volume and basal area. Of the site, climatic, and soil‐test variables, C/N ratio in the surface soil was the only one that significantly increased precision of the core equations. The best combined equation explained 46% of the variation in percent volume response. The anaerobic N mineralization test failed to make a significant contribution to the core equation and had a lower correlation with response than did the C/N ratio. Stand variables remain the most reliable predictors of fertilizer response in this region; any improvement from including soil data for N or organic matter is not justified because of their additional cost.

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

“…In one analysis involving only 5-yr growth periods, the maximum direct response of Douglas-fir to fertilization was concluded to occur in the first 5-yr period after fertilization (Wang 1990). In contrast, 2-yr remeasurements led to the conclusion that growth response peaked between the 2 nd and 4 th growing season after fertilization (Miller et al 1989).…”

Nitrogen-fertilization impacts on carbon sequestration and flux in managed coastal Douglas-fir stands of the Pacific Northwest

“…In this case, fertilization of plots with lower site indices resulted in more C sequestered in standing trees. Though C sequestration in particular has not been studied a great deal in relation to site, it is often observed that stands with lower site indices respond more to N-fertilization (Ballard, 1984;Miller et al, 1989;Hopmans and Chappell, 1994).…”

Growth and nutritional responses of western hemlock to fertilization: A review

“…Based on these studies, single applications of N at 224 kg/ha were estimated, over 10 years, to increase gross volumes by 22-25 m 3 /ha in unthinned stands and by 27-32 m 3 /ha in thinned stands (Chappell et al 1992). These substantial responses by Douglas-fir to N fertilization have been corroborated by other long-term studies (Omule 1990;Brix 1993a;Weetman et al 1997) and by shorter-term studies relating response to site characteristics (Miller et al 1989;Carter et al 1998). Growth of Douglas-fir has generally not increased when elements other than N were added alone or in combination with N (Gessel et al 1981;Weetman et al 1997;Carter et al 1998).…”