Predicting Douglas fir growth and response to nitrogen fertilization in western Oregon
1989
Miller, R.E. | McNabb, D.H. | Hazard, J.
The accurate prediction of which Douglas fir [Pseudotsaga 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.
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