Effect of urea fertilization on allometric relations in young Douglas-fir trees

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Abstract
The effect of nitrogen fertilization on allometric relations was examined for 23-yr-old site III Douglas-Fir (P. menziesii (Mirb. Franco) trees 2 yr after urea fertilization. Logarithmic regression equations using stem diameter (cm) to predict tree biomass components (kg) were not significantly (P = 0.05) different between fertilized and control trees for total foliage, total branch, dead branch, stembark or stemwood. New foliage and new twig components, were higher in fertilized trees than in control trees. Analysis of data from this and earlier studies suggest fertilization will increase leaf biomass per tree relative to control trees on sites having low N availability; however, this response will decrease with increasing N availability. Regression equations based on regional analysis of unfertilized trees yield estimates of foliage biomass for average trees on average sites. If N fertilization brings the site above average in terms of nitrogen availability than these regression equations will underestimate foliage biomass. On sites that are initially very N deficit. N fertilization will bring the site closer to average in terms of N availability, resulting in more accurate predictions of foliage biomass for fertilized stands than for control stands.