Nitrogen Relations in a Douglas-fir Plantation

Abstract

A 42-year-old Western Washington Douglas-fir [Pseudotsuga menziessii (Mirb.) Franco] plantation which was N-deficient was studied in relation to the application of N applied as urea to the forest floor, and carbohydrate applied to the forest floor to widen the C/N ratio and thus intensify the existing N deficiency. After 1 year, needle size was measured and needles were also analyzed for pigment content N, P, K, Ca, Mg, and Mn. As a result of N fertilization needle size and weight had increased. Needle retention (years) was increased in the N treatments and reduced in the carbohydrates treatments. This effect was reflected in the quantities of leaf litter-fall. Nitrogen concentrations were increased in treatments where N was applied as were pigment contents but other nutrients decreased in concentration probably as a result of growth dilution. While there was no significant decrease in needle N concentration where carbohydrate was applied, the N uptake was reduced because of needlesize and retention timechanges. The results are discussed in relation to nutrient diagnostic work.