Late-Season Fertilization, Mineral Nutrient Reserves, and Retranslocation in Planted Douglas-fir ( Pseudotsuga menziesii (Mirb.) Franco) Seedlings

Abstract Bare-root Douglas-fir ( Pseudotsuga menziesii (Mirb.) Franco) seedlings were given three levels of nitrogen (N), phosphorus (P), and potassium (K) fertilization (low, medium, high) during July to October of their second growing season in a nursery. The following January seedlings were lifted, an initial sample was obtained, and the remaining seedlings were grown in a sand culture under three nutrient regimes: (1) no essential nutrients except Ca (0); (2) all essential nutrients except N (All - N); (3) all essential nutrients (All). After 8 weeks, seedlings were separated into new shoots, needles, stems, old roots, and new roots. Dry weight and nutrient content of these parts were determined and changes during growth calculated by subtracting initial sample values. In a second similar experiment seedlings were grown in an artificial soil in the greenhouse and harvested at 14-day intervals so that dry weight and nutrient content changes could be followed. Seedlings increased in dry weight by 49 percent over 8 weeks with no net uptake of N, P, or K. During this time some 40 percent of the N, 30 percent of the P, and 44 percent of the K in the seedlings was retranslocated to support new shoot and root growth. In the absence of external N, P, and K supply, needles, stems, and old roots all lost these nutrients. Needles appeared to be the most important N storage site since they continued to retranslocate N in the presence of an external supply and, similarly, old roots were the important P and K storage site and retranslocated P and K when these nutrients were supplied externally. Seedlings from high fertilizer treatment in the nursery had higher N concentrations and content than seedlings from the other two treatments. They also showed a higher relative growth rate (RGR) in sand culture, but not in artificial soil. Late-season fertilization increased the number of new roots produced after 14 and 28 days and hastened bud burst. These effects were attributed to increased mineral nutrient reserves. Forest Sci. 31:485-496.