Relationship between spacing and nitrogen fertilization of seedlings in the nursery, seedling mineral nutrition, and outplanting performance

Abstract

Three nitrogen (N) levels and six seedling spacings were applied factorially in a randomized block design, of four replications, to coastal and interior varieties of Douglas-fir (Pseudotsugamenziesii (Mirb.) Franco), Sitka spruce (Piceasitchensis (Bong.) Carr), and lodgepole pine (Pinuscontorta Dougl.) in the nursery. After growing for 2 years in the nursery, seedling dimensions, dry weights, and needle N, phosphorus (P), and potassium (K) concentrations were determined. At the same time, samples of 400 trees per treatment were planted into the forest using the nursery design. Three years after outplanting, survival and height measurements were obtained for all species and, in addition, dry weights and nutrient concentrations for the coastal Douglas-fir. Both an increase in N fertilizer and wider spacing increased 2-year-old (2–0) seedling needle percent N, except in lodgepole pine. Wider spacing also increased percent P in interior Douglas-fir and percent K in lodgepole pine. Within the highest N fertilizer level the amount of variation in needle percent N accounted for by spacing was 75% in coastal Douglas-fir, 78% in interior Douglas-fir, and 92% in Sitka spruce. Needle percent N in 2–0 coastal Douglas-fir and Sitka spruce was positively correlated with survival (r2 = 0.24 and 0.35, respectively), total height (r2 = 0.48 and 0.61, respectively), and current height growth (r2 = 0.31 and 0.51, respectively) after 3 years in the forest. The relationships of spacing to 2–0 seedling percent N, and of 2–0 seedling percent N to outplanting performance indicated that, at least for Douglas-fir and Sitka spruce, spacing influenced outplanting performance through its effect on N nutrition. During 3 years in the forest, coastal Douglas-fir trees had maintained closely similar relative growth rates (RGR) in all treatments. Consequently, relative differences in 2–0 seedling dry weight between treatments of about threefold between 0.6- and 12-cm spacing and twofold between 60 and 235 kg N/ha fertilizer levels, had been maintained. After 3 years in the forest there were no differences in nutrient concentrations between nursery N treatments, indicating that reserves available at lifting had been dissipated, but nutrient concentrations tended to decrease with wider spacing. This suggested some unidentified wide spacing effect was promoting growth and causing nutrient dilution in the tissue.