BIOMASS AND NUTRIENT ELEMENT DYNAMICS IN DOUGLAS-FIR - EFFECTS OF THINNING AND NITROGEN-FERTILIZATION OVER 18 YEARS

The effects of thinning (two-thirds of basal area removed) and N ferti lization (448 kg N/ha as urea) on biomass and nutrition of a 24-year-o ld Douglas-fir (Pseudotsuga menziesii (Mirb.) France) stand at Shawnig an Lake were studied over 18 years. At years 0, 9, and 18 after treatm ents, the aboveground biomass and N, P, K, Ca, and Mg contents of stem wood, stem bark, foliage, and dead and live branches were determined ( kg/ha), and increments in these properties (kg ha(-1)-year(-1)) were c alculated for the 0-9 and 9-18 year periods. Foliar biomass was increa sed by both treatments during the first period and also by thinning in the second period. Aboveground net primary production (ANPP) per unit of foliage biomass (foliage efficiency) was increased by treatments i n the 0-9 year period. The combined effects of increased foliage mass and foliage efficiency resulted in increased total biomass production. Thinning and fertilization increased the uptake of all elements excep t for P with fertilization. This increase may have contributed to the long-term increase in stem growth. Retranslocation of elements before foliage shedding was important for tree nutrition, but was not improve d by fertilization during the 9-18 year measurement period. The effici ency of N use in dry matter production (ANPP/unit of N uptake) was dec reased by fertilization. This implied that poor sites would respond be tter to fertilization than rich sites.