Effect of understory competition on distribution and recovery of 15 N applied to a western red cedar–western hemlock clear-cut site

Fertilizer labeled with 15 N was used to study the fate of N in forest soil–plant systems with (control) and without competition (treated) from an ericaceous evergreen shrub, salal (Gaultheriashallon Pursh), on a western red cedar (Thujaplicata Donn ex D. Don)–western hemlock (Tsugaheterophylla (Raf.) Sarg.) clear-cut site on northern Vancouver Island. Fertilizer was applied in April 1991 at 200 kg N•ha −1 as (NH 4 ) 2 SO 4 (3.38044% 15 N enrichment) to single-tree plots of 1 m radius. Four-year-old western red cedar, western hemlock, and Sitka spruce (Piceasitchensis (Bong.) Carrière) were used and the plots were destructively sampled after two growing seasons (October 1992). The distribution of 15 N within trees was virtually unaffected by the treatment but displayed differences among species. The majority of the 15 N in a tree was found in the current-year needles. Because of the dilution effect, 15 N abundances in the above ground tree components were not different between treatments but 15 N contents were significantly increased by salal removal. The pattern of and treatment effect on total N distribution were similar to those of 15 N. Total recovery by trees of applied 15 N was 7.7, 17.8, and 10.3% in the treated plots planted with cedar, hemlock, and spruce, respectively. The corresponding values for the control plots were 4.1, 2.0, and 4.9%. Understory in the control plots immobilized 14.8, 24.6, and 13.5% of the applied N for plots planted with the respective species. Total recoveries in soil and vegetation ranged from 57 to 87%, of which 59 to 82% was recovered in the soil compartments. Results clearly showed that trees competed poorly with the understory vegetation for the applied fertilizer N.