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

Fertilizer labeled with 15N 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 (Gaultheria shallon Pursh), on a western red cedar (Thuja plicata Donn ex D. Don) -- western hemlock (Tsuga heterophylla (Raf.) Sarg.) clear-cut site on northern Vancouver Island. Fertilizer was applied in April 1991 at 200 kg N.ha-1 as (NH4)2SO4 (3.38044% 15N enrichment) to single-tree plots of 1 m radius. Four-year-old western red cedar, western hemlock, and Sitka spruce (Picea sitchensis (Bong.) Carriere) were used and the plots were destructively sampled after two growing seasons (October 1992). The distribution of 15N within trees was virtually unaffected by the treatment but displayed differences among species. The majority of the 15N in a tree was found in the current-year needles. Because of the dilution effect, 15N abundances in the above ground tree components were not different between treatments but 15N contents were significantly increased by salal removal. The pattern of and treatment effect on total N distribution were similar to those of 15N. Total recovery by trees of applied 15N 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.