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

Fertilizer labeled with N-15 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 (Gaultherin 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)(2)SO4 (3.38044% N-15 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 N-15 within trees was virtually unaffected by the treatment but displayed differences among species. The majority of the N-15 in a tree was found in the current-year needles. Because of the dilution effect, N-15 abundances in the above ground tree components were not different between treatments but N-15 contents were significantly increased by salal removal. The pattern of and treatment effect on total N distribution were similar to those of N-15. Total recovery by trees of applied N-15 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 fur the applied fertilizer N.