Understorey competition affects tree growth and fate of fertilizer-appliedN-15 in a Coastal British Columbia plantation forest: 6-year results

Growth of planted seedlings in cutovers dominated by salal (Gaultheria shal lon Pursh) is poor largely because of low N availability and understorey co mpetition. In this paper, the response of tree growth and fertilizer recove ry to understorey competition was studied. The trees were four years old wh en ((NH4)-N-15)(2)SO4 (200 kg N/ha, 3.38004% enrichment) was applied in 199 1 to single-tree plots, with either understorey removed from (treated) or l eft (control) in the plots. Half of the plots were either sampled after two (1992) or six (1996) growing seasons. Understorey competition continued to significantly reduce height and diameter growth between 1992 and 1996, exc ept diameter growth for western redcedar (Thuja plicata Donn.). Nitrogen an d N-15 concentration in both tree and understorey components decreased from 1992 to 1996 and N concentration in 1-year-old foliage in 1996 (but not in 1992) was significantly lower in the control than in the treated plots, in dicating that the site was low in N supply and the effect of fertilizer app lication on tissue N concentration did not last for 6 years. Results strong ly indicated that the trees or understorey vegetation had no net uptake of fertilizer N beyond the second growing season. Understorey vegetation compo nents played a significant role in the uptake and recycling of fertilizer N in this forest ecosystem.