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

Abstract

Growth of planted seedlings in cutovers dominated by salal (Gaultheria shallon Pursh) is poor largely because of low N availability and understorey competition. In this paper, the response of tree growth and fertilizer recovery to understorey competition was studied. The trees were four years old when (15NH4)2SO4 (200 kg N/ha, 3.38004% enrichment) was applied in 1991 to single-tree plots, with either understorey removed from (treated) or left (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, except diameter growth for western redcedar (Thuja plicata Donn.). Nitrogen and 15N 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, indicating that the site was low in N supply and the effect of fertilizer application on tissue N concentration did not last for 6 years. Results strongly indicated that the trees or understorey vegetation had no net uptake of fertilizer N beyond the second growing season. Understorey vegetation components played a significant role in the uptake and recycling of fertilizer N in this forest ecosystem.