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The nitrogen nutrition of western redcedar, western hemlock, and salal : subtitle species-specific mechanisms for accessing nitrogen in cedar-hemlock forests on northern Vancouver Island Bennett, Jennifer Nicole

Abstract

Old-growth cedar-hemlock (CH) forests on northern Vancouver Island are characterized by thick forest floors with large total nitrogen (N) contents and low extractable NO₃" and NH₄⁺ + concentrations. Western redcedar (Thujaplicata Donn.), western hemlock (Tsuga heterophylla (Raf.)Sarge), and salal (Gaultheria shallon Pursh) growing in these forests are N-limited but show different productivities. Salal forms a vigorous understorey and cedar grows better than hemlock. The co-occurrence of cedar, hemlock and salal in N-poor CH forests and their respective productivities may be the result of the three species: 1. utilizing different organic and inorganic N forms. 2. having different rooting distributions allowing them to access spatially separated N pools. In this thesis, I attempted to evaluate the potential importance of these two mechanisms for providing N to cedar, hemlock and salal in these forests. In two pot trials, the abilities of the three species to take up and/or access different organic and inorganic N compounds were examined to determine species-specific differences in N form use. Cedar, hemlock and salal were all able to take up ¹⁵N & ¹³C-labelled glutamic acid intact and showed similar abilities to access ¹⁵N-labelled NO₃",NH₄⁺ + , glutamic acid, protein and protein-tannin compounds. Nitrate and NH₄⁺ + accounted for the largest proportions of N absorbed by the three species and were the most available sources of N during a 20-day period. To examine if differences in access to spatially separated N pools is possible, the vertical fine root distributions of cedar, hemlock and salal were measured in three old-growth CH forests. Salal and hemlock had the highest root densities in the upper forest floor horizons. In contrast, cedar showed a more even distribution of fine roots in the forest floor horizons and the upper 10 cm of mineral soil. These correlated patterns suggest that salal and hemlock are forced to share a common pool of resources because of overlapping depletion zones around the roots, whereas cedar probably has access to a greater proportion of N present in the lower forest floor and mineral soil horizons. It appears that accessing spatially separated N pools may be more important than differing organic and inorganic N uptake patterns in CH forests, and may, in part, explain the different productivities of cedar, hemlock and salal in these systems.

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