Carbon isotope discrimination in western hemlock and its relationship to mineral nutrition and growth

Western hemlock (Tsuga heterophylla [Raf.] Sarg.) is a major component of temperate rainforests in coastal British Columbia. Forest fertilization can enhance the growth of forest trees, but results are inconsistent for western hemlock. We investigated the relationship between delta C-13 (foliage and stemwood), growth response and tree nutritional status in this species. To establish a sampling protocol for stemwood, we first assessed spot-to-spot variation around and along the bole, which exceeded 1 parts per thousand. We utilized the reaction wood (high lignin content) and adjacent normal wood in two curved western hemlock stems to evaluate whether this variation was related to wood composition. There was a consistent 3.43 parts per thousand difference between lignin and holocellulose, but the isotopic mass balance of whole wood was conserved and, therefore, did not vary with lignin content. Therefore, extraction of cellulose or holocellulose prior to analysis can introduce (not remove) bias. In a detailed study of a third stem, circumferential and longitudinal variation in delta C-13 was associated with spiral grain indicating limited physiological mixing of isotopic signatures originating from the crown. Wood was subsequently pooled from four cardinal positions around each stem. Eight even-aged western hemlock stands were selected and fertilized with different combinations of nitrogen (N), phosphorous (P) and a blend of S, K, Mg, Zn and Cu. Fertilization was effective in increasing foliar N, P, K and S depending on treatment. At the end of the first growing season after fertilization, the effect of treatments on foliar delta C-13 was nearly significant (P = 0.054), but did not persist into a second year. Effects on tree-ring delta C-13 were more obvious and persisted for about 3 years, averaging similar to 0.2-0.4 parts per thousand over this period, depending on treatment. Combinations of N, P and blend had the greatest effect, consistent with relative increases in basal area increment. Effects of fertilizer additions on delta C-13, though clear, were superimposed on larger site and annual weather-related patterns in delta C-13. Large tree-to-tree variation in delta C-13 was positively correlated with basal area increment, both before and after treatment imposition, suggesting that high water-use efficiencies are associated with greater growth.