Carbon isotope discrimination in Tsuga heterophylla and its relationship to mineral nutrition and growth

Western hemlock (Tsuga heterophylla) is one of the most important tree species in coastal British Columbia. Forest fertilization is a method used by foresters to enhance the growth of forest trees, but results are inconsistent for western hemlock. The objective of my research was to explore the use of carbon isotope analysis as a physiological tool to diagnose the nutritional status and potential response of western hemlock to fertilization. Normally cellulose is isolated from wood samples to be analyzed for carbon isotope analysis (expressed as 813C values), but this is a tedious and likely unnecessary process. Reaction wood (high lignin content) and adjacent normal wood in two western hemlock saplings was analysed to evaluate the possible effects of wood composition on 813C. The 8 C values of the lignin and cellulose fractions differed by 3.43 %c ± 0.26 (mean ± SD; n = 40). 813C values of lignin and cellulose from different disk positions were more variable in one sapling than the other. The isotopic mass balance of whole wood was conserved and therefore did not vary with lignin content indicating that use of whole wood, rather than cellulose, is suitable for isotopic analysis. Eight pure western hemlock stands selected and experimentally fertilized in an earlier study were used as source for foliage and stemwood samples. Fertilization treatments applied to these stands in mid to late May of 1995 were as follow: (1) control, (2) N (225 kg/ha), (3) N (225 kg/ha) + P (100 kg/ha), (4) N (225 kg/ha) + P (500 kg/ha), (5) N (225 kg/ha) + P (100 kg/ha) + blend (230 kg/ha), and (6) N (225 kg/ha) + P (500 kg/ha) + blend (230 kg/ha). The blend included additions of S, K, Mg, Zn and Cu. 813C values within foliage and stemwood were analysed after fertilization. At the end of first growing season after fertilization, the effect of treatments on foliar 8 C was almost significant (P = 0.0539) and there was an interaction between sites and treatments. At the end of second growing season, no interaction between site and treatment was evident and the effect of treatments on foliar 813C was not significant. Foliar SO4-S (Sulfate) levels at the end of the first and second growing seasons following fertilization were reduced by either N or N + P fertilization treatments. The change in 813C values of tree rings from before to after nutrient additions was significantly affected by treatment and site and there was an interaction between sites and treatments. N applied alone had no significant effect on change in wood 813C. The greatest change (0.33 %o) was in the NP100 (treatment 3 above) level of treatment. Relative change in six-year basal area increment (BAI) was significantly affected by treatments and sites and there was an interaction between sites and treatments. N applied alone had no significant effect on relative change in basal area increment whereas NP100B (treatment 5 above) had the greatest effect. The relative change in basal area increment was also significantly affected by sites.Forestry, Faculty ofGraduat