An investigation into the factors contributing to the growth-check of conifer regeneration on Northern Vancouver Island

Conifer plantations established on cutovers in the CWHb1 zone on Northern Vancouver Island grow well initially, but coincident with the reinvasion of salal (Gaultheria shallon Pursh.) on sites formerly dominated by western red cedar and western hemlock (CH phase) the trees become chiorotic and growth stagnates. These symptoms are not seen on sites dominated by western hemlock and amabalis fir (HA phase). This study examined some site and soil chemical factors which could be responsible for differences in forest productivity by: 1) documenting physical differences between the CH and HA phases; 2) documenting morphological and chemical differences between the organic horizons found in forest floors of CH and HA phases using classical wet chemistry techniques and 13C nuclear magnetic resonance (NMR) spectroscopy; 3) determining seasonal trends in free phenolic acid concentrations of soils under salal on CH cutovers by high speed centrifugation and High Performance Liquid Chromatography, and 4) determining if solutions of phenolic acids at field concentrations or of salal leachates have a negative effect on conifer seed germination, growth and short term 32phosphorus uptake. The HA phase was found to occur on higher topographic positions making them drier and more susceptible to windthrow events than the CH phase which occur on adjacent, lower slope positions. This windthrow process appears to rejuvenate the site by mixing organic horizons with mineral soil, increasing aeration, breaking hardpans, and generally improving site conditions. Six distinct humus horizons were identified on the basis of origin and on degree of decomposition. The proportion of humus horizon types on the CH reflected ecosystem maturity and lack of disturbance; that of the HA indicated repetitive windthrow events. The CH humus horizons tended to have higher concentrations of K, Ca, Mn, available S, lipids, and total and labile polysaccharides, as well as a higher pH. The HA humus horizons were found to be higher in available N and P and tended to have a lower C/N ratio for the more well— humified horizons. Tannin signals from 13C NMR spectroscopy were found in the Fm horizons of both CH and HA, but the intensity was greater for the CH. The source of tannins appears to be salal, as strong tannin peaks were identified in salal roots, leaves, flowers, berries and litter. Tannins are known to inhibit decomposition and mineralization processes. Concentrations of phenolic acids originating from angiosperms (presumably salal) were significantly higher in summer months, coincident with greater physiological activity of salal. Phenolic acids are known to cause root membrane dysfunctioning in some situations. The germination values of seeds and the biomass of seedlings of Sitka spruce, western hemlock and western red cedar tended to be lower given treatments of either a phenolic acid solution at field concentrations or a salal leachate solution compared to a control of distilled water. The uptake of 32P by the excised roots of Sitka spruce and western hemlock were higher for the phenolic acid and salal leachate solutions than for the control, indicating P was probably limiting even after only 12 weeks. Uptake of 32P by fine roots of mature western red cedar and western hemlock was significantly reduced by the phenolic acid and salal leachate solutions. In conclusion, this study provides evidence to suggest that the growth-check of conifer regeneration involves a number of interacting factors. These include: 1) the presence of hardpans and compacted mineral soils which contribute to annual periods of anaerobic soil conditions; 2) the presence of large pockets of nutrient poor woody humus, and 3) the presence of tannins, lipids and phenolic acids in active humus horizons which may be contributing to decreased decomposition, mineralization and nutrient uptake in trees.

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