The effects of fertilization with nitrogen (N) alone, and in combination with sulphur (S), on the growth and foliar nutrition of six immature, managed Douglas-fir (Pseudotsuga menziesii var. glauca [Beissn.] Franco) stands in the Interior Cedar-Hemlock (ICH) biogeoclimatic zone of British Columbia are reported 6 years after treatment. Results indicate that interior Douglas-fir stands growing on circummesic sites within the ICH zone are generally responsive to fertilization. Disregarding results from one installation that was damaged by Armillaria root disease, average net volume response following fertilization with N alone was 13.5 m3/ha (range: 6.5-24 m3/ha) compared to the control treatment. Six-year volume gains from N+S additions averaged 16 m3/ha (range: 10-23 m3/ha). In relative terms, stand volume responses to fertilization with N and N+S averaged 24% (range: 8-41%) and 28% (range: 16-39%), respectively. Growth projections generated by the tipsy growth and yield program indicate that the accelerated stand development following a single fertilizer application will likely reduce biological rotations (i.e., culmination of mean annual increment) and technical rotations (e.g., minimum harvestable age) by 2-3 years. Relative growth responses compare favourably with results from Douglas-fir fertilization studies in other jurisdictions.
Pre- and post-fertilization foliar nutrient analyses indicate that several of the sites were marginally S deficient, and that S status deteriorated 1 year following N fertilization. Added S was readily taken up, thereby maintaining a favourable N:S balance in trees fertilized with N+S. Despite improvements in foliar S status, the incremental growth benefits of added S may be too small on most sites to justify the extra expense involved in blending and applying N+S fertilizers in large-scale aerial operations.
Results from this study, and others, indicate that pre-fertilization levels of foliar N and sulphate S (SO4) may have utility in selecting candidate stands and in making appropriate fertilizer prescriptions. For example, the largest growth responses following fertilization may be expected in stands with pre-fertilization foliar N levels less than 11.5 g/kg (< 13 g/kg when using dry combustion analytical methods). Also, low foliar N combined with small amounts of pre-fertilization foliar SO4 (< 200 mg/kg) may indicate that additional growth gains can be achieved by blending S with N in fertilizer prescriptions. Finally, results from this study apparently support previous claims from the Inland Northwest that low pre-fertilization foliar potassium (K) levels (< 6 g/kg) and elevated N:K foliar concentration ratios (> 2) may increase the susceptibility of fertilized Douglas-fir to mortality losses from Armillaria root disease. Additional Douglas-fir fertilizer trials are needed to test and refine these preliminary guidelines for wet-belt Douglas-fir in the interior of British Columbia.
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Updated October 24, 2008