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Post-harvest floor changes and nitrogen mobilization in an Engelmann spruce-subalpine fir forest David, Clive Addison


Engelmann spruce-subalpine fir [Picea engelmannii Parry ex Engelm.-Abies lasiocarpa (Hook.) Nutt.] (ESSF) forests occupy large portions of western North America, and of British Columbia (B.C.) in particular. These areas represent a harsh environment for plant growth. The ESSF forests of B.C. have presented serious problems of regeneration following harvesting; several factors stimulated speculation as to whether N supply limitations were involved. This study was intended to highlight the post-harvest N dynamics of an ESSF forest, and the implications of the latter for silvicultural practices. Its general objectives included characterization of the post-harvest assart effect, and investigation of the N status and growth of advance regeneration. These were achieved by means of a comparative study of an age sequence of harvested sites. The assart effect lasted for at least eight years after harvesting, with a peak of change between years three and six. There were no major physical changes in the forest floor. Low C/N ratios between 19 and 32 were believed to have contributed to increased N availability. ESSF forests may have a generally higher level of N availability than previously supposed. The advance regeneration benefited from the assart effect. Nutrient uptake appeared to increase generally from at least three years after harvesting; increases of up to 78% were noted for N. There appeared to be no general macronutrient or micronutrient limitation to growth. However, evidence of S deficiencies was encountered in some trees. Moreover, the critical levels used for N may be in need of revision. A revised critical level of 1.40% for foliar N concentrations is proposed for subalpine fir advance regeneration. If this is accurate, regeneration may have been at least temporarily N-limited from year eight after harvesting. A more rigorous investigation of these possibilities is needed. The cutting method applied to the sites approximated a one-cut shelterwood method. The method as encountered in this study should not be considered a viable silvicultural option for similar ESSF forests. Its successful application would involve some degree of forest floor manipulation to improve seedbed conditions and soil microclimatic regimes. The findings of this study demonstrate that the environmental and biological characteristics of ESSF forests make high levels of planning and care a prerequisite for the success of silvicultural practices. The question of what comprises realistic growth and yield expectations of second-rotation stands in the ESSF zone needs to be addressed urgently.

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