Thinning and nitrogen fertilization in a grand fir stand infested with western spruce budworm. IV. An ecosystem management perspective
1992
Waring, R.H. | Savage, T. | Cromack, K. Jr | ,
A stand of grand fir, Abies grandis (Dougl.) Lindl., was thinned and (or) fertilized with 350 kg/ha of nitrogen during an outbreak of the western spruce budworm, Choristoneura occidentalis Freeman. With thinning, the leaf area index was reduced by > 30%. Tree growth efficiency, measured as wood production per unit of leaf area, increased by 60% within 2 years. Thinning combined with fertilization improved growth efficiency by 140%. Fertilization without thinning brought a much delayed response in growth, awaiting some beneficial effects of defoliation. Fertilization provided tree roots in the upper soil horizons access to nitrogen during the spring, increasing the concentration of nitrogen in emerging foliage from 1 to 1.5% of dry weight and free amino acids levels 4-fold. Foliage enriched in amino acids increased shoot growth 3-fold, more than compensating for any nutritional benefits to insect larvae. From an ecosystem perspective, managers should recognize that allowing pine forests to be replaced with fir through fire protection and selective logging has increased the nitrogen demand beyond that readily supplied in the ponderosa pine/true fir type. Fertilizing with one application of nitrogen at the time of an insect outbreak may reduce mortality and associated fire hazard through a period of up to 5 years.
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