Index of Vigour in Norway Spruce (Picea abies Karst.)

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Abstract
The relative sapwood increment at breast height, which indirectly gives the stemwood increment per unit of leaf mass or leaf area, was used as an index of vigour in Norway spruce (Picea abies Karst.). Four methods to delineate sapwood and heartwood sections were tried on increment cores from twenty tree species. A method based on differential translucence proved the best in most cases and was used in thirteen stands of Norway spruce. The index showed a curvilinear decrease with increasing tree age, attaining almost constant values as tree age exceeded 35 years. An age correction is needed when indices of stands at rather different ages are compared, and a comparison between stands and/or indices from different years can be made by sampling a minimum of about thirty cores from fifteen trees per stand. The within-stand variation was found to be smaller in the vigour indices than in annual ring measurements per se. The vigour index can be used to express the state of resistance in single trees or stands to insect attacks and disease. If the index is used to indicate the effect of environmental factors, the stands involved should have similar leaf-area indices and soil conditions. The present index of vigour may be a valuable tool in ecological studies of competition within woody vegetation, the effect of environmental factors (including air pollution) on trees, and interrelations between trees and herbivores.

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