Effects of Light Intensity on Tracheid Dimensions in Picea sitchensis

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Abstract
In seedlings of Picea sitchensis grown in constant conditions, or within older trees in the field, light intensity had no net effect on the wall thickness of tracheids produced at the same point in time. This appears to be due to a balanced regulatory system, effects of light intensity on rate of accumulation of wall volume per leaf being offset by differences in rate of xylem increment, and differences in wall material per tracheid being nullified in their effects on wall thickness by effects on tracheid diameter. Mean tracheid wall thickness across the growth ring increased with light intensity, due to increase in proportion of late-wood associated with the longer duration of cambial activity at higher light intensity, duration of wall thickening increasing during the season. Duration of wall thickening did not vary with light intensity. The rate of increase in wall volume was limited by light intensity (and hence possibly by substrate availability) at all light intensities in the field, but in seedlings in controlled conditions the rate of wall production was no greater at 20 000 lx than at 6700 lx.

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