Effects of Environmental Change on Wood Production and Wood Structure in Picea sitchensis Seedlings

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Abstract
Changes in environment that would be expected to reduce substrate availability, decrease the rate of xylem increment, tracheid diameter and wall thickness, in seedlings of Picea sitchensis. But after reaching a minimum about 10 days to 3 weeks after environmental change, xylem increment, tracheid wall thickness, and in some instances tracheid diameter, increase again up to about 4 to 6 weeks after transfer. This recovery parallels a recovery in net assimilation rate, and is associated with an increase in photosynthetic efficiency. Although the light intensity and temperature treatments imposed were more drastic than those that would normally be expected in the field, they had remarkably little net effect on tracheid dimensions. On the other hand change in photoperiod produced greater changes in tracheid dimensions, photoperiodic effects being superimposed on effects of change in substrate availability. It is suggested that adaptation to reduced substrate will buffer the plant against climatic fluctuations during the growing season, while the photoperiodic response will ensure preparation for winter dormancy. It is concluded that the major seasonal changes in tracheid dimensions are unlikely to be caused by variation in substrate availability. Changes in light intensity and temperature have minor effects on wood production and structure through effects on substrate availability, but the major seasonal trends are more likely to be associated with changes in growth regulation production.