Changes in the Growth Pattern of White Pine Trees Undergoing Suppression

The phenotypic responses of suppressed trees were studied in a 60—year—old white pine stand. The first evidence of suppression is a tree—wide decrease in the production of secondary xylem. Height growth is relatively insensitive. During the latter 30 years of life suppressed trees made 50.7% of their total height growth but only 19.1% of their total diameter growth. Eventually the cambium fails to cut off xylem at the base of the tree. With time, the sheath of new—formed xylem retreats from the based giving rise to an increasingly large zone of missing rings. Girdling experiments indicate that diminished supplies of food and growth regulators move through the zone of missing rings to support primary root growth. Apparently the phloem remains functional in the zone of missing rings. Either the phloem can function for periods greater than 1 or 2 years, or the cambium cuts off phloem alone in the missing ring zone. In the normal cambial cycle, phloem alone is produced during the last part of the growing seas...