Changes in the Growth Pattern of White Pine Trees Undergoing Suppression

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Abstract
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 season. It is proposed that a physiological threshold exists below which growth conditions are inadequate for the production of secondary xylem but remain favorable for the production of primary root growth and possibly for secondary phloem. This physiological mechanism is of considerable ecological significance since, as suppression proceeds, the plant invests a higher and higher proportion of its decreasing energy supply in primary growth or in all tissues that require annual renewal thus prolonging its survival. Hence, the growth of white pine, while undergoing suppression, illustrates the principle of phenotypic adaptability.