INFLUENCE OF AGE AND GROWTH-RATE ON RADIAL ANATOMY OF ANNUAL RINGS OFTHUJA-OCCIDENTALIS L (EASTERN WHITE CEDAR)

Increment cores and stem disks were collected from Thuja occidentalis L. (eastern white cedar) and divided into young (<30 yr) and old (>300 yr) age classes. A further subdivision of the material based on growt h rate (annual rings <0.5 mm versus annual rings greater than or equal to 0.5 mm) was made. This allowed for analysis of the effect of tree aging independent of growth rate. Number of tracheids per annual ring, tracheid radial diameter, and ratio of latewood to earlywood did not vary with tree age. In contrast, the annual rings of both young and ol d slow-growing trees had fewer tracheids, decreased mean tracheid diam eter (both earlywood and, to a lesser extent, latewood) and an increas ed proportion of smaller latewood tracheids. In fast-growing trees reg ardless of age, tracheid radial diameter was larger in earlywood and r emained relatively constant (plateau phase) for most of the width of t he annual ring. Radial diameter then declined sharply, before the init iation of latewood. In slow-growing trees, initial earlywood tracheid radial diameter was smaller and the plateau phase short or absent, and the decline in tracheid radial diameter for both fast- and slow-growi ng trees was not coupled with latewood production. It is proposed that plasticity of tracheid radial diameter is a safety feature that reduc es the risk of embolisms for trees growing on xeric sites.