Differential axis architecture in Thuja occidentalis (eastern white cedar)

Quantitative architectural variation was investigated among the axes of Thuja occidentalis. Forty axes of each axis order were randomly sampled and for each axis: length, excurrency index, basal diameter, number of daughter axes, number of basal reiterated axes, dorsiventrality index, and percentage of length partitioned into photosynthetic (bearing green leaves) and nonphotosynthetic (nongreen) components was determined. The crown exhibited six axis orders. Both axis length and basal diameter declined with increasing axis order, and crown form was excurrent. The number of daughter axes was highest on axis order 1 (trunk) and lowest on axis order 5. Axis orders 2 through 4 had a similar number of daughter axes. The frame of the tree was composed of axis orders 1 and 2. These axes were radially symmetrical in midpoint cross section, exhibited indeterminate growth, and only a small percentage of their length was devoted to photosynthetic tissue. Axis orders 3 through 6, taken together, had many characteristics of compound leaves. They were dorsiventrally flattened, exhibited determinate growth (axis terminated by a cone or abscised), and the majority of their length was devoted to photosynthetic tissue. In autumn these axes were often abscised as a unit. Both male and female cones were located terminally on axis orders through 6, with 82.7% of male cones occurring on axis order 6, and 86.0% of female cones occurring on axis order 5. The architectural unit of T. occidentalis resembled that of Thuja plicata. Attims' model most closely resembles the architectural pattern of these two species.