ARCHITECTURE AND CROWN GEOMETRY IN TABEBUIA ROSEA (BIGNONIACEAE)

In the tropical tree Tabebuia rosea, seedlings form an erect unbranched stem with rhythmic growth. Three distinct and predictable geometrical stages are then recognized in crown development. Sympodial branching begins with the arrest of the terminal bud of the trunk and symmetric outgrowth of a pair of subtending lateral buds. During an intermediate phase, branching becomes asymmetric at about Order 5. At each sympodial bifurcation there is differentiation between vigorous, relatively straight main branches (leaders) and less vigorous laterals forming regular pseudomonopodial branch complexes, which collectively constitute the cup-shaped crown. Finally, dormant lateral buds in the lowest bifurcation of the trunk are released and reiterate the original crown form. Ultimately an erect, apparently monopodial tree is formed by a set of superimposed cup-shaped crowns. Crown development of Tabebuia is unique because it involves predictable ontogenic changes in branching patterns. Crowns of open-grown Tabebuia consist of relatively few, wide branch tiers, crowns of forest grown trees are tall and narrow. Analysis of the adaptive geometry of wide vs. narrow crowns through computer simulation illustrates the precise cost of mechanical support for terminal leaf rosettes at successive developmental phases and suggests that tall, erect, narrow, and multi-tiered crowns are more efficient than wide open crowns.