Crown geometry of plantation-grown American sycamore and its simulation

Branching variables were measured on plantation-grown American sycamore (Platanus occidentalis L.) as part of a study to describe and model branch-level crown geometry. The analyses were assumed to define a complete description of sycamore crown geometry.

Results show that sycamore produces two types of primary proleptic branches: those that develop either the current year (Type I) or the subsequent year (Type II). Several subsequent analyses treated these branch types separately. Type I branching potential varied with tree and through time, and the number formed was associated with extension growth. Type I branches form on the lower trunk annual growth increment (AGI), form at a slightly more acute angle than Type II branches, and, after the first year, do not differ in extension growth from Type II branches.

Environmental and genetic factors affected extension growth. Leader growth was predicted from either calculated soil moisture values or one of three drought indices, and from class variables which were assumed to represent fertilization/cultivation and tree genetics. Variables related to crown geometry and crown growth history were used to analyze relative branch growth (branch AGI/leader AGI). Node density was examined. Node position analysis revealed three phases of growth: increasing, almost constant, and declining internode distance. One or more phases were absent on many branches. Phyllotaxis was studied. For leaves or primary branches, the mean angle of divergence was 145.7 degrees. The direction of the spiral reversed on 27.5% of the tree AGIS; a phenomenon independent of tree and time. Second and higher-order branches formed at mean angles of 90 and 270 degrees. On these AGIS, the circular variance increased acropetally to 13 nodes, or 90 cm, at which point the phyllotaxis appeared spiral. The number of daughter branches formed, their angle of divergence from the mother section, and their subsequent inclinations were predicted from variables related to branch geometry, crown position, and tree genetics.

Results of the branching analysis were used to develop a FORTRAN computer model which simulates the three-dimensional development of sycamore crowns. The model was validated by comparing output for leader growth and branching characteristics with field observations. Future model uses are discussed.