3-D growth patterns of trees: Effects of carbon economy, meristem activity, and selection

A functional-structural plant growth model was used to explore how selection might influence the ontogenetic patterns in three-dimensional (3-D) growth of trees. The 3-D plant structure is defined by the orientation of metamers. The dynamics in the 3-D plant structure depend on the production of metamers and/or leaf pipes and the loss of such plant components. In the simulations, metamer and leaf-pipe traits were kept constant, so all ontogenetic changes depended on the spatial arrangement of metamers and/or leaf pipes. This study explores the consequences of three new assumptions for ontogenetic changes in 3-D plant structure: (1) meristems are produced at the positions where branches fall, thus enabling a tree to maintain a viable meristem population within the crown; (2) metamers are placed at meristem positions in the 3-D structure where the carbon benefit over the expected life span of a leaf pipe is maximized; (3) the carbon allocation to reproduction maximizes the long-term reproductive output. In combination with the constraints set by the morphology of metamer and leaf pipe, the carbon economy, and light conditions, these assumptions explain how selection may cause a sigmoid expansion phase and a stable steady-state phase; adaptive responses in 3-D structure during ontogeny; limits to tree size (including height); constant allometric scaling during the expansion phase; different scaling for trees in different light environments; and responses in optimal reproductive allocation to forest light environments. These results support the idea that selection for maximizing the net carbon gain determines how trees change in 3-D tree structure during ontogeny and, at the same time, how they acclimate in 3-D structure in response to light gradients.