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.