Using L−Systems for Modeling the Architecture and Physiology of Growing Trees: The L−PEACH Model

Carbohydrate partitioning represents a central problem of process−based models of tree growth because of the coupling between carbon partitioning, growth, and architecture. PEACH was an early, sink−driven, carbohydrate partitioning model for simulating reproductive and vegetative growth of fruit trees. Carbon partitioning in that model was based on the hypothesis that a tree grows as a collection of semi−autonomous but interacting sinks (organs), and that these organs compete for resources. Organs of the same type were clustered into composite compartments, such as roots, fruit, or stems. Carbon was allocated to compartments depending on their competitive ability with respect to other compartments, and relative proximity to carbon sources. Biomass growth was dependent on an experimentally derived growth potential for each organ type. This approach made it possible to avoid the empirical allocation coefficients, functional balance rules, and allometric relationships that were common to most other tree models at the time. However, as pointed out by Le Roux et al., the PEACH model almost entirely ignored the interaction between tree architecture and carbon allocation. In addition, each organ type was treated collectively as a single compartment, and thus all organs of the same type grew at the average rate for that organ. Because of these limitations, there was no potential to simulate differences in organ size or quality as a function of location in the canopy. It was also impossible to use this model structure to simulate the function of individual organs and capture the influence of their performance on patterns of carbon partitioning. Overcoming these limitations requires a more detailed model of carbon economy, in which growth and function of each organ is modeled individually within an architecturally explicit model of canopy growth.