Canonical modelling: an approach for intermediate-level simulation of carbon allocation in functional-structural plant models

Functional-structural models that include detailed mechanistic representation of underlying physiological processes can be difficult and expensive to build, and the resulting models are often very complicated. This is particularly true when representing carbon allocation, as the various processes involved are relatively poorly understood. Purely empirical models, on the other hand, are simpler and easier to construct, but are of limited use in simulating, predicting and explaining the way that plants adapt and respond to varying environmental conditions. In this chapter, we discuss an intermediate approach to modelling plant function that can simulate plant responses, including changes in carbon allocation patterns, without requiring a detailed knowledge of the underlying physiology. In this approach, plant function is modelled using a ‘canonical’ modelling approach, where processes such as carbon allocation are represented by a number of fluxes between compartments, and these fluxes are in turn represented using flux functions of a standard mathematical form. The values of the parameters of these flux functions are then determined by fitting the global output of the model to global data, rather than attempting to make the functions represent underlying processes in a quantitatively accurate way. Here we demonstrate the canonical modelling using an example involving the cotton plant, where two alternative hypotheses explaining observed compensation after defoliation are represented. We discuss some potential advantages of this canonical approach over both more mechanistic and more empirical approaches to representing carbon allocation, and conclude that canonical modelling offers a useful, flexible and relatively simple way of simulating plant function at an intermediate level of abstraction