title: A mathematical framework for modelling cambial surface evolution using a level set method.
abstract: OBJECTIVE
  During their lifetime, tree stems take a series of successive nested shapes. Individual tree growth models traditionally focus on apical growth and architecture. However, cambial growth, which is distributed over a surface layer wrapping the whole organism, equally contributes to plant form and function. This study aims at providing a framework to simulate how organism shape evolves as a result of a secondary growth process that occurs at the cellular scale.
  
  
  METHODS
  The development of the vascular cambium is modelled as an expanding surface using the level set method. The surface consists of multiple compartments following distinct expansion rules. Growth behaviour can be formulated as a mathematical function of surface state variables and independent variables to describe biological processes.
  
  
  RESULTS
  The model was coupled to an architectural model and to a forest stand model to simulate cambium dynamics and wood formation at the scale of the organism. The model is able to simulate competition between cambia, surface irregularities and local features. Predicting the shapes associated with arbitrarily complex growth functions does not add complexity to the numerical method itself.
  
  
  CONCLUSIONS
  Despite their slenderness, it is sometimes useful to conceive of trees as expanding surfaces. The proposed mathematical framework provides a way to integrate through time and space the biological and physical mechanisms underlying cambium activity. It can be used either to test growth hypotheses or to generate detailed maps of wood internal structure.
authors.affiliation: New Zealand Forest Research Institute Ltd, Private Bag 3020, Rotorua 3046, New Zealand. damien.sellier@scionresearch.com
authors.forename: Damien
authors.initials: D
authors.lastname: Sellier
authors.forename: Michael J
authors.initials: MJ
authors.lastname: Plank
authors.forename: Jonathan J
authors.initials: JJ
authors.lastname: Harrington
mesh_terms: D000465:Algorithms; D058506:Cambium / Q000254:growth & development* / Q000502:physiology; D003198:Computer Simulation; D005052:Eucalyptus / Q000254:growth & development / Q000502:physiology; D008954:Models, Biological; D018547:Plant Stems / Q000254:growth & development / Q000502:physiology; D013997:Time Factors; D014934:Wood / Q000254:growth & development / Q000502:physiology; D052584:Xylem / Q000254:growth & development / Q000502:physiology