SIMULATION OF BRANCH GROWTH IN THE PINACEAE - INTERACTIONS OF MORPHOLOGY, PHENOLOGY, FOLIAGE PRODUCTIVITY, AND THE REQUIREMENT FOR STRUCTURAL SUPPORT, ON THE EXPORT OF CARBON

Five interacting processes that determine the life cycle and productivity of a branch are simulated. Morphological development describes branch segment (branchlet) replication. Foliage development is controlled by production rate and longevity on each branchlet. Production of photosynthate, calculated as net biomass increment, may vary with branchlet age. Phenology defines the yearly brnachlet growth period and so influences foliage amount produced. Branchwood thickening is incremented to maintain a specified deflection profile. The export of potential biomass increment from the branch is estimated. These five processes together define branch growth as a balance between positive and negative feedbacks. In the early years of branch growth the more rapid the foliage development, the more available biomass increment there is to produce more foliage. However, as the branch elongates and foliage is held further from the tree, greater foliage amounts comes to require greater amounts of branchwood for support. The potential biomass increment exported from the branch to the trunk first increases, as foliage increases, and then decreases as the demand for supporting branchwood increases. Simulations show how the set point between the two feedback systems can be specified by phenology and morphology. For export an optimum length of time can be defined for branchlet growth when there is a balance between the amount of production invested in new foliage and the additional export to the trunk obtained from it. The necessity to simulate branchlet production as non-stationary over the branch lifetime is demonstrated. Two morphological systems for this are investigated; APICAL which specifies branchlet proliferation only according to length and order of parent branchlets and SEARCH which simulates plasticity in development through the inhibition of branchlet production according to local crowding. For the SEARCH algorithm there is an interaction with branchlet phenology that has a substantial influence on both the timing and amount of export to the trunk. © 1990 Academic Press Limited.