Theoretical study of variables affecting the export of assimilates from branches of Picea

A theoretical analysis was done to identify the features of conifer branch development, based on a model for Picea sitchensis (Bong.) Carr., that affect the ability to export assimilates, assuming (i) a branch's requirement for assimilates is determined by its need for mechanical support, and (ii) the production of assimilates is a simple function of total shoot length. There were three salient findings. First, there is an optimum and maximum shoot length for assimilate export by unbranched shoots. Second, the less a shoot extends itself each year, the more assimilate it may export, all else being equal. Third, there is a larger surplus of assimilates for export when a shoot develops laterals. A combination of lateral branching, deflection and decreased elongation can enable branches to continue growing and exporting assimilates for many years, as occurs at the bases of tree crowns.