ALLOMETRIC ANALYSIS OF SITKA SPRUCE BRANCHES - MECHANICAL VERSUS HYDRAULIC DESIGN PRINCIPLES

The geometry of tree branches can have considerable effect on their ef ficiency in terms of carbon export per unit carbon investment in struc ture. The purpose of this study was to evaluate different design crite ria using data describing the form of Picea sitchensis branches. Allom etric analysis of the data suggests that resources are distributed to favour shoots with the greatest opportunity for extension into new spa ce, with priority to the extension of the leader. The distribution of allometric relations of links (branch elements) was tested against two models: the pipe model, based on hydraulic transport requirements, an d a static load model based on the requirement of shoots to provide me chanical resistance to static loads. Static load resistance required t he load parameter to be proportional to the link radius raised to the power of 4. This was shown to be true within a 95% statistical confide nce limit. The pipe model would require total distal length to be prop ortional to link radius squared but the measured branches did not conf orm well to this model. The comparison suggests that the diameters of branch elements were more related to the requirements for mechanical l oad. The cost of following a hydraulic design principle (the pipe mode l) in terms of mechanical efficiency was estimated and suggested that the pipe model branch would not be mechanically compromised but would use structural resources inefficiently. Resource allocation among bran ch elements was found to be consistent with mechanical stability crite ria but also indicated the possibility of allocation based on other cr iteria, such as potential light interception by shoots. The evidence s uggests that whilst branch topology increments by reiteration of units of morphogenesis, the geometry follows a functional design pattern.