Do Developing Plant Branches Conform to a Single Allometric Growth Rule?

ABSTRACT The use of physical models to predict patterns of organismal growth has long interested biologists. Models based on hydraulic or biomechanical principles have been invoked to predict the scaling of tree branching networks, but most assume self‐similarity throughout the network and don't consider more than a single physical constraint. A more recent model (flow similarity) predicts that the allometry of terminal tree branch dimensions will differ from the bole and basal branches, centering around values derived from hydraulic considerations. Here we evaluate the predictions of the flow similarity model for the growth of terminal stems of woody plants from 52 species sampled across the seed plant phylogeny and spanning over four orders of magnitude in stem volume. We find that the slopes describing terminal stem allometry across seed plants display limited variation, despite nearly 400 million years of divergent evolution across the examined lineages. Phylogenetic analysis reveals stable trait means and a strong pull towards specific values, potentially reflecting stabilising selection. Interestingly, these mean values are largely consistent with the flow similarity model predictions. Our results suggest that the ontogeny of terminal stems in seed plants follow similar allometric scaling principles which operate across evolutionary and ecological scales.