Ecological implications of the determination of branch hierarchies.

•  The performance of the whole plant is largely dependent on its ability to allocate limited resources to branches that perform best throughout its life. Here, the hypothesis that the fate of young branches is determined by their growth rates and not merely by their relative physical sizes or net photosynthetic outputs was tested. •  The development of asymmetrical two-branch plants was followed after either one or both of the branches were restrained for short periods. •  The larger branch was invariably dominant in unrestrained or bilaterally restrained plants. However, when the larger branch was restrained while the smaller branch was not, the branch hierarchy inverted despite the pronounced photosynthetic advantage of the larger branch over its smaller counterpart. •  It is suggested that growth rates are more important than physical size or photosynthetic output in young plants, where they could serve as better predictors of the overall future performance of the branch. It is speculated that rate-sensitivity has been selected for when plastic responses cannot adequately track environmental changes in real time.