Plant allometry is key for determining the role of forests in global carbon cycles, through the calculation of tree biomass using proxy measurements such as tree diameters or heights. Metabolic ecology theory (MET) considers the general principles that underpin allometry, but MET scaling relationships have been challenged on their lack of fit to empirical data and global applicability. We postulated that MET scaling is applicable only for plant tissues combining conductive and supportive functionality (tracheids), but as plants evolved tissues of specialized conductive functionality (vessels) their allometry progressed into more complex relationships. According to this principle, we deducted generalized MET (gMET) relationships with mechanistically deducted coefficients. Our gMET models proved to have exceptional empirical support against global datasets, achieving unbiased predictions across biomes worldwide. These results prove gMET models to be a crucial improvement to MET-based allometry, providing a universally applicable theoretical framework for worldwide estimations of forest carbon.