Convergence and differentiation of tree radial growth in the Northern Hemisphere

Tree radial growth records both ontogenetic growth trends and environmental information. Although the former is usually excluded from climate-growth analyses, its gradual changes have a more profound effect on forest carbon sequestration. Elucidating the kinetic mechanism behind this intrinsic trend will greatly improve our understanding and prediction of climate-growth relationships. The iterative growth model (IGM) and its extensions link organism growth, lifespan, and respiration, providing important insights into this trend. Here, we extended the IGM to the tree-ring scale (IGMR) and used tree-ring width datasets across the Northern Hemisphere to analyse the constraints of tree ontogenetic growth trends on radial growth rate (along the radius gradient). On this basis, we further elucidated the convergence and differentiation of these growth trends across different climate types and clades. The results showed that: (i) ontogenetic growth trends of trees cause the radial growth rate to follow a typical unimodal pattern along the radius gradient. (ii) This unimodal pattern is a function of tree radial size, metabolic exponent, and maintenance metabolic rate, constraining the response of tree radial growth to climate. (iii) Ontogenetic growth trends result in an inherent trade-off between tree radial growth rate and lifespan. In conclusion, different growth drivers do not alter the constraining effect of tree size on radial growth. Ontogenetic growth trends should be considered in future studies of climate-growth relationships.