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Across plant species and biomes, a conserved set of leaf traits govern the economic strategy 11 used to assimilate and invest carbon. As plants age, they face new challenges that may 12 require shifts in this leaf economic strategy. In this study, we investigate the role of the 13 developmental transition, vegetative phase change (VPC), in altering carbon economics as 14 plants age. We used overexpression of miR156, the master regulator of VPC, to modulate the 15 timing of VPC in Populus tremula x alba, Arabidopsis thaliana and Zea mays to understand 16 the impact of this transition on leaf economic traits, including construction cost, payback 17 time, and return on investment. Here we find that VPC regulates the shift from a low-cost, 18 quick return juvenile strategy to a high-cost, high-return adult strategy. The juvenile strategy 19 is advantageous in light-limited conditions, whereas the adult strategy provides greater 20 returns in high-light. The transition between these strategies is correlated with the 21 developmental decline in the level of miR156, suggesting that is regulated by the 22 miR156/SPL pathway. Our results provide an eco-physiological explanation for the existence 23 of juvenile and adult leaf types, and suggest that natural selection for these alternative 24 economic strategies could be an important factor in plant evolution. 25 26