Root functional change achieves water source separation under vegetation succession

Abstract Water source separation can be one of strategies for different plant species to coexist in a community. This study first demonstrates mechanisms of water source separation during vegetation succession. An isotope‐incorporated mechanistic model was employed to simulate the xylem water isotopic composition and the root water uptake profile for a Pinus densiflora – Quercus myrsinaefolia mixed stand undergoing secondary succession and a pure P .  densiflora stand before succession. In the mixed stand, the model successfully simulated the xylem water isotopic composition on the assumption that the root surface resistance per unit length of root ( r rs * ) decreases with increasing depth for P .  densiflora but is constant for Q .  myrsinaefolia . Uptake fraction for P .  densiflora was greater in deeper zones but in shallower for Q .  myrsinaefolia . In contrast, in the pure stand, a constant r rs * for P .  densiflora gave good reproducibility in isotope simulation and shallow water uptake dominated. These findings highlight root functional change as a mechanism of water source separation; pre‐existing P .  densiflora trees decrease their deep‐root r rs * to compensate for an increase in shallow‐root r rs * . This mechanism was caused by competition against invading Q .  myrsinaefolia trees.