Patterns and dynamics of canopy–root coupling in tropical tree saplings vary with light intensity but not with root depth

Carbon (C) dynamics in canopy and roots influence whole-tree carbon fluxes, but little is known about canopy regulation of tree-root activity. Here, the patterns and dynamics of canopy–root C coupling are assessed in tropical trees. Large aeroponics facility was used to study the root systems of Ceiba pentandra and Khaya anthotheca saplings directly at different light intensities. In Ceiba, root respiration (R_r) co-varied with photosynthesis (A_n) in large saplings (3-to-7-m canopy–root axis) at high-light, but showed no consistent pattern at low-light. At medium-light and in small saplings (c. 1-m axis), R_r tended to decrease transiently towards midday. Proximal roots had higher R_r and nonstructural carbohydrate concentrations than distal roots, but canopy–root coupling was unaffected by root location. In medium-sized Khaya, no R_r pattern was observed, and in both species, R_r was unrelated to temperature. The early-afternoon increase in R_r suggests that canopy–root coupling is based on mass flow of newly fixed C in the phloem, whereas the early-morning rise in R_r with A_n indicates an additional coupling signal that travels faster than the phloem sap. In large saplings and potentially also in higher trees, light and possibly additional environmental factors control the diurnal patterns of canopy–root coupling, irrespective of root location.