A study of the effects of nutrients and water supply (2 × 2 factorial experiment) was conducted in a 12‐yr‐old stand of loblolly pine (Pinus taeda L.) during a period in which soil moisture was not augmented by irrigation because of frequent rain events. Information on the responses of sapwood‐to‐leaf area ratio and early‐to‐late wood ratio, to four years of treatments led to the hypothesis that the combination of increased nutrient and water supply (IF treatment) will increase tree transpiration rate per unit leaf area (EC,1) above EC,1 in the control (C), as well as increasing EC,1 above that when either the supply of water (I) or of nutrients (F) is increased. We further hypothesized that canopy transpiration (EC) will rank IF > F > I = C, based on the ranking of leaf area index (L) and assuming that the ranking of EC,1 is as first hypothesized. We rejected our first hypothesis, because F had lower EC,1 than the other treatments, rather than IF having higher values. We could not reject the second hypothesis; the ranking of average daily EC was 1.8 mm for IF, 1.2 mm for F, and 0.7 mm for both C and I (se < 0.1 mm for all treatments). Thus, it was the lower EC,1 of the F treatment, relative to IF, that resulted in ranking of EC similar to that hypothesized. Lower EC,1 in F trees was found to relate to lower canopy stomatal conductance, even though soil moisture conditions during the time of the study were similar in all treatments. Only trees in the F treatment absorbed a substantial amount of water (25%) below 1 m in the soil. These results indicate a “carry‐over” effect of irrigation when combined with fertilization that increases EC in irrigated trees, relative to unirrigated trees, even under conditions when soil moisture is high and similar in all treatments.

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-7m canopy-root axis) at high-light, but showed no consistent pattern at low-light. At mediumlight 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.

Despite the important role of tropical forest ecosystems in the uptake and storage of atmospheric carbon dioxide (CO2), the carbon (C) dynamics of tropical tree species remains poorly understood, especially regarding belowground roots. This study assessed the allocation of newly assimilated C in the fast-growing pioneer tropical tree species Ceiba pentandra (L.), with a special focus on different root categories. During a 5-day pulse-labelling experiment, 9-month-old (~3.5-m-tall) saplings were labelled with 13CO2 in a large-scale aeroponic facility, which allowed tracing the label in bulk biomass and in non-structural carbohydrates (sugars and starch) as well as respiratory CO2 from the canopy to the root system, including both woody and non-woody roots. A combined logistic and exponential model was used to evaluate 13C mean transfer time and mean residence time (MRT) to the root systems. We found 13C in the root phloem as early as 2 h after the labelling, indicating a mean C transfer velocity of 2.4 ± 0.1 m h−1. Five days after pulse labelling, 27% of the tracers taken up by the trees were found in the leaves and 13% were recovered in the woody tissue of the trunk, 6% in the bark and 2% in the root systems, while 52% were lost, most likely by respiration and exudation. Larger amounts of 13C were found in root sugars than in starch, the former also demonstrating shorter MRT than starch. Of all investigated root categories, non-woody white roots (NRW) showed the largest 13C enrichment and peaked in the deepest NRW (2–3.5 m) as early as 24 ± 2 h after labelling. In contrast to coarse woody brown roots, the sink strength of NRW increased with root depth. The findings of this study improve the understanding of C allocation in young tropical trees and provide unique insights into the changing contributions of woody and non-woody roots to C sink strengths with depth.

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