Belowground carbon use in woody plants under experimental warming: uneven evidence for fine-root, root-exudation, and mycorrhizal responses.

How warming changes belowground carbon use in woody plants remains poorly understood. Most experiments measure fine-root, root-exudation, and mycorrhizal responses separately, providing limited evidence on how these responses vary together within the same experiment. We synthesized 13 primary publications from 11 independent experiments: eight field, one outdoor pot, and two controlled experiments. Our synthesis showed no significant increase in fine-root biomass or production. Root respiration, mortality, and N concentration increased, whereas root P and non-structural carbohydrate concentrations decreased. Root-exudation measurements were reported differently across studies, limiting direct comparisons. Root-exudation responses also varied with nutrient treatment, season, and warming duration, while exudate quantity and composition did not always change together. Most mycorrhizal studies measured colonization or infection rather than direct plant-fungal carbon and nutrient exchange. Among the mycorrhizal studies included here, an isotope-based C-for-N ratio proxy varied with N source and sampling time. When multiple belowground responses were measured within the same experiment, concurrent increases in fine-root activity, root exudation, and mycorrhizal association occurred only under particular nutrient and temporal conditions and were not a general response to warming. Opposing responses did not demonstrate compensation; no study showed that an increase in one pathway offset a decrease in another in terms of nutrient acquisition or plant performance. Testing whether warming reallocates carbon among fine roots, root exudation, and mycorrhizal fungi will require coordinated measurements of carbon supply, comparable belowground carbon fluxes, plant nutrient acquisition, and competing carbon sinks within the same experiment.