Seasonal variations drive short‐term dynamics and partitioning of recently assimilated carbon in the foliage of adult beech and pine

(13) CO2 pulse-labelling experiments were performed in situ on adult beeches (Fagus sylvatica) and pines (Pinus pinaster) at different phenological stages to study seasonal and interspecific short-term dynamics and partitioning of recently assimilated carbon (C) in leaves. Polar fraction (PF, including soluble sugars, amino acids and organic acids) and starch were purified from foliage sampled during a 10-d chase period. C contents, isotopic compositions and (13) C dynamics parameters were determined in bulk foliage, PF and starch. Decrease in (13) C amount in bulk foliage followed a two-pool exponential model highlighting (13) C partitioning between 'mobile' and 'stable' pools, the relative proportion of the latter being maximal in beech leaves in May. Early in the growing season, new foliage acted as a strong C sink in both species, but although young leaves and needles were already photosynthesizing, the latter were still supplied with previous-year needle photosynthates 2 months after budburst. Mean (13) C residence times (MRT) were minimal in summer, indicating fast photosynthate export to supply perennial organ growth in both species. In late summer, MRT differed between senescing beech leaves and overwintering pine needles. Seasonal variations of (13) C partitioning and dynamics in field-grown tree foliage are closely linked to phenological differences between deciduous and evergreen trees.