Carbon isotope composition of respired CO₂ in woody stems and leafy shoots of three tree species along the growing season: physiological drivers for respiratory fractionation.

This article investigates the carbon isotope composition of respired CO₂ (δ¹³C_R) and bulk organic matter (δ¹³C_B) in different organs (leafy shoots and woody stems) of three tree species—deciduous Norway maple (Acer platanoides L.) and pedunculate oak (Quercus robur L.), and evergreen white cedar (Thuja occidentalis L.)—across spring and late summer. Using Cavity Ring-Down Laser Spectroscopy, the study found that maple and oak shoots exhibited ¹³C-enriched respired CO₂ relative to bulk organic matter during spring but not in late summer or stems, whereas cedar showed no significant variation between organs or seasons and had ¹³C-depleted respired CO₂. The results suggest organ- and season-specific differences in respiratory substrates and metabolic pathways, with deciduous species showing greater seasonal variability linked to starch concentration and growth activity, while evergreen cedar displayed more stable respiratory isotope signatures. These findings highlight complex interactions in post-photosynthetic carbon isotope fractionation influenced by species-specific physiology and phenology.