Carbon allocation dynamics in conifers and broadleaved tree species revealed by pulse labeling and mass balance

Fixed carbon (C) is a central resource, whose dynamic allocation by the tree determines its ability to establish, grow and compete with other species. Traditionally, tree C allocation has been estimated by isotopic or mass balance approaches. Here we developed a setup of 13CO2 pulse-labeling, with a flux-coupled, three-phase detection, allowing us to follow C quantitatively. We examined C allocation patterns in 2-year-old potted saplings of five evergreen forest tree species that co-occur in a typical mixed forest in the Mediterranean region: two early succession conifers (Pinus, Cupressus) and three late succession broadleaf species (Quercus, Ceratonia, Pistacia). Across the five species, C moved from leaves to stem and fine roots following an exponential decay, with parameters that are consistent with the ecological role of each species. Eight days post-labeling, conifers allocated ~30% of the C belowground, and broadleaves <10%. In contrast, leaf C residence time was ~4 days in conifers and only ~1 day in broadleaves. Root exudation was quantified and shown to be small. We integrated our results into a compartmental model of 13C distribution in the tree, enhancing our understanding of divergent C allocation strategies among tree species within the same ecosystem.