Forests represent the majority of terrestrial primary productivity, and therefore play an important role in global carbon budgets, however not much is known about internal carbon dioxide (CO2) efflux, from the tree roots through the xylem to the foliage. Researchers in Georgia (US) studied an experimental Populus deltoides plantation to determine how much CO2 is transported from the roots, into the xylem and up into the leaves, compared to the efflux from the soil into the atmosphere.
They observed a diurnal pattern in dissolved CO2 concentrations in the xylem, which were highest before transpiration occurred, and where CO2 transport from the soil into the xylem exceeded soil efflux (into the atmosphere) during daylight. This transport route is hypothesized by the authors to be a built in, carbon-recycling mechanism to compensate for respiratory losses.
Their results point to a potential underestimation in below ground carbon allocation, and highlights a need for future studies on the processes controlling carbon allocation in a forest ecosystem.
Aubrey and Teskey (2009), “Root-derived CO2 efflux via xylem stream rivals soil CO2 efflux”