Who cares about tree carbon allocation?

Most biological C resides in trees. Humans have been increasing atmospheric levels of CO2, exerting feedbacks on both humans and trees. Now humanity seeks for ways to increase C sequestration, and hence understanding tree C allocation is pivotal to all life on Earth. In 2015 we presented the first full description of tree C allocation dynamics that accounts for all C fluxes and pools in a tree. In the years that passed, we have been applying the mass balance and isotopic 13C labeling approaches to study tree C allocation in the field and greenhouse, at levels spanning from ecological to molecular. We have been advancing knowledge on tree C allocation to (I) more tree species, representing more functional groups. (II) more growing conditions, including heat, drought, and elevated CO2. (III) research focused on belowground allocation: root growth, exudation, transfer to mycorrhizal fungi and beyond. (IV) research focused on molecular mechanisms of C storage and carbohydrate management in specific tree tissues.

Among the many recent findings, we showed that (1) about half of all assimilated C is respired back to the atmosphere, but variations between tree species are large. (2) in the Mediterranean, conifers allocate more C belowground than evergreen broadleaf species. (3) under drought, while C sequestration (source) decreases significantly, C sinks remain the same, but partition less to respiration and more belowground. (4) under drought and heat, C sinks rely on decomposition of C reserves such as starch. (5) under elevated CO2, C sequestration increases in proportion to CO2 level, but not tree growth. C allocation patterns change in a species-specific manner. (6) CO2 responses continue up until other minerals run out. (7) a lot of uncertainty revolves around belowground C allocation. Root growth dynamics measured in the field are distinctive from stem or leaf growth. (8) root exudation accounts for 10% of all assimilated C but is decoupled from assimilation dynamics. (9) C transfer to mycorrhizal fungi is more rapid and diverse than previously thought. (10) part of the mycorrhizal C finds its way to neighboring trees through the mycorrhizal network. (11) at the molecular level, daily starch metabolism gene expression is different from the stress-mode starch metabolism pattern, operating unique beta amylases and starch synthases. (12) many other gene families are involved in C allocation, e.g., vacuole hexose transports, which regulate glucose levels in leaf and stem cells. (13) overexpressing one of these transporter genes produced fast-growing mutant poplar trees.

Taken together, these findings are crucial to all life on Earth, and particularly to us humans, at any level: scientists; stakeholders; and the wide public. Who cares about tree carbon allocation? Everybody!