Carbon storage pools of trees: fixed or flexible?

Most of the carbon (C) reserves in trees are stored in the living parenchyma of stems and roots and exhibit characteristic variations with phenology, growth and environmental stress. Up to date, it is only partially understood how the formation and re-mobilization of stored C in sapwood is regulated and synchronized over tissues and long distances. Mechanistic concepts of C storage therefore often assume simple bucket-models, where the amount of stored C equals the net-balance between C assimilation and the sum of all C sink activities (e.g., respiration and growth).Here, we summarize results from previous experimental and observational studies in our group, that tested the reaction of non-structural carbohydrate reserve pools in trees to situations of limited photosynthetic C supply. Overall, these studies suggested that C reserve concentrations in sapwood follow abrupt changes of the net C-source-sink balance in the short-term and at severe C starvation, but they consistently reach homeostatic levels that are very similar across different C-source-sink conditions over longer time periods. According to our findings, we propose that C reserve pool sizes in tree are determined and closely controlled rather than the simple net-result of C source vs. -sink activities. With respect to mechanistic C models of trees, this suggests that C reserves should be rather treated as a fixed C-sink than a variable parameter.