Origin and fate of carbon and nitrogen reserves in trees

This scientific commentary refers to ‘Tracing carbon and nitrogen reserve remobilization during spring leaf flush and growth following defoliation’ by Hart et al. (https://doi.org/10.1093/treephys/tpae015). Non-structural carbohydrates (NSC) stored across the organs of trees fluctuate over the seasons to compensate for the temporal variations between carbon (C) uptake and C loss via respiration or growth (Martínez-Vilalta et al. 2016). Similarly, storage and remobilization of nitrogen (N) are also required because seasonal growth and N uptake can be temporally uncoupled (Millard and Grelet 2010). However, tree growth is rarely limited by C but more commonly by water deficit, low temperature and nutrient availability (Körner 2013). In turn, the uptake of N by the roots is influenced by and provides feedback into the C allocation processes (Joseph et al. 2021, Schönbeck et al. 2021). Therefore, analysing the dynamics of NSC together with those of nutrients, such as N, can help disentangle confounding limitations on tree growth or the factors underlying co-variation among NSC concentrations, C assimilation and growth (Thompson et al. 2023). Hart et al. (2024) investigated the remobilization of C and N reserves during spring leaf flush and after an experimental defoliation event in aspen (Populus tremuloides Michx.) saplings. The researchers elegantly tracked the fate of C and N originating in both the roots and stems using a novel combination of experimental grafting and isotopic labelling. Hart and colleagues (2024) addressed fundamental questions about the dynamics of C and N in trees such as (i) determining which organs serve as sources and which as sinks, (ii) quantifying the amount of matter remobilized to sinks and the amount stored in sources and (iii) understanding the form in which C is transported and used, or retained in storage organs.