Tree allocation dynamics beyond heat and hot drought stress reveal changes in carbon storage, belowground translocation and growth

Summary Heatwaves combined with drought affect tree functioning with as yet undetermined legacy effects on carbon (C) and nitrogen (N) allocation. We continuously monitored shoot and root gas exchange, δ 13 CO 2 of respiration and stem growth in well‐watered and drought‐treated Pinus sylvestris (Scots pine) seedlings exposed to increasing daytime temperatures (max. 42°C) and evaporative demand. Following stress release, we used 13 CO 2 canopy pulse‐labeling, supplemented by soil‐applied 15 N, to determine allocation to plant compartments, respiration and soil microbial biomass (SMB) over 2.5 wk. Previously heat‐treated seedlings rapidly translocated 13 C along the long‐distance transport path, to root respiration ( R root ; 7.1 h) and SMB (3 d). Furthermore, 13 C accumulated in branch cellulose, suggesting secondary growth enhancement. However, in recovering drought‐heat seedlings, the mean residence time of 13 C in needles increased, whereas C translocation to R root was delayed (13.8 h) and 13 C incorporated into starch rather than cellulose. Concurrently, we observed stress‐induced low N uptake and aboveground allocation. C and N allocation during early recovery were affected by stress type and impact. Although C uptake increased quickly in both treatments, drought‐heat in combination reduced the above–belowground coupling and starch accumulated in leaves at the expense of growth. Accordingly, C allocation during recovery depends on phloem translocation capacity.