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

Abstract

- 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, δ13CO2 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 13CO2 canopy pulse-labeling, supplemented by soil-applied 15N, to determine allocation to plant compartments, respiration and soil microbial biomass (SMB) over 2.5 wk. - Previously heat-treated seedlings rapidly translocated 13C along the long-distance transport path, to root respiration (Rroot; 7.1 h) and SMB (3 d). Furthermore, 13C accumulated in branch cellulose, suggesting secondary growth enhancement. However, in recovering drought-heat seedlings, the mean residence time of 13C in needles increased, whereas C translocation to Rroot was delayed (13.8 h) and 13C 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.