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, δ13 CO2 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 CO2 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 (Rroot ; 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 Rroot 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.
Highlights
The Earth’s forests play an important role in the global carbon (C) cycle by removing a large amount of CO2 from the atmosphere (Pan et al, 2011; Le Quereet al., 2018)
Gas exchange declined under drought as observed in lower net assimilation (Anet) and shoot dark respiration (Rshoot-night) compared to control seedlings (Tukey’s honestly significant difference (HSD), P > 0.1; Fig. 3a,b)
The larger stress impacts under drought-heat were observed in low Ψneedle (− 2.70 Æ 0.13 MPa; P < 0.01; Table 3), stem diameter shrinkage (Tukey’s HSD, P < 0.01; Fig. 3d), and 4.6°C warmer maximum needle temperatures compared to the heat treatment (P < 0.001)
Summary
The Earth’s forests play an important role in the global carbon (C) cycle by removing a large amount of CO2 from the atmosphere (Pan et al, 2011; Le Quereet al., 2018). Reduced stomatal conductance limits evaporative leaf cooling, leading to higher leaf temperatures (Birami et al, 2018; Drake et al, 2018) It decreases CO2 uptake via photosynthesis (Rennenberg et al, 2006; Ruehr et al, 2016; Birami et al, 2018), which can influence C and nitrogen (N) allocation dynamics (Ruehr et al, 2009; Blessing et al, 2015; Schonbeck et al, 2020) and growth rates (Bauweraerts et al, 2014; Teskey et al, 2015; Ruehr et al, 2016).
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