Diverging responses of water and carbon relations during and after heat and hot drought stress in Pinus sylvestris

Forests are increasingly affected by heatwaves, often co-occurring with drought, with consequences for water and carbon (C) cycling. However, our ability to project tree resilience to more intense hot droughts remains limited. Here, we used single tree chambers (n = 18) to investigate transpiration (E), net assimilation (A(net)), root respiration (R-root) and stem diameter change in Scots pine seedlings in a control treatment and during gradually intensifying heat or drought-heat stress (max. 42 degrees C), including recovery. Alongside this, we assessed indicators of stress impacts and recovery capacities. In the heat treatment, excessive leaf heating was mitigated via increased E, while under drought-heat, E ceased and leaf temperatures reached 46 degrees C. However, leaf electrolyte leakage was negligible, while light-adapted quantum yield of photosystem II (F '(v)/F '(m)) declined alongside A(net) moderately in heat, but strongly in drought-heat seedlings, in which respiration exceeded C uptake. Drought-heat largely affected the hydraulic system as apparent in stem diameter shrinkage, declining relative needle water content (RWCNeedle) and water potential (psi(Needle)) reaching -2.7 MPa, alongside a 90% decline of leaf hydraulic conductance (K-Leaf). Heat alone resulted in low functional impairment and all measured parameters recovered quickly. Contrary, following drought-heat, the recovery of K-Leaf was incomplete and stem hydraulic conductivity (K-S) was 25% lower than the control. However, F '(v)/F '(m) recovered and the tree net C balance reached control values 2 days post-stress, with stem increment rates accelerating during the second recovery week. This indicates a new equilibrium of C uptake and release in drought-heat seedlings independent of hydraulic impairment, which may slowly contribute to the repair of damaged tissues. In summary, Scots pine recovered rapidly following moderate heat stress, while combined with drought, hydraulic and thermal stress intensified, resulting in functional damage and slow recovery of hydraulic conductance. This incomplete hydraulic recovery could critically limit evaporative cooling capacities and C uptake under repeated heatwaves.

Automated summary

Forests are affected by heatwaves and drought, impacting water and carbon cycling. In this study, researchers investigated the response of Scots pine seedlings to heat and drought-heat stress. They found that in the heat treatment, transpiration increased to mitigate leaf heating, while in the drought-heat treatment, transpiration ceased and leaf temperatures reached high levels. The hydraulic system was largely affected by drought-heat, resulting in stem diameter shrinkage and decreased leaf hydraulic conductance. Recovery was faster in the heat treatment compared to the drought-heat treatment, with incomplete hydraulic recovery in the latter. This incomplete recovery could limit evaporative cooling capacities and carbon uptake in repeated heatwaves.