Unrevealing 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 the resilience of trees to an intensification of hot droughts remains limited. Here, we used single tree cuvettes (n=18) allowing us to investigate transpiration (E), net assimilation (Anet), root respiration (Rroot) and stem diameter change in Scots pine seedlings during gradually intensifying heat or drought-heat stress (max. 42{degrees}C), and post-stress. Further, we assessed indicators of stress impacts and recovery capacities. Under heat stress, well-watered seedlings prevented overheating of leaves effectively via increased E, while under drought-heat leaf temperatures increased to 46{degrees}C. However, leaf electrolyte leakage was negligible, but Fv/Fm declined alongside Anet moderately in heat but strongly in drought-heat seedlings, in which respiration exceeded C uptake. Further, the decrease of needle water potential ({psi}Needle) to -2.7 MPa and relative needle water content (RWCNeedle) under drought-heat reflected a decline of leaf hydraulic conductance (KLeaf) by 90% and stem hydraulic conductivity (KS) by 25%. Alongside, we observed pronounced stem diameter shrinkage. Heat stress alone resulted in low functional impairment and all measured parameters recovered fast. In contrast, larger impacts following combined heat and drought led to the incomplete recovery of KLeaf and KS. Despite Anet tended to be reduced albeit Fv/Fm had recovered, the seedlings net C balance reached control values 2 d after stress release and stem growth rates exceeded control rates in the 2nd week post-stress. This indicates that a new equilibrium of C uptake and release was maintained at the tree level, slowly supporting regaining of stress-induced losses. In summary, we highlight that under moderate heatwaves with low functional impairment, recovery is fast in Scots pine, while in combination with drought hydraulic and thermal stress are intensified, resulting in functional damage and delayed recovery processes. The incomplete recovery of hydraulic conductance indicates limited water transport capacities that could become critical under repeated heat events.