Fine precision scale

Minimum leaf conductance (Kerstiens, 1996 ) was measured as described in Pearcy and Zimmermann (2000 ). One leaf per individual was cut before dawn when stomata were assumed to be still closed. The cut petiole was immediately sealed with melted candle wax, and the leaf area was scanned using a flatbed scanner, followed by analysis using Pixstat (Schleppi, 2021 ). The leaves were stuck to a lab tape run between two lab stands, standing in a small dark climate chamber with stable T (26°C) and humidity (60%) and the ventilation on. Every 15–20 min, the leaves were taken from the climate chamber and weighed in a dark room using a fine‐precision scale (Mettler‐Toledo). This procedure was repeated eight times. g_min (mmol m⁻² s⁻¹) was calculated as cuticular transpiration per mole fraction VPD, assuming the leaf internal air to be fully saturated (Pearcy & Zimmermann, 2000 ).

Pressure‐volume curves were determined using the bench‐dehydration method (Koide et al., 2000 ). Before dawn, a leaf from the top of the crown was cut off and immediately sealed in a plastic bag (Whirlpak) that was previously exhaled. Predawn water potential (kPa) was measured directly using a Scholander‐type pressure chamber (PMS Instrument Company, Model 1505D). The same leaf was immediately weighed using a fine‐precision scale (Mettler‐Toledo), placed in a plastic bag, and allowed to dry progressively in the open plastic bag on a lab bench. The procedure of measuring water potential, weighing, and drying was repeated with increasing drying time intervals (from 10 s to 1 h) for the two Quercus species until achieving water potentials of about −4 MPa or until water potential reached a plateau. For Fagus sylvatica, the procedure was repeated continuously without letting the leaves dry on the bench due to the rapid water loss and a corresponding drop in leaf water potential. Subsequently, the leaves were individually put in a paper bag and dried in an oven at 60°C for 24 h to determine the dry mass. Leaf water potential at turgor loss point (Ψ_TLP, MPa) was calculated after Koide et al. (2000 ). At midday, another leaf was cut off from the same individuals, and midday water potential (Ψ_md) was measured.