Leaf-level gas exchange and water potential
We measured the leaf-level light-saturated net photosynthesis (Anet, µmol m-2 s-1) and stomatal conductance (gs, mol m-2s-1) on one fully developed leaf (or multiple needles for conifers) per tree from a 50cm to 1m-long (for oak and pine species, respectively) sun-exposed branch. The branches were sampled using an extension pole, directly placed in a water bucket, and recutted under water to restore water flow (Lange et al., 1986). Within 15 min after sampling, Anet and gs were measured using one infrared gas exchange analyzer (LI-6800 or LI-6400, LICOR Biosciences, USA). The measurements were done between 9 am and 1 pm (local time). The relative humidity inside the chambers was set to 50% (to match the average daily ambient environmental conditions during the measurements), the CO2 concentration to 400 ppm, the photosynthetic photon flux density (PPFD) to 1500 μmol m-2 s-1 (to ensure saturating light conditions), and the air temperature inside the cuvette from 20 to 30°C depending of the sampling dates (to fit the mean midday air temperature during the measurements; Fig. S2). Each leaf was measured when the gas exchange values were stable (i.e., after max. 5 minutes). On the same day as gas exchange measurements, one twig per tree was collected before sunrise (Ψpd) and at midday (Ψmd) to measure the leaf water potential (MPa) with a Scholander-type pressure chamber (M1505D, PMS Instruments, USA). The difference between Ψpd and ΨmdΨ) was calculated to describe the tree’s stomatal regulation strategy.