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 Ψmd (ΔΨ) was
calculated to describe the tree’s stomatal regulation strategy.