High heat tolerance, evaporative cooling, and stomatal decoupling regulate canopy temperature and their safety margins in three European oak species
Résumé
Abstract
Heatwaves and soil droughts are increasing in frequency and intensity, leading many tree species to exceed their thermal thresholds, and driving wide‐scale forest mortality. Therefore, investigating heat tolerance and canopy temperature regulation mechanisms is essential to understanding and predicting tree vulnerability to hot droughts. We measured the diurnal and seasonal variation in leaf water potential (Ψ), gas exchange (photosynthesis
A
net
and stomatal conductance
g
s
), canopy temperature (
T
can
), and heat tolerance (leaf critical temperature
T
crit
and thermal safety margins TSM, i.e., the difference between maximum
T
can
and
T
crit
) in three oak species in forests along a latitudinal gradient (
Quercus petraea
in Switzerland,
Quercus ilex
in France, and
Quercus coccifera
in Spain) throughout the growing season. Gas exchange and Ψ of all species were strongly reduced by increased air temperature (
T
air
) and soil drying, resulting in stomatal closure and inhibition of photosynthesis in
Q. ilex
and
Q. coccifera
when
T
air
surpassed 30°C and soil moisture dropped below 14%. Across all seasons,
T
can
was mainly above
T
air
but increased strongly (up to 10°C >
T
air
) when
A
net
was null or negative. Although trees endured extreme
T
air
(up to 42°C), positive TSM were maintained during the growing season due to high
T
crit
in all species (average
T
crit
of 54.7°C) and possibly stomatal decoupling (i.e.,
A
net
≤0 while
g
s
>0). Indeed,
Q. ilex
and
Q. coccifera
trees maintained low but positive
g
s
(despite null
A
net
), decreasing Ψ passed embolism thresholds. This may have prevented
T
can
from rising above
T
crit
during extreme heat. Overall, our work highlighted that the mechanisms behind heat tolerance and leaf temperature regulation in oak trees include a combination of high evaporative cooling, large heat tolerance limits, and stomatal decoupling. These processes must be considered to accurately predict plant damages, survival, and mortality during extreme heatwaves.
Auteurs, date et publication :
Auteurs Alice Gauthey , Ansgar Kahmen , Jean‐Marc Limousin , Alberto Vilagrosa , Margaux Didion‐Gency , Eugénie Mas , Arianna Milano , Alex Tunas , Charlotte Grossiord
Publication : Global Change Biology
Date : 2025
Volume : 30
Issue : 8
Pages : e17439