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Extreme temperature and leaf damage in Puéchabon. June 28, 2019

The 28th of June 2019 has been identified as the hottest day in French history, and the extreme heat wave that happened in Southern France that day resulted in maximum temperature above 45°C for the first time since meteorological records are available. The record for hottest temperature in the Puéchabon experimental site was also broken that day with our meteorological station recording a maximum temperature of 43.4°C at 5 pm and a 6-hour period above 40°C. The last record of 40.7°C happened on August 12, 2003 and was almost 3°C below.
This extreme and unprecedented heat wave have had a strong and rapid impact on vegetation and the oaks in Puéchabon are now exhibiting serious leaf damage. More than 70% of the trees on the experimental site show signs of leaf whitening and drying to a certain extent, while approximately 20% of them have lost all of their new leaves produced during the spring 2019.

Photo 1 : View from the Eddy-Covariance flux tower showing trees whitened by the heat stress.

The consequence of the heat stress on the oak leaves is particular and differs from what could be observed in 2016 and 2017 following severe drought stress (see for comparison : First, leaf damage following heat stress was most detrimental to the new leaves produced this spring and that were not completely mature when the heat stress occurred (average defoliation of 31% for the 2019 leaf cohort against 12% for the 2018 cohort). Second, the symptoms of heat stress are a whitening of the leaves starting from the edge and that does not result in a complete and immediate desiccation of the whole leaf lamina. Third, water potential measured on twigs bearing damaged leaves was generally close to the water potential measured on twigs bearing intact leaves, so heat stress does not seem do induce xylem cavitation, at least after one week. We may thus hypothesize that the physiological mechanisms of heat stress damage are different from the ones involved in drought stress damage.

Photo 2 : oak leaves at different stages of heat damage. It appears that the edges of the leaf lamina are damaged first, with the central leaf vein remaining hydrated, consistently with the twig water potential measurements.

Photo 3 : A branch of tree that experienced leaf damage on approximately 40% of its 2019 leaves. Note the white color of the damaged leaves and the partial drying on the edges of the leaf lamina.

Photo 4 : A tree that has been damaged on almost all of its 2019 leaves, only green leaves that remain are the old 2018 leaves shadowed by the 2019 growth

Photo 5 : All the leaves near the apex of this branch have been totally damaged, some of the lower leaves that were protected from the sun have been only partially damaged

Photo 6 : On some trees the leaves have already turned completely brown, but still with the characteristic two colors between the lamina edge on the central vein.

Less than two weeks after the heat stress, it is still too soon to guess the long-term consequences of this damage for the trees. It must be noted however that extreme weather (drought in 2016 and 2017, heat in 2019) has already resulted in widespread leaf damage in 3 years over 4 since 2016. Thanks to their evergreen habit and strong resilience, the trees have so far always recovered from these defoliations. Will this one be different ?