Seasonal variations of Quercus pubescens isoprene emissions from an in natura forest under drought stress and sensitivity to future climate change in the Mediterranean area
Authors: Genard-Zielinski Anne-Cyrielle , Boissard Christophe , Ormeño Elena , Lathière Juliette , Reiter Ilja M. , Wortham Henri , Orts Jean-Philippe , Temime-Roussel Brice , Guenet Bertrand , Bartsch Svenja , Gauquelin Thierry , Fernandez Catherine 
Published: August 2018

Abstract. At a local level, biogenic isoprene emissions can greatly affect the air
quality of urban areas surrounded by large vegetation sources, such as in the
Mediterranean region. The impacts of future warmer and drier conditions on
isoprene emissions from Mediterranean emitters are still under debate.
Seasonal variations of Quercus pubescens gas exchange and isoprene
emission rates (ER) were studied from June 2012 to June 2013 at the O3HP
site (French Mediterranean) under natural (ND) and amplified (AD, 32 %)
drought. While AD significantly reduced stomatal conductance to water vapour
throughout the research period excluding August, it did not significantly
preclude CO2 net assimilation, which was lowest in summer (≈-1 µmolCO2 m−2 s−1). ER followed a significant seasonal
pattern regardless of drought intensity, with mean ER maxima of 78.5 and
104.8 µgC gDM-1 h−1 in July (ND) and August (AD)
respectively and minima of 6 and < 2 µgC gDM-1 h−1 in October and April respectively.
The isoprene emission factor
increased significantly by a factor of 2 in August and September under AD
(137.8 and 74.3 µgC gDM-1 h−1) compared with ND (75.3
and 40.21 µgC gDM-1 h−1), but no significant
changes occurred on ER. Aside from the June 2012 and 2013 measurements,
the MEGAN2.1 (Model of Emissions of Gases and
Aerosols from Nature version 2.1) model was able to assess the observed ER variability only when its
soil moisture activity factor γSM was not operating and regardless
of the drought intensity; in this case more than 80 % and 50 % of ER
seasonal variability was assessed in the ND and AD respectively. We suggest
that a specific formulation of γSM be developed for
the drought-adapted isoprene emitter, according to that obtained for Q. pubescens in
this study (γSM= 0.192e51.93 SW with SW the soil
water content). An isoprene algorithm (G14) was developed using an
optimised artificial neural network (ANN) trained on our experimental dataset (ER + O3HP
climatic and edaphic parameters cumulated over 0 to 21 days
prior to the measurements). G14 assessed more than 80 % of the observed
ER seasonal variations, regardless of the drought intensity. ERG14 was
more sensitive to higher (0 to −7 days) frequency environmental changes under
AD in comparison to ND. Using IPCC RCP2.6 and RCP8.5 climate scenarios, and
SW and temperature as calculated by the ORCHIDEE land surface model,
ERG14 was found to be mostly sensitive to future temperature and
nearly insensitive to precipitation decrease (an annual increase of up to 240 % and
at the most 10 % respectively in the most severe scenario). The main impact
of future drier conditions in the Mediterranean was found to be an
enhancement (+40 %) of isoprene emissions sensitivity to thermal stress.