Abstract
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.
Highlights
A large number of Mediterranean deciduous and evergreen trees produce and release isoprene (2-methyl-1,3-butadiene, C5H8)
Given that this study focuses on isoprene emissions from Q. pubescens, we fixed the vegetation with the corresponding plant functional types (PFTs) “temperate broad-leaf summer green tree”
The AD plot soil water deficit remained significant until the end of the experiment (Mann– Whitney, P < 0.05 in June 2012, P < 0.001 from July 2012 to June 2013), the rain exclusion system was not activated between December 2012 and June 2013
Summary
A large number of Mediterranean deciduous and evergreen trees produce and release isoprene (2-methyl-1,3-butadiene, C5H8). The role of isoprene remains a subject of debate, it seems likely that C5H8 helps plants to optimise CO2 assimilation during temporary and mild stresses, especially during the growing and warmer periods (Brilli et al, 2007; Loreto and Fineschi, 2015). The major role of isoprene in plant defence probably explains its large annual global emissions (440–660 TgC yr−1, Guenther et al, 2006), forming the largest quantity of all biogenic volatile organic compounds (BVOCs) emitted. Present in the atmosphere at the ppb or ppt level, isoprene has a broad impact on atmospheric chemistry, both in the gas phase (especially in the O3 budget of some urbanised areas, Atkinson and Arey, 2003) and in the particulate phase (secondary organic aerosols formation, Goldstein and Steiner, 2007), and on biosphere–atmosphere feedbacks. Given the broad impacts of isoprene on atmospheric chemistry, considerable efforts have been made to (i) understand the physiological mechanisms responsible for isoprene synthesis and emission and the different environmental parameters that control their variability, in order to (ii) develop isoprene emission models that can account for the broadest possible range of environmental conditions
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