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

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 ($\approx -\mathrm{1}$ µmol${}_{{\mathrm{CO}}_{\mathrm{2}}}$ 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 g${}_{\mathrm{DM}}^{-\mathrm{1}}$ h⁻¹ in July (ND) and August (AD) respectively and minima of 6 and < 2 <span class="inline-formula">µgC g${}_{\mathrm{DM}}^{-\mathrm{1}}$ 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 g${}_{\mathrm{DM}}^{-\mathrm{1}}$ h⁻¹) compared with ND (75.3 and 40.21 µgC g${}_{\mathrm{DM}}^{-\mathrm{1}}$ 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 γ_SM 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 γ_SM be developed for the drought-adapted isoprene emitter, according to that obtained for Q. pubescens in this study (γ_SM= 0.192e^51.93 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. ER_G14 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, ER_G14 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.