Simulating precipitation decline under a Mediterranean deciduous Oak forest: effects on isoprene seasonal emissions and predictions under climatic scenarios

Abstract

Abstract. Seasonal variations of Q. pubescens physiology and isoprene emission rates (ER)were studied from June 2012 to June 2013 at the O 3 HP site (French Mediterranean) under natural (ND) and amplified (+30%, AD) drought. While AD significantly reduced the stomatal conductance to water vapour over the season excepting August, it did not significantly limit CO 2 net assimilation, which was the lowest in summer. ER followed a 20 significant seasonal pattern, whatever the drought intensity, with mean ER maxima of 78.5 and 104.8 µgC g DM -1 h -1 in July (ND) and August (AD) respectively. Isoprene emission factor increased significantly by a factor of 2 in August and September under AD (137.8 and 74.3 µgC g DM -1 h -1 ) compared to ND (75.3 and 40.21 µgC g DM -1 h -1 ), but no changes occurred onER. An isoprene algorithm (G14) was developed using an optimised artificial neural network 25 trained on our experimental dataset (ER + O 3 HP climatic and edaphic parameters cumulated over 0 to 21 days before measurements). G14 assessed more than 80% of the observed ER seasonal variations, whatever the drought intensity. In contrast, ER was poorly assessed under water stress by MEGAN empirical isoprene model, in particular under AD. Soil water (SW) content was the dominant parameter to account for the observed ER variations, regardless the 30 water stress treatment. ER was more sensitive to higher frequency environmental changes under AD (0 to -7 days) compared to ND (7 days). Using IPCC RCP2.6 and RCP8.5 climate scenarios, SW and temperature calculated by the ORCHIDEE land surface model, and G14, an annual 3 fold ER relative increase was found between present (2000-2010) and future (2090-2100) for RCP8.5 scenario compared to a 70% increase for RCP2.6. Future ER 35Biogeosciences Discuss.,

Resistance of native oak to recurrent drought conditions simulating predicted climatic changes in the Mediterranean region

“…A similar study was performed in 2012 (first year of AD) on isoprene emissions from the same trees (Genard‐Zielinski et al, ) and highlighted an increase in this compound, whereas we observed a decrease after the third and fourth years of AD. Decreases in isoprene emissions could be problematic for Q. pubescens as this compound is known to act as an antioxidant and maintain the stability of thylakoid membranes under oxidative conditions (Velikova et al, ).…”

“…antioxidant system.A similar study was performed in 2012 (first year of AD) on isoprene emissions from the same trees(Genard-Zielinski et al, 2017) and highlighted an increase in this compound, whereas we observed a decrease after the third and fourth years of AD.Decreases in isoprene emissions could be problematic for Q. pubescens as this compound is known to act as an antioxidant and maintain the stability of thylakoid membranes under oxidative conditions(Velikova et al, 2011). The antioxidant system of Q. pubescens could be affected by the isoprene decrease through compensatory mechanisms(Munne-Bosch, 2005), but this was apparently not the case because no strong trade-off (indicated by a negative correlation) was detected between isoprene and the major antioxidants (e.g., α-tocopherol and xanthophyll cycle antioxidants).…”

“…These results suggest that isoprene could act as an additional antioxidant favoured only in the earlier steps of drought as highlighted in our studies (first, third, and fourth years). The release of assimilated carbon in isoprene could confirm this hypothesis, as this parameter was higher during the first year under AD (Genard‐Zielinski et al, ) whereas it was the same between treatments after the third and fourth years.…”

Effect of mid-term drought on <i>Quercus pubescens</i> BVOCs' emission seasonality and their dependency on light and/or temperature

“…The O 3 HP site was created in 2009 in order to study the impact of climate change on a Q. pubescens forest. Using a rainfall exclusion device (an automated monitored roof deployed during chosen rain events) set up over part of the O 3 HP canopy, it was possible to reduce natural rain by 30 % and to extend the drought period in an attempt to mimic the current climatic model projections for 2100 (Giorgi and Lionello, 2008;IPCC, 2013;Polade et al, 2014). Two plots were considered in the site: a plot receiving natural precipitation where trees grew under natural drought (300 m 2 surface, used as control plot) and a second plot submitted to amplified drought (232 m 2 surface).…”

“…Likewise, the capacity of current L + T and T algorithms to predict emission shifts under different drought scenarios in the context of climate change needs to be addressed for isoprene and highly volatile compounds. This is of particular interest for the Mediterranean area where the most severe climatic scenario of the IPCC predicts an intensification of summer drought consisting of a rain reduction that could locally reach 30 %, an extension of the drought period as well as a temperature rise of 3.4 • C (Giorgi and Lionello, 2008;IPCC, 2013;Polade et al, 2014) for 2100.…”