## 2 Materials and Methods
### MEGAN-MOHYCAN: Biogenic VOC Emission Modelling  
The biogenic emissions of isoprene, monoterepenes, and 2-methylbutenol are estimated using MEGAN [5; 48] coupled to the MOdel for HYdrocarbon emissions by the CANopy (MOHYCAN; [49; 68]), a multi-layer canopy environment model. The net emission rates \(F\) (\(\mu\)g m\({}^{-2}\) h\({}^{-1}\)) into the above-canopy atmosphere are estimated as follows:  
\[F=\varepsilon\cdot\gamma_{0},\;\;\text{with}\;\gamma_{0}=C_{CE}\cdot\gamma_{PT }\cdot LAI\cdot\gamma_{A}\cdot\gamma_{SM}\cdot\gamma_{CO_{2}}, \tag{1}\]  
where the emission factor \(\varepsilon\) (\(\mu\)g m\({}^{-2}\) h\({}^{-1}\)) represents the emission rate at standard conditions as defined by [48]. Deviations from those conditions are accounted for by the activity factors encapsulated in \(\gamma_{0}\) representing the response of biogenic emissions to their major identified environmental and phenological drivers. Activity factors account for leaf temperature and photosynthetic photon flux density (\(\gamma_{PT}\)), leaf area index (LAI), leaf age (\(\gamma_{A}\)), drought stress (\(\gamma_{SM}\)) and CO\({}_{2}\) inhibition (\(\gamma_{CO_{2}}\)). The temperature and light response algorithm incorporates the influence of current and past conditions, as well as the attenuation of light by leaves within the canopy. The calculation of \(\gamma_{PT}\) is performed by MOHYCAN, the canopy environment model, detailed by [49]. The adjustment factor C\({}_{\text{CE}}\) is specific for each canopy environment model and is set to 0.52 for MOHYCAN, so that \(\gamma_{0}=1\) for standard canopy conditions. The response of biogenic isoprene emissions to drought in the MEGAN model is accounted for primarily in the \(\gamma_{SM}\) activity factor through the direct impact of soil water stress on the emissions. Moreover, drought leads to enhanced stomatal resistances and therefore higher leaf temperatures accounted for in \(\gamma_{PT}\) defined in MOHYCAN. For severe drought conditions, however, the impact of \(\gamma_{SM}\) outweighs the impact of higher leaf temperatures, and can even lead to a complete shutdown of the emissions [33]. Currently, the MEGAN model provides the users with two algorithms of \(\gamma_{SM}\) for the response of emissions to drought. In MEGANv2.1, the impact of drought is estimated based on the volumetric soil water content (denoted \(\theta\)) and \(\theta_{W}\). The factor \(\gamma_{SM}\) accounts for direct deviations due to changes in \(\theta\) as follows:  
\[\gamma_{SM}= \begin{cases}1&\theta>\theta_{1}\\ \frac{\theta-\theta_{W}}{\Delta\theta}&\theta_{W}<\theta<\theta_{1}\\ 0&\theta<\theta_{W}\end{cases}  \tag{2}\]  
The original value of \(\Delta\theta\) (0.06 m\({}^{3}\) m\({}^{-3}\); [48]) was derived empirically based on the study of [33] conducted on potted plants, with one plant species (_Populus deltoides Bartr._) and soil type. The value \(\Delta\theta\) was later lowered to a more conservative 0.04 m\({}^{3}\) m\({}^{-3}\)[5], because emissions were being shut off in regions where there was no extreme drought, and will further be referred to as the default value for this parameter. The threshold point \(\theta_{1}\) is defined as \(\theta_{W}+\Delta\theta\). As emphasized by [49], the soil water content should be paired consistently with the wilting point given the importance of the wilting point in the determination of soil moisture. In the third version of the model (MEGANv3; [69]), isoprene emissions respond simultaneously to water stress and photosynthesis, as provided by the Community Land Model (CLM; [70]) in the Community Earth System Model (CESM; [71]), which is coupled to MEGAN. The parameterization of the drought stress activity factor \(\gamma_{SM}\) in MEGANv3 (hereafter denoted \(\gamma_{SM}^{*}\)) is based on CLM maximum rate of carboxylation (\(V_{CMAX}\)) and soil moisture stress function (\(\beta_{t}\)). A thorough description of these parameters is provided in [70] and the parameterization is fully discussed in [69].