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Canopy emissions of volatile hydrocarbons such as isoprene and monoterpenesplay an important role in air chemistry. They depend on variousenvironmental conditions, are highly species-specific and are expected to beaffected by global change. In order to estimate future emissions of theseisoprenoids, differently complex models are available. However, seasonaldynamics driven by phenology, enzymatic activity, or drought stress stronglymodify annual ecosystem emissions. Although these impacts depend themselveson environmental conditions, they have yet received little attention inmechanistic modelling.  In this paper we propose the application of a mechanistic method forconsidering the seasonal dynamics of emission potential using the "SeasonalIsoprenoid synthase Model" (Lehning et al., 2001). We test this approach withthree different models (GUENTHER, Guenther et al., 1993; NIINEMETS,Niinemets et al., 2002a; BIM2, Grote et al., 2006) that are developed forsimulating light-dependent monoterpene emission. We also suggest specificdrought stress representations for each model. Additionally, the proposedmodel developments are compared with the approach realized in the MEGAN(Guenther et al., 2006) emission model. Models are applied to a MediterraneanHolm oak (<i>Quercus ilex</i>) site with measured weather data.  The simulation results demonstrate that the consideration of a dynamicemission potential has a strong effect on annual monoterpene emissionestimates. The investigated models, however, show different sensitivities tothe procedure for determining this seasonality impact. Considering a droughtimpact reduced the differences between the applied models and decreasedemissions at the investigation site by approximately 33% on average overa 10 year period. Although this overall reduction was similar in all models,the sensitivity to weather conditions in specific years was different. Weconclude that the proposed implementations of drought stress and internalseasonality strongly reduce estimated emissions and indicate themeasurements that are needed to further evaluate the models

Process-based simulation of seasonality and drought stress in monoterpene emission models