Technical note: A simple theoretical model framework to describe plant stomatal “sluggishness” in response to elevated ozone concentrations

Elevated levels of tropospheric ozone, inline-formulaO₃, cause damage to terrestrial vegetation, affecting leaf stomatal functioning and reducing photosynthesis. Climatic impacts under future raised atmospheric greenhouse gas (GHG) concentrations will also impact on the net primary productivity (NPP) of vegetation, which might for instance alter viability of some crops. Together, ozone damage and climate change may adjust the current ability of terrestrial vegetation to offset a significant fraction of carbon dioxide (inline-formulaCO₂) emissions. Climate impacts on the land surface are well studied, but arguably large-scale modelling of raised surface level inline-formulaO₃ effects is less advanced. To date most models representing ozone damage use either inline-formulaO₃ concentration or, more recently, flux-uptake-related reduction of stomatal opening, estimating suppressed land–atmosphere water and inline-formulaCO₂ fluxes. However there is evidence that, for some species, inline-formulaO₃ damage can also cause an inertial “sluggishness” of stomatal response to changing surface meteorological conditions. In some circumstances (e.g. droughts), this loss of stomata control can cause them to be more open than without ozone interference. To both aid model development and provide empiricists with a system on to which measurements can be mapped, we present a parameter-sparse framework specifically designed to capture sluggishness. This contains a single time-delay parameter inline-formula $M7inlinescrollmathml{\mathrm{italic\; \tau }}_{chem{\mathrm{normal\; O}}_{\mathrm{normal\; 3}}}$ 17pt12ptsvg-formulamathimg53fe492292bd68c0ca020554daff6225 bg-15-5415-2018-ie00001.svg17pt12ptbg-15-5415-2018-ie00001.png , characterizing the timescale for stomata to catch up with the level of opening they would have without damage. The larger the value of this parameter, the more sluggish the modelled stomatal response. Through variation of inline-formula $M8inlinescrollmathml{\mathrm{italic\; \tau }}_{chem{\mathrm{normal\; O}}_{\mathrm{normal\; 3}}}$ 17pt12ptsvg-formulamathimgb0d6cfa8e85d1ef2bda977da215c8efc bg-15-5415-2018-ie00002.svg17pt12ptbg-15-5415-2018-ie00002.png , we find it is possible to have qualitatively similar responses to factorial experiments with and without raised inline-formulaO₃, when comparing to reported measurement time series presented in the literature. This low-parameter approach lends itself to the inclusion of ozone-induced inertial effects being incorporated in the terrestrial vegetation component of Earth system models (ESMs).