Abstract
Tropospheric ozone concentrations have increased by 60–100% in the Northern Hemisphere since the 19th century. The phytotoxic nature of ozone can impair forest productivity. In addition, ozone affects stomatal functions, by both favoring stomatal closure and impairing stomatal control. Ozone-induced stomatal sluggishness, i.e., a delay in stomatal responses to fluctuating stimuli, has the potential to change the carbon and water balance of forests. This effect has to be included in models for ozone risk assessment. Here we examine the effects of ozone-induced stomatal sluggishness on carbon assimilation and transpiration of temperate deciduous forests in the Northern Hemisphere in 2006-2009 by combining a detailed multi-layer land surface model and a global atmospheric chemistry model. An analysis of results by ozone FACE (Free-Air Controlled Exposure) experiments suggested that ozone-induced stomatal sluggishness can be incorporated into modelling based on a simple parameter (gmin, minimum stomatal conductance) which is used in the coupled photosynthesis-stomatal model. Our simulation showed that ozone can decrease water use efficiency, i.e., the ratio of net CO2 assimilation to transpiration, of temperate deciduous forests up to 20% when ozone-induced stomatal sluggishness is considered, and up to only 5% when the stomatal sluggishness is neglected.
Highlights
Tropospheric ozone concentrations have increased by 60–100% in the Northern Hemisphere since the 19th century
We focused on O3-sensitive temperate deciduous forests exposed to realistic O3 concentrations in the Northern Hemisphere
We investigated how O3 uptake changed the parameters of the photosynthesis-stomatal model, which is widely used in many land-surface schemes in climate models[5,9]
Summary
Our study suggests a simple way to include O3-induced stomatal sluggishness in the Ball-Woodrow-Berry model. Higher O3 concentrations, e.g., 44.6 ± 4.7 nmol mol−1 as an average of daytime values over China, resulted in 9.1 ± 2.0% and 8.0 ± 1.6% reductions in the “sluggishness run” and “no sluggishness run”, respectively Such a stronger impact on carbon assimilation, when O3-induced stomatal sluggishness was included, was due to enhanced stomatal O3 uptake, which led to a further negative impact on photosynthesis. According to our “sluggishness run”(Fig. 2c), we estimated a 2-3% increase of WUE by a 8–10 nmol mol−1 decrease in O3 concentrations, while only a ~1% increase of WUE was found in the “no sluggishness run” (Fig. 2c) This result suggests that a significant part of the WUE trend at North American sites (corresponding to about one-tenth of the observed WUE trend) may be explained by O3 effects, when O3-induced stomatal sluggishness is included. This implies that stomatal sluggishness is essential to assessing impacts of air quality to terrestrial ecosystems under present and future atmospheric conditions
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