Ozone-induced stomatal sluggishness changes carbon and water balance of temperate deciduous forests

Abstract

Tropospheric ozone concentrations have increased by 60-100% in the Northern Hemisphere since the 19(th) 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.

Figure 1

Changes of g_min over a range of cumulative O₃ uptake (CUO) used for the “sluggishness run” and “no sluggishness run” of SOLVEG-MRI-CCM2. Data points of g_min were obtained from an analysis of measurements in June, August and October 2012 (see Fig. S1) at the O₃-FACE experiment on Siebold’s beech in Japan (blue circle: ambient O₃; red circle: elevated O₃). Obtained g_min were fitted by a sigmoid function for “sluggishness run” (solid line): g_min = 0.03 + 0.09/[1 + exp{–0.21·(CUO – 24.7)}], R² = 0.89. Dashed line shows no change of g_min (g_min = 0.03 mol m⁻² s⁻¹) and was used for “no sluggishness run”.

Figure 2

Percent change of modelled net CO₂ assimilation, transpiration and water use efficiency in temperate deciduous forests in the Northern Hemisphere in relation to daytime mean O₃ concentration or cumulative canopy O₃ uptake (years 2006-2009). a, net CO₂ assimilation, b, transpiration, and c, water use efficiency were simulated by the offline coupling simulation of SOLVEG-MRI-CCM2. Effects of O₃-induced stomatal sluggishness were included (black open circles and red lines) or excluded (gray circles and gray lines). The percentage of change of each parameter was calculated relative to “control run” (no O₃ effect).

Figure 3

Percent changes of reduction in modelled water use efficiency (WUE) in relation to daytime mean O₃ concentration or canopy cumulative O₃ uptake for several regions in the Northern Hemisphere (years 2006–2009). The percentage of reduction of WUE relative to “control run” (no O₃ effect) was calculated by the offline coupling simulation of SOLVEG-MRI-CCM2 including O₃-induced stomatal sluggishness. Plots and bars represent mean values and standard deviations, respectively. The five regions, i.e., Europe, North America, East Asia (without China), China and Central/West Asia are defined in Fig. S3.