Exploring new strategies for ozone-risk assessment: A dynamic-threshold case study

Area of Science:

• Forest Ecology
• Atmospheric Chemistry
• Plant Physiology

Background:

• Tropospheric ozone is a significant air pollutant detrimental to forest ecosystems.
• Phytotoxic Ozone Dose (POD) is used to assess ozone risk based on stomatal ozone uptake.
• Understanding ozone's impact on forest health is crucial for ecosystem management.

Purpose of the Study:

• To evaluate the effects of ozone on a Holm oak forest in central Italy.
• To test and compare different ozone impact response functions within a multi-layer canopy model.
• To determine if a clear phytotoxic ozone threshold exists and if it varies seasonally.

Main Methods:

• Implemented and tested four flux-based ozone impact response functions using the AIRTREE model.
• Validated model performance against observed Gross Primary Productivity (GPP) from Eddy Covariance CO2 flux data.
• Tested six different detoxifying thresholds (0-5 nmol O3 m⁻² s⁻¹) to assess ozone tolerance.

Main Results:

• Species-specific response functions improved model accuracy (RMSE reduced by up to 8.5%) compared to Plant Functional Type (PFT) based functions.
• A linear response function with a 1 nmol m⁻² s⁻¹ threshold best simulated annual GPP, but ozone tolerance varied seasonally.
• A dynamic threshold approach reduced GPP overestimation by 213 g C m⁻² y⁻¹ and RMSE by up to 7.7%.

Conclusions:

• Seasonal variations in ozone tolerance necessitate the use of dynamic or seasonal thresholds for accurate ozone damage prediction.
• Nonlinear ozone response functions may fully account for detoxification, suggesting a 0 nmol O3 m⁻² s⁻¹ threshold in some cases.
• Further species-specific manipulative experiments are needed to develop robust response functions for diverse plant species.