Though the air may appear clear, ozone pollution poses a serious risk to forests across the United States. Recent research links elevated ozone levels to decreased survival rates in many tree species. This new analysis advances understanding by assessing mature trees in natural conditions rather than focusing solely on seedlings in laboratory settings.

Comprehensive Study Linking Ozone Exposure to Tree Health

Scientists combined extensive forest inventory data with detailed measurements of tropospheric ozone exposure to identify species-specific thresholds where ozone begins to harm trees. Covering 88 species and about 1.5 million individual trees, the study offers unprecedented insight into how ozone affects growth and survival in mature forests.

Methodology Behind the Research

Led by Nathan Pavlovic of Sonoma Technology Inc. and Charles Driscoll of Syracuse University, the team integrated forest data with air quality records to examine correlations between ozone exposure and tree performance. Unlike prior work that relied on seedling experiments in controlled environments, this study focused on fully grown trees thriving in their native ecosystems.

Limitations of Seedling-Focused Studies

Earlier U.S. studies mainly utilized seedlings to measure ozone impact, including a synthesis of 16 species that established biomass loss curves. While useful, these experiments could not capture how trees decades into maturity respond to ozone, which is critical for forest management and conservation.

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Quantifying Ozone Risk with Critical Exposure Levels

Researchers introduced the concept of a “critical level,” indicating the ozone exposure at which noticeable declines in growth or survival occur. They used the W126 metric, a cumulative, summertime-weighted measure emphasizing daytime ozone peaks, which pose greater threats to vegetation.

Separate models for growth and 10-year survival reveal species-specific W126 thresholds linked to a 5% decrease in growth and a 1% reduction in survival. These figures help forest managers prioritize species most at risk.

Policy Context of Ozone Thresholds

These critical levels align with U.S. Environmental Protection Agency (EPA) practices, which have long incorporated W126 in vegetation welfare assessments. Scientific advisory groups have similarly used such thresholds when recommending secondary ozone standards aimed at minimizing biomass loss.

Regional Differences: West Versus East

The study highlights stark regional contrasts. From 2016 to 2018, parts of the western U.S., especially west of the Rockies, exceeded critical ozone levels for nearly all tree species, affecting both growth and survival. In contrast, the eastern U.S. showed limited growth impacts with survival effects mostly confined to sensitive species or localized high-exposure areas, consistent with declining ozone peaks recorded in the East since the early 2000s.

Why Seedlings and Mature Trees Respond Differently?

Seedling tests, while controlled, fail to reflect complex field factors such as drought, heat, soil diversity, and competition. Some species that seem ozone-resistant in seedlings may become vulnerable when ozone stress combines with environmental challenges found in mature forests.

This study accounts for these mediating factors, producing thresholds that better represent real-world conditions and aid foresters in selecting resilient species for planting and regeneration.

Implications for Forest Management and Policy

Practically, if a region’s summertime W126 ozone level surpasses a species’ survival threshold, forest managers might adjust planting plans, thin stands more aggressively, or favor species less sensitive to ozone while maintaining ecosystem goals.

For policymakers, the challenge lies in aligning ozone standards with ecosystem health. Current secondary standards under the Clean Air Act use an eight-hour human health metric, which does not correspond directly with vegetation damage driven by seasonal ozone accumulation.

International Variations in Ozone Metrics

Globally, Europe often reports ozone exposure via AOT40 metrics, while the U.S. primarily relies on W126. Scientific consensus is moving toward flux-based metrics that track ozone uptake by plants. This variation underscores the importance of developing ecosystem-specific measures to better protect forests.

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Complexities of Ozone’s Impact on Trees

No single metric can capture all damage pathways. W126 focuses on summertime ozone peaks, vital for crops, but conifers with long growing seasons accumulate ozone differently, which may explain their heightened sensitivity in western forests.

Factors such as limited rural monitoring, wildfire smoke chemistry, and drought-induced changes in leaf stomata add uncertainty. Still, the established thresholds offer a practical foundation for improved monitoring and modeling.

Advancing Science for Better Climate and Carbon Projections

Ongoing research includes Earth system models that more accurately represent ozone’s effects on photosynthesis and water use. These advances will refine regional carbon and climate predictions and help translate ozone pollution controls into tangible benefits for forest health.

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