Ozone pollution threatens tree survival across western U.S.

The air can look clear and still carry a problem. Across the United States, ozone has been linked to lower chances of survival for some trees, and a new analysis finally shows how much risk different species face.

Researchers paired long term forest records with measured exposure to tropospheric ozone to set species specific thresholds for harm. The study spans 88 species and roughly 1.5 million trees, a scale that moves the conversation beyond seedlings and lab chambers.

Study on ozone pollution

Nathan Pavlovic of Sonoma Technology Inc. and Charles Driscoll of Syracuse University brought together forest inventory data and air quality archives to estimate how exposure links to slower growth and lower survival. Their team focused on mature trees observed in place, not seedlings in controlled settings.

Earlier work in the United States leaned heavily on seedling experiments, including a 16 species synthesis that set response curves for biomass loss. That seedling paper became a touchstone, but it could not tell us how older trees respond after decades in the field.

Ozone effects on tree survival

The team used the concept of a critical level to summarize risk, the exposure at which a defined drop in growth or survival appears.

The researchers expressed exposure with “W126,” a cumulative, summertime-weighted metric for ozone exposure that emphasizes higher concentrations during daylight hours, reflecting their greater potential to damage vegetation.

They modeled growth and 10 year survival separately, which matters because a small shift in survival compounds over a century long rotation. The paper reports species specific W126 levels for a 5 percent drop in growth and a 1 percent drop in survival, allowing managers to see which trees blink first.

The numbers they put forth sit in a wider policy context. The EPA has long evaluated vegetation protection using W126 in its welfare reviews, and its advisory panel, CASAC, has considered thresholds associated with 1 to 2 percent biomass loss in trees when weighing secondary standards.

Ozone impact in west vs east

“Recently (2016-2018), portions of the western United States exceeded O3 CLs (or ozone critical levels, are the exposure thresholds at which a specific percentage decline in tree growth or survival is expected to occur) for nearly all tree species for both growth and survival,” wrote Pavlovic and colleagues.The clearest pattern appears west of the Rockies. 

In the East, the analysis found little evidence of widespread growth impacts at current levels, with survival effects limited to sensitive species and pockets with higher exposure.

That picture is consistent with national monitoring records showing strong declines in extreme ozone across the eastern United States since the 2000s.

Seedlings versus mature trees

Seedling studies provide clean experiments, but they cannot fully replicate heat, drought, soil variation, and competition in mature stands. The earlier 16 species seedling synthesis captured broad sensitivity classes, yet some species that look tolerant in chambers can be more vulnerable in place when ozone stacks with water stress and heat.

The new analysis, by design, keeps those mediating factors in the model to produce field relevant exposure thresholds.

That makes the species list more useful for foresters weighing which coniferous evergreens to plant and where deciduous hardwoods may hold up better under ozone.

What this means for forests and policy

A practical use case is simple. If a county’s summertime W126 sits above a species’ survival threshold, managers can alter planting mixes, accelerate thinning, or shift regeneration toward less sensitive species that still meet ecological goals.

Policy makers face a different question. Secondary ozone standards under the Clean Air Act are meant to protect crops, materials, and ecosystems, but the current form, built around an eight-hour human health metric, does not map neatly to vegetation outcomes that depend on seasonal accumulation.

Internationally, Europe often reports AOT40 exceedance while the United States leans on W126, and scientific bodies favor flux based vegetation metrics that track uptake. That split underscores why an ecosystem specific exposure metric remains important in any standard that intends to protect living landscapes.

Ozone pollution and tree survival

No single metric captures every pathway to damage, and W126 emphasizes summertime peaks that matter a great deal for crops. The longer growing seasons of evergreen conifers can raise cumulative uptake, which may help explain why western forests emerge as more sensitive in the new maps.

Uncertainty also comes from interpolation in places with sparse rural monitors, from wildfire smoke chemistry, and from how drought changes stomatal behavior. Even so, the thresholds offer a practical yardstick that can be updated as monitoring improves and as exposure models evolve.

The broader science keeps moving, including new Earth system model schemes that better represent ozone injury to photosynthesis and water use.

Better process representation should make regional carbon and climate projections more realistic, and helps translate exposure reductions into real ecological gains.

The study is published in Journal of Geophysical Research: Atmospheres.

—–

Like what you read? Subscribe to our newsletter for engaging articles, exclusive content, and the latest updates. 

Check us out on EarthSnap, a free app brought to you by Eric Ralls and Earth.com.

—–