Air quality improvements are needed to protect sensitive U.S. tree species

Over the last twenty years, policies in the United States have been effective at reducing air pollution which has positively benefitted forests. Although forests are now better protected, sensitive tree species still experience harmful impacts - demonstrating further needs to reduce air pollution.
Air quality improvements are needed to protect sensitive U.S. tree species

Air quality policies

Forests and tree species can be harmfully impacted by air pollution. For example, atmospheric nitrogen and sulfur that falls back to the ground can acidify soils and alter forest populations. In the 1980s, acid rain became one of the most important environmental issues facing U.S. ecosystems. In the northeast, there were large tree die-offs in areas downwind from coal-fired power plants, raising national awareness on the issue.

A series of regulations resulted and were aimed at lowering the power plant emissions that were directly contributing to the acidic conditions. The Clean Air Act Amendments of 1990 did this by targeting emissions of nitrogen and sulfur oxides in multiple phases. Subsequently, other regulations or rules such as the formerly applicable Clean Air Interstate Rule (CAIR), the Mercury and Air Toxics Standards (MATS), the Cross-State Air Pollution Rule (CSAPR), and the SO2 Data Requirements Rule (DRR) further reduced power plant emissions. These policies and more stringent vehicle emissions standards have largely led to better air quality across the country.

The damage of insects and acid rain precipitation on spruce-fir forests in North Carolina, USA along the Appalachian Mountain Range.
Photo credit: Kevin Simpson

Now that thirty years have passed since the first of these air quality policies, it is worth evaluating how forests have reacted to these changing conditions. In our paper, we assessed health indicators (growth and survival) for ninety-four common tree species to understand forest responses to decreasing air pollution and whether these declines have been large enough. This investigation is timely as the U.S. Environmental Protection Agency is currently deciding whether to strengthen air quality standards for ecosystems or welfare-based standards.

Positive forest health trends but risks remain

Many tree species have responded positively to declining nitrogen and sulfur deposition. While nitrogen can act as a nutrient that causes increased growth, too much of any one thing can be harmful. When there is excess nitrogen, opportunistic or fast-growing species can proliferate, leading to potential shading and losses of nitrogen-sensitive species. On the other hand, sulfur is less complicated and generally leads to decreased growth and increased mortality.

We found that the growth and survival rates of trees have increased by as much as +47% since 2000. This is an incredibly positive result that stems directly from the ability to control nitrogen and sulfur emissions at the source - demonstrating how effective air quality policies can be for protecting terrestrial ecosystems.

The trends of the nitrogen and sulfur deposition between 2000 to 2019. The growth and survival rates of tree species are also shown over this timeframe.
The average deposition trends and tree species' effects across the contiguous U.S. since 2000 for (a) nitrogen impacts to growth, (b) nitrogen impacts to survival, (c) sulfur impacts to growth, and (d) sulfur impacts to survival. As nitrogen and sulfur deposition have decreased, responses have generally been positive. Relevant air quality rules and regulations are also shown using shading (e.g., CSAPR). Colored bands around the trends are the standard deviation.

However, we also found that sensitive species are still harmed at rates that could be considered adverse for forests. In this area of research, there are no well-established levels of effect that the community views as acceptable. We selected 1% declines in survival and 5% declines in growth as thresholds to protect against. This translates to roughly 5% fewer trees and 21% less aboveground carbon storage over 100 years - a conservation target I felt was worth preventing.

We also had to select how much of the forest to protect. Some researchers may want to protect all species, while others may be interested in only protecting 80% of the forest; we chose 95% to focus on sensitive species. We found that large swathes of the U.S. do not meet the health indicator cutoffs for 95% of the forest. This begs the question...what more needs to be done?

Further declines are needed for sensitive species

We can look at the effects happening under real-world conditions, but we can also calculate how much air pollution needs to change to prevent adverse effects. Using this approach, we found that over half of the lower forty-eight states need lower nitrogen deposition so sensitive species do not experience harmful mortality rate declines. Roughly half of the contiguous U.S. also needs modest sulfur deposition decreases to protect sensitive species for both health indicators.

The deposition decreases needed in the contiguous U.S. to prevent (a) a 5% decline in growth because of nitrogen deposition, (b) a 1% decline in survival because of nitrogen deposition, (c) a 5% decline in growth because of sulfur deposition, and (d) a 1% decline in survival because of sulfur deposition. State boundaries generated by the US Census Bureau. Species’ surfaces modified from Wilson et al. (2013).


Our results have important implications for policymaking. We provide a new approach for turning the knobs in ecological risk assessments. It has historically been difficult to target a level of pollution given the complexities of the natural world and because air quality standards are based on concentrations in the air, rather than deposition. One complication with this approach is that relationships between concentration and deposition are not uniform across landscapes.

While national consistency is needed for welfare-based standards, our findings demonstrate that a singular target pollution level might not be sufficiently protective across the U.S., even for deposition-based approaches. A strategy without these considerations could lead to forest biodiversity loss as some species become more dominant, resulting in a potential homogenization of the community. We should not miss the trees for the forest in this case - there are cascading consequences in doing so which can have long-lasting impacts.


2019 TIGER/Line Shapefiles (US Census Bureau, 2020).

Wilson, B. T., Lister, A. J., Riemann, R. I. & Griffith, D. M. Live Tree Species Basal Area of the Contiguous United States (2000–2009) (USDA, 2013).


Cover photo courtesy of U.S. EPA. This photo may not be used to infer or imply EPA endorsement of any product, company, or position. The views expressed here are those of the authors and do not necessarily represent the views or policies of the U.S. EPA, U.S. Forest Service, or Sonoma Technology, Inc.

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