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Sensitive tree species remain at risk despite improved air quality benefits to US forests

Nature Sustainability volume 6, pages 1607–1619 (2023)Cite this article

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Abstract

Atmospheric nitrogen (N) and sulfur (S) deposition can significantly affect forest biodiversity and production by altering the growth and survival of trees. Three decades of air quality regulations in the United States have led to large reductions in oxides of N (44–81%) and S (50–99%) emissions and associated deposition. Here we evaluated the magnitude and extent of effects over 20 years from atmospheric N and S deposition on the growth and survival of 94 tree species—representing 96.4 billion trees and an average of 88% of forest basal area across the contiguous United States (CONUS). Overall, species’ growth and survival rates have responded positively to declining deposition, but we find that decreases of at least 2.5 kg ha−1 yr−1 N are needed across 19.8% (growth) and 59.5% (survival) of the CONUS to prevent detrimental effects to sensitive species. Reduced forms of N (NHx = NH3 + NH4+) are now the dominant form of N deposition in 45.4% of the CONUS—notably in agricultural regions—and exclusively need to be reduced by ≥5.0 kg ha−1 yr−1 N in some areas. Further S deposition decreases of ≥1.0 kg ha−1 yr−1 S are needed in 50.4% (growth) and 56.2% (survival) of the CONUS to protect sensitive species and, notably, evergreen trees. Total basal area is increasing in much of the country (85.2%) because of N fertilizing effects, but these growth increases could result in biodiversity loss. Our findings can be used to evaluate past successes of air quality policies and the future benefits of air pollution reductions to terrestrial ecosystems.

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Fig. 1: Annual trends of average N and S deposition and the fifth percentile effects on species’ growth rates and survival probability.
Fig. 2: Fifth percentile species-level effect from N and S deposition on the growth and survival rates of tree species between 2000 and 2019.
Fig. 3: Recent three-year average (2017–2019) N and S deposition effects on the fifth percentile of all species nationally.
Fig. 4: Deposition changes needed from 2017–2019 levels to prevent detrimental effects for sensitive species across states.
Fig. 5: The oxidized and reduced N deposition levels at FIA plots compared with deposition changes needed to protect 95% of trees.
Fig. 6: The total basal area impacts across the CONUS.

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Data availability

The metadata for datasets generated in this study are available in a data repository (https://doi.org/10.23719/1529199). The repository contains contact information to obtain all of the raw data, processed summary data and raster data. The tree species parameters that were analysed during this study are included in ref. 12 (and its supplementary information files). The continuous grids of tree species that were analysed during this study are included in the ref. 42 dataset. The 2000–2019 air pollution raster surfaces (v. 2018.2) that were used during this study are publicly available from the National Atmospheric Deposition Program53 (https://nadp.slh.wisc.edu/committees/tdep/). The FIA dataset is available from the US Forest (https://www.fs.usda.gov/research/products/dataandtools/forestinventorydata).

Code availability

Analyses were performed in both Python and R. Relevant scripts and README files are available on a Github repository (https://github.com/Justin-Coughlin/air_pollution_effects_trees).

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Acknowledgements

We thank J. Lynch, J. T. Smith and J. Miller for commenting on earlier versions of this manuscript and the National Atmospheric Deposition Program’s Critical Loads of Atmospheric Deposition Committee, which has provided useful insight on this research. We thank K. Horn, J. Phelan and R. Dalton for providing data and/or minor feedback on results. We thank J. James for initial script development assistance and T. Wilson for his helpful advice on the use of the US Forest Service’s continuous live-tree basal area dataset. This work was supported with funding from the US Environmental Protection Agency under the Air Climate and Energy National Program within the Office of Research and Development. This research was originally supported by the US Geological Survey’s John Wesley Powell Center for Earth System Analysis and Synthesis (forecasting forest response to N deposition: integrating data from individual plant responses to soil chemistry with a continental-scale gradient analysis, 2013). The funders had no role in study design, data collection and analysis, decision to publish or preparation of the manuscript. The views expressed in this manuscript are those of the authors and do not necessarily reflect the views or policies of the US Environmental Protection Agency or the US Forest Service.

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Authors and Affiliations

  1. Air and Radiation Division, US Environmental Protection Agency Region 5, Chicago, IL, USA

    Justin G. Coughlin

  2. Office of Research and Development, US Environmental Protection Agency, Washington, DC, USA

    Christopher M. Clark & Robert D. Sabo

  3. Northern Research Station, USDA Forest Service, Burlington, VT, USA

    Linda H. Pardo

  4. Southern Region, USDA Forest Service, Asheville, NC, USA

    Jeremy D. Ash

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  1. Justin G. Coughlin
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Contributions

J.G.C., C.M.C. and R.D.S. designed the research. J.G.C. analysed the data. J.G.C., C.M.C., R.D.S., L.H.P. and J.D.A. wrote the manuscript.

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Correspondence to Justin G. Coughlin or Christopher M. Clark.

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Nature Sustainability thanks Samuel Simkin, Carly Stevens and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

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Extended data

Extended Data Fig. 1 Kolmogorov-Smirnov (K-S) test plots comparing endpoint effects across phenotypes in 2000 and 2019.

Cumulative density function (CDF) plots showing the effect for N-influenced growth rates in 2000 (a), N-influenced growth rates in 2019 (b), N-influenced survival rates in 2000 (c), N-influenced survival rates in 2019 (d), S-influenced growth rates in 2000 (e), S-influenced growth rates in 2019 (f), S-influenced survival rates in 2000 (g), and S-influenced survival rates in 2019 (h). Deciduous (blue) and evergreen (green) trees are differentiated. A two-sided K-S test was used on data within the 95% confidence interval of the sample size. The K-S statistic and corresponding p-value are also shown.

Extended Data Fig. 2 Histograms of 2000 and 2019 endpoint effects by wood products.

Ridgeline histogram plots of N-influenced growth rates (a), N-influenced survival rates (b), S-influenced growth rates (c), and S-influenced survival rates (d). The upper panels display distributions of effects from deposition in 2000 and lower panels are effects from deposition in 2019. Different wood product uses are shown where B is strictly building materials, UP is unfinished wood products, UP+B is unfinished wood products and building materials, and Neither is neither of those types of wood products44.

Extended Data Fig. 3 ΔDg5 and ΔDs1 values across the CONUS for N and S deposition.

Maps of ΔNg5 (a), ΔNs1 (b), ΔSg5 (c) and ΔSs1 (d) values (kg ha-1 yr-1 N or S) across the CONUS compared to 2017–2019 average N and S deposition. State boundaries generated by the US Census Bureau62. Species’ surfaces modified from ref. 42.

Extended Data Fig. 4 The weighted basal area effects (a-d) and the total basal area impact (e-h due to 2017–2019 average N and S deposition.

The basal area weighted effect (%, BAP) due to 2017–2019 average N or S deposition in each 250-m grid cell for N-influenced growth (a), N-influenced survival (b), S-influenced growth (c), and S-influenced survival rates (d). The total basal area impact using BAT (m2 ha-1) during the same time period are also shown for N-influenced growth (e), N-influenced survival (f), S-influenced growth (g), and S-influenced survival rates (h). State boundaries generated by the US Census Bureau62. Species’ surfaces modified from ref. 42.

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Figs. 1–23, Tables 1–3, results and uncertainty evaluations.

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Coughlin, J.G., Clark, C.M., Pardo, L.H. et al. Sensitive tree species remain at risk despite improved air quality benefits to US forests. Nat Sustain 6, 1607–1619 (2023). https://doi.org/10.1038/s41893-023-01203-8

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