Potential Impact of Ozone on Coniferous Forests of the Interior Southwestern United States

Abstract

Despite the well-documented negative impacts of ozone on the health of coniferous forests in southern California and the significant growth experienced by southwestern cities over the past several decades, the ozone/forest dynamic in the interior portion of the southwestern United States has been largely ignored. Primarily through a review of literature pertaining to most aspects of ozone and its impact on forest health, this article provides insights on the ozone/forest dynamic within coniferous forests of the interior Southwest. It is suggested that ozone absorption in southwestern coniferous forests may equal that in southern California, owing to the long-distance transport of atmospheric pollutants into the interior Southwest and the presence of the North American monsoon. Nevertheless, research gaps identified in this article suggest a need for future research on ozone exposure levels and the ozone sensitivities of conifer species and varieties in southwestern coniferous forests.

Key Words:

• climate
• coniferous forests
• ozone
• southwestern United States

Acknowledgments

I am grateful to the editor and the reviewers for their very thorough comments. In addition, I would like to thank Andrew Comrie, John Kupfer, Michael Solem, and Anne Sutherland for examining various portions of the manuscript. Finally, I am indebted to Katherine Hankins for her constant encouragement, as well as her reviews of many versions of the manuscript.

Notes

Notes: The monitors' notations refer to U.S. EPA site identification numbers, which are as follows:

¹060710005442011,

²040133002442011,

³040132005442011,

⁴040139701442011,

⁵040190021442011,

⁶320030073442011,

⁷040058001442011, and

⁸350011012442011.

1. Forest health is defined in a pathological sense as the incidence of biotic and abiotic factors affecting a population of trees within a forest ecosystem (Ferretti 1997).

2. National wilderness areas and national parks were afforded the highest degree of protection from air pollution-caused deterioration in future years by the Clean Air Act Amendments of 1977. Federal land managers in these Class I air-quality areas are authorized to protect air-quality-related values. Therefore, the land managers advise the state or federal government if emissions from a proposed project may adversely impact a wilderness area (Bunyak 1993).

3. The climate data pertain to the Palisade Ranger Station near Tucson, Arizona, and they were obtained from the Western Regional Climate Center, as noted in Table 1.

4. Population estimates for relevant MSAs were obtained from the U.S. Census Bureau, as noted in Table 1.

5. Estimates of daily vehicle miles traveled for relevant MSAs were obtained from the U.S. Environmental Protection Agency, as noted in Table 1.

6. Ozone exposure indices are used to weight ozone concentrations, thereby providing a more realistic measure of the severity of the exposure of conifers to ambient ozone. Two indices, SUM60 and SUMW126, are employed in this study. SUM60 is the sum of all hourly concentrations greater than or equal to 60 parts per billion (ppb), while SUMW126 is the sum of sigmoidally weighted hourly ozone concentrations; less weight is given to lower ozone concentrations. Hourly ozone concentrations were weighted using the following equation:

where M and A were assigned values of 4403 and 126 ppm⁻¹, respectively. W _i is the weighting factor for concentration i, and C _i is the concentration of i (Lefohn and Runeckles 1987).

7. Estimates of anthropogenic emissions of VOCs and NO_x for relevant MSAs were obtained from the U.S. Environmental Protection Agency, as noted in Table 1.

8. Ozone-exposure levels in coniferous forests near Albuquerque, Las Vegas, and Tucson are estimated using the exposure levels at urban-fringe sites situated at or near the bases of forested mountains, which include the Sandia Mountains, Spring Mountains, and Rincon Mountains, respectively. Multiplying the Mt. Ord to eastern Phoenix ratio of average daily and daytime ozone exposures—which are 1.4 (daytime SUMW126), 2.1 (daily SUMW126), 1.5 (daytime SUM60), and 2.0 (daily SUM60)—by the urban-fringe exposure levels provide rough estimates of exposure levels in these three mountain ranges. The accuracies of the estimated ozone exposure levels are unknown; thus, the values should be treated with much less validity than observed values.