Ozone Sensitive Plant Species on National Park 

Service and U.S. Fish and Wildlife Service Lands: 

Results of a June 24-25, 2003 Workshop

Baltimore, Maryland 

National Park Service Air Resources Division 

U.S. Fish and Wildlife Service Air Quality Branch

November 2003 
Ozone Sensitive Plant Species on National Park 

Service and U.S. Fish and Wildlife Service Lands: 

Results of a June 24-25, 2003 Workshop

Baltimore, Maryland

Prepared by:

Ellen Porter, Air Resources Division, National Park Service

U.S. Department of the Interior

National Park Service Air Resources Division, Denver, Colorado

U.S. Fish and Wildlife Service Air Quality Branch, Denver, Colorado

November 2003

NPS D1522

Natural Resource Report NPS/NRARD/NRR-2003/01

Acknowledgements:

Drs. Art Chappelka, Howie Neufeld, Donald Davis, Robert Kohut and Pat Temple 

provided scientific expertise at the Baltimore Workshop. Dr. Gretchen Smith, Jim 

Renfro, Dr. David Peterson, Ed Jepsen, Dr. John Skelly, and Dr. William Manning 

provided additional scientific expertise and peer review. The author wishes to thank 

Tonnie Maniero, Tamara Blett, and Kristi Morris for helpful editing comments.

This report is available at: www2.nature.nps.gov/ard/pubs/index.htm

Cover photos by Dr. Donald Davis

Contents

Summary..................................................................................................1

Background ..............................................................................................1

Workshop Goals and Results.....................................................................3

Appendix A: Baltimore Workshop Participants..........................................5

Appendix B: Additional Reviewers ...........................................................7

Appendix C: Lists of Sensitive, Bioindicator, and Suspect Species.............9

Sensitive Species ................................................................................ 13

Bioindicator Species ........................................................................... 16

Suspect Species .................................................................................. 19

References Cited.................................................................................... 21

1

Summary

The National Park Service (NPS) convened a workshop in June 2003 to review and 

update lists used by the NPS and the U.S. Fish and Wildlife Service (FWS) to identify

ozone sensitive plant species. At the workshop, ozone effects scientists made 

recommendations for the lists; subsequent to the workshop, an additional group of 

scientists provided review and comments to the lists. The results are presented in the 

following report.

Background

The NPS and FWS, both Department of Interior agencies, are responsible for identifying 

and protecting air quality related values (AQRVs) on their lands. AQRVs include 

resources that may be affected by air pollution and include vegetation, wildlife, water, 

soils, and visibility. One of the most widespread air pollutants is ozone, which harms 

vegetation as well as human health (USEPA 1996). Ozone is not emitted directly from 

smokestacks or vehicles, but is formed when other pollutants, primarily nitrogen oxides 

and volatile organic compounds, react in the atmosphere in the presence of sunlight, 

usually during the warm summer months (USEPA 1996). Ozone causes considerable 

damage to vegetation throughout the world, including agricultural crops and native plants 

in natural ecosystems (USEPA 1996). The Environmental Protection Agency (EPA) has 

established an ozone standard to protect human health; however, EPA has not set a 

standard to protect vegetation and there is much evidence to suggest that the human 

health-based standard is not protective of sensitive vegetation (Heck and Cowling 1997).

Ozone enters plants through leaf stomata and oxidizes plant tissue, causing changes in 

biochemical and physiological processes. Both visible foliar injury (e.g., stipple and 

chlorosis) and growth effects (e.g., premature leaf loss, reduced photosynthesis, and 

reduced leaf, root, and total dry weights) can occur in sensitive plant species (Skelly 

2000). In a natural ecosystem, many other factors can ameliorate or magnify the extent 

of ozone injury at various times and places such as soil moisture, presence of other air 

pollutants, insects or diseases, and other environmental stresses. Ozone effects on natural 

vegetation have been documented throughout the country, particularly in many areas of 

the eastern U.S. and in California. A relatively small number of national parks and 

national wildlife refuges have been surveyed for ozone injury. As a result, injury has 

been documented in Great Smoky Mountains, Shenandoah, Lassen Volcanic, 

Sequoia/Kings Canyon, and Yosemite National Parks (NPS 2000a; NPS 2002), and 

Edwin B. Forsythe, Cape Romain, Moosehorn, Seney, and Mingo National Wildlife 

Refuges (Davis 2001; Davis 2003a; Davis 2003b; Davis 2003c; Davis 2003d).
2

Workshop Goals and Results

In the 1990s, NPS and FWS developed lists of ozone sensitive plant species for many 

parks and refuges and in 2000 published these lists in the Federal Land Managers’ Air 

Quality Related Values Workgroup (FLAG) report (NPS 2000b). As new information 

has become available, NPS and FWS recognized a need to review, revise, and update the 

lists of sensitive plant species. In June 2003, NPS sponsored a workshop in Baltimore, 

Maryland, for this purpose. Workshop participants (Appendix A) had extensive 

experience in ozone field surveys and ozone fumigation experiments conducted with 

species that occur on NPS and FWS lands. Participants reviewed the existing NPS and 

FWS lists of sensitive species and, using expert judgment based on knowledge of 

scientific literature, fumigation experiments, and field experience, revised and updated 

the lists, sorting ozone-sensitive species into two lists:

Sensitive species: species that typically exhibit foliar injury at or near ambient ozone 

concentrations in fumigation chambers and/or are species for which ozone foliar injury 

symptoms in the field have been documented by more than one expert observer.

Bioindicator species (subset of sensitive species): sensitive species that exhibit foliar 

symptoms in the field at ambient ozone concentrations that can be easily recognized as 

ozone injury by subject matter experts; species whose ozone sensitivity has been 

confirmed at realistic ozone concentrations in exposure chambers; species that are widely 

distributed regionally; and, species that are easily identified in the field.

Both lists are limited in number of species because, to date, relatively few species from 

natural ecosystems have been fumigated in chambers or examined in the field for ozone 

symptoms. Because of these limitations, the lists also include a small number of exotic 

species, e.g., Ailanthus altissima. Although A. altissima is undesirable in parks and 

refuges, it serves as an indicator of possible ozone injury to native species and can 

therefore be useful to resource managers. Continuing fumigations and field surveys will 

likely identify many other ozone sensitive species and the lists will be revised 

accordingly. In addition, the lists include only species that display foliar injury in 

response to ozone. Other species may be sensitive to ozone, but respond with growth and 

reproduction effects, which are very difficult to document under field conditions.

The list of sensitive species will enable parks and refuges to identify their AQRVs that 

are sensitive to ozone. Bioindicator species, because of the attributes described above, 

are recommended for use in field surveys in parks and refuges where the risk for ozone 

injury to plants is considered moderate or high. The workshop participants cautioned that 

field diagnoses of injury on any plants, including bioindicator plants, should only be 

attempted by trained observers because other agents, including insects and disease, also 

produce foliar symptoms.
3

A third list was developed for species suspected to be sensitive to ozone :

Suspect species: for suspect species, there is some evidence of sensitivity; however, 

suspect species do not meet the criteria listed above for bioindicator or sensitive species. 

For example, suspect species may have shown foliar injury in fumigations at

unrealistically high ozone concentrations or in very limited field observations, or they 

may be species for which evidence from different observers is conflicting. Suspect 

species should not be included on park or refuge lists of ozone sensitive species. 

However, researchers may want to consider suspect species as potential candidates for 

future investigation.

 

After the Baltimore meeting, the three lists were distributed for review by an additional 

group of ozone researchers (Appendix B). Their comments were incorporated into the 

three final lists for bioindicator, sensitive, and suspect species (Appendix C). The NPS is 

using the lists of sensitive species (which includes bioindicator species) in a risk 

assessment for approximately 270 parks in the NPS Inventory and Monitoring Program. 

Risk assessments for individual parks consider 1) presence of sensitive species, 2) ozone 

concentrations and cumulative doses, and 3) soil moisture status (as indicated by the 

Palmer Z Index). The results from these assessments will assist park managers in 

determining the need for future ozone monitoring and vegetation surveys.
4
5

Appendix A: Baltimore Workshop Participants

Dr. Art Chappelka

School of Forestry and Wildlife Sciences

Auburn University

Auburn, AL 36849-5418

chappah@auburn.edu

Dr. Howie Neufeld

Department of Biology

572 Rivers St.

Appalachian State University

Boone, NC 28608

neufeld@appstate.edu

Dr. Donald Davis

211 Buckhout Lab

Penn State University

University Park, PA 16802

ddd2@psu.edu

Dr. Robert Kohut

Boyce Thompson Institute

Cornell University

Ithaca, NY 14853

rjk9@cornell.edu

Dr. Patrick Temple

USDA Forest Service

PSW Research Station

4955 Canyon Crest Drive

Riverside, CA 92507

ptemple@fs.fed.us
6
7

Appendix B: Additional Reviewers

Dr. Gretchen Smith

University of Massachusetts

Department of Natural Resources Conservation

160 Holdsworth Way

Amherst, MA 01003-4210

gcsmith@forwild.umass.edu

Jim Renfro

Great Smoky Mountains National Park

1314 Cherokee Orchard Road

Gatlinburg, TN 37738

jim_renfro@nps.gov

Dr. David Peterson

USDA Forest Service

PNW Research Station

400 N. 34th Street, Suite 201

Seattle, WA 98103

peterson@fs.fed.us

Ed Jepsen

Wisconsin Department of Natural Resources

Bureau of Air Management

101 S. Webster Street, PO Box 7921

Madison, WI 53707

jepsee@dnr.state.wi.us

Dr. John Skelly

Department of Plant Pathology

108 Buckhout Laboratory

Pennsylvania State University

University Park, PA 16803

jms34@psu.edu

Dr. William Manning

Department of Microbiology

University of Massachusetts

203 Morrill Science Center IVN

639 North Pleasant Street

Amherst, MA 01003-9298

wmanning@microbio.umass.edu
8
9

Appendix C: Lists of Sensitive, Bioindicator, and Suspect Species
10
11

PLANT SPECIES IN NPS/FWS UNITS SENSITIVE* TO OZONE 

*Species considered “sensitive” are those that typically exhibit foliar injury at or near 

ambient ozone concentrations in fumigation chambers and/or are species for which ozone 

foliar injury symptoms in the field have been documented by more than one observer.

Scientific Name Common Name Category

Aesculus octandra Yellow buckeye Sensitive

Ailanthus altissima Tree-of-heaven Sensitive 

Alnus rubra Red alder Sensitive 

Alnus rugosa Speckled alder Sensitive

Amelanchier alnifolia Saskatoon serviceberry Sensitive 

Apios americana Groundnut Sensitive

Apocynum androsaemifolium Spreading dogbane Sensitive 

Apocynum cannabinum Dogbane, Indian hemp Sensitive

Artemisia douglasiana Mugwort Sensitive

Artemisia ludoviciana Silver wormwood Sensitive

Asclepias exaltata Tall milkweed Sensitive 

Asclepias incarnata Swamp milkweed Sensitive

Asclepias syriaca Common milkweed Sensitive 

Aster acuminatus Whorled aster Sensitive 

Aster macrophyllus Big-leaf aster Sensitive 

Cercis canadensis Redbud Sensitive 

Clematis virginiana Virgin’s bower Sensitive

Corylus americana American hazelnut Sensitive

Eupatorium rugosum White snakeroot Sensitive

Fraxinus americana White ash Sensitive 

Fraxinus pennsylvanica Green ash Sensitive

Gaylussacia baccata Black huckleberry Sensitive

Krigia montana Mountain dandelion Sensitive

Liquidambar styraciflua Sweetgum Sensitive

Liriodendron tulipifera Yellow-poplar Sensitive 

Lyonia ligustrina Maleberry Sensitive

Oenothera elata Evening primrose Sensitive 

Parthenocissus quinquefolia Virginia creeper Sensitive

Philadelphus coronarius Sweet mock orange Sensitive

Physocarpus capitatus Ninebark Sensitive 
12

Physocarpus malvaceus Pacific ninebark Sensitive

Pinus banksiana Jack pine Sensitive

Pinus jeffreyi** Jeffrey pine Sensitive 

Pinus ponderosa*** Ponderosa pine Sensitive 

Pinus pungens Table-mountain pine Sensitive

Pinus radiata Monterey pine Sensitive

Pinus rigida Pitch pine Sensitive

Pinus taeda Loblolly pine Sensitive

Pinus virginiana Virginia pine Sensitive

Platanus occidentalis American sycamore Sensitive 

Populus tremuloides Quaking aspen Sensitive 

Prunus serotina Black cherry Sensitive 

Prunus virginiana Choke cherry Sensitive

Quercus kelloggii California black oak Sensitive

Robinia pseudoacacia Black locust Sensitive

Rhus copallina Winged sumac Sensitive

Rhus trilobata Skunkbush Sensitive 

Rubus allegheniensis Allegheny blackberry Sensitive

Rubus canadensis Thornless blackberry Sensitive

Rubus cuneifolius Sand blackberry Sensitive 

Rubus parviflorus Thimbleberry Sensitive

Rudbeckia laciniata Cutleaf coneflower Sensitive 

Salix gooddingii Gooding’s willow Sensitive

Salix scouleriana Scouler’s willow Sensitive

Sambucus canadensis American elder Sensitive 

Sambucus mexicana Blue elderberry Sensitive 

Sambucus racemosa Red elderberry Sensitive

Sapium sebiferum Chinese tallowtree Sensitive 

Sassafras albidum Sassafras Sensitive

Solidago altissima Goldenrod Sensitive 

Spartina alterniflora Smooth cordgrass Sensitive

Symphoricarpos albus Common snowberry Sensitive 

Vaccinium membranaceum Huckleberry Sensitive

Verbesina occidentalis Crownbeard Sensitive 

Vitis labrusca Northern fox grape Sensitive 
13

Vitis vinifera European wine grape Sensitive 

** P. jeffreyi and P. ponderosa may hybridize, making identification difficult.

*** P. ponderosa var. ponderosa is the more sensitive variety; P. ponderosa var. 

scopulorum is not as sensitive.

Updated: 27 October 2003
14
15

PLANT SPECIES IN NPS/FWS UNITS CONSIDERED GOOD FIELD 

BIOINDICATORS* FOR OZONE INJURY SURVEYS

*Bioindicator species for ozone injury meet all or most of the following criteria:

- species exhibit foliar symptoms in the field at ambient ozone concentrations that can be 

easily recognized as ozone injury by subject matter experts 

- species ozone sensitivity has been confirmed at realistic ozone concentrations in exposure 

chambers

- species are widely distributed regionally

- species are easily identified in the field

Scientific Name Common Name Category

Ailanthus altissima Tree-of-heaven Bioindicator

Alnus rubra Red alder Bioindicator

Alnus rugosa Speckled alder Bioindicator

Apios americana Groundnut Bioindicator

Apocynum androsaemifolium Spreading dogbane Bioindicator

Artemisia douglasiana Mugwort Bioindicator

Artemisia ludoviciana Silver wormwood Bioindicator

Asclepias exaltata Tall milkweed Bioindicator

Asclepias syriaca Common milkweed Bioindicator

Aster acuminatus Whorled aster Bioindicator

Aster macrophyllus Big-leaf aster Bioindicator

Cercis canadensis Redbud Bioindicator

Corylus americana American hazelnut Bioindicator

Eupatorium rugosum White snakeroot Bioindicator

Fraxinus americana White ash Bioindicator

Gaylussacia baccata Black huckleberry Bioindicator

Liriodendron tulipifera Yellow-poplar Bioindicator

Lyonia ligustrina Maleberry Bioindicator

Oenothera elata Evening primrose Bioindicator

Physocarpus capitatus Ninebark Bioindicator 

Physocarpus malvaceum Pacific ninebark Bioindicator

Pinus jeffreyi** Jeffrey pine Bioindicator

Pinus ponderosa*** Ponderosa pine Bioindicator

Platanus occidentalis American sycamore Bioindicator

Populus tremuloides Quaking aspen Bioindicator

Prunus serotina Black cherry Bioindicator
16

Rhus trilobata Skunkbush Bioindicator

Rubus allegheniensis Allegheny blackberry Bioindicator

Rubus canadensis Thornless blackberry Bioindicator

Rudbeckia laciniata Cutleaf coneflower Bioindicator

Salix scouleriana Scouler’s willow Bioindicator

Sambucus canadensis American elder Bioindicator

Sambucus mexicana Blue elderberry Bioindicator

Sambucus racemosa Red elderberry Bioindicator

Sapium sebiferum Chinese tallowtree Bioindicator 

Symphoricarpos albus Common snowberry Bioindicator

Vaccinium membranaceum  Huckleberry Bioindicator

Verbesina occidentalis Crownbeard Bioindicator 

Vitis labrusca Northern fox grape Bioindicator

Vitis vinifera European wine grape Bioindicator

**  Pinus jeffreyi and P. ponderosa may hybridize, making identification difficult.

*** P. ponderosa var. ponderosa is the more sensitive variety; P. ponderosa var. 

scopulorum is not as sensitive.

****Sensitivity of Vaccinium membranaceum has been demonstrated in chambers, but 

not in the field, possibly because of low ozone levels throughout its range. 

Updated: 27 October 2003
17

PLANT SPECIES IN NPS/FWS UNITS SUSPECTED* OF BEING SENSITIVE

TO OZONE

*Species suspected of being sensitive to ozone are species for which there is some 

evidence of sensitivity, but species does not meet criteria for sensitive species (i.e., 

sensitive species typically exhibit foliar injury at or near ambient ozone concentrations in 

fumigation chambers and/or are species for which ozone foliar injury symptoms in the 

field have been documented by more than one observer). Suspect species are also those 

species for which evidence from different observers is conflicting. NOTE: THIS LIST 

SHOULD NOT BE USED TO IDENTIFY OZONE-SENSITIVE SPECIES IN PARKS. 

IT CAN BE USED TO IDENTIFY SPECIES OF INTEREST FOR FUTURE 

RESEARCH.

Scientific Name Common Name Category

Acer macrophyllum Bigleaf maple Suspect

Acer negundo Boxelder Suspect

Acer rubrum Red maple Suspect

Acer saccharinum Silver maple Suspect

Aconitum uncinatum Wild monkshood Suspect (GRSM**)

Aesculus glabra Ohio buckeye Suspect

Amelanchier laevis Allegheny serviceberry Suspect (GRSM**)

Anemone quinquefolia Wood anemone Suspect (GRSM**)

Anemone virginiana Thimbleweed Suspect (GRSM**)

Aristolochia durior Dutchman’s pipe Suspect (GRSM**)

Asclepias quadrifolia Four-leaved milkweed Suspect

Aster curtisii Curtis’s aster Suspect (GRSM**)

Aster divaricatus White-wood aster Suspect (GRSM**)

Aster engelmannii Engelmann’s aster Suspect

Aster infirmus Entire-leaved aster Suspect (GRSM**)

Aster puniceus Purple-stemmed aster Suspect

Aster umbellatus Flat-toppped aster Suspect

Betula alleghaniensis Yellow birch Suspect

Betula populifolia Gray birch Suspect

Bidens frondosa Beggar-ticks Suspect (GRSM**)

Bromus tectorum Cheatgrass Suspect

Calycanthus floridus Sweet-shrub Suspect (GRSM**)

Campsis radicans Trumpet creeper Suspect

Cephalanthus occidentalis Buttonbrush Suspect (GRSM**)
18

Cladrastis lutea Yellowwood Suspect

Clitoria mariana Butterfly pea Suspect (GRSM**)

Cornus florida Flowering dogwood Suspect 

Diervilla sessilifolia Bush-honeysuckle Suspect (GRSM**)

Gaylussacia ursina Bear huckleberry Suspect (GRSM**)

Gentiana amarella Northern gentian Suspect

Geum radiatum Mountain avens Suspect (GRSM**)

Gillenia trifoliata Bowman’s-root Suspect (GRSM**)

Glyceria nubigena Manna grass Suspect

Hamamelis virginiana Witch-hazel Suspect (GRSM**)

Helianthus divaricatus Woodland sunflower Suspect

Helianthus glaucophyllus White-leaf sunflower Suspect (GRSM**)

Helianthus microcephalus Small wood sunflower Suspect (GRSM**)

Helianthus strumosus Paleleaf woodland sunflower Suspect

Hexastylis arifolia Heartleaf Suspect (GRSM**)

Hieracium paniculatum Panicled hawkweed Suspect (GRSM**)

Impatiens capensis Spotted touch-me-not Suspect (GRSM**)

Larix decidua European larch Suspect

Larix leptolepis Japanese larch Suspect

Lindera benzoin Spicebush Suspect 

Menziesia pilosa Minnie-bush Suspect (GRSM**)

Oxydendrum arboreum Sourwood Suspect (GRSM**)

Pinus nigra Austrian pine Suspect

Pinus strobus Eastern white pine Suspect

Prenanthes altissima Rattlesnake root Suspect (GRSM**)

Prunus americana Wild plum Suspect

Rhamnus alnifolia Alder-leaved buckthorn Suspect (GRSM**)

Rhododendron bakeri Cumberland azalea Suspect (GRSM**)

Rhododendron calendulaceum Flame azalea Suspect (GRSM**)

Rhododendron catawbiense Catawba rhododendron Suspect (GRSM**)

Rhus glabra Smooth sumac Suspect

Rhus typhina Staghorn sumac Suspect

Rubus argutus Highbush blackberry Suspect

Rubus idaeus Red raspberry Suspect

Rudbeckia hirta Black-eyed susan Suspect
19

Rugelia nudicaulis Rugel’s ragwort Suspect

Saxifraga arguta Saxifrage Suspect

Senecio serra Tall butterweed Suspect

Silphium asteriscus Rosin-weed Suspect (GRSM**)

Smilax glauca Glaucous catbrier Suspect (GRSM**)

Smilax rotundifolia Greenbrier Suspect (GRSM**)

Solidago roanensis Roan’s goldenrod Suspect (GRSM**)

Spiraea x vanhouttei Vanhoutte spirea Suspect

Stachys clingmanii Clingman’s hedge-nettle Suspect (GRSM**)

Stewartia ovata Mountain stewartia Suspect (GRSM**)

Syringa x chinensis Chinese lilac Suspect

Syringa vulgaris Common lilac Suspect

Tilia americana American basswood Suspect

Tilia euchlora Crimean linden Suspect

Tilia heterophylla White basswood Suspect (GRSM**)

Tilia platyphyllos Bigleaf linden Suspect

Toxicodendron radicans Poison-ivy Suspect

Trautvetteria caroliniensis Tassel-rue; False bugbane Suspect (GRSM**)

Vitis aestivalis Summer grape Suspect (GRSM**)

Vitis riparia Riverbank grape Suspect

Vitis vulpina Frost grape Suspect (GRSM**)

**GRSM – injury noted in Great Smoky Mountains NP.

Updated: 27 October 2003
20
21

References Cited

Davis, D.D. 2001. Evaluation of Ozone Injury on Vegetation in the Mingo National 

Wildlife Refuge, Missouri, 2000 Observations. Submitted to U.S. Fish and Wildlife 

Service, Air Quality Branch, Denver, Colorado. 41 pp.

Davis, D.D. 2003a. Evaluation of Ambient Ozone Injury on the Foliage of Vegetation in 

the Cape Romain National Wildlife Refuge, South Carolina, 2002 Observations. 

Submitted to U.S. Fish and Wildlife Service, Air Quality Branch, Denver, Colorado. 

33 pp.

Davis, D.D. 2003b. Evaluation of Ozone Injury on Vegetation within the Edwin B. 

Forsythe National Wildlife Refuge, New Jersey, 2002 Observations. Submitted to U.S. 

Fish and Wildlife Service, Air Quality Branch, Denver, Colorado. 44 pp.

Davis, D.D. 2003c. Evaluation of Ozone Injury on Vegetation in the Moosehorn National 

Wildlife Refuge, Maine, 2002 Observations. Submitted to U.S. Fish and Wildlife Service, 

Air Quality Branch, Denver, Colorado. 35 pp.

Davis, D.D. 2003d. Evaluation of Ozone Injury on Vegetation in the Seney National 

Wildlife Refuge, Michigan, 2002 Observations. Submitted to U.S. Fish and Wildlife 

Service, Air Quality Branch, Denver, Colorado. 38 pp.

Heck, W.W. and E.B. Cowling. 1997. The need for a long term cumulative secondary 

ozone standard – an ecological perspective. Environmental Management January:23-33.

National Park Service. 2000a. Technical Information in Support of the Department of the 

Interior’s Request for a Rule to Restore and Protect Air Quality Related Values. National 

Park Service Air Resources Division Publication. (18 November 2003); 

http://www2.nature.nps.gov/ard/epa/index.htm).

National Park Service. 2000b. Federal Land Managers' Air Quality Related Values 

Workgroup (FLAG) Phase I Report. (18 November 2003; 

www2.nature.nps.gov/ardnew/psd/flag). 

National Park Service. 2002. Air Quality in the National Parks. Second Edition. National 

Park Service, Air Resources Division, Lakewood, Colorado. Published September 2002, 

NPS Document number D-2266. U.S. Department of the Interior, Washington D.C.

Skelly, J.M. 2000. Tropospheric ozone and its importance to forests and natural plant 

communities of the northeastern United States. Northeastern Naturalist 7:221-236.

U.S. Environmental Protection Agency. 1996. Air Quality Criteria for Ozone and Related 

Photochemical Oxidants. Vol. 2. EPA/600/P-93/004bF. U.S. Environmental Protection 

Agency, Office of Research and Development. Research Triangle Park, North Carolina.