United States

Department of 

Agriculture

Forest Service

Northern 

Research Station

General Technical

Report NRS-103

Ozone Injury to Forests 

Across the Northeast and 

North Central United States, 

1994 - 2010

Gretchen C. Smith

Randall S. Morin

George L. McCaskill
Visit our homepage at: http://www.nrs.fs.fed.us/

Published by: For additional copies:

U.S. FOREST SERVICE U.S. Forest Service 

11 CAMPUS BLVD SUITE 200 Publications Distribution

NEWTOWN SQUARE PA 19073 359 Main Road

 Delaware, OH 43015-8640

 Fax: (740)368-0152

August 2012 Email: nrspubs@fs.fed.us

Abstract

Ozone is a highly toxic air contaminant that has been shown to decrease tree growth and 

cause signifi cant disturbance to forested ecosystems. Ozone also causes distinct foliar 

injury symptoms to certain species (bioindicator plants) that can be used to detect and 

monitor ozone stress (biomonitoring) in the forest environment. In the early 1990s, the U.S. 

Forest Service, in partnership with the U.S. Environmental Protection Agency, developed 

and implemented a suite of forest health indicators to respond to emerging demands for a 

comprehensive assessment of the health of U.S. forests. This report focuses on the states 

in the Northern Research Station-Forest Inventory and Analysis region, which has the 

longest record of ozone biomonitoring in the country, from 1994 through 2010. The results 

of 17 years of ozone injury detection provide indisputable evidence that ozone-induced 

foliar injury symptoms occur routinely on ozone-sensitive bioindicator plants across much 

of the forested landscape and in areas previously thought to be relatively ozone free. This 

report provides state-level information on where ozone stress occurs and whether ozone 

stress is increasing or decreasing over time. It also provides state-level estimates of the 

acres of forest land and volume of ozone-susceptible species at risk of ozone impact.

Cover Photos

Ozone injury, clockwise from top left:

 Maple, photo by Robert L. Anderson, U.S. Forest Service, bugwood.org

 Yellow-poplar, photo by U.S. Forest Service, Region 8, bugwood.org

 Black cherry, photo by Tim Tigner, Virginia Department of Forestry, bugwood.org

 Blackberry, photo by Robert L. Anderson, U.S. Forest Service, bugwood.org

Manuscript received for publication 24 February 2012
1

INTRODUCTION

In the early 1990s, the U.S. Forest Service (USFS), in partnership with the U.S. 

Environmental Protection Agency (EPA), developed and implemented a suite 

of forest health indicators to respond to emerging demands for a comprehensive 

assessment of the health of U.S. forests (Anonymous 1995). The ozone (O3) indicator 

was developed by the USFS Forest Health Monitoring (FHM) program to address 

specific concerns about the negative effects of O3 pollution on forest health and 

productivity. Ozone is a highly toxic air contaminant that has been shown to decrease 

tree growth and cause significant disturbance to forested ecosystems (Bytnerowicz 

et al. 2008, Karnosky et al. 2007, Krupa and Legge 1995, Laurence and Anderson 

2003, Percy et al. 2003)1

. Ozone also causes distinct foliar injury symptoms to 

certain species (bioindicator plants) that can be used to detect and monitor O3 stress 

(biomonitoring) in the forest environment (Smith et al. 2003, 2008).

Ozone detection with bioindicator plants does not identify specific levels of O3

present in ambient air but does identify if conditions are favorable for O3 injury 

to occur. Favorable conditions are dependent on plant susceptibility to O3, the 

concentration and duration of exposure to O3, and the external environment in 

which the plant is growing (Kohut 2005, Krupa et al. 2001, Schaub et al. 2003). 

Of the many environmental conditions that influence plant-pollutant interactions, 

soil moisture status is often considered the most critical because stomatal closure 

during periods of drought or low soil moisture can severely limit O3 uptake. With 

few exceptions, field-based studies have shown that increases in both O3 uptake 

and foliar injury correlate with increased availability of soil water in the forest 

environment (Chappelka and Samuelson 1998, McLaughlin et al. 2007, Showman 

1991). However, the controlling influence of soil moisture on O3 uptake may 

depend on the severity of water stress as well as other plant-based factors that 

influence the balance between moisture input (precipitation) and moisture output 

(evapotranspiration) for individual plants or sites (Zhang et al. 2010). Generally, 

O3 concentrations in excess of 60 ppb O3 for several hours will cause injury to 

ozone-sensitive plants provided there are no external growth conditions (e.g., low 

soil moisture) that cause stomates to close (Mansfield 1998, U.S. EPA 2007). For 

certain genotypes of otherwise sensitive O3 species, injury may also be reduced 

by a plant’s inherent ability to construct and maintain internal defense and repair 

mechanisms following O3 uptake.

A useful bioindicator plant may be a tree, woody shrub, or herb species as long as 

the species responds to ambient levels of O3 pollution with distinct visible foliar 

symptoms that are easy to diagnose (Chappelka and Samuelson 1998). Field 

studies and controlled fumigation experiments were used to identify ozone-sensitive 

species and characterize the specific foliar response for each species used in the 

The Authors

GRETCHEN C. SMITH 

is the National Ozone Field 

Advisor with the University of 

Massachusetts, Department of 

Environmental Conservation, 

Amherst, MA.

RANDALL S. MORIN and 

GEORGE L. MCCASKILL are 

research foresters with the Forest 

Inventory and Analysis (FIA) 

program, Northern Research 

Station, Newtown Square, PA.

1

An earlier document provides detail on O3 formation, sources, and transport and a 

comprehensive review of O3 impacts on trees. The rationale for O3 biomonitoring is 

discussed along with procedures for data collection and quality control and detailed 

information on the sampling and estimation techniques used to summarize data and 

interpret findings (Smith et al. 2008).
2

USFS biomonitoring program (Brace et al. 1999, Davis 

et al. 1982, Krupa and Manning 1988). Many of these 

species show a highly variable response to O3 under field 

conditions because of intraspecific genetic variation in 

O3 sensitivity and variable site or microsite factors. Still, 

acknowledging the complexity and variability inherent 

to any field-based study, ozone-induced foliar injury 

symptoms occur routinely on sensitive bioindicator 

species during the growing season and detection of foliar 

injury is diagnostic for the presence of phytotoxic O3

concentrations in U.S. forests (Eckert et al. 1999, Kohut 

2007, Krupa et al. 1998, Manning 2003).

Across the country, O3 surveys are managed by the 

USFS Forest Inventory and Analysis (FIA) program and 

implemented in three of the four regional units covered 

by the Northern, Southern, and Pacific Northwest 

Research Stations (http://fia.fs.fed.us/regional-offices/).2

Currently, the national grid consists of more than 1,005 

field sites in 40 states. At every site, the amount and 

severity of injury to the foliage of sensitive plants is used 

to formulate a plot-level injury index referred to as the 

ozone biosite index or BI (Smith et al. 2007). Plot data 

provide detection-level information on where O3 stress 

200 100 0 200 Miles

.

Figure 1.—Northern Research Station 

Forest Inventory and Analysis (NRS-FIA) 

ozone biomonitoring grid.

Projection: Albers, NAD83.

Sources: FIA 2010.

Geographic base data are provided by 

the National Atlas of the USA. FIA data 

and tools are available online at

http://www.fia.fs.fed.us/tools-data/

Cartography: R.S. Morin. Dec. 2011

2

The FIA Intermountain region does not participate in the 

ozone biomonitoring program.

3

The USFS Forest Inventory and Analysis program (FIA) 

collects data annually on a base grid that includes the integrated 

forest inventory and forest health plots known as Phase 2 

(P2) and Phase 3 (P3), respectively. The ozone biomonitoring 

plots are part of P3 in FIA. Data collected on P3 plots provide 

estimates of health and condition for the Nation’s forests.

is occurring across a state or region and whether stress is 

increasing or decreasing over time.

The USFS biomonitoring survey includes a standardized 

protocol for sampling and site selection designed to 

maximize the opportunity for O3 injury detection during 

the growing season (U.S. Forest Service 2006). Sampling 

occurs on a unique national grid that consists of a single 

panel of O3 biomonitoring sites that are measured every 

year. A map of the portion of the O3 grid covered by 

the Northern Research Station, Forest Inventory and 

Analysis region (NRS-FIA) is shown in Figure 1. Before 

2002, biosite locations were co-located with forested 

ground plots3

 and, as a result, were restricted to the best 

possible opening or open canopy area in a wooded area 

(Fig. 2). These less than optimal areas for O3 intrusion 

were replaced in 2002 with the implementation of an 

improved ozone grid designed to decouple the O3 sample 

from the FIA base grid (Fig. 3). The changes in grid and 

site selection procedures allowed for larger openings and 

improved species and plant counts at each survey location.
3

Core reports for the O3 indicator include tabular data 

describing plot-level counts and injury indices by 

state and year and mapped data derived from spatially 

interpolated indices of O3 impact (e.g., BI) and O3

air quality (e.g., SUM064

). The SUM06 index data 

were obtained from the EPA, which monitors hourly 

ambient O3 concentrations at stations across the Nation. 

Assessment of risk to the forest resource from ambient 

Water

Nonforest

Forest

Figure 2.—NRS-FIA ozone biomonitoring sites, 1994-2001.

Projection: Albers, NAD83.

Sources: FIA 2010, NLCD 2006.

Geographic base data are provided by 

the National Atlas of the USA. FIA data 

and tools are available online at

http://www.fia.fs.fed.us/tools-data/

Cartography: R.S. Morin. Dec. 2011

Water

Nonforest

Forest

Figure 3.—NRS-FIA ozone biomonitoring sites, 2002-2010.

Projection: Albers, NAD83.

Sources: FIA 2010, NLCD 2006.

Geographic base data are provided by 

the National Atlas of the USA. FIA data 

and tools are available online at

http://www.fia.fs.fed.us/tools-data/

Cartography: R.S. Morin. Dec. 2011

O3 exposure is based on categorized BI data following 

spatial interpolation to the forested ground plots on the 

FIA base grid (Smith et al. 2008).

4

SUM06 is defined as the sum of all hourly average ozone 

concentrations greater than or equal to 0.06 ppm that occur 

between June 1 and August 31, a widely recognized threshold 

for ozone injury to sensitive plants.
4

Regional Grid Development

This report focuses on the states in the NRS-FIA region, 

which has the longest record of O3 biomonitoring in 

the country, from 1994 through 2010. The sample area 

includes 24 states, which are typically divided into two 

subregions. The Northeast (NE) subregion includes 

the New England States (Vermont, New Hampshire, 

Maine, Massachusetts, Connecticut, Rhode Island), New 

York, Ohio, and the Mid-Atlantic States (New Jersey, 

Pennsylvania, Delaware, Maryland, West Virginia), while 

the North Central (NC) subregion includes the Lake 

States (Michigan, Wisconsin, Minnesota), the East NC 

States (Indiana, Illinois, Iowa, Missouri), and the Plain 

States (Kansas, Nebraska, North Dakota, South Dakota). 

Field implementation began in 1994 in the six New 

England States and in the Lake States. Over the next 5 to 

7 years, the NE plot network expanded into the MidAtlantic States and then Ohio (1997) and finally New 

York (1999), while the NC implementation moved from 

Indiana (1996) west to Missouri (2000) and finally into 

the Plain States. By 2002, the ozone grid in NRS-FIA 

was complete including all 24 states on the improved 

ozone grid.

Regional Air Quality

Ozone air quality for the NE and NC States varies 

considerably across the landscape without regard 

for state boundaries or otherwise useful groupings 

of states or physiographic regions. Both the NE and 

NC States include areas with relatively clean air 

characterized by background concentrations of O3 that 

are inconsequential to plant health. Both subregions also 

include areas characterized by low, moderate, and high 

ambient O3 exposures during the growing season where 

the risk of injury varies with site- and weather-related 

factors that influence whether the toxic O3 molecule 

is able to enter the plant through open stomates. A 

regional map of average 3-month growing season 

SUM06 values for 1994 to 2009 (Fig. 4) describes most 

of northern New England and the northern Plains States 

as relatively clean (SUM06 < 8 ppm-hrs) with air quality 

worsening north to south from Maine to Maryland and 

from North Dakota to Kansas. Relatively low ozone 

exposures (SUM06 8-15 ppm-hrs) characterize more 

northerly portions of Wisconsin, Michigan, New York, 

and Massachusetts, while moderate to high seasonal 

O3 exposures (SUM06 >15 ppm-hrs) are found in all 

or part of the East North Central States of Missouri, 

Iowa, Illinois, and Indiana, continuing east across 

Ohio, Pennsylvania, Maryland, and Delaware, and on 

into New Jersey. Although each O3 season is unique 

and some years are substantially worse than others, the 

states typically characterized by the highest seasonal O3

exposures every year include Ohio and the Mid-Atlantic 

States in the NE, and all or part of Illinois and Indiana 

in the NC. In 2002, one of the worst ozone years (Fig. 

4), high O3 concentrations extended beyond what was 

typical into Missouri and parts of Nebraska, Kansas, and 

southern Michigan, while blanketing most of the East 

North Central and Mid-Atlantic States, and southern 

New England. In contrast, 2004 was a year with 

relatively clean to low O3 air (Fig. 4) extended across all 

states in NRS-FIA except in the more populated areas of 

Pennsylvania, New Jersey, Maryland, and Delaware.

OBJECTIVES

The 17-year record of biomonitoring in NRS-FIA 

(1994-2010) provides a unique opportunity to examine 

changes and trends in the ozone indicator over the long 

term. To that end, the primary objective of this report 

is to summarize the core ozone indicator data for all 

24 states in the NRS-FIA region. Tables and maps are 

used to summarize information by state and by year. 

Comparisons are made between states or groups of states 

in clean, low, moderate, and high ozone exposure areas. 

Spatial interpolation is used to extend the information 

collected at biosites to the forest population and to 

estimate the acres of forest land and the volume of 

susceptible tree species at risk of O3 impact.

A second objective is to examine relationships between 

injury and exposure within the context of variable site 

moisture conditions and wet vs. dry years. Long-term 

trends in foliar injury and air quality are discussed for the 

region as a whole with reference to national air quality 

standards and climate change.
5

Mean SUM06

> 8

8 - 15

15 - 25

> 25

Mean SUM06

< 8

8 - 15

15 - 25

> 25

Projection: Albers, NAD83.

Sources: EPA 2009.

Geographic base data are provided by 

the National Atlas of the USA. FIA data 

and tools are available online at

http://www.fia.fs.fed.us/tools-data/

Cartography: R.S. Morin. Dec. 2011

Figure 4.—Spatial interpolation of mean 3-month cumulative 

ozone concentrations (SUM06), in ppm-hrs, for the 16-year 

(1994 to 2009) sampling period and mean 3-month cumulative 

ozone concentrations (SUM06) for 2002 and 2004. 

Mean SUM06

< 8

8 - 15

15 - 25

> 25

2002

2004
6

The following forest health assessment questions 

developed specifically for the ozone indicator provide the 

framework for this report:

1. How many biosites are evaluated/injured?

2. How many plants are evaluated/injured? 

3. What species are used for biomonitoring? 

4. Do the injury data indicate that phytotoxic 

concentrations of ozone are present in eastern 

forests? 

5. Do the injury data indicate that ozone air quality 

is changing over time? 

6. If so, is it improving or deteriorating? 

7. Where is the injury most severe, or frequent? 

8. What amount of forest land is subject to levels of 

ozone pollution that may cause injury?

9. What volume of ozone-sensitive species is at risk, 

and where is it?

10. What is the relationship between ambient ozone 

concentrations and the injury data?

11. Are there explanatory variables that improve the 

interpretability of the injury data?

METHODS

Sample Area

As of 2010, the NRS-FIA biomonitoring grid included 

466 ground locations in 24 states (Fig. 3). The number 

of years of biomonitoring ranges from 8 to 17 depending 

on the start year for each state (Table 1). The field sites 

vary in size and do not have set boundaries. They are 

defined by the presence of ozone-sensitive indicator 

species indigenous to the NE and NC States (Table 2).

Ozone sampling occurs on a unique national grid based 

on the Environmental Monitoring and Assessment 

design (White et al. 1992). Biosite locations are 

mapped; geographic coordinates are recorded; and site 

characteristics are described in terms of size, elevation, 

terrain position, aspect, soil drainage, soil depth, and 

site disturbance. Crews are trained to select undisturbed, 

wide open areas (>3 acres) with more than 30 plants of 

more than three bioindicator species. The approximate 

5

As defined by Smith et al. (2008): visible symptoms on 

bioindicator plants indicate that O3 is present at concentrations 

that cause injury and that predisposing conditions (e.g., 

adequate site moisture) are coincident.

locations of the plants used for evaluations are drawn 

on the site map so that the same population of plants is 

evaluated by regular and quality assurance (QA) crews 

on return visits to the site. Throughout this document, 

site, biosite, and plot are used interchangeably to refer 

to the ground locations where ozone-sensitive plants are 

evaluated every year.

Foliar Injury, the BI, and Risk 

Assessment

Ozone injury and our ability to detect injury increase 

over the course of the field season. For these reasons, 

injury assessment is limited to 3 to 4 weeks (from late 

July through mid-August) within which the ozone 

indicator is considered stable. Crews are trained and 

certified in O3 injury recognition every year and submit 

injured leaf vouchers to a regional expert for validation of 

the field results.

The site-level biosite index (BI) is derived from the 

amount, severity, and incidence of ozone-induced foliar 

injury to ozone-sensitive bioindicator plants at each 

biosite (Smith et al. 2007). The BI values describe a 

gradation of plant injury response that quantifies the 

degree of O3 injury conditions5

 on the biomonitoring 

plots. The BI is the average score (amount * severity) 

for each species averaged across all species on the biosite 

multiplied by 1,000 to allow risk categories to be defined 

by integers (Table 3). The BI is calculated as:

BBBB = 1000 �mm−1�nnjj

−1

mm

jj=1

� aapp

nnjj≥10

pp=1

sspp �

where

BI = biosite index

m = number of species evaluated

nj

 = number of plants of the j∙th species evaluated

apj = proportion of the injured leaves on the pth plant of 

the j

∙th species

spj = average severity of injury on the pth plant of the j

∙th

species
7

Table 1.—Number of years of biomonitoring, number of years with ozone injury, and year 

biomonitoring was started for the Northeast (NE) and North Central (NC) States, 1994-2010

Region and state Number of years Start yeara

Biomonitoring Ozone injury 

detected

Northeast States:

 Maine 17 9 1994

 New Hampshire 17 15 1994

 Vermont 17 17 1994

 Massachusetts 17 17 1994

 Connecticut 17 16 1994

 Rhode Island 17 17 1994

 New Jersey 17 15 1994

 Maryland 17 17 1994

 Delaware 15 14 1995

 Pennsylvania 14 14 1995

 West Virginia 16 16 1995

 Ohio 14 14 1997

 New York 12 12 1999

North Central States:

 Wisconsin 17 16 1994

 Michigan 17 16 1994

 Minnesota 17 5 1994

 Indiana 15 15 1996

 Illinois 14 14 1997

 Iowa 11 7 2000

 Missouri 11 8 2000

 Kansas 9 5 2002

 Nebraska 9 1 2002

 North Dakota 9 0 2002

 South Dakota 9 1 2002

a

Data were not collected in Delaware in 1996 or in Pennsylvania in 1996 and 1997.

Table 2.—List of species sampled on the ozone biomonitoring plots

Common name Scientific name

Common and tall milkweed Asclepias spp.

Black cherry Prunus serotina

Blackberry Rubus allegheniensis

Spreading dogbane Apocynum androsaemifolium

White ash Fraxinus americana

Sassafras Sassafras albidum

Yellow-poplar Liriodendron tulipifera

Big-leaf aster Aster macrophylum

Sweetgum Liquidambar styraciflua

Pin cherry Prunus pennsylvanica

Mountain snowberry Symphoricarpos oreaphilus

Quaking aspen Populus tremuloides

Ponderosa pine Pinus ponderosa

Western wormwood Artemesia ludoviciana

Mugwort Artemesia douglasiana

Skunk bush Rhus trilobata
8

The BI value is used to assess the probable risk of O3

injury to all plants in the general area represented by 

each biosite (Coulston et al. 2003, Smith et al. 2008). BI 

values are grouped into four categories of injury (none, 

low, moderate, and high) that reflect both decreasing 

air quality and increasing risk of ozone injury to the 

forest resource (Table 3). These categories were proposed 

by Smith (1995) and have been used consistently in 

reporting results from FIA ozone surveys (Campbell et 

al. 2000, 2007; Rose and Coulston 2009; Smith et al. 

2008).

The inverse distance squared weighting technique 

was used for interpolating the BI data to generate an 

O3 risk map that is applied to the FIA base grid to 

extend information collected at the biosite to the forest 

population. This approach is used to assign a BI value to 

each P2 ground plot and tree and then estimate, for each 

state, the acres of forest land and the volume of ozonesusceptible tree species in each BI risk category (none, 

low, moderate, and high). Each tree species measured on 

FIA plots was assigned a sensitivity ranking (sensitive, 

moderately sensitive, insensitive, or unknown) based on 

the literature review provided by Smith et al. (2007). 

The list of ozone-sensitive tree species includes but is not 

limited to the following species: red maple (Acer rubrum), 

yellow birch (Betula alleghaniensis), green ash (Fraxinus 

pennsylvanica), eastern white pine (Pinus strobus), 

quaking aspen (Populus tremuloides), and the established 

bioindicator species including black cherry, white ash, 

yellow-poplar, and sassafras.

Table 3.—Classification scheme for the FIA biosite index (BI) describing the associated categories of foliar injury 

response, relative air quality, risk assumption, and probable impact

Bioindicator response category Relative air 

quality

Assumption 

of risk

Probable impact

BI = 0 < 5

Good None

Tree-level response: Visible injury to highly 

sensitive species, e.g., black cherry Little or no foliar injury

BI = 5.0 < 15

Moderate Low

Tree-level response: Visible injury to moderately 

sensitive species, e.g., yellow-poplar Light to moderate foliar injury

BI = 15 < 25 Unhealthy for 

sensitive species Moderate

Tree-level response: Visible and invisible injury

Moderate to severe foliar injury

BI ≥ 25

Unhealthy High

Ecosystem-level response: Visible and invisible 

injury leading to structural and functional changes Severe foliar injury

Ozone Exposure

SUM06 and N100 are two cumulative O3 exposure 

indices that are used to characterize ambient ozone 

exposures over a specified time period for assessing 

vegetation effects (U.S. EPA 2007). The inverse distance 

squared weighting technique was used for interpolating 

hourly ozone data obtained from the EPA (http://www.

epa.gov/air/data/index.html) across the landscape and 

assigning an average growing season (June, July, and 

August) SUM06 (the sum of all hourly average ozone 

concentrations ≥ 0.06 ppm) value to each biosite and 

year. The same database is used to assign an N100 (the 

number of hours of ozone ≥ 100 ppb) value to each 

biosite and year. The SUM06 measure provides an 

indication of chronic O3 stress for the growing season, 

and N100 is an indication of peak O3 concentrations. 

Descriptive ozone exposure categories for this report are 

derived from the seasonal mean values of the SUM06 

and N100 statistics as follows: Clean (SUM06 < 8 ppmhrs; N100 < 5 hrs), Low O3 (8 ≤ SUM06 < 15 ppm-hrs; 

5 ≤ N100 < 15 hrs), Moderate O3 (15 ≤ SUM06 < 25 

ppm-hrs; 15 ≤ N100 < 25 hrs), and High O3 (SUM06 ≥ 

25 ppm-hrs; N100 ≥ 25 hrs).

Site Moisture

The two moisture indices used in this report are the 

Palmer Drought Severity Index (PDSI) and a plant 

moisture availability index (MI). Data sources include 

the National Climatic Data Center (http://www.ncdc.

noaa.gov/paleo/pdsidata.html) and the PRISM Climate 
9

Group (http://www.prism.oregonstate.edu/). The PDSI 

quantifies long-term drought as derived from current 

and cumulative weather patterns (http://www.ncdc.noaa.

gov/oa/ncdc.html). The MI is derived from the ratio 

of monthly precipitation (moisture input) to potential 

evapotranspiration (moisture output) and quantifies 

the potential moisture available to plants (Rose and 

Coulston 2009). Average growing season (June, July, 

and August) PDSI and MI values were used to describe 

and compare moisture conditions by overlaying the 

annual PDSI and MI raster maps with biosite locations 

and assigning moisture values to each one; PDSI 

specifically to indicate soil moisture stress, and MI to 

indicate potential plant moisture deficits. Both PDSI and 

MI values were grouped into three categories: PDSI to 

represent wet (PDSI ≥ 1.25), near normal (-1.25 < PDSI 

< 1.25), and dry (PDSI ≤ -1.25) soil moisture, and MI 

to describe conditions of moisture surplus (MI ≥ 0.15), 

moisture balance (-0.15 < MI < 0.15), and moisture 

deficit (MI ≤ -0.15).

Analysis

Descriptive statistics presented here include the numbers 

of biosites and plants evaluated and injured by state, 

region, and year; the percentage of biosites in each BI 

injury category by state, region, and year; and the overall 

assessment of risk to the forest resource from ambient 

O3 exposure for each state. Changes in BI over time are 

examined by calculating the regional mean BI by year for 

1994 through 2010.

Additional descriptive statistics include the percentage 

of biosites in each ozone exposure (SUM06 and N100) 

and site moisture (PDSI and MI) category by region and 

year. Regional trends (1994 to 2009) in the calculated 

average growing season SUM06 and N100 are reported, 

as are trends in growing season precipitation (1994 to 

2010) above and below the average for each state and 

year. Linear regression analysis was used to examine the 

relationship between ozone exposure and BI. Regression 

statistics were computed using mean ozone exposure 

and BI values within ozone exposure classes (clean, 

low, moderate, and high). Regression residuals and 

influence diagnostics including Cooks D and DFBETAs 

were evaluated to ensure normality and homogeneity of 

residuals as well as no undue influence from individual 

observations.

RESULTS AND DISCUSSION

As previously noted, field implementation began in 

New England and spread south to the Mid-Atlantic 

States, and west to the NC States; the number of years 

of biomonitoring ranged from 9 (4 states) to 11-16 (9 

states) to 17 (11 states). From 1994 to 2010, O3 injury 

was detected at least once in each participating state in 

NRS-FIA except North Dakota (Table 1). In the 13 

NE States, injury was detected every year (Vermont, 

Massachusetts, Rhode Island, Maryland, Pennsylvania, 

West Virginia, Ohio, New York) or almost every year 

(New Hampshire, Connecticut, New Jersey, Delaware) 

in every state except Maine where injury was detected 

just over half the time (9 of 17 years). In the 11 NC 

States, injury was detected every year in Indiana and 

Illinois, every year but one in Wisconsin and Michigan, 

and more than half the time in Iowa, Missouri, and 

Kansas. Injury was detected in only 5 of 17 years in 

Minnesota and in only 1 of 9 years in Nebraska and 

South Dakota.

During the implementation phase on the 1994-

2001 grid, numbers of sites per state and number of 

visits per plot varied by start year and by state-driven 

intensification of the grid in certain states including 

Vermont, Pennsylvania6

, Delaware, and Wisconsin 

(Fig. 5). With the implementation of the 2002 grid, 

most intensified plots were dropped and the number of 

sites per state and number of visits per plot tended to 

level out across the region (Fig. 6). In 2010, more than 

46,000 plants were evaluated for ozone injury at 470 

ground locations in NRS-FIA, up from an original count 

of about 4,800 plants at 118 locations in 1994 (Table 4).

6

Special studies included an intensified grid in the Allegheny 

National Forest (ANF) and, by request of the State 

Cooperator, intensified grids in Delaware, Wisconsin, and 

Vermont. The ANF grid was maintained through 2010.
10

1 or 2

3 or 4

5 or 6

7 or 8

Water

Nonforest

Forest

Number of Visits

1 or 2

3 or 4

5, 6, or 7

8 or 9

Water

Nonforest

Forest

Number of Visits

Projection: Albers, NAD83.

Sources: FIA 2010, NLCD 2006.

Geographic base data are provided by 

the National Atlas of the USA. FIA data 

and tools are available online at

http://www.fia.fs.fed.us/tools-data/

Cartography: R.S. Morin. Dec. 2011

Projection: Albers, NAD83.

Sources: FIA 2010, NLCD 2006.

Geographic base data are provided by 

the National Atlas of the USA. FIA data 

and tools are available online at

http://www.fia.fs.fed.us/tools-data/

Cartography: R.S. Morin. Dec. 2011

Figure 5.—Number of visits to ozone biomonitoring sites, 1994-2001.

Figure 6.—Number of visits to ozone biomonitoring sites, 2002-2010.
11

COMMON QUESTIONS ABOUT 

BIOMONITORING

What do the sites look like?

Grid changes implemented in 2002 allowed greater 

flexibility in the selection of ground locations for 

biomonitoring, which is reflected in the summarized site 

attribute data (Table 5). In the first 8 years (1994-2001), 

about half (29 to 64%) of the biosites were greater 

than or equal to 1.214 ha (3 acres) in size compared to 

more than 80 percent in the more recent 9-year period 

(2002-2010). Similarly, the number of sites with three 

or more species steadily increased over time, reaching 

highs of 90 to 97 percent from 2002 on. Generally, 

other basic site characteristics did not change as a result 

of the more flexible O3 grid; in all years, trained crews 

tended to locate biosites on flat land (53 to 70%) with no 

appreciable aspect (69 to 87%), and approximately half 

of the sites (47 to 61%) were at elevations below 304.8 m 

(1,000 feet). Regardless of slope or elevation, crews selected 

open areas where the soil was well drained (>89%) with 

no obvious bedrock exposure (>92%) and no evidence of 

human activity or natural disturbance that might cause soil 

compaction (>82%). The data suggest that biosites in all 

Table 4.—Number of biosites and number of plants evaluated by year 

for the NE and NC States, 1994-2010

Year

NE States NC States

Number of 

biosites

Number of 

plants

Number of 

biosites

Number of 

plants

1994 86 3,590 32 1,293

1995 147 6,049 137 5,245

1996 126 5,468 103 4,469

1997 151 7,156 123 5,764

1998 269 11,915 196 10,592

1999 372 19,467 188 14,237

2000 269 17,371 290 21,089

2001 341 29,399 233 17,445

2002 230 22,893 260 23,253

2003 229 22,793 269 23,628

2004 227 22,376 267 24,392

2005 232 23,360 240 23,130

2006 233 23,239 237 23,391

2007 227 22,823 236 22,777

2008 217 21,682 240 23,147

2009 228 23,454 239 23,512

2010 231 23,988 239 22,760

states in the NRS-FIA region are relatively similar with 

respect to coded site attributes, although there are some 

obvious differences in the range of elevations and terrain 

positions that are largely determined by landscape features 

within each state or physiographic region (data not shown).

What species are used for 

biomonitoring?

The most common species evaluated on NE and NC 

biosites included blackberry, common and tall milkweed, 

black cherry, white ash, and spreading dogbane, generally 

in the range of 2,000 to 4,000 plants per year on the 

full grid (Tables 6, 7). Sassafras was also sampled widely 

(600 to 1,500 plants/year) in the NE and NC States as 

was yellow-poplar in NE and big-leaf aster in NC. Less 

commonly occurring species included pin cherry and 

sweetgum in NE and a variety of western bioindicator 

species in NC including ponderosa pine, mountain 

snowberry, skunkbush, and western wormwood that are 

found only in the more western portions of the Plain 

States. Field images of ozone-induced foliar injury to 

some of the more common bioindicator species are 

available at the FIA ozone indicator Web site: http://nrs.

fs.fed.us/fia/topics/ozone/default.asp.
12

Table 5.—Percent of total biosites in each site attribute category, number of biosites evaluated, and percent of injured biosites by year for the 24-state NRS-FIA region, 1994-2010 Site Attribute Percent of total biosites in each site attribute category

1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 2010

Plot SizeBiosites ≥ 1.2 hectares (3 acres) 55 29 36 48 43 53 59 64 81 84 80 81 82 83 88 85 86

Biosites < 1.2 hectares (3 acres) 45 71 64 52 57 47 41 36 19 16 20 19 18 17 12 15 14

Number of Species≥ 3 species 32 24 37 40 34 50 65 74 90 91 92 94 96 97 95 96 92

< 3 species 68 76 63 60 66 50 35 26 10 9 8 6 4 3 5 4 8

AspectNo aspect (slope<5%) 82 71 79 74 83 87 81 74 73 76 72 73 71 73 69 70 73

Northeast facing slope 9 12 10 10 8 6 11 13 13 12 13 14 14 14 15 15 13

Southwest facing slope 9 17 11 16 9 7 8 13 14 12 15 13 15 13 16 15 14

Terrain PositionFlat land unrelated to slope 58 53 64 60 64 53 63 62 62 65 61 64 58 63 66 70 75

Lower slope, Bench, Upper Slope 42 47 36 40 36 47 37 38 38 35 39 36 42 37 34 30 25

ElevationLow (≤ 304.8 meters; 1000 ft) 63 48 47 52 51 50 59 61 58 59 58 57 58 60 59 55 54

Moderate to High (> 304.8 m;1000 ft) 37 52 53 48 49 50 41 39 42 41 42 43 42 40 41 45 46

Soil DrainageSoil is well drained 89 95 93 91 94 96 96 96 95 96 93 95 94 95 95 96 97

Soil is generally wet 7 3 4 5 4 3 3 4 3 3 5 1 2 2 4 3 2

Soil is excessively dry 4 2 3 4 2 1 1 - 2 1 2 4 4 3 1 1 1

Soil DepthBedrock is not exposed 94 96 96 95 92 95 96 97 97 98 97 96 97 97 96 97 98

Bedrock is exposed; soils shallow 6 4 4 5 8 5 4 3 3 2 3 4 3 3 4 3 2

Site DisturbanceNone 82 88 90 92 98 98 99 99 99 89 89 90 94 94 97 95 92

Overuse: humans or natural disturbance 18 12 10 8 2 2 1 1 1 11 11 10 6 6 3 5 8

Total biosite count 118 284 229 274 465 560 559 574 490 498 494 472 470 463 457 467 499

Percent injured biosites 56 26 36 27 48 31 48 36 33 27 30 27 34 28 23 19 27
13

Table 6.—The number of evaluated and injured plants by species and year for the NE States, 1994-2010

1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 2010 Totals

Big-leaf aster

Evaluated 0 0 0 0 60 258 13 135 32 120 60 413 301 165 391 300 210 2,458

Injured 0 0 0 0 4 0 0 0 0 10 9 3 1 0 3 0 0 30

Black cherry

Evaluated 739 771 965 906 1,995 3,424 2,745 4,944 4,008 3,612 3,335 3,638 3,766 3,644 2,531 2,950 3,066 47,039

Injured 176 147 132 101 356 100 201 172 235 111 105 86 137 35 39 9 10 2,152

Blackberry

Evaluated 1,252 1,952 1,477 1,851 2,750 3,343 3,358 5,122 4,395 4,676 4,428 3,956 4,296 3,432 4,303 4,506 4,844 59,941

Injured 396 246 247 172 799 230 447 458 449 256 226 155 210 151 111 8 85 4,646

Dogbane

Evaluated 154 407 259 661 1,070 3,049 2,177 4,935 3,495 3,318 3,336 4,122 3,841 4,110 3,835 4,346 4,469 47,584

Injured 15 12 50 17 38 95 29 67 79 28 65 39 51 56 38 17 49 745

Milkweed

Evaluated 933 1,644 1,650 1,591 3,378 5,695 4,003 5,722 4,111 4,502 4,453 4,591 4,438 4,637 4,566 4,580 4,954 65,448

Injured 227 108 184 75 515 114 288 177 220 169 369 103 163 124 103 35 89 3,063

Pin cherry

Evaluated 0 413 167 162 205 250 218 536 624 1,206 630 537 658 491 118 276 432 6,923

Injured 0 5 18 0 9 8 10 1 4 29 0 0 0 12 0 0 0 96

Sassafras

Evaluated 0 157 156 493 492 805 1,243 1,948 1,275 1,478 1,242 1,407 1,292 1,229 1,102 1,130 1,201 16,650

Injured 0 0 10 0 11 6 54 12 3 2 22 5 39 5 1 0 2 172

Sweetgum

Evaluated 43 30 30 141 279 361 255 220 447 437 469 441 499 489 493 315 430 5,379

Injured 19 0 0 1 69 49 6 5 9 28 1 52 25 52 11 4 3 334

White ash

Evaluated 448 590 639 882 1,254 1,868 2,207 4,350 3,665 3,312 3,450 3,326 3,186 3,467 2,917 3,218 3,269 42,048

Injured 83 56 75 67 121 24 101 103 46 23 71 45 62 83 27 53 75 1,115

Yellow-poplar

Evaluated 11 137 145 481 553 444 952 464 841 867 973 929 940 969 654 911 1,113 11,384

Injured 7 4 15 12 54 1 10 0 22 23 23 14 10 8 3 18 13 237

Totals

Evaluated 3,580 6,101 5,488 7,168 12,036 19,497 17,171 28,376 22,893 23,528 22,376 23,360 23,217 22,633 20,910 22,532 23,988 3,580

Injured 923 578 731 445 1,976 627 1,146 995 1,067 679 891 502 698 526 336 144 326 923

Percent 25.8 9.5 13.3 6.2 16.4 3.2 6.7 3.5 4.7 2.9 3.9 2.2 3 2.3 1.6 0.6 1.4 25.8
14Table 7.—The number of evaluated and injured plants by species and year for the NC States, 1994-2010

1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 2010 Totals

Aspen

Evaluated 0 0 0 0 0 0 0 0 44 11 0 0 0 0 65 72 60 252

Injured 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0

Big-leaf aster

Evaluated 60 960 426 589 1,388 1,380 1,126 759 768 917 1,103 772 809 681 751 722 771 13,982

Injured 0 5 0 1 20 14 4 4 0 4 1 0 0 0 0 0 0 53

Black cherry

Evaluated 266 780 735 863 1,714 2,218 4,016 3,359 2,983 3,225 3,257 2,593 2,606 2,372 2,037 1,948 1,972 36,944

Injured 11 14 34 15 34 47 251 182 65 24 41 8 33 4 8 0 3 774

Blackberry

Evaluated 412 1,519 645 1,669 1,848 2,626 3,225 2,281 2,315 2,103 2,206 1,647 2,439 2,243 2,256 2,013 2,767 34,214

Injured 0 38 80 211 245 210 128 45 23 57 12 27 36 8 53 37 84 1,294

Dogbane

Evaluated 170 656 772 983 1,758 2,675 3,258 2,329 3,913 3,934 4,335 4,251 3,855 4,124 4,162 4,443 4,176 49,794

Injured 3 16 22 6 80 67 82 51 15 24 23 26 46 33 22 19 43 578

Milkweed

Evaluated 394 1,282 1,355 1,624 2,999 3,620 6,023 4,646 5,368 5,358 5,409 5,174 5,083 5,100 5,357 5,274 5,412 69,478

Injured 44 60 44 34 269 285 534 254 126 97 166 88 105 80 126 103 206 2,621

Mugwort

Evaluated 0 0 0 0 0 0 0 0 0 0 0 0 300 282 30 30 0 642

Injured 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0

Pin cherry

Evaluated 0 18 30 52 30 130 301 260 382 366 395 431 312 247 249 243 312 3,758

Injured 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0

Ponder. pine

Evaluated 0 0 0 0 0 0 0 0 182 247 135 270 240 185 200 169 217 1,845

Injured 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
15

Sassafras

Evaluated 0 0 84 324 272 495 1,336 1,092 863 856 1,145 1,133 1,075 1,038 1,142 979 989 12,823

Injured 0 0 0 0 3 0 9 12 0 0 6 6 3 3 2 9 0 53

Skunkbush

Evaluated 0 0 0 0 0 0 0 0 299 334 280 319 300 368 375 423 373 3,071

Injured 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 0 1

Snowberry

Evaluated 0 0 0 0 0 0 0 0 1,499 824 673 1,311 1,219 1,132 922 946 914 9,440

Injured 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0

Sweetgum

Evaluated 0 0 30 87 110 116 143 211 254 302 252 256 248 221 230 263 260 2,983

Injured 0 0 0 21 54 0 6 1 3 2 0 2 2 15 3 10 1 120

White ash

Evaluated 21 30 202 528 1,036 1,350 2,616 2,150 2,693 2,293 4,260 4,166 4,181 4,024 3,998 3,938 3,864 41,350

Injured 0 0 3 34 29 40 67 8 13 7 17 18 21 12 29 59 15 372

Wormwood

Evaluated 0 0 0 0 0 0 0 0 548 531 554 662 382 390 370 457 384 4,278

Injured 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0

Yellow-poplar

Evaluated 0 0 130 412 363 437 565 358 290 339 298 273 252 311 292 307 289 4,916

Injured 0 0 77 43 60 4 56 35 2 22 5 3 12 9 27 43 29 427

Totals

Evaluated 1,323 5,245 4,409 7,131 11,518 15,047 22,609 17,445 22,401 21,640 24,302 23,258 23,301 22,718 22,436 22,227 22,760 289,770

Injured 58 133 260 365 794 667 1,137 592 247 237 271 178 258 164 270 281 381 6,293

Percent 4.38 2.54 5.9 5.12 6.89 4.43 5.03 3.39 1.1 1.1 1.12 0.77 1.11 0.72 1.2 1.26 1.67 2.17

Table 7.—continued

1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 2010 Totals
16

The average number of plants evaluated per plot was 

similar for all species in the NE and NC States indicating 

that crews were following recommended plant selection 

criteria and generally maximizing plant counts for every 

available bioindicator species. However, there were 

differences in the percent injured plants and in the 

species-level foliar injury index (sppBI) both among 

species and between NE and NC (Tables 8, 9).

In the NE States, both the percent injured plants and 

the species BI were highest in the earliest sample years 

from 1994 to 1998, showing a sharp dropoff in 1999 

that persisted through 2010 with very few exceptions 

(blackberry in 2002; big-leaf aster in 2004). The 

percentage of injured plants was highest in 1994 for all 

species, ranging from 9.7 percent for spreading dogbane 

to 63.6 percent for yellow-poplar with all other species 

above 18 percent. The associated BI values for 1994, 

however, were relatively low except for sweetgum and 

blackberry, indicating that although the injured plant 

count was high the severity of injury was not. The 

percentage of injured plants was high again for most NE 

species in 1998, a year when BI values also peaked for 

seven of the nine sampled species. Black cherry appeared 

to be the most ozone-responsive species for NE showing 

more than 10 percent injured plants every year from 

1994 to 1998.

In the NC States, although the percent injured plants 

and species BI were high for sweetgum in 1998 and 

yellow-poplar in 1996, the injury index values for 

most species and years were much lower than in NE, 

particularly for the five most frequently sampled species 

(i.e., black cherry, blackberry, dogbane, milkweed, and 

white ash). As in NE, the percent injured plants and BI 

were highest for all species in the earlier years, tending 

to stay relatively high through 2001 when injury values 

dropped off and stayed low through 2010. There was 

some overlap in relatively high species BI in 1998 for 

three of the same species that showed high BI in NE (i.e., 

blackberry, milkweed, sweetgum). Still, no single species 

showed a consistently strong response to ozone (>10% 

percent injured plants), although milkweed is perhaps 

the most reliable bioindicator species for NC given its 

abundance and relatively high percentage of injured 

plants in 1994 to 2001.

Is ozone injury present?

The summary information demonstrates that the 

majority of the bioindicator species evaluated by NC and 

NE field crews over 1994 to 2010 showed evidence of 

ozone-induced foliar injury every year in some part of the 

sampling region, thus answering yes to the question, “Are 

phytotoxic ozone concentrations present in the forests 

of the NC and NE subregions?” Additional site-level 

injury statistics, summarized by state and year, provide 

information on where that injury is occurring and where 

it is most severe or frequent (Tables 10 - 13).

In 1994, the first year of sampling in NE, ozone injury 

was detected on 100 percent of the sampled sites (Table 

10) in Connecticut, and more than half of the sampled 

plants (66%) showed injury symptoms (Table 11). 

Other NE States with a high percentage of injured sites 

in 1994 included New Jersey (78%), Maryland (100%), 

Massachusetts (79%), New Hampshire (83%), Rhode 

Island (100%), and Vermont (86%). Both Maryland 

(51%) and New Jersey (49%) also had high percentages 

of injury to sampled plants, but the percentages of 

injured plants in Massachusetts (19%), New Hampshire 

(21%), Vermont (23%), and Rhode Island (20%) 

were all similarly lower. In Maine, the only other state 

collecting injury data in 1994, O3 injury was detected 

on only 24 percent of the sampled sites and the percent 

injured plants was a meager 6 percent. The majority of 

sites in Connecticut continued to show injury every year 

from 1995 through 1998 and again from 2000 through 

2007. However, the percentage of injured plants dropped 

below 30 percent and eventually below 20 percent until 

the final year (2010) when for the first time no ozone 

injury was detected on any site or plant in Connecticut. 

The percentages of injured sites and plants were also 

relatively low in the years leading up to 2010 and in 

1999 when injury was detected on only one-third of 

the sites in Connecticut on only 1 percent of plants. A 

similar pattern of ups and downs in the numbers of sites 

and plants with injury was typical of other NE States 

except for Maine where injury was detected in only 9 of 

the 17 years and never on more than 8 percent of the 

evaluated plants.
17

Table 8.—The percent of plants with injury and the biosite index (BI) by species and year for the NE States, 1994-2010

1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 2010

Big-leaf aster

% Injured Plants 0 0 0 0 6.7 0 0 0 0 8.3 15 0.7 0.3 0 0.8 0 0

Species BI - - - - 17.6 - - - - 1.2 1.6 0.2 0 0 3.7 - -

Black cherry

% Injured Plants 23.8 19.1 13.7 11.2 17.8 2.9 7.3 3.5 5.9 3.1 3.2 2.4 3.6 1 1.5 0.3 0.3

Species BI 4.3 11.1 7.4 5.5 19.8 1.4 4.4 2.2 4.9 1.6 1.3 0.9 1.5 0.4 0.7 0.1 0.1

Blackberry

% Injured Plants 31.6 12.6 16.7 9.3 29.1 6.9 13.3 8.9 10.2 5.5 5.1 3.9 4.9 4.4 2.6 0.2 1.8

Species BI 41.7 8.4 31.7 12.8 56.3 9.8 23.1 11.6 17.5 3.6 8.1 5.3 5.2 3.8 1.2 0.3 4.4

Dogbane

% Injured Plants 9.7 2.9 19.3 2.6 3.6 3.1 1.3 1.4 2.3 0.8 1.9 0.9 1.3 1.4 1 0.4 1.1

Species BI 12.4 0.1 3.1 4.1 2.5 2.2 0.8 1.4 1.7 0.7 0.9 0.7 0.7 0.6 0.3 0.3 1.2

Milkweed

% Injured Plants 24.3 6.6 11.2 4.7 15.3 2 7.2 3.1 5.4 3.8 8.3 2.2 3.7 2.7 2.3 0.8 1.8

Species BI 7.1 1.8 3.1 2.9 14.9 1.1 6.3 3.4 3.3 1.6 4.2 0.9 1.8 1 1.1 0.8 1.6

Pin cherry

% Injured Plants 0 1.2 10.8 0 4.4 3.2 4.5 0.2 0.6 2.4 0 0 0 2.4 0 0 0

Species BI - - - - - - - -

Sassafras

% Injured Plants 0 0 6.4 0 2.2 0.8 4.3 0.6 0.2 0.1 1.8 0.4 3 0.4 0.1 0 0.2

Species BI - - 3.6 - 2.5 1.2 0.7 0.4 - 0.1 0.7 0.2 1.2 0.3 0.2 - 0.1

Sweetgum

% Injured Plants 44.2 0 0 0.7 24.7 13.6 2.4 2.3 2 6.4 0.2 11.8 5 10.6 2.2 1.3 0.7

Species BI 45.1 - - 0.1 38.2 14.9 0.1 1.4 0.7 1.9 0.4 4.9 1.9 6.8 3.3 0.6 4

White ash

% Injured Plants 18.5 9.5 11.7 7.6 9.7 1.3 4.6 2.4 1.3 0.7 2.1 1.4 1.9 2.4 0.9 1.7 2.3

Species BI 6.3 9.8 24.1 11.9 16.6 0.9 7.8 4.1 2.4 0.7 3.5 1.4 2 2.5 2.2 2.9 3.4

Yellow-poplar

% Injured Plants 63.6 2.9 10.3 2.5 9.8 0.2 1.1 <0.1 2.6 2.7 2.4 1.5 1.1 0.8 0.5 2 1.2

Species BI 3.8 0.1 0.9 0.7 6.3 0.1 0.3 0.2 0.7 0.5 1 0.2 0.6 0.1 0.6 1.3 1.3
18

Table 9.—The percent of plants with injury and the biosite index (BI) by species and year for the NC States, 1994-2010 

1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 2010

Big-leaf aster

% Injured Plants 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0

Species BI - 0.7 - 0.2 0.8 0.9 0.2 0.4 - 0.3 - - - - - - -

Black cherry

% Injured Plants 4.1 1.8 4.6 1.7 2 2.1 6.3 5.4 2.2 0.7 1.3 0.3 1.3 0.2 0.4 0 0.2

Species BI 2.4 0.5 1.3 0.6 0.5 0.4 1.6 1.6 1.4 0.2 0.8 0.1 0.6 0.1 <0.1 - 0.3

Blackberry

% Injured Plants 0 2.5 12.4 12.6 13.3 8 4 2 1 2.7 0.5 1.6 1.5 0.4 2.3 1.8 3

Species BI - 1.8 33.9 21.3 33.1 10.3 6.9 4.4 1.7 2 1.7 1.9 2.3 0.8 2.9 2.3 4.2

Dogbane

% Injured Plants 1.8 2.4 2.8 0.6 4.6 2.5 2.5 2.2 0.4 0.6 0.5 0.6 1.2 0.8 0.5 0.4 1

Species BI 0.2 1.8 3.9 0.2 2.5 0.9 2.4 3.7 0.5 0.9 0.2 0.6 2.4 0.5 0.7 0.6 0.8

Milkweed

% Injured Plants 11.2 4.7 3.2 2.1 9 7.9 8.9 5.5 2.3 1.8 3.1 1.7 2.1 1.6 2.4 2 3.8

Species BI 1.4 1.3 1.4 0.2 5.9 3.7 4.9 3.5 2.3 0.8 2.4 2 1.2 0.7 0.7 0.4 1.8

Pin cherry

% Injured Plants 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0

Species BI - - - - - - - - - - - - - - - - -

Sassafras

% Injured Plants 0 0 0 0 1.1 0 0.7 1.1 0 0 0.5 0.5 0.3 0.3 0.2 0.9 0

Species BI - - - - 0.1 0 0.3 0.7 - - 0.2 0.1 0.1 0.4 0.1 0.2 -

Sweetgum

% Injured Plants 0 0 0 24.1 49.1 0 4.2 0.5 1.2 0.7 0 0.8 0.8 6.8 1.3 3.8 0.4

Species BI - - - 34.5 41.2 - 0.2 3.2 0.9 1.9 - 0 0.1 8.9 0.4 2.3 0.1

White ash

% Injured Plants 0 0 1.5 6.4 2.8 3 2.6 0.4 0.5 0.3 0.4 0.4 0.5 0.3 0.7 1.5 0.4

Species BI - - 0.6 2.9 2.5 2 2.5 0.2 0.4 0.1 1.2 0.8 0.3 0.3 1.2 1.2 1

Yellow-poplar

% Injured Plants 0 0 59.23 10.44 16.53 0.92 9.91 9.78 0.69 6.49 1.68 1.1 4.76 2.89 9.25 14.01 10.03

Species BI - - 137.3 6.2 28.3 0.8 10.8 7.3 0.7 2 0.5 1 0.5 0.8 6.3 19 6.6
19

Table 10.—Number of evaluated biosites with ozone-induced foliar injury by NE State and year

Year

State 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 2010

Connecticut Evaluated 4 7 7 7 5 6 7 6 7 7 5 8 5 7 7 8 5

 Injured 4 6 6 4 5 2 6 6 7 7 3 7 5 5 3 2 0

Delaware Evaluated - 1 - 3 3 11 9 10 11 11 11 8 11 11 12 8 10 Injured - 0 - 2 3 10 6 8 4 11 8 6 8 7 8 2 4

Maine Evaluated 29 34 27 25 28 24 19 22 17 18 18 18 18 18 18 18 18

 Injured 7 3 4 0 7 0 0 0 0 0 0 4 3 0 1 1 3

Maryland Evaluated 9 9 5 5 12 10 7 27 8 9 9 9 9 8 9 8 8

 Injured 9 9 5 4 12 3 2 16 3 2 3 9 5 4 1 2 2

Massachusetts Evaluated 14 15 13 16 18 18 14 8 9 9 9 9 10 9 9 10 8

 Injured 11 6 3 11 14 10 9 1 2 4 2 4 10 2 3 1 1

New Hampshire Evaluated 6 23 24 21 25 23 20 19 5 5 5 8 6 5 5 5 5

 Injured 5 10 7 1 10 4 1 3 0 0 1 3 2 1 1 1 3

New Jersey Evaluated 9 6 6 7 11 9 9 2 10 7 8 9 8 8 8 8 8

 Injured 7 5 4 0 11 1 1 0 4 1 5 5 7 6 3 4 4

New York Evaluated - - - - - 85 9 56 37 37 36 35 35 38 41 36 35 Injured - - - - - 12 3 14 16 11 15 5 13 12 12 3 13Ohio Evaluated - - - 19 19 18 19 34 34 34 34 34 34 35 34 36 40

 Injured - - - 10 12 4 5 8 8 9 12 10 7 4 4 5 9

Pennsylvania Evaluated - 12 - - 100 129 100 104 50 50 48 59 67 56 43 58 62

 Injured - 5 - - 51 24 63 39 33 17 19 17 31 24 7 9 17

Rhode Island Evaluated 1 5 5 5 5 4 5 4 5 5 4 5 5 5 5 5 6

 Injured 1 5 5 5 5 3 4 4 4 3 4 1 5 3 3 3 4

Vermont Evaluated 14 18 16 17 17 21 22 18 17 18 20 18 14 13 11 12 13 Injured 12 5 13 9 9 7 6 4 5 5 4 4 5 1 4 4 2

West Virginia Evaluated - 18 23 26 26 14 29 30 27 27 28 28 28 28 32 31 28

 Injured - 2 11 2 19 2 3 3 13 12 6 6 6 15 9 7 9
20Table 11.—Number of plants evaluated and injureda on biosites by NE State and year

Year

State 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 2010

Connecticut Evaluated 212 304 382 411 331 405 491 514 654 701 470 867 519 643 713 659 570

 Injured 140 78 110 30 72 5 49 79 66 46 68 76 106 26 19 11 0

Delaware Evaluated - 30 - 244 211 864 283 825 1065 1298 1176 845 1215 1235 1283 875 1373

 Injured - 0 - 16 68 159 34 80 29 212 143 80 99 73 91 4 21

Maine Evaluated 962 1371 962 1098 1517 1063 799 940 1597 1539 1620 1632 1676 1810 1780 1729 1781

 Injured 54 32 75 0 15 0 0 0 0 0 0 12 23 0 2 2 9

Maryland Evaluated 392 316 248 240 414 338 261 1820 1109 762 833 671 844 922 830 932 909

 Injured 202 79 76 57 86 29 27 143 49 31 35 94 18 16 1 14 9

Massachusetts Evaluated 469 492 528 424 1,129 942 720 451 728 822 630 927 841 770 823 859 821

 Injured 88 56 22 53 146 63 41 25 15 16 6 15 71 3 26 10 3

New Hampshire Evaluated 325 1,001 1,032 887 1,205 1,014 947 1,214 405 258 321 790 524 478 498 590 536

 Injured 68 67 27 2 88 8 7 14 0 0 5 7 4 1 1 3 50

New Jersey Evaluated 347 238 216 210 991 810 817 178 802 700 619 579 676 678 674 682 855

 Injured 171 91 40 0 264 1 9 0 36 5 92 42 67 54 13 13 21

New York Evaluated - - - - - 3,582 474 5,679 3,415 3,087 3,271 3,055 3,476 3,405 3,923 3,789 3,857

 Injured - - - - - 99 12 66 169 73 167 16 53 115 88 14 81

Ohio Evaluated - - - 1,417 926 810 1,713 3,114 3,175 3,366 3,357 3,060 3,124 3,208 3,270 3,092 3,731

 Injured - - - 69 72 22 41 92 42 29 73 29 29 22 13 20 18

Pennsylvania Evaluated - 478 - - 2,299 7,177 7,402 11,147 5,548 5,598 5,303 6,245 5,807 5,430 3,607 5,801 6,483

 Injured - 51 - - 652 140 778 405 439 97 195 64 118 116 13 31 72

Rhode Island Evaluated 25 313 292 336 332 251 349 286 461 426 384 507 481 500 512 693 693

 Injured 5 90 72 65 97 44 41 39 42 19 23 12 68 15 17 16 14

Vermont Evaluated 858 1,169 1,049 1,092 946 1,382 1,544 1,335 1,409 1,639 1,752 1,435 1,235 1,186 1,120 1,105 1,188

 Injured 195 22 200 139 98 40 74 29 23 25 18 8 13 15 16 5 7

West Virginia Evaluated - 359 759 797 1,614 829 1,571 1,831 2,525 2,465 2,640 2,747 2,799 2,558 2,829 2,738 2,357

 Injured - 12 109 14 318 17 33 56 157 103 66 47 29 70 41 12 30

aInjury validated by expert.
21

Table 12.—Number of evaluated biosites with ozone-induced foliar injury by NC State and year

Year

State 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 2010

Illinois Evaluated - - - 9 19 26 40 43 27 28 29 28 28 32 31 31 31

 Injured - - - 9 18 22 35 14 5 15 17 17 24 18 22 20 25

Indiana Evaluated - - 8 10 17 19 26 31 22 31 28 26 23 25 25 25 26

 Injured - - 7 6 16 13 23 23 7 12 11 10 10 13 13 17 18

Iowa Evaluated - - - - - - 8 7 7 7 7 7 4 7 7 8 7 Injured - - - - - - 0 0 0 4 0 1 4 5 3 1 1Kansas Evaluated - - - - - - - - 9 12 12 13 13 12 13 13 12

 Injured - - - - - - - - 0 1 1 0 0 2 1 4 0Michigan Evaluated 12 40 27 30 59 43 54 49 45 45 45 45 45 48 50 50 51

 Injured 2 8 3 0 7 15 30 30 18 7 13 13 13 8 6 9 14Minnesota Evaluated 4 32 40 40 59 46 41 26 29 29 29 29 29 29 29 28 29

 Injured 0 0 0 1 1 0 1 1 0 0 0 0 0 0 0 1 0

Missouri Evaluated - - - - - - 57 38 39 38 38 38 38 38 38 41 40

 Injured - - - - - - 19 12 9 4 7 0 1 1 2 0 0Nebraska Evaluated - - - - - - - - 8 7 8 8 8 8 8 8 8 Injured - - - - - - - - 0 0 0 0 0 0 0 0 1North Dakota Evaluated - - - - - - - - 8 8 8 8 8 8 8 8 8 Injured - - - - - - - - 0 0 0 0 0 0 0 0 0South Dakota Evaluated - - - - - - - - 12 12 12 13 13 13 12 12 12

 Injured - - - - - - - - 0 0 0 0 0 1 0 0 0Wisconsin Evaluated 16 65 28 34 42 54 64 39 55 57 55 33 31 29 29 29 29

 Injured 8 9 14 9 25 41 49 20 25 13 22 12 6 4 3 0 5
22

Table 13.—Number of plants evaluated and injureda on biosites by NC State and year

Year

State 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 2010

Illinois Evaluated - - - 451 758 1934 3149 2919 2703 1951 2236 2436 2354 2289 2321 2673 2474 Injured - - - 171 220 154 246 64 22 66 74 44 88 56 122 73 130

Indiana Evaluated - - 589 729 822 1441 2245 2440 1384 1809 1857 2002 1896 2161 2581 2458 2501

 Injured - - 193 84 223 176 352 177 27 85 47 68 53 58 109 156 134

Iowa Evaluated - - - - - - 312 326 598 643 666 768 420 776 664 655 633

 Injured - - - - - - 0 0 0 6 0 3 10 11 8 2 3

Kansas Evaluated - - - - - - - - 822 1060 965 1084 1119 994 893 866 6561

 Injured - - - - - - - - 0 1 1 0 0 7 2 32 0

Michigan Evaluated 512 1447 1236 1189 2353 3359 4238 4271 4649 4467 4648 4643 4902 4531 4412 4302 4673

 Injured 11 54 31 0 22 120 195 163 62 30 50 35 65 24 40 49 107

Minnesota Evaluated 129 588 647 1059 2594 1963 1617 1157 1633 2565 2729 2863 3084 3028 2538 2649 2971

 Injured 0 0 0 1 5 0 2 1 0 0 0 0 0 0 0 1 0

Missouri Evaluated - - - - - - 2,912 2,581 4,057 3,062 2,990 3,365 3,764 3,662 4,028 4,191 4,163

 Injured - - - - - - 83 83 29 7 24 0 4 2 9 0 0Nebraska Evaluated - - - - - - - - 835 720 816 855 781 816 931 801 863

 Injured - - - - - - - - 0 0 0 0 0 0 0 0 3North Dakota Evaluated - - - - - - - - 866 695 752 775 720 720 881 875 925

 Injured - - - - - - - - 0 0 0 0 0 0 0 0 0South Dakota Evaluated - - - - - - - - 753 1,264 1,170 1,406 1,432 1,340 1,278 1,273 1,142

 Injured - - - - - - - - 0 0 0 0 0 1 0 0 0Wisconsin Evaluated 652 3,210 1,997 2,336 4,065 5,540 6,616 3,751 4,953 5,392 5,473 2,933 2,809 2,460 2,694 2,800 2,904

 Injured 47 79 36 40 252 195 218 104 107 45 75 24 38 5 6 0 6

aInjury validated by expert.
23

In the NC States, biomonitoring began in 1994 in 

only Minnesota (no injury detected), Wisconsin (injury 

detected on 8 of 16 sites), and Michigan (injury detected 

on 2 of 12 sites). In the first year of biomonitoring in 

Indiana (1996) and Illinois (1997), injury was detected 

on 88 percent and 100 percent of the sites, respectively 

(Table 12). As other NC States entered the program, the 

highest percentages of injured sites and plants continued 

to occur in Indiana and Illinois with occasional high 

percentages of injured sites in Michigan and Wisconsin, 

particularly from 1998 to 2002. The percentage of 

injured plants tended to be lower than those recorded 

in the NE States, ranging from 12 to 38 percent in 

Indiana and Illinois in the earliest years, but then falling 

to generally less than 10 percent in all NC States and to 

less than 2 percent in all states except Indiana and Illinois 

after 2002 (Table 13).

Both the number (Fig. 7) and percent (Fig. 8) of visits 

with injury when viewed on a regional scale tended to 

reflect the regional air quality data (Fig. 4) with some 

important exceptions. For example, high percentages 

of visits with injury occurred in northern Vermont and 

northern Wisconsin even though relatively low SUM06 

concentrations are typical of these areas. On the other 

hand, Kansas is characterized by moderate SUM06 

exposures, but the number and percent of visits with 

injury were minimal except perhaps in 2009 when injury 

was detected on 4 of 13 evaluated sites. For the most 

part, however, the areas where average growing season 

SUM06 values were relatively high were also the areas 

where crews detected ozone injury more than half the 

time over the sampling period. The areas of greatest 

injury incidence include Illinois, Indiana, and southern 

Michigan of the NC States, and virtually all of the NE 

States except for northern New Hampshire and Maine.

Along with injury incidence and distribution, injury 

severity, as indicated by the site-level BI values, must be 

considered when assessing O3 impact. The percentage 

of sites in each BI foliar injury class is presented in the 

appendix by state and year (Appendix 1: Table 1A-NE, 

Table 1B-NC). The categorized BI classes are none, 

low, moderate, and high injury indicating increasing 

probability of ozone impact (Table 3). In the none and 

low BI classes, there is little risk of O3 impact other than 

visible foliar injury, but as the BI increases, the risk of 

impact shifts from visible injury to sensitive species to 

less visible effects on growth and vigor and, at the highest 

level, the increased probability of adverse impacts on 

population structure and function. 

In 1994, as previously noted, injury was detected on 

all evaluated sites in Connecticut. More importantly 

with respect to O3 impact, all of those sites fell into the 

moderate ozone impact category. More severe injury 

was recorded in New Jersey where the majority of sites 

fell into the moderate (33%) and high (22%) BI classes. 

In other NE States, even though injury was detected 

on many sites in 1994, most of the injury was slight, 

indicating little or no risk of ozone impact other than 

visible foliar symptoms to the most sensitive plants. 

From 1994 to 1998, Maryland and New Jersey stand 

out as states where more than half of the evaluated sites 

sustained moderate to high foliar injury every year, as did 

Connecticut for 3 of those 4 years, and West Virginia 

and Rhode Island for 1 year. After 1998 and continuing 

through 2010, the majority of evaluated sites in all NE 

States fell into the none and low BI categories every year 

except for sites in Delaware and Rhode Island in 1999.

In the NC States, sites with a high percentage of plants 

in the moderate high BI classes were typical of Indiana 

in 1996 and 1998 and of Illinois in 1997 and 1998 

with a small percentage of sites in the high BI class 

also occurring in Wisconsin, Michigan, and Missouri 

on occasion. Similar to the NE States, the majority of 

evaluated sites in all NC States fell into the lowest BI 

class from 1999 through 2010 except in 2001 in Indiana 

when the majority of sites were split between the lowest 

BI class and the three higher BI categories.

The regional summary table for percentage of biosites 

in each BI foliar injury class (Table 14) indicates that 

moderate to high foliar injury symptoms were more 

often recorded in the earliest sampling years (1994 to 

1998) in both NE and NC. The years with the highest 

percentage of biosites in the higher BI categories were 

1994, 1996, and 1998 in NE and 1996, 1997, and 1998 

in NC. The regional average biosite index was highest 
24

Number of Visits

with Injury

0 - 1

2 - 4

5 - 9

10 - 17

Water

Nonforest

Forest

Percent of Visits

with Injury

0 - 15

15 - 40

40 - 70

70 - 100

Water

Nonforest

Forest

Projection: Albers, NAD83.

Sources: FIA 2010, NLCD 2006.

Geographic base data are provided by 

the National Atlas of the USA. FIA data 

and tools are available online at

http://www.fia.fs.fed.us/tools-data/

Cartography: R.S. Morin. Dec. 2011

Projection: Albers, NAD83.

Sources: FIA 2010, NLCD 2006.

Geographic base data are provided by 

the National Atlas of the USA. FIA data 

and tools are available online at

http://www.fia.fs.fed.us/tools-data/

Cartography: R.S. Morin. Dec. 2011

Figure 7.—Number of visits to ozone biomonitoring sites with injury, 1994-2010.

Figure 8.—Percent of visits to ozone biomonitoring sites with injury, 1994-2010.
25

Table 14.—Percentage of biosites in each foliar injury category (BI)a in the NE and NC States by year, 1994-2010

Biosite Index 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 2010

0 to 4.9 (least injured) Northeast 43.1 66.2 49.5 75.2 44.6 84.1 85 77.7 78.6 89 82.6 85.5 76.8 90.8 91.2 88.9 86

 North Central 95.2 95.4 75.4 75.6 59.7 81.3 86.7 88.2 96.4 97.4 96.5 98.3 94.3 98 96.5 94.8 95.8

5.0 to 14.9 Northeast 10.1 13.5 18.4 7.2 9.7 3.9 8 12.4 12.6 9.5 9.6 6.2 15.2 6.7 8.6 9.8 7.7

 North Central 2.8 3.2 0.9 6.8 7.2 13.5 8.4 7.4 3.1 1.7 2.7 0.9 4.8 1.9 2.5 4.6 2.9

15 to 24.9 Northeast 28.7 10.1 11.7 6.8 14.7 5.2 4 4.6 4.3 0.9 3.7 2.7 6.3 1.8 0.2 1.7 5.1

 North Central 2.1 0 0 2.2 8.3 1.8 1.8 2 0.2 0 0.5 0.2 0.9 0 0.5 0.2 0.5

≥25 (most injured) Northeast 18.1 10.2 20.4 10.8 30.9 6.7 3 5.3 4.5 1.3 4.1 5.4 1.7 0.7 0 1.5 2.7

 North Central 0 1.3 23.7 15.3 24.8 3.3 3 2.4 0.2 0.9 0.2 0.5 0 0 0.4 0.4 0.7

Average BI 12.6 3.6 10.6 5.5 17.6 2.9 4.7 3.2 2.9 1.1 2.1 1.5 1.6 1.1 1 0.9 1.6

aBI index and risk categories: 0 to 4.9 = little or no foliar injury and no risk of ozone impact; 5 to 14.9 = low foliar injury and low risk; 15 to 24.9 = moderate foliar injury and moderate risk; 

≥ 25 = severe foliar injury and high risk of ozone impact.
26

in 1998, reflecting peak foliar injury scores in both NE 

and NC. Even in years with high injury (e.g., 1998), 

more than half of all sites fell into the none and low 

BI categories, suggesting that areas of high O3 risk are 

localized in relatively small pockets of poor air quality 

and predisposing conditions.

Regional BI data were also summarized by SUM06 

ozone exposure class. For all sites, and for all injured 

sites (BI > 0), the average site-level foliar injury index 

(BI) increased as the SUM06 ozone exposure level 

increased from clean to high (Fig. 9). Similarly, the 

percent injured biosites increased with increasing O3

exposure class, at least for the earliest (1994-1998) and 

most damaging sampling period (Fig. 10). In the middle 

(1999-2004) and more recent (2005-2010) sampling 

periods, the percent injured biosites increased initially 

from the clean to the low or moderate exposure classes, 

but then tended to level off or decline slightly. This 

response reflects changing conditions in regional air 

quality that are discussed in some detail in the following 

section on trends.

Has injury changed over time?

Sixteen-year trends (1994-2009) in foliar injury were 

examined separately for biosites grouped in clean, low, 

moderate, and high ozone exposure zones. Within each 

ozone exposure group, annual mean values for BI were 

calculated and plotted over the 16 years from 1994 to 

2009 (Fig. 11). Regression procedures were used to 

determine if the slope of the line for BI within each 

exposure group was significantly different from zero 

and, if so, the slope direction (Table 15).

The SUM06 groupings described above for O3 exposure 

imply certain injury thresholds for ozone air quality, 

both in terms of the visible foliar injury that crews 

rate in the field and the possibility of growth impacts. 

Biosites grouped in the clean ozone exposure group are 

expected to sustain little or no foliar injury (BI close to 

zero) and no risk of growth impacts. However, biosites 

in the low, moderate, and high O3 exposure categories 

may have a wide range of BI values depending on a 

0

5

10

15

20

25

30

35

40

45

<8.0 8 - 14.9 15 - 24.9 ≥25

Average Biosite Index

SUM06 Ozone Exposure Group (ppm-hrs)

All biosites

Biosites with injury

0

10

20

30

40

50

60

<8.0 8.0 - 14.9 15.0 - 24.9 ≥25

Percentage of Biosites with Injury

SUM06 Ozone Exposure Group (ppm-hrs)

1994 to 1998

1999 to 2004

2005 to 2009

Figure 9.—Biosite index by SUM06 ozone exposure class, 

1994-2009.

Figure 10.—Percentage of biosites with injury by SUM06 

ozone exposure class for the early (1994-1998), middle 

(1990-2004), and more recent (2005-2009) sampling periods.

0

5

10

15

20

25

0

10

20

30

40

50

Mean Biosite Index

Year

Clean Low Moderate

High SUM06 N100

Figure 11.—Mean BI (biosite index) for clean, low, moderate, 

and high ozone exposure sites, and mean SUM06 and N100 

values by year, 1994-2009.
27

variety of site conditions (e.g., soil moisture), exposure 

factors (e.g., air temperature), and plant response 

mechanisms (e.g., O3 scavenging and repair processes) 

that control whether ozone in the air actually moves into 

the plant through open stomates and causes injury.

Seasonal mean foliar injury values for biosites in the 

clean ozone exposure group were all quite low (BI < 4.9) 

and, although there appeared to be a slight downward 

trend in BI over the 16-year sample period, this trend 

was not significant (p = 0.085; Table 15). In contrast, 

biosites in the moderate and high ozone exposure 

groups showed a significant downward trend (α = 0.05) 

in seasonal mean BI over the 16-year period with the 

significance of the trend increasing from the moderate 

(p = 0.031) to high (p < 0.006) ozone exposure group 

(Table 15). Changes in the 3-year rolling average of the 

BI (data not shown) showed similar results, indicating 

that annual fluctuations in BI, especially over 1994 to 

1998, were not unduly influencing the significance of 

the downward trend.

When seasonal mean values for SUM06 and N100 were 

plotted over the 16 years from 1994 to 2009 along with 

the categorized BI data, it was clear that both of the 

cumulative ozone exposure indices were declining over 

time as well, and this may largely explain the downward 

trends in foliar injury. However, it is also true that the 

year-to-year fluctuations in the seasonal BI values do not 

correspond well with the fluctuating SUM06 and N100 

values. For example, the spike in seasonal BI in 1994 

and 1996 in the high O3 group does not correspond to 

a spike in ozone exposures although SUM06 and N100 

Table 15.—Regression model statistics for 16-year trends 

in mean foliar injury (BI) for biosites grouped in clean, 

low, moderate, and high ozone exposure categories, 

1994-2009

Ozone Exposure 

Group

BI (foliar injury) 

p-values R2 Slope direction

Clean 0.085 0.2 negative 

Low 0.076 0.21 negative 

Moderate 0.031 0.31 negative 

High 0.006 0.49 negative 

values for both years are above accepted thresholds 

for ozone injury (Lefohn et al. 1997). Similarly, O3

exposures were high in 1999 and highest in 2002, 

years when BI values were relatively low in all exposure 

groups. Additionally, regression procedures examined 

the relationship between seasonal mean BI and SUM06 

within each ozone exposure group and found no 

significant results.

An overall downward trend in injury incidence (percent 

injured plants) and severity (species BI) was also noted 

by species over the 17-year survey period as previously 

described (Tables 8, 9). The average percent injured 

plots also declined somewhat over 1994 to 2010, more 

so in NE than in NC (Fig. 12). In 1994, injury was 

detected on more than 60 percent of the evaluated sites 

in NE and on about one-third of the sites in NC. Except 

in 1999, the percent injured sites remained above 30 

percent in NE through 2007, dropping to 25.2 percent 

in 2008 and 18.1 percent in 2009 before increasing to 

28.9 percent in 2010. In NC, percent injured biosites 

rose initially from 31.2 percent in 1994 to 54.1 percent 

in 2000 before dropping back down to less than 26 

percent from 2002 through 2010. It would appear that 

despite an obvious drop in the severity of O3 injury to 

ozone-sensitive bioindicator plants as reflected in the 

BI trend data (Fig. 11), O3 injury was still detected in 

2010 on more than 25 percent of the evaluated sites 

regionwide.

0

10

20

30

40

50

60

70

Percent of Biosites with Injury

Year

NE

NC

Figure 12.—Percent of biosites with injury by year for the NE and 

NC States, 1994-2010.
28

Does site moisture influence injury?

There were both wet and dry O3 seasons over the 

course of the 1994 to 2010 sampling period (http://

www.ncdc.noaa.gov/oa/climate/research/cag3/state.

html). In NE, the trend for most states over the 17 

years of biomonitoring was from dry to wet with the 

majority of years before 2003 having below average 

growing season precipitation and the majority of years 

from 2003 forward having above average growing season 

precipitation (Appendix 2: Table 1). NC States were less 

uniform with respect to growing season precipitation 

trends. Six states (Nebraska, Kansas, Missouri, Iowa, 

Illinois, and Indiana) showed a dry to wet trend very 

similar to NE, while the remaining four more northerly 

states showed either no trend or a slight wet to dry trend 

over 1994 to 2010. For most states with relatively high 

seasonal O3 exposures (e.g., the Mid-Atlantic States and 

Indiana and Illinois), the years when average growing 

season precipitation was lowest (1995, 1999, 2002, 

and 2005) corresponded to the years with the highest 

seasonal O3 exposures. This finding is expected given 

that prolonged periods of hot, dry weather tend to drive 

O3 formation and lead to temperature inversions that 

trap O3 at ground level.

The moisture indices (PDSI and MI) intersected with 

the biosite ground locations provide a more site-specific 

indicator of wet and dry ozone seasons (1994-2007). 

Both PDSI and MI fluctuated from one year to the 

next (Figs. 13, 14). Soil moisture was low in 1995, 

limiting (PDSI < -1.25) in 1999 in high O3 areas, and 

it was near normal for most of the remaining years with 

particularly wet soil moisture conditions prevailing in 

1996 and 2004. Plant moisture deficits (MI < -0.15) 

were indicated for 11 of the 14 years of this sample with 

the most extreme plant moisture deficits occurring in 

1995, 1999, 2002, 2005, and 2007 especially in high O3

areas. By either measure, 1999 was clearly the driest year 

of the 14-year period, with 1995 a close second. The 

dry years correspond to a dropoff in foliar injury from 

1994 to 1995 and again from 1998 to 1999 even though 

seasonal mean ozone concentrations increased from one 

year to the next (Fig. 11). By the same token, injury 

increased from 1995 to 1996, a relatively wet year, 

despite a drop in seasonal mean O3 concentrations.

-3

-2

-1

0

1

2

3

4

19941995199619971998199920002001200220032004200520062007

Mean PDSI

Year

Low ozone sites

High ozone sites

-0.45

-0.4

-0.35

-0.3

-0.25

-0.2

-0.15

-0.1

-0.05

0

0.05

19941995199619971998199920002001200220032004200520062007

Mean MI

Year

Low ozone sites

High ozone sites

Figure 13.—Mean Palmer Drought Severity Index (PDSI) 

values for low and high ozone exposure groups by year, 

1994-2007.

Figure 14.—Mean plant moisture index (MI) value for low 

and high ozone exposure sites by year, 1994-2007.

For this report, data summarized by ozone exposure 

(SUM06) and site moisture (PDSI and MI) classes 

demonstrate that regardless of ozone exposure level 

(clean, low, moderate, high), there is a reduced percent of 

injured biosites (Figs. 15, 16) and a much lower BI (Figs. 

17, 18) in areas with moisture deficits (PDSI < -1.25; 

MI < -0.15) than in areas with adequate or surplus soil 

and plant moisture conditions. Regionwide, the highest 

BI and percent injured plots occur under high ozone-wet 

conditions, followed by high ozone-dry, low ozone-wet, 

and finally low ozone-dry (Table 16).

These findings suggest that although high ambient 

O3 concentrations are the driving force behind the 

proportion of biosites that sustain injury and the severity 

of that injury in any given year, the site moisture 

conditions are also a very strong influence on the 

biomonitoring data. In Maryland, for example, moderate 

to high injury was detected at every site from 1994 
29

through 1998 under conditions of high O3 exposure 

(seasonal mean SUM06 >25 ppm-hr) and variable site 

moisture (wet and dry). In 1999, after two growing 

seasons of below average rainfall, mean site moisture 

values indicated severe drought (PDSI = -3.22) and 

severe plant moisture deficits (MI = -0.45), and although 

seasonal mean SUM06 concentrations remained high 

(SUM06 = 39 ppm-hr), no injury was detected on seven 

of nine evaluated sites. Similar results were obtained in 

other states in 1999 and again in 2002, another very 

dry year regionwide (PDSI = -3.90; MI = -0.53), but 

generally only in high O3 areas such as Indiana and 

Illinois and the Mid-Atlantic States. In areas or years of 

low to moderate O3 exposures, or areas and years lacking 

extreme moisture deficits, the interrelationships among 

Figure 15.—Percent of biosites with injury by ozone exposure group for dry, near normal, and wet soil 

moisture condition (PDSI) sites, 1994-2007.

0

10

20

30

40

50

60

70

Clean (< 8.0) Low (8 - 14.9) Moderate (15 -

24.9)

Percent of Biosites with Injury

High (≥ 25)

SUM06 Ozone Exposure Group (ppm-hrs)

Deficit (MI ≤ -0.15)

Balance (-0.15 < MI ≤ 0.15)

Surplus (MI > 0.15)

0

2

4

6

8

10

12

14

16

18

Clean (< 8.0) Low (8 - 14.9) Moderate (15 -

24.9)

High (≥ 25)

Mean Biosite Index

SUM06 Ozone Exposure Group (ppm-hrs)

Dry: PDSI ≤ -1.25

Near Normal: -1.25 < PDSI ≤ 1.25

Wet: PDSI > 1.25

Figure 16.—Percent of biosites with injury by ozone 

exposure class for plant moisture deficit, balance, and 

surplus condition (MI) sites, 1994-2007.

Figure 17.—Mean biosite index (BI) by ozone exposure class 

for dry, near normal, and wet soil moisture (PDSI) sites, 

1994-2007.

0

5

10

15

20

25

30

Clean (< 8.0) Low (8 - 14.9) Moderate (15 -

24.9)

High (≥ 25)

Mean Biosite Index

SUM06 Ozone Exposure Group (ppm-hrs)

Deficit (MI ≤ -0.15)

Balance (-0.15 < MI ≤ 0.15)

Surplus (MI > 0.15)

Figure 18.-—Mean biosite index (BI) by ozone exposure class 

for plant moisture deficit, balance, and surplus condition (MI) 

sites, 1994-2007.
30

injury, exposure, and site moisture are less predictable 

(Smith 2011, Wang et al. 2012).

What is the risk of ozone injury?

Spatial interpolation of the BI data is performed to 

generate a regional map of O3 risk to plants as defined 

by Smith et al. (2008). In this report, 17 years of BI data 

were used to create an interpolated bioindicator response 

surface (Fig. 19) that was then used to interpret the risk 

of ozone impact to the forest resource across the NRSFIA region. Using procedures outlined by Bechtold and 

Patterson (2005), the BI map was merged with P2 tree 

plot data to generate state-level estimates of acres of forest 

land and volume of ozone-susceptible species in each BI 

category from least injured and no risk of impact to low, 

moderate, and high levels of injury and probable impact. 

The areas of greatest risk included Illinois, Indiana, West 

Table 16.—Calculated means for foliar injury (BI) and percent injured biosites for 

sites grouped in wet and dry categories of soil (PDSI) and plant (MI) moisture in 

areas of low and high ozone

PDSI Soil Moisture MI Plant Moisture

SUM06 Biosite Index % Injured Biosites Biosite Index % Injured Biosites

Wet Dry Wet Dry Wet Dry Wet Dry

Low 1.4 0.4 20.2 9.2 2.2 0.3 25.3 12.6

High 6.2 2.7 44 27.2 7.7 4.1 51.6 34

Figure 19.—Spatial interpolation of the mean biosite index for 1994 to 2010.

Projection: Albers, NAD83.

Sources: FIA 2010.

Geographic base data are provided 

by the National Atlas of the USA. FIA 

data and tools are available online at

http://www.fia.fs.fed.us/tools-data/

Cartography: R.S. Morin. Dec. 2011

Virginia, Maryland, Pennsylvania, and New Jersey. There 

were also large and small pockets of high O3 risk in 

Missouri, Wisconsin, Michigan, Ohio, New York, and 

all of New England except Maine.

In Connecticut, the majority of ozone-sensitive tree 

volume (1,543 million cubic feet) is at risk of O3 impact 

(Table 17); about 1.2 million acres (63%) of forest land 

with 62 percent of the tree volume of ozone-susceptible 

trees are at risk (Table 18), although the risk level is low 

(BI = 5.0-14.9). In Maryland, 0.61 million acres (23%) 

with 22 percent of the volume of ozone-susceptible trees 

are at high risk of O3 impact (BI ≥ 25), about half that 

amount is at moderate risk (BI 15-24.9), and 1.0 million 

acres (38%) with 39 percent of the tree volume is at low 

risk of ozone impact. Less than one-third (27%) of the 

susceptible tree volume is at no risk. New Jersey is the 
31

Table 17.—Estimated volume in million cubic feet of live trees 5 inches in diameter and greater on forest land by biosite index risk category, ozone-sensitivity category, and NE State, 2010

Biosite Indexa Biosite Indexa

State and ozone- 0 to 4.9 5.0 to 14.9 15.0 to 24.9 ≥25 State and ozone- 0 to 4.9 5.0 to 14.9 15.0 to 24.9 ≥25

sensitivity categoryb (no risk) (low risk) (moderate risk) (high risk) sensitivity categoryb (no risk) (low risk) (moderate risk) (high risk)

Connecticut New York Unknown 265 561 0 0 Unknown 4,252 672 49 19 Insensitive 590 838 0 0 Insensitive 12,091 1,420 121 36

 Moderately sensitive 158 450 0 0 Moderately sensitive 966 114 8 2

 Sensitive 783 1,093 0 0 Sensitive 14,232 1,929 113 95

Delaware Ohio Unknown 100 16 0 0 Unknown 3,209 853 134 82

 Insensitive 79 6 0 0 Insensitive 2,761 755 93 90

 Moderately sensitive 64 6 0 0 Moderately sensitive 901 212 19 27

 Sensitive 532 47 8 0 Sensitive 5,842 1,267 220 149Maine Pennsylvania Unknown 1,553 33 0 0 Unknown 4,889 1,644 297 246 Insensitive 12,976 229 0 0 Insensitive 7,226 2,435 354 220 Moderately sensitive 1,179 19 0 0 Moderately sensitive 1,322 525 84 52

 Sensitive 9,213 249 0 0 Sensitive 11,204 4,404 707 463Maryland Rhode Island Unknown 340 504 170 232 Unknown 22 45 0 0

 Insensitive 209 366 228 301 Insensitive 65 123 4 0

 Moderately sensitive 181 255 57 158 Moderately sensitive 79 111 4 0

 Sensitive 1,122 1,592 482 911 Sensitive 100 303 32 0

Massachusetts Vermont Unknown 527 167 29 0 Unknown 508 287 92 116

 Insensitive 2,052 524 54 0 Insensitive 2,953 1,606 566 428 Moderately sensitive 577 171 2 0 Moderately sensitive 254 128 58 63

 Sensitive 3,752 952 74 0 Sensitive 2,300 1,256 393 324

New Hampshire West Virginia Unknown 392 235 74 41 Unknown 4,308 2,406 400 254 Insensitive 2,498 1,356 243 87 Insensitive 4,665 2,647 585 385 Moderately sensitive 387 190 19 13 Moderately sensitive 1,359 974 148 80

 Sensitive 3,056 1,629 173 52 Sensitive 6,250 3,268 687 523New Jersey Total 135,580 43,495 7,045 5,568 Unknown 242 611 45 5 aBiosite index based on interpolated values for each FIA plot.

bOzone-sensitivity categories are based on both field observations and fumigation trials (Smith et al. 2007). Insensitive 252 372 59 14

 Moderately sensitive 183 326 32 6

 Sensitive 560 1,313 128 94
32

only other NE State besides Connecticut and Maryland 

with less forest land in the no risk category than in the 

combined elevated risk categories. Other NE States 

with from 0.07 to 0.50 million acres of forest land with 

some percentage of tree volume at high risk of ozone 

impact include New Hampshire, Vermont, New Jersey, 

New York, Ohio, Pennsylvania, and West Virginia. 

These states also include some percentage of tree volume 

and forest land in the low and moderate risk categories 

although the majority of trees are at no risk. In Delaware 

and Maine, the majority of ozone-susceptible trees and 

forest land are not at risk of ozone impact although both 

states have small percentages of tree volume in the low 

risk category.

In Indiana, about 1.8 million acres of forest land with 

42 percent of the tree volume of susceptible species are 

classified at low risk of O3 impact, about 0.8 million 

acres with 18 percent of the tree volume are at moderate 

risk, and 0.6 million acres with 11 percent of the tree 

volume of susceptible species are classified at high risk of 

O3 impact (Tables 19, 20). This leaves less than onethird of the forest land and susceptible tree volume in 

Indiana at no risk of ozone impact. Other NC States 

with small percentages (2-3%) of forest land in the high 

risk category include Illinois, Michigan, Missouri, and 

Wisconsin, but in all of these states most of the ozonesusceptible tree volume is at no risk of ozone impact. 

Similarly, most of the forest land in Minnesota is at no 

risk of ozone impact although there are 0.08 million 

acres of forest land with 0.6 percent of the susceptible 

tree volume at low risk and 0.02 million acres with 0.1 

percent of the susceptible tree volume at moderate risk. 

All of the forest land in Iowa, Kansas, Nebraska, North 

Dakota, and South Dakota is at no risk of O3 impact.

Modeling Risk

It has been argued that to assess O3 risk to a given 

population of plants, one needs to know the effective O3

dose for that plant population. Effective ozone dose is a 

term used to define the sensitivity of a plant per unit of 

O3 uptake and is thus intended to reflect the inherent 

and often complex traits that define plant response 

including avoidance, tolerance, detoxification, and repair 

(Matyssek et al. 2008, 2010; Paoletti et al. 2010). At 

any given site, a given plant’s response to O3 is likely 

to be further compounded by local weather conditions, 

insect and disease pests, plant age, and many other 

factors (Zhang et al. 2010). Unraveling and quantifying 

(modeling) these factors may be an impossible task. 

Biomonitoring provides an alternative and much 

simplified approach to risk assessment. Documenting 

the response of ozone-sensitive bioindicator plants to 

ambient ozone concentrations in the natural forest 

environment provides the needed integration of biotic 

and abiotic factors for risk assessment. The bioindicator 

plant reveals the effective ozone dose and thus the 

process is simplified to one of careful observation and a 

commitment to high sample size and standardization.

The 17 years of data summarized in this report 

demonstrate that plant response to O3 is highly variable. 

Every ozone season is unique, and no two plants, no 

matter how close together they are growing or whether 

they arise from the same rhizome or are different in 

genus and species, will show the same visible response to 

O3. This fact is inherent to a plant-based injury response 

index and strengthens rather than detracts from the 

reliability of the data. The plant-to-plant and year-to-year 

variability is representative of the natural environment 

and, therefore, provides an accurate and defensible 

assessment of O3 impact to the forest resource.

SUMMARY AND CONCLUSIONS

Summary statistics in this report validate Web-based 

reports from the U.S. EPA (2007) and U.S. DOI (2003) 

that peak ozone concentrations (> 100 ppb O3) during 

the growing season are on the decline although mid-level 

values (60 to 90 ppb O3) are not. Additional evidence of 

changing trends in ozone air quality comes from studies 

conducted both within and outside the United States. 

Lefohn et al. (2008 and 2010) reported a reduction 

in the frequency of higher hourly average ozone 

concentrations from both urban and rural monitoring 

stations in the U.S., while also noting that lower hourly 

average ozone concentrations are shifting toward more 

moderate levels. Similar findings are reported from 

the UK and other European countries (Dawnay and 

Mills 2009, Percy et al. 2003). Reductions in peak 

ozone concentrations are due to the implementation of 
33

Table18.—Estimated area in acres of forest land by biosite index risk 

category and NE State, 2010

Biosite Indexa

0 to 4.9 5.0 to 14.9 15.0 to 24.9 ≥25

State (no risk) (low risk) (moderate risk) (high risk)

Connecticut 702,391 1,199,166 0 0

Delaware 310,277 27,288 3,003

Maine 17,393,361 249,086 0 0

Maryland 683,811 1,000,443 371,332 608,932

Massachusetts 2,556,978 754,101 63,472 0

New Hampshire 2,873,376 1,515,164 200,635 71,922

New Jersey 596,085 1,257,367 147,887 69,189

New York 14,975,772 1,801,806 132,415 76,272

Ohio 6,231,378 1,593,082 223,923 182,336

Pennsylvania 11,558,304 4,120,829 635,146 409,943

Rhode Island 111,639 254,418 12,058 0

Vermont 2,762,448 1,363,859 468,390 414,566

West Virginia 7,190,437 4,148,528 912,397 497,527

Total 67,946,256 19,285,137 3,170,658 2,330,686

a

Biosite index based on interpolated values for each FIA plot.

Table 19.—Estimated area in acres of forest land by biosite index risk 

category and NC State, 2010

Biosite Indexa

0 to 4.9 5.0 to 14.9 15.0 to 24.9 ≥25

State (no risk) (low risk) (moderate risk) (high risk)

Illinois 3,912,578 888,020 54,109 6,833

Indiana 1,144,739 1,840,471 756,703 548,420

Iowa 3,026,078 0 0 0

Kansas 2,437,401 0 0 0

Michigan 18,566,996 1,239,615 153,391 43,415

Minnesota 17,175,228 84,331 21,984 0

Missouri 15,029,386 364,182 50,292 49,906

Nebraska 1,520,468 0 0 0

North Dakota 772,427 0 0 0

South Dakota 1,883,024 0 0 0

Wisconsin 16,249,748 526,245 68,486 27,924

Total 81,718,074 4,942,864 1,104,965 676,498

a

Biosite index based on interpolated values for each FIA plot.
34

Table 20.—Estimated volume in million cubic feet of live trees 5 inches in diameter and greater on forest land by biosite index risk category, ozone-sensitivity category, and NC State, 2010

Biosite Indexa Biosite Indexa

State and ozone- 0 to 4.9 5.0 to 14.9 15.0 to 24.9 ≥25 State and ozone- 0 to 4.9 5.0 to 14.9 15.0 to 24.9 ≥25

sensitivity categoryb (no risk) (low risk) (moderate risk) (high risk) sensitivity categoryb (no risk) (low risk) (moderate risk) (high risk)

Illinois Missouri Unknown 3,033 648 34 7 Unknown 8,126 297 44 28

 Insensitive 1,549 430 14 8 Insensitive 5,538 150 32 16 Moderately sensitive 869 196 10 0 Moderately sensitive 4,789 53 6 9

 Sensitive 1,664 307 26 1 Sensitive 1,444 54 3 19Indiana Nebraska Unknown 582 1,079 383 328 Unknown 829 0 0 0

 Insensitive 502 984 449 284 Insensitive 81 0 0 0

 Moderately sensitive 241 392 133 87 Moderately sensitive 35 0 0 0

 Sensitive 1,041 1,427 634 413 Sensitive 1,076 0 0 0

Iowa North Dakota Unknown 2,511 0 0 0 Unknown 256 0 0 0

 Insensitive 934 0 0 0 Insensitive 19 0 0 0

 Moderately sensitive 301 0 0 0 Moderately sensitive 52 0 0 0

 Sensitive 647 0 0 0 Sensitive 384 0 0 0

Kansas South Dakota Unknown 2,094 0 0 0 Unknown 226 0 0 0

 Insensitive 136 0 0 0 Insensitive 88 0 0 0

 Moderately sensitive 90 0 0 0 Moderately sensitive 32 0 0 0

 Sensitive 777 0 0 0 Sensitive 1,940 0 0 0

Michigan Wisconsin Unknown 2,389 306 100 3 Unknown 2,389 77 4 0

 Insensitive 14,084 826 90 21 Insensitive 9,802 313 40 9

 Moderately sensitive 1,611 228 33 13 Moderately sensitive 1,938 60 12 4

 Sensitive 11,749 825 134 76 Sensitive 8,784 280 42 8

Minnesota Total 112,938 9,033 2,251 1,334 Unknown 3,384 11 3 0 aBiosite index based on interpolated values for each FIA plot.

bOzone-sensitivity categories are based on both field observations and fumigation trials (Smith et al. 2007). Insensitive 5,963 32 13 0

 Moderately sensitive 1,527 3 2 0

 Sensitive 7,433 56 10 0
35

successful pollution control policies that have reduced 

emissions of ozone precursor pollutants such as nitrogen 

oxides (NOx), carbon monoxide (CO), and volatile 

organic compounds (VOCs). Working against this trend 

are the equally important increases in human-made 

sources of these pollutants (from Asia and India, for 

example) that increase NOx, CO, and VOCs emissions 

on a global scale (Percy et al. 2003). The projected result 

for forest areas around the globe is a reduction in the 

magnitude of peak ozone exposure accompanied by an 

increase in long-term chronic exposure. Long-range 

transport of polluted air masses on the local scale from 

urban to rural areas, and on the global scale from Asia 

to North America and from North America to Europe, 

also contributes to higher background concentrations. 

This finding suggests we must remain vigilant in our 

O3 monitoring efforts and devise new ways to measure 

and model relatively small effects of ozone on our most 

sensitive populations (Bennett et al. 2006, Matyssek et 

al. 2010).

Other researchers have reported that both ozone uptake 

and foliar injury correlate with increased availability 

of soil water in the forest environment (Davis and 

Orendovici 2006, Mclaughlin et al. 2007, Orendovici 

et al. 2010, Rose and Coulston 2009, Showman 1991, 

Zhang et al. 2010). In this 24-state ozone survey, the 

field sample is much larger than that of any previous 

field-based study, and the range of site conditions, 

from genotype to soil type, is highly variable both in 

space (Maine to Kansas) and time (1994-2010). Not 

surprisingly then, although we found certain clusterings 

of the data that support the concept that site moisture 

has a strong influence on plant response to ozone, 

we could not document a consistent and predictable 

relationship among injury, ozone, and site moisture 

conditions. The injury indices calculated from the field 

data suggest that the percentage of injured sites and the 

severity of foliar injury are highest on sites categorized 

as high ozone-wet, followed by high ozone-dry, low 

ozone-wet, and low ozone-dry, but these differences are 

not statistically significant (Wang et al. 2012). There is 

just too much variability in the plant/ozone interaction 

under natural conditions, at least on the scale reported 

here; and this is compounded by the fact that under 

natural conditions, so many of the injury values are 

at or near zero (Wang et al. 2012), thus generating a 

skewed dataset. And yet, it is clear that injury drops 

dramatically in areas of high ambient O3 (e.g., MidAtlantic States) when drought conditions prevail as 

happened in 1999, and that injury is minimized when 

seasonal ozone concentrations, especially peak (N100) 

O3 concentrations, drop below a certain threshold as 

in 2004 through 2009. Without additional research, 

we may not be able to predict ozone injury, but we can 

say with confidence that there will be little or no injury 

when and where there are extreme plant moisture deficits 

and where low levels of ozone combine with low plant 

available moisture.

In conclusion, the results of 17 years of ozone injury 

detection provide indisputable evidence that ozoneinduced foliar injury symptoms occur routinely on 

ozone-sensitive bioindicator plants across much of the 

forested landscape of the NC and NE States, and in 

areas previously thought to be relatively ozone free. This 

report provides state-level information on where O3 stress 

occurs and whether O3 stress is increasing or decreasing 

over time, and it provides state-level estimates of the 

acres of forest land and volume of ozone-susceptible 

species at risk of O3 impact. Summarized results for the 

1994 to 2010 time period are as follows:

•	 Ozone injury occurs frequently (70 to 100% 

of visits) on many sites in most NC and NE 

States including Missouri, Wisconsin, Michigan, 

Illinois, Indiana, Ohio, West Virginia, 

Maryland, Delaware, Pennsylvania, New 

York, New Jersey, Connecticut, Rhode Island, 

Massachusetts, New Hampshire, and Vermont.

•	 Ozone injury occurs only infrequently (<40% of 

visits) on most sites in South Dakota, Nebraska, 

Kansas, Iowa, Minnesota, and Maine, and not at 

all in North Dakota. 

•	 Many thousands of ozone-sensitive plants are 

evaluated every year with the percent injured 

plants ranging from 26 percent in 1994 to less 

than 1 percent in 2009 in NE, and from 7 

percent in 1998 to less than 1 percent in 2007 

in NC.
36

•	 Although the percent injured plants and the BI 

(injury severity) declined from 1994 to 2010, 

the percent injured sites showed a less obvious 

downward trend. 

•	 Although seasonal mean SUM06 concentrations 

and N100 values show a declining trend from 

1994 to 2010, ambient ozone concentrations 

remain at plant-damaging levels; one in four 

evaluated sites continues to show injury every 

year even though the severity of that injury has 

declined over time. 

•	 The seasonal mean SUM06 and N100 values 

tend to mirror each other in year-to-year 

fluctuations although N100 values drop close 

to zero by 2004 while SUM06 values remain at 

more moderate levels through 2007 and 2008.

•	 For sites in high O3 areas (seasonal mean 

SUM06 >25 ppm hr), there is a direct 

correlation between injury (BI) and ozone 

exposure. 

•	 Sites with injury (BI > 0) and sites with no 

injury (BI = 0) occur at all O3 exposure levels 

(clean, low, moderate, and high). However, at all 

ozone exposure levels, there is a lower percentage 

of injured sites and less severe injury in dry vs. 

wet areas.

•	 The years with the highest soil moisture and 

plant moisture stress correspond to the years 

with the lowest BI in areas of high seasonal O3

exposure (SUM06 > 25 ppm hrs). 

•	 Based on 17 years of data, the areas of greatest 

risk of impact from ambient ozone exposures 

during the growing season include Illinois, 

Indiana, and the Mid-Atlantic States of West 

Virginia, Maryland, Pennsylvania, and New 

Jersey. 

•	 Pockets of high ozone risk to the forest resource 

also occur in Missouri, Wisconsin, Michigan, 

Ohio, New York, Connecticut, Rhode Island, 

Massachusetts, New Hampshire, and Vermont.

•	 There is little or no risk of O3 impact to the 

forest resource in Maine, Minnesota, Iowa, 

Kansas, Nebraska, South Dakota, and North 

Dakota. 

•	 The documented trends in O3 exposure (SUM06 

and N100) and injury (BI) indicate that 

ozone air quality for the NC and NE States is 

improving over time.

To find additional guidance on ozone biomonitoring, 

data analysis, and interpretation, and to access reports 

from other FIA regions, see the downloadable material 

on the ozone Web site: http://nrs.fs.fed.us/fia/topics/

ozone/default.asp

Acknowledgments

The authors would like to thank John Coulston and 

Ed Jepsen for their expert advice and support over 

the years; Mitchell Pennabaker and Jay Lackey for 

assistance with crew training and quality assurance; 

and the forest agencies for the NE and NC states, the 

State coordinators, regional trainers, field crews, and 

quality assurance staff who collect and verify ozone data. 

Thanks also to John Coulston, Robert Long, Scott Pugh, 

and Anita Rose for serving as reviewers and providing 

insightful, constructive comments.
37

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40

Table 1A.—Percentage of biosites by bioindicator response category by NE State and year (the four BI categories are intended to describe 

increasing risk of probable ozone impact to the forest resource as follows: no risk (BI <5); low risk (BI = 5-15); moderate risk (BI = 15-25); and 

high risk (BI > 25) of ozone impact)

Biosite Index Biosite Index

Year and State 0 to 4.9 5.0 to 14.9 15.0 to 24.9 ≥25 Year and State 0 to 4.9 5.0 to 14.9 15.0 to 24.9 ≥25

1994 1997

 Connecticut 0.0 0.0 100.0 0.0 Connecticut 71.4 14.3 14.3 0.0

 Maine 93.1 6.9 0.0 0.0 Delaware 100.0 0.0 0.0 0.0

 Maryland 0.0 11.1 22.2 66.7 Maine 100.0 0.0 0.0 0.0

 Massachusetts 57.1 21.4 14.3 7.1 Maryland 40.0 0.0 0.0 60.0

 New Hampshire 50.0 16.7 16.7 16.7 Massachusetts 81.3 6.3 12.5 0.0

 New Jersey 44.4 0.0 33.3 22.2 New Hampshire 100.0 0.0 0.0 0.0

 Vermont 57.1 14.3 14.3 14.3 New Jersey 100.0 0.0 0.0 0.0

 Ohio 57.9 10.5 15.8 15.8

1995 Rhode Island 20.0 40.0 20.0 20.0

 Connecticut 42.9 14.3 14.3 28.6 Vermont 64.7 0.0 11.8 23.5

 Delaware 100.0 0.0 0.0 0.0 West Virginia 92.3 7.7 0.0 0.0

 Maine 94.1 2.9 0.0 2.9

 Maryland 11.1 33.3 33.3 22.2 1998

 Massachusetts 80.0 20.0 0.0 0.0 Connecticut 0.0 0.0 60.0 40.0

 New Hampshire 91.3 4.3 4.3 0.0 Delaware 33.3 0.0 33.3 33.3

 New Jersey 33.3 0.0 16.7 50.0 Maine 92.9 7.1 0.0 0.0

 Pennsylvania 83.3 8.3 0.0 8.3 Maryland 16.7 25.0 0.0 58.3

 Rhode Island 20.0 60.0 20.0 0.0 Massachusetts 72.2 27.8 0.0 0.0

 Vermont 77.8 5.6 16.7 0.0 New Hampshire 80.0 4.0 8.0 8.0

 West Virginia 94.4 0.0 5.6 0.0 New Jersey 9.1 9.1 9.1 72.7

 Ohio 52.6 15.8 0.0 31.6

1996 Pennsylvania 61.0 7.0 1.0 31.0

 Connecticut 28.6 14.3 14.3 42.9 Rhode Island 20.0 0.0 40.0 40.0

 Maine 88.9 7.4 0.0 3.7 Vermont 70.6 5.9 5.9 17.6

 Maryland 0.0 0.0 20.0 80.0 West Virginia 26.9 15.4 19.2 38.5

 Massachusetts 84.6 7.7 7.7 0.0

 New Hampshire 83.3 12.5 4.2 0.0 1999

 New Jersey 33.3 16.7 33.3 16.7 Connecticut 100.0 0.0 0.0 0.0

 Rhode Island 20.0 80.0 0.0 0.0 Delaware 18.2 27.3 18.2 36.4

 Vermont 50.0 18.8 12.5 18.8 Maine 100.0 0.0 0.0 0.0

 West Virginia 56.5 8.7 13.0 21.7 Maryland 80.0 0.0 10.0 10.0

 Massachusetts 88.9 0.0 5.6 5.6

 New Hampshire 95.7 4.3 0.0 0.0

 New Jersey 100.0 0.0 0.0 0.0

 New York 96.5 2.4 0.0 1.2

 Ohio 83.3 11.1 0.0 5.6

 Pennsylvania 92.2 2.3 1.6 3.9

 Rhode Island 50.0 0.0 25.0 25.0

 Vermont 95.2 4.8 0.0 0.0

 West Virginia 92.9 0.0 7.1 0.0

Appendix 1
41

Table 1A.—continued

Biosite Index Biosite Index

Year and State 0 to 4.9 5.0 to 14.9 15.0 to 24.9 ≥25 Year and State 0 to 4.9 5.0 to 14.9 15.0 to 24.9 ≥25

2000 2003

 Connecticut 57.1 28.6 14.3 0.0 Connecticut 71.4 28.6 0.0 0.0

 Delaware 44.4 22.2 22.2 11.1 Delaware 54.5 36.4 9.1 0.0

 Maine 100.0 0.0 0.0 0.0 Maine 100.0 0.0 0.0 0.0

 Maryland 100.0 0.0 0.0 10.0 Maryland 77.8 11.1 0.0 11.1

 Massachusetts 92.9 7.1 0.0 0.0 Massachusetts 100.0 0.0 0.0 0.0

 New Hampshire 100.0 0.0 0.0 0.0 New Hampshire 100.0 0.0 0.0 0.0

 New Jersey 100.0 0.0 0.0 0.0 New Jersey 100.0 10.0 0.0 0.0

 New York 100.0 0.0 0.0 0.0 New York 94.6 2.7 2.7 0.0

 Ohio 78.9 5.3 10.5 5.3 Ohio 94.1 5.9 0.0 0.0

 Pennsylvania 69.0 9.0 6.0 16.0 Pennsylvania 98.0 0.0 0.0 2.0

 Rhode Island 80.0 20.0 0.0 0.0 Rhode Island 80.0 20.0 0.0 0.0

 Vermont 86.4 9.1 0.0 4.5 Vermont 94.4 5.6 0.0 0.0

 West Virginia 93.1 3.4 0.0 3.4 West Virginia 92.6 3.7 0.0 3.7

2001 2004

 Connecticut 33.3 33.3 33.3 0.0 Connecticut 40.0 40.0 20.0 0.0

 Delaware 70.0 30.0 0.0 0.0 Delaware 54.5 27.3 9.1 9.1

 Maine 100.0 0.0 0.0 0.0 Maine 100.0 0.0 0.0 0.0

 Maryland 55.6 22.2 3.7 18.5 Maryland 100.0 0.0 0.0 0.0

 Massachusetts 87.5 0.0 12.5 0.0 Massachusetts 100.0 0.0 0.0 0.0

 New Hampshire 89.5 5.3 5.3 0.0 New Hampshire 80.0 0.0 0.0 20.0

 New Jersey 100.0 0.0 0.0 0.0 New Jersey 62.5 12.5 12.5 12.5

 New York 96.4 1.8 0.0 1.8 New York 83.3 11.1 0.0 5.6

 Ohio 82.4 5.9 0.0 11.8 Ohio 91.2 8.8 0.0 0.0

 Pennsylvania 82.7 7.7 4.8 4.8 Pennsylvania 85.4 12.5 0.0 2.1

 Rhode Island 25.0 50.0 0.0 25.0 Rhode Island 100.0 0.0 0.0 0.0

 Vermont 94.4 5.6 0.0 0.0 Vermont 95.0 5.0 0.0 0.0

 West Virginia 93.3 0.0 0.0 6.7 West Virginia 82.1 7.1 7.1 3.6

2002 2005

 Connecticut 75.0 25.0 0.0 0.0 Connecticut 75.0 25.0 0.0 0.0

 Delaware 72.7 18.2 9.1 0.0 Delaware 62.5 25.0 0.0 12.5

 Maine 100.0 0.0 0.0 0.0 Maine 100.0 0.0 0.0 0.0

 Maryland 75.0 0.0 0.0 25.0 Maryland 44.4 22.2 0.0 33.3

 Massachusetts 100.0 0.0 0.0 0.0 Massachusetts 100.0 0.0 0.0 0.0

 New Hampshire 100.0 0.0 0.0 0.0 New Hampshire 100.0 0.0 0.0 0.0

 New Jersey 80.0 10.0 10.0 0.0 New Jersey 66.7 0.0 11.1 22.2

 New York 81.1 5.4 2.7 10.8 New York 100.0 0.0 0.0 0.0

 Ohio 88.2 11.8 0.0 0.0 Ohio 97.1 2.9 0.0 0.0

 Pennsylvania 58.0 22.0 8.0 12.0 Pennsylvania 98.3 1.7 0.0 0.0

 Rhode Island 20.0 60.0 20.0 0.0 Rhode Island 80.0 0.0 20.0 0.0

 Vermont 94.1 0.0 5.9 0.0 Vermont 94.4 0.0 0.0 5.6

 West Virginia 77.8 11.1 0.0 11.1 West Virginia 92.9 3.6 3.6 0.0
42

Table 1A.—continued

Biosite Index Biosite Index

Year and State 0 to 4.9 5.0 to 14.9 15.0 to 24.9 ≥25 Year and State 0 to 4.9 5.0 to 14.9 15.0 to 24.9 ≥25

2006 2009

 Connecticut 0.0 80.0 20.0 0.0 Connecticut 87.5 12.5 0.0 0.0

 Delaware 72.7 9.1 9.1 9.1 Delaware 100.0 0.0 0.0 0.0

 Maine 94.4 5.6 0.0 0.0 Maine 94.4 5.6 0.0 0.0

 Maryland 100.0 0.0 0.0 0.0 Maryland 87.5 12.5 0.0 0.0

 Massachusetts 70.0 30.0 0.0 0.0 Massachusetts 90.0 10.0 0.0 0.0

 New Hampshire 100.0 0.0 0.0 0.0 New Hampshire 80.0 20.0 0.0 0.0

 New Jersey 50.0 12.5 25.0 12.5 New Jersey 75.0 12.5 0.0 12.5

 New York 97.1 2.9 0.0 0.0 New York 100.0 0.0 0.0 0.0

 Ohio 100.0 0.0 0.0 0.0 Ohio 94.4 2.8 2.8 0.0

 Pennsylvania 92.5 3.0 4.5 0.0 Pennsylvania 94.8 3.4 0.0 1.7

 Rhode Island 40.0 40.0 20.0 0.0 Rhode Island 60.0 40.0 20.0 0.0

 Vermont 92.9 7.1 0.0 0.0 Vermont 91.7 8.3 0.0 5.6

 West Virginia 89.3 7.1 3.6 0.0 West Virginia 100.0 0.0 0.0 0.0

2007 2010

 Connecticut 85.7 14.3 0.0 0.0 Connecticut 100.0 0.0 0.0 0.0

 Delaware 81.8 9.1 9.1 0.0 Delaware 100.0 0.0 0.0 0.0

 Maine 100.0 0.0 0.0 0.0 Maine 94.4 5.6 0.0 0.0

 Maryland 100.0 0.0 0.0 0.0 Maryland 87.5 0.0 12.5 0.0

 Massachusetts 100.0 0.0 0.0 0.0 Massachusetts 100.0 0.0 0.0 0.0

 New Hampshire 100.0 0.0 0.0 0.0 New Hampshire 40.0 20.0 20.0 20.0

 New Jersey 75.0 12.5 12.5 0.0 New Jersey 50.0 25.0 12.5 12.5

 New York 92.1 2.6 0.0 5.3 New York 91.4 5.7 0.0 2.9

 Ohio 97.1 2.9 0.0 0.0 Ohio 100.0 0.0 0.0 0.0

 Pennsylvania 87.5 10.7 1.8 0.0 Pennsylvania 80.6 17.7 1.6 0.0

 Rhode Island 80.0 20.0 0.0 0.0 Rhode Island 100.0 0.0 20.0 0.0

 Vermont 92.3 7.7 0.0 0.0 Vermont 92.3 7.7 0.0 0.0

 West Virginia 89.3 7.1 0.0 3.6 West Virginia 82.1 17.9 0.0 0.0

2008

 Connecticut 71.4 28.6 0.0 0.0

 Delaware 83.3 16.7 0.0 0.0

 Maine 100.0 0.0 0.0 0.0

 Maryland 100.0 0.0 0.0 0.0

 Massachusetts 88.9 11.1 0.0 0.0

 New Hampshire 100.0 0.0 0.0 0.0

 New Jersey 75.0 25.0 0.0 0.0

 New York 90.2 9.8 0.0 0.0

 Ohio 97.1 2.9 0.0 0.0

 Pennsylvania 97.7 2.3 0.0 0.0

 Rhode Island 100.0 0.0 0.0 0.0

 Vermont 90.9 9.1 0.0 0.0

 West Virginia 90.6 6.3 3.1 0.0
43

Table 1B.—Percentage of biosites by bioindicator response category by NC State and year (the four BI categories are intended to describe 

increasing risk of probable ozone impact to the forest resource as follows: no risk (BI <5); low risk (BI = 5-15); moderate risk (BI = 15-25); and 

high risk (BI > 25) of ozone impact) 

Biosite Index Biosite Index

Year and State 0 to 4.9 5.0 to 14.9 15.0 to 24.9 ≥25 Year and State 0 to 4.9 5.0 to 14.9 15.0 to 24.9 ≥25

1994 2000

 Michigan 91.7 8.3 0.0 0.0 Illinois 72.5 20.0 2.5 5.0

 Minnesota 100.0 0.0 0.0 0.0 Indiana 67.7 12.9 6.5 12.9

 Wisconsin 93.8 0.0 6.3 0.0 Iowa 100.0 0.0 0.0 0.0

 Michigan 85.7 12.2 2.0 0.0

1995 Minnesota 96.2 3.8 0.0 0.0

 Michigan 92.5 5.0 0.0 2.5 Missouri 93.0 3.5 1.8 1.8

 Minnesota 100.0 0.0 0.0 0.0 Wisconsin 92.2 6.3 0.0 1.6

 Wisconsin 93.8 4.6 0.0 1.5

2001

1996 Illinois 90.7 9.3 0.0 0.0

 Indiana 12.5 0.0 0.0 87.5 Indiana 46.2 30.8 7.7 15.4

 Michigan 92.6 3.7 0.0 3.7 Iowa 100.0 0.0 0.0 0.0

 Minnesota 100.0 0.0 0.0 0.0 Michigan 90.7 3.7 3.7 1.9

 Wisconsin 96.4 0.0 0.0 3.6 Minnesota 100.0 0.0 0.0 0.0

 Missouri 92.1 5.3 2.6 0.0

1997 Wisconsin 97.4 2.6 0.0 0.0

 Illinois 11.1 11.1 11.1 66.7

 Indiana 70.0 20.0 0.0 10.0 2002

 Michigan 100.0 0.0 0.0 0.0 Illinois 92.6 7.4 0.0 0.0

 Minnesota 100.0 0.0 0.0 0.0 Indiana 86.4 13.6 0.0 0.0

 Wisconsin 97.1 2.9 0.0 0.0 Iowa 100.0 0.0 0.0 0.0

 Kansas 100.0 0.0 0.0 0.0

1998 Michigan 91.1 6.7 2.2 0.0

 Illinois 10.5 15.8 36.8 36.8 Minnesota 100.0 0.0 0.0 0.0

 Indiana 5.9 11.8 0.0 82.4 Missouri 92.3 5.1 0.0 2.6

 Michigan 98.3 1.7 0.0 0.0 Nebraska 100.0 0.0 0.0 0.0

 Minnesota 98.3 1.7 0.0 0.0 North Dakota 100.0 0.0 0.0 0.0

 Wisconsin 85.7 4.8 4.8 4.8 South Dakota 100.0 0.0 0.0 0.0

 Wisconsin 98.2 1.8 0.0 0.0

1999

 Illinois 65.385 26.923 3.846 3.846

 Indiana 63.158 21.053 5.263 10.526

 Michigan 83.721 13.953 0 2.326

 Minnesota 100 0 0 0

 Wisconsin 94.444 5.556 0 0
44

Table 1B.—continued

Biosite Index Biosite Index

Year and State 0 to 4.9 5.0 to 14.9 15.0 to 24.9 ≥25 Year and State 0 to 4.9 5.0 to 14.9 15.0 to 24.9 ≥25

2003 2006

 Illinois 92.9 3.6 0.0 3.6 Illinois 96.4 3.6 0.0 0.0

 Indiana 80.6 12.9 0.0 6.5 Indiana 82.6 13.0 4.3 0.0

 Iowa 100.0 0.0 0.0 0.0 Iowa 75.0 25.0 0.0 0.0

 Kansas 100.0 0.0 0.0 0.0 Kansas 100.0 0.0 0.0 0.0

 Michigan 100.0 0.0 0.0 0.0 Michigan 88.9 8.9 2.2 0.0

 Minnesota 100.0 0.0 0.0 0.0 Minnesota 100.0 0.0 0.0 0.0

 Missouri 100.0 0.0 0.0 0.0 Missouri 97.4 2.6 0.0 0.0

 Nebraska 100.0 0.0 0.0 0.0 Nebraska 100.0 0.0 0.0 0.0

 North Dakota 100.0 0.0 0.0 0.0 North Dakota 100.0 0.0 0.0 0.0

 South Dakota 100.0 0.0 0.0 0.0 South Dakota 100.0 0.0 0.0 0.0

 Wisconsin 98.2 1.8 0.0 0.0 Wisconsin 96.8 0.0 3.2 0.0

2004 2007

 Illinois 89.7 6.9 3.4 0.0 Illinois 96.9 3.1 0.0 0.0

 Indiana 89.3 10.7 0.0 0.0 Indiana 84.0 16.0 0.0 0.0

 Iowa 100.0 0.0 0.0 0.0 Iowa 100.0 0.0 0.0 0.0

 Kansas 100.0 0.0 0.0 0.0 Kansas 100.0 0.0 0.0 0.0

 Michigan 88.4 9.3 2.3 0.0 Michigan 97.9 2.1 0.0 0.0

 Minnesota 100.0 0.0 0.0 0.0 Minnesota 100.0 0.0 0.0 0.0

 Missouri 94.7 2.6 0.0 2.6 Missouri 100.0 0.0 0.0 0.0

 Nebraska 100.0 0.0 0.0 0.0 Nebraska 100.0 0.0 0.0 0.0

 North Dakota 100.0 0.0 0.0 0.0 North Dakota 100.0 0.0 0.0 0.0

 South Dakota 100.0 0.0 0.0 0.0 South Dakota 100.0 0.0 0.0 0.0

 Wisconsin 100.0 0.0 0.0 0.0 Wisconsin 100.0 0.0 0.0 0.0

2005 2008

 Illinois 96.4 3.6 0.0 0.0 Illinois 90.3 9.7 0.0 0.0

 Indiana 96.2 0.0 0.0 3.8 Indiana 80.0 16.0 0.0 4.0

 Iowa 100.0 0.0 0.0 0.0 Iowa 100.0 0.0 0.0 0.0

 Kansas 100.0 0.0 0.0 0.0 Kansas 100.0 0.0 0.0 0.0

 Michigan 88.9 6.7 2.2 2.2 Michigan 92.0 2.0 6.0 0.0

 Minnesota 100.0 0.0 0.0 0.0 Minnesota 100.0 0.0 0.0 0.0

 Missouri 100.0 0.0 0.0 0.0 Missouri 100.0 0.0 0.0 0.0

 Nebraska 100.0 0.0 0.0 0.0 Nebraska 100.0 0.0 0.0 0.0

 North Dakota 100.0 0.0 0.0 0.0 North Dakota 100.0 0.0 0.0 0.0

 South Dakota 100.0 0.0 0.0 0.0 South Dakota 100.0 0.0 0.0 0.0

 Wisconsin 100.0 0.0 0.0 0.0 Wisconsin 100.0 0.0 0.0 0.0
45

Table 1B.-—continued

Biosite Index Biosite Index

Year and State 0 to 4.9 5.0 to 14.9 15.0 to 24.9 ≥25 Year and State 0 to 4.9 5.0 to 14.9 15.0 to 24.9 ≥25

2009 2010

 Illinois 96.8 3.2 0.0 0.0 Illinois 96.8 3.2 0.0 0.0

 Indiana 56.0 40.0 0.0 4.0 Indiana 69.2 23.1 3.8 3.8

 Iowa 100.0 0.0 0.0 0.0 Iowa 100.0 0.0 0.0 0.0

 Kansas 100.0 0.0 0.0 0.0 Kansas 100.0 0.0 0.0 0.0

 Michigan 90.0 8.0 2.0 0.0 Michigan 88.2 5.9 2.0 3.9

 Minnesota 100.0 0.0 0.0 0.0 Minnesota 100.0 0.0 0.0 0.0

 Missouri 100.0 0.0 0.0 0.0 Missouri 100.0 0.0 0.0 0.0

 Nebraska 100.0 0.0 0.0 0.0 Nebraska 100.0 0.0 0.0 0.0

 North Dakota 100.0 0.0 0.0 0.0 North Dakota 100.0 0.0 0.0 0.0

 South Dakota 100.0 0.0 0.0 0.0 South Dakota 100.0 0.0 0.0 0.0

 Wisconsin 100.0 0.0 0.0 0.0 Wisconsin 100.0 0.0 0.0 0.0
46

Appendix 2 Table 1.—State-level precipitation data for the June through August growing season obtained from the National Climate Data Center at the following Web site: http://www.ncdc. noaa.gov/oa/climate/research/cag3/state.html (mapped data are summarized below to highlight, for each state, the years above and below average precipitation, the driest and wettest years, and the overall trend in growing season precipitation for the 1994 to 2010 sampling period) NE States Above average precipitation Below average precipitation Driest 2nd Driest Wettest 2nd Wettest Trend Maine 1996, 1998, 2004-2006, 2008-2010 1994-1995, 1997, 1999-2003, 2007 19952 2001 2009 2008 Dry to wet New Hampshire 1998, 2003-2006, 2008-2009 1994-1997, 1999-2002, 2007, 2010 1995 1999 2006 2008 Dry to wet

 Vermont 1994, 1996-1998, 2000, 2002-2010 1995, 1999, 2001 1999 2001 1998 2008 Dry to wet

 Massachusetts 1994, 1996, 1998, 2000-2001, 2003-2004, 2006, 2008-2009 1995, 1997, 1999, 2002, 2005, 2007, 2010 1995 1999 2009 2006 Dry to wet Connecticut 1994, 1996-1998, 2000-2004, 2006, 2008-2009 1995, 1999, 2005, 2007, 2010 1999 1995 2009 2006 Dry to wet

 Rhode Island 1994, 1996-1998, 2000-2001, 2003-2004, 2006, 2008-2010 1995, 1999, 2002, 2005, 2007 1999 1995 2006 2009 Dry to wet

 New York 1994, 1996, 1998, 2000, 2003-2006, 2008-2010 1995, 1997, 1999, 2001-2002, 2007 1999 1995 2006 2009 Dry to wet

 New Jersey 1994, 1996-1997, 2000, 2003-2004, 2006-2007, 2009 1995, 1998-1999, 2001-2002, 2005, 2008, 2010 20102 1995 2009 2003 Dry to wet Delaware 1994, 1996, 2000-2001, 2003-2004, 2006, 2009 1995, 1997-1999, 2002, 2005, 2007-2008, 2010 1995 1999 2003 2001 Dry to wet

 Maryland 1994, 1996, 2000-2001, 2003-2004, 2006, 2009 1995, 1997-1999, 2002, 2005, 2007-2008, 2010 20022 1995 2003 1996 - Pennsylvania 1994, 1996, 2000, 2003-2004, 2006-2007, 2009 1995, 1997-1999, 2001-2002, 2005, 2008, 2010 1995 1999 2003 2006 Dry to wet

 West Virginia 1994, 1996, 1998, 2000-2001, 2003-2004, 2006, 2008-2010 1995, 1997, 1999, 2002, 2005, 2007 1999 2002 2003 1996 -

 Ohio 1994-1995, 1997-1998, 2000, 2003-2004, 2006-2010 1996, 1999, 2001,2002, 2005 2002 1999 2003 1998 Dry to wetNC States Indiana 1996-1998, 2000-2001, 2003-2006, 2008-2010 1994-1995, 1999, 2002, 2007 2002 1999 1998 2003 Dry to wet

 Illinois 1998, 2000-2001, 2004, 2006-2010 1994-1997, 1999, 2002, 2005 2005 1999 2010 2000 Dry to wet Iowa 1994, 1998-2000, 2002, 2007-2010 1995-1997, 2001, 2003-2006 2003 2001 2010 2008 Dry to wet Missouri 1995, 1998, 2000-2001, 2004-2005, 2008-2010 1994, 1996-1997, 1999, 2002-2003, 2006-2007 2002 2007 1998 2008 Dry to wet

 Michigan 1994-1996, 1999-2000, 2002, 2004, 2008-2010 1997-1998, 2001, 2003, 2005-2007 1998 2003 1994 2010 Wet to dry Wisconsin 1994-2002, 2007-2008, 2010 2003-2006, 2009 2003 2005 2010 1999 Wet to dry

 Minnesota 1994-1995, 1997-2000, 2002, 2005, 2010 1996, 2001, 2003-2004, 2006-2009 2006 2003 2002 2010 Wet to dry North Dakota 1994-1995, 1996-2002, 2005, 2008, 2010 1996, 2003-2004, 2006-2007, 2009 2006 2003 2005 2002 Wet to dry

 South Dakota 1994-1995, 1996-1999, 2001, 2005, 2007-2010 1996, 2000, 2002-2004, 2006 2006 2003 2010 2009 Dry to wet Nebraska 1994, 1996, 1998-1999, 2005, 2007, 2009-2010 1995, 1997, 2000-2004, 2006, 2008 2002 2003 2010 2009 Dry to wet

 Kansas 1995-1999, 2004-2010 1994, 2000-2003 2002 2003 2005 2004 Dry to wet
Printed on Recycled Paper

Smith, Gretchen C.; Morin, Randall S.; McCaskill, George L. 2012. Ozone injury to 

forests across the northeast and north central United States, 1994 - 2010. 

Gen. Tech. Rep. NRS-103. Newtown Square, PA: U.S. Department of Agriculture, 

Forest Service, Northern Research Station. 46 p.

Ozone is a highly toxic air contaminant that has been shown to decrease tree growth 

and cause signifi cant disturbance to forested ecosystems. Ozone also causes distinct 

foliar injury symptoms to certain species (bioindicator plants) that can be used to 

detect and monitor ozone stress (biomonitoring) in the forest environment. In the early 

1990s, the U.S. Forest Service, in partnership with the U.S. Environmental Protection 

Agency, developed and implemented a suite of forest health indicators to respond to 

emerging demands for a comprehensive assessment of the health of U.S. forests. This 

report focuses on the states in the Northern Research Station-Forest Inventory and 

Analysis region, which has the longest record of ozone biomonitoring in the country, 

from 1994 through 2010. The results of 17 years of ozone injury detection provide 

indisputable evidence that ozone-induced foliar injury symptoms occur routinely on 

ozone-sensitive bioindicator plants across much of the forested landscape and in 

areas previously thought to be relatively ozone free. This report provides state-level 

information on where ozone stress occurs and whether ozone stress is increasing or 

decreasing over time. It also provides state-level estimates of the acres of forest land 

and volume of ozone-susceptible species at risk of ozone impact.

KEY WORDS: biomonitoring, FIA, forest health, indicator species, ozone

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