United States
Environmental Protection
Agency
Atmospheric Research and
Exposure Assessment Laboratory
Research Triangle Park, NC 27711
Research and Development
EPA/620/SR-94/028 January 1995
EPA Project Summary
Forest Health Monitoring 1991
Statistical Summary
The interagency Forest Health Moni-
toring program has been implemented
in 12 eastern states. The full report is
the first comprehensive statistical re-
port by the program. It provides statis-
tical summaries of tree health and forest
condition measurements that were
made at 628 field locations in 1991.
Based on an analysis of selected mea-
surements of tree crown condition, tree
crowns appeared to be healthy for
nearly all of the species and forest
types that were sampled. The regional
status of overstory tree species diver-
sity and stand density is also summa-
rized. Summaries of selected off-plot
data describe climate, forest pests, and
air pollution in the regions where the
field measurements were made. Future
reports will consider additional states
and more measurements as they be-
come available.
This Project Summary was developed
by EPA's Atmospheric Research and
Exposure Assessment Laboratory, Re-
search Triangle Park, NC, to announce
key findings of the research project
that is fully documented in a separate
report of the same title (see Project
Report ordering information at back).
Background
In response to the diverse and growing
public concerns about potential human
impacts on our environment, the U.S. For-
est Service and the U.S Environmental
Protection Agency (EPA) have initiated a
cooperative national program of Forest
Health Monitoring (FHM). Although not es-
tablished in all parts of the nation, FHM
has already provided valuable information
about forest health in some areas. The
objective is to provide a many-sided view
of forest health that will assist the public
and other decision makers in setting pri-
orities and making informed choices aimed
at reducing the ecological risks of human
impacts.
The purposes of the full report are to
describe the current approach and activi-
ties, to summarize the data that were col-
lected in 1991, and to outline some new
directions that are being explored for pos-
sible implementation later. The report is
also a test of an assessment process by
which data from many sources will be
brought together for analyses. It is antici-
pated that suggestions received from the
readers will enable FHM to tailor future
reports to better meet the needs of infor-
mation users. The appendices to the full
report contain tables and charts so that
readers may explore particular items of
interest. A description of the statistical pro-
cedures used to prepare the report is also
provided.
Procedures
There are now 925 plots in the FHM
national network, of which 628 plots are
forested. This is about 16% (excluding
Alaska) of the projected total number of
forested plots that will be installed nation-
wide over the next several years. The
installed plots are located systematically,
using a probability sampling design,
throughout the forests of 12 states in the
eastern U.S. During the late summer of
1991, these plots were visited by trained
field crews to make selected measure-
ments of forest health. Over 45,000 trees
and seedlings of more than 100 species
in 10 major forest types were measured
by state and federal personnel. Through a
rigorous quality assurance program, these
data were found to meet data quality ob-
jectives. The data and supporting docu-
mentation have been entered into the FHM
databases.
At each one-hectare forested plot loca-
tion, four fixed-radius (7.32 meters) sub-
plots were established. Within these
subplots, forest condition measurements
were made of tree size, species, fre-
quency, damage, and crown condition.
Additional measurements, such as slope
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and aspect, were made to characterize
the forested plot locations. Forest condi-
tion measurements were analyzed by us-
ing statistical procedures, and the results
were expressed in tables, charts, and cu-
mulative distribution functions. The cumu-
lative distribution functions of the crown
condition measurements were further ana-
lyzed to estimate the proportions of the
sampled populations with different health
status.
Data summaries and maps were also
prepared for three types of off-plot data:
climate, forest insects and diseases, and
air pollution. For these off-plot data types,
selected measurements that are made by
other monitoring programs were summa-
rized in a way that will permit future analy-
ses of the possible determinants of forest
health as indicated by the on-plot mea-
surements. It was not possible to use the
FHM statistical estimators for the off-plot
data types; summary procedures that were
appropriate for each particular off-plot da-
tabase were used instead.
Results and Discussion—
On-Plot Data
Although conclusions about regional for-
est health are tenuous when they are
based on just a few measurements taken
in just one year, the following general
findings emerged from statistical analyses
of the 1991 on-plot data:
Tree Size and Stand Density
Tree size and stand density mea-
surements are important for many analy-
ses of forest health and productivity. The
full report summarizes stand density (ex-
pressed as basal area per unit area) and
tree frequency (number of stems per unit
area) for selected subpopulations that were
measured in 1991. Results are presented
in the form of cumulative distribution func-
tions. These data will be the basis for
assessments of tree and stand growth
after the plots are remeasured in later
years.
Tree Crown Conditions
Tree damage and crown condition were
measured on all trees > 12.7 cm dbh on
forested subplots. The full report presents
summary tables and cumulative distribu-
tion functions of selected measurements,
aggregated by species, forest type, and
crown group. Subpopulations of species,
forest types, and crown groups with a
sufficient sample size were also delineated
into categories of optimal, nominal,
subnominal, and poor condition for se-
lected measurements, based on expert
judgment or published "threshold" values
for those categories.
The measurement procedures are de-
scribed in detail in the full report. Briefly,
the following crown measurements were
made:
Crown transparency
A measure of the amount of sun-
light that passes through foliated
portions of a crown.
Crown dieback
A measure of the mortality of rela-
tively young branches that are in
the upper, sunlight-exposed por-
tions of the crown.
Crown position
A measure of the relative physical
location of a tree crown in a stand
of trees.
Crown density
A measure of the two-dimensional
appearance of crown fullness and
symmetry.
Crown ratio
A measure of the amount of an
entire tree bole that is foliated.
A Crown Defoliation Indicator (GDI) model
was also used to describe overall crown
condition as a function of the crown trans-
parency and crown dieback measure-
ments.
The interpretation of tree crown condi-
tion measurements depends not only on
the ecological significance of a particular
measured value, but also on the perceived
importances of relatively high or relatively
low values. The approach taken here to
setting the threshold values of condition
was conservative. Where possible, the
setting of threshold values considered ex-
pected differences among species, size
classes, crown positions, and typical habi-
tat characteristics.
A cursory evaluation of the cumulative
distribution function analysis results indi-
cated that most of the tree species, all
examined forest types, and most crown
groups examined were in good condition.
It was recognized that since most of the
population for species and crown groups
were found to be nominal, the thresholds
might be too high. If this is true, then the
percentages of the population that were in
a subnominal condition should be of
greater concern.
Research both off the detection moni-
toring plots and using data from the de-
tection monitoring plots is needed to set
consistent and tailored threshold values
for each species, forest type, and/or crown
group.
Tree Species Diversity
The full report quantifies the tree spe-
cies diversity of the overstory community
by using three well known diversity indi-
ces—the number of species present, the
natural exponent of Shannon's index, and
the reciprocal of Simpson's index. Data
for trees > 12.7 cm diameter at breast
height were used to calculate the diversity
indices. Cumulative distribution functions
of the indices were used to quantify the
1991 regional status of overstory tree spe-
cies diversity in the northeast and south-
east U.S.
The cumulative distribution functions of
the diversity indices were not significantly
different within either geographic region.
Using the exponent of Shannon's index to
illustrate the results for the northeast re-
gion, cumulative probability values of 0.25,
0.50, and 0.75 were obtained when the
index was 2.2, 3.6, and 4.6, respectively.
The corresponding index values for the
southeast region were 1.6, 2.9, and 4.6,
respectively.
Although statistical relationships do not
imply cause and effect, some associa-
tions were explored among forest charac-
teristics and the values of the exponent of
Shannon's index. In the southeast region,
high overstory tree species diversity val-
ues were most closely associated with an
oak forest type, natural stand origin, saw
timber size class, no recent disturbances,
and more than one plot condition code. In
contrast, low values were most closely
associated with a pine forest type, planted
stand origin, seedling/sapling size class,
recent cutting disturbance, and one plot
condition code. Interpretations of these
statistical associations were difficult be-
cause of confounding or correlations
among the forest characteristics.
In the northeast region, the possible
associations of diversity values and forest
characteristics were even more difficult to
investigate because there was relatively
little variation in forest characteristics. For
example, the northern hardwoods forest
type had sample plots distributed over the
full range of diversity index values, but
stand origins and size classes were the
same for most of those plots. Although
few pine plots were sampled in the north-
east, their proportion decreased as diver-
sity values increased. In contrast to the
southeast region, recent disturbances were
not associated with diversity values in the
northeast region.
The cumulative distribution functions of
the two geographic regions were most
different at lower diversity values. The dif-
ference was not statistically significant and
was explainable by higher proportions of
the pine forest type and planted stand
origin in the southeast in comparison to
the northeast. The number of species was
more sensitive than the exponent of
Shannon's index to regional differences.
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Results and Discussion—
Off-Plot Data
Three types of off-plot information were
introduced into the FHM databases in
1991. Data from the Forest Service (for-
est insects and diseases), EPA (ozone
and wet deposition), and the National Oce-
anic and Atmospheric Administration (pre-
cipitation, temperature, and significant
weather events) were summarized by FHM
analysts. In future reports, these and other
summaries will be used to explore pos-
sible regional associations between on-plot
measurements and particular stresses that
are of concern. The highlights of the auxil-
iary data summaries follow:
Major Forest Insects and
Diseases (Eastern U.S.)
A compilation of information from vari-
ous state and federal sources identified
four problems of special interest. A new
disease (cause unknown) of blackgum has
been found in the Appalachian Mountains
in three states. Thousands of cabbage
palms have died from unknown causes
along the Florida Gulf coast. Populations
of the black twig borer (an introduced am-
brosia borer) are increasing and affecting
a large number of tree species. Dogwood
anthracnose, first discovered in 1987, is
expanding rapidly throughout the range of
flowering dogwood.
The report summarizes the status of a
large number of common forest insects
and diseases. Detailed insect information
is provided for the hemlock wooly adelgid,
the hemlock loopers, the eastern spruce
budworm, the southern pine beetle, and
the gypsy moth. Detailed information is
also provided for fusiform rust, the littleleaf
disease syndrome, the oak decline syn-
drome, and the beech bark disease. Ob-
servations of other insects and diseases
are organized according to forest types.
Evidence of damage from weather events
and ozone is mentioned.
Climate
The climate report summarizes selected
climate conditions and events that are
known from experience to affect forest
health. Data from a variety of sources
were summarized into databases and
maps of precipitation, hurricane occur-
rence, high wind events, ice storms, and
late spring hard freezes. The period from
October 1990 through September 1991 is
summarized for the regions where FHM
plots were measured in 1991.
Highlights include severe tropical storms
in the fall, a record warm winter with low
snowfall, an early spring, a severe spring
ice storm, and a long and hot summer
punctuated by Hurricane Bob.
Air Pollution
Maps of interpolated ozone data for low
elevations in the eastern U.S. and maps
and regional summary statistics for se-
lected ions in precipitation revealed the
general trends described below. Emissions
data, dry deposition, and pollutants that
are typically point source-oriented were
not included in the full report.
A summary of ozone data for the years
1985-1989 was made using the seasonal
(April through October) "W126 index" that
places an emphasis on peak ozone hourly
average concentrations. In general, ozone
concentrations vary among years, but over
the five-year period there were two re-
gions that tended to give higher values of
the W126 index. One region extends con-
tinuously along the mid-Atlantic seacoast
and west to the Appalachian Mountains.
The second region is smaller and is cen-
tered over the Ohio River valley near the
Ohio-Indiana border. Ozone data sources
included the EPA Aerometric Information
Retrieval System, the National Dry Depo-
sition Network, and the Mountain Cloud
Chemistry Program.
Data from the National Atmospheric
Deposition Program were used to prepare
summaries of (precipitation-weighted)
mean pH, sulfate deposition, nitrate depo-
sition, and ammonium deposition for 1990
(January through December). Precipitation
was most acidic, and sulfate and nitrate
deposition was highest, in a corridor that
extended roughly from eastern Michigan
and southern Indiana to New York and
southern New England. Precipitation acid-
ity and deposition of sulfate and nitrate
generally decrease with distance away
from this corridor. This general pattern is
similar to that observed in earlier years.
Nearly the entire eastern U.S. receives
precipitation that is more acidic than "nor-
mal rainfall." There was not a clear spatial
pattern of ammonium deposition over the
study area.
Conclusion
The full report describes the current ap-
proach and activities and summarizes the
data that were collected in 1991. Looking
to the future, FHM plans the following
events and improvements by 1994:
Agreements will be reached with
additional state and federal agen-
cies to increase the scope of FHM
participation and to improve the ef-
ficiency of data collection and the
depth of data analysis.
The plot network will be expanded
from 12 to 18-24 states, including
states from the western and
north-central regions of the nation.
The core set of measurements
made at each plot location will be
expanded so that more aspects of
forest condition can be monitored.
New measurements must first pass
a rigorous peer-review process, and
candidates include measures of soil,
wildlife habitat, and foliage chemis-
try.
The collection of off-plot forest in-
sect and disease information will
be standardized.
The set of off-plot databases will
be augmented to include informa-
tion from satellite sensors and soil
surveys.
The assessment capabilities will be
developed further and made avail-
able to data analysts around the
nation. Annual statistical summa-
ries will be prepared, and the data
will be made available for in-depth
interpretive reports to address spe-
cific forest health issues.
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Ralph Baumgardner is the EPA Project Officer (see below).
The complete report, entitled "Forest Health Monitoring 1991 Statistical Summary,"
(Order No. PB95-136172; Cost: $27.00, subjectto change) will be available only
from
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at
Atmospheric Research and Exposure Assessment Laboratory
U.S. Environmental Protection Agency
Research Triangle Park, NC 27711
United States
Environmental Protection Agency
Center for Environmental Research Information
Cincinnati, OH 45268
Official Business
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