Health of North American Forests
L’état de santé des forêts nord-américaines
Sanidad de los Bosques de América del Norte
Produced by the Atmospheric Changes and Forests Study Group
of the North American Forestry Commission in cooperation with:
Établi en collaboration par le Groupe d’étude sur les changements atmosphériques
et les forêts de la Commission forestière pour l’Amérique du Nord :
Informe producido por el Grupo de Estudio de Cambios Atmosféricos y Bosques
de la Comisión Forestal de América del Norte, en cooperación con:
Canada/Canadá
J. Peter Hall
Laszlo Magasi
Lester Carlson
United States/États-Unis/Estados Unidos
Ken Stolte
Elvia Niebla
Mexico/Mexique/México
Maria de Lourdes de la I. de Bauer
Carlos E. Gonzalez-Vicente
Tomás Hernández-Tejeda
North American Forestry Commission Report/
Rapport de la North American Forestry Commission/
Informe de la Comisión Forestal de América del Norte
Published by / Publié par / Publicado por
Canadian Forest Service / Service canadien des forêts
Science and Sustainable Development Directorate/
Direction des sciences et du développement durable
Ottawa, 1996
Cover: Aspen in an alpine meadow. (CFS—Petawawa National Forestry Institute)
Couverture : Peuplier dans un pré alpestre. (SCF—Institut forestier national de Petawawa)
Tapa: Álamo temblón en un prado alpestre. (CFS—Petawawa National Forestry Institute)
Contents / Table des matières / Indice
Health of North American Forests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4
North American Biomes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Inside back cover
L’état de santé des forêts nord-américaines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
Biomes nord-américains . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Page 3 de la couverture
Sanidad de los Bosques de América del Norte . . . . . . . . . . . . . . . . . . . . . . . . . . . 36
Biomasas en América del Norte . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Reverso de la contratapa
Appendices/Annexes/Anexos . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53
© Her Majesty the Queen in Right of Canada 1996
Catalogue No. Fo42-252/1996
ISBN 0-662-62141-7
Copies of this publication may be obtained free of charge from:
Natural Resources Canada
Canadian Forest Service
Ottawa, Ontario K1A 0E4
A microfiche edition or photocopies of this publication may be purchased from:
Micromedia Ltd.
240 Catherine St., Suite 305
Ottawa, Ontario K2P 2G8
Editing and Production: Paula Irving
Design and Layout: Danielle Monette
Canadian Cataloguing in Publication Data
Main entry under title:
Health of North American forests = L’état de santé des forêts
nord-américaines = Sanidad de los bosques de América del Norte
(North American Forestry Commission Report)
Text in English, French and Spanish.
Includes bibliographical references.
ISBN 0-662-62141-7
Cat. no. Fo42-252/1996
1. Forest health—North America.
2. Sustainable forestry—North America.
3. Forest conservation—North America.
4. Forest management—North America.
I. Hall, J. Peter.
II. Canadian Forest Service. Science and Sustainable Development Directorate.
III. North American Forestry Commission.
IV. Title: L’état de santé des forêts nord-américaines.
V. Title: Sanidad de los bosques de América del Norte.
VI. Series.
SD387.S87H42 1995
Printed on
recycled paper
634.9’6
C95-980298-3E
PRINTED IN CANADA
Printed on alkaline
permanent paper
Health of North American Forests
Contents
Abstract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
6
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
6
Forest Biomes in North America . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
6
Pollution and Forests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
8
Acid Deposition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
8
Ground Level Ozone . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
8
Nitrate Pollution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
9
Stress in Forests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
9
Forest Harvesting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
Forest Management . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
Pest Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
Assessing Forest Health . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
Health of North American Forests . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
1. Temperate Needleleaf Forests or Woodlands. . . . . . . . . . . . . . . . 11
2. Temperate Broadleaf Forests or Woodlands . . . . . . . . . . . . . . . . 12
3. Mixed Mountain and Highland Systems . . . . . . . . . . . . . . . . . . . 13
4. Subtropical and Temperate Rain Forests . . . . . . . . . . . . . . . . . . . 14
5. Evergreen Sclerophyllous Forests . . . . . . . . . . . . . . . . . . . . . . . . . 14
6. Tropical Dry Deciduous Forests or Woodlands . . . . . . . . . . . . . . 15
7. Tropical Humid Forests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
Selected Bibliography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
Appendix I. Major Tree Species in Each Biome . . . . . . . . . . . . . . . . 53
Appendix II. Damaging Agents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60
Appendix III. Parasitic Plants and Hosts . . . . . . . . . . . . . . . . . . . . . . 64
6 Health of North American Forests
Abstract
Forests are environmentally and economically important
in North America, and forest health is monitored to enable
policy decisions to be made on factors affecting forest health.
Forests are subject to several stresses, natural and human, and
these stresses have various effects. Data for monitoring systems
are used to separate the causes of observed damage to forests.
North American forests are divided into seven biomes and
forest health is described for each. Mortality in North American
forests is generally within the normal range for forests and
is mostly caused by natural thinning. A variety of known insects, diseases, and abiotic factors killed trees and damaged
tree crowns. Some forests are stressed by land use practices.
Incidence of pollution-like symptoms occurred in several biomes and on many species. Where these symptoms were observed,
trees are being studied to determine the cause and extent of
damage.
Introduction
This publication is a result of a cooperative effort by the
Atmospheric Changes and Forests Study Group of the North
American Forestry Commission (NAFC). The report is an
overview of the state of the health of North American forests
based on information from Canada, the United States, and
Mexico. The forests described range from the boreal conifers
to the tropical hardwoods, and from coastal rain forests to
mountain timberline forests.
These forest ecosystems are a source of clean air and water
and are areas for homes for many of our citizens. They provide
recreational opportunities and a habitat for wildlife, and yield
pulp and paper, lumber, food, and firewood. Consequently,
North Americans are very concerned about the health of these
ecosystems. The sustainable development of these forests is
essential to our economic, biological, and spiritual health.
Forests are subject to natural and anthropogenic stresses
that act singly or collectively to affect forest health. There is
no universally accepted definition of forest health, except for
poor health, which is evident when forest ecosystems are deteriorating or declining. When forests are damaged by insects,
diseases, fire, or climatic events, they are probably only temporarily unhealthy because they are adapted to these stresses and
they can recover from them.
Stresses such as air pollution or potential climate change
may permanently damage forest health, because forests have
not adapted to these additional stresses. Anthropogenic air pollution as a primary or contributing source of stress is present to
varying degrees in most North American forest ecosystems.
Pollution has been affecting European forests for many years.
In Canada, air pollution has damaged conifer forests in British
Columbia and Ontario, and birch forests near the eastern coast.
In the United States, the red spruce forests in the Appalachian
Mountains and ponderosa pine in the San Bernardino and
Sierra Nevada mountains have been affected. The pine and fir
forests in the valley of Mexico have also been damaged by
atmospheric pollutants. Forests in all three countries remain
potentially at risk from atmospheric pollution. To protect our
forests, we must know the state of their health.
This report describes the mortality and crown condition of
trees in these forests as affected by various stresses. The Atmospheric Changes and Forests Study Group expects to report on
the state of the health of North American forests regularly.
Forest Biomes in North America
The health of forests is discussed in seven forested biomes
or natural regions as described for the Man and Biosphere
Program (see the map on the inside back cover). The forest
types, major species, and forest stresses are described briefly
for each biome.
1. Temperate Needleleaf Forests or Woodlands
This biome dominates the northern latitudes from the Arctic
tree line south to the Great Lakes and the hardwood forests of
the eastern United States. Forests in this biome consist of
the boreal and mixedwood species with a variety of conifers
and hardwoods. The major species include white spruce, black
spruce, tamarack, balsam fir, eastern hemlock, white pine, jack
pine, lodgepole pine, sugar maple, white birch, yellow birch,
aspen, and balsam poplar. Many of these ecosystems are dominated by wildfires, and the regeneration of black spruce and
jack pine depends largely on wildfire. These forests have been
harvested for many years and large clearcuts predominate in
the north. In the south, extensive areas have been converted to
agricultural use.
2. Temperate Broadleaf Forests or Woodlands
This biome is composed primarily of hardwood species
and a mixture of conifers dominates in the southeast. It includes
forests in southern Canada and in the United States east of the
Prairies. It is bordered on the north by the temperate needleleaf
forests and in the east and south by the Atlantic Ocean and the
Gulf of Mexico. The major broadleaf species include beech,
yellow poplar, hickory, yellow birch, black cherry, sugar maple,
red maple, and oaks. The major conifers include white pine,
eastern hemlock, red spruce, and balsam fir. In the southern
Health of North American Forests
part of the biome, loblolly pine, slash pine, shortleaf pine,
white oak, and scarlet oak predominate. This is a species-rich
biome with many forest types.
Much of the original forest was cleared for settlement
after 1600. During the last century, much of the agricultural
land has reverted to forest. The forests are now affected by a
wide variety of insects, diseases, and pollutants.
3. Mixed Mountain and Highland Systems
These are continental interior forests ranging from latitude
15 to 60°N on sharply changing topographical relief. Typically
these forests begin about 2000 m above sea level and go up to
about 3500 m in the southwest, and begin near sea level farther
north and go up to around 3000 m. Dominant species in the
northern part of the biome include Engelmann spruce, alpine
fir, lodgepole pine, Douglas-fir, subalpine fir, ponderosa pine,
white fir, red fir, trembling aspen, and oaks.
In the south, the species composition changes to a rich
mixture of species. So far, 128 different species of oaks have
been identified in Mexico, and there may be more species to be
described and classified. Similarly, 54 different species of
pines are known in these forests, along with 9 species of true
firs, 4 species of Douglas-fir, 2 species of spruce, and 3 species
of cypress.
Many of these ecosystems are fire-dominated, with additional localized harvesting and land clearing activities. The
forests are affected by insects, diseases, and parasitic plants.
Some species are affected by pollutants near Mexico City,
Jalisco (Guadalajara), and Nuevo Leon (Monterrey).
4. Subtropical and Temperate Rain Forests
This biome consists of the Pacific coastal forests from
Alaska to California and is dominated by western red cedar,
western hemlock, Sitka spruce, Douglas-fir, Amabilis fir,
yellow cypress, mountain hemlock, alpine fir, subalpine fir,
redwoods, ponderosa pine, white fir, and red fir. These forests
are valuable and have been extensively harvested. Insect defoliators, bark beetles, and stem and root diseases also affect these
forests. Forest wildfires are not common, but forest management
activities have been quite intensive.
5. Evergreen Sclerophyllous Forests
Forests in this biome are characterized generally as dryland
forests. Typically the species are small and have sclerophyllous (leathery) evergreen needles or leaves. Growth rates are
typically slow because of low rainfall and the low fertility of
the soils. The dominant tree species are pinyon, juniper, oak,
manzanita, ceanothus, and chamise. These forests occupy higher
7
elevations in desert areas and lower elevations and warmer
slopes of mountains in the more southern latitudes. Some of
these forest communities are well adapted to frequent wildfires
which maintain these species in the biome.
6. Tropical Dry Deciduous Forests or Woodlands
This biome occurs in western and southern Mexico and in
the Yucatan Peninsula. Again there is a rich species mix with
27 different species of Acacia; A. coulteri and A. farnesiana
are typical. Other species include Alvaradoa amorphoides,
Bursera spp., Ceiba acuminata, C. aesculifolia, Lysiloma divaricata, Prosopis laevigata, Fraxinus purpusili, Piscidia communis, Lysiloma bahamensis, Pithecellobium dulce, Cyrtocarpa
procera, Cordia dodecandra, Tabebuia chysantha, and Gliricidia sepium. These forests grow from 0 to 1900 m, and the
annual mean temperature is 20–29°C. There is one dry season
(5–8 months) and one rainy season. Most forests are on rocky
soils and hillsides of low fertility. Some of these species have a
high economic value. The main problems for these forests are
deforestation, wildfire, and grazing.
7. Tropical Humid Forests
This biome comprises the coastal forests in Mexico from
Veracruz to the southern Yucatan Peninsula. The dominant
tree species are Dialium guianense, Terminalia amazonia, Swietenia macrophylla, Cedrela mexicana, Vochysia hondurensis,
Andira galeottiana, Swartzia panamensis, Calophyllum brasiliense, Guatteria anomala, Aspidosperma megalocarbon,
Manilkara zapota, Bursera simaruba, Dendropanax arboreus,
Tabebuia rosea, Alchornea latifolia, Guarea excelsa, Celtis
monoica, Brosimum alicastrum, Pimenta dioica, Platymiscium
yucatanum, Robinsonella mirandae, Astronium graveolens,
Zanthoxylum kellermanii, Ceiba pentandra, Sabal mexicana,
Trichilia minutiflora, Bucida buceras, Metopium brownei, Terminalia oblonga, Virola guatemalensis, Zuelania guidonia,
Sterculia mexicana, Barnoullia flammea, Blepharididium mexicanum, Pseudolmedia oxyphyllaria, Ficus insipida, Bravaisia
integerrima, and Quararibea funebris.
There are many epiphytic plants of the Bromeliaceae and
Orchidaceae families. In some cases, there are 34 different
species of plants in 2000 m2 and 735 trees/ha. These forests
grow in flat lands, and deep, rich, well-drained soils. The species
mentioned grow from sea level up to 1000 m. The average
precipitation is from 1500 to 3000 mm, and the annual mean
temperature is above 29°C.
These forests have been extensively clearcut for agriculture and grazing. There is shift cultivation practiced in which
the trees are cut and burned, and the soil is cultivated for a few
years then abandoned. After that, the vegetation regrows and
8 Health of North American Forests
the cycle begins again. Land use change is thus a major factor
affecting the health of these forests.
Wildfires are common in these forests. Sometimes the
clearcut areas are used as grasslands. Erosion in areas without
vegetation can be severe due to the heavy rains. Some damaging
insects and diseases are present, and damage on plants by pollutants emitted by oil refineries is common.
Pollution and Forests
Air pollution was first recognized in the nineteenth century as potentially harmful. Because natural ecosystems have a
built-in resilience to stress, it took many years for air pollutionrelated changes in biological organisms to become obvious.
However, the science community now accepts that air pollution
affects forests, lakes, agricultural crops, cultural monuments,
and human health.
Air pollution from individual point sources will kill trees
and destroy complete ecosystems typically somewhere within
25–50 km of the source. In North America, considerable damage was done to forests from point sources during the nineteenth and early twentieth century. More recently, concerns
have arisen regarding the possible effects of both point-source
and regional pollution in decline and death of trees and forest
ecosystems.
Damage caused by regional transport of pollutants is difficult to detect, and forest declines in areas of low and high levels
of acid deposition have occurred. The effects of pollution damage have been frequently duplicated under laboratory conditions;
however, under ambient levels of pollution, the mechanism of
cause and effect is not clear and has seldom been demonstrated.
Removal of the stress by reduction of emissions, however,
usually results in the recovery of the affected vegetation, suggesting a cause–effect relationship.
Many pollutants are affecting forests in North America;
acidic deposition, ozone, nitrates, various particles, and heavy
metals are best known.
Transported pollutants affecting forests:
Gaseous
Sulfur compounds
Nitrogen compounds
Photo-oxidants
Hydrocarbons
Pollutant mixtures
Particulates
Acidic (S and N) compounds
Heavy metals
Hydrocarbons
Solutions
Oxidants
Acid rain/fog
Hydrocarbons
Direct/indirect effects of air pollution:
Direct effects
1.
2.
3.
4.
5.
6.
7.
nutrient leaching from foliage
ozone damage and decreased photosynthetic efficiency
ozone damage and increased foliar leaching
sulfur dioxide damage
foliar fertilization and increased winter injury
foliar fertilization and altered nutrient allocation
growth altering substances
Indirect effects
8. nutrient leaching from soil
9. aluminum mobility and toxicity
10. heavy metal toxicity
Concerns have also recently been raised about volatile
organic compounds (VOC) and UV-B radiation. These pollutants originate from various anthropogenic sources and are
transported over long distances and over extensive areas.
Acid Deposition
Acid deposition consists of wet and dry deposition of SOx
and NOx. Acid deposition/acid rain affects vegetation by interacting with the leaf surface. It affects soils by inhibiting nutrient
uptake by trees (nitrogen/phosphorus) and releasing toxic (aluminum) elements. Acidic deposition levels of 20 km • ha-1 • yr-1
or greater adversely affect lakes and streams in North America.
Biomes 1 and 2 are exposed to pollutants in this way. Air
pollution is an additional stress on North American forests
beyond the normal damaging factors. These added stresses can
predispose forest trees to damage from other insects and
diseases that otherwise might cause less damage. Emissions of
acidic pollutants may have peaked and declined slightly over
much of North America; however, in all areas pollutants are
still accumulating in soils, water, and vegetation. In some areas
emissions of pollutants are still increasing.
Ground Level Ozone
Tropospheric (ground level) ozone is produced in the earth’s
lower atmosphere from photochemical reactions involving
oxides of nitrogen and volatile organic compounds (NOx/VOC)
in the presence of sunlight. Ozone damages plants by entering
through the open stomates and affects mesophyll cells in the
leaf. It builds up in the lower atmosphere especially on hot
days in cities and flows out into the surrounding countryside.
Health of North American Forests
Thresholds for damage to vegetation is 50–80 parts per billion.
Agricultural leaf crops are widely affected causing considerable economic losses annually. This tropospheric ozone is
increasing at a rate of 1–2% annually.
Ozone air pollution in the temperate broadleaf forests of
biome 2 is of concern. Ozone levels are highest in the midAtlantic and upper southeastern coastal plain areas. The effects
are magnified in the southeastern part of the biome. Plants are
exposed longer because they flush early, and humidity levels
are higher, facilitating the uptake of ozone.
Ozone-sensitive tree species such as black cherry, sassafras,
and yellow poplar are injured in both the upper and middle of
the crowns in more than 60% of trees. High ozone exposures
cause ozone injury when relative humidity and soil moisture
are high. During periods of drought, damage is limited because
plant stomates close.
In biomes 2 and 3, the temperate broadleaf and mixed
mountain and highland biomes, trees are frequently exposed to
phytotoxic concentrations of ozone. Ozone stress typically
occurs in the mixed-coniferous forests at higher elevations
(>1500 m). In the San Bernardino Mountains of southern California, and the southern and central Sierra Nevada Mountains farther north, ozone is particularly injurious to ponderosa
pine, Jeffrey pine, and to a lesser extent to black oak. Damaged
foliage, loss of needles, and growth reductions have occurred.
A similar situation to the one described for the San Bernardino Mountains has been detected in southern mountainous
areas of Mexico City where at altitudes of 2850–3500 m, Pinus
hartwegii, one of the nine native species to the Valley of Mexico,
was highly sensitive to ozone. During the last 3 years, the O 3
monitoring data for the ozone have shown values that almost
daily exceed the Air Quality Standards and very frequently
remain for several hours a day, well above the threshold for
damage to vegetation.
The observed damage on pines first at Ajusco and later at
the national park Desierto de los Leones located farther southwest of Mexico City is severe and the population of P. hartwegii
in certain areas is declining or dying. The patterns of growth of
P. hartwegii using standard dendrochronological techniques
show a marked decline in growth since the early 1970s at both
sites (Ajusco and Desierto de los Leones) and there are no
signs of subsequent recovery. In terms of typical oxidantinduced symptoms, there is a variability in the response of the
pine species in the area and among individuals within species.
Nitrate Pollution
The source of nitrate pollution includes automobile, agricultural, and industrial emissions. There are increasing concerns
about nitrate deposition because its levels are increasing at the
same time as sulfate levels are decreasing in many areas.
9
Continued deposition of nitrogen will increase the amount
of nitrogen in forest ecosystems, and depending on the nature
of the soil will increase the capacity for nitrification and the
movement of nitrogen to surface water. Vegetation growth can
be expected to increase where nitrogen is a limiting factor, and
where the deposition is not accompanied by elements interfering
with nutrient uptake and/or where it introduces additional
stress. Continued nitrogen deposition affects other components
of the ecosystem including mycorrhizae, insects, pathogens,
and wildlife. Present ecosystem interactions may be changed in
unpredictable ways. Nitrogen deposition is of particular concern
in biome 2 and the southern parts of biome 1.
In biome 5, evergreen sclerophyllous forests, nitrate emissions (NOx) are greatest near major cities. While sulfate emissions (SOx) are generally decreasing, nitrate emissions have
been steadily increasing over the last century. The effect of
increased deposition of nitrates is to fertilize trees. When this
happens the trees do not harden off properly for their dormant
season. This can result in extensive foliar damage from low
temperatures or droughts because the trees are not able to
respond to their changed environment.
The factors associated with mortality in the past have now
been supplemented by pollutants of various types. The pollutants
therefore are causing additional stress by making trees more
susceptible to insects, diseases, climatic extremes, or nutrient
stresses. Pollutants may also trigger a decline in the health of
forests already stressed by other factors.
Stress in Forests
Stress is a constant and natural part of the forest ecosystem.
Stress varies over time and space, and individual trees and
forests react to it by tolerating and recovering, or by declining
and dying. Chronic and acute stresses affect forest ecosystems.
Chronic stresses influence forest ecosystems over long periods
and over large areas. Examples include climate, soil parent material, topography, and competition. Acute stressors are more
limited in space and time, and include insects, diseases, pollution, storms, and fire.
Most natural stresses do not damage forest ecosystems
beyond repair, and they recover to a state resembling the original conditions. Only in severe cases is an ecosystem driven
over its threshold of resilience, in which case it acquires a new
equilibrium, or it declines and degrades completely. Largescale forest declines are often thought to have been triggered
by stress events (drought, frost, pollution) and may have been
aggravated by a predisposing condition of many years of other
stresses including pollutants.
Natural competition causes stress and occurs when individual trees compete for growing space, solar radiation, water,
10 Health of North American Forests
or essential nutrients. In natural forests more trees die of stress
from competition than from any other factor. The “normal”
mortality in unmanaged forests from natural thinning is about
1–3% annually. Other natural stresses may cause higher levels
of mortality. Some of these are biological (animals, fungi, insects, nematodes, bacteria); others are climatic (drought, floods,
storms). Wildfires cause extensive tree mortality but usually
result in a new forest becoming established and are not considered harmful to forest health in the long term.
Human (anthropogenic) stressors such as air pollution,
harvesting, exotic forest pests, and forest management activities
all add to natural stresses. Human-caused disturbances are common to forests that have been extensively exploited.
Two instances of forest decline in eastern North America
illustrate the interplay of natural and anthropogenic causes. In
eastern Canada, a well-known decline and dieback sequence
occurred in yellow birch beginning about 1925. Extensive
areas of mortality occurred in disturbed and undisturbed stands
and on various physiographic site types. Various insects and
diseases were suggested as causes as were weather events,
particularly droughts and unusually mild winters, but no cause
and effect link was determined. Since approximately 1950,
however, there has been a gradual recovery from the decline,
and the species is now generally healthy.
A case of an interaction between a foliar pathogen (Lophodermium sp.)and oxidant gases has been described as present
in the mountainous forest “El Ajusco,” located south of Mexico
City. Here Pinus hartwegii shows severe foliage damage in
addition to the characteristic pollution-induced symptoms
chlorotic banding and mottling of the leaves. Abundant fruiting
bodies of Lophodermium sp. are present on the affected foliage.
The magnitude of the damage is directly related to the age of
the leaves and to the side of the tree exposed to the urban zone.
In a high elevation forest park near Mexico City, the
Desierto de los Leones, where the O3-induced symptoms on
P. hartwegii are evident at certain altitudes and exposures, a
massive forest decline of sacred fir was observed for the first
time in 1982. It was similar to tree damage and decline described
in Europe and North America. Trees located in the windward
zone, exposed to air masses from Mexico City, were the most
severely affected, especially trees at the distal ends of ravines.
Within 2–3 years after the symptoms appeared, the trees were
dead.
Although there is much less information in relation to air
pollution effects on Abies than on pines, it was concluded that
the polluted air from Mexico City was an important factor in
the decline of fir. However, drought, insects, mites, pathogens,
and poor forest management have collectively contributed to
this decline. The areas where the sacred fir trees died have
been reforested with different pine species as well as with
sacred fir. Most of those trees show already very clear oxidant-
type symptoms and some of them are in poor health. Thus
under those special conditions the zone can be considered a
trial selection for tree tolerance to O3.
Decline has occurred in red spruce forests over the past
century in the Appalachian Mountains, and recent growth records
suggest that it is far more widespread than previously. Severe
droughts in the mid-1960s might have initiated the decline.
However, since the decline occurred before the drought, and in
nondrought areas, drought was probably not the primary cause.
Recent research has indicated that a combination of site factors, climate, and sulfate/nitrate pollution are collectively causing
the decline.
Forest Harvesting
Forest harvesting has occurred in much of the North
American forests. Harvesting interrupts the natural succession
in the forest, affects the water balance, causes soil erosion and
compaction from harvesting equipment, and leaves logging
slash which provides fuel for wildfires. Fires are suppressed to
protect commercially valuable forests particularly in the temperate needleleaf forests. These actions have resulted in a successional preference for balsam fir rather than the fire-tolerant
species jack pine and black spruce. The resulting balsam fir
forests are prone to damage from defoliating insects such as
the spruce budworm and the hemlock looper. All these factors
result in areas of forest under increased stress. Harvesting of
trees has often been done in ways that adversely affect forest
health. Harvesting methods such as selective cutting and
protection of wildlife habitat, old growth forests, streams, and
soils all help avoid stress on forests.
Forest Management
Increases in the intensity of forest management may result
in stress, particularly by conversion of stands from one species
or species group to another. This may lead to an ecologically
unsustainable forest and may at the same time be aesthetically
displeasing. Examples include extensive conifer tree plantations in biomes 1 and 2 and in the southern parts of biome 4.
Large monocultures are also often more susceptible to infestation by damaging insects and fungi than forests composed of a
mixture of species. For example, second-generation stands of
Norway spruce frequently have reduced growth rates compared
with the previous forests on the same site. This is caused by
lower pH resulting from spruce growing on soils that previously
supported deciduous species. This causes nutrient stress on the
new forests and makes them vulnerable to other stresses such
as pollutants.
Land was originally cleared for agriculture in North America, cultivated, and then reverted to forests as the land became
Health of North American Forests
impoverished. At present many of these lands support forests
in the early successional stages and are very often under nutrient
stress because many of the original nutrients were removed by
other crops.
Modern forest management emphasizes the ecological
integrity of the forest over timber production. This approach is
now being widely adopted in temperate and boreal forests, and
criteria and indicators are being developed by councils directed
by the United Nations Council on Economic Development.
Pest Introduction
There are several insects and diseases accidentally introduced to North America in the nineteenth century. They were
not particularly damaging in their native habitat but caused
extensive problems in their new environment because natural
controls were lacking. Examples include chestnut blight, Dutch
elm disease, beech bark disease, and balsam woolly adelgid.
Both chestnut blight and Dutch elm disease have had such a
devastating effect that the host species, American chestnut
and American elm, have ceased to be a significant part of the
Carolinian forest. These ecosystems have changed and now
contain a different mixture of species.
The balsam woolly adelgid has killed much of the higher
elevation Fraser fir in eastern North America, leaving extensive areas of dead trees that are being replaced by other species.
These stresses have had major effects on forest health. Forest
ecosystems have responded by developing into different ecosystems; the original forest types have probably disappeared
forever.
11
Current forest conditions and changes are measured annually. Tree mortality is evaluated; the rate considered normal for
an unmanaged forest is 1–3%, caused primarily by thinning in
natural stands. Higher levels may be caused by insects, disease,
drought, frost, etc., singly or in combination. Forests that had
normal or excess mortality from these causes would not be
considered unhealthy. However, forests that had mortality from
unknown causes or from other than normal causes for which
forests are adapted would be considered in poor health.
Tree growth may be used to evaluate the effects of pollutant stress on forests. In many instances it is difficult to detect
changes in growth caused by pollution among the other factors
affecting growth. Unless there is massive reduction in growth
in polluted areas compared with growth in nonpolluted areas, it
is difficult to use growth rates to detect pollution effects.
Tree condition is evaluated for conifers and hardwoods.
The types of damage from insects and diseases are described.
The cause of defoliation and dieback is attributed to insects,
diseases, or other factors.
The symptoms of air pollution damage are easily confused
with symptoms of damage from natural causes, and the correct
identification depends on the experience of personnel trained
to distinguish symptoms of air pollution damage from symptoms
of abnormal climatic conditions, nutrient deficiencies, and the
effects of insects and diseases. Symptoms are explained for
both conifers and hardwoods. When damage not easily explicable by natural causes is observed, research is necessary to
determine the cause of the damage.
Health of North American Forests
Assessing Forest Health
To ensure forest sustainability, we first must know how
our forests are functioning in an environment that has been to
some extent changed by human activity. With this knowledge
action can be taken to protect forests. These questions are asked
during monitoring: Is this forest/tree different from the normal
healthy forest/tree and if so what has caused the change? Are
the changes caused by insects, diseases, drought, frost, or some
unknown factor?
Forest management agencies have established permanent
sample plot systems in representative forest ecosystems, and
results from these plots are synthesized for the major species in
each of the seven biomes. Data collected are supplemented by
the results of additional surveys conducted by national and
local forest management organizations. There are several, localscale monitoring and research programs to determine the condition of the forest, measure annual changes, and evaluate trends
in forest condition.
The health of forest ecosystems is described for each biome.
The assessment of forest health is based on tree mortality, tree
condition, and pollution symptoms. The causes of mortality
and crown damage are also described.
1. Temperate Needleleaf Forests or Woodlands
The major stresses acting on this biome are various defoliating insects, root rots, and localized effects of harvesting and
wildfire. Symptoms of air pollution have seldom been observed
on trees in this biome.
Tree Mortality
Tree mortality among most species in this coniferdominated biome is mostly in the normal range of 1–3% for
unmanaged forests. Causes of mortality were natural thinning,
blowdown, and sporadic attacks by forest insects such as the
spruce budworm, the balsam woolly adelgid, and the ragged
spruce gall adelgid.
12 Health of North American Forests
Normal levels of mortality also prevailed in the hardwoods,
although damage levels were higher than in the conifers.
Mortality of white birch, a species that had been stressed by
severe foliage browning before 1986, returned to normal levels
compared with previous years. Mortality in the aspen species
was higher than normal and was caused by natural thinning,
blowdown, drought, late spring frosts, defoliating insects, and
root and stem rots.
Tree Condition
The conifers were generally in good condition. Local insect
outbreaks or weather conditions affected some species. In
particular, the spruce budworm, twig aphids, and mites caused
defoliated spruces. Pine species were healthy and had no significant crown damage/defoliation. Some damage from canker
and needle cast disease caused loss of needles on lodgepole
pine, and a combination of insect pests, drought, and poor site
quality affected the condition of red pine. A common rot, brown
cubicle rot, also caused mortality in white spruce. Other damaging insects included the pitch nodule maker and a long-horned
beetle; diseases included armillaria root rot and gall rust.
Balsam fir, a host species for several defoliating insects,
was quite healthy. The 50-year cycle of the eastern spruce budworm peaked in the 1980s and has since been declining. Some
trees previously weakened by insects have died and there are
localized infestations of the eastern spruce budworm and the
hemlock looper. Eastern larch is occasionally infected with
European larch canker causing gradual deterioration in crown
condition, branch mortality, and occasional dead trees. As a
result of cankers and other defoliating insects, decline of eastern larch is occurring in some regions, and mortality can be
expected to increase above normal levels in the future.
Birch crowns were affected to varying degrees by drought,
storm damage, various insect pests (hemlock looper and the
spearmarked black moth) and diseases, leaf spot, and root rots.
Aspen crowns were damaged by insects, diseases, summer
drought, and late spring frosts. Forest tent caterpillar and the
large aspen tortrix have caused most of the crown damage.
The crowns of largetooth aspen were generally healthy and
had occasional marginal discoloration and chlorosis caused by
drought.
Pollution Symptoms
In eastern Canada, severe foliage browning has led to the
death of many northern white birch trees. The browning damage is related to the presence of acidic coastal fog which damages
the foliage and causes branch and eventually tree mortality.
Weakened trees die from this and other forest stresses such as
armillaria root rot, having first been predisposed by the pollutants. In addition to the pollution damage on birch, occasional
needle flecking and chlorosis were also observed on white
pines and red spruce. These symptoms are consistent with
those caused by ozone, but the actual cause has not yet been
determined.
2. Temperate Broadleaf Forests or Woodlands
The temperate broadleaf forest biome of the eastern United
States encompasses a wide variety of forest ecosystems, of
which oak–hickory is the major forest type extending from
Texas in the south to the Dakotas in the north, and east to
southern New England. These forests have been extensively
stressed by damage from introduced pest species and their
species composition has been altered during the past few
decades. This forest type was formerly chestnut–oak, when
American chestnut dominated these forests. Chestnut blight,
introduced in 1900, has eliminated American chestnut, reducing
it to an understory species seldom reaching tree size.
Tree Mortality
Mortality was low in these forests, in the normal range of
1–3%, and was caused by natural thinning as the stands mature
and develop. Exceptions included higher than normal mortality
in sugar maple caused by Armillaria sp. after the trees were
stressed by other natural factors. Balsam woolly adelgid populations caused extensive mortality of Fraser fir in the southern
Appalachians.
Tree Condition
The crowns of most conifer species were healthy. There was
some defoliation and discoloration caused by the occasional
presence of insects and diseases on the foliage. On hardwoods,
there were occasional trace amounts of twig or branch dieback
and marginal chlorosis or discoloration, and some damage from
insects, diseases, and abiotic factors.
American beech was healthy except for stem wounds or
nectria canker, an introduced pathogen that severely damages
beech. Red oak crowns were damaged slightly by pear thrips
and localized drought stress.
The crowns of sugar maple were mostly healthy. Trees are
recovering from insect infestations and drought in various
parts of the range. Symptoms of these stresses included discoloration and scorch of leaves and chlorosis, both symptoms
consistent with drought.
The major biotic stresses on this biome include gypsy
moth, the southern pine beetle, eastern spruce budworm, balsam
woolly adelgid, fusiform rust, and dogwood anthracnose. Gypsy
moth is currently a major pest in these forests and continues to
defoliate hardwoods over extensive areas and in isolated
patches. It was introduced in 1869 and has since spread south
and west, feeding mainly on oaks. Stressed trees are often
attacked by two-lined chestnut borer and armillaria root rot. By
Health of North American Forests
1991, gypsy moth had defoliated over 1.6 million ha and
infested another 50 million ha.
In the southern part of the biome, southern pine beetle
continues to affect southern pines. More than 24 000 infestations occurred in 1991. Dogwood anthracnose occurs in the
mountains and piedmont areas of several southern states and is
spreading throughout these areas. The disease causes defoliation
and mortality; in areas above 1000 m all trees may be killed.
Oak decline is another major stress factor on the oak–
hickory forest. It is a complex initiated by several factors:
drought, site condition, foliage diseases, and insect defoliation.
Fusiform rust continues to be the most serious disease in
this biome. Approximately 6.8 million ha of forest were affected.
Annosus root disease continues to be the most serious root
disease in the southern regions, weakening trees and making
them susceptible to pine bark beetles.
Pollution Symptoms
In 1990, sulfate and nitrate deposition levels were highest
in the eastern United States. Ozone levels over a 10-year period
were highest in the mid-Atlantic and upper southeastern
coastal plain states. Black cherry and sweetgum were damaged
on three quarters of monitoring plots. Over 60% of black cherry,
sassafras, and yellow poplar were damaged in the upper and
middle of the crowns.
Ozone injury was recorded from late May in the southeast
part of the biome to mid-July in the northeastern part of the
biome. Ozone caused greater damage to species in the southeast part of the biome because of higher exposures for longer
periods compared with northern areas. The trees have longer
periods of susceptibility because they flush earlier. The higher
humidity levels in this biome also facilitate gas exchange and
the uptake of ozone into the leaves. In summary, this biome
has a wide variety of economically and aesthetically important tree species affected by a wide variety of insects, diseases,
and climatic factors. Pollution damage affects the forests regionally and causes considerable damage to the more sensitive
species.
3. Mixed Mountain and Highland Systems
These continental interior forests are often fire-dominated.
Some harvesting, land conversion to agriculture, weather damage, and occasional pollution events also cause stress to these
forests.
Tree Mortality
Mortality was at normal levels for most species in the
northeastern part of the biome. Exceptions occurred for lodgepole pine and Douglas-fir which had higher than average mortality caused by natural thinning and root rot, respectively.
13
There is not a quantification of the mortality percentage of
the Mexican forests; however, it is higher than average mortality reported for the United States and Canada and is caused by
bark beetles throughout most of the country. In addition, parasitic plants, tree diseases, and insects cause significant losses.
Tree Condition
The crowns of most trees sampled in this biome were
affected by many insects and diseases. Weather effects caused
some needle damage or loss at levels considered normal for this
biome. Needle flecking, a discoloration caused by snow deposition and reflected sunlight, was common at higher elevations.
Needle discoloration in alpine fir was caused by various diseases,
for example, Lirula abietis-concoloris (Mayr ex Dearn.) Darker,
and a secondary fungus, Stegopezizella balsameae (Davis) Syd.
Foliage of Douglas-fir was damaged by Swiss needle cast and
snow blight. Some crown damage was recorded on lodgepole
pine from atropellis canker and pine needle cast disease.
Bark beetles, Dendroctonus mexicanus, D. adjuntus, and
D. frontalis, caused most of the damage on mixed-coniferous
forests in Mexico, where they are affecting 8644 ha. Parasitic
plants such as Arceuthobium sp., Psittacanthus sp., and Phoradendron sp. are present on conifer and broadleaf trees. The
first is known as dwarf misletoe and is affecting 1.8 million ha
just in Mexico. These tree mistletoes are widely distributed in
the biome. Other foliar injuries are caused by defoliating insects:
Evita hyalinaria, Zadiprion sp., Neodiprion sp., Baliosus sp.,
Malacosoma incurvum azteca, and Pterophylla. There are also
borer insects of branches and buds such as Stylosus sp.,
Eucosma sp., Rhyacionia sp., Conophthorus sp., and Paranthrene dolli. The fungus diseases affecting small areas of Mexican forests but without significant damage are Heterobasidium
annosum, Phellinus pini, and Fusarium subglutinans. These
diseases are considered high risk and have the potential to
spread.
Pollution Symptoms
In biome 3, ozone stress typically occurs in the mixedconiferous forests occurring between 1500 and 2500 m in
California, and to a lesser degree some mountain forests in
southern Arizona up to 4000 m. Ozone is particularly injurious
to ponderosa pine and Jeffrey pine, and less injurious on black
oak. Loss of needle whorls, and growth reductions were the
foliar injury symptoms observed in the San Bernardino and
Sierra Nevada mountains.
In this biome, sulfate emissions peaked in the early 1960s
and by the early 1980s peak levels decreased by 20%. While
sulfate emissions are generally decreasing, nitrate emissions
have been steadily increasing. Nitrate emissions tend to be
greatest near major cities in California, Washington, Colorado,
and Arizona. No damage to forests has been reported.
14 Health of North American Forests
Conifers are more affected by ozone than broadleaf species;
Pinus hartwegii has been described as the most sensitive species
showing chlorotic mottling, banding, severe defoliation, branch,
mortality, and poor natural regeneration. Other species affected
include Pinus patula, P. montezumae, P. teocote, P. radiata,
and Abies religiosa. In the Valley of Mexico, Abies religiosa
shows a severe branch mortality, defoliation, and a reddishbrown color on the upper leaf surface of the older leaves. Some
conifers have photochemical oxidant damage near urban areas.
There is white flecking on pine trees caused by insects that
resemble pollution symptoms. Different species of broadleaf
trees and shrubs are evident in downwind areas near urban
areas and frequently have reddish-brown stippling on the upper
side of the leaves resembling symptoms of oxidant damage.
4. Subtropical and Temperate Rain Forests
These forests contain the largest and oldest trees in the
world. Species such as Douglas-fir, Sitka spruce, and sequoia
are world famous for their size and age. Fires have been the
major factor altering forest ecosystems but they have occurred
rarely. More recently, drought, harvesting, and exclusion of
fire have combined to render the current ecosystems more
vulnerable to pests, diseases, and fires. Fire prevention programs
allow a large buildup of fuel, thus increasing the risk of future
fires that may be more destructive than if normal ecosystem
development were allowed. Major forest pests are western
spruce budworm, Douglas-fir tussock moth, mountain pine
beetle, Douglas-fir beetle, spruce beetle, fir engraver, western
pine beetle, and pine engraver beetle.
Tree Mortality
Mortality has been within the normal levels in these forests
and was caused by natural thinning, attack by bark beetles, and
by occasional blowdown.
Tree Condition
Crowns of conifer species appeared healthy and had no
damage. Western red cedar and western hemlock were generally healthy and had some crown thinning, chlorosis, and traces
of foliage discoloration due to crown shading, suppression,
and storm damage. The western spruce budworm and storms
damaged Douglas-fir. Discoloration and chlorotic lower crown
foliage from Swiss needle cast and snow blight fungus also
occurred sporadically.
Pollution Symptoms
In general, there were few symptoms of pollution observed
in these forests. Some occasional needle flecking occurs in
localized situations but this has not been shown to have been
caused by pollution.
5. Evergreen Sclerophyllous Forests
The topography of this biome is mountainous, and forests
are found on lower slopes and river valleys. Forest distribution
is limited largely by rainfall and soil conditions.
The biome contains a wide variety of species, and conifers
dominate the landscape. Major conifers include western white
pine, ponderosa pine, lodgepole pine, grey pine, Jeffrey pine,
Coulter pine, sugar pine, singleleaf pinyon pine, incense cedar,
Douglas-fir, white fir, and mountain hemlock. Major hardwoods
include California black oak, canyon live oak, interior live oak,
blue oak, Oregon white oak, Engelmann oak, dogwood, big
leaf maple, willow, and trembling aspen.
Forests in this biome have been widely affected by forest
fires which are a major factor in successional patterns. Human
habitation also affects forests largely because of land use changes
from forest to settlement, agriculture, and grazing. Forest harvesting occurs widely.
Tree Mortality
Tree mortality is frequently at levels higher than expected
from natural thinning in unmanaged stands. This is caused by
the effects of a variety of insects and diseases. The major insect
pests affecting forests include the Jeffrey pine beetle, the lodgepole needleminer, mountain pine beetle, western pine beetle,
pine engravers, black pineleaf scale, and the California flatheaded borer.
Diseases affecting forests include the annosus root disease, dwarf mistletoe, black stain root disease, and white pine
blister rust. Abiotic damage caused by ozone damage on pines
also affects many forests.
A combination of multiple organisms or an interaction of
site and stand conditions and insects causes most of the mortality in commercial forests of this biome. These interactions
are usually between bark beetles, dwarf mistletoe, root diseases,
and overstocking after disturbance.
Tree Condition
Crowns of both conifers and hardwoods are affected by
defoliating insects. These pests are mostly species-specific and
damage trees at endemic and outbreak levels. A common stress
on the trees is drought and this aggravates the damage caused
by other stresses. Symptoms of ozone damage on pines are
common in certain areas.
Pollution Symptoms
In this biome, ozone is a major factor in forest health, particularly on Jeffrey pine and ponderosa pine. In the 1960s, ozone
was identified as the factor causing damage to pine forests in
the San Bernardino Mountains east of Los Angeles. In 1971, the
first report of ozone injury to pines in the Sierra Nevada Moun-
Health of North American Forests
tains was published. Damage symptoms are chlorotic mottling on
pine needles. Ozone injury occurs throughout most of the Sierra
Nevada Mountains, and damage increases from north to south.
6. Tropical Dry Deciduous Forests or Woodlands
This is a species-rich biome in western and southern Mexico
that has distinct wet and dry seasons. These forests are affected
by several anthropogenic stresses, deforestation, wildfire, and
grazing.
Tree Mortality
There is not a quantitative estimate of mortality at present.
These forest types are largely free of insect and disease damage and of air pollution. Tree mortality is normal for unmanaged forests in this biome and is caused by wildfires, grazing,
and land use change for agriculture and livestock uses.
Tree Condition
The crowns of the broadleaf species are generally in good
condition. There are no important conifer species in this type
of forest. Occasionally insects or diseases affect some species
without significant damage. Drought and poor site conditions
stress most tree species but are not important factors of damage. In southern Mexico, there is a bark beetle, Dendroctonus
frontalis, attacking Pinus pringlei and P. oocarpa in the States
of Oaxaca and Guerrero.
Pollution Symptoms
Because most of these tree species are living under dry
conditions, there is not significant foliar damage. There are not
specific studies on this particular kind of vegetation in Mexico,
although a large area of the country is within this ecosystem.
More detailed studies are needed to explain the factors affecting
these forests.
7. Tropical Humid Forests
This biome consists of the southern and coastal Mexican
forests containing many species in their diverse ecosystems.
Human influence is strong; many areas are cleared for agriculture and settlement. Pollution from point sources is common.
Tree Mortality
There is no estimate of mortality available in this biome.
Causes of tree mortality include natural and induced thinning,
as well as forest clearing for agriculture and livestock. In this
biome, the heavy rains can easily cause soil erosion. Several
local or regional tree diseases and insects are also important.
Damage resembling that caused by sulfate (SO2) on trees in
the areas is probably caused by emissions from oil refineries.
15
Tree Condition
The crowns of broadleaf and conifer species appear generally healthy and have little damage. Occasionally storms and
wildfires cause relatively severe injury to these forests. Some
insects present are Hypsipyla grandella, Xyleborus sp., and
Chrysobothris sp., attacking Cedrela mexicana and Swietenia
macrophylla in nurseries, plantations, and natural regeneration; Rhyacionia frustana attacks Pinus oocarpa, P. pringlei,
and P. caribaea; and Hylesia frigida attacks some species of
pines. Different species of rodents, ants, and locusts are affecting
nurseries and plantations. A few tree pathogens attacking these
forests are Puccinia spp. on pines, and true mistletoes such as
Psittacanthus sp., Phoradendron sp., and Struthantus sp. on
broadfleaf species.
Pollution Symptoms
In general, there are no symptoms of air pollution on these
forests; however, near oil refineries, damage similar to SO2
occurs on some species. Leaf chlorosis or necrosis occurs intervenally. There are no specific studies dealing with this symptomatology to confirm this particular kind of damage. Research
is needed in this area of air pollution effects on vegetation in
the tropical humid forests.
Conclusions
To protect our forests and ensure their survival and sustainability for the use and pleasure of future generations, we need
to be aware of the state of their health. If this is known and if
they are in danger, action can be taken to preserve and protect
them. Forests are essential to the continued economic and spiritual health of all North Americans.
Forests are subjected to natural and anthropogenic stresses
that may cause fundamental damage and changes to forest
ecosystems. Monitoring the state of the health of the forests is
done by assessing the mortality rates, and the condition of the
crowns to determine what if anything is damaging them. The
results of these monitoring activities are used to assess the
health of the forests.
Tree mortality in North American forests was generally in
the normal range of 1–3% and is caused largely by competition
and suppression within stands. The effects of insects, diseases,
drought, and storms were observed frequently, and occasionally
caused higher rates of mortality. Trees in some areas improved
and recovered from previous damaging stresses.
Defoliation and dieback of tree crowns were observed
frequently and were caused primarily by known damaging
agents: insects, diseases, drought, frost, etc. Classic symptoms
of air pollution were found in some forest ecosystems. Damage
from acid fogs and ozone occurred on several species in several
16 Health of North American Forests
biomes indicating the potential for extensive damage in the
absence of corrective action. In addition, trees might have been
weakened or stressed by air pollution and this stress may not
yet be evident in the monitoring programs. Overall, the assessments indicate that there is no large-scale decline in the health
of forests that can be attributed solely to atmospheric pollution.
Nevertheless, continued monitoring is necessary to ensure that
the health of these forests is protected.
This publication is a general overview of the situation; for
more detailed results, the reader is referred to the national
publications on the subject.
Bauer, L.I., de; Hernández-T., T.; Manning, W.J. 1985. Ozone
causes needle injury and tree decline in Pinus hartwegii at
high altitudes in the mountains around Mexico City. J. Air
Pollut. Control Assoc. 8: 838.
Bauer, L. I. de; Krupa, S.V. 1990. The Valley of Mexico:
Summary of observational studies on its air quality and
effects on vegetation. Environ. Pollut. 65: 109–118.
Becker, M.G.; Landmann, G.; Levy, G. 1989. Silver fir decline
in the Vosges mountains [France]: role of climate and
silviculture. Water Air Soil Pollut. 48: 77–86.
Selected Bibliography
Berrang, P.; Karnosky, D.F.; Bennett, J. P. 1991. Natural selection for ozone tolerance in Populus tremuloides: an evaluation of nationwide trends. Can. J. For. Res. 21: 1091–1097.
Alarcón, M.A. 1993. Estudio Dendrocronológico de Pinus
hartwegii Lindl. en el Suroeste de México. Tesis M.C.
Montecillo, Méx.
Berry, C.R. 1973. The differential sensitivity of Eastern White
Pine to three types of air pollution. Can. J. For. Res. 3:
184–187.
Allen, D.C.; Barnett, C.J.; Millers, I.; Lachance, D. 1992.
Temporal change (1988–1990) in sugar maple health, and
factors associated with crown condition. Can. J. For. Res.
22 (11): 1776 –1784.
Binkley, D.; Valentine, D. 1991. Fifty-year biogeological
effects of green ash, white pine and Norway spruce in a
replicated experiment. For. Ecol. Manage. 40: 13–25.
Alvarado, R.D.; Bauer, L.I. de. 1991. Ataque de Lophodermium sp. en poblaciones naturales de Pinus hartwegii de
“El Ajusco”, México, bajo el efecto de gases oxidantes.
Micologia Neotropical Aplicada 44: 99–109.
Alvarado, R.D.; Bauer, L.I. de; Galindo, A.J. 1993. Decline of
sacred fir (Abies religiosa) in a forest park south of
Mexico City. Environ. Pollut. 80: 115–121.
Andrade, E.V.; Cibrian, T.D. 1980. Evaluación de problaciones
de muérdago enano (Arceuthobium globosum) Hawks y
Wiens y A. vaginatum Willd) en bosques de Pinus hartwegii
Lindl en Zoquiapan, Edo. de México. Mem. Prime Simp.
Nal. sobre Parasitología Forestal. Uruapan, Mich.,
México. p. 238–253.
Auclair, A.N. 1987a. The distribution of forest declines in
eastern Canada. Pages 307–319 in Kairukstis, L., S. Nilsson,
and A. Straszak (eds.), Proc. Workshop on Forest Decline
and Reproduction: Regional and Global Consequences.
IIASA, A-2361, Laxenburg, Austria.
Auclair, A.N. 1987b. The climate change theory of forest
decline. Pages 1–29 in Lavender, D.P. (ed.) Proc. IUFRO
Conf. on Woody Plant Growth in a Changing Physical and
Chemical Environment. Univ. British Columbia, Vancouver,
British Columbia.
Bauer, L.I., de; Hernández-T., T.; Alvarado, R.D. 1987. Forest
decline in southern areas of Mexico City. Abstracts of the
XIV International Botanical Congress. Berlin. p. 404.
Blank, L.W. 1985. A new type of forest decline in Germany.
Nature 314: 311–314.
Bormann, F.H. 1985. Air pollution stresses on forests. Bioscience 35(7): 434–441.
Braathe, P. 1957. Is there a connection between the birch dieback and the March thaw of 1936? For. Chron. 33: 357–363.
Bravo, H.A. 1987. La contaminación del aire en México.
Editorial Universo Veintiuno, A.C. México. 296 p.
Chappelka, A.H.; Renfro, J. 1992. Biomonitoring ozone injury
on hardwood species in Great Smoky Mountains National
Park. Interim Report—1992. Cooperative Agreement CA5460-2-9009, Subagreement No. 1. 41 p.
Chevonne, B.I.; Linzon, S.N. 1988. Tree decline in North
America. Environ. Pollut. 50: 87–99.
Cibrian-Tovar, D.; Ebel, B.H.; Yates III, H.O.; MendezMontiel, J.T. 1986. Cone and seed insects of the Mexican
conifers (Insectos de conos y semillas de las coníferas de
México). USDA For. Serv., Southeast. For. Expt. Sta.,
Asheville, North Carolina. 110 p.
Clarke, J; Barter, G.W. 1958. Growth and climate in relation to
yellow birch. For. Sci. 4(4): 343–364.
Cowling, E.B. 1989. Recent changes in chemical climate and
related effects on forests in North America and Europe.
Ambio 18(3): 167–171.
Cramer, H.H. 1984. On the predisposition to disorders of middle European forests. Pflanzenschutz-Nachr. 37(2): 97–207.
Health of North American Forests
D’Eon, S.P.; Magasi, L.P.; Lachance, D.; DesRochers, P. 1994.
ARNEWS—Canada’s National Forest Health Monitoring
Plot Network. Manual on Plot Establishment and Monitoring (Revised). Can. For. Serv., Petawawa Natl. Forest.
Inst., Inf. Rep. PI-X-117.
Eager, C.; Adams, M.B. 1992. Ecology and decline of Red
Spruce in the Eastern United States. Springer-Verlag,
New York. 417 p.
Fraser, G.A. 1989. Acid rain control: Potential commercial
forestry benefits to Canada. Forestry Canada, Ottawa,
Ont., Inf. Rep. E-X-42. 31 p.
Garner, J.H.B.; Pagano, T.; Cowling, E. 1989. An evaluation
of the role of ozone, acid deposition and other airborne
pollutants in the forests of eastern North America. USDA
For. Serv., Southeast. For. Expt. Sta., Asheville, North
Carolina, Gen. Tech. Rep. SE-59. 172 p.
Gibson, I.A.S.; Salinas-Quinard, R. 1985. Notas sobre
enfermedades forestales y su manejo. Bol. Tec. Inst. Nac.
Invest. For. No. 16. México. 196 p.
Gregorius, H.R. 1989. The importance of genetic multiplicity
for tolerance of atmospheric pollution. Pages 163–172 in
Scholz, F., H.R. Gregorius, and D. Rudin (eds.), Genetic
effects of air pollutants in forest tree populations. SpringerVerlag, Berlin, Heidelberg.
17
Hennessey, T.C.; Dougherty, P.M.; Tauer, C.G. 1987. Forest
Decline—Assessing Impacts of a Changing Environment on
Forest Productivity. Heron Publishing, Victoria, B.C. 102 p.
Hernández-T., T. 1984. Efecto de los gases oxidantes sobre
algunas especies del género Pinus nativas del Valle
México. Tesis M.C., México.
Hernández-T., T.; Bauer, L.I., de. 1986. Photochemical oxidant
damage on Pinus hartwegii at the Desierto de los Leones,
D. F. Phytopathology 76(3): p. 377.
Hertel, G.D.; Eagar, C.; Medlarz, S.A.; McFadden, M.W.
1992. The effects of acidic deposition and ozone of forest
tree species in the eastern United States: Results from the
Forest Response Program. Pages 54–65 in Huettl and
Mueller-Dombois (eds.), Forest Decline in the Atlantic
and Pacific Region. Springer-Verlag, Berlin, Heidelberg.
Hiratsuka, Y.; Zalasky, H. 1993. Frost and other climate-related
damage of forest trees in the Prairie Provinces. Forestry
Canada, Northwest Region, Inf. Rep. NOR-X-331. 25 p.
Hofacker, T.H.; Fowler, R.F.; Turner, L.; Webster, K.; Rieffe,
M. 1992. Forest Insect and Disease Conditions in the
United States in 1991. USDA For. Serv., Forest Pest
Management, AB-2S, Washington, D.C. 139 p.
Houston, D.A. 1987. Forest tree declines of past and present:
current understanding. Can. J. Plant Pathol. 9: 349–360.
Hain, F.P. 1987. Interactions of insects, trees and air pollutants. Tree Physiol. 3: 93–102.
Huerta-C., J. 1982. 100 maderas mexicanas. Pub. Esp. Inst.
Nac. Invest. For. México. 9 p.
Hall, J. Peter 1993. ARNEWS annual report 1992. Natural
Resources Canada, Canadian Forest Service, Ottawa, Ont.,
Inf. Rep. ST-X-7, 50 p. (English, French, and Spanish
under one cover)
Jauregui, O.E., 1958. El aumento de la turbiedad del aire en la
Ciudad de México. Rev. Ing. Hidr. en Méx. Vol. 12(3).
Hall, J. Peter; Addison, P.A. 1991. Response to air pollution:
ARNEWS assesses the health of Canada’s forests. Forestry
Canada, Ottawa, Ont., Inf. Rep. DPC-X34. 42 p. (English,
French and Spanish under one cover)
Hanisch B.; Kilz, E. 1990. Monitoring of forest damage. Verlag,
Eugen, Ulmer, Stuttgart. 334 p.
Hawbolt, L.S.; Skolko, A.J. 1948. Investigation of yellow
birch dieback in Nova Scotia in 1947. J. For. 46: 659–671.
Hawksworth, F.G., 1980. Los muérdagos enanos (Arceuthobium)
y su importancia en la silvicultura de México. Mem.
Primer Simp. Nal. sobre Parasitología forestal. Uruapan,
Mich., México. p. 207–208.
Hawksworth, F.G.; Wiens, D. 1972. Biology and classification
of dwarf mistletoes (Arceuthobium) Agr. Handbook No.
401, USDA, Washington, D.C. 234 p.
Keller, W.; Pitblado, J.R.; Conroy, N.I. 1986. Water quality
improvements in the Sudbury, Ontario, Canada area related
to reduced smelter emissions. Water, Air Soil Pollut. 31:
765–774.
Magasi, L.P. 1989. White birch deterioration in the Bay of Fundy
region, New Brunswick, 1979–1988, Forestry Canada,
Maritimes Region, Fredericton, N.B., Inf. Rep. M-X-175.
Malhotra, S.S.; Blauel, R.A. 1980. Diagnosis of air pollutant
and natural stress symptoms on forest vegetation in
western Canada. Can. For. Serv., Nor. For. Res. Centre,
Edmonton, Alta., Inf. Rep. NOR-X-228. 84 p.
Mangis, D.; Baron, J.; Stolte, K. 1991. Acid rain and air pollution
in desert park areas. Proceedings of a workshop and management recommendations. Tucson, Arizona, May 16–18,
1988. Technical Report NPS/NRAQD/NRTR-91/02. 129 p.
Manion, P.D. 1981. Tree disease concepts. Prentice-Hall Inc.,
Englewood Cliffs, N.J.
18 Health of North American Forests
McLaughlin, S.B. 1985. Effects of air pollution on forests. J.
Air Pollut. Control. Assoc. 35: 512–534.
Millers, I.D.; Allen, D.C.; Lachance, D. 1992. Sugar maple
crown conditions improve between 1988 and 1990. USDA
For. Serv./Forestry Canada, NA-TP-039-2.
Millers, I.; Lachance, D., Burkman, W.G.; Allen, D.C. 1991.
North American Sugar Maple Decline Project: Organization and Field Methods. USDA For. Serv., Northeast. For.
Expt. Stat., Gen. Tech. Rep. NE-154. 27 p.
Mueller-Dombois, D., 1987. Natural dieback in forests. Bioscience 37(8): 575–583.
National Acid Precipitation Assessment Program (NAPAP).
(n.d.). Diagnosing injury to eastern forest trees. NAPAP
Forest Responses Program, Vegetation Survey Research
Cooperative, USDA, U.S.F.S. 122 p.
Pearson, R.G.; Percy, K.E. 1990. The 1990 Canadian longrange transport of air pollutants and acid deposition
assessment report. Part 5. Terrestrial effects. Fed./Prov.
Res. Monitor. Coord. Comm., Downsview, Ont. 105 p.
SARH-INIFAP. 1982b. Contribuciones al estudio ecológico de
las zonas cálidohúmedas de Mexico (4). Bol. Esp. Inst.
Nac. Invest. For. México. 7 p.
SARH-SFF. 1994. Resumen ejecutivo: Principales realizaciones
1988–1993 e informe anual de avance prográmatico 1993.
México. 20 p.
Schindler, D.W. 1988. Effects of acid rain on freshwater ecosystems. Science 239: 149–157.
Shadwick, D.S.; Smith, L.A. 1992. Selected air quality and
deposition data summaries. Pages 8.1–8.13 in Forest
Health Monitoring, 1992. Forest Health Monitoring 1991
Statistical Summary. U.S. Environmental Protection
Agency, Washington, D.C.
Smith, S.L.; Dale, J.; DeNitto, G.; Marshall, J.; Owen, D.
1994. California Forest Health: Past and Present. USDA,
For. Serv., R5-FPM-PR-001. 70 p.
Sociedad Mexicana de Entomología. 1980. Memoria del
primer simposio nacional sobre parasitología forestal.
Editora Educación. Uruapan, Mich., México. 324 p.
Pennington, T.D.; Sarukhan, J. 1968. Manual para la identificación
de campo de los principales árboles tropicales de Mexico.
Imprenta Benjamín Franklin, S.A. México, D.F. 413 p.
Stone, L.L.; Skelly, J.M. 1974. The growth of two forest tree
species adjacent to a periodic source of air pollution.
Phytopathology 64: 773–778.
Phillips, S.O.; Skelly, J.M.; Burkhart, H.E. 1977. Eastern
white pine exhibits growth retardation by fluctuating air
pollutant levels: interaction of rainfall, age and symptom
expression. Phytopathology 67: 721–725.
Treshow, M.; Anderson, F.K.; Harner, F. 1967. Responses of
Douglas-fir to elevated atmospheric fluorides. For. Sci. 13:
114–120.
Rehfuess, K.E. 1987. Perceptions on forest diseases in central
Europe. Forestry 60(1): 1–11.
Udvardy, M.D.F., 1975. A classification of the biogeographical
provinces of the world. IUCN Occasional Paper No. 18,
Morges, Switzerland. 48 p.
Rzedowski, J. 1978. Vegetación de México. Ed. Limusa.
México. 432 p.
USDA. 1994. America’s Forests: 1994 Health Update. USDA
For. Serv., Agric. Inf. Bull. 696. 20 p.
SARH. 1988a. Memoria de los simposia nacionales de
parasitología forestal II y III. Pub. Esp. Inst. Nac. Invest.
For. No. 46. México. 463 p.
Vásquez-Collazo, I.; Pérez-Chavez, R.; Pérez-Chavez, R.
1992. Efecto del parasitismo del muérdago (Psittacanthus
schiedeanus) (Cham & Schlecht) Blume en el desarollo de
tres especies del género Pinus. Ciencia Forestal, No. 40,
Vol. 7: 48–64.
SARH. 1988b. Memoria del IV simposio nacional sobre
parasitología forestal. Tomo I. Pub. Esp. Inst. Nac. Invest.
For. Agropec. No. 59. México. 556 p.
SARH. 1988c. Memoria del IV simposio nacional sobre parasitología forestal. Tomo II. Pub. Esp. Inst. Nac. Invest.
For. Agropec. No. 60. México. p. 557–996.
SARH-INIFAP. 1981. Memoria de la I reunión sobre plagas y
enfermedades forestales. Pub. Esp. Inst. Nac. Invest. For.
No. 32. México. 66 p.
SARH-INIFAP. 1982a. Contribuciones al estudio ecológico de
las zonas cálidohúmedas de México (2). Bol. Esp. Inst.
Nac. Invest. For. México. 5 p.
Waring, R.H. 1987. Characteristics of trees predisposed to die.
Bioscience 37(8): 569–574.
Wellman, F.L., 1964. Parasitism among neotropical phanerogams. Annu. Rev. Phytopath. 2 : 43–56.
White, J.C. Ed. 1988. Acid Rain. The relationship between
Sources and Receptors. Elsevier, New York, USA. 223 p.