People have multiple effects on forest ecosystems. Human impacts include land conversion and harvesting, suppression of natural fire cycles and floods, and the introduction of non-native species, especially pathogens. These in turn influence ecological processes and ultimately forest dependent plant and animal species.
Air pollutants are suspected to have a significant cumulative impact on forest ecosystems by affecting regeneration, productivity, and species composition over extensive areas. Correlating forest inventory and health statistics with air pollution data provides additional information on the effects of these pollutants. Increased ultra-violet radiation, caused by changes in the earth=s atmosphere, also has been shown to damage plants.
Monitoring of forest structure or macro species such as vertebrates (as for Criterion 1) tends to detect changes in ecological processes only decades after they have begun. On the other hand, monitoring of very short-lived species associated with specific ecological processes such as decomposition and nutrient cycling provides an immediate indication of more subtle changes in ecological processes with potential importance to forests in the long run.
INDICATOR 15: Area and percent of forest affected by processes or agents beyond the range of historic variation, e.g., by insects, disease, competition from exotic species, fire, storm, land clearance, permanent flooding, salinization, and domestic animals.
Disturbance plays an important role in forest ecosystems. In fact, without disturbance, forests would eventually cease to function due to excessive buildup of biomass and the end of necessary nutrient cycling. The key question for this indicator is whether the magnitude of disturbance we are witnessing is outside of the range of historic variation.
New factors affecting and changing forests in the last century are exotic pests as well as pollution from industry and urbanization. Changes in land uses have also been significant, including conversion to residential, agriculture, and other uses.
Wildfire has been and continues to be one of the most significant influences on the composition and structure of forests in the United States, especially in the West. After European immigrants landed on the continent, wildfires were routinely suppressed. Previously, warm, dry forest ecosystems, comprising more than half of the forest area in the interior West, had been maintained by frequent, low-intensity fires. At higher elevations, cooler and moister forests experienced less frequent, stand-replacing fires.
A variation in these disturbances is assumed to have occurred prior to settlement of the continent by Europeans. Actions of indigenous peoples, however, contributed to the historic variation. Disturbance agents beyond the range of historic variation have been identified from past and present forest vegetation trends (species, structure, successional patterns), and trends in the direct effect of a disturbance agent, such as acres of mortality or defoliation per year.
Monitoring and recording of disturbances in our forests has taken place only over the past 20 to 80 years, depending on the disturbance agent. Observations are lacking for more distant time frames, precluding us from knowing with certainty what the historical conditions and trends may have been. Most of the information on current status and trends presented for this indicator comes from Forest Service records over the past 50 years. Insect- and disease-caused mortality or defoliation data have been gathered in ground surveys, aerial surveys, and insect trapping networks.
Insect and disease data have also been collected by the Forest Inventory and Analysis (FIA) program for State and private forest lands and by national forest inventory programs for Federal lands, along with other forest vegetation data. Both sets of data are available on about a 10-year cycle for re-measurement.
Fire data for 11 western States are available from the Intermountain Fire Sciences Laboratory. These data have been compiled from Forest Service Smokey Bear Reports, 1916-1982; and from individual agency reports, 1983-1994, which have been compiled by the Bureau of Land Management, National Interagency Fire Center, Boise, Idaho. Data collected prior to 1931 do not include National Park Service or Indian Reservation lands; and data collected prior to 1926 apparently include only forested areas. National fire data area are available in Fire and Aviation Management, US Forest Service, National Office. Those data were compiled from regional reports and include fire data from the Forest Service, Department of the Interior, and State and private lands.
Other data pertinent to disturbance agents and processes, such as resulting impacts of tree mortality and growth, and tree damage and crown condition, are available from the Forest Health Monitoring Program. Data on specific disturbance agents have not been collected. Plots are remeasured on a 4-year cycle; and remeasurement data are currently available from some States.
Data indicating land clearance, salinization, and other human caused disturbance may be available from localized sources throughout the country, but it was not possible to gather such detail for this report.
Data Summary and Interpretation
Native Insects and Diseases: In 1995, over 90 million acres of forested land in the United States were affected by southern pine beetle, mountain pine beetle, spruce budworm (eastern and western), spruce beetle, dwarf mistletoe, root disease, and fusiform rust. This is a conservative estimate, since not all forested land had been surveyed or monitored for insect and disease damage. In addition, several million more acres contain trees affected by native insects and diseases other than those listed above.
The southern pine beetle prefers loblolly and shortleaf pine, but will attack and colonize all species of southern yellow pine, whether in pure pine stands or in stands that are mixed with hardwoods. Mountain pine beetle attacks lodgepole, ponderosa, sugar, and white pines. The acreage affected by this insect has steadily declined since reaching a historical high in the early 1980's.
The eastern spruce budworm is a native insect inhabiting over 150 million acres of spruce and fir forests in North America, including 12 million acres within the United States. Large outbreaks of this insect have been recorded since the 1770's in the eastern United States and Canada. The biggest outbreak in this century occurred in the 1920's and 1930's, due in part to the extensive harvesting of spruce. Vast areas of balsam fir were killed, and budworm did not return until the 1940's, peaking in the 1970's. Spruce forests in West Virginia decreased to about 110 thousand acres in 1986, due, in part, to spruce budworm-caused mortality as well as logging and fires in the early 1900's. In Alaska, an increasing population of eastern spruce budworm has affected extensive areas containing over-mature spruce.
The western spruce budworm is native to mixed conifer forests of the West, feeding primarily on Douglas-fir and true firs. After widespread outbreaks in the 1980's, throughout its range, western spruce budworm populations collapsed in the early 1990's and remain at low levels. Major changes in outbreak patterns have occurred since fire suppression and harvesting of non-host species became widespread in the early 1900's. In the Rocky Mountains, recent outbreaks are more severe, and in the Northern Rockies they also last longer. In Oregon=s Blue Mountains, outbreaks have become more frequent and more severe since the early 1990's, although no change in outbreak duration was found.
Spruce beetle occurs throughout the range of spruce in North America. Infestations have risen exponentially in recent years, with more than 1 million acres of spruce beetle activity detected in 1986. Current spruce beetle activity in Alaska is also higher than historic levels.
Over 28 million acres in the western States are affected by dwarf mistletoes, parasitic plants that cause growth reduction, top killing, deformity, and mortality in conifers. Over 40 percent of Douglas-fir, 26 percent of ponderosa pine, and 21 percent of true fir east of the Cascades is infected. Dwarf mistletoes have increased over the past century east of the Cascades due to the suppression of fires. Fire was an effective sanitizing agent, especially for severely infected trees and stands that were more fire-prone.
In the western United States, root diseases can be found on over 6 percent of commercial forest lands of all ownerships, causing growth loss and mortality on more than 9 million acres. In Oregon and Washington, an estimated 7 to 8 percent of forested lands (three million acres) is affected by root disease. In Idaho and Montana, almost 15 percent of commercial forest land (3.3 million acres) has root disease. In the eastern U.S., root disease has been reported on over 1.8 million acres. In the Southeast, annosum root disease contributes significantly to pine mortality, reduced growth, and increased vulnerability to bark beetle attacks. There are an estimated 163.5 million acres at high or moderate risk to annosum root disease in the Southeast.
Fusiform rust is a disease that flourishes across the southeast, killing or deforming millions of slash and loblolly pine each year. The rust fungus requires both pine and oak trees to complete its life cycle; spores produced on oak leaves infect pine and subsequent spores produced on the pine, in turn, infect oak. Currently, over 13 million acres have trees infected with fusiform rust. The current level of fusiform rust is probably far outside the natural range of variation due to dramatic alterations to the natural pine forests in the southeast by intensive pine management and fire control.
Exotic Insects, Diseases, and Plants: There are over 4500 exotic free-living species in the U.S. today - approximately 2 to 8 percent of plants, insects, pathogens are introduced - some beneficial and some harmful. Of the 70 major insect pests fount in US forests, 19 are exotic. The corresponding proportion of forest pathogens may be even greater. Some forested ecosystems are more impacted and threatened by exotics than others. Hawaii is an extreme example, where two-thirds of the plant species in Hawaii Volcanoes National Park are non-native.
White pine blister rust was first found in the west in 1921 and by 1956 had spread to most of the white pine forests in the western United States. It is now distributed across the entire range of western white pine from British Columbia to New Mexico and across the major part of the range of other susceptible white pine such as sugar, limber, and whitebark pine. Up to 95 percent of the original stands of western white pine and sugar pine have been killed or damaged by white pine blister rust. Indirect impacts are also experienced as wildlife habitats are modified. For example, whitebark pine, a source of food and habitat for many species of wildlife including the grizzly bear, is considered a keystone species in high-altitude ecosystems. Suppression of fires has increased competition from other species and, at the same time, reduced the clearings in stands with whitebark pine and thus the number of places for seeds to be cached by the Clark's nutcracker, a bird that plays a key role in whitebark pine survival.
There are over 600,000 acres (254,000 ha) of Port-Orford-cedar limited primarily to coastal northern California and southern Oregon. A little over 30,000 acres (12,250 ha) are infested with root disease that was introduced in 1952 and spread primarily by movement of contaminated water and soil. A significant portion of large Port-Orford-cedar has been killed in local areas and has had important ecological impacts in reducing stream shading, long-term wildlife habitat, and large woody material in streams.
The European gypsy moth was introduced to Massachusetts in 1869. In 1995, over 1.4 million acres of hardwood forests were defoliated from New England to Virginia and Michigan. On the west coast, European gypsy moths have been trapped periodically since 1974. In 1991, the Asian form of the gypsy moth was introduced in the Northwest, and two years later in the East. Neither form of gypsy moth has become established in the Northwest and the Asian form was eradicated in North Carolina. In 1996, gypsy moth defoliation declined more than 85 percent from 1995 levels due to spread of a fungal pathogen throughout most of the gypsy moth population. In areas where the gypsy moth is spreading into new areas, the fungus has not yet caught up to the moth.
The hemlock woolly adelgid was reported on the west coast in the 1920s, but did very little damage to western forests. In 1950, the insect was introduced to the east coast. Approximately 25 percent of the 1.3 million hectares of host type have been infested in the East and some scientists believe the entire range of eastern hemlock is at risk within the next 20 to 30 years. Loss of hemlock from eastern forests will have impacts on riparian areas where hemlocks help regulate water temperature, on wildlife species that depend on hemlock stands for shelter, nesting, and foraging habitat.
Beech bark disease was found killing American beech in eastern Maine in the early 1930s, as it spread into the United States from the Canadian Maritime. By the early 1980's it had spread through New York and into northeastern Pennsylvania. The merchantable timber volume loss is significant, and the mortality continues.
Fire: The frequency and scope of wildfire has changed dramatically since European settlers increasingly laid claim to the lands in the West in the mid-1800's. Livestock grazing and the significant curtailment of burning by Native Americans were inexorably changing the landscape. By the 1930's, the Forest Service and other Federal agencies were bringing more and more effective fire suppression to the land. Fewer and fewer acres of wildfire were recorded in the 1950's, 60's, and 70's. Beginning in the 1980's, wildfires in the West began to increase again. Over 3 million acres burned in 1994, over 1 million in 1995, and an estimated 5 million or more in 1996. The primary, overriding reason for increased fires and fire susceptibility is, ironically, the unprecedented success of decades of fire suppression. That success brought increased forest density and biomass, changes in forest composition (diminished populations of certain fire-adapted tree species such as aspen) and the resulting increases in insect and disease susceptibility and mortality, and buildup of fuels.
In the West, wildfire occurrence, patterns, and severity are probably considered, by most, to be beyond the range of historic variation. We assume here that historic variation included the very significant influence that Native Americans had with forest vegetation.
Fire behavior has not changed in southern pine forests to the extent it has in western forests over the last century. Despite increased fire suppression efforts beginning in 1930's, arson-caused fire (for pasture and insect control) and prescribed fire in managed stands kept fire at a higher level than it would otherwise have been. The proportion of forest burned is similar to historic conditions, 10 to 30 percent annually, although altered in timing and frequency. These seasonal changes have led to significant changes in southern pine forest community structure, composition, and biological diversity. A reduction in the extent of fire in some parts of the Southeast has resulted in increased oak populations which, in turn, have led to increased levels of fusiform rust.
Other Disturbances: Major weather disturbances seem to have increased in frequency and intensity in recent years, including hurricanes, droughts, and flood events, resulting in destruction of millions of acres of forest. The effects of weather on forests may also be more significant now than in the past, at least partially due to management activities which increase the vulnerability of the ecosystem. After the immediate damage of the event itself, additional mortality in weakened stands results from subsequent insect or disease attacks. Several western states including California and eastern Oregon and Washington, experienced a drought between the mid-1980s and mid-1990s. Outbreaks of some insects were particularly high during these years especially on sites that were dry and had high tree density. No data was gathered for this report about trends in frequency and severity of these weather events.
There is mounting evidence that global climate change has occurred in the last 100 years. Proposed theoretical causes of present and future changes are almost exclusively anthropogenic, resulting from increased use of petroleum, coal, and natural gas; deforestation and burning of vast tracts of land; and increased production of methane from domesticated livestock. Some scientists believe they have clear evidence showing that, on average for the world, temperatures have increased by about .6 degrees centigrade during this century. Climate models predict a global surface temperature increase averaging 1.0 to 3.5 degrees centigrade and a 50 centimeter rise in sea level by 2100. Some parts of the scientific community do not believe that global climate change is a cause for great concern.
INDICATOR 16: Area and percent of forest land subject to specific levels of air pollutants (e.g., sulfates, nitrates, ozone) or ultraviolet B that may cause negative impacts on the forest ecosystem.
A common approach for evaluating ecosystems has been to consider two broad community types, aquatic and terrestrial. This is an especially convenient division for air pollutant purposes because the important group of pollutants that affect terrestrial species (e.g., ozone and hydrogen fluoride) are, in general, different from those that affect aquatic species (e.g., sulfates, nitrates, and metals). For example, pollutants such as acid fog composed of nitrates and sulfates, are injurious to plant leaves and can contribute to watershed acidification. Consequently, categorizing monitoring programs as benefitting purely one component of the ecosystem is not always possible. For simplicity, however, we will interpret the data separately for aquatics versus terrestrial systems.
Lichens especially are good bio-indicators of forest health because they are affected very directly by pollution. They rely on atmospheric sources for their nutrition and thus are very sensitive to changes in the atmosphere. Reports of the analyses from some of these studies show lichens can be affected by oxides of sulfur and nitrogen, hydrogen fluoride, metals, organics, radionucleides and perhaps to a lesser degree by sulfated, nitrates, ozone and ammonia.
Negative impact is interpreted to mean a substantial change in the function of an ecosystem or components of an ecosystem due to the influence of air pollutants. Obvious changes are loss of species or even loss of tolerant genotypes.
The National Acid Deposition Program (NADP) is a nationwide network of precipitation chemistry monitoring sites. First begun in 1978, the network currently consists of about 200 sites of average weekly data.
Average sulfate ion deposition has declined around the Ohio River Valley, most noticeably between 1989 and 1995 (see Figure 4-1 for 1995). Sulfur deposition has crept lower in the west, except along the northwest coast where there has been essentially no change. Nitrate ion deposition has declined at most locations in the east especially in the southeastern US. Nitrate deposition in the west has remained about the same (see Figure 4-2).
University of Georgia and Environmental Protection Agency Ultraviolet Monitoring Network has data for only four sites and it is limited in scope. The USDA UV-B Radiation Monitoring Program was initiated in 1992 to determine the geographical distribution and temporal trends of UV-B radiation in the US. Monitoring is conducted at three sites but averaged data and trend data are not yet available. The Mercury Deposition Network is a national network started in 1993 and currently consists of 30 sites measuring weekly average total and methyl-mercury in precipitation samples. These data are readily available from the Network.
Fig. 4-1. Sulfate ion deposition.
Along the Appalachian chain (and
other high elevation regions) depositions of acid compounds (sulfates and nitrates) and toxic trace elements is enhanced by the effect of the mountains. Mountains are frequently bathed in clouds which may result in atmospheric loading three to ten time greater than valley or lowland deposition. In addition, during the summer, mountain forests receive a higher dose of ozone (and possibly uvb..) than low elevation forests. The potential for multi-pollutant environmental damage to Fig. 4-2. Nitrate Ion Deposition.
mountain forests may influence forest
health and the susceptibility of
mountain forests to pathogens.
Research at Mount Mansfield, Vermont indicates that some high elevation soils no longer act as net sinks for nitrate. Additional work needs to be conducted in other high forest catchments to determine the extent of this phenomenon. One of the key questions on mercury contamination in remote areas is how much mercury deposits in the catchments of these remote areas, and how much of the mercury is exported to downstream aquatic systems.
Aerometric Monitoring consists of federal, state and local governments conducting ambient air (aerometric) monitoring of six pollutants for which exist national ambient air quality standards. Hundreds of sites operate across the US, mostly in or near populated areas. Of the six criteria pollutants, ozone is most important for forest health and is highest in the eastern US, California and some southern locations.
National annual average hourly ozone concentrations show a declining trend from 1985 to 1992. Since most of these sites are in urban areas and no other trend data exist, rural and forested ozone concentrations must be assumed by inference also to be declining. The 1992-94 data suggest that the average ozone concentrations have leveled off at about 110 ppb (.110 ppm). The Central Valley in California and the Los Angeles basin are violating the hourly average standard (120 ppb), as are much of the northeast seaboard and scattered other southern and Midwestern urban areas. The north central plains States do not regularly exceed the primary standard. While ozone concentrations are declining, the pollutant is still above an acceptable threshold in most of the east. The southern California air basin and the surrounding forested slopes stretch into the southern Sierra experience the highest persistent ozone concentrations of anywhere in the country. Many studies over the last 30 years have demonstrated that ponderosa pine, Jeffrey pine and other less wide-spread species of the region suffer from reduced energy flow and nutrient cycling.
The Interagency Monitoring of Protected Visual Environments (IMPROVE) network is one of the highest quality air monitoring programs for forested areas in the US. Thirty IMPROVE sites and about a dozen more IMPROVE-look-alike sites are in place today. The data are used to monitor visual air quality in National Wildernesses, Parks and Refuges.
The North American Maple Project (NAMP) is an international program instituted by the US Forest Service and the Canadian Forest Service in 1987. Its objectives were to: 1) determine the rate of change in sugar maple condition; 2) determine if changes in sugar maple condition differ due to management or level of atmospheric pollution; and 3) identify possible causes of sugar maple decline.
Nine years of annual, quality controlled, crown condition data indicates that over the natural range of sugar maple, the resource is generally healthy. There are a number of localized problems where causal agents (drought or insects) caused significantly worse health ratings. There is no significant difference in sugar maple health based on management regime, and trees in both sugarbushes and natural stands responded similarly to stress. Sugar maples exposed to high levels of sulfate and nitrate deposition had significantly worse health ratings than trees exposed to medium or low levels of deposition.
The Forest Health Monitoring Program (FHM) operates lichen and ozone monitoring in a portion of the 21 states that have the FHM permanent plot sampling grid. A full lichen census is available in some states and ozone-sensitive plants are monitored in some. Both of these data sets are available but not fully summarized yet. Other efforts are more sporadic and decentralized.
Data on the average fine sulfate aerosol concentrations for two recent successive three year periods, suggest that a slight reduction in sulfur aerosol has occurred in the west. The addition of sites in the mid-Atlantic area and Alabama revealed higher concentrations than the highest observed in the area previously but even most continuous sites showed no significant change in concentration over the six year period. Average fine aerosol nitrate concentrations seem to have declined most noticeably in California while the lack of detail in 1989 in the east may have masked a higher deposition then.
Although the National Acid Precipitation Assessment Program (NAPAP) report concludes that the vast majority of forests in the US and Canada are not in decline, the stress imposed by acidic deposition may contribute with other more significant factors, including climate, pests and disease, to the decline of some communities such as the red spruce in the higher reaches of the northern Appalachian Mountains.
NAPAP also concludes that although acid deposition may contribute in the long term to the leaching of nutrients from soils with low buffering capacities, there is no evidence that this has yet affected terrestrial biota.
Our ability to relate declines in growth and yield to exposures to pollutants such as ozone are limited because of the substantial gaps in our knowledge of air quality in many forested areas due to a shortage of monitors. Ambient levels of ozone do appear to be decreasing, but some forests may or may not show any positive effect immediately from this decline because of secondary impacts.
This indicator demonstrates some improvement in the overall health of forested ecosystems. For aquatic systems the situation is less clear because of the buffering capacity of soils and water systems. However, sulphate and nitrate deposition appear to be declining as well.
INDICATOR 17: Area and percent of forest land with diminished biological components indicative of changes in fundamental ecological processes (e.g., soil, nutrient cycling, seed dispersion, pollination) and/or ecological continuity (monitoring of functionally important species such as nematodes, arboreal epiphytes, beetles, fungi, wasps, etc.).
This indicator is intended to provide insight into whether key ecological components or processes, or ecological continuity, are changing in a negative way, suggesting the sustainability of a forest ecosystem may be declining or in jeopardy. This report focuses on ecological components and processes, because there are some definitions available to do so, and the lack of definitions concerning functionally important species as indicators of ecological continuity. Recent research supports the concern for nutrient loss and suggests acid rain can induce soil aluminum toxicity in some cases.
Key ecological processes may be defined as those essential to the long-term health and sustainability of forest ecosystems. Generally they are processes that are the most essential and fundamentally affecting all species. For example, nutrient cycling is an essential process because diminished cycling of nutrients will cause degradation of forest ecosystems.
The primary literature databases used for this search were TreeCD, Agricola, Biological Abstracts, and CAB. From Tree CD, Agricola, CAB, and World Wildlife literature data bases voluminous records are available but there was insufficient time to complete a review of the data. Little data exists on area and percent of forests where ecological processes have been affected.
The impact of biomass removal on ecosystem nutrient reserves has been recognized for a century. Initially, scientists predicted nitrogen to be a nutrient of concern in harvest removals, though today estimates suggest that natural and pollution nitrogen inputs to ecosystems generally balance removals in harvests. Modern research stresses the depletion of phosphorus and calcium from U.S. forest ecosystems. However, nitrogen depletion has been a concern in frigid conifer ecosystems of the northeast and northwest.
Harvest-caused nutrient losses in runoff and leaching are common, though carefully planned logging can limit impacts to about four times background levels in Eastern forest ecosystems. Specific areas with vulnerable soils are those in the coastal plain and piedmont regions in the eastern U.S. In the western U.S. scientists identify ecosystems with steep slopes as prone to debris avalanches following harvests. These events increase erosion dramatically. Early research at the Hubbard Brook long term ecological research site in New Hampshire stimulated great concern over harvesting because of potential impacts of nutrient losses on water quality. Later research revealed that the measured nitrate losses in streamwater at Hubbard Brook were unusually large compared to other ecosystems.
Great concern for atmospheric deposition of acid rain was met with heightened awareness as areas of forest declines were reported in northeast Adirondack and southern Appalachian spruce - fir and northeastern sugar maple forest ecosystems in the 1970's and 1980's. The U.S. National Acid Precipitation Assessment Program reported several findings in 1991 that relate to the health of forest ecosystems:
Acid nitrogen and sulfur deposition patterns are concentrated in the northeastern U.S. where industrial outputs are greatest. New York, Pennsylvania, and Ohio appear to exhibit the largest wet deposition of acid rain there. In the southern region Texas, Florida, Louisiana, and Tennessee produce large quantities of nitrogen and sulfur pollutants. Western region pollutant deposition remains low, though scientists there report many soils in the Sierra Nevada to be sensitive or very sensitive to damage from acid deposition.
There is substantial susceptibility to degradation from acid rain in the South as well. Ten percent of southern soils are sensitive to aluminum toxicity with additional acidification and 59 percent of soils there are susceptible to increased leaching of critical nutrients. Fifty-six percent of the Maryland landscape receives acid rain in excess of the expected critical load. Southeastern Appalachian spruce soils are more susceptible to acid rain damage than the northeastern spruce-fir ecosystem soils. Scientists in the northeastern region suggest acid deposition may be causing forest declines in spruce-fir ecosystems of the northern Appalachians and Adirondacks and in sugar maple ecosystems, though most evidence is inconclusive.
There were significant distribution losses of United States forested wetlands between 1940 and 1980. The greatest losses of forested wetlands during the 1940 to 1980 period in descending magnitude occurred in the north central (1,900,000 hectares), south central(1,900,000 ha), southeast (500,000 ha), and rocky mountain (355,000 ha excluding Arizona and New Mexico) regions in the U.S.
Summary Table 4-1 defines the percent change in ANPP (annual net primary productivity) as the change from maximum values achieved in early stand development compared to values for older forest stands.
In general, there is adequate data to determine the human activities and natural influences that disturb the forest and affect many key ecological processes. However lack of monitoring of the relevant biological components precludes a reliable estimate of the area of forest land affected by changes in fundamental biological processes.
The maintenance of forest ecosystem health and vitality have been measured by three indicators: processes and agents of disturbance; land and forest area susceptible to air pollutants; and forest land with diminished biological components indicative of fundamental changes in ecological processes and continuity. The following highlights reflect the significant trends and indications of this criterion.
Disturbance is part of the natural forest ecosystems. In fact, without disturbance, forests would eventually cease to function due to excessive buildup of biomass and the end of necessary nutrient cycling. The effects of native insects and diseases, fire, exotic pests (introduced insects, diseases, and plants), and weather, to some extent, on forest vegetation were addressed. In many
cases, the combined occurrence of more than one disturbance produced the changes in forest composition that we observe today.
There are substantial gaps in knowledge of air quality in many forested areas, limiting our ability to relate declines in growth and yield to exposures to pollutants such as ozone.
Lack of data precludes the analysis of area of forests affected with diminished biological components or ecological processes. In some cases, data does exist, but it has not yet been analyzed in enough depth to allow conclusions to be drawn. Long term monitoring and analysis of these elements needs to be designed and implemented.
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