Authored By: K. W. Stolte, D. Darr
Kenneth Stolte and David Darr
USDA Forest Service Southern Research Station and National Office
A variety of forces and influences has shaped forest ecosystems throughout the evolutionary history of trees and forests. Native insects and pathogens, extreme weather events, and cyclic fires are part of normal ecosystem structure and function that includes weakening, death, regeneration, recycling, and replacement of trees and forests. Forests are adapted to endemic levels of native insects and pathogens that periodically become epidemic and temporally cause greater effects that recede with time. Periodic extremes of weather or long-term shifts in climate are also natural occurrences and forests adapt in a variety of ways. Fire is also an essential component of forest ecosystems, and provides many beneficial effects when it occurs within normal cycles of frequency and severity.
When these normal processes interfere with human use of forests, the condition may be undesirable but is not ecologically unhealthy or unsustainable.
Evaluating risk to forest health and sustainability necessitates enumeration of the type and magnitude of major stressors (national or regional-scale stressors), and knowledge of the impacts of stressors on forest ecosystems. Understanding the mechanism and severity of stressor effects facilitates understanding the effects of multiple stressors acting additively or synergistically on forest ecosystems. Exotic invasive species (particularly insects, pathogens, and plants); fragmentation and land-use change; air pollution (gases, acids, fertilizers, and climate change); changes in ecosystem processes due to management activities such as fire suppression and selective harvests; and exacerbated populations of native species due to compositional or functional changes in forest ecosystem comprise one list of the top 5 stressors that have, or have potential to, affect tens of millions of acres of forest lands in the U.S.
This paper evaluates the magnitude of effects on forests by many of these major stressors, and area and percent of forests where key ecosystem processes have been altered but the causal agent(s) are unknown. These stressors and effects on forest ecosystems are delimitated by Criterion 3 of the Montreal Process Criteria and Indicators (MPCI). Criterion 3 contains the biotic and abiotic stressors (Indicator 3.a.), air pollution stressors (Indicator 3.b.), and changes in biological indicators of ecological processes or continuity (Indicator 3.c.).
Relatively large areas of forestland in the U.S. have been altered from historic conditions (pre-1600) or relevant reference conditions by conversion of forestland to agriculture, pastures, urban, or other uses. An estimated 1,045,435,000 acres of forests (about 46 percent of the total land area) covered the U.S. prior to 1630 (pre-European settlement), and currently 748,923,000 acres of forestland remain, a loss of 296,512,000 acres or 28.4 percent of the original forest. Most of the lost of forest land occurred in the 18th and 19th centuries—since 1900 the area of forest cover has increased due to improved agricultural methods, use of other materials for building and fuels, etc. Currently much of the land lost to urbanization each year comes from agricultural or pasture lands where forests had already been cleared. Urban sprawl and air pollution are 2 undesirable side-effects of industrialization and population growth that have had varying effects on the capacity to maintain healthy forest ecosystems. Urban sprawl is moving high-impact development and other human influences directly into or near forests on millions of acres in the East and West. This incursion of humans fundamentally changes the nature of forest ecosystems, diminishing the ability to maintain capacities for biodiversity (Criterion 1), productivity (Criterion 2), ecosystem health (Criterion 3), and some aspects of socioeconomic benefits (Criterion 6) of the MPCI. At the same time, this movement of people into the forest provides directly for some other socioeconomic benefits that humans seek from the natural resources.
Globally, the period 1996 to 2000 was part of the warmest decade (1991–2000) in the historical record, and 1998 was the warmest year since 1861. This observation suggests that temperatures in U.S. forests have exceeded both the range of historic and recent variation. Current analyses of relevant data specifically addressing the measurable effects of climate change have indicated that 4,600 square miles of pinyon pine forests in the Southwest have experienced high mortality associated with unusually high temperatures associated with drought. Unusually severe weather events (e.g., 1998 ice storm that damaged 17.5 million acres of Northeastern forests), widespread droughts in 1999 and 2000, and other unusual climatic and weather events may be caused by global climate change.
The introduction and spread of exotic, invasive species (insects, pathogens, plants, and animals) currently threaten many forest ecosystems. Exotic insects and pathogens have decimated native tree species such as American chestnut, butternut, American elm, eastern hemlocks, dogwoods, American beech, white pine, and others. An estimated 117 exotic insect species have been introduced into the U.S. from the late 1800’s to 2000 and have caused varying degrees of damage to host tree species and forests. Gypsy moth has caused periodic defoliation and death of trees over huge areas of Eastern forests, affecting over 26 million acres of forests in 1980-1982, the peak years of defoliation. Gypsy moth continues to spread into new areas of broadleaved, deciduous forests, altering the composition of the affected forests. Hemlock wooly adelgid was first introduced into the western U.S. and now causes extensive mortality of Eastern and Carolina hemlocks along coastal sections extending from New Hampshire to the Carolinas. The potential for this insect to spread further east and south throughout the range of eastern hemlock is high.
Exotic pathogens have greatly altered the composition, structure, and function of some forests. Introduced, nonnative pathogens have devastated several native U.S. tree species and caused broad, negative ecological effects. Chestnut blight and Dutch elm disease have eliminated two major tree species (American chestnut and American elm, respectively) from Eastern forests, causing major structural and functional changes in those ecosystems. Chestnut blight introduced in 1904 has virtually eliminated American chestnut from Eastern forests where it once was one of the most common trees. White pine blister rust has steadily spread throughout the East and West to affect all five-needle pines in the United States. It was introduced in 1906 in the eastern U.S. and spread throughout the range of host species in the East. In 1926 it was introduced into southwest Washington and has since spread east to South Dakota and south to California and New Mexico. It has changed the way Eastern and Western white pines are managed, and disrupted ecosystem functions wherever the susceptible tree species are significant components of the forest. Butternut canker is estimated to have killed 77 percent of Butternut trees in North Carolina and Virginia, and is a threat to the survival of this species throughout its range. Dogwood anthracnose affects flowering dogwoods in more then 22 eastern States, and has killed most of the dogwoods above 3000 feet elevation and in cool shaded areas below 3000 feet.
Sudden Oak Death disease, European pine shoot beetle, and Asian longhorned beetle are other recently introduced pathogens and insects that are damaging trees or forests in the U.S. Southern pine beetle, spruce beetle, fusiform rust, western spruce budworm, and mountain pine beetle are some of about a dozen native insects or pathogens that have continuing negative effects on the health of varying areas of forests during the latter part of the 20th century that have exceeded normal levels. The occurrence, severity, and spread of these damaging agents can be positively or negatively affected by management activities. Active timber management can sometimes promote forest health and reduce damage by enhancing the overall vigor of trees in a forest or by changing the forest composition. Altered species composition and density in a forest that results in less vigorous forests that are more susceptible to an insect or disease (pathogen) outbreak can result from management decisions that preclude natural processes or avoid all timber management and favors preservation of forest for other purposes.
Kudzu, tree of heaven, and empress tree are a few examples of introduced plants that kill or replace native trees in Eastern forests and cause a general change in forest composition and function. New exotic species that change forest composition, structure, and function continue to be introduced into the U.S. as a result of ever-increasing global commerce despite our knowledge that these damaging agents have been negatively affecting forest health and sustainability for more then a hundred years.
Air pollutants that affect forest health and are of greatest concern fall into three broad categories: (1) acidifying agents (nitrates, sulfates, and other anions), (2) fertilizing agents (N-based compounds, nutrient cations, etc.), and (3) oxidizing agents (primarily ozone). Decreases in the protective stratospheric ozone layer by chlorides, methane, and other gases cause an increase in ultraviolet-B (a type of radiation from the sun) that is a related concern because sparse data is available to evaluate the nature and severity of effects on forest ecosystems, and the area of forests affected. Forest ecosystems are exposed to elevated levels of several air pollutants, although the level of exposure, the specific pollutants, and the vulnerability of tree and other species varies by region. Elevated sulfur and nitrogen deposition (major components of acidic deposition) is highest in the North and South, while exposure to ozone is greatest in the South and parts of the Southwest. The effects of air pollution on forest ecosystem health are often associated with large uncertainty. Readily observable injury, often characterized by visible symptomology, is found in parts of the Southwest and Eastern U.S. Elevated ozone exposures causing visible foliar injury and reducing growth of sensitive plant species, and changes in soil chemistry from acidic precipitation and nitrogen fertilization are the most probable candidates for negatively affecting large areas of forest ecosystems.
The response of forest ecosystem processes to known or unknown stressors relies on biological indicators as surrogate measures of effects on these processes because it is difficult or impossible to directly evaluate ecosystem processes at large spatial scales. Thus defoliation and crown dieback gives an estimate of the essential process of photosynthesis, carbon fixation, and photosynthetic efficiency (related to growth) since the amount of foliage is a limit on the amount of carbon that can be fixed. Similarly, mortality volumes are related to key processes such as reproduction, carbon cycling, and seral development. Significant changes in tree crown conditions and mortality volume indicators related to key ecological processes were found over large areas of Western and Eastern forests. It was not possible to associate all observed negative effects with specific causal agents at this time.
Changes in ecological condition from altered fire regimes affect many ecological processes. Changes in historic fire cycles affected all regions, with moderate (condition 2) to substantial (condition 3) changes in historic fire regimes affecting large areas. The absence of fire from many forests for nearly 100 years has exacerbated forest health problems including dwarf mistletoe in many western forests, oak decline in the Ozark and Ouachita Mountains, mountain pine beetle in Western pine forests, and Western spruce budworm in Douglas-fir and true fir forests in the West. Catastrophic fires raged from 1938 to 1950, burning an average of 24.9 million acres per year for 13 years, compared to the period 1951 to 1978 where 4.9 million acres per year burned, a reduction of 80 percent in acres burned. Thus, a highly successful management strategy enhanced some aspects of forest health (e.g., carbon sequestration) for many decades ultimately led to the unintended consequence of reducing overall forest health and sustainability on vast areas of forest lands. Changes in historic fire regimes due to fire suppression, selective harvests, and other management activities have significantly altered fire regimes on 372,037,000 acres (60 percent) of the 620,306,000 acres of forestland in the conterminous 48 States in the U.S. Historic fire regime changes led to unusually hot and extensive fires when the areas eventually burned—7.4, 3.7, and 8.4 million acres of Western forestlands burned in 1988, 1997, and 2000, respectively, despite modern fire-fighting technology.
Fire is sometimes a damaging agent that adversely affects forest productivity with respect to human values. During much of the 20th century, fire prevention and fire suppression greatly reduced tree mortality and soil erosion that had formerly occurred on vast areas of southern and western forests. Such management preserved forests for many desirable uses and contributed to the increasing timber productive capacity of forests throughout the United States (Criterion 2). Fire suppression in areas naturally adapted to relatively frequent fires (especially many Southern and Western pine forests) results in altered species composition and increased density of trees per acre. These changes created increased fuel loads and other conditions that were conducive to large, high intensity fires, as reflected in increasing annual burned acreage in many years since the 1980s following 3 decades of relatively low annual burned acreage.
Monitoring Methods Session - Tuesday Afternoon
corresponding author:
Kenneth Stolte
USDA Forest Service
Southern Research Station
3041 E. Cornwallis Road
Research Triangle Park, NC 27709
919-549-4022
kstolte@fs.fed.us