What Is A Healthy Forest?
SOURCE: Health of Idaho's Forests - USDA Forest Service - Northern Region, Report No. 99-4
Regardless of how people view Idaho's forests, the health of these forests is vital. But what is a healthy forest? Healthy compared to what? By what criteria? There are many definitions and concepts because how one views forest health is a reflection of personal values.
In urban forests or in campgrounds, agents of change, like disease, fire, insects and weather damage are often undesirable. They put our facilities as well as visitors at some level of risk. However, in wilderness areas these same elements are considered desired components of a functioning ecosystem. It is our use or objective in managing the forest that determines how we view these agents of change as desirable or undesirable.
In searching for defining elements of a healthy forest, we might consider a forest unhealthy if it loses the ability to maintain or replace its unique species or functions. One way scientists have assessed whether a system is unhealthy is by comparing current conditions with the normal range of dynamics the system has experienced through the past. This concept is referred to as the historic range of variability. Change can be determined using techniques such as permanent monitoring plots, fire history analyses, old historical photo records or studies of pollen and charcoal layers in bogs or lakes. These various pieces of information are then integrated with our understanding of the dynamics of the ecosystem.
The ability of the forest to sustain itself ecologically and provide what society wants and needs is what defines a healthy forest. Maintaining the balance between forest sustainability and production of goods and services is the challenge for owners and managers of the state's forests.
- Ecological: A healthy forest maintains its unique species and processes, while maintaining its basic structure, composition and function.
- Social: A healthy forest has the ability to accommodate current and future needs of people for values, products and services.
These components are inextricably linked. Forests cannot meet social needs without possessing the sustained capacity to grow, reproduce, recycle nutrients, and carry out other ecological functions.
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Change is fundamental to all ecosystems. Change can occur suddenly or over such a long period that no change is apparent in the short term.
The process of vegetation change is called forest succession. "Disturbances," notably fire, insects, disease, climate and human activity, influence the direction and rate of change. Without disturbance, forests change, but at a different rate and direction.
Prior to European settlement, fire was the primary means of vegetative change, ignited by either American Indians or lightning. Settlement brought new agents of modification. Timber harvesting and fire suppression were meant to provide income and products or to protect people and property. However, other changes were unintentional, such as the introduction of damaging diseases, insects or vegetation. Some unanticipated side effects of intentional activities also have proved to be a negative, such as overly dense forest resulting from wildfire suppression.
Fortunately vast areas of Idaho still exhibit intact forests of native tree species. While all native tree species are present, in some areas proportions have changed substantially. Still these forests provide a habitat for a large number of native birds and other animals. These forests are also highly valued for their recreational opportunities, wilderness, and commodities they produce.
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Western White Pine
Aspen Forest Types
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Forest Health Issues
Forest health issues are rooted both in their ecological as well as social aspects. A forest is a dynamic system, continually changing in response to disturbances. Some disturbances help maintain native species and historic conditions. Others threaten them. Thus, there are limits to which a forest can recover from disturbances, especially exotic ones.
Ecological integrity is defined as the forest's ability to renew itself, or the ability to renew itself, or the ability to withstand disturbances and recover through time and across the landscape. If the forest is to have the potential to meet social needs, including wildlife habitat, clean water and products such as wood and recreation opportunities, then the integrity of the ecosystem must be retained.
The following sections address seven issues facing Idaho's people and their forests:
The introduction of foreign plants, animals, and microorganisms is one of the most disruptive influences on ecosystems. Sometimes a non-native species will find conditions highly favorable in its new location. With natural enemies left behind, populations expand unchecked until the species becomes a pest.
The result can be that non-native species eliminate native plants or animals from an ecosystem, greatly altering how the ecosystem functions. This is happening in Idaho, where a number of invaders have damaged the state's ecology and economy.
Water originating in forested areas throughout Idaho is valued for many reasons, from its use for domestic needs to providing habitat for anadromous fish species. The forested lands adjacent to streams and rivers serve to collect and purify the water, funneling it through a network of stream channels into the river systems.
The ability of the forests to collect and purify water is affected by the condition of the forest and the occurrence of disturbances that change the structure, composition and pattern of forest vegetation.
Because of its wide spread implications, water quality has become a major forest management issue. In some watersheds riparian areas and stream channels have been negatively impacted directly by logging, by fires, by road building, by dams and by mining. Indirectly, these same events occurring on upland areas may also effect water quality and streamside conditions.
Some relationships between water quality and condition of the upland vegetation are poorly quantified. However, we know changes in the amount, structure, composition of vegetation, both live and dead, within a watershed may effect several different aspects of water quality.
Some aspects of water quality effected by vegetation included the amount of water flowing out of the watershed, the retention of snow-pack, and the amount of sediment carried by water, the water's temperature and nutrient content. Variation in these characteristics over time and across a watershed is normal and desirable for the proper function of the system. The variation is a function of the amount of plant cover alive and dead in the form of litter, duff and woody debris, successional stage, pattern and structure of the vegetation across the watershed. Changes in the vegetative condition may be the result of fire, harvest, insect or disease activities, developments including roads, mining or subdivisions. Concerns are raised when the variation of these attributes exceeds the normal variation.
It is beyond the scope of this report to assess the health of Idaho's watersheds but it is important to recognize the links between watershed health and forest health.
Watershed Health Implications
Wildland Interface Development
While natural disturbance events, like fires and insect outbreaks, are common and even healthy for many forests, they present more difficult situations in development areas. If forests deteriorate, some people are effected by the aesthetic loss of forest cover and for other reasons and values for which they move to a wildland location. Another issue is the fire hazard and threat to life and personal property presented by abundant dead or dying trees. While urban areas throughout the Interior West have experienced population booms in the past decade, so have rural areas. Many people continue to seek rural locations with nearby recreational opportunities. While some counties are growing faster than others in Idaho, the state as a whole has been growing at an estimated rate of 18 percent per year since 1990.
Only a few counties are experiencing low growth rates. Much of the development that supports this influx of people is in, or adjacent to, forested lands. While some of that development is taking place near Idaho's larger population centers, there is also a substantial amount of new dispersed housing in rural counties.
Valley County, in the central portion of the state, is a good example of the growth phenomenon. The county is estimated to be expanding at a rate of about 31 percent. Much of the land within the county's borders is both forested and government owned. About 20 percent of the land base are in private ownership and, therefore, potentially available for residential development. Nearly all of that development is in close proximity to the surrounding forest lands.
The problem in terms of fire management is obvious. The probability of human-ignited fire is greater where there is more people, and there is an ever-increasing population in the wildland interface. More fire starts in conjunction with dense forests and hot or windy weather conditions, increases the possibility of fires capable of destroying homes and putting human lives at risk.
Wildland Interface Development Implications
Data shows the total inventory volume of growing stock in Idaho's timberland totals 36.6 billion cubic feet, an increase of 12 percent between 1952 and 1987. Average net annual growth was 816 million cubic feet. Of that growing stock, 76 percent is in National Forest System (NFS) lands.
The volumes of western white pine, western larch and ponderosa pine have decreased. Ponderosa pine and western white pine, historically the two most important timber species in the state, declined by nearly 4 billion cubic feet between 1952 and 1987. Ponderosa pine decreased by 40 percent and western white pine by 60 percent.
Douglas-fir increased by 15 percent and now composes 31 percent of the growing stock.
An aggregation of Engelmann spruce, western larch, western red cedar, and western hemlock increased by 30 percent. Although western larch is included among the class experiencing an increase, it most likely decreased since the acreage in larch type decreased.
Net growth can be compared with tree removals to estimate net change. Overall, net growth was nearly three time tree removals, but there were substantial differences by ownership. On NFS lands, the growth was more than four times greater than the removals; whereas on other ownership's it was about one and one-half times greater than removals.
Other factor effecting forest growth include disease, insects and fire, some more severely at times and locations than others.
Overall, net growth appears positive for most species, with some exceptions. On the Boise and Payette National Forests, mortality from insects, disease and fire exceeded growth for the period 1988-1992.
Mountain pine beetle was the cause of extensive mortality of lodgepole pine on the Targhee, Sawtooth, and Caribou National Forests during the 1970's and 1980's. In northern Idaho, mountain pine beetle in the early part of the century and later in combination with blister rust caused major losses of western white pine.
Root disease is extensive in many locations where white pine and other species were replaced by Douglas-fir and true firs. Timber volumes in infested stands are reduced by about 50 percent.
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Insects and Disease
Disease agents (pathogens) and insects affect forests in various ways. They are essential to the function of dynamic ecosystems: they serve to thin out some of the trees, recycle nutrients, create habitat and provide food to many wildlife species. They can also negatively affect resource values and ecosystem functions.
Thus, their effects may be viewed as beneficial, depending on the management objectives of the owner. Key questions involve how insect and pathogen activities affect the things we value, both in the short and long-term. In this report, we will focus on only a few native pathogens and insects in Idaho, specifically those having the most significant effects on current forest conditions.
From the resource perspective, tree mortality and growth loss can be highly significant. The two affect timber growth and reduce desirable forest cover in recreation areas. They can present hazards to visitors, reduce the ability of forest canopies to intercept snow and prevent excessive runoff, change wildlife habitat and influence various other commodities and amenities.
Fire, insects and disease are regulators of forest change. With wildlife suppression, insects, pathogens and humans have become the major agents of change. In particular, they play enhanced roles in succession, decomposition and nutrient recycling. Insects and pathogens are highly adapted to particular forest conditions, i.e. species composition, age, density, and others. So as forests change in composition and structure, They become more susceptible to some agents and less susceptible to others.
It, therefore, should not be surprising that some insects and pathogens have become less common and some more common as forests change. Given the current susceptibility of some stands, the considerable disease and insect caused changes and resource impacts are expected to continue.
Implications for Native Insects & Diseases
Wildlife and Fire Management
Idaho's forests evolved with and adapted to fire. All are in some way "fire dependent." Reduced fire frequencies, the result of suppressing natural fire starts combined with the elimination of native American burning during much of the current century, have altered forest compositions and structure.
Fire is a normal part of the forest ecosystem and is essential to sustaining forests. It functions to reduce surplus biomass, recycle nutrients, set the stage for regenerating forests and in combination with other disturbance mechanisms maintains a diverse forest landscape.
Yet, severe stand replacing fires over large areas may be incompatible with our current human settlement and uses of the forest. Such large severe fires threaten human lives, buildings, air quality, wildlife, wildlife habitat, timber, water quality and quantity, and recreational opportunities. In addition, when such fires occur on the steep granitic soils of central Idaho, they can cause serious erosion and landslides that further threaten human lives, buildings and natural resources.
Historically, fire patterns varied greatly in different locations. In forests at lower elevations and on dry sites at middle elevations where ponderosa pine was once the major forest component, fire intervals averaged 6 to 35 years.
These fires usually burned the understory and maintained forests in an open park-like condition with grassy undergrowth. Here, forests were primarily made up of large, widely spaced pine and larch, which had thick bark and were fire resistant. Occasionally the fires were more severe and would kill much of the forest
In forests at higher elevations and in moist, middle elevations, fire intervals were longer, ranging from 40 to 200 years. In these areas, fire was generally two types, mixed severity where it created a mosaic of forested conditions, in parts of the burned area some fire resistant trees survived, but the understory and thinned barked trees were burned, in other portions very little was affected.
(right) Fire is a normal part of the forest ecosystem and is essential to sustaining forests.
The fires that killed only part of the forest are very important in the development of many old-growth forests in the moist types. The second type of fire at these higher elevations was stand replacement, in which essentially all the trees were killed. In the Northern Rockies, stand replacement fires commonly occurred in western white pine stands on the average every 150-200 years. These were often in hot dry years or fires driven by strong wind events or both.
However, many decades of fire prevention, fire suppression, and timber harvesting have changed the fire regimes throughout the western United States, including Idaho. Our sup-pression efforts have been, until recently at least, quite successful.
There is growing concern that we are becoming less successful in our suppression efforts, as fuels continue to accumulate in unburned and otherwise unmanaged parts of the landscape. Furthermore, many now question the ecological desirability of suppressing all fires, especially surface and mixed severity fires.
Many forested areas now have high fuels, given the accumulation of trees and dead wood in the forest from decades of fire suppression, and are considered at risk of severe wildfire. When lightning storms ignite multiple fires in dry weather cycles control becomes extremely difficult and expensive and can cover large areas. In the dry forest types, fires can be more severe than in the past. Since 1984, the number of acres burned annually by forest fires have increased substantially in Idaho.
Biological Diversity of Idaho's Forests
In Idaho, threatened populations of a few prominent species, such as the grizzly bear or woodland caribou, serve to highlight the larger issues of species diversity. Bio-diversity may be viewed as a subset of forest health known as "habitat health."
Sustained healthy habitats for wildlife, vascular plants, and non-vascular plants (e.g., lichens, fungi, and bryophytes) is an important measure of all plant communities, including forests. Bio-diversity is, therefore, a critical forest health issue. However, it remains a difficult element to measure.
In Idaho, broad-scale species diversity has been most affected by human interventions in disturbance regimes, such as where fire suppression and some timber harvest patterns and prescriptions are evident. Exotic plant, disease, and insect introductions also influence species diversity.
Many of these interventions and introductions have been discussed in previous sections of this report. Often these components work together to limit the amount and diversity of native species populations.
In terms of forest landscapes, native diversity is best maintained with a variety of forest type and stand structure conditions. While old-growth forest may support a greater diversity of species at one location, landscape diversity is best supported by a strategy that provides a mixture of age conditions from young to old. The challenge is maintaining the mix of young, mid-age, and old forests which supports diversity across the entire landscape.
Previous management practices in this region have affected successional stages overall by reducing the percentages of young and old stands, while increasing the percentage of mid-age forests. Furthermore, great reductions in some forest types, such as western white pine, aspen, and ponderosa pine, will likely result in reduced regional diversity through the loss of plant and animal communities which thrive in these forest types.
Implications for Biological Diversity of Idaho's Forests
The issues identified in this report are a product of monitoring, research and management experience. For a land manager or owner, what can be done to address those issues? How can the ecologic integrity of forests be restored so that the land can meet individual and diverse objectives? The loss of integrity can be traced largely to three actions:
1) Addition of foreign agents. Exotic plants, pathogens, insects and other agents can profoundly affect ecosystems. Examples in Idaho include white pine blister rust and a growing number of noxious weeds. These agents now threaten a number of native species.
2) Withholding fire. Fire is a key process in the western "fire adapted forests". Without fire, forests continue to change with pathogens and insects playing a larger and different role. Fire prepares the site for regeneration of the shade intolerant species. The trend is loss of these species that were historically most abundant, and increase in medium-sized forests of shade tolerant species. It appears that forests are becoming less diverse and more homogeneous.
3) Direct influence. Humans have directly affected forests by harvesting, mining, road building and other forms of development. Harvesting, especially selective harvest of high-value trees, has decreased the amount of shade-intolerant pines and western larch, and reduced the amount of older age forests.
These changes and others occurred for several reasons:
- Information was lacking on how forests would respond to these human influences. Monitoring and research during the past several decades has put us in a better position to predict the effects of our actions.
- Some management practices instituted for desirable goals ended up producing other unforseen, undesirable results.
An example, was the past practice of removing or burning most of the woody debris after a harvest to reduce the risk of wildfire. We have since learned this can deplete some nutrients and remove habitat for some animals important to the proper function of a healthy forest. Adjusting management activities to respond to new information and knowledge when things go wrong can minimize and mitigate the effects of unanticipated consequences. Such response is referred to as "adaptive management."
- Public values changed. Practices that were tolerated, even seen as desirable, are now considered unacceptable. Also, there is no public consensus on what values should be used to manage forests. Given that situation, the wisest management policy might be one which maintains options where possible.
It is clear that some changes in management policies and practices will be needed. Inaction or passive management will allow some of the problems to worsen. Managment actions implemented using our understanding of forest functions, like fire regimes, seems to be a prudent course to pursue.
Much remains to be learned, but strategies are being developed that offer promise. Each particular landscape has a different solution or combination of solutions depending on the type and number of health problems that exist and the values for which the land is managed.
Solutions to these health problems involve a combination of management activities:
(1) prevention, to keep exotic species from becoming established or spreading farther, to prevent wildfire through fuels reduction, to prevent disease and insect damage through hazard reduction;
(2) integrated management, to deal with exotic agents now firmly established, to reestablish appropriate levels and functions of native insects and diseases;
(3) suppression, of fire, diseases and insects when the alternative is unacceptable;
(4) restoration, of damaged watersheds, of fire in the ecosystem, of tree species and structures that have become scarce;
(5) monitoring, to track broad vegetation trends, to evaluate the effectiveness of treatments, make adaptations as we contine to learn, and to detect emerging problems.
All of these elements that address forest health problems require using our current understanding of how ecosystems work and what objectives are to be achieved. Many of these forest health issues have developed over a period of decades and will require commitment to long-term activities or projects to restore forest health. Other issues can be addressed in relatively short periods of time.
Management Implications Summary
Forrest Health Issues Summary
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