Forests are dynamic associations of living organisms that undergo constant change. By definition, they are plant communities dominated by trees, which provide the main physical structures and carry out most of the primary production (conversion of sunlight energy, air, water, and minerals to food through photosynthesis) in the system. Most of the forests in the northern part of the U.S. have developed since the retreat of the glaciers some 10,000 to 14,000 years ago. Thus, their development is closely allied with the arrival and growth of human populations, which began about that same time. As the glaciers melted, great floods and windstorms shaped the landscape, and species (including trees and other plants, animals, and humans) migrated into places that became available and hospitable to them. People shaped their environment by hunting animals, harvesting plants for food, medicine, fuel, and building materials, and by using fire to create openings, change grazing patterns, affect hunting, and protect themselves from enemies or wildfires. The forests changed in response to species migrations, climate changes, and human influences, as well as in response to natural disturbances such as wind, temperature extremes, and fire. Forest Type Different forest structures can be defined in many ways, and exhibit somewhat different characteristics in different forest types, but for this report we will use five general structural types to describe forest conditions. They are: (right) How old is old? One note about "old-growth" forests. Scientists have Iona recognized that all forest types go through several structural stages as different trees become established, grow, compete with each other for space, water, nutrients, and sunlight, and then die. This process, called succession, takes place in reasonably predictable patterns, driven as it is by the growth and aging of the keystone tree species. Early ecologists theorized that most forests proceeded through these successional stages until they reached the final, or climax stage where they remained more or less in equilibrium. Much of our popular vision of forests is based on this early theory, and many therefore conclude that if we leave a forest alone long enough, it will settle into a relatively stable condition, often called "old growth" or "ancient forests." To avoid debates over "how old is old?" and "aren't there large, old trees in other structures?" we have called this condition a "complex" structure, after Oliver et al. (1997). More important, however, is the fact that most modem ecologists have abandoned the idea that "climax forest" represents a state of equilibrium. Instead, it has been shown that the most common forest condition is for portions of a forested region to be affected by a major disturbance event, such as a windstorm or a fire, which can either bum on the ground and kill young trees or bum in the tree canopy killing the large trees. Rather than reaching a common "old growth" stage across large areas, it is now generally agreed that forests---before they were affected by human-driven forces---had a mixture of different structural conditions across the landscape. These forests' diversity kept isolated disturbances from becoming wide-scale disasters. It also provided a variety of forest environments to which different groups of soil organisms, plants, animals, and people became adapted. Thus, the diversity of forest conditions across large areas becomes a major contributor to the biological diversity of a region, and to its ability to provide the different habitats needed by the full range of species. Management and Non-management Modem forest management changes forests significantly through tree planting, timber harvest and other silvicultural manipulation, road building, fragmentation, and fire management. Management aimed at timber production has cleared forest areas through clearcutting (creating open structures), planted trees (hastening succession), maintained low-stress tree growth conditions through thinning (avoiding some problem: overly dense structures), and harvested trees as they reached econoiruc maturity to restart the cycle (usually avoiding both the complex and savanna structures). Such management may produce excellent results in terms of timber productivity, but it can create large areas that lack structural and biological diversity. (left) Clearcuts create open structures and rapid reforestation, but they can introduce fragmentation and harm aesthetics. Forest management that avoids human manipulation to the extent possible---such as setting aside parks and other reserves---may retain savanna, understory, and complex structures, at least for a while. If the protection includes the elimination of American Indian fire practices (as it virtually always does) and suppression of wild-fires (as has been common in many areas), the long-term result is likely to be a forest that no longer has structural diversity. In forests where fire was the dominant disturbance factor, its removal or suppression allows areas to grow thick with small trees and brush, changing species composition, and replacing savanna, open, and understory structures with dense structures. Species that depend on open conditions or fire disturbances may be replaced by species adapted to surviving in the shaded conditions of the dense stands, and total diversity will drop since dense forests have the fewest species in most cases. Satellite images of some of America's most remote wilderness areas now show unusually large areas of uniform forest structural conditions. The associated risk is that a forest formerly disturbed in relatively small patches will become uniformly disturbed over relatively large areas. Species that, in the past, survived disturbances by moving from patch to patch to find suitable habitat may not be able to move the greater distances required by a widespread disturbance. Forest protection is, of course, forest management. On the other band, so is forest nonmanagement, where noncommercial forests---recovering from earlier timber harvest or agricultural use---are left totally alone. The results, however, are much the same. Non-management may also create large areas of dense structure. Without natural fire as a periodic disturbance, and with owners who lack an economic incentive to carry out silvicultural treatments like thinning or prescribed fire, these areas can become dominated by dense structures and lack the diversity that could help shield them from future large-area problems. Another important aspect of forest management is the regulation of wildlife populations. Where some wildlife species have been introduced intentionally for sport others have been eradicated to prevent damage. Wildlife management means, for the most part, managing population levels. In the U.S., wildlife management is primarily done by state wildlife agencies that regulate hunting and fishing seasons and bag limits to assure a sustainable wildlife resource. Focusing on a few main species prized by hunters---deer and turkeys, for example---may be very important from the viewpoint of the wildlife agency whose support comes largely from those recreational users. From a forest health viewpoint, however, too many deer may be a major problem. Settling this conflict will call for broad cooperation and compromise between the different agencies and groups involved. To make matters more complex, the fact is that forest management methods affect wildlife just as much as wildlife management affects forests. Poorly constructed forest roads or ill-designed timber sales can lead to major landslides or erosion that damages the spawning beds of valuable fish species. Removing tree cover from stream banks eliminates the shading that helped keep water temperatures down, and the warmer water may become inhospitable to prized species. Clearcutting large areas of mature forest has been shown to cause a short-term release of nutrients while the site is undergoing the rapid post-harvest decomposition of organic matter due to both the slash on the ground and the additional warmth and moisture in the soil; then there can be a period of several years when the young fast-growing trees effectively capture most of the available nutrients, leaving fewer to "leak" into the streams. These rapid nutrient "fluxes" (high and low levels in the stream) may affect fish populations, particularly if the rapid regrowth period extends for several years on sites that are marginal or deficient in key nutrients, causing stream nutrient levels to fall below the thresholds required by a healthy aquatic system. Again, forest managers and wildlife biologists need to understand these complex interactions and work together to prevent unwanted impacts. Looks are Deceiving The evaluation of existing structural stages is one important way to judge the health of a forest system, and the role of people in creating that condition. If a forest region lacks one or more of the structures it historically contained, or a single structure is overly dominant, it may have lost a great deal of its diversity. If this lack of diversity proceeds to the point where some species are suppressed or eliminated because of a lack of suitable habitat, the forest has clearly failed to sustain its complexity and is, by the definition given Previously, in an unhealthy condition. Proving that a given forest region is in an unhealthy condition can be difficult, however. The forest may look green and lush, with trees appearing to be in the prime of health. That such an area is greatly oversimplified in terms of diversity and structure may seem a minor problem at the moment. If these areas go through a period of stress, however, and suffer major dieback over large areas, the forest health problem becomes plain for all to see. The problem is not simply one of green trees being healthy and brown trees being sick. The harder challenge for ecologists and managers is to demonstrate that a problem exists before that dieback occurs, and then find publicly acceptable ways to alter the forest so that the problem is avoided. This is a lot harder than it sounds, and reasonable people can differ greatly on what a certain condition means, and what steps might be taken to reduce the risks involved. People who look at today's very visible problems in the Blue Mountains of Oregon should remember how healthy these places looked in the 1960s; while those who trumpet the recovery of hardwood forests in the East should consider the problems these dense forests may face in the future. As difficult as it is to assess the potential future of a particular place, it is likely to be better to take small steps to steer gently toward a desired future condition than to wait until it is so late that major damage is caused, or major actions (often more subject to overcorrection) are launched. Broad Trends Affecting Forest Health Reviewing the reports of forest conditions across the United States, in context of what we understand today about forest ecosystems, leads to the conclusion that there are six broad trends tied to how people are influencing forests that may significantly affect forest health and sustainability. They are: (right) Homes and roads break up forest continuity. In identifying these trends one should resist the temptation to think they occur separately in neat compartments. Where one trend is underway, another is likely at work. That interplay---combined with other forces that impact forest health---further complicates our discussion.