of forests (DellaSala et al. 1995a). For instance, Costanza (1992) has stated, "To be healthy and sustainable, a system must maintain its metabolic activity level as well as its internal structure and organization (a diversity of processes effectively linked to one another) and must be resilient to outside stresses over a time and space frame relevant to that system." The author further proposes that an index of ecosystem health should include three parts: vigor, organization, and resilience. Vigor is measured by the productivity, or output, of timber, food, recreational opportunities, species populations, or other products of forest ecosystems. Organization is measured by the complexity of forest structures and by the diversity of the species present. Resilience describes the response of a system to disturbance.
Natural disturbances are a normal part of the long-term functioning of healthy forest ecosystems. The intensity of natural disturbances is generally inversely related to their frequency. Most disturbances of high intensity and low frequency, such as sustained high winds from hurricanes or wildfires under high winds in dense, dry forests, cannot be prevented by humans. Such disturbances tend to occur over a large geographic area and across land ownerships. Low-intensity natural disturbances, such as insects, diseases, and ground-level fires in other forest types, often can be managed by humans to either prevent or reduce the damage. These disturbances often occur at a local level and within one ownership.
The U.S. Department of Agriculture's (USDA) Forest Service (1993) has identified a number of issues in three forest-health categories: potentially acceptable resource situations, potentially deteriorating resource situations, and potentially serious resource situations. Nonfederal forests would be included in the categories of potentially deteriorating and potentially serious resource situations and would be important in any responses to them.
The 28 percent increase in the number of forest landowners over the past 15 years (Moulton and Birch 1995) and the associated decrease in the average size of ownership parcels of nonfederal forestlands (Chapter 2) document a clear trend toward increased parceling of nonfederal forestland. The impact of parceling might not affect the health of any one ecosystem. In fact, some evidence indicates that timber-harvesting behavior is inversely related to parcel size, the smaller owners being less likely to harvest (Moulton and Birch 1996). However, the recognition that ecosystems have functions in aggregate has important implications for the management of a single forest tract.
In relation to biodiversity, a decrease in the area of a particular forest tract should result in a decrease in the number of species the ecosystem can support, according to the theory of island biogeography and accepted species-area relationships (Soule 1991). However, assumptions of such equilibrium theories might be unrealistic. First, the productive potential of nonfederal forests has changed over time and future changes promise to be even more dramatic. Second, evidence is increasing that biodiversity is affected by landscape scale dynamics.