There are a number of forest ecosystem health indicators that can be used to assess the ecological condition of a forest ecosystem, and that can contribute to the effective development of ecological performance measures. Depending on the spatial scale used, indicators range from changes in dissolved oxygen and specific conductivity, to larger dimension indicators such as the area and percent of forestland with significant compaction resulting from human activities (an example of a Montreal process indicator).
First developed in 1994, the Montreal Process, a voluntary agreement between a twelve countries, developed a framework for monitoring, assessing and reporting forest conditions, with the goal of ensuring sustainable forest management (Technical Notes on Implementation of the Montreal Process Criteria and Indicators, 2009). Some indicators developed through the Montreal Process which may be applied to developing ecological performance measures include: area and percent of forest affected by biotic processes and agents (e.g., disease, insects, invasive species) beyond reference conditions; area and percent of forest affected by abiotic agents (e.g., fire, storm, land clearance) beyond reference conditions, and forest fragmentation (indicator that provides information on the extent to which forests are being fragmented over time by human activities and natural processes) (Technical Notes on Implementation of the Montreal Process Criteria and Indicators, 2009).
Forest managers and wildlife scientists use several indicators (e.g., mortality and tree growth data) to investigate key forest ecosystem attributes such as sustainability, productivity, and aesthetics (USDA Forest Service, 2005). For example, the indicators discussed below were initially developed and measured by the Forest Health Monitoring (FHM) Program in the 1990s. In 1999, they were incorporated into the Forest Service Forest Inventory and Analysis (FIA) Program (Stolte et al., 2002).
Tree Growth Indicator - This indicator is used to determine if the balance between growth and mortality is adequate to sustain a forest ecosystem. It can also be used to analyze any unusual spatial or temporal pattern of stand growth rates (USDA Forest Service, 2005).
Lichen Indicator – Lichens, part fungi, part algae or cyanobacteria are extremely sensitive to environmental stressors. These organisms quickly respond to changes in forest structure, air quality, and climate since they completely rely on atmospheric sources of nutrition. For example, the composition of a forest lichen community is one of the best biological indicators of nitrogen and sulfur-based air pollution. The lichen indicator represents an early sign of improving or deteriorating air quality (USDA Forest Service, 2005).
Soil Quality Indicator – The productivity, hydrology, and species composition of forests can be impacted by stressors affecting the natural function of the soil. The physical and chemical properties of the soil as well as the extent of erosion and compaction influence soil quality and its ability to sustain biological productivity, and support a healthy plant and animal community (USDA Forest Service, 2005).
Tree Mortality Indicator – Similar to the tree growth indicator, the tree mortality indicator can be used to determine if there are any unusual spatial or temporal patterns in mortality rates. This indicator can help analyze the ecosystem balance between tree growth and tree mortality (USDA Forest Service, 2005).
Crown Condition Indicator – This indicator is an essential component of forest ecosystem structure that directly affects the composition, processes, and vigor of the understory plant and animal communities. Tree crowns are influenced by a variety of physiological factors (such as age and light, water, and nutrient availability), and represent indicators of overall forest health. Large, dense crowns are often associated with vigorous growth rates, while small, sparse crowns suggest unfavorable site conditions (USDA Forest Service, 2005).
Natural Communities Indicator - The vegetation indicator is designed to quantify the proportion of native and introduced species by assessing the type, abundance, and spatial arrangement of all trees, shrubs, herbs, grasses, and ferns. This information can further be used to distinguish community types, predict succession, and measure vegetation changes. Some ecosystem properties such as productivity, response to disturbance, and use by wildlife, are intrinsically correlated to the forest community type found in both the overstory and the understory. The elements that ensure wildlife habitat such as the presence of snags and large tress are essential in ensuring a diverse mix of habitat types, contributing to higher levels of species richness and ecosystem diversity (Yearsley and Parminter, 1998). Therefore, the vegetation indicator plays a crucial role in helping forest managers to extrapolate other forest health indicators on broader areas, and assess trends over time with respect to forest’ response to stressors such as pollution, changes in land use, and disturbance events (USDA Forest Service, 2005).
Ozone Indicator – This indicator is designed to detect and monitor ozone stress in the forest environment. Ozone pollution not only causes direct foliar injury to many plant species, but also has been shown to reduce tree growth, alter species composition, and predispose trees to insect and disease attacks (USDA Forest Service, 2005).
Down Woody Materials Indicator - The Down Woody Materials (DWM) indicator is often used to estimate potential carbon pools, assess fuel loadings, and evaluate fire risk. This indicator includes forest ecosystem components such as coarse woody debris, fine woody debris, duff, litter, shrubs/herbs, slash piles, and fuelbed depths. The DWM indicator serves as a broad indicator of forest health, and can also be used to establish relationships between forest types, past management actions, and current levels of coarse woody debris.
Other useful indicators include: concentration of pollutants, level of genetic connectivity, structural diversity, species richness, fish and reptile distribution, song bird distribution, native plant occurrences, and others. Often times, a vegetation indicator is used to measure and monitor the type, abundance, and vertical position of vascular plant species. Changes in species abundance and forest vertical structure, as well as changes in trends in plant species richness, can be a very useful indicator of forest health (Stolte et al., 2002).