For centuries, those who manage forests have been confronted with a simple question: how productive is the site? Unfortunately, answering this question is not simple. As managers, we must consider many different factors.
What is our objective, i.e., fiber production, water conservation, maintaining biodiversity?
What physical factors (soils, disturbance) control productivity?
How do these physical factors interact with the species present?
How do these physical factors influence the internal ecosystem processes?
Over the years, many different methodologies have developed to address this simple, yet elusive question. These include basic estimates of site quality using the height of canopy trees or ground-flora assemblages, to complex systems that integrate multiple factors across multiple scales.
Background
Regardless of the methodology, all forest classifications strive to elucidate the relationships among physical site factors and the vegetation. Perhaps the best way to describe these relationships is with hierarchy theory -- the idea that ecosystems are structured top-down, with the higher levels mediating the development of the lower levels. Described in this way, hierarchy theory provides an excellent way to understand the interrelationships among plant communities and the environment. Additionally, it provides for an excellent description of an ecosystem -- a hierarchically structured volume of earth, air, and water, having particular developmental histories within which organisms live and interact with each other (Barnes et al. 1998).
Click here to learn more about hierarchy theory and ecosystems
Vegetation classifications
Site index
- Until the 1970s the goal of forest classification was to determine site quality. Consequently, the focus was on the growth of individual trees. One of the most ubiquitous measures of site quality is site index - the height of the dominant portion of a forest stand at a specified standard age. Although site index indicates the productivity of an individual stand, it does not indicate why a stand has good or poor productivity. It assumes that the tree height or growth integrates the 'effect' of the site. In other words, the use of site index curves is analogous to a phytometer - plants are a measure of the site. However, research has shown that the growth curves are not the same for all sites, especially those that have been in production for long periods of time. Additionally, genetic improvements to trees, such as loblolly pine, also change the height-growth curves, making earlier site indices obsolete.Click here to see a site index curve for loblolly pine in the southeastern United States.
Cover type maps
- One of the most widely applied classification schemes are cover type maps that segregate the landscape based on differences in canopy cover. The timber industry and non-industrial organizations (e.g. The Nature Conservancy) have relied heavily on vegetation classifications and have developed many different schemes over the past fifty years. For example, most national forests and individual states have Forest Inventory and Analysis (FIA) programs designed to monitor changes in vegetative composition and structure over time.Although there is considerable variation associated with the detail of information collected for vegetative classifications, all essentially provide data concerning the size, age, and relative abundance of individual tree species. Estimates of standing volume and future growth and information relating to stand history are sometimes included in vegetation classifications. When all of this information is combined, vegetation classifications provide valuable information about current forest composition and structure. When these stand characteristics are mapped, they can provide insight into the composition and structure of the local population, stand, and ecosystem-community level, and the landscape and regional mosaic. Although many vegetation classifications are not linked with other ecosystem components, some forest products companies are developing computer systems to link vegetative inventory data with soil and topographic classifications.
Click here to see a typical cover type map.
Soil classifications provide detailed soils information. In theory, any soil classification should incorporate all factors relating to soil development, ranging from geomorphic-physiographic influences to plant-animal influences. In other words, soils are a function of climate, topography, biota, parent materials, and time. However, in practice many of the factors constraining soil development are overlooked for those that most directly affect soil profile development. Regardless, soil classifications and maps can be extremely useful, providing information relating to geomorphology, water quality, forest productivity, biodiversity, and equipment use.
Soil classifications, primarily developed for agricultural purposes, have been implemented for many regions of the United States. However, USDA soil surveys are sometimes poorly developed for forested landscapes compared with those dominated by agricultural land-uses.
Habitat type classifications use indicator species, usually understory or ground-flora species, to infer the vegetative community that would occur as a climax association. Due to the relatively large remnants of undisturbed vegetation in the western United States, habitat typing is the most widely applied ecological classification scheme in this region. Habitat typing is also common in portions of the Lake States.
Habitat type: all the land capable of producing the same plant association at climax.
Plant association: climax plant (forest) community type distinguished by the groups of understory species in combination with late-successional overstory species.
A habitat type classification is used because:
Generally, it is believed that habitat typing is an improvement over traditional vegetation classifications because it has a broader ecological basis.
All habitat type classifications are based on several premises, which are usually determined before individual stands are classified. These include:
Habitat type classifications have been widely applied to millions of hectares of the Rocky Mountains, Intermountain Region, Pacific Northwest, and portions of the Southwest. Habitat typing has also been applied at the state level in the Lake States. However, these classifications have been applied to specific regions or national forests and generally have not been framed around a regional hierarchical framework or mapped. Habitat types usually generally classified in the field on a site-by-site basis. However, in some areas specific coverages of habitat types have been delineated. Additionally, seral stages of habitat types have also been classified in parts of the western United States. Unfortunately, habitat type classifications in many cases fail to account for herbivory by animals, making classification of chronically disturbed sites difficult.
Landtype association classifications are successive stratification of the landscape based on the interactions and controlling influences of environmental factors (sensu state factors of Jenny 1961). These state factors are arranged hierarchically, and include climate, geology, physiography, topography, and soils. Many such hierarchical ecological classification systems have been developed in the U.S. modeled after Wertz and Arnold’s (1972) Land System Inventory.
In an LTA classification, the landscape is stratified into hierarchical segments based on physiography, geology, soils, topography, and vegetation. These include:
The vegetative component is generally not mapped, but is considered when delineating the lowest level, the landtype. However, in practice, vegetation is generally not used to segregate LTAs since species composition and structure is more a function of past disturbance rather than site potential.
Most LTA classification have been developed and applied in the southern Appalachians and Cumberland Plateau regions of the southeastern United States. LTA classifications provide a sound biological basis for forest management because they recognize inherent site differences and soil-related site hazards at the landtype level. Similar to habitat type classifications, these relationships are determined a priori, meaning that the inter-relationships among ecosystem components are considered before classification units are delineated. However, the focus is generally on differences in physiography and topography, rather than vegetation.
Click here to see a stylized landtype association classification.
The primary rationale for this type of classification is that the landscape is "conceived as ecosystems, large and small, nested within one another into a hierarchy of spatial sizes" (Rowe and Sheard 1981). These ecosystems have structure as determined by the relationships among physiography, soils, and vegetation.
Thus, an ecosystem (regardless of the scale) has the following characteristics:
Although ecosystem classifications are similar to LTA classifications, there are inherent differences among the two classification schemes. Primarily, while LTA classifications focus heavily on the environmental factors influencing ecosystem development, ecosystem classification lends equal weight to all components, including vegetation. The focus of ecosystem classification is on the three-dimensional landscape and all the integrated ecosystem components. Like the other ecological approaches, these relationships among components are determined before ecosystems are delineated and mapped. Although mapping of ecosystems is generally based on the inter-relationships among physiography, soils, and vegetation, the more stable components of the classification, e.g., physiography and soils, can be used to map ecosystems successfully in disturbed landscapes.
Ecosystem classification systems were first developed and used to manage forests in the southwestern German state of Baden-Württemberg. Ecosystem classifications have also been developed for many regions of the eastern United States. Initial applications of ecosystem classifications in the early 1980s were conducted for small tracts in the Upper Peninsula of Michigan (McCormick Experimental Forest, Sylvania Wilderness Area). Since these classifications were first implemented, many more ecosystem classifications have been developed in the eastern United States. The USDA Forest Service has adopted an Ecological Classification and Inventory (EC&I) initiative designed to develop ecosystem classifications for all National Forests within the Eastern Region. The goal of the EC&I program is to provide resource managers with a tool to implement management initiatives at different spatial scales, including the landscape level. Although the methodologies are slightly different for each National Forest, the trend has been to develop and implement an ecosystem classification, then map ecosystems over a longer period of time. Ecosystem classifications have been developed and applied to the Ottawa, Huron-Manistee, Nicolet, Chippewa, and Superior National Forests of the Lake States, and the Wayne and Hoosier National Forests of the Central Hardwoods Region. Additionally, several ecosystem classifications have been developed for the southeastern United States on both public and private lands. As with habitat typing, ecosystem classifications have been developed locally, and methodologies have not been standardized nationally. However, there are national ecological hierarchies within which any forestland classification could be placed, e.g., Bailey’s EcoRegions (Bailey 1995).
Click here to see an ecosystem classification map.
Summary
Although several different varieties of forestland classification schemes are currently being used by both industrial and non-industrial organizations, most industrial operations rely on vegetation classifications. Although there is considerable variation in the development and implementation of these classifications, each has the same objective – to determine site productivity from vegetation as it relates to many different objectives such as fiber production, water quality, wildlife, recreation, maintenance of biodiversity. Except for soil classifications, all describe the composition of the forest community quite well. However, all the classification schemes fail to incorporate many of the structural and functional aspects into how they stratify forest ecosystems that are necessary to manage forests holistically. For example, cover type maps provide little information on the vertical structure or how nutrients cycle within the forest ecosystem. While this information may not be necessary to manage for fiber production, it is essential for maintaining biodiversity and long-term productivity.