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The following is an electronic version of

National Hierarchical Framework of Ecological Units

David T. Cleland, Peter E. Avers, W. Henry McNab, Mark E. Jensen, Robert G. Bailey,

Thomas King, and Walter E. Russell

Full citation:

Cleland, D.T.; Avers, P.E.; McNab, W.H.; Jensen, M.E.; Bailey, R.G., King, T.; Russell, W.E. 1997.

National Hierarchical Framework of Ecological Units. Published in, Boyce, M. S.; Haney, A., ed. 1997.

Ecosystem Management Applications for Sustainable Forest and Wildlife Resources. Yale University

Press, New Haven, CT. pp. 181-200.

INTRODUCTION

To implement ecosystem management, we need basic information about the nature and distribution of

ecosystems. To develop this information, we need working definitions of ecosystems and supporting

inventories of the components that comprise ecosystems. We also need to understand ecological patterns

and processes and the interrelationships of social, physical, and biological systems. To meet these needs,

we must obtain better information about the distribution and interaction of organisms and the

environments in which they occur, including the demographics of species, the development and

succession of communities, and the effects of humans activities and land use on species and ecosystems

(Urban et al. 1987). Research has a critical role in obtaining this information.

This chapter presents a brief background of regional land classifications, describes the hierarchical

framework for ecological unit design, examines underlying principles, and shows how the framework can

be used in resource planning and management. The basic objective of the hierarchical framework is to

provide a systematic method for classifying and mapping areas of the earth based on associations of

ecological factors at different geographic scales. The framework is needed to improve our efforts in

national, regional, and forest level planning; to achieve consistency in ecosystem management across

National Forests and regions; to advance our understanding of the nature and distribution of ecosystems;

and to facilitate interagency data sharing and planning. Furthermore, this framework will help us evaluate

inherent capabilities of land and water resources and the effects of management on them.

Ecological units delimit areas of different biological and physical potentials. Ecological unit maps can be

coupled with inventories of existing vegetation, air quality, aquatic systems, wildlife, and human elements

to characterize complexes of life and environment, or ecosystems. This information on ecosystems can be

combined with our knowledge of various processes to facilitate a more ecologically sound approach to

resource planning, management, and research.

Note that ecological classification and mapping systems are devised by humans to meet human needs

and values. Ecosystems and their various components often change gradually, forming continua on the

earth's surface which cross administrative and political boundaries. Based on their understanding of

ecological systems, humans decide on ecosystems boundaries by using physical, biological, and social

considerations.

We recognize that the exact boundaries for each level envisioned in this process and developed in map

format may not fit every analysis and management need. Developing boundaries of areas for analysis,

however, will not change the boundaries of ecological units. In some cases, an ecological unit may be the

analysis area. In other cases, watersheds, existing conditions, management emphasis, proximity to

special features (for example natural, wilderness, or urban areas), or other conditions may define an

analysis area. In these cases, ecological units can be aggregated or divided if necessary to focus on

relevant issues and concerns.

BACKGROUND: REGIONAL LAND CLASSIFICATIONS

Hierarchical systems using ecological principles for classifying land have been developed for geographical

scales ranging from global to local. Using a bioclimatic approach at a global scale, several researchers

have developed ecological land classifications: Holdridge (1967), Walter and Box (1976), Udvardy (1975),

and Bailey (1989a,b). Wertz and Arnold (1972) developed land stratification concepts for regional and land

unit scales. Other ecologically based classifications proposed at regional scales include those of Driscoll

et al. (1984), Gallant et al. (1989), and Omernik (1987) in the United States and those of Wiken (1986)

and the Ecoregions Working Group (1989) in Canada. Concepts have also been presented for ecological

classification at subregional to local scales in the United States (Barnes et al. 1982), Canada (Jones et al.

1983, Hills 1952), and Germany (Barnes 1984).

Each of these systems have strong points that contribute to the strength of the national hierarchy. But no

single system has the structure and flexibility necessary for developing ecological units at continental to

local scales. The concepts and terminology of the national system draw upon this work to devise a

consistent framework for application throughout the United States.

ECOLOGICAL UNIT DESIGN

The primary purpose for delineating ecological units is to identify land and water areas at different levels of

resolution that have similar capabilities and potentials for management. Depending on scale, ecological

units are designed to exhibit similar patterns in: (1) potential natural communities, (2) soils, (3) hydrologic

function, (4) landform and topography, (5) lithology, (6) climate, and (7) natural processes such as nutrient

cycling, productivity, succession, and natural disturbance regimes associated with flooding, wind, or fire.

It should be noted that climatic regime is an important boundary criterion for ecological units, particularly at

broad scales. In fact, climate, as modified by topography, is the dominant criterion at upper levels. Other

factors, such as geomorphic process, soils, and potential natural communities, take on equal or greater

importance than climate at lower levels.

It follows, then, that ecological units are differentiated and maps designed by multiple components,

including climate, physiography, geology, soils, water, and potential natural communities. These

components may be analyzed individually, and then combined, or multiple factors may be simultaneously

evaluated to classify ecological types, which are then used in ecological unit design. The first option may

be increasingly used as geographic information systems (GIS) become more available. The

interrelationships among independently defined components, however, will need to be carefully evaluated,

and the results of layering component maps may need to be adjusted to identify units that are both

ecologically significant and meaningful to management. When various disciplines cooperate in devising

integrated ecological units, products from existing resource component maps can be modified, and

integrated interpretations can be developed (Avers and Schlatterer, 1991).

Ecological unit inventories are generally designed and conducted in cooperation with the Natural Resource

Conservation Service, Agricultural Experiment Stations of Land Grant Universities, Bureau of Land

Management, and other appropriate state and federal agencies. Mapping conventions and soil

classification meet standards of the National Cooperative Soil Survey.

Table 1. National hierarchy of ecological units

Planning and

analysis scale

Ecological

Units

Purpose, objectives, and general use

Ecoregion

Global

Continenta

Regional

Domain

Division

Province

Broad applicability for modeling and sampling.

Strategic planning and assessment. International planning.

Subregion Section

Subsection

Strategic, multiforest, statewide, and multiagency analysis and

assessment.

Landscape Landtype

association

Forest or areawide planning, and watershed analysis.

Land unit Landtype

Landtype

phase

Project and management area planning and analysis.

Hierarchy can be expanded by user

to smaller geographical areas and

more detailed ecological units if

needed.

Very detailed project planning.

CLASSIFICATION FRAMEWORK

The National Ecological Unit Hierarchy is presented in Tables 1, 2, and 3. The hierarchy is based on

concepts and terminology developed by numerous scientists and resource managers (Hills 1952, Crowley

1967, Wertz and Arnold 1972, Rowe 1980, Allen and Starr 1982, Barnes et al. 1982, Forman and Godron

1986, Bailey 1987, Meentemeyer and Box 1987, Gallant et al. 1989, Cleland et al. 1992). The following is

an overview of the differentiating criteria used in the development of the ecological units. Table 2

summarizes the principal criteria used at each level in the hierarchy.

Table 2. Principal map unit design criteria of ecological units.

Ecological unit Principal map unit design criteria

Domain Broad climatic zones or groups (e.g., dry, humid, tropical)

Division Regional climatic types (Koppen 1931, Trewatha 1968)

Vegetational affinities (e.g., prairie or forest)

Soil order

Province Dominant potential natural vegetation (Kuchler 1964)

Highlands or mountains with complex vertical climate-vegetation-soil zonation

Section Geomorphic province, geologic age, stratigaphy, lithology

Regional climatic data

Phases of soil orders, suborders, or great groups

Potential natural vegetation

Potential natural communities (PNC) (FSH 2090)

Subsection Geomorphic process, surficial geology, lithology

Phases of soil orders, suborders, or great groups

Subregional climatic data

PNC—formation or series

Landtype association Geomorphic process, geologic formation, surficial geology, and elevation

Phases of soil subgroups, families, or series

Local climate

PNC—series, subseries, plant associations

Landtype Landform and topography (elevation, aspect, slope gradient, and position)

Phases of soil subgroups, families, or series

Rock type, geomorphic process

PNC—plant associations

Landtype phase Phases of soil subfamilies or series

Landform and slope position

PNC—plant associations or phases

Note: The criteria listed are broad categories of environmental and landscape components. The actual

classes of components chosen for designing map units depends on conditions and relative importance of

factors within respective geographic areas.

Table 3. Map scale and polygon size of ecological units.

Ecological unit Map scale range General polygon size

Domain 1:30,000,000 or smaller 1,000,000s of square miles

Division 1:30,000,000 to 1:7,500,000 100,000s of square miles

Province 1:15,000,000 to 1:5,000,000 10,000s of square miles

Section 1:7,500,000 to 1:3,500,000 1,000s of square miles

Subsection 1:3,500,000 to 1:250,000 10s to low 1,000s of square

miles

Landtype association 1:250,000 to 1:60,000 1,000s to 10,000s of acres

Landtype 1:60,000 to 1:24,000 100s to 1,000s of acres

Landtype phase 1:24,000 or larger <100 acres

Ecoregion Scale

At the Ecoregion scale, ecological units are recognized by differences in global, continental, and regional

climatic regimes and gross physiography. The basic assumption is that climate governs energy and

moisture gradients, thereby acting as the primary control over more localized ecosystems. Three levels of

ecoregions, adapted from Bailey (1980), are identified in the hierarchy:

1. Domains, subcontinental divisions of broad climatic similarity, such as lands that have the dry climates

defined by Koppen (1931), which are affected by latitude and global atmospheric conditions. For example,

the climate of the Polar Domain is controlled by arctic air masses, which create cold, dry environments

where summers are short. In contrast, the climate of the Humid Tropical Domain is influenced by

equatorial air masses and there is no winter season. Domains are also characterized by broad differences

in annual precipitation, evapotranspiration, potential natural vegetation, and biologically significant

drainage systems. The four Domains are named according to the principal climatic descriptive features:

Polar, Dry, Humid Temperate, and Humid Tropical.

2. Divisions, subdivisions of domains determined by isolating areas of definite vegetational affinities (for

example, prairie or forest) that fall within the same regional climate, generally at the level of the basic

types of Koppen (1931) as modified by Trewartha (1968). Divisions are delineated according to: (a) the

amount of water deficit (which subdivides the Dry Domain into semi-arid, steppe, or arid desert), and (b)

the winter temperatures, which have an important influence on biological and physical processes and the

duration of any snow cover. This temperature factor is the basis of distinction between temperate and

tropical/subtropical dry regions. Divisions are named for the main climatic regions they delineate, such as

steppe, savannah, desert, Mediterranean, marine, and tundra.

3. Provinces, climatic subzones, controlled primarily by continental weather patterns such as length of dry

season and duration of cold temperatures. Provinces are also characterized by similar soil orders. The

climatic subzones are evident as extensive areas of similar potential natural vegetation such as those

mapped by Kuchler (1964). Provinces are named typically using a binomial system consisting of a

geographic location and vegetative type such as Bering

Tundra, California Dry-Steppe and Eastern Broadleaf Forests (Bailey et al. 1985).

Highland areas that exhibit altitudinal vegetation zonation and that have the climatic regime (seasonality of

energy and moisture) of adjacent lowlands are classified as provinces (Bailey et al. 1985). The climatic

regime of the surrounding lowlands can be used to infer the climate of the highlands. For example, in the

Mediterranean division along the Pacific Coast, the seasonal pattern of precipitation is the same for the

lowlands and highlands except that the mountains receive about twice the quantity. The provinces are

named for the lower-elevation and upper-elevation (subnival) belts, for example, Rocky Mountain forest-

alpine meadows.

Subregional Scale

Subregions are characterized by combinations of climate, geomorphic process, topography, and

stratigraphy that influence moisture availability and exposure to radiant solar energy, which in turn directly

control hydrologic function, soil-forming processes, and potential natural community distributions. Sections

and Subsections are the two ecological units mapped at this scale.

1. Sections, broad areas of similar sub-regional climate, geomorphic process, stratigraphy, geologic origin,

topography, and drainage networks. Such areas are often inferred by relating geologic maps to potential

natural vegetation "series" groupings such as those mapped by Kuchler (1964). In recent years, numerical

analyses of weather station and remotely sensed climatic information have assisted in determining

Section boundaries. Boundaries of some sections approximate geormorphic provinces (for example, Blue

Ridge) as recognized by geologists. Section names generally describe the predominant geomorphic type

or feature upon which the ecological unit delineation is based, such as Flint Hills, Great Lakes Morainal,

Bluegrass Hills, Appalachian Piedmont.

2. Subsections, smaller areas within Sections with similar surficial geology, lithology, geomorphic process,

soil groups, subregional climate, and potential natural communities. Subsection boundaries usually

correspond with discrete changes in geomorphology. Names of Subsections are usually derived from

geologic features, such as Plainfield sand dune, Tipton till plain, and granite hills.

Landscape Scale

At the landscape scale, ecological units are defined by general topography, geomorphic process, surficial

geology, associations of soil families, and potential natural communities, patterns, and local climates

(Forman and Godron 1986). These factors affect biotic distributions, hydrologic function, natural

disturbance regimes, and general land use. Local landform patterns become apparent at this level in the

hierarchy, and differences among units are usually obvious to on-the-ground observers. At this level,

terrestrial features and processes may also have a strong influence on ecological characteristics of

aquatic habitats (Platts 1979, Ebert et al. 1991).

Landtype association ecological units represent this scale in the hierarchy. These are groupings of

landtypes or subdivisions of subsections based on similarities in geomorphic process, geologic rock types,

soil complexes, stream types, lakes, wetlands, subseries or plant association vegetation communities.

Repeatable patterns of soil complexes and plant communities are useful in delineating map units at this

level. Names of Landtype Associations are often derived from geomorphic history and vegetation

community.

Land Unit Scale

At the basic land unit scale, ecological units are designed and mapped in the field based on properties of

local topography, rock types, soils, and potential natural vegetation. These factors influence the structure

and composition of plant communities, hydrologic function, and basic land capability. Landtypes and

landtype phases are the ecological units mapped at this scale.

1. Landtypes, subdivisions of landtype associations or groupings of landtype phases based on similarities

in soils, landform, rock type, geomorphic process, and plant associations. Land surface form that

influences hydrologic function (for example, drainage density, dissection, and relief) is often used to

delineate different landtypes in mountainous terrain. Valley bottom characteristics (for example,

confinement) are commonly used in establishing riparian landtype map units. Names of landtypes include

an abiotic and biotic component (USDA Forest Service Handbook 2090.11).

2. Landtype Phase, subdivisions of Landtypes based on topographic criteria (for example, slope-shape,

steepness, aspect, position), hydrologic characteristics, associations and consociations of soil taxa, and

plant associations and phases that influence or reflect the microclimate and productivity of a site.

Landtype phases are often established based on interrelationships between soil characteristics and

potential natural communities.In riparian mapping, landtype phases may be established to delineate

different stream-type environments (Herrington and Dunham, 1967). Naming is similar to landtypes.

The Landtype Phase is the smallest ecological unit recognized in the hierarchy. However, even smaller

units may need to be delineated for very detailed project planning at large scales (Table 1). Map design

criteria depend on project objectives.

Plot Data

Point or plot sampling units are used to gather ecological data for inventory, monitoring, and quality

control, and for developing classifications of vegetation, soils or ecological types. This plot data feeds into

databases for analysis, description, and interpretation of ecological units (Keane et al. 1990). Plots, while

not mappable, can be shown on maps as point data.

UNDERLYING PRINCIPLES

Ecosystem Concept

Ecosystems are places where life forms and environment interact; they are three-dimensional segments

of the Earth (Rowe 1980). Tansley introduced the term ecosystem in 1935, first articulating the explicit

idea of ecological systems composed of multiple abiotic and biotic factors (Major 1969). The ecosystem

concept brings the biological and physical worlds together into a holistic framework within which ecological

systems can be described, evaluated, and managed (Rowe 1992). The structure and function of

ecosystems are largely regulated along energy, moisture, nutrient, and disturbance gradients. These

gradients are affected by numerous environmental and biological factors including climate, geology, soils,

flora, fire, and wind, and these factors vary at different spatial and temporal scales (Barnes at al. 1982,

Jordan 1982, Spies and Barnes 1985). Ecological systems therefore exist at many spatial scales, from the

global ecosphere down to regions of microbial activity. Using multiple biotic and abiotic factors, the

National Hierarchy of Ecological Units organizes the environmental components of ecosystems into an

orderly set of spatial scales based on measurable features and processes. The National Hierarchy thus

takes the infinite variety of ecosystems and places them into a limited number of discrete, practical units

that are mappable, repeatable, and distinguished from one another by differences in various structural or

functional characteristics.

At global, continental, and regional scales, ecosystem patterns correspond with climatic regions, which

change mainly due to latitudinal, orographic, and maritime influences (Bailey 1987, Denton and Barnes

1988). Within climatic regions, landforms modify macroclimate (Rowe 1984, Smalley 1986, Bailey 1987),

and affect the movement of organisms, the flow and orientation of watersheds, and the frequency and

spatial pattern of disturbance by fire and wind (Swanson et al. 1988). Within climatic - geomorphic

regions, water, plants, animals, soils, and topography interact to form ecosystems at Land Unit scales

(Pregitzer and Barnes 1984). Thus ecological systems exist at many spatial scales, from the global

ecosphere down to regions of microbial activity. The challenge of ecosystem classification and mapping is

to distinguish natural associations of ecological factors at different spatial scales, and to define ecological

types and map ecological units that reflect these different levels of organization.

And while the association of these multiple biotic and abiotic factors is all important in defining ecosystems

and ecological units, all factors are not equally important at all spatial scales. At coarse scales, the

important factors are largely abiotic, while at finer scales both biotic and abiotic factors are important.

Furthermore, the level of discernible detail, the number of factors contributing to ecosystems, and the

number of variables used to characterize these factors progressively increase at finer scales. Hence the

data and analysis requirements, as well as the investments for ecosystem classification and mapping,

also increase for finer-scaled activities.

The conditions and processes occurring across larger ecosystems affect and often override those of

smaller ecosystems, and the properties of smaller ecosystems emerge in the context of larger systems

(Rowe 1984). Moreover, environmental gradients that affect ecological patterns and processes change at

different spatial scales. Thus, it is useful to conceive of ecosystems and their underlying biophysical

environments as occurring in a nested geographic arrangement, with smaller ecosystems embedded in

larger ones (Allen and Starr 1982, O'Neill et al. 1986, Albert et al. 1986). This spatial hierarchy is

organized in decreasing orders of scale by the dominant factors affecting ecological systems. Ecosystems

become networked, however, when non-adjacent systems exhibit similar structure and function with

respect to specific biota (for example, sedentary plants as opposed to wide ranging animals) and various

processes; hence the networking of ecological systems is scale dependent (Allen and Hoekstra 1992).

Networking of ecosystems occurs most often at lower levels of the hierarchy, and depends upon

requirements, environmental tolerances, and dispersion mechanisms of biota, as well as other factors that

affect biotic-abiotic interactions within and across local, landscape and regional ecosystems.

Life and Environmental Interactions

Life-forms and environment have interacted and codeveloped at all spatial and temporal scales, one

modifying the other through feedback. Appreciating these interactions is integral to understanding

ecosystems. At a global scale, for example, scientists have theorized that the evolution of cyanobacteria,

followed by terrestrial plants capable of photosynthesis, carbon fixation and oxygen production converted

the earth's atmosphere from a hydrogen to an oxygen base and still sustain it today. At a continental

scale, the migration of species in response to climate change, and the interaction of their environmental

tolerances and dispersal mechanisms with landform controlled migration routes formed today's patterns in

the distributions of species. At a landscape scale, life-forms, environment, and disturbance regimes have

interacted to form patterns and processes. For example, pyrophilic communities tend to occupy droughty

soils in fire-prone landscape positions, produce volatile foliar substances, and accumulate litter, thereby

increasing their susceptibility to burning. At yet finer scales, vegetation has induced soil development over

time through carbon and nutrient cycling, enabling succession to proceed to communities with higher

fertility requirements.

In each of these examples, life forms and environment have modified one another through feedback to

form ecological patterns and processes. These types of relationships underscore the need to consider

both biotic and environmental factors while classifying, mapping and managing ecological systems.

Spatial and Temporal Variability

The structure and function of ecosystems change through space and time. Consequently, we need to

address both spatial and temporal sources of variability while evaluating, classifying, mapping, or

managing ecosystems (Delcourt et al. 1983, Forman and Godron 1986). At a land unit scale, for example,

the fertility of particular locations changes through space because of differences in soil properties or

hydrology, and at ecoregion scales, conditions vary from colder to warmer because of changes in

macroclimate. These relatively stable conditions favor certain assemblages of plants and animals while

excluding others because of biotic tolerances and such processes as competition. These environmental

conditions are classified as ecological types and mapped as ecological units.

Within ecological units, ecosystems may support vegetation that is young, mature, or old, and they may be

composed of communities that are early, mid-, or late successional. These relatively dynamic conditions

also benefit certain plant and animal species and assemblages. Conditions that vary temporally are

classified and mapped as existing vegetation, wildlife, water quality, and so forth.

These examples illustrate that ecological units do not provide all the information needed to classify, map

and manage ecosystems. Ecological units address the spatial distributions of relatively stable associations

of ecological factors that affect ecosystems. When combined with information on existing biotic conditions

and ecological processes, the National Hierarchy of Ecological Units provides a means of addressing

spatial and temporal variations that affect the structure, function and management potentials of

ecosystems. Adding our knowledge of processes to this information will enable us to evolve better

information.

USE OF ECOLOGICAL UNITS

Ecological units provide basic information for natural resource planning and management. Ecological unit

maps may be used for activities such as delineating ecosystems, assessing resources, conducting

environmental analyses, and managing and monitoring natural resources.

Ecosystem Mapping

To map ecosystems, or places where life and environment interact, we need to combine two types of

maps: maps of existing conditions of biota that change readily through time, and maps of potential

conditions of environments that are relatively stable. Existing conditions change due to particular

processes that operate within the bounds of biotic and environmental, or ecological, potentials. Existing

conditions are inventoried as current vegetation, wildlife, water quality, and so forth. Potential conditions

are inventoried as ecological units. When these maps are combined, biotic distributions and ecological

processes can be evaluated, and results can be extrapolated to similar ecosystems. The integration of

multiple biotic and abiotic factors, then, provides the basis for defining and mapping ecosystems.

Fundamental base maps are key to mapping ecosystems and integrating resource inventories. These

maps include the primary base map series, showing topography, streams, lakes, ownership, political

boundaries, cultural features, and other layers in the cartographic features file. On this base, the next set

of layers could include terrestrial or aquatic ecological units. Next would be layers of information on

existing vegetation, wildlife populations, fish distribution, cultural resources, demographics, economic

data, and other information needed to delineate ecosystems to meet planning and analysis needs.

GIS will provide a tool for combining these separate themes of information, and representing the physical,

biological, and social dimensions to define and map ecosystems. But scientists and managers using this

technology must actually integrate information themes, comprehend processes, and formulate

management strategies. These tasks will not be accomplished mechanically.

Resource Assessments

The hierarchical framework of ecological units can provide a basis for assessing resource conditions at

multiple scales. Broadly defined ecological units (for example, ecoregions) can be used for general

planning assessments of resource capability. Intermediate scale units (for example, landtype

associations) can be used to identify areas with similar natural disturbance regimes (for example, mass

wasting, flooding, fire potential). Narrowly defined land units can be used to assess site specific conditions

including distributions of terrestrial and aquatic biota; forest growth, succession, and health; and various

physical conditions (for example, soil compaction and erosion potential, water quality).

High resolution information obtained for fine scale ecological units can be aggregated for some types of

broader scale resource assessments. Resource production capability, for example, can be estimated

based on potentials measured for landtype phases, and estimates can be aggregated to assess ranger

district, national forest, regional, and national capabilities.

Environmental Analyses

Ecological units provide a means of analyzing the feasibility and effects of management alternatives. To

discern the effects of management on ecosystems, we often need to examine conditions and processes

occurring above and below the level under consideration (Rowe 1980). For example, the effects of timber

harvesting are manifest not only at a land unit scale, but also at micro-site and landscape scales. Although

the direct effects of management are assessed at the land unit scale, indirect and cumulative effects take

place at different points in space or time, often at higher spatial scales. We can minimize conflicting

resource uses (for example, remote recreational experiences versus developed motorized recreation,

habitat management for area sensitive species versus edge species) if we consider the design and effects

of projects at several scales of analysis. Ecological units defined at different hierarchical levels will be

useful in conducting multi-scaled analyses for managing ecosystems and documenting environmental

effects (Brenner and Jordan 1991, Jensen et al. 1991).

Watershed Analysis

The national hierarchy provides a basis for evaluating the linkages between terrestrial and aquatic

systems. Because of the interdependence of geographic components, aquatic systems are linked or

integrated with surrounding terrestrial systems through the processes of runoff, sedimentation, and

migration of biotic and chemical elements. Furthermore, the context of water bodies affects their

ecological significance. A lake embedded within a landscape containing few lakes, for example, functions

differently from one embedded within a landscape composed of many lakes for wildlife, recreation for

people, and other ecosystem values. Aquatic systems delineated in this indirect way may have many

characteristics in common, including hydrology and biota (Frissell et al. 1986). Overlays of hierarchical

watershed boundaries on terrestrial ecological units are useful for many watershed analysis efforts. In this

case, the watershed becomes the analysis area, which is both superimposed by and composed of a

number of ecological units which affect important hydrologic processes such as water runoff and

percolation, water chemistry, and ecological function due to context.

Desired Future Conditions

Desired future conditions (DFC’s) portray the land or resource conditions expected if goals and objectives

are met. Ecological units will be useful in establishing goals and methods to meet DFC’s. When combined

with information on existing conditions, ecological units will help us project responses to various

treatments.

Ecological units can be related to past, present, and future conditions. Past conditions serve as a model of

functioning ecosystems and provide insight into natural processes. It is unreasonable, for example, to try

to restore systems like oak savannas or old-growth forests in areas where they did not occur naturally.

Moreover, natural processes like disturbance or hydrologic regimes are beyond human control. Ecological

units will be helpful in understanding these processes and in devising DFC’s that cab attained and

perpetuated.

Desired future conditions can be portrayed at several spatial scales. We can minimize conflicting resource

uses (for example, remote recreational experiences versus developed motorized recreation, habitat

management for area-sensitive species versus edge species) if we consider the effects of projects at

several spatial scales of analysis. Ecological units will be useful in delineating land units at relevant spatial

scales for planning DFC’s.

Resource Management

Information on ecological units will help establish management objectives and will support such

management activities as the protection of habitats of sensitive, threatened, and endangered species, or

the improvement of forest and rangeland health to meet conservation, restoration, and human needs. For

example, information on current productivity can be compared to potentials determined for landtype

phases, and areas producing less than their potential can be identified (Host et al. 1988). Furthermore,

long term sustained yield capability can be estimated based on productivity potentials measured for fine

scale ecological units.

Monitoring

Monitoring the effects of management requires baseline information on the condition of ecosystems at

different spatial scales. Through the ecological unit hierarchy, managers can obtain information about the

geographic patterns in ecosystems. They are thus in a position to design stratified sampling networks for

inventory and monitoring. Representative ecological units can be sampled and information can then be

extended to analogous unsampled ecological units, thereby reducing cost and time in inventory and

monitoring.

By establishing baselines for ecological units and monitoring changes, we can protect landscape,

community, and species level biological diversity; and other resource values such as forest productivity,

and air and water quality. The results of effectiveness and validation monitoring can be extrapolated to

estimate effects and set standards in similar ecological units.

Evaluation of air quality is an example of how the National Hierarchical Framework of Ecological Units can

be used for baseline data collection and monitoring. The Forest Service is developing a National Visibility

Monitoring Strategy that addresses protection of air quality standards as mandated by the Clean Air Act

(USDA Forest Service 1993). Key to this plan is stratification of the United States at the subregion level of

the national hierarchy into areas that have similar climatic, physiographic, cultural, and vegetational

characteristics. Other questions dealing with effects of specific airborne pollutants on forest health, such

as correlation of ozone with decline of ponderosa pine and other trees in mixed conifer forest ecosystems

in the San Bernardino Mountains of southern California, will require establishment of sampling networks in

smaller ecological units at landscape or lower levels.

Contemporary and Emerging Issues

The National Hierarchy of Ecological Units is based on natural associations of ecological factors. These

associations will be useful in responding to contemporary and emerging issues, particularly those that

cross administrative and jurisdictional boundaries. Concerns regarding biological diversity, for example,

can be addressed using the ecological unit hierarchy (Probst and Crow 1991). Conservation strategies

can be developed using landscape-level units as coarse filters, followed by detailed evaluations and

monitoring conducted to verify or adjust landscape designs. We can rehabilitate ecosystems and

dependent species that have been adversely affected by fire exclusion, fragmentation, or other results of

human activities if we grow to understand the natural processes that species and ecosystems

codeveloped with, and then mimic those processes through ecosystem management.

Species may become rare, threatened, or endangered because their habitat is becoming degraded,

because they are specialists endemic to a particular area, or because they are at the edge of their natural

range. In the first two instances, protection and recovery efforts are warranted. In the latter case, however,

it may be futile to try to maintain biota where they are predisposed to decline. At a minimum, populations

at the edge of their range can be evaluated for genetic diversity, and recovery programs can be

administered accordingly. Species and community distributions can often be related to ecological units,

which can be useful in their inventory and protection.

The emphasis on sustaining and restoring the integrity of ecosystems may aid in arresting the decline of

biological diversity and preempt the need for many future protection and recovery efforts Developing basic

information on the nature and distribution of ecosystems and their elements will enable us to better

respond to issues like global warming, forest health and sustainability, and biological diversity.

The hierarchical framework of ecological units was developed to improve our ability to implement

ecosystem management. This framework, in combination with other information sources, is playing an

important role in national, regional, and forest planning efforts; the sharing of information between forests,

stations, and regions; and interregional assessments of ecosystem conditions.

Regions and stations, with national guidance, are coordinating their design of ecological units at higher

levels of the national hierarchy. Development of landscape and land unit maps is being coordinated by

appropriate regional, station, forest and ranger district level staff. As appropriate, new technologies (for

example, remote sensing, GIS, expert systems) should be used in the design, testing and refinement of

ecological unit maps.

The classification of ecological types and mapping of ecological units pose a challenge to integrate not

only information, but also the concepts and tools traditionally used by various disciplines. The effort brings

together the biological and physical sciences that have too often operated independently. Specialists like

foresters, fishery and wildlife biologists, geologists, hydrologists, community ecologists and soil scientists

will need to work together to develop and implement this new classification and mapping system. The

results of these concerted efforts will then need to be applied in collaboration with planners, social

scientists, economists, archaeologists and the many other specialties needed to achieve a truly ecological

approach to the management of our nation's National Forests and Grasslands.

LITERATURE CITED

Albert, D.A.; Denton S.R.; Barnes, B.V. 1986. Regional landscape ecosystems of Michigan. School of

Natural Resources, University of Michigan, Ann Arbor. 32 p.

Allen, T.F.H.; Starr, T.B. 1982. Hierarchy: perspectives for ecological complexity. Chicago: The University

of Chicago Press. 310 p.

Allen, T.F.H.; Hoekstra, T.W. 1992. Toward A Unified Ecology. New York: Columbia University Press. 384

Avers, P.E.; Schlatterer, E.F. 1991. Ecosystem classification and management on national forests. In:

Proceedings of the 1991 symposium on systems analysis in forest resources.

Bailey, R.G. 1980. Descriptions of the ecoregions of the United States. Washington DC: U.S. Department

of Agriculture, Forest Service. Miscellaneous Publication 1391. 77 p.

Bailey,R.G.; Zoltai,S.C.; Wiken, E.B. 1985. Ecological Regionalization in Canada and the United States.

Geoforum 16 (3):265-275.

Bailey, R.G. 1987. Suggested hierarchy of criteria for multi-scale ecosystem mapping. Landscape and

Urban Planning 14:313-319.

Bailey, R.G. 1989a. Ecoregions of the continents (map). Washington, DC: U.S. Department of Agriculture,

Forest Service. Scale 1:30,000,000.

Bailey, R.G. 1989b. Explanatory supplement to the ecoregions map of the continents. Environmental

Conservation 15(4):307-309.

Baldwin, J.L. 1973. Climates of the United States. Washington DC: U.S. Department of Commerce.

NOAA. U.S. Government Printing Office. 113 p.

Barnes, B.V.; Pregitzer K.S; Spies, T.A; Spooner, V.H. 1982. Ecological forest site classification. Journal

of Forestry 80:493-498.

Barnes, B.V. 1984. Forest ecosystem classification and mapping in Baden-Wurttemberg, West Germany.

In: Forest land classification: experience, problems, perspectives. Proceedings of the symposium;

1984 March 18-20; Madison, WI:49-65.

Brenner, R.N.; Jordan, J.K. 1991. The role of an ecological classification system in forest plan

development and implementation. In: Proceedings of the 1991 symposium on systems analysis in

forest resources, 1991 March 3-6; Charleston, SC.

Cleland, D.T.; Crow, T.R.; Avers, P.E.; Probst, J.R. 1992. Principles of land stratification for delineating

ecosystems. In: Proceedings of Taking an Ecological Approach to Management National

Workshop. 1992 April 27-30; Salt Lake City, Utah. 10 p.

Cowardin, L.M.; Carter, V.; Golet, F.C.; LaRoe, E.T. 1979. Classification of wetlands and deepwater

habitats of the United States. Washington, DC: U.S. Department of the Interior, U.S. Fish and

Wildlife Service. 131 p.

Crowley, J.M. 1967. Biogeography in Canada. Canadian Geographer 11: 312-326.

Delcourt, H.R.; Delcourt, T.A; Webb, T. 1983. Dynamic plant ecology: the spectrum of vegetative change

in space and time. Quant. Sci. Rev. 1:153-75.

Denton, D.R.; Barnes, B.V. 1988. An ecological climatic classification of Michigan: a quantitative

approach. Forest Science 34:119-138.

Driscoll, R.S.; et. al. 1984. An ecological land classification framework for the United States. Washington

DC: U.S. Department of Agriculture, Forest Service. Miscellaneous Publication 1439. 56 p.

Ebert, D.J.; Nelson, T.A.; Kershner, J.L. 1991. A soil-based assessment of stream fish habitats in coastal

plains streams. Proceedings of warmwater fisheries symposium. 1991 June 4-8; Phoenix, AZ.

Ecoregions Working Group. 1989. Ecoclimatic regions of Canada, first approximation. Ecological Land

Classification Series No. 23. Ottawa: Environment Canada. 119 p. and map at 1:7,500,000.

Fenneman, N.M. 1938. Physiography of eastern United States. McGraw-Hill Book Company. 714 p.

Forman, R.T.T.; Godron, M. 1986. Landscape Ecology. New York: Wiley and sons.

Frissell, C.A.; W.J. Liss, W.J.; Warren, C.E.; Hurley, M.C. 1986. A hierarchical framework for stream

habitat classification: viewing streams in a watershed context. Environmental Management 10:

199-214.

Gallant, A.L.; Whittier, T.R.; Larsen, D.P.; Omernik, J.M.; Hughes, R.M. 1989. Regionalization as a tool for

managing environmental resources. U.S. Environmental Protection Agency. EPA/600/3-89/060.

Corvallis, OR. 152 p.

Herrington, R.B; Dunham, D.K. 1967. A technique for sampling general fish habitat characteristics of

streams. Ogden, UT: U.S. Department of Agriculture, Forest Service. Research Paper INT-41.

Intermountain Forest and Range Experiment Station. 12 p.

Hills, G.A. 1952. The classification and evaluation of site for forestry. Ontario Department of Lands and

Forests. Resource Division Report 24.

Hix, D.M. 1988. Multifactor classification of upland hardwood forest ecosystems of the Kickapoo River

watershed, southwestern Wisconsin. Canadian Journal of Forest Research 18:1405-1415.

Holdridge, L.R. 1967. Life zone ecology. Tropical Science Center. San Jose, CA. 206 p.

Host, G.E.; Pregitzer, K.S; Ramm, C.W.; Hart, J.B.; Cleland D.T. 1987. Landform mediated differences in

successional pathways among upland forest ecosystems in northwestern Lower Michigan. Forest

Science 33:445-457.

Host, G.E.; Pregitzer, K.S; Ramm, C.W.; Lusch, D.T.; Cleland D.T. 1988. Variations in overstory biomass

among glacial landforms and ecological land units in northwestern Lower Michigan. Canadian

Journal of Forest Research 18:659-668.

Jensen, M.C.; McMicoll, C.; Prather, M. 1991. Application of ecological classification to environmental

effects analysis. Journal of Environmental Quality 20:24-30.

Jones, R.K. et al. 1983. Field guide to forest ecosystem classification for the clay belt, site region 3e.

Ontario, Canada: Ministry of Natural Resources. 123 p.

Jordan, J.K. 1982. Application of an integrated land classification. In: Proceedings, Artificial Regeneration

of Conifers in the Upper Lakes Region. 1982 October 26-28; Green Bay, WI: 65-82.

Keane, R.E.; Jensen, M.E.; Hann, W.J. 1990. Ecodata and Ecopac: analytical tools for integrated resource

management. The Compiler. 8(3): 24-37.

Koppen, J.M. 1931. Grundriss der Klimakunde. Berlin: Walter de Grayter. 388 p.

Kuchler, A.W. 1964. Potential natural vegetation of the conterminous United States. American Geographic

Society Special Publication 36. 116 p.

Major, J. 1969. Historical development of the ecosystem concept. In: (Van Dyne, G.M. ed.) The

ecosystem concept in natural resource management. New York: Academic Press: 9-22.

McNab, W.H. 1987. Rationale for a multifactor forest site classification system for the southern

Appalachians. In: Proceedings of 6th Central Hardwood Forest Conference. 1987 February 24-26,

Knoxville, TN: 283-294.

Meentemeyer, V. and E.O. Box. 1987. Scale effects in landscape studies. In: (Turner, M.G. ed.)

Landscape heterogeneity and disturbance. New York: Springer-Verlag: 15-34.

Miller, P.R.; Taylor, O.C.; Wilhour, R.G. 1982. Oxidant air pollution effects on a western coniferous forest

ecosystem. Environmental Research Brief. EPA-600-/D-82-276. 10 p.

Omernik, J.M. 1987. Ecoregions of the conterminous United States. Annals of the Association of

American Geographers 77:118-125.

O'Neill, R.V.; DeAngelis, D.L.; Waide, J.B.; Allen, T.F.H. 1986. A hierarchical concept of ecosystems.

Princeton, N.J; Princeton University Press.

Platts, W.S. 1980. A plea for fishery habitat classification. Fisheries 5(1):2-6.

Platts, W.S. 1979. Including the fishery system in land planning. Ogden, UT: U.S. Department of

Agriculture, Forest Service. Gen. Tech. Rep. INT-60. Intermountain Forest and Range Experiment

Station. 37 p.

Pregitzer, K.S.; Barnes, B.V. 1984. Classification and comparison of upland hardwood and conifer

ecosystems of the Cyrus H. McCormick Experimental Forest, upper Michigan. Canadian Journal

of Forest Research. 14:362-375.

Probst, J.R.; Crow, T.R. 1991. Integrating biological diversity and resource management. Journal of

Forestry. 89:12-17.

Rowe, J.S. 1980. The common denominator in land classification in Canada: an ecological approach to

mapping. Forest Chronicle 56:19-20.

Rowe, J.S. 1984. Forest Land Classification: limitations of the use of vegetation. Proceedings of the

symposium on forest land classification. 1984 March 18-20; Madison, WI: 132-147.

Rowe, J.S. 1992. The ecosystem approach to forest management. Forest Chronicle 68: 222-224.

Russell, W.E.; Jordan, J.K. 1991. Ecological classification system for classifying land capability in

midwestern and northeastern U.S. national forests. In: Proceedings of the symposium, Ecological

Land Classification: applications to identify the productive potential of southern forests. Charlotte,

NC, January 7-9. Ashville, NC: U.S. Department of Agriculture, Forest Service Gen. Tech. Report

SE-68.

Smalley, G.W. 1986. Site classification and evaluation for the Interior Uplands. Atlanta, GA: U.S.

Department of Agriculture, Forest Service. Tech. Pub. R8-TP9. Southern Region.

Spies, T.A.; Barnes, B.V. 1985. A multifactor ecological classification of the northern hardwood and

conifer ecosystems of Sylvania Recreation Area, Upper Peninsula, Michigan. Canadian Journal of

Forest Research 15:949-960.

Swanson, F.J.; Kratz, T.K.; Caine, N.; Woodmansee, R.G. 1988. Landform effects on ecosystem patterns

and processes. BioScience 38:92-98.

Trewartha, G. T. 1968. An Introduction to Climate, 4th ed, New York, NY: McGraw-Hill. 408 p.

Urban, D.L.; O'Neil, D.; Shugart, H.H. 1987. Landscape ecology. BioScience 37:119-127.

U.S. Department of Agriculture. 1981. Land resource regions and major land resource areas of the United

States. Washington, DC: Agricultural Handbook 296. U.S. Government Printing Office. 156 p.

Udvardy, M.D.F. 1975. A classification of the biogeographical provinces of the world. Occasional Paper

18. International Union for Conservation of Nature and Natural Resources, Morges, Switzerland.

48 p.

U.S. Department of Agriculture, Forest Service. 1976. Land systems inventory guide. Northern Region.

U.S. Department of Agriculture, Forest Service. 1993. Draft National Visibility Monitoring Strategy.

Washington, DC.

U.S. Department of Agriculture, Forest Service Handbook 2090. Washington, DC.

U.S. Department of Agriculture, Forest Service Manual 2060. Washington, DC.

Walter, H.; Box, E. 1976. Global classification of natural terrestrial ecosystems. Vegetatio 32:75-81.

Wertz, W.A.; Arnold, J.A. 1972. Land Systems Inventory. Ogden, UT: USDA Forest Service,

Intermountain Region. 12 p.

Wiken, E.B. (compiler). 1986. Terrestrial ecozones of Canada. Ecological Land Classification Series No.

19. Hull, PQ: Environment Canada, Hull.