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Seeing the Forest and the Trees: Ecological Classification for Conservation

The classification that this publication describes was authored by: Mark Anderson Patrick Bourgeron Mark T. Bryer Rex Crawford Lisa Engelking Don Faber-Langendoen Mark Gallyoun Kathleen L. Goodin

Dennis H. Grossman Sally Landaal Kenneth J. Metzler Karen D. Patterson Milo Pyne Marion S. Reid Lesley Sneddon Alan S. Weakley

and by ecologists in the network of Natural Heritage programs and Conservation Data Centers (see back cover).

Citation: Maybury, Kathleen P., editor. 1999. Seeing the Forest and the Trees: Ecological Classification for Conservation. The Nature Conservancy, Arlington, Virginia. ISBN 0-9624590-2-X Copyright 1999 The Nature Conservancy

Acknowledgements Don Faber-Langendoen, Alan Weakley, Marion Reid, and Dennis Grossman, deserve special acknowledgement for their ideas, careful review, and other contributions to this publication. Thoughtful review and comments were also provided by Jonathan Adams, Mark Anderson, Mark Bryer, Patrick Comer, Christine A. De Joy, Beth Duris, Stephanie R. Flack, Kathleen L. Goodin, Sally Landaal, Julie Lundgren, Karen D. Patterson, Milo Pyne, Carol Reschke, Lesley Sneddon, and Bruce A. Stein. Deborah A. Gries provided research assistance. Jim Drake, Jonathan L. Haferman, and Stuart Sheppard assisted with maps. Design and Production: Nicole S. Rousmaniere

Table of Contents Preface ............................................................................................................................. 2

Introduction ....................................................................................................................... 3

Why Communities? ............................................................................................................. 4

Why Is a Classification System Important? .............................................................................. 6

What Is the USNVC? .......................................................................................................... 7

What Is the Current Status of the USNVC? ........................................................................... 10

How Is the USNVC Being Used? ........................................................................................ 12 Within the Conservancy ............................................................................................. 12 Beyond the Conservancy ............................................................................................ 19

Summary and Future Challenges ......................................................................................... 23

Appendices: The Nuts and Bolts of the USNVC System ........................................................... 24 Appendix A—Key Attributes ....................................................................................... 24 Appendix B—Structure and Type Definition ................................................................... 26 Appendix C—Partners in Development and Application .................................................. 29

Cited References .............................................................................................................. 36

I

n the early summer of 1991, a biologist invento-

Classifications such as the one developed in North Caro-

rying a pine savanna on the North Carolina

lina have now been compiled, integrated, and expanded

Coastal Plain came across a 3-foot-tall plant he couldn’t

into a system that is applicable across the nation: the U.S.

identify. The puzzling plant species, a member of the sedge

National Vegetation Classification, or USNVC. The prod-

family, turned out to be unknown to science—and all the

uct of a two-decade-long collaboration between the Con-

more intriguing because its nearest close relatives were

servancy and the network of Natural Heritage programs,†

found to be boreal species that occur almost 500 miles

the USNVC greatly enhances our ability to recognize,

away. Subsequent inventories at this same small site have

assess, and conserve natural communities everywhere we

documented the presence of an astounding 500 species

work, in the United States and beyond. It represents the

of vascular plants, as well as large populations of several

first U.S. community classification system that is national

globally endangered plants and invertebrates.

in scope and detailed enough in its consideration of

For those dedicated to preserving the nation’s biodiversity, remarkable discoveries like these are usually

natural diversity to be useful in making local, site-specific conservation decisions.

a call to action, galvanizing efforts to forge

In the case of the North Carolina site, a

partnerships with members of the local

community classification system allowed bi-

community, negotiate land deals, and initiate protection agreements. These efforts

Preface

ologists to recognize and document that at least one plant community found there was

are often imbued with a sense of urgency,

extremely rare on a global basis. That real-

as many sites of exceptional biological value

ization was a principal factor in the decision

face encroaching development or other threats. In this

to make the site a Conservancy preserve long before com-

case, however, no sense of crisis ever arose; nothing out

prehensive inventories of species had been conducted there.

of the ordinary happened at all. At the time the scientist

In this report, we present several examples of how

discovered the anomalous sedge, large portions of the site

scientists within and beyond the Conservancy are using

had been a Conservancy preserve for more than five years.

the USNVC to accomplish the best possible conservation.

Ties to the community were already strong, and protec-

Our hope is to make the classification, and its enormous

tion efforts had long been part of the day-to-day work of

potential for improving conservation decisions, accessible

the Conservancy’s North Carolina Field Office.

to a wider audience of conservation practitioners. Essen-

How did the Conservancy come to identify this

tially, we want this tool to be in the hands of those who

specific place as being of critical conservation concern years

need it, wherever they need it, to carry out effective and

before many of the important species discoveries had been

efficient conservation.

made? In large part, the answer has to do with a scientific

 The ecologists of

approach created expressly to meet conservation needs:

The Nature Conservancy,

a classification system for ecological communities.

August 1998



The Natural Heritage network is an informal designation of state and other programs that work cooperatively to collect and manage information on rare species and natural communities.

2

Seeing the Forest and the Trees: Ecological Classification for Conservation

W

hat types of natural vegetation exist across

This standardized approach is allowing assess-

the landscape? Which types are intrinsically

ments of conservation status, trends, and management

rare or have been severely degraded by human activities?

practices for ecological communities across local,

How do we identify the best remaining occurrences of natu-

regional, and national landscapes. It has enhanced the

ral communities across their geographic ranges? To direct

Conservancy’s ability to identify the most important

our limited conservation resources to the specific places

sites for conserving our nation’s biodiversity, and it is

where they will have the greatest impact, we must have

playing an important role in our efforts to identify a

clear answers to questions such as these—answers that

portfolio of conservation sites representative of each

ultimately hinge on how we define and categorize the rich

ecoregion.† Beyond the Conservancy and Heritage pro-

ecological diversity that is one of our nation’s greatest trea-

grams, the system’s utility has been widely appreciated:

sures. To answer these questions, The Nature Conservancy,

it is now accepted as the standard for classification,

in partnership with the network of Natural Heritage pro-

inventory, and mapping work in all U.S. federal agen-

grams, has developed a scientifically sound, consistent, and

cies, including the National Park Service, the National

flexible classification system that can

Forest Service, and the Fish and

be applied to terrestrial ecological

Wildlife Service. These agencies,

communities throughout the world. The system can be used to classify

Introduction

all types of vegetated communities,

along with other academic and professional conservation and management organizations, are

from verdant wetlands to arid deserts nearly lacking in plant

increasingly becoming the Conservancy’s partners in

life, and from the most pristine old-growth forests to

the ongoing development and application of the clas-

cultivated annual crop fields. Using this system, a team of

sification and its provision to a burgeoning number of

Conservancy and Heritage ecologists has now completed a

users and contributors.

first iteration of the natural vegetation types of the United

This report briefly describes the classification

States. This represents the first time the country’s natural

system and identifies major opportunities for applying

terrestrial communities have been classified using a single

it to meet our current conservation challenges, as well as

system on a scale fine enough to be useful for the conser-

those that lie ahead.

vation of specific sites.



An ecoregion is a relatively large unit of land and water delineated by the biotic and abiotic factors that regulate the structure and function of the communities within it. It provides a unit of geography that is more relevant than political units for organizing and prioritizing conservation planning efforts.1

Seeing the Forest and the Trees: Ecological Classification for Conservation

3

A

ttempts to conserve biological diversity can

species-by-species approach to conservation. Saving

be directed at different biological and eco-

individual species once they are on the brink of extinc-

logical levels, ranging from genes to species to commu-

tion often demands large amounts of time, societal

nities and ecosystems. Communities can be described

concern, and money. Such an approach is clearly neces-

as assemblages of species that co-occur in defined areas

sary for species that are facing particular threats, and it

at certain times and that have the potential to interact

is also reasonable for large mammals, birds, and plants

2,3,4

with one another.

But com-

that are of particular signifi-

munities are more than the

cance to humans, especially

sum of their species; they also represent the myriad biological and environmental interactions that are inherently a part of

Why Communities?

each unique natural system.

those that inspire a sense of awe or affinity. But there are an estimated 10 million to 100 million species on Earth, only a fraction of which are known

Thus, by describing, tracking, and preserving commu-

to science. 5 These include the little, less glamorous

nities, ecologists can protect a complex suite of organ-

species that create soils, pollinate plants, and play

isms and interactions not easily identified and protected

numerous other ecological roles. Protecting the vast

through other means.

majority of them will require a broader, natural com-

In addition, a consensus has emerged within the conservation community about the inadequacy of a

munity-based approach that conserves habitats and species assemblages as a whole.

The best approach to protecting many of our most imperiled species, such as the elusive and rare San Gabriel Mountain slender salamander, is to preserve the habitats on which they depend. Natural assemblages of plants are widely regarded as biological expressions of the complex factors that make up a particular habitat—factors such as climate, soils, natural disturbance processes, and the structure of the plants themselves. Thus, natural plant communities can be used as a “coarse filter” for protecting numerous species, even the less glamorous species whose very existence—let alone habitat needs—we may have yet to discover.

San Gabriel Mountain slender salamander

© Kate Spencer

4

Seeing the Forest and the Trees: Ecological Classification for Conservation

The ecological community concept has been recognized as an important conservation planning tool for the following reasons: • Communities have inherent value that is worth conserving. They encompass a unique set of interactions among species and contribute to important ecosystem functions. • Communities can be used as surrogates for species and for ecological processes, particularly in species-rich and data-poor areas such as the tropics. • By protecting communities, we protect many species not specifically targeted for conservation. This is especially important for poorly known groups such as fungi and invertebrates. • Monitoring change over time is often most meaningful when done at the level of communities. Changes may be detected in overall species abundance, including the proportion of non-native species; in structure, such as the development of old-growth characteristics; and in function, such as alterations in nutrient cycling. • Communities are an important tool for systematically characterizing the current pattern and condition of

© James R. Snyder

ecosystems and landscapes.

More Than the Sum of Its Parts Tropical forest communities may comprise an especially high number of species. Like all communities, they also comprise a complex array of interactions among species and between species and their environments. Shown here: a seasonally flooded tropical forest, south Florida.

Seeing the Forest and the Trees: Ecological Classification for Conservation

5

T

he Conservancy and the Natural Heritage network

concluded that a basic question—to what extent the natu-

have recognized ecological communities as

ral ecosystems in the United States have been reduced in

important elements of conservation for many years. The

area or degraded in quality due to human activities—

best, most viable occurrences of these communities,

could only be answered “by a relatively crude approach

along with occurrences of rare and imperiled species,

because a systematic approach to understanding these

have formed the basis for protection decisions through-

systems at a national scale was not yet available.”6 Other

out the Conservancy’s history.

basic questions, such as, What are the key environmental

In the past, despite the recognized importance of

factors in relationship to a particular community across

ecological communities, no accepted framework for

its entire range? and, What is the complete geographic

national or international community classification existed.

distribution of a particular community? often could not be answered at all.

Without a standard approach, community protection decisions were made only on a state-bystate or agency-by-agency basis, based on independently developed classifications. These classifications worked effectively for the conservation of important areas within states or jurisdic-

Why Is a Classification System Important?

Recognizing the need for a national and international system for classifying terrestrial communities to carry out its mission effectively, the Conservancy, in conjunction with the Natural Heritage network, undertook development of the U.S. National Vegetation Classification

tions, but from a national and international perspective, they risked unnecessarily redun-

(USNVC) system. A team of more than 100 Conservancy

dant protection of a few communities and inadequate

and Heritage ecologists established the standardized

protection of many others. In addition, in the absence of

classification framework and defined the first iteration

a common classification, the results of many inventory

of U.S. communities within it.† In the coming years, the

and monitoring programs, such as those conducted in

USNVC will be continuously refined and developed by

national forests and parks, state forests and parks, or fish

the Conservancy and an expanding network of partners

and wildlife refuges, could not be integrated or compared.

who are using the system to accomplish their conserva-

As recently as 1995, several prominent researchers

tion and management goals.



More detailed information about the classification system, including its development, status, and applications, can be found in the recent publication International Classification of Ecological Communities: Terrestrial Vegetation of the United States.7 A first iteration of the list of community types can be found in the second volume of that publication.8 Both volumes are available on the World Wide Web at www.tnc.org.

6

Seeing the Forest and the Trees: Ecological Classification for Conservation

T

he USNVC system blends the features of many

vegetation structure and that occur under similar habi-

existing classification systems that are most

tat conditions.

useful for conservation. It essentially represents a

The association concept encompasses both the

structured compilation of an enormous amount of

dominant species (those that cover the greatest area) and

fine-scale state and local information on vegetation,

diagnostic species (those found consistently in some

and an integration of this information with a modified

vegetation types but not others) regardless of whether

version of UNESCO’s worldwide framework for coarse-

they are large trees or diminutive understory plants.

scale vegetation classification.9

This means associations can reflect a greater ecological

Terrestrial ecological communities are classified

specificity than can a “cover type” or other type based

based on vegetation as it currently exists across the land-

solely on the dominant species of the upper stratum.

scape. Because of their conservation objectives, the

Dominant cover species are often widespread, and may

Conservancy and the Natural Heri-

occur with many different species

tage network are classifying and

over large, heterogeneous land-

describing only the more natural types of vegetation. The USNVC framework, however, may be used

What Is the USNVC?

scapes.10,11 For example, in northern Minnesota and adjacent parts of Canada, on moist bedrock sub-

to classify all existing vegetation,

strates the “generalist” species jack

including heavily human-influ-

pine (Pinus banksiana) occurs with

enced types such as those in developed areas, crop-

an understory of balsam fir (Abies balsamea), whereas

lands, and places severely altered by past logging or

on dry sandplains jack pine grows with bearberry (Arcto-

farming. (See Appendix A for additional information

staphylos uva-ursi) under its canopy.12 These two distinct

about the key attributes of the USNVC.)

plant assemblages, and the profoundly different environ-

In the USNVC, terrestrial vegetation is classified

mental conditions under which they occur, undoubtedly

within a nested, seven-level hierarchy, the finest level of

represent differences of real ecological significance.

which is called the association. (See Appendix B for a com-

The differences are captured by including dominant

plete explanation of the system’s hierarchical structure.)

and diagnostic species in both the over- and understory,

The confluence of three interrelated criteria—species

as well as habitat conditions, in the association concept.

composition, structure, and habitat—conceptually

Despite their relatively high degree of ecological speci-

defines an association: it represents those plant assem-

ficity, associations must repeat across the landscape.† Indi-

blages that exhibit similar total species composition and

vidual occurrences of the same or different associations,

The association concept is intended to be fine enough to be useful for identifying specific, ecologically meaningful sites for conservation, but broad enough to be connected to landscape-scale processes and patterns. †

A few associations that are restricted to specific and unusual environmental conditions are exceptions to this general rule.

Seeing the Forest and the Trees: Ecological Classification for Conservation

7

© Charlie Ott and Gunther Matschke

Hawaiian cloud forests m

Death Valley m

Towering redwoods m

The Everglades m

Tallgrass prairies Encompassing the boreal and the tropical, alpine summits and deserts well below sea level, the United States is blessed with a great and beautiful variety of natural vegetation. Conceptualizing—even in a general way—the diversity and pattern of this vegetation across the landscape is a daunting prospect. Doing so in a way that helps us understand relatively small-scale but ecologically meaningful patterns is even Death Valley National Monument

more of a challenge.

however, may range greatly in size. For example, some west-

characteristics of the leaves, such as seasonality, shape,

ern grasslands occur naturally in patches of tens of thou-

and texture. These features are referred to as physiognomic

sands of acres. In contrast, southern Appalachian “beech

characteristics, and they are generally much more useful

gaps,” characterized by stunted, gnarled beech trees,

than species composition for rapidly categorizing veg-

often occur in sharply bounded mountaintop patches of

etation over large geographic areas (or in areas where

only a few acres.

little is known about the species composition).

Associations are grouped into the next level of the

At the coarsest level of the USNVC hierarchy,

USNVC hierarchy, the alliance, primarily on the basis of

the class, vegetation is divided into seven types: Forest,

having common dominant species in the upper stratum

Woodland, Shrubland, Dwarf-shrubland, Herbaceous,

of the vegetation. Alliances, in turn, are nested into

Nonvascular, and Sparse Vegetation. A summary of the

progressively coarser levels of the hierarchy, primarily

USNVC’s seven levels is provided in the box on the

based on characteristics related to the structure (height

facing page.

and spacing) and overall shape of the plants, and to

8

Seeing the Forest and the Trees: Ecological Classification for Conservation

© Julie Moore, TNC

A Summary of the U.S. National Vegetation Classification System LEVEL

PRIMARY BASIS FOR CLASSIFICATION

EXAMPLE (SEE PHOTO)

Class

Structure of vegetation

Woodland

Subclass

Leaf phenology

Evergreen Woodland

Group

Leaf types, corresponding to climate

Temperate or Subpolar Needle-Leaved Evergreen Woodland

Subgroup

Relative human impact (natural/semi-natural, or cultural)

Natural/Semi-natural

Formation

Additional physiognomic and environmental factors, including hydrology

Saturated Temperate or Subpolar Needle-Leaved Evergreen Woodland

Alliance

Dominant/diagnostic species of the uppermost or dominant stratum

Longleaf Pine -- (Slash Pine, Pond Pine) Saturated Woodland Alliance †

Association

Additional dominant/diagnostic species from any strata

Longleaf Pine / Little Gallberry / Carolina Wiregrass Woodland † The Longleaf Pine / Little Gallberry / Carolina Wiregrass Woodland. This association is found on flat, poorly drained sites on the Coastal Plain of North and South Carolina.



Rules for naming alliances and associations: The names of dominant and diagnostic species are used in alliance and association names. Those species occurring in the same stratum are separated by a hyphen ( - ); those occurring in different strata are separated by a slash ( / ). Species found less consistently either in all associations of an alliance, or in all occurrences of an association, are placed in parentheses. In most cases, the word “alliance” appears in alliance names to distinguish them from associations. Examples of alliance names: American Beech - Southern Magnolia Forest Alliance [Fagus grandifolia - Magnolia grandiflora Forest Alliance]; Longleaf Pine / Oak Species Woodland Alliance [Pinus palustris / Quercus spp. Woodland Alliance]. Examples of association names: Subalpine Fir / Grouseberry Forest [Abies lasiocarpa / Vaccinium scoparium Forest]; Saltgrass - (Saltmarsh Dropseed) Herbaceous Vegetation [Distichlis spicata - (Sporobolus virginicus) Herbaceous Vegetation].

Seeing the Forest and the Trees: Ecological Classification for Conservation

9

T

he number of currently defined USNVC types at

environments does this type occur? How much varia-

each level of the system’s hierarchy and the per-

tion (in structure and in species composition) is recog-

centage of associations in each class are presented

nized within the type? How does this type differ from

below. The number of types at the coarser levels of the

similar types? and, How does this type respond to dis-

hierarchy will not change significantly as the classifica-

turbances, both natural and human induced?

tion develops. However, changes will occur at the finest

Conservancy ecologists are providing answers to

levels as the existing alliances and associations are

these questions in the form of detailed descriptions of

refined and new types are added through additional

alliances and associations. Thus far, descriptions have

inventories and analyses.

been completed for about 80 per-

Each of the currently defined associations has a minimum set of information associated with it, including the association’s conservation status, and a list of states and U.S. Forest Service ecoregions

13

where it is known or thought to occur. However, to understand

What Is the Current Status of the USNVC?

cent of the currently defined alliances and about 40 percent of the associations. An example of a typical description appears on the facing page. It depicts the Blackjack Oak Eastern Red Cedar / Little Bluestem - Orange-Grass St. John’s -Wort

alliances and associations in depth and to recognize

Wooded Herbaceous Vegetation Association. Otherwise

them in the field, ecologists will need answers to more

known as the Shawnee Sandstone Glade, this midwestern

detailed questions about specific types. Important

savanna is naturally restricted to unusually thin-soiled

questions regarding each type include: In what types of

places where droughty conditions prevail.

Total Number of Vegetation Types at Each Level of the USNVC Level Class Subclass Group Formation Alliance Association

Number of Types 7 22 62 231 1,642 4,515

Note: Data shown are current as of August 1998 and represent types in the Natural/Semi-natural Subgroup only.

10

Dwarf-shrublands 3%

Herbaceous Vegetation 27%

Shrublands 16%

Woodlands 18%

Nonvascular Vegetation