3.0 Vegetation Classification: Background

3.1 What is Classification?

The objective of classification is to group together a set of
observational units on the basis of their common attributes (Kent and
Coker 1992). The end product of a classification should be a set of
groups derived from the units of observation where, typically, units
within a group share more attributes with one another than with units in
other groups. For vegetation classification, the unit of observation is
typically the \"stand,\" defined as a relatively homogeneous area with
respect to species composition, structure, and function.

The process of classifying a particular type of vegetation on the
landscape requires a clearly defined objective for the classification
and a familiarity with the variability across its range. If the
objective of a study is to create an independent vegetation
classification system, attribute data on species, cover, vegetation age
and structure, leaf characteristics, bark characters, dispersal
mechanisms and life history traits should be collected and organized. If
the objective is to classify ecosystems, data on the key environmental
features such as soils, hydrology, landform, etc., need to be collected.
The biological and environmental information to be collected, organized,
and described must be carefully chosen to meet the objectives of the
classification.

**3.1.1 Community Units and Continua**

Within the Anglo-American ecological tradition, there has been a
disinterest in classification *per se*. Beginning with the viewpoint of
Gleason (1917, 1926), extended by others, including Whittaker (1956,
1962) and Curtis (1959), it is held that vegetation units cannot be
defined; species comprising a community respond individually to
environmental gradients and to each other. Whittaker (1962) referred to
this viewpoint as the \"individualistic dissent.\" The question often
became polarized between the \"continuum concept\" and the \"community
unit concept.\" The argument is still presented in such a polarized
light today, despite efforts to broaden the discussion (Moravec 1992,
Roughgarden 1989).

Despite the polarized viewpoints, several features of communities are
widely recognized (Mueller-Dombois and Ellenberg, 1974):

> • Similar species combinations recur.
>
> • No two stands (or sampling units) are exactly alike.
>
> • Species assemblages change more or less continuously if one samples
> a geographically widespread community throughout its range.

Thus, recurring species combinations are variously correlated with their
environment, and these shift geographically. Austin (1991) considered
that vegetation units will be most interpretable within certain
landscape regions. In sum, an ordering is possible, but within limits.
Vegetation classifications often require a predetermined consistency
that does not do justice to the complexity and variability of the units.
The same may be said for land classifications. The goal of
classification is to determine the relative degree of similarity and
dissimilarity among units while recognizing that the communities are
distributed on a continuum across the landscape.

3.2 Review of Different Approaches to Classification

Many vegetation classification systems have been developed, but three
have gained widespread acceptance: physiognomic classifications,
floristic classifications, and site or ecosystem classifications (Howard
and Mitchell, 1985). The intent of all three is to provide a systematic
ordering of vegetation or ecosystem pattern and to relate these patterns
to ecological processes. Following is a brief survey of various
classification systems and a description of their strengths and
limitations.

**3.2.1 Vegetation Classifications**

***3.2.1.1 Physiognomic Methods***

Beginning in the nineteenth century, with the work of plant geographers
such as Humboldt, Warming, and Grisebach, vegetation classification
focused on the outward appearance or physiognomy of the vegetation.
Broadly speaking, physiognomy refers to structure (height and spacing of
the vegetation) and life forms of the dominant species (the gross
morphology and growth aspect of the plants). In addition, physiognomy
refers to characters of seasonality, leaf shape, phenology, duration,
etc. These features are easily recognized in the field and can be
applied with little knowledge of the flora. In addition, they permit
generalizations about the vegetation at a coarse, often worldwide scale.

The basic unit of several physiognomic classification systems is the
\"formation.\" a \"community type defined by dominance of a given growth
form in the uppermost stratum (or the uppermost closed stratum) of the
community, or by a combination of dominant growth forms\" (Whittaker
1962). In practice, formations are defined by varied, conventionally
accepted combinations of growth-form dominance and characteristics of
the environment. \"Cold-deciduous alluvial forests,\" \"evergreen
subdesert shrublands,\" and \"alpine meadows\" are examples of
formations.

The predominance of certain physiognomic types in a region tend to
correspond to major climatic zones. Thus, physiognomic categories are
often expressions of macroclimate, soils, and vegetation (Holdridge
1947, Walter 1985, Howard and Mitchell 1985). As a result, broad-leaved
evergreen trees tend to be found in tropical climates, evergreen
needle-leaved trees tend to be found in boreal climates, etc.
Physiognomic features provide a fast, efficient way to categorize
vegetation, can often be linked to remote sensing signatures, and are
useful for initial reconnaissance of areas requiring survey.
Physiognomic classification systems generally emphasize a divisive (or
\"top-down\") approach, subdividing coarse vegetation patterns into
units suitable for small-scale assessment. In addition, physiognomy
reflects the effects of disturbance and management (such as grazing or
fire), though in a relatively coarse way.

In the twentieth century, the physiognomic traditions of Warming and
others were expanded in several directions (as described in detail by
Whittaker 1962 and Shimwell 1971). In Europe, Brockman-Jerosch and Rubel
(1912) and Rubel (1930) emphasized physiognomy together with species
dominance. Their methods were expanded by Fosberg (1961), Ellenberg and
Mueller-Dombois (1967) and the United Nations Educational, Scientific
and Cultural Organization (UNESCO 1973). In the United States, Clements
(1916, 1928), and later Braun (1947, 1950) identified broad-scale
regional formations, described by major dominants sharing the same
physiognomy. More appropriately called \"vegetational regions,\" these
units described what were thought to be the \"climatic climax types,\"
areas of vegetation that were typical, mature phases of the vegetation.
Other recent descriptions of vegetation in the United States that
emphasize physiognomic units can be found in Vankat (1979), Barbour and
Billings (1988), and Barbour and Christensen (1993). In Great Britain,
the work of Moss (1913), Clements (1916), Watt (1934), and Tansley
(1939) described both climatic climaxes and edaphic climaxes, areas of
vegetation occurring on different soils within the same climate
(poly-climax types). In the tropics, structural profiles of the
vegetation were described in detail and physiognomic units characterized
the layers (Richards 1952, Beard 1955, Cain *et al*. 1956).

Mapping standards improved as cartographic techniques summarizing
vegetation structure through symbols were developed by Dansereau (1951)
and Kuchler (1949, 1967). Kuchler\'s (1964) work led to a physiognomic
vegetation map of the United States that has received widespread use and
management application (Klopatek *et al*. 1979, Crumpacker *et al*.
1988).

***3.2.1.2 Floristic Methods***

Whereas most physiognomic methods emphasize attribute patterns of
dominant species groups in the vegetation, floristic methods
characterize the species themselves. The basic floristic unit is the
\"association,\" defined by Flahault and Schroter (1910) as \"a plant
community of definite floristic composition, presenting a uniform
physiognomy, and growing in uniform habitat conditions.\" This
definition implies that associations that share a certain physiognomy
would be grouped together into the same formations.

In defining associations, some floristic methods focus on species that
occur constantly throughout a set of stands, while others emphasize
indicator or diagnostic species, species that are dominant or restricted
to these stands. Floristic methods require intensive field sampling,
detailed knowledge of the flora, and careful tabular analysis of stand
data to determine the constant or diagnostic species groups. Floristic
methods reflect local and regional patterns of vegetation and are more
detailed than physiognomic methods. They also provide detailed
descriptions of biotic communities regardless of their successional
stage or origin. As such, they are typically organized by an
agglomerative (or \"bottom-up\") approach, with lower units being
combined into higher ones. Floristic composition is often correlated
with soil or landform patterns. Thus, floristic units have been used
frequently as indicators of ecosystem processes and are a useful
component of ecosystem classifications (Mueller-Dombois and Ellenberg
1974, Rowe 1984, and Strong *et al*. 1990).

Early twentieth century ecologists who favored a strict floristic system
included members of what has been termed the Zurich-Montpellier
Tradition in central Europe (see Shimwell 1971). The most well known
among them is Braun-Blanquet (1928, 1932, 1951), whose work established
a formal approach to the floristic classification of vegetation. The
Braun-Blanquet system has been explained in detail by Poore (1955),
Becking (1957), Whittaker (1962), Mueller-Dombois and Ellenberg (1974),
and Westhoff and van der Maarel (1973). Initially, floristic data
(composition and cover of species) are collected from stands using plot
methods. The plot, a relèvé, is placed in an area of the stand that is
considered to be representative of the vegetation of the entire stand.
The plot data are then compiled into tables (species by plots), and the
species are sorted to identify those that co-occur in certain patterns.
Based on this analysis of the plot data, stands can be grouped into
associations. The associations can then be compared to one another to
determine which groups of species best exemplify the association, either
by being dominant or restricted to the association.

Species that are common to several associations can be used to assemble
the associations into broader groups. For example, the Braun-Blanquet
approach groups plant associations with common diagnostic species into
units called \"alliances.\" In this way associations can be arranged
into a hierarchy based on floristic composition. Mueller-Dombois and
Ellenberg (1974) note that the association concept has become more
narrowly defined as more information is gathered in a region. They
consider the alliance level, where species with more widespread
distribution are used to identify groupings, a more easily defined unit
at the regional level and useful for orientation with respect to
floristic composition.

Ecologists in northern Europe initially emphasized floristic differences
between the vegetation layers rather than the overall floristic list,
but they subsequently adopted an approach similar to that of
Braun-Blanquet (Whittaker 1962). In England, less effort was expended in
formalizing the use of floristics, and more on basic description for the
purposes of vegetation dynamics (Tansley 1939, Watt 1947). Recent
efforts by Rodwell (1991) emphasized species constancy to define
associations, and represents a substantial contribution to a fully
developed floristic classification of British vegetation. Until
recently, floristic classifications in the United States have only
focussed on very local areas.

***3.2.1.3 Potential versus Existing Vegetation***

When identifying objectives for a classification, it is important to
decide whether the classification is intended to portray existing
vegetation or potential natural vegetation (PNV). Classifications
emphasizing existing vegetation determine their vegetation units based
on the current characteristics of the vegetation regardless of the stage
of development. Stands are classified according to their characteristics
at the time the sample is collected. The selection of the stands for
sampling, however, may be weighted to those considered most natural.

Classifications emphasizing potential natural vegetation use vegetation
characteristics that represent the most mature and stable endpoints of
vegetation development. In the words of Tuxen (1956, in Mueller-Dombois
and Ellenberg 1974), potential vegetation becomes \"the vegetation
structure that would become established if all successional sequences
were completed without interference by man under the present climatic
and edaphic conditions (including those created by man).\" Thus the
vegetation units are hypothetical units that are thought to indicate a
site\'s potential for developing certain kinds of vegetation. These
units are based on known current relationships between vegetation and
site characteristics, such as soils or landform. They can be used to
great advantage by land managers faced with a landscape where much of
the vegetation has been removed. However, PNV units are limited by the
current knowledge of vegetation-site relationships, and the ability of
vegetation *per se* to infer site characteristics. They also emphasize
hypothesized climax vegetation, a concept fraught with theoretical
difficulties.

The best known portrayal of potential natural vegetation is that of
Kuchler (1964), who mapped the potential natural vegetation of the
United States at a scale of 1:3,168,000 and (in 1985) at a scale of
1:7,500,000. This map is limited in its focus to only mature types.
Thus, for example, extensive natural stands of trembling aspen are not
portrayed on the map because these are not considered climax types.

**3.2.2 Site Classifications**

***3.2.2.1 Site Classifications Emphasizing Vegetation***

Site classifications are intended to reflect the potential of a
particular site to support various types of vegetation. A number of
different site classification systems have used vegetation only to
determine the site potential, usually with reference to successional
trends or productivity. In this sense, these systems focus on potential
natural vegetation.

Site classifications emphasizing vegetation have been developed in
concert with the development of physiognomic and floristic
classifications. Cajander (1909, in Shimwell 1971) noted how the same
understory composition could occur under different canopy dominants in a
system of \"forest site types.\" He inferred that ground vegetation is
more representative of site factors than are canopy dominants and worked
with others to describe ecological series of communities along
environmental gradients.

A widespread approach to site classification using vegetation is that of
the habitat type classification system (Daubenmire 1952, Pfister and
Arno 1980, Kotar *et al*. 1988). This system focuses on natural climax
or near climax vegetation with an emphasis on all understory species as
a faithful reflection of site characteristics. Relationships between
vegetation and the soils or landform factors are established during and
after the classification process, but these factors are not used to
define the vegetation units (Komarkova 1983). The units described are
natural ones, but the emphasis is on determining vegetation units that
represent \"ecologically equivalent landscapes\" (Kotar *et al*. 1988).
Insofar as they describe the floristic composition of part of the
natural vegetation, namely climax stands, the units of the habitat type
are fairly equivalent to the plant association concept (Komarkova 1983).
The intent is to use these descriptions to visit sites that are not at
climax and, by examining their understory composition, to infer their
ecological potential.

Somewhat different from the habitat type approach is that of ecological
species groups, which are species that show similar \"ecological
behavior.\" Generally these species belong to the same layer of
vegetation (e.g., the herb layer, nonvascular layer, or shrub layer).
The method presumes that communities are combinations of plant species
whose composition is dependent on the local environment (Mueller-Dombois
and Ellenberg 1974). The community unit identified can, at times, be
very similar to the plant association level, whereby the ecological
species groups are the diagnostic species for the association. However,
it is also possible that the same association could contain several
ecological species groups (Mueller-Dombois and Ellenberg 1974). The
ecological species group information can either be used by itself to
indicate site characteristics, in which case the system partially
resembles the habitat type system, or it can be integrated with other
measured site factors as part of an ecosystem classification (Pregitzer
and Barnes 1982, Cleland *et al*. 1994).

***3.2.2.2 Site Classifications Emphasizing Multiple Factors***

Site classification systems that use multiple factors have as their
focus the subdivision of land into major and minor land types or
landscape ecosystems. They have been developed primarily for land
managers who need to integrate resource management, biological
conservation, and restoration planning. They are also used for
comparisons of productivity, species distributions, and interactions.
These systems are most appropriate for classifying ecosystems, defined
by the dynamic interactions of the biotic and physical components.
Ecosystems are treated as \"layered, volumetric segments of the
biosphere\" (Barnes *et al*. 1982, Rowe 1984). As with vegetation
classifications, emphasis is placed on units that are more or less
homogeneous both as to form and structure, but in this case with respect
to all factors of the land and the vegetation supported thereon (Rowe
1961).

An ecosystem approach to classification, namely that the plant community
is considered together with its environment, was implicit in Clements
work (1916), but was defined explicitly by Tansley (1935) and similarly
by Sukachev (1945) as \"biogeocoenosis.\" Central to the application of
the approach is that all parts of the system are included. In some
systems, each part---vegetation, soils, climate and landform - is first
studied independently and then combined (Jones *et al*. 1983, Sims *et
al*. 1989, Driscoll *et al*. 1984). For others, it is considered
essential that the parts be combined at the outset, since it is their
joint interactions on the landscape that define the units. It is
difficult to bring together all of the multiple factors jointly beyond
the local level and understand their interactions. Thus, the units are
considered hypotheses in need of further testing (Albert *et al*. 1986).
Mapping is a key step in the process (Rowe 1984, Zonneveld 1989).
Bailey\'s ecoregional map of the United States (1976, 1994) is more like
the independent approach, as he relies heavily on separate climatic,
physiographic, and vegetation maps and then reconciles their boundaries.
The work of Albert *et al*. (1986) and Cleland *et al*. (1994) represent
more of the combined approach.

The biogeoclimatic zone system of Krajina (1965, in Mueller-Dombois and
Ellenberg 1974) is another system in which vegetation is emphasized in
defining landscape or ecosystem units. These zones are defined as
geographic areas that are predominantly controlled by the same
macroclimate and contain similar soils and (climatic climax) vegetation.
The definition of the zones at lower scales utilizes vegetation units
that are defined by the plant association concept (Pojar *et al*. 1987).
At higher levels, climatic zones and topographic position are used to
help group vegetation units into the biogeoclimatic zones.

**3.2.3 Land Cover Classifications**

Land cover classifications are primarily intended for land management or
resource planning. They emphasize conspicuous features of the land
surface, and can be combined with land-use maps to convey an overall
perspective on what is visually present on the land. As such, they often
rely on characters that can be seen by remote sensing images (Witmer
1978).

To a certain extent, land cover classifications can draw from units
defined by physiognomic classifications (Anderson *et al*. 1972). For
example, forest cover types are a \"descriptive classification on forest
land based on present occupancy of an area by tree species. They are
named by characteristic dominants that recur over tens of thousands of
hectares,\" (Eyre 1980). Since physiognomic units also emphasize the
dominant features of the vegetation (see above), there is some overlap
in perspective.

**3.2.4 Combined Classification Approaches**

There are many commonalities among these classification systems. For
example, site classifications include considerable vegetation
information that is collected in the same way that would be used for
vegetation classification (Mueller-Dombois and Ellenberg 1974, Pregitzer
and Barnes 1982). Similarly, habitat type classifications define plant
associations in a similar manner to that of the floristic system of
Braun-Blanquet. Furthermore, site classifications that bring together
independent vegetation, soil, and landform classifications rely on the
independent classification of these variables as their starting point
(Jones *et al*. 1983, Sims *et al*. 1989).

***3.2.4.1 Physiognomic--Floristic Approaches***

The principle underlying physiognomic classification is that each
specific life form has a strategy (Stearns 1976) which has been selected
under similar ecological pressures, and that the composition of life
forms in a vegetation type is governed by these strategies (Monsi 1960,
Raunkier 1904, Walter 1973, Whittaker 1975). Since physiognomic
attributes are borne by individual species, recognition of a
physiognomic assemblage depends on the co-occurrence of species in a
given area. The co-occurrence of species leads to specific physiognomic
vegetation types that can be delineated as discrete units in the
landscape. As such, the physiognomic types can be related to floristic
classifications that include the total composition.

The advantages of the separate components of the physiognomic and
floristic approaches to classifying vegetation have been presented
above. An important reason for combining these approaches is that
vegetation is most thoroughly described by both structure and floristic
composition. Physiognomic systems are easily recognized in the field,
can be applied with little knowledge of the flora, permit
generalizations of vegetation patterns over large areas, and can be
linked to remote sensing signals to facilitate vegetation mapping. These
attributes allow the identification of patterns where little is known
about an area, or more detailed survey is impractical. Floristic
information, however, is almost always used for detailed site analyses,
whether for studying environmental gradients, ecological site factors,
or describing and forming classification units. Patterns of succession,
disturbance, history (including paleo-ecology), and natural assemblages
are better assessed through floristic composition than physiognomy.

A fully developed classification is most readily developed by combining
physiognomy and floristics. This type of system allows the geographic
orientation of physiognomic characters to be tied to the more local site
specific information of the floristic characters. In combination, these
systems can satisfy a broader range of objectives for use of the
classification system. In particular, the combined
physiognomic--floristic approach has the desirable attribute of
producing mappable units with significant ecological meaning.

The rationale for such a coupling of systems has been developed over the
years (e.g., Rubel 1930, Ellenberg 1963, Webb *et al*. 1970, Wergner and
Spangers 1982, Westhoff 1967, Westhoff and Held 1969, Borhidi 1991).
These studies have found a very good fit between floristic and
physiognomic classifications of the same areas because both types of
attributes are borne by individual species. Whittaker (1962, p. 137),
despite his hesitation on the usefulness of vegetation classifications,
provided guidelines on the development of a physiognomic--floristic
system, when such systems were warranted. He fully expected that plant
associations, ecological species groups, and habitat types could be used
to develop flexible, but consistent community units. In the United
States, Driscoll *et al*. (1984) recommended the development of a joint
system using the physiognomic units of UNESCO (1973) and the floristic
units of habitat types, of which an example has recently been provided
by Dick Peddie (1993) in New Mexico. Strong *et al*. (1990) in Canada
also proposed a combined physiognomic--floristic approach. The list of
plant communities which was used to map the vegetation of Australia\'s
National Parks and Reserves was developed by Specht *et al*. (1974)
using a joint physiognomic--floristic approach.