Vegetatio 118: 131-137, 1995.
© 1995 Kluwer Academic Publishers.
Printed in Belgium.
Canadian wetlands: Environmental gradients and classification
S.c. Zoltai 1 & D.H. Vitt2
I
Forestry Canada, Northern Forestry Centre, Edmonton, Alberta, Canada T6H 3S5
Botanic Garden, The University of Alberta, Edmonton, Alberta, Canada T6G 2El
2 Devonian
Key words: Bogs, Ecological gradients, Fens, Marshes, Peatlands, Shallow lakes, Swamps,
tion, Water chemistry
Vegetation-Class!fi§f~'-:
Abstract
The Canadian Wetland Classification System is based on manifestations of ecological processes in natural wetland
ecosystems. It is hierarchical in structure and designed to allow identification at the broadest levels (class, form,
type) by non-experts in different disciplines. The various levels are based on broad physiognomy and hydrology
(classes); surface morphology (forms); and vegetation physiognomy (types). For more detailed studies, appropriate
characterization and subdivisions can be applied. For ecological studies the wetlands can be further characterized
by their chemical environment, each with distinctive indicator species, acidity, alkalinity, and base cation content.
For peatlands, both chemical and vegetational differences indicate that the primary division should be acidic,
Sphagnum-dominated bogs and poor fens on one hand and circumneutral to alkaline, brown moss-dominated rich
fens on the other. Non peat-forming wetlands (marshes, swamps) lack the well developed bryophyte ground layer of
the fens and bogs, and are subject to severe seasonal water level fluctuations. The Canadian Wetland Classification
System has been successfully used in Arctic, Subarctic, Boreal and Temperate regions of Canada.
Wetland classification
Classification is a process of organizing information by
forming groups within which the individual elements
are similar in some respects, but are distinctly different
from the elements of the other groups. However, the
purpose of classification must be clearly defined, as the
same objects, in this case wetlands, can be grouped differently, depending on the needs of the intended user.
The perceptions of a wildlife or waterfowl biologist,
an agronomist, a peat producer, a hydrologist, or an
engineer can be vastly different when considering the
same wetland. It was natural, therefore, that a number
of wetland classification systems have been developed
in Canada by different interest groups.
Some ecologists developed a phytosociological
classification of peatland vegetation (cf. Dansereau &
Segadas-Vianna 1952; Gauthier & Grandtner 1975).
Meanwhile, the mire classification developed in
Europe (Osvald 1925) has been used to include vegetation indicators and environmental gradients in Canada
(cf. Sjors 1952, 1969; Vitt et at. 1975; Gauthier 1980;
Damman 1986). The same principles were incorporated into a classification of Ontario wetlands (Jeglum
et al. 1974). A classification of the wetlands of the
Canadian prairies was developed (Millar 1976) primarily to fulfil the needs of waterfowl ecologists. An
organic soil classification system was developed by
the Canada Soil Survey Committee (1978), characterizing the peatlands by their soil properties. Yet another
classification system was developed to serve the engineering community (Radforth 1969), based on various
surface morphology and peat structure features.
This variety of classification approaches resulted
in restricted communication between the various user
groups. The Canadian Wetland Classification System
(CWCS) has been developed to facilitate communication between different disciplines concerned about
this segment of the natural landscape (Tarnocai 1988).
This classification system is based on categorizing
important ecosystem processes, such as water budget (amount and seasonality), carbon budget (peat
132
accumulation), and environmental parameters, such
as water quality (dissolved solids) and quantity, which
interact to allow the development of distinctive ecosystems. Such a classification is synthetic and was chosen because this places the emphasis on environmental
processes that determine the development of different
types of wetlands. The genesis and dynamics of wetlands are ill...·!" --t at the highest levels of the classification by grouph.o similar wetland functions. This aspect
is v:ery ~mportant for all wetlands, but especially for
pea,tlands that literally create their own environment
': oveFalOiigperiod of time.
The classification system has a hierarchical structure where the various categories represent different
levels of complexity. The higher levels have been
assigned multispectral (ecosystemic) definitive and
descriptive criteria, the lower levels having more specific criteria (Zoltai et al. 1975). The three higher
categories have readily recognizable definitive criteria
which allow their use by non-specialists. The formal
classification ends at a level which is fairly specific, but
still too generalized for specialized purposes. Specialists are encouraged to develop their classification that
is compatible with the ewcs, based on their interests:
ecosystems, floristics, hydrology, soils, engineering
qualities, etc. The ewcs provides a common platform
that allows the exchange of data and results between
different disciplines, using a common language.
The CWCS differs in its basic philosophy from the
classification system developed for the US Fish and
Wildlife Service (Cowardin et at. 1979). The CWCS
is based chiefly on wetland functions: interrelationships of the biotic and abiotic components of the wetland ecosystems. The resulting units have implications
of the hydrology, water quality, climate, vegetation
interactions and chronology (genesis) of the particUlar wetlands. The USFWS system, on the other hand,
is an objective approach that relies largely on observable features which require few process oriented decisions. It is designed to accommodate a wide range
of environments, from deep waters to bogs and from
rocky substrates to peatlands. This system answers the
questions of what and where, but does not provide a
framework for understanding the rationale of wetland
development.
In the ewcs a wetland is defined as land that is
saturated with water long enough to promote wetland
or aquatic processes as indicated by poorly drained
soils, hydrophytic vegetation, and various kinds of
biological activity that are adapted to a wet environment (Tarnocai et al. 1988). Wetlands include organic
MESOTROPHIC
Fig. 1. The five classes of the Canadian Wetland Classification
System in relation to the important chemical gradients. The vertical
axis is a qualitative expression of the influence of water depth.
wetlands (peatlands) and mineral wetlands (generally
non-peaty).
The broadest category of wetlands, wetland classes
are recognized on the basis of overall genetic origin of
the wetland ecosystem. Five wetland classes have been
recognized: bogs, fens, swamps, marshes and shallow
open waters (Fig. 1). They are defined and described
on the basis of distinctive abiotic parameters such as
hydrologic regime, water chemistry, or mineral material, which interact with the biota to form characteristic
vegetation cover, and in some classes, peat.
In the next lower category are the wetland forms,
which are based on surface morphology and pattern,
water quality, relationship to open water, and morphology of the underlying mineral soil, as expressed by the
ecosystems that are established on the wetlands. This is
an open-ended category, with new wetland forms being
described and defined as needed. The third category,
wetland types, is based on vegetation physiognomy.
The wetland is then named to identify all three categories, for example: a wooded (type) raised (form)
bog (class); or shrubby (type) patterned poor (form)
fen (class).
Wetland characteristics
Quantitative and qualitative differences in ecological
processes, operating within given climatic conditions,
are primarily responsible for differences in wetlands.
Whereas it is often difficult to measure ecological processes themselves, it is relatively easy to determine the
manifestations of these processes in terms of ecosystem characteristics. These characteristics in turn can
133
be used to define a synthetic classification system.- The
classification itself serves to reflect the fundamental
ecological processes.
The classification of wetlands should be based on
the factors that control wetland processes. Most of
these factors are present as gradients, and these are
often non-linear in form. Although these gradients are
often correlated with one another, they do not necessarily vary at similar rates or quantities. Despite these
complications, wetlands can be grouped into fundamental types that reflect basic ecological processes
(Figures I, 2). The Canadian System recognizes five
classes of wetlands that form nodes along several complex hydrological, chemical, and biotic gradients.
Shallow open waters are wetlands that exist whenever the water levels are sufficient to create habitats
for aquatic and floating vegetation. Although seasonal fluctuations in water level may expose the bottom
substrate, aquatic processes characterize this wetland.
Chemistry of the water does not differentiate this wetland class from the remaining four (Fig. 1), but may be
important in determining the type of shallow water vegetation that is present. These wetlands form a transition
to truly aquatic ecosystems and are largely influenced
by the adjoining aquatic system (Vitt & Slack 1975).
Marshes are treeless wetlands that are subject to relatively large seasonal water level fluctuations. These
wetlands are strongly influenced by either ground
water or surface water and have relatively high
amounts of water flow. As a result, available nutrients
(here considered only as nitrogen and phosphorus) are
abundant and vascular plant production is high, however decomposition rates are also high. Bryophytes are
nearly lacking, owing to their inability to compete with
vascular plants under eutrophic conditions and when
fluctuating water levels are present. The influence of
surface and ground waters generally serves to increase
base cations and HCO:;- , thus creating a well buffered
wetland. Trees are lacking due to either regional climatic conditions and/or the seasonally wet conditions.
As a result, marshes are wetlands that reflect eutrophic, temporally variable conditions that prohibit a well
developed bryophyte layer, largely due to high vascular plant production. Rapid decomposition, along with
the poorly developed ground layer, inhibits significant
peat accumulation. Chemical differences greatly influence the vegetation composition of marshes. Freshwater marshes are dominated by calcium and bicarbonate (and are alkaline systems), while saline marshes
have sodium and sulphate as their dominant ions (and
are thus saline and non-alkaline). Tidal marshes, with
very different vegetation, are dominated by sodium and
chloride ions (and thus are also saline and non-alkaline
wetlands) (Fig. 2).
Swamps have many similarities to marshes in that
swamps have strong seasonal water level fluctuations
and relatively strong water flow. Like marshes, the
bryophyte ground layer is poorly developed or lacking in most swamps. Both production and decomposition is generally high, and the seasonal lowering of
the water level permits the establishment of a welldeveloped tree or shrub layer. When peat is produced
in some quantity, it is well decomposed; however, in
general, peat accumulation is small. Tree development
often inhibits water flow and some swamps can become
acidic, especially in areas of acidic ground water. These
acidic treed wetlands rarely develop a bryophyte dominated ground layer due to the fluctuating water levels,
but stable water levels under dense conifer cover allow
the formation of a moss carpet. Peat accumulation is
small owing to rapid decomposition. Vegetationally,
swamps can be quite diverse, from deciduous alderash, to evergreen white cedar, to densely forested black
spruce, to cypress swamps farther south. All of these
wetlands, however, are characterized by the presence
of trees or tall shrubs, generally little peat accumulation
and extreme seasonal water level fluctuation.
Peatlands are distinct from non-peat forming wetlands by a combination of hydrological and biotic factors that function together to create conditions suitable
for decreased plant production and decreased decomposition. The stability of seasonal water levels restricts
total water flow through a wetland and allows a ground
layer of bryophytes to develop. Available nutrient
levels are low, as nutrients are accumulated in nonavailable forms by this ground layer. As a result, production of the vascular plant component is reduced, but
production of the ground layer increases due to active
nutrient sequestering by the bryophyte layer (Bayley
et at. 1987). Stable water levels and decreased nutrient
availability lead to a decrease in decomposition and
peat accumulation increases.
Fens are always bryophyte dominated wetlands that
are influenced by the chemistry of the surrounding
mineral soil deposits. Fens having bicarbonate (thUS
they are alkaline) as their dominant anion and calcium as their dominant cation are rich fens. These wetlands are characterized by brown mosses largely of the
134
OLIGOTROPHIC
MESOTROPHIC
EUTROPHIC
TOTAL NUTRIENT AVAILABILITY -+
PRODUCTION -+
DECOMPOSITION ~
DTreed
c:?J
Sphagnum
Q
Brown Mosses
Fig. 2. The relationship of bog, fen, marsh, and swamp wetland classes to the major chemical, biotic, and hydrologic gradients. Fens and
marshes are divided into several wetland forms in order to illustrate the pattern of variability of these classes. Saline wetlands may be either
fens or marshes.
family Amblystegiaceae, and an abundance of sedges.
Extreme-rich fens have pH above 7.0 (and are basic)
and often have deposits of marl in their wetter areas,
while moderate-rich fens have pH above 5.5 and below
7.0 (and are acid). These rich fens are strongly influenced by water from surrounding uplands and from the
larger watershed, and generally have sufficient flow to
allow the wetlands to be mesotrophic. Although data
are scarce, it appears that production of the ground
layer in rich fens is similar to that in poor fens and
bogs (Vitt 1990), while production of the vascular plant
component may be greater than in these latter wetlands
due to somewhat greater decomposition allowing more
nutrients to be made available. Poor fens (and bogs)
are acidic, non-alkaline wetlands that are dominated by
the genus Sphagnum. Sphagnum, through its abilities
to acidify and hold large quantities of water, restricts
both water flow and nutrient availability, which in turn
decrease vascular plant production and decomposition.
Poor fens, as rich fens, are influenced by geogenous
water. The presence of a well developed ground layer 'of Sphagnum serves to stabilize the wetland and
increase the acidity. From both vegetational and chemical viewpoints, poor fens are more similar to bogs than
they are to rich fens (MaImer et al. 1992); however,
the hydrology of poor fens has greater similarities to
that of rich fens.
Bogs are Sphagnum-dominated, ombrotrophic wetlands. They are acidic, largely due to humic acid
production during decomposition processes (Hemond
1980) and to cation exchange due to uronic acid production from Sphagnum, and non-alkaline. The base
cation content is limited and ombrotrophy greatly
restricts water flow. Nutrients are largely tied up in
Sphagnum and accumulated peat and mineralization
processes are reduced. Thus the rate of production
in the ground layer remains relatively similar to that
of fens seemingly due to internal cycling of nutrients
within the Sphagnum plants, while vascular plant production is limited.
Wetlands in Canada form distinct types along complex gradients. Especially important are: (1) Hydrology (in particular, seasonal water level fluctuation and
amount of water flow through wetlands); (2) Chemistry (in particular nutrient [N, P] availability, acidity
[hydrogen ion content], alkalinity [bicarbonate content], and base cation content rCa, Mg, Na, K]). Nutrient availability leads to classification of wetlands as
oligotrophic, mesotrophic or eutrophic; and (3) Biotic
135
(in particular the development of a ground layer dominated by either brown mosses or Sphagnum and the
presence of a tree layer). Proper classification of wetlands will lead to proper management and any classification system must recognize the complex interrelationships between the gradients formed from these
controlling hydrological, chemical, and biotic factors.
The ewcs has been used as the framework for evaluating wetlands for conservation or use in Canada (Bond
et at. 1992).
The needs of particular disciplines can be accommodated by combining the CWCS with parameters
that are important to that discipline. It is evident from
wetlands of west-central Canada that data sets of these
characteristics can be used to formulate wetland sites
into an ecologically relevant classification (Tables 1
and 2).
Inventorying wetlands
It must be clearly understood that wetland classification and the inventorying of wetlands are two distinct
and separate operations. Although the wetland classification should be used as an inventory and mapping
criterion, the purpose of the inventory, and the desired
detail and scale of the inventory map will dictate the
kinds of wetlands that will be recognized. Therefore,
a universal, global inventory of wetlands is possible
only if these parameters are determined in advance.
This is not an easy task, as circumstances vary greatly
around globe. In some countries with limited wetland
resources, the few wetlands may be very well documented, whilst in other countries, such as Canada, vast
tracts of wetlands are virtually untouched and never
studied.
The wetlands of Canada have not been accurately
inventoried. A small scale map was generated (National Wetlands Working Group (1986) from available published and unpublished information. In recent years,
however, a number of wetland inventories were initiated by various provinces that provide more accurate
information. The mapping units are generally compatible with or were inspired by the CWCS. The wetland
resources of New Brunswick were inventoried at a
scale of 1:50 000 (Airphoto Analysis Associates Consultants Ltd. 1975). The Ontario portion of the Hudson
Bay Lowlands were mapped at a scale of 1:250 000
(Pala & Boissonneau 1982). The peatlands of southern Quebec have been mapped and an atlas published
(Buteau 1989) at a scale of 1:250 000; the inventorying
of the wetlands of northern Quebec is under way. The
wetlands of Alberta (with the exclusion of southern
parts of the province) have been mapped at 1:250 000
scale, and a summary map was published at a scale of
1:1000000 (Vitt 1992). The mapping of the wetlands
of Manitoba is currently under way. In addition to wetland inventory maps, the surficial geology maps of the
northern Mackenzie River valley show the extent of
fens and bogs at a scale of 1:250000 (Duk-Rodkin &
Hughes 1992).
In our experience, technology must be combined
with field work in order to obtain reliable data on the
extent and kind of wetlands. Remotely sensed data in
the form of aerial photographs or spectral data (passive or active systems) are necessary to cover areas of
poor access. Although computer-enhanced remotely
sensed information shows promise (Pala & Boissonneau 1982; Anon 1989), the evaluation and validation
of this information is necessary before confidence can
be established. This rapidly developing technology,
however, dictates that this option must be monitored
for possible future use.
The use of aerial photos is a well-established and
tried method of wetland inventory (Buteau 1989; Vitt
1992). Wetland types can be readily identified to a
detail limited only by the scale of the photos. The thickness of the peat cannot be determined from air photos
with confidence, although relationships between peat
depth and topography can be established locally within climatic regions. Ground truthing is necessary to
determine the vegetation types, cation and nutrient
levels, in addition to peat thickness and stratigraphy
measurements. Ideally, a multi-tiered approach should
be applied: (1) field work limited to a small portion of
the wetlands; (2) expansion of this knowledge to large
areas through air photo interpretation; (3) extension of
the results of air photo study to a regional scale through
interpretation of remotely sensed information.
We feel confident that the CWCS can be applied
to Boreal, Subarctic, and Arctic regions of the Northern Hemisphere. The definition of the wetland classes
is compatible with that of northern European wetland
ecologists. The wetland forms are especially useful for
air photo interpretation, and new forms can be defined,
as needed. The broad vegetation physiognomy, used
to identify wetland types, should not pose any diffiCUlty. However, we lack the necessary experience to
judge the relevance of CWCS to wetlands of lower
latitudes.
136
Table 1. Variability in surface water chemistry in the wetland classes from a small region (Elk Island National
Park) of central Alberta, Canada (Nicholson 1992; n = 331).
Bog
Fen
Swamp
Marsh
3.5-3.6
5.2-6.4
pH
Reduced Conductivity* (J.LS cm- I )
Ca (mg I-I)
4.0-6.2
5.9-6.1
16-27
40-160
230-330
160-530 (900)**
4-7
2-33
Na (mg I-I)
2-3
2-5
26-43
5-22
27-65
3-125 (800)**
Organic N (J.Lg I-I)
NO;- (J.LgI- I )
2900-3000
1350-2850
2000-3000
200-2500(6400)**
13-20
8-23
7-10
9-175
160-250
23-80
28-146
73-130 (6400)**
350-480
135-400
220-650
250-520 (1100)**
NHt (J.LgI- I )
P (total)(J.Lg I-I)
* Conductivity minus the effect due to hydrogen ions (Sjors 1952)
* * ( ) Data from saline marshes.
Table 2. Average chemical compOSItion of surface water in various peatlands in
west-central Canada (Zoltai & Johnson 1987). All data are from point measurements
at mid-summer. Standard error of the mean in brackets.
Bogs
Poor fens
Moderate-
Extreme-
rich fens
rich fens
No. sites
71
33
147
18
Depth to water table
37
12
11
5
(1.6)
below surface (cm)
(1.8)
(4.0)
(1.2)
pH
4.5
(.02)
4.8
(.04)
5.8
(.05)
6.5
Reduced conductivity* (J.LS cm- I )
62
(3)
53
(7)
212
(12)
374
Ca(mgl- I )
2.04
(.18)
2.90
(.63)
24.98
(1.59)
53.60
(5.29)
Mg(mgl- I )
0.87
(.12)
1.19
(.32)
10.16
(.69)
14.20
(1.24)
Na(mgl- 1)
2.59
3.89
(.28)
(.93)
4.75
(.36)
6.54
(.87)
0.17
(.018)
0.14
(.024)
0.13
(.046)
0.12
(.020)
1.42
1.26
(.19)
1.42
(.11)
0.97
(.20)
P(mgl- 1)
K(mgl- 1)
(.10)
(.12)
(33)
* Conductivity minus the effect due to hydrogen ions (Sjors 1952).
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