2 Functional Characteristics and Indicators of Boreal Peatlands

Dale H. Vitt

2.1 Introduction

Globally, peatlands occupy about 4 × 106 km2 (Gorham 1991; Joosten andClarke 2002), with the boreal and subarctic peatland area estimated to beapproximately 3.46 × 106 km2, or about 87 % of the world’s peatlands. Sixcountries have greater than 50,000 km2 of peatland and these account for93 % of the world’s peatlands – five of these countries are predominantlyboreal. Russia contains 1.42 × 106 km2, Canada 1.24 × 106 km2, the USA625,000 km2, Finland 96,000 km2, and Sweden 70,000 km2; in addition,Indonesia has an estimated 270,000 km2 (Joosten and Clarke 2002).

The term peatland, commonly used in the North American literature, isused interchangeably in this chapter with the European term mire.Although recently Joosten and Clarke (2002) have given slightly differentmeanings for these two terms, here I consider the terms to be synony-mous. Peatlands are ecosystems in which net primary production (NPP)has exceeded decomposition over thousands of years, and as a resultorganic matter, rich in carbon, has accumulated over time, and peat isformed. Although peat-forming plant communities occur in most of theworld’s nine zonobiomes (Walter 1979), they are most prevalent in zono-biome VIII (cold-temperate boreal climate), or more commonly termedthe boreal forest. Zonobiome VIII is characterized by short summers andlong winters, coniferous, evergreen vegetation, and podzolic upland soils(Chap. 1). Precipitation is variable, ranging from annual total precipitationof 2,000 to about 250 mm; however, because of short cool frost-free sea-sons even when precipitation is quite low it exceeds potential evapotran-spiration.

Ecological Studies,Vol. 188R.K.Wieder and D.H.Vitt (Eds.)Boreal Peatland Ecosystems© Springer-Verlag Berlin Heidelberg 2006

2.2 Peatland Initiation

Across the boreal zone, peatlands originate by four processes (Chap. 3).The most common appears to by paludification (or swamping) whereinpeat forms on previously drier, vegetated habitats over inorganic soils andin the absence of a body of water, generally owing to a regional water tablerise and associated climatic moderation. Additionally, local site factorsmay have strong influences on paludification (Almquist-Jacobson andFoster 1995; Anderson et al. 2003). Secondly, peat may form directly onfresh, moist, nonvegetated mineral soils. This primary peat formationoccurred directly after glacial retreat or on former inundated land that hasrisen owing to isostatic rebound. Thirdly, shallow bodies of water maygradually be filled in by vegetation that develops floating or groundedmats – these mats filling in, or terrestrializing, the former aquatic habitat.Both lake water chemistry and morphometry, as well as species of plantsin the local area, influence the rates and pathways of vegetation succession(Kratz and DeWitt 1986; Vitt and Slack 1975; Wilcox and Simonin 1988).Fourthly, peat may form and be deposited on shallow basins once occu-pied by early Holocene lakes. These former lake basins, lined with vege-tated impervious lake clays, provide hydrologically suitable sites forsubsequent peat development. Although the last process has not been rec-ognized previously, I discuss it briefly as it may be common in glaciatedareas on flat terrain.

2.3 Critical Factors for Peatland Diversification

Beginning in the early 1900s, many authors have described peatlands anddiscussed the factors that they perceived as of primary importance topeatland development and persistence. Many of these early studiesincluded boreal peatlands or peatlands just south of the boreal zone. Fun-damental classic papers include those by Cajander (1913) in Finland,Weber (1911) in Germany, Clements (1916) in the USA, Lewis and Dowd-ing (1926) in Canada, von Post and Granlund (1926) in Sweden, Katz(1930) in Russia, and Tansley (1939) in Great Britain. Later in that century,studies by DuReitz (1949), Sjörs (1950), Ivanov (1957), Ruuhijärvi (1960),Eurola (1962), and Heinselman (1963) had international importance.These papers (and many others) have provided much of the basis for ourunderstanding of peatland function and diversity. Here I attempt toreview these primary factors within the framework of the boreal zone.

Five factors appear to have great significance on how peatlands func-tion– hydrology, climate, chemistry, substrate, and vegetation/flora. All of

D.H. Vitt10

these have been used to varying degrees to develop classifications of peat-lands and to explain how peatlands are distributed.

2.3.1 Hydrology

Peatland persistence depends on a constant, long-term water supply, andthe origin of this water influences the form and function of peatlands. Fol-lowing the ideas of Weber (1911), von Post and Granlund (1926), and Sjörs(1948), water arriving at a peatland can be derived from a variety ofsources. Diagrams and useful discussions of these hydrological types canbe found in Damman (1986) and Mitsch and Gosselink (2000). Some peat-lands are influenced only by waters derived from rain and snow (ombroge-nous), whereas other peatlands are dominated by ground and surfacewaters (geogenous). Geogenous waters can be of three types depending onthe movement and source: (1) stagnant water –generally soil water, butstagnant bodies of water are included here as well (topogenous); (2) flow-ing water – especially sheet flow on gentle slopes, including seepages andsprings (soligenous); (3) flood water – especially from water courses thatresult in lateral flow away from the direction of stream flow (limno-genous).

Geogenous waters of all three types contain dissolved cations andanions (“minerals”) and this minerotrophy (DuReitz 1954) exerts a stronginfluence on the vegetation, flora, and function of peatlands, and formsthe ecological basis for the term fen. Ombrogenous waters have lower con-centrations of dissolved cations and anions and provide the ombrotrophicconditions suitable for the development of a bog. Thus, from a hydrologi-cal perspective, fens are geogenous and bogs are ombrogenous. From anecosystem point of view, fens are minerotrophic, while bogs are ombro-trophic. In addition to the fundamental role played by hydrology(Chap. 13), both bogs and fens are also strongly influenced by local cli-mate, regional substrate and geomorphology, prevalent regional vegeta-tion and flora, and water chemistry, and as a result occur on the landscapein a number of different types. These individual peatland types oftenoccur together and form large peatland complexes containing a wide vari-ety of peatland types (Vitt et al. 2003).

2.3.2 Climate

Ruuhijärvi (1960) and Eurola (1962) for Finland, Damman (1979) for east-ern North America, Botch and Masing (1979a, b) for Russia, Sjörs (1983)for Sweden, and Vitt et al. (2000) for western Canada have provided mapsor diagrams for the broad geographical distribution patterns of the major

Functional Characteristics and Indicators of Boreal Peatlands 11

peatland complexes in the boreal zone. Detailed reviews of these regionalpeatland complexes are in Gore (1983) and Keddy and Fraser (2005). InRussia, the Vasyugan peatland complex covers about 5.27 × 106 ha (Ini-sheva and Golovatskaya 2002) and is the largest continuous peatland sys-tem in the world (Botch and Masing 1983). In Canada, two large areas ofpeatlands are present: the Hudson Bay Lowlands, with extensively paludi-fied fens, and the MacKenzie River Basin peatland complex in northwest-ern Canada, with permafrost bogs to the north and fen complexes to thesouth. In Fennoscandia, extensive aapamire complexes dominate theboreal zone.

At the regional scale, peatland complexes vary with climate. Across theboreal and subarctic zonobiome, the principal peatland complexes aregenerally correlated with climatic gradients of precipitation and tempera-ture (Fig. 2.1). Climate varies from relatively cool and moist conditions inthe south to climates with cold, relatively dry regimes in the north. In rel-atively cool, moist climates, ombrogenous plateau bogs and soligenousblanket fens dominate. These peatland complexes are abundant in coastalNorway, extreme southern Finland, Scotland, and coastal British Colum-bia and Newfoundland, Canada, as well as coastal Maine and southwest-ern Alaska, USA. Generally northward and inland, these coastal zones arefollowed by a zone of patterned bogs, with regional dominance of concen-tric and eccentric raised bogs (or basin bogs) especially dominant in moresouthern, subcontinental boreal areas such as southern Sweden and Fin-land, Atlantic Canada, Maine, and western (European) Russia. The pat-terned bogs are treeless or nearly so, dominated by species of Sphagnum(S. tenellum, S. capillifolium, S. austinii, and/or S. cuspidatum) that aremore or less restricted to oceanic/subcontinental climates, and that aregenerally associated with wet, minerotrophic laggs (or moats) and sur-rounding wooded fens.As climate becomes more continental, trees and/orshrubs become more abundant and water becomes less available at thebog surface; thus patterns are decreased. Some scattered trees (at least inNorth America and Siberia), the presence of scattered pools of water, anda lack of hydrological patterning characterize these subcontinental bogs.Glaser and Janssens (1986) proposed a model of peatland developmentthat attempted to place these regional peatland types into a broad succes-sional framework.As with the more oceanic peatland complexes, marginalwooded fens (Canada) and wooded swamps (the Sogua in Siberia; E. Lap-shina, personal communication) may be extensively developed aroundsubcontinental bogs. Across the midboreal and southern boreal zoneswhere continentality is further increased, bogs and fens characteristicallyoccupy isolated basins or form peatland complexes containing mixturesof fens and bogs. Continental bogs lack pools of water, are extensivelywooded (at least in North America but less so in Siberia), and lack domi-nance by the more oceanic/subcontinental species of Sphagnum. Domi-

D.H. Vitt12

nating in the midboreal and extending into the northern boreal forest isthe aapamire zone. In the broad sense, aapamires are minerotrophic, wetpeatland complexes. Aapamire complexes may have patterned fens (flarkaapamires), fen lawns (lawn aapamires), and/or small bogs (Laitinen et al.2005). These large patterned or unpatterned fen complexes always containmire expanse vegetation (Sjörs 1948) at their centers (Laitinen et al. 2005)and are extremely variable in chemistry and vegetation depending onlocal substrate conditions (see later). The aapamire zone is extensivelydeveloped in central Fennoscandia and northwestern Siberia, as well as toa lesser degree across Alaska and Canada. In continental Canada, exten-sive aapamire complexes contain open, patterned or unpatterned, fensalong with associated densely wooded bog islands and peninsulas. At thenorthern and most continental end of the climatic gradient are tworegional peatland complex types influenced by permafrost. Peat moundsand ridges with permafrost cores contained within an extensive matrix ofunfrozen minerotrophic fen characterize palsa mires. Patterned palsamires with parallel ridges, each with an ice core and alternating with

Functional Characteristics and Indicators of Boreal Peatlands 13

-5°

0°

+10°

Ave

rage

Tem

pera

ture

(°C

)

P recipitation (mm)200

+5°

-10°

600 1000 3500

Aapamire

C ontinental bog

S ubcontinental bog

P lateaubog

P atternedbog

Marsh

Open fen

P alsa-mire

P eatplateau

B lanketfen

Fig. 2.1. Relationship between climate (temperature and precipitation) and majorpeatland complexes of the boreal forest (derived from patterns presented by Sjörs1983, Ruuhjärvi 1983, Botch and Masing 1983, Vitt et al. 2003, Damman 1977, andZoltai and Pollett 1983; climate diagrams from Lieth 1999)

unfrozen pools of water usually filled with carpets of Sphagnum, maydominate the landscape. Palsa mires appear to be associated with areaswhere surface water is abundant. Peatland complexes having peat plateausor large solitary permafrost peat islands, often within a minerotrophic fenmatrix, replace palsa mires in areas where surface water is less abundantand permafrost more extensive. In the subarctic forest-tundra, these per-mafrost islands coalesce to form extensive frozen landscapes containingonly small residual pockets of unfrozen peat (collapse scars; Horton et al.1979). The latter are common in central Canada and northwestern Siberia,whereas the former are distributed widely in northern Fennoscandia,Russia, Alaska, and Canada. Finally, whereas western Siberia (east of theUral Mountains) west of the Yensiei River has abundant peatlands, thearea east of the Yenisei River is rather mountainous and peatland cover isless (E. Lapshina, personal communication)

2.3.3 Vegetation and Flora

In the 1940s, DuReitz (1954) described minerotrophic, acidophilous,Sphagnum-dominated plant communities that are relatively poor inspecies, and he termed peatlands having these communities “poor fens.”He also recognized that there exist minerotrophic plant communities hav-ing species with little tolerance for acidity. These “rich fen” communitiescontain quite a few species with high fidelity to circumneutral or calcare-ous conditions and are relatively rich in species. He termed these circum-neutral fens “moderate rich fens” and the calcareous fens as “extreme richfens” (DuReitz 1954). Rich fens for the most part lack a significant cover ofSphagnum; thus, the peat mosses dominate only in ombrotrophic bogsand minerotrophic poor fens (but see later). Rich fens, on the other hand,have ground layers dominated by a number of true mosses that owing totheir reddish-brown coloration have been called brown mosses. This divi-sion in ground-layer type as a critical difference between bogs and poorfens, compared with rich fens, has been emphasized by several authorsrecently (Gorham and Janssens 1992; Vitt 2000; Wheeler and Proctor2002) and all of these authors have suggested that the fundamental differ-ences in the flora between Sphagnum-dominated and brown-moss-domi-nated peatlands should provide the basic criterion for our classification ofpeatlands. However, it has long been noted that some circumneutral richfens can have some abundance of mesotrophic Sphagnum species and itmay be that Sphagnum-dominated rich fens may also deserve specialrecognition.

Since most species of the ground layer (bryophytes and lichens) havedisjunct, rather broad distributions, especially in the boreal region, theyallow direct comparisons of peatlands between Asia, Europe, and North

D.H. Vitt14

America. As far as I know, all species of peatland bryophytes occur in atleast two of these three boreal regions. In contrast, many of the abundantvascular plants occur in only one (or sometimes two) of these geographi-cal areas. Tree species that dominate peatlands in North America (Piceamariana in bogs and fens and Larix laricina in fens) do not occur in Eura-sia. Pinus sylvestris and Picea abies are restricted to Europe and westernAsia, whereas Larix gmelinii, Larix sibirica, Pinus obovata, and Pinus sibir-ica are known only from Asia. Species of Betula are largely differentbetween the three geographical regions; for example, Betula pubescens ischaracteristic of fens in Siberia, while B. glandulosa and B. glanduliferaare characteristic in North America. The dwarf shrubs have important dif-ferences, including Ledum groenlandicum (endemic to North America)and Calluna vulgaris and Erica tetralix [endemic to Eurasia (Calluna vul-garis is also known from Newfoundland)]. Additionally, many of the fieldlayer species are restricted to either North America or Eurasia, and few ofthe Carex species are disjunct between North America and Eurasia. How-ever, despite these floristic differences in the vascular plants, several criti-cal species do occur in both Eurasia and North America, includingChamaedaphne calyculata, Ledum palustre, several species of Vaccinium,including V. oxycoccos and V. vitis idaea, and Rubus chamaemorus.

2.3.4 Vegetation Comparisons

Although ombrotrophic bogs are characteristically uniform throughoutthe boreal zone, several key geographical differences do occur. In borealNorth America, Picea mariana dominates the tree layer; thus, all continen-tal boreal bogs have a dense canopy beneath which Ledum groenlandicumnormally dominates.As continentality decreases, Picea mariana decreasesin cover until North American oceanic bogs become treeless, or have scat-tered individuals of Thuja plicata on the west coast (Vitt et al. 1990). Incontrast, in eastern Asia, species of Larix (e.g., L. gmelinii and L. sibirica)dominate in bogs and fens, while in boreal Europe and western Asia, bogsare treeless or have only scattered individuals of Pinus sylvestris, with treecover increasing with continentality. In continental Siberia dwarf Pinussylvestris covers the bogs. Across the boreal zone, fens vary from com-pletely treeless (open) to shrub-dominated (mostly Betula and/or Salixspecies) to wooded (with open canopy); however, the occurrence of Larixlaricina only in North America allows most boreal fens on the continent tohave significant tree cover, whereas in Eurasia, fens are either open ordominated by a series of different flowering plant species. None of thesetree species, except Larix laricina, has the ability to tolerate the high waterlevels present in aapamires; as a result, patterned aapamires in NorthAmerica typically have a greater abundance of trees than they do in Eura-

Functional Characteristics and Indicators of Boreal Peatlands 15

sia. Also, abundant in Europe and western Russia are wooded swamps(Sogua or black alder swamps), while in western Siberia wooded swampsare highly variable, but have high species richness, especially in the fieldand ground layers.

In summary, in terms of vegetation, North American bogs and fens,especially throughout the continental boreal areas, are characterized by ahigher abundance of trees, while Eurasian peatlands are far more oftenopen or only have scattered individuals present. Comparisons betweencontinents are hampered by intercontinental floristic differences in thevascular plants, especially the lack of several key indicators on one or theother continents (e.g., Calluna vulgaris, Erica tetralix, Ledum groen-landicum). Intercontinental similarities in bryophyte floras, coupled withtheir highly specific habitat requirements (Gignac et al. 1991), make theseground-layer species highly suitable as peatland indicators.

2.3.5 Floristic Indicators

Perhaps the best integrators of long-term variability in minerotrophy andombrotrophy are the plants that exist at individual peatland sites. Lists oftypical indicator species for bogs and/or fen types are available in Ruuhi-järvi (1960) and Eurola (1962) for Finland, Sjörs (1983) for Sweden, andChee and Vitt (1989) for western Canada. Whereas the flora and vegeta-tion of peatlands have been well studied (Dierssen and Dierssen 2001) andare not dealt with in detail here, the fungi (Chap. 6) and fauna (Chap. 5)are less well understood and are treated in more detail. Integration ofindicators from these lists produces a short list of critical indicators ofpeatland types of the boreal region that largely span North America,Europe, and Asia (Fig. 2.3). Especially noteworthy are the following: forombrotrophy, Rubus chamaemorus, Sphagnum fuscum, and Calluna vul-garis (Eurasia only) are usually reliable. Warnstorfia fluitans is character-istic of bog pools. For minerotrophy, Betula glandulosa (North Americaonly), Sphagnum riparium, and numerous regional Carex species, includ-ing C. lasiocarpa, C. limosa, and C. rostrata, are indicated. Especially note-worthy as critical indicators are species of the moss family Amblyste-giaceae, particularly species that historically have been placed in thegenus Drepanocladus (sensu lato). Currently these are usually distributedamong several genera: Warnstorfia fluitans (bogs); W. exannulatus (poorfens); Drepanocladus aduncus (eutrophic habitats – marshes); Scorpidiumcossonii (extreme rich fens); Hamatocaulis vernicosus and H. lapponicus(moderate rich fens).

D.H. Vitt16

2.3.6 Substrate

Whereas bogs are hydrologically separated from influences of localgroundwater, fens are seasonally in contact with surface water (lake,stream, spring runoff over frozen ground) and/or groundwater. The sur-rounding bedrock and soil chemistry determine the chemistry of thewater flowing into the fen. For example, the highly impervious, graniticand metamorphic bedrock of eastern Canada, Fennoscandia, and theglaciated, alluvial sediments of western Siberia provide inputs poor inminerals and relatively rich in hydrogen ions, whereas the carbonate-richsedimentary bedrock of Alaska, western Canada, southwestern Siberia,and western Europe provides inputs rich in calcium, magnesium, and car-bonates (alkalinity), but poor in hydrogen ions. Thus, brown-moss-domi-nated fens dominate in carbonate-rich areas, while bogs and poor fensdominate in areas with acidic substrates.

2.3.7 Chemistry

Here fertility refers only to the dissolved inorganic forms of the nutrientsnitrogen (NO3

–, NH4+) and phosphorus (ortho-P). Across the boreal zone,

it seems that most pristine peatlands are nitrogen-limited. Only some verywet, rich fens or fens associated with lakes may be phosphorus-limited(Chap. 11). Nitrogen and phosphorus concentrations of upper pore watersof bogs are not significantly different from those of fens (Vitt et al. 1995),as would be expected for such nutrient-limited systems. However, greaterwater flow in fens may result in greater seasonal nutrient input. Thus, bogsand some fens (owing to either minimal flow or nutrient-poor surround-ing substrates) can be termed oligotrophic. Fens influenced by morenutrient-rich inflows and higher flow rates can have a mesotrophic nutri-ent status. Communities with eutrophic nutrient status are restricted tonon-peat-forming wetlands (marshes and southern swamps).

This oligotrophic–eutrophic nutrient gradient is independent of the“mineral” gradient composed of pH, base cations (Na+, K+, Ca2+, Mg2+),and associated anions (HCO3

–, CO32–, SO4

2–, Cl–) (Vitt and Chee 1990). Theformation of peat and the differentiation of various bog and fen typesoccurs along this mineral gradient (see Fig. 10.1 in Vitt 2002). Along thepeatland gradient, pH varies from 3.0 to 9.0, while concentrations of totalbase cations vary from nearly zero to more than 1,000 mg L–1. This mineralgradient, especially pH, has been used by many as the fundamental basisfor the differentiation and comparison of peatlands (Sjörs 1950; Naka-mura et al. 2002; Tahvanainen 2004). However, this pH gradient is compro-mised through additional minerals that covary and may be influenced by

Functional Characteristics and Indicators of Boreal Peatlands 17

pH itself; thus, as hydrogen ions decrease, base cations increase.Alkalinityand the form in which carbon is dissolved in the water are a function ofpH (Wetzel 1983), with bicarbonate (alkalinity) increasing from zero at orbelow pH 5.5 to about150–200 mg L–1 HCO3

– + CO32– at pH 8.0. The com-

plete absence of bicarbonate alkalinity below pH 5.5 is a fundamentaldividing point in the habitat limits of many peatland species. Fens withsurface waters having pH values less than 5.5 and little or no alkalinity aredominated by oligotrophic species of Sphagnum, and minerotrophic poorfens can be defined utilizing both chemical and floristic features. In fenswith pH above 5.5 and the presence of alkalinity, Sphagnum abundancedecreases and true mosses mostly dominate the ground layer.

In summary, boreal peatlands develop and persist owing to a complexset of regional and local factors. In particular, hydrology, climate, chem-istry, substrate, and flora/vegetation all contribute to the type of peatlandthat develops at a site and to the functioning of individual peatlands overtime. What follows is an outline of functional criteria for both wetlandsand for the dominant peatland types of the boreal forest. Grades are func-tional levels of organization that may be achieved through unrelated andvaried processes. Here, the term grade, long used in systematics, is appliedto peatlands in order to allow a more functional (ecosystem) view of peat-

D.H. Vitt18

Wetland Peatland Fen Bog

Marsh

Swamp

Poor

fen

Moderate

rich fen

Extreme

rich fen

Continental bog

Bog with internal lawn

Peat plateau

Palsa bog

Oceanic

bog

GRADES

Geo

geno

usH 2

0Sup

ply

Seaso

nally

Wet

Satur

ated

Soils

Positi

veH 2

OBal

ance

Stabl

eW

ater

Table

Mos

s dom

inan

ce

Cat

otel

m

Salin

ityD

ecre

ases

Olig

otro

phy

Trees

Drie

r Surfa

ce

Incr

ease

dA

crot

elm

Om

brot

roph

y

Spha

gnum

Local

ly

Con

tinuo

us

Ice

Frost

Mou

nds

Perm

afro

st

Pools

Clo

sed

Can

opy

No

Alk

alin

ity

pH<

5.5

Spha

gnum

Eutro

phy

Mes

otro

phy

Alk

alin

ityPre

sent

pH>

5.5

Mes

otro

phic

Spha

gnum

*

Mar

l

Iceco

re

Exten

sive

ice

Con

tinen

tal C

limat

e

Fig. 2.2. Grades (functional levels of organization) and criteria (bars) that define themajor boreal wetland types. Open bars define attributes for all units placed abovethe branch

land organization. Specific criteria identify each grade in Fig. 2.2, while theapparent floristic indicators for these criteria are provided in Fig. 2.3. Thisapproach attempts to tie individual floristic indicators to appropriate cri-teria for each functional wetland/peatland grade.

2.4 The Functional Grades of Boreal Wetlands

2.4.1 The Wetland Grade

In the boreal zone, wetlands form when soils are saturated throughout theyear or are seasonally wet over much or all of the growing season. Watersthat influence the wetland have been in contact with nonwetland soils andthus are enriched with a variety of minerals and nutrients. The local wet-land area is influenced by recharge hydrology. These conditions allow

Functional Characteristics and Indicators of Boreal Peatlands 19

Wetland Peatland Fen Bog

Marsh

Swamp

Poor

fen

Moderate rich fen

Extreme rich fen

Continental bog

Bog with internal lawn

Peat plateau

Palsa bog

Oceanic bog

GRADES

S.le

nens

e

Cla

dina

spp.

S.lin

dber

giiS.

balti

cum

mos

ses*

feat

her

Myl

ia

Mes

otro

phic

Spha

gnum

*

Brach

ythe

cium

mild

eanu

m

Cin

clid

ium

styg

ium

Cat

osco

pium

nigr

itum

Scor

pidi

umsc

orpi

oide

s

Scor

pidi

umco

sson

ii

Tom

enth

ypnu

mni

tens

Betul

agl

andu

losa

/pum

ila

War

nsto

rfia

fluita

ns

Rubus

cham

aem

orus

Dre

pano

clad

us

Plagi

omni

umsp

p.

adun

cus

anom

ala

War

nsto

rfia

exan

nula

tus

S.te

nellu

m

Car

exsp

p.(la

sioc

arpa

/lim

osa)

S.fu

scum

Ham

alic

aulis

vern

icos

us

Cal

lierg

ongi

gant

eum

Cam

pyliu

mstel

latu

m

Fig. 2.3. Some floristic indicators for the boreal wetland grades. Species are based onmy experience mainly in North America, but in most cases will be appropriate forEurasia. Feather mosses are Pleurozium schreberi and Hylocomium splendens.Mesotrophic sphagnums are S. subsecundum, S. warnstorfii, S. teres, S. subnitens, andS. contortum

organic matter to accumulate to a variable degree, but variable watertables, high nutrient inputs, and wetland plant species allow carbon andnitrogen to be mineralized at rates that prevent development of the anaer-obic portion of the peat column (catotelm) and the buildup of peat. Whenthese conditions are present, wetlands of two types may form – swampsand marshes. Wetland ecosystems are characterized by the developmentof complex woody canopies (deciduous swamps) or by a dense field layerof herbaceous species (marshes) as is characteristic of the southern eco-tones of the boreal forest. Indicators of boreal deciduous swamps includewoody vegetation dominated by a variety of small trees (e.g., Acer, Alnus,Ilex, Myrica, Nemopanthus, Rhus, Salix), an understory rich in herbs, andthe lack of an abundant ground layer of bryophytes. Boreal evergreenswamps have closed canopies of Thuja occidentalis or Picea mariana (inNorth America), Pinus sylvestris (in Fennoscandia), or Pinus sibirica,Pinus obovata, Pinus sylvestris, or Larix sibirica (in Russia) as well asabundant herbaceous vegetation and high diversity of bryophytes.Swamps sometimes possess shallow, woody peat layers. Marshes lack tall,woody vegetation and are characterized by species of broad-leafed Carex,Calamagrostis, Phragmites, Scirpus, and Typha. Ground vegetation ispoorly developed.

2.4.2 The Peatland Grade

The formation of extensive, peat-forming ecosystems of the boreal zone isassociated with several features. Climatically, there is a regionally positivewater balance at least during the growing season; this allows local watertables to stabilize.Young landscapes may be rich in Na+ ion and flushing ofthe landscape over time may decrease Na+ ion concentrations in favor ofCa2+ ion. Decreases in Na+ and stabilization of water tables allowbryophyte colonization and persistence that in turn create conditionssuitable for catotelm development.

2.4.3 The Fen Grade

The presence of geogenous water defines fens; however, a well-developedground layer that functions to sequester nutrients and decreases openwater evaporation, as well as development of an anaerobic peat column(catotelm), leads to decreases in wetland nutrient status (mesotrophy).Watershed substrates that contribute high amounts of base cations, thepresence of alkalinity, and low hydrogen ion concentrations from inflow-ing waters lead to the development of rich fens dominated by true mosses.Substrates with few base cations, little or no alkalinity, and high concen-

D.H. Vitt20

trations of hydrogen ion lead to the development of poor fens dominatedby peat mosses. Abundant soligenous waters (especially flowing overfrozen surface peats in the spring) contribute to the formation of pattern-ing (aapamires).

2.4.4 The Bog Grade

Achievement of ombrotrophy is associated with increased oligotrophy,increased aerobic peat column (acrotelm), drier surface conditions, dom-inance of oligotrophic Sphagnum species, and, at least in boreal NorthAmerica, with the presence of an open canopy of trees. Autogenic factorsbecome more important relative to allogenic ones (Bauer and Vitt 2003).Pools of open water are characteristic of oceanic bogs, whereas these arenot present in continental climatic conditions. Bogs may be influenced bypermafrost owing to the insulative properties of Sphagnum and the rela-tively thick acrotelm. In regions with little free water, isolated frostmounds occur in wooded bogs that when melted produce internal lawns(Vitt et al. 1994). Farther north, extensive ice allows peat plateaus todevelop. In areas with more available water, palsas form.

2.5 Conclusions

Zonobiome VIII is a mosaic of lakes, upland evergreen and deciduous for-est, and peatlands. Most of the world’s peat-forming ecosystems occur inthe boreal zone where they play important roles in carbon sequestration,erosional control, and landscape filtration. Peatlands are uniquely unbal-anced ecosystems that are sensitive to the influences of hydrology, climate,and surrounding substrate. Peat-forming wetlands form two functionallevels of organization: fens and bogs. Both of these grades develop deepdeposits of peat and stabilize the landscape for long periods of time. Bothare characterized by well-developed catotelms and ground layers domi-nated by bryophytes.

Acknowledgements. I have gained much from conversations and field work withKelman Wieder. Our joint trip to central Siberia (funded by NSF, USA) considerablyenhanced my understanding of boreal peatlands, as did earlier trips of mine toFennoscandia and Great Britain (funded by NSERC, Canada). Stephen Zoltai and Ispent many hours discussing northern wetlands and he was instrumental in thedevelopment of peatland ecology in Canada. Sandra Vitt produced the graphics forwhich I am grateful. Elena Lapshina added valuable comments about Siberia.

Functional Characteristics and Indicators of Boreal Peatlands 21

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