8 Primary Production in Boreal Peatlands

R. Kelman Wieder

8.1 Introduction

Primary production is arguably the most important ecological function ofpeatland ecosystems, as long-term and substantial peat accumulation, thehallmark of peatlands,depends on inputs of new plant organic matter,viz.,net primary production. Moore and Bellamy (1974) state that “Peatlandsare, by definition, unbalanced systems in which the rate of production oforganic material by living organisms exceeds the rate at which these com-pounds are respired and degraded.”Similarly, Crum (1992) notes that “It isonly when plant growth exceeds decomposition that organic matter accu-mulates as sodden peat.” Such claims, while seemingly self-evident, areoverstatements that reveal a decided peatland bias. Indeed, the accumula-tion of organic matter in soils, as a result of an excess of production overdecomposition over long periods of time,is essentially universal across ter-restrial ecosystems. Even in systems with tiny soil organic matter storescompared with peatlands, these stores represent the integrated long-termexcess of net primary production over decomposition.

What makes peatlands unique then is the magnitude of the long-termimbalance between net primary production and decomposition that hasled to the impressive and massive quantities of organic matter presentlystored in these ecosystems worldwide. In boreal peatlands, the huge quan-tities of peat are especially impressive given that the accumulation hasoccurred over such a short period of time, 5,000–7,000 years, during theHolocene. Commonly, peat accumulation in boreal environments isascribed to the slow rates of decomposition in the cold, anoxic, sometimesacidic and sometimes nutrient-deficient catotelm layer (deeper, perma-nently water-saturated peat layer) of the peat deposit (Clymo 1984;Chap. 7). Clymo (1984) gives an overall estimate of the exponential decayrate for the catotelm of 0.00014 year–1 (equivalent to a residence time ofnearly 7,150 years), which provides a quantitative measure of the excep-tionally slow turnover of organic matter in the catotelm. However, peat

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

accumulation in the acrotelm (surface peat, above the catotelm) is verysensitive to even small changes in both net primary production anddecomposition (Wieder 2001). The fate of net primary production in theacrotelm determines the quantity and quality of organic matter that iseventuallytransferred into the catotelm (Clymo 1984). Thus, it is ratherfutile to attribute peat accumulation primarily to net primary productionor decomposition throughout the entire peat column; it is the balancebetween these two functions that is important.

To understand factors that can influence past, present, and future peataccumulation, it is critical that we understand factors that can influencepast, present, and future peatland net primary production. This chapterreviews net primary production in boreal peatland ecosystems by vegeta-tion type, with a focus on what is known about the factors that influencenet primary production. For the most part, studies that have reportedgrowth responses in metrics that are not useful for estimating net primaryproduction (e.g., growth in length of Sphagnum or other mosses, growthin leader length or basal area for trees) have not been incorporated intothis synthesis.

8.2 Sphagnum Production

Understandably, most research on peatland production has centered onSphagnum species. Of the vascular and nonvascular plant species thatcontribute to peat accumulation, Sphagnum is the most important, at leastfor bogs and poor fens (Chap. 2). Clymo and Hayward (1982) note that thegenus Sphagnum probably stores more carbon in its recognizable plantremains as peat than any other plant genus worldwide. Methods for mea-suring Sphagnum production are well established (Clymo 1970). Recentreviews have compiled data on Sphagnum production for Canadian peat-lands (Campbell et al. 2000) and globally (Gunnarsson 2005). Gunnarsson(2005) reports that globally, Sphagnum production of individual species atindividual sites ranges from 8 to 1,450 g m–2 year–1, with an overall meanof 259 g m–2 year–1 (±206 standard deviation; 68 sites), with most of thesites from boreal and subboreal peatlands in the northern hemisphere. Inlight of these recent reviews, this section will focus on a synthesis of thefactors that influence peatland Sphagnum growth and production.

8.2.1 Nitrogen

Although Sphagnum production in boreal peatlands has generally beenthought to be nitrogen-limited (Chap. 10), early studies in the UK sug-

R.K. Wieder146

gested that elevated atmospheric nitrogen deposition could alter compet-itive interactions among Sphagnum species, as well as inhibit the growthof some Sphagnum species, ultimately leading to the loss of these speciesfrom UK peatlands (Press and Lee 1982; Woodin et al. 1985; Press et al.1986; Chap. 10). More recent studies have revealed that although borealSphagnum-dominated ombrotrophic bogs in pristine regions in generalare nitrogen-deficient, the response of the Sphagnum layer to enhancedatmospheric nitrogen deposition is triphasic. With increasing atmos-pheric nitrogen deposition (1) nitrogen limits Sphagnum production suchthat increased nitrogen deposition leads to increased moss growth/pro-duction, (2) nitrogen is no longer limiting, but increases in nitrogen depo-sition are transferred to increases in the organic nitrogen concentration inliving Sphagnum tissues, and (3) the living Sphagnum layer becomesnitrogen-saturated such that inorganic nitrogen bypasses the livingSphagnum layer and is transferred downward (Lamers et al. 2000;Berendse et al. 2001; Limpens and Berendse 2003; Limpens et al. 2003a;Bragazza et al. 2004). For Sphagnum fuscum, phase 1 may span nitrogendeposition values up to 16 kg ha–1 year–1, with higher nitrogen depositionleading to diminished production (Vitt et al. 2002).

Field nitrogen fertilization studies have produced mixed results, in partattributable to variation in preexisting background nitrogen depositionlevels. Fertilization studies conducted where background nitrogen depo-sition was low (western Canada, northern Minnesota, and northern Swe-den) have generally shown enhanced nitrogen uptake and attendantincreased Sphagnum growth/production (Bayley et al. 1987; Rochefort etal. 1990; Aerts et al. 1992; Li and Vitt 1997; Chapin et al. 2004), although onestudy in northern Sweden found no effect of nitrogen fertilization onSphagnum production (Aerts et al. 2001). Where background nitrogendeposition is high, nitrogen fertilization has produced either no effect orrepression of Sphagnum production (Austin and Wieder 1987; Aerts et al.1992, 2001; Gunnarsson and Rydin 2000), suggestive of either phase 2 orphase 3 of the triphasic response.

8.2.2 Other Nutrient Effects on Production

8.2.2.1 Sulfur

Tallis (1964) suggested that acid rain (elevated deposition of acidity andnitrogen- and sulfur-containing compounds) across the UK could becausally related to an observed demise of peatland Sphagnum species.Subsequent research demonstrated that atmospheric sulfur compounds(SO2, HSO3

–, SO42–) indeed could exert inhibitory influences on the photo-

synthesis, growth, and survival of Sphagnum species (Ferguson et al. 1978;

Primary Production in Boreal Peatlands 147

Ferguson and Lee 1979, 1980, 1983).Although the dominant form of sulfurin atmospheric deposition, SO4

2–, appears to have little or no inhibitoryinfluence on Sphagnum growth/production (Ferguson et al. 1978; Austinand Wieder 1987; Gunnarsson et al. 2004), sulfate deposition can influenceother ecosystem-level processes in boreal peatlands (Chap. 12).

8.2.2.2 Phosphorus and Potassium

Under conditions of high nitrogen deposition, nitrogen limitation of theSphagnum layer in peatlands may shift to phosphorus limitation (Aerts etal. 1992; Chap. 11). A field fertilization study (Limpens et al. 2004) con-firmed that at moderate and high nitrogen deposition sites Sphagnumgrowth (height growth and net primary production) is phosphorus-lim-ited, and also showed that when phosphorus is added to the Sphagnumlayer, Sphagnum can take up some of the excess atmospherically depositednitrogen that otherwise would be transported downward below the grow-ing moss layer. However, in a fertilization and liming experiment in sub-boreal northern Minnesota (Chapin et al. 2004), phosphorus addition didnot ameliorate the negative effects of the high nitrogen fertilization onSphagnum fuscum/capillifolium production. Under conditions of elevatednitrogen deposition and elevated atmospheric CO2, nitrogen and phos-phorus concentrations of Sphagnum species were unaffected, while potas-sium concentrations decreased, suggesting a shift to potassium limitationof growth (Hoosbeek et al. 2002).A synoptic study of Sphagnum nitrogen-to-phosphorus and nitrogen-to-potassium ratios also suggested a shift tophosphorus and/or potassium limitation under conditions of high atmos-pheric nitrogen deposition (Bragazza et al. 2004). High nitrogen deposi-tion, coupled with enhanced phosphorus availability, may alter competi-tive interactions between Sphagnum species, leading to increaseddominance by relatively minerotrophic species, such as Sphagnum fallax(Limpens et al. 2003b).

8.2.3 Carbon Dioxide

With increasing interest in the effects of elevated atmospheric CO2 levels onterrestrial ecosystems (Norby et al. 1999), some research has focused onphotosynthetic responses of peatland Sphagnum to elevated CO2 (Silvola1990; Jauhiainen and Silvola 1999). Laboratory studies of peatland Sphag-num growth responses to elevated CO2 and elevated nitrogen deposition(Jauhiainen et al. 1994, 1999; van der Heijden et al. 2000) have producedsomewhat ambiguous results, contributed in part by both interspecies andintraspecies variability in Sphagnum responses. All of this work has

R.K. Wieder148

focused on European locations. The European Bog Ecosystem ResearchInitiative (BERI) represents the only field CO2 enhancement experiment inboreal peatlands, implemented using free-air CO2 enrichment (FACE)technology. After 3 years of exposure to elevated CO2, Sphagnum (mainlyS. magellanicum, S. balticum, S. papillosum, and S. fallax; Hoosbeek et al.2002) net primary production was unaffected at all four BERI sites (Hoos-beek et al.2001).It remains uncertain whether increasing atmospheric CO2concentration,in and of itself,will have any substantial effect on Sphagnumnet primary production in boreal peatlands.

8.2.4 Climate and Moisture

The distribution of mosses, and in particular of Sphagnum species, hasbeen shown to be closely related to climatic factors, such as mean annualprecipitation and mean annual temperature (Gignac et al. 1998). In light ofthe wide distribution of Sphagnum mosses, attempts have been made torelate Sphagnum production to broad climatic gradients (Damman 1979;Wieder and Lang 1983; Moore 1989; Rochefort et al. 1990; Vitt 1990). Themost geographically comprehensive effort in this regard is that of Gun-narsson (2005), who compiled literature data to assess patterns in Sphag-num production in relation to taxonomic sections (Cuspidata>Acutifo-lia), field microhabitat (hummock, lawn, carpet; about 200, 260, and370 g m–2 year–1, respectively), geographic factors (latitude, altitude, dis-tance to the sea), and broad climatological factors. Mean annual tempera-ture and mean annual precipitation explained over 30 % of the total vari-ation in production across all Sphagnum species (Gunnarsson 2005).

On a more local scale, moisture availability appears to influence Sphag-num production. Sphagnum plants appear to grow best during the cooland humid conditions that characterize spring and autumn, with slowergrowth during hotter, drier summer months (Brock and Bregman 1989;Lindholm 1990). For poor fens in northern Quebec, production of hum-mock sphagna (58–73 g m–2 year–1) was greater than lawn sphagna(9–19 g m–2 year–1) (Bartsch and Moore 1985) and in an poor fen inOntario (Canada), production of Sphagnum species was greater on hum-mocks than on either hollows or on “midhummocks” (Rochefort et al.1990). In both of these studies, differences in Sphagnum net primary pro-duction could be attributed both to differences in the relative abundanceof Sphagnum species along these microtopographic gradients and to dif-ferences in the production of individual Sphagnum species depending onmicrotopographic location (Vitt 1990). Gunnarsson’s (2005) broad geo-graphic synthesis revealed that Sphagnum production was greater on car-pets than on lawns or on hummocks, attributable in part to peatlandmicrotopographic moisture gradients.

Primary Production in Boreal Peatlands 149

8.3 True Moss Production

There is a paucity of data on production of true mosses in boreal peat-lands. In a nonpermafrost bog in Manitoba, Reader and Stewart (1972)reported production of Aulacomnium palustre and Polytrichum juniper-inum as 5.4 and 35 g m–2 year–1, respectively. Across two boreal and threemontane rich fens in Alberta, Canada, Vitt (1990) reported productionvalues for Tomenthypnum nitens, Campylium stellatum, Drepanocladusrevolvens, and Scorpidium scorpioides of 55–131, 58–78, 65–109, and60–170 g m–2 year–1, respectively. Thormann and Bayley (1997) reportedproduction for Aulacomnium palustre and Drepanocladus aduncus in anAlbertan moderate rich fen in two different years of 136–405 and27–121 g m–2 year–1, respectively. In another moderate rich fen in Alberta,Busby et al. (1978) reported production of Hylocomium splendens andTomenthyphum nitens of 79 and 190 g m–2 year–1.

Vitt (1990) examined microtopographic effects on overall moss (nat-ural communities consisting predominantly of Sphagnum fuscum, Sphag-num magellanicum, Sphagnum angustifolium, Tomenthypnum nitens,Campylium stellatum, Drepanocladus revolvens, and Scorpidium scorpi-oides) production in bogs, poor fens, boreal rich fens, and montane richfens of Alberta, Canada. Across all peatland types, production was higherin carpets than in lawns. For bogs, production was lower in hummocksthan in lawns, but in poor fens and rich fens, production in hummockswas greater than or equal to production in lawns.

Responses of boreal peatland-dwelling true mosses to broad climaticgradients has received little attention. The distribution and abundance ofseveral boreal peatland true mosses have been successfully modeled usingmean annual temperature and mean annual precipitation (Gignac et al.1991a, b; Gignac 1992).Vitt (1990) examined annual growth increments ofH. splendens and Polytrichum strictum from herbarium specimens col-lected from locations across nonarctic North America. Growth of thedrought-tolerant, ectohydric H. splendens was highly variable across itsboreal range, but was correlated with both precipitation and continental-ity. In contrast, growth of the less drought tolerant, endohydric, Poly-trichum strictum was more constant and could not be related to climaticvariability. Despite these efforts, the net primary production response oftrue mosses to broad climatic gradients is not well understood.

Little effort has been made to examine nutrient effects on true mossproduction, although there is some evidence that Tomenthypnum nitensproduction may be limited by nitrogen (Bayley et al. 1987; Rochefort andVitt 1988), at least in geographical locations where ambient atmosphericnitrogen deposition is low. In a cutover bog in the Swiss Jura Mountains(Mitchell et al. 2002), Polytrichum strictum production over a 3-year

R.K. Wieder150

period was stimulated by elevated nitrogen levels (535 and 931 g m–2,respectively, in control and elevated nitrogen treatments, difference sig-nificant), but was similar in control and elevated CO2 treatments (525 and629 g m–2, respectively, difference not significant).

While Gunnarsson’s (2005) synthesis suggested that true moss produc-tion in boreal peatlands was generally lower than Sphagnum productionfor western Canadian peatlands, Vitt (1990) concluded that Sphagnumspecies and brown mosses had similar production values. In rich fenecosystems dominated by true mosses, Sphagnum mosses may be a rela-tively minor contributor to overall moss production. Our understandingof the chemical and climatic factors that influence true moss productionin boreal peatlands is deficient.

8.4 Herbaceous Species and Field Layer Production

Herbaceous species are much more abundant in minerotrophic fens thanin ombrotrophic bogs although production data for total herbaceousspecies in either boreal bogs or boreal fens are sparse. Total herb produc-tion at an Albertan bog, measured in three different years, was 8.5, 14.5,and 34 g m–2 year–1, and was contributed by Rubus chamaemorus, Smi-lacina trifolia, and Eriophorum vaginatum (Szumigalski 1995; Thormann1995). Grigal et al. (1985) reported total herb production for a raised anda perched bog in Minnesota of 14 and 22 g m–2 year–1, respectively. Rangesfor total herb production for an extremely rich fen (mainly from Scirpuscyperinus, Muhlenbergia glomeratum, Triglochin spp., and Juncus spp.),moderately rich fens (mainly from Carex spp., Menyanthes trifoliata,Potentilla palustris, and Smilacina trifolia), and a poor fen (mainly fromCarex spp., Menyanthes trifoliata, and Smilacina trifolia) in Alberta were81.4–96.9, 52.3–302.0, and 51.6–55.3 g m–2 year–1, respectively (Szumigal-ski 1995; Thormann 1995). No data have been reported for belowgroundherbaceous species production in boreal bogs.

In a fertilization and liming experiment in two subboreal peatlands innorthern Minnesota (Chapin et al. 2004), bog graminoid (Carex oligo-sperma and E. spissum) aboveground production was unaffected by anytreatment and fen graminoid (mainly Carex exilis, with smaller contribu-tions from Rhynchospora alba, Scheuchzeria palustris, Rhynchosporafusca, and Carex lasiocarpa) aboveground production was elevated onlyby the high nitrogen and high phosphorus fertilization with lime in thethird year of the study.

Carex rostrata is an indicator of minerotrophy in peatlands, and as suchis found in fens, but not bogs (Chap. 2).Aboveground production of Carexrostrata has been reported as 90, 116, and 176±15 (standard error)

Primary Production in Boreal Peatlands 151

g m–2 year–1 at a moderately rich fen in Quebec (Bartsch and Moore 1985),a shrubby peatland in Manitoba (Reader and Stewart 1972), and amesotrophic fen in southern Finland (Saarinen 1996), respectively. At thelast site, Carex rostrata rhizomes, coarse root, and fine root productionwas 113±32, 82±22, and 1,053±135 g m–2 year–1, such that abovegroundproduction was only 12 % of total production (Saarinen 1996). At themoderately rich fen in Quebec (Bartsch and Moore 1985), Carex aquatilisproduction was 164 g m–2 year–1.

There is a paucity of recent research on the factors that influenceherbaceous species production in boreal peatlands. In contrast, some ofthe earliest research on nutrient limitation in peatland ecosystemsfocused on the responses of herbaceous species. These early studies(Tamm 1954; Gore 1961 a, b; Goodman and Perkins 1968 a, b) found noconsistent influence of nitrogen, phosphorus, potassium, calcium, or mag-nesium on growth (data were not presented as net primary production)for peatland species including the herbaceous species E. vaginatum,Molinea caerulea, and Dactylis glomerata. More recently, however,enhanced atmospheric nitrogen deposition and associated increasednitrogen availability in European bogs has been implicated as a possiblecause of increased invasion of the grass Molinea caerulea, as well as thesmall tree Betula pubescens (Limpens et al. 2003a), although a shift tophosphorus limitation may limit vascular plant success (Tomassen et al.2004). Herbaceous production in boreal peatlands and the factors thatinfluence it remain poorly understood.

8.5 Shrub Production

Although shrubs, especially in the Ericaceae, can be a dominant featureof many peatland types, production data for the total shrub layer ineither boreal bogs or boreal fens are sparse. Total shrub production at abog in Alberta, measured in three different years, was 77, 97, and117 g m–2 year–1, and was contributed mainly by Ledum groenlandicum,Vaccinium vitis-idaea, Andromeda polifolia, Chamadeaphne calyculata,and Oxycoccos quadripetalus (Szumigalski 1995; Thormann 1995).Reader and Stewart (1972) reported total shrub production for a Mani-toba bog of 316 g m–2 year–1, contributed mainly by Chamaedaphne caly-culata, Ledum groenlandicum, V. vitis-idaea, O. quadripetalus, andKalmia polifolia. Grigal et al. (1985) reported total shrub production fora raised and a perched bog in Minnesota of 200 and 43 g m–2 year–1,respectively, contributed mainly by Ledum groenlandicum andChamaedaphne calyculata. Ranges for total shrub production for awooded moderately rich fen (mainly from Andromeda polifolia,

R.K. Wieder152

B. pumila, and Salix pedicularis), a shrubby moderately rich fen (mainlyfrom Andromeda polifolia and Salix pedicularis), and a shrubby poor fen(mainly from B. pumila, Salix pedicularis, and Andromeda polifolia) inAlberta were 96–120, 7–52, and 112–157 g m–2 year–1, respectively (Szu-migalski 1995; Thormann 1995). No data have been reported for below-ground shrub production in boreal peatlands. Myrica gale, with root-associated nitrogen fixers, can form dense stands in peatlands of NorthAmerica and Europe (Skene et al. 2000). At a temperate-zone peatland incentral Massachusetts, USA, aboveground and belowground productionof Myrica gale was reported as 392 and 157 g m–2 year–1, respectively,with nitrogen fixation providing 43 % of the nitrogen supporting thisproduction (Schwintzer 1983).

Little research has focused on the factors that influence shrub produc-tion in boreal peatlands. Fertilization (phosphorus and potassium) anddrainage of a bog in southern Finland dramatically increased above-ground production of the shrub layer (defined by the author as compris-ing small Pinus sylvestris, B. pendula, and B. pubescens) from 2.4 to12.5 g m–2 year–1, as well as belowground production of the shrub layerfrom 0.7 to 3.1 g m–2 year–1 (Vasander 1982). Fertilization with nitrogen,phosphorus, and potassium increased production of Chamaedaphne caly-culata from 31 to 115 and from 55 to 198 g m–2 year–1 in the first and sec-ond years of treatment at Caribou Bog in Maine, USA, but did not influ-ence production at a nearby weakly minerotrophic poor fen (averageproduction in control and fertilized sites over 2 years of 294 g m–2 year–1;Bartsch and Schwintzer 1994). Lack of a response at the fen site appears tohave been related to an inherently higher nutrient availability. To examineaboveground net primary production responses, a subboreal bog and apoor fen in northern Minnesota were fertilized with two levels of nitrogenand phosphorus, with or without lime additions (Chapin et al. 2004). Forthe bog, Chamaedaphne calyculata production was enhanced by phos-phorus, but not nitrogen, addition, although this effect disappeared withliming. The bog shrub Ledum groenlandicum exhibited a complex fertil-ization and liming interaction, while the bog shrubs V. oxycoccus andK. polifolia and the fen shrub Andromeda glaucophylla had no clearresponses to fertilization or liming. Overall, for either the bog or the fen,total shrub layer production in any of the treatments, however, did not dif-fer from control shrub layer production (Chapin et al. 2004). As withherbaceous production, shrub production in boreal peatlands and the fac-tors that influence it remain poorly understood.

Primary Production in Boreal Peatlands 153

8.6 Tree Production

Production of tree species on boreal peatlands has received little atten-tion, and data on belowground production are especially sparse. Becausethere are only a few dominant tree species in boreal peatlands worldwide,the information that exists can be easily addressed by species.

Black spruce, Picea mariana, is a dominant tree species in ombro-trophic peatlands (bogs) of North America, and it also occurs as a majorspecies in poor fens (Chap. 2). Picea mariana is widely distributed acrossboreal Canada, growing not only in peatlands, but also on a wide variety ofnonpeatland soil types (Viereck and Johnston 1990; Montague andGivnish 1996). Because Picea mariana is a very important pulpwoodspecies in eastern North America, some research has focused on factorsaffecting its growth. Height-growth patterns for Picea mariana differbetween peatland and upland stands in Ontario (Payandeh 1978, 1989),and probably across Canada more generally. In upland sites, soil moistureand aeration influence growth (Jeglum 1974; Lieffers and Rothwell 1986;Dang and Lieffers 1989; Hillman et al. 1990; MacDonald and Yin 1999; Royet al. 1999), while water chemistry as reflective of the ombrotrophic–minerotrophic status of peatlands has a major influence on Picea marianagrowth (Heinselman 1970). Picea mariana growth can be enhanced byfertilization with nitrogen and phosphorus in upland (van Nostrand1979) as well as peatland (Alban and Watt 1981; Payandeh 1989; Wells1993) sites.

Tamarack, Larix laricina, is a dominant tree species in treed fens ofboreal North America and can be found across a wide range along theombrotrophic–minerotrophic gradient (Heinselman 1970; Sims et al.1979; Montague and Givnish 1996; Chap. 2). Larix laricina is widely dis-tributed across boreal Canada. Although Larix laricina grows on a varietyof soil types, because of its tolerance for high soil moisture, acidity, andlow soil temperature, Larix laricina tends to be more abundant in peat-lands than in surrounding uplands (Johnston 1990). Growth sensitivity tosoil moisture has been demonstrated as postdrainage growth release(MacDonald and Yin 1999). Larix laricina is an important pulpwoodspecies, and its growth depends on both soil moisture/aeration and nutri-ent status (Glerum and Pierpoint 1968; Tilton 1977; Johnston 1990).

Scots pine, Pinus sylvestris, is the world’s most widely distributed pinespecies, although it is native to Europe and Asia and was introduced inNorth America (Skilling 1990). Although Pinus sylvestris grows on a vari-ety of soil types in upland areas, it also grows as a dominant tree speciesin boreal peatlands of Europe and Asia (Chap. 2), where its growth is gen-erally stunted (Steven and Carlisle 1959), probably because of unfavorablesoil moisture/aeration conditions (Seppälä 1969). The sensitivity of peat-

R.K. Wieder154

land Pinus sylvestris to growing season temperature and precipitationsuggested that this species might provide sensitive historical climatereconstructions through dendrochronological analyses, but observedtemporal patterns were more pronounced for upland than peatland trees(Linderholm et al. 2002). Pinus sylvestris root biomass increases withdrainage (Heikurainen 1955; Paavilainen 1966, cited in Finér 1991).Drainage of Finnish pine bogs may result in small increases in carbonmineralization in peat, but the resulting carbon losses are outstripped bystimulated aboveground net primary production (increased litterfall) andfine root production of Pinus sylvestris (Minkkinen and Laine 1998).Pinus sylvestris seedling root biomass and length were enhanced by NPKand PK fertilization (Paavilainen 1967, 1968, cited in Finér 1991), althoughno root growth response to NPK or PK fertilization was found for a standwith 85-year-old trees (Finér 1991).

Although forestry-motivated research has examined factors that influ-ence Picea mariana, Larix laricina, and Pinus sylvestris growth, suchresearch has focused more on upland stands that tend to be more produc-tive than peatlands. In addition, such research has focused on above-ground growth only and has not made an effort to convert forestry-rele-vant growth information to ecologically relevant net primary productiondata. Data on peatland tree production are sparse.

Aboveground net primary production data for Picea mariana have beenreported for a permafrost bog in northern Alaska (77 g m–2 year–1; Billings1987) and for permafrost plateaus in northern Manitoba (32–140 g m–2

year–1; Camill et al. 2001) where production increased as the mean annualtemperature increased from –5 to –1 °C.Production has been estimated fornonpermafrost bogs in northern Alberta (27 and 54 g m–2 year–1; Szumigal-ski and Bayley 1996 and Szumigalski 1995, respectively) and in southeast-ern Manitoba (58 and 302 g m–2 year–1 in the bog forest and muskeg zones,respectively, of a single peatland; Reader and Stewart 1972). Picea marianaproduction has been reported for a few subboreal peatlands, viz., for araised and a perched bog in subboreal northern Minnesota (100 and310 g m–2 year–1; Grigal et al. 1985) and for Waterloo Black Spruce Bog insoutheastern Michigan (176 g m–2 year–1; Ulrich 1980).Aboveground Piceamariana production estimates for fens are limited to a single moderatelyrich fen in northern Alberta (5.6 g m–2 year–1; Szumigalski 1995).

Even sparser than the data for aboveground production of trees inboreal peatlands are data for belowground production. Using the rootingrowth approach at an Albertan peatland dominated by Picea marianaand Larix laricina, Conlin and Lieffers (1993) demonstrated that rootgrowth for both species began in July and continued through mid-Sep-tember for Larix laricina and mid-October for Picea mariana. The datawere not expressed as production, but are roughly equivalent to 200 and340 g m–2 year–1 for Larix laricina and Picea mariana, respectively (Conlin

Primary Production in Boreal Peatlands 155

R.K. Wieder156

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and Lieffers 1993). These values are considerably larger than those esti-mated for a permafrost bog in northern Alaska (32 g m–2 year–1; Billings1987). In a detailed study of root dynamics at three Finnish peatlands,Finér and Laine (1998) reported Pinus sylvestris root production in the top20 cm of a meso-oligotrophic tall sedge fen, an oligotrophic tall sedge pinefen, and an ombrotrophic dwarf-shrub pine bog to be 63, 64, and112 g m–2 year–1, respectively, and 221, 261, and 310 g m–2 year–1, respec-tively, using sequential root harvesting techniques and two differentmethods of calculation. These values for Pinus sylvestris are larger thanthose obtained by Backéus (1990) for an ombrotrophic Swedish bog (51and 86 g m–2 year–1 in hummocks and lawns, respectively), and smallerthan those obtained for a Finnish tall sedge fen using a pulsed-labeling of14C (1,100 g m–2 year–1; Saarinen 1996).

Pinus sylvestris is the only major peatland tree species for which severalstudies have estimated both aboveground and belowground production atthe same sites over the same time period, with most of these results beingcompiled by Vasander (1982). Both aboveground and belowground pro-duction values vary considerably, with aboveground production exceed-ing belowground production by factors of 1.5–7.8 (Table 8.1).

8.7 Conclusions

Most of the research on net primary production in boreal peatlands hasfocused on Sphagnum mosses, whose production seems to be relatedbroadly to coarse climatic factors (mean annual temperature and meanannual precipitation) as well as to atmospheric nitrogen deposition. Muchless effort has focused on production of true mosses, even though thesespecies may dominate some peatlands, notably rich fen ecosystems. Fewdata are available for production of herbaceous or shrub species in borealpeatlands and even less is known about the factors that control theirgrowth/production. Although considerable research has focused on netprimary production of boreal peatland tree species, much of this has beendirected toward understanding growth on drained and/or fertilized sitesin relation to forestry practices. The extent to which these findings haverelevance to tree growth/production in unmanaged settings remainsuncertain. Probably because of methodological difficulties and con-straints (Finér and Laine 2000; Hansen et al. 2000), measurements ofbelowground production of vascular plants in boreal peatlands are few.Our understanding of the factors that influence belowground productionand root turnover is meager and represents a major knowledge gap giventhat root-related processes may play an important major role in peatlandcarbon balance (Silvola et al. 1996; Crow and Wieder 2005).

Primary Production in Boreal Peatlands 157

Few studies have quantified net primary production for all vegetationlayers at a single site. Total aboveground net primary production forNorth American boreal and subboreal bogs averaged 462 g m–2 year–1

(range 280–755 g m–2 year–1; five sites), and for fens averaged 319 g m–2

year-1 (range 114–710 g m–2 year–1; ten sites) (data compiled and presentedin Thormann and Bayley 1997). However, none of these studies measuredbelowground production. The only study of which I am aware in which allcomponents, aboveground and belowground, of net primary productionwere quantified is that of Vasander (1982). For a virgin bog, a drained bog,and drained and fertilized bogs in southern Finland, total abovegroundnet primary production was 238, 136, and 354 g m–2 year–1, respectively,and total belowground production was 139, 78, and 266 g m–2 year–1,respectively (Vasander 1982).

Over the past 50 years, considerable effort has been devoted to quanti-fying peatland net primary production and to understanding the bioticand abiotic factors that influence this key ecosystem functional property.Most of the research has focused on Sphagnum mosses, to the extent thatit has become possible to synthesize global patterns in Sphagnum produc-tion (Gunnarsson 2005). For other species or groups of species, however,limited data make this sort of synthesis premature. Because so few studieshave tried to quantify whole ecosystem net primary production, we seema long way from uncovering consistent patterns within or across differentboreal peatland types with regard to the relative contribution of mosses,herbs, shrubs, and trees to either aboveground or total net primary pro-duction, or with regard to the relative importance of aboveground versusbelowground contributions to total net primary production. Althoughombrotrophic bogs, minerotrophic poor fens, and minerotrophic richfens, each with their own unique plant community composition, occuracross the boreal region, within the boreal region, climatic gradients intemperature and precipitation exist, along with anthropogenically influ-enced gradients in nitrogen deposition. A complete view of how thesebroad gradients influence whole ecosystem net primary production in dif-ferent peatland types remains elusive.

Clearly, the fate of the huge quantity of carbon stored in the borealpeatlands of the northern hemisphere will hinge on how the offsettingprocesses of net primary production and decomposition are influencedby future climatic conditions. As models of peatland carbon cycling andaccumulation continue to be developed (Hilbert et al. 2000; Frolking et al.2001; LaFleur et al. 2003; Belyea and Malmer 2004), a more comprehensiveview of peatland net primary production will be needed. Despite over ahalf a century of interest and research on net primary production inboreal peatlands, considerable additional effort will be needed to developan overarching understanding of this key ecosystem process, in terms ofboth rates and controlling factors.

R.K. Wieder158

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