Dynamics of plant-mediated organic matter and nutrient cycling

following water-level drawdown in boreal peatlands

Raija Laiho1 and Harri VasanderPeatland Ecology Group, Department of Forest Ecology, University of Helsinki, Helsinki, Finland

Timo PenttilaFinnish Forest Research Institute, Vantaa Research Centre, Vantaa, Finland

Jukka LainePeatland Ecology Group, Department of Forest Ecology, University of Helsinki, Helsinki, Finland

Received 22 November 2002; revised 12 February 2003; accepted 17 March 2003; published 22 May 2003.

[1] If boreal peatlands face drought more often due to climatic warming, the responses ofvegetation may drastically change the functions of the ecosystem. We assessed theeffects of water-level drawdown on plant-mediated organic matter (OM) and nutrientfluxes in a chronosequence of undrained and drained, originally sparsely treed fens. Thechronosequence mimicked the reduced growing season moisture predicted by currentclimate change scenarios. In a pristine state, OM and nutrient fluxes were characterizedby annual cycling through graminoids and mosses. Water-level drawdown initiated a‘‘forest succession,’’ in which the OM and nutrient cycles shifted from being dominatedby graminoids and mosses to dominance by arboreal vegetation in two decades.Simultaneously, the quantity and tissue type composition of annual litterfall, as well as thequantity and allocation of annual nutrient uptake, changed radically. The changes mayhave contrasting but as yet unexplored implications for the carbon and nutrient balancesof these sites. INDEX TERMS: 1615 Global Change: Biogeochemical processes (4805); 1851

Hydrology: Plant ecology; 1890 Hydrology: Wetlands; KEYWORDS: climate change, litterfall, nutrient uptake,

vegetation change, wetland

Citation: Laiho, R., H. Vasander, T. Penttila, and J. Laine, Dynamics of plant-mediated organic matter and nutrient cycling

following water-level drawdown in boreal peatlands, Global Biogeochem. Cycles, 17(2), 1053, doi:10.1029/2002GB002015, 2003.

1. Introduction

[2] Water level, or soil moisture, is the major factorcontrolling plant community composition [Glaser et al.,1990; Jeglum and He, 1995], carbon (C) allocation [Welt-zin et al., 2000], organic matter (OM) decomposition[Bridgham et al., 1991; Szumigalski and Bayley, 1996],and C fluxes [Moore and Dalva, 1993; Funk et al., 1994;Kettunen et al., 1999] in peatlands. Consequently, thefunction and element balances of peatland ecosystemsmay drastically change in response to alterations in waterlevel. Climate change scenarios predict warmer temper-atures and reduced growing season rainfall in parts of theboreal zone [Gitay et al., 2001]. These changes couldcause a drawdown of the water level of peatlands [Rouletet al., 1992] which, although largely an indirect influenceof climate change, may be the main agent of ecosystem

change in the boreal and subarctic peatlands, outweighingthe impacts of rising temperature and longer growingseasons [Gorham, 1991].[3] In wet pristine peatlands, annual cycling of OM and

nutrients through the ground vegetation, mainly graminoidsand mosses, is important for all element cycles [Bernard andHankinson, 1979; Urban and Eisenreich, 1988; Verry andUrban, 1992]. The poor aeration of the soil impedes thegrowth of unspecialized vascular plants, including most treespecies [Jeglum, 1974; Macdonald and Yin, 1999], and thetree stand, when present, is usually sparse and has a minorrole in the ecosystem function. Vegetation physiognomyusually clearly depends on the soil moisture regime [Verry,1997].[4] After water-level drawdown, the soil, and conse-

quently, the whole ecosystem enter a dynamic state ofchange: a secondary succession that may last for decades[Laine et al., 1995a, 1995b; Vasander et al., 1997]. Becausevegetation is an important mediator in the element balances[Hobbie, 1996; Aerts et al., 1999], changes in the C andnutrient cycling through vegetation have to be understoodand taken into account when estimating ecosystemresponses to climate change. Short measurement periods

GLOBAL BIOGEOCHEMICAL CYCLES, VOL. 17, NO. 2, 1053, doi:10.1029/2002GB002015, 2003

1Also at Finnish Forest Research Institute, Joensuu Research Centre,Joensuu, Finland.

Copyright 2003 by the American Geophysical Union.0886-6236/03/2002GB002015$12.00

22 - 1

fail to capture the effects of the new succession on vegeta-tion dynamics with their consequent implications for ele-ment balances, however.[5] In parts of the boreal zone, large areas of peatlands

have been drained to improve forest growth. These peat-lands were usually sparsely forested (tree-covered) in theirpristine state. An important point here is that ditching hascommonly been the only operation done: the original treestand (albeit often sparse) has formed the basis for theproduction forest stand. These drained peatlands give usan excellent opportunity to examine the long-term effectsof water-level drawdown on peatland ecosystem function[e.g., Lieffers, 1988; Glenn et al., 1993; Martikainen etal., 1993; Roulet et al., 1993; Laine et al., 1995a;Nykanen et al., 1998; Minkkinen et al., 1999; Silvan etal., 2000]. In addition to climate change scenarios, theresults of drainage studies are currently needed forestimating the effects of land use changes on the Cbalance.[6] The aim of this study was to analyze long-term

(several decades) ecosystem responses to water tablelowering in terms of changes in OM and nutrient cycling.Our method was to combine results from chronosequenceand repeated measures studies with those of dendrochro-nological tree growth analysis. More specifically, wewanted to evaluate the dynamics following water-leveldrawdown of (1) plant biomass, (2) annual litterfall, (3)plant biomass nutrient pool, (4) annual nutrient cyclethrough the vegetation, and (5) total nutrient uptake byvegetation and its allocation to aboveground and below-ground biomass. We will use nitrogen (N) and potassium(K), two elements with differing cycling and depositionpatterns, as examples.

2. Materials and Methods

2.1. Study Area

[7] The peatlands chosen for this study were situated insouthern Finland (61�350–61�520N, 24�050–24�250E). Thearea is part of the southern boreal coniferous forest zone andthe eccentric raised bog region. The mean annual temper-ature in the region is +3�C and that of July is +16�C. Themean annual temperature sum (accumulated mean daily

temperatures �+5�C) varies between 1150 and 1250degree-days. The annual precipitation is ca. 710 mm, ofwhich ca. 240 mm is snowfall.

2.2. Site Description

[8] Six sites forming a drainage succession chronose-quence were chosen for detailed measurements (Table 1).Two of the sites were undrained, while the rest had beendrained 8, 22, 30, and 55 years before the beginning of theinvestigation period (summer 1991 for sites 2 and 5, 1992for other sites). The drained sites had not been furthermanaged after ditching. The initial lowering in the waterlevel in our study sites, caused by ditching, was 15–20 cm(Table 1). Later on in the forest succession, increasedevapotranspiration has induced a further lowering of thewater levels down to 50–70 cm during the growing seasonsin our older drained sites.[9] All sites represented tall-sedge pine fen communities

before drainage [in the sense of Laine and Vasander, 1996].The sites were subjectively selected from the material (84sites) of an earlier study dealing with the effect of drainageon peat properties and vegetation [Laiho and Laine, 1994a,1995; Laine et al., 1995a; Weiss et al., 1998; Laiho et al.,1999]. The original material had been carefully selected toallow comparison between various sites at different stagesof the postdrainage succession. For details of the selection,see the work of Laiho and Laine [1994a]. The sites of thepresent study were chosen to obtain as wide a drainage agevariation as possible within the chosen site type, and so thatthe tree stand volumes on the drained sites would be close tothe average of that drainage age. The two undrained sitesrepresented the average and the higher end of tree standvolume variation [Heikurainen, 1971].[10] In their natural, undisturbed state, tall-sedge pine fens

are wet sites with stunted Scots pine (Pinus sylvestris L.)growing on low hummocks, often with some birch (Betulapubescens Ehrh.). The lawn between the hummocks isdominated and characterized by sedge species, such asCarex lasiocarpa Ehrh and/or Carex rostrata Stokes. Alsotypical is the dwarf birch Betula nana L. The moss layer iscontinuous and dominated by one or the other of theSphagnum recurvum collective species, most often Sphag-num fallax (Klinggr.) Klinggr., or Sphagnum angustifolium

Table 1. General Features of the Study Sites at the Beginning of the Investigation Perioda

Site

1 2 3 4 5 6

Year of drainage . . . . . . 1984 1970 1961 1937Stand volume, m3 ha�1 6 35 10 76 102 150Shrub biomass,b g m�2 41 (8)A 76 (11)A 288 (30)B 103 (32)AC 54 (20)A 186 (27)BC

Graminoid biomass,b g m�2 58 (8)A 72 (6)A 60 (10)A 22 (5)B 15 (3)B 5 (1)B

Moss biomass,b g m�2 534 (20)A 349 (15)B 644 (21)C 137 (40)D 193 (28)D 452 (53)AB

WT,c cm 10 10 20 50 50 60Peat N,d % of dry mass 1.5 2.2 1.7 2.2 2.4 2.0Peat K,d % of dry mass 0.042 0.052 0.044 0.028 0.026 0.043

aFor details of vegetation characteristics, see the work of Laiho [1996], Laiho and Finer [1996], and Laiho and Laine [1997].bAboveground, dry mass (105�C), standard error of mean (20 subsamples) given in parentheses. Different superscripts indicate differences that are

significant at the 0.05 level (one-way ANOVA and Tamhane’s T2 for pairwise comparisons).cAverage water table depth during growing season.dTotal concentration in the 0–20 cm peat layer.

22 - 2 LAIHO ET AL.: PLANT-MEDIATED OM AND NUTRIENT CYCLING

(Russ.) C. Jens. These sites are minerotrophic and arecommonly found at the slopes or margins of raised bogs,as well as in aapa mires. With its counterparts, tall-sedgepine fen is a very common mire site type throughout theboreal zone [e.g., Pyavchenko, 1963]. In North America,ecologically equivalent sedge fens usually have Larix lar-icina (Du Roi) C. Koch as the major tree species [e.g.,Harris et al., 1996].[11] After drainage of these sites, sedges and mire herbs

disappear fairly soon, and dwarf shrubs such as B. nana andLedum palustre L. gain dominance. Mire dwarf shrubs giveway to Vaccinium myrtillus L. and Vaccinium vitis-idaea L.as the shading from the tree stand canopy increases.Cottongrass (Eriophorum vaginatum L.) may remain asthe only relatively abundant graminoid. Sphagnum mossesare gradually replaced by such forest species as Pleuroziumschreberi (Brid.) Mitt., even though Sphagnum russowiiWarnst. and other Sphagnum species may still be abundanton old drained sites. Changes in plant species compositionafter drainage are described in more detail by Laine et al.[1995a]. The detailed lists of plant species with coverages ofthe study sites in the beginning of the investigation periodhave been published by Laiho [1996].[12] On undrained sites, sample plots were laid out in the

middle of an area where vegetation uniformly representedthe chosen site type. Plot sizes were 600 and 900 m2. Ondrained sites, the sample plots covered a whole stripbetween ditches, with one ditch counted in the plot area.Plot size varied from 1158 to 1328 m2 depending on theditch spacing, which varied from 30 to 37 m.

2.3. Plant Biomass and Nutrient Pools

[13] Twenty equally spaced subplots were sampled ateach site for ground vegetation biomass. Shrubs wereharvested on an area of 0.5 m2 per subplot, graminoidsand mosses on an area of 0.19 m2. Zero level for samplingwas the upper level of the rooting zone, i.e., the layer wherethe topmost fine roots could be found. This correspondedapproximately to the bottom of the green part of the mosslayer. Sampling was done in late June, at the time ofmaximum annual aboveground biomass. Sampling wasrepeated at site 2 to check the stability of the nutrient poolvalues, which turned out to be within ±6–12% units.Ground vegetation biomass at the beginning of the inves-tigation period was reported by Laiho [1996]. We used themean value from two undrained sites to represent thepredrainage situation.[14] Nutrient concentrations were measured for the shrub,

the graminoid, and the moss components separately. Totalconcentrations of P, K, Ca, and Mg were measured using anICP analyzer (ARL 3580) after nitric acid-perchloric aciddigestion. Nitrogen concentrations were measured using aLeco CHN 600 analyzer.[15] All trees in the sample plots were measured for

diameter at breast height (DBH) (diameter at 1.3 m), barkthickness, total height, and height to the base of the crown.Six to 14 biomass sample trees per plot were chosen on thebasis of stratified random sampling to cover the whole rangeof diameters, and tree species (pine and birch). Tree biomassmeasurements have been described by Laiho and Laine

[1997]. Element concentrations were measured for eachbiomass component (wood, bark, live branches, deadbranches, and needles/leaves) in each sample tree, usingthe same methods as for ground vegetation. The totalnutrient content in each sample tree was calculated as thesum of the component pools. Equations were developed toestimate the total biomass and nutrient content of a singletree using DBH as the independent variable (Table 2).[16] We estimated the change in aboveground tree bio-

mass over time for each plot using dendrochronologicalgrowth analysis. The past annual DBH was reconstructedfor each sample tree back to 5 years before drainage usingtheir annual ring data. The previous year’s diameter incre-ment at breast height was then regressed against the currentDBH, using KPL software [Heinonen, 1994]. Applying theregressions to the recorded diameters at breast heightproduced the DBH of each tree a year earlier. This proce-dure was repeated either until the fifth predrainage year oruntil the tree’s DBH became zero. Thus estimates of theannual DBH distributions of the tree stands were obtainedfor as far back as the fifth predrainage year. The recon-structed annual DBH data were divided by tree species into1-cm classes. The mean DBH of each class were fitted intothe biomass and nutrient content equations (Table 2). Thebiomasses and nutrient contents thus obtained for each DBHclass were multiplied by the number of trees in that class.These values were then summed up to give the totalbiomasses and nutrient pools of the tree stands each year.[17] Fine root (Ø � 10 mm) biomass at the beginning of

the investigation period was measured and reported byLaiho and Finer [1996]. Sixteen to 20 peat cores weresystematically taken from each plot from 0–30 cm depth in

Table 2. Equations for Estimating Aboveground (Dry) Biomass

and Nutrient Contents of Single Treesa

Ci p1 p2 p3 R2 s.e.e. n

Pine: Undrained Sites (Ci = p1 DBHp2)Biomass 0.094 (0.043) 2.255 (0.170) 0.986 3.2 14N 0.079 (0.055) 2.714 (0.253) 0.996 8.4 5P 0.017 (0.009) 2.237 (0.198) 0.987 0.6 9K 0.158 (0.095) 2.033 (0.224) 0.980 4.0 9Ca 0.090 (0.045) 2.603 (0.186) 0.991 7.0 9Mg 0.057 (0.033) 2.008 (0.217) 0.982 1.3 9

Pine: Drained Sites (Ci = p1 DBHp2 + A DBHp3)Biomass 0.106 (0.028) 2.369 (0.083) 0.987 14.8 38N 0.921 (0.513) 1.988 (1.173) 0.979 63.6 18P 0.202 (0.120) 1.547 (0.191) 0.931 5.4 33K 0.168 (0.055) 2.100 (0.103) 1.047 (0.127) 0.970 10.5 33Ca 1.040 (0.503) 1.692 (0.154) 0.957 34.3 33Mg 0.062 (0.021) 2.116 (0.107) 0.982 5.0 33

Birch (Ci = p1 DBHp2)Biomass 0.155 (0.071) 2.226 (0.191) 0.983 2.9 15N 0.295 (0.230) 2.534 (0.288) 0.990 17.9 5P 0.151 (0.126) 1.677 (0.332) 0.922 2.2 11K 0.023 (0.012) 2.974 (0.195) 0.987 4.0 11Ca 0.415 (0.219) 2.025 (0.225) 0.984 5.2 10Mg 0.122 (0.055) 1.870 (0.177) 0.979 1.5 11

aCi, kg for biomass, g for nutrients; DBH, diameter at 1.3 m (cm); A,dummy variable adjusting the estimate for recently drained sites (A = 1 iftime since water-level drawdown <10 years, otherwise A = 0); s.e.e.,standard error of estimate. Asymptotic standard errors of the estimatedparameters p1–p3 are given in parentheses.

LAIHO ET AL.: PLANT-MEDIATED OM AND NUTRIENT CYCLING 22 - 3

late June. Zero level was defined the same as for the groundvegetation sampling. The cores were divided into three 10-cm-deep subsamples, starting from the surface. Roots wereextracted by hand. Living roots were separated into threespecies groups: Scots pine, shrubs and birch, and Cyper-aceae (Carex L. spp. sedges and cottongrass, E. vaginatumL.), and four diameter classes:<1, 1–2, 2–5, and 5–10 mm,except for Cyperaceae roots almost all of which had adiameter smaller than 1 mm. Element concentrations weremeasured for each species group with the same methods asfor the ground vegetation.[18] We estimated coarse root (Ø > 10 mm) biomass for

trees using equations developed by Laiho and Finer [1996]for Scots pine, and by Finer [1989, Appendix 19; with thecorrection that +a2d should be +a2 ln d] for birch. Theseequations estimate the total dry mass of stump and coarseroots in single trees, using DBH as an independent value.The element contents of coarse roots were estimated usingthe element concentrations data of Finer [1989] measuredfor the same tree species and site type as in this study. BothFiner’s and our data included nutrient concentration valuesmeasured from the stump (cutting level) of the sample trees.Thus we were able to adjust Finer’s concentrations forcoarse roots by a stump-level concentration ratio in ourversus her sample trees. Accumulation of biomass andnutrients in coarse roots was estimated using the recon-structed annual DBH data.

2.4. Litterfall

[19] Aboveground tree litterfall was measured for 5 con-secutive years (1 June 1993–31 May 1998) at sites 2, 4, and5, and for 2 years at site 6, which was later harvested. Finewoody debris (FWD; twigs and branches) was collected intwenty 0.5-m2 traps (wooden frames, 0.5 m � 1 m) per site,laid systematically across the plots on top of the forest floor.Nonwoody debris was collected in nine 0.5-m2 traps (poly-ethene funnels, diameter 0.8 m) per site, suspended 1 mabove ground. Litter from these traps was sorted intoneedles, leaves, cones, and ‘‘other nonwoody litter.’’ Twigsand branches were excluded from these samples. Traps wereemptied once a month during the snowless period (usuallyfrom May to November); wintertime litterfall was collectedin the first sampling point in the spring. The sorted sampleswere dried at 70�C. Their moisture content was measuredby drying a subsample at 105�C. Nutrient concentrations inthe litter materials were measured using the same methodsas for plant biomass.[20] We estimated the annual amount and nutrient content

of nonwoody litterfall from ericaceous shrubs using litterfallto biomass ratios obtained by Haland [1994] for similarshrub cover. The amount of litterfall in his data was 6.2% ofthe maximum standing stock biomass, the ratios fornutrients varying between 2.6% for K and 14.7% for Ca.Twig litterfall from shrubs was obtained from the FWDtraps, and is included in FWD in this paper.[21] The annual aboveground litterfall from graminoids

was estimated to be 86% of the annual maximum biomassof shoots, as measured by Saarinen [1998] for C. rostratain a mesotrophic fen in the same region as our sites. Thenutrient content of this litter was estimated based on the

results of Bernard et al. [1988] and Bernard and Hankinson[1979] as a proportion of the nutrient content of themaximum standing stock biomass. Bernard and Hankinson[1979] measured decreases of 73–79% in the nutrientcontent in C. rostrata between maximum (summer) andminimum (winter) biomass. Of these decreases, weassumed 50% to be caused by litterfall and 50% by trans-location for N, P, and K, and 100% by litterfall for Ca andMg, based on the work of Auclair [1982] and Bernard et al.[1988].[22] The annual moss litterfall was estimated based on the

results of Vasander [1982], who measured moss biomassand production in the drainage succession of a bog. In allhis sites (n = 14), which were undrained and drained withvarious microsites, moss turnover rates of the mixed speciescompositions similar to ours followed:

turnover ¼ 0:662 1� e�0:008biomass� �

ð1Þ

with R2 = 0.924 and standard error of 0.0403. This impliesthat moss biomass is a good indicator of the productivity ofthe moss community in the drainage succession. Wheremoisture conditions are favorable for moss production,moss biomass is also high and vice versa. For estimating thenutrient content in moss litter, we used the concentrationgradient between the living and dead moss segmentsobserved by Pakarinen [1978]. We used living-to-deadratios of 3 for K, 1.5 for N, 1.4 for P, and 1.0 for Ca and Mg.These are averages of Pakarinen’s values for hummock andhollow species. The ratios follow the generally observedpattern of nutrient mobility.[23] The estimation of fine root litter production was

related to species composition and successional stage. Forsedges in the undrained sites, we applied turnover of 1.2(based on late summer biomass) from Bernard [1974] whopresented data for C. rostrata in Minnesota with a four-seasons climate similar to ours. In the youngest drained site,practically all graminoids were E. vaginatum, which isknown to renew practically all its fine roots annually[e.g., Chapin et al., 1979], i.e., the minimum turnoverwould be 1. N. Silvan et al. (unpublished data) suggest thatthe turnover of E. vaginatum roots would fall between 1 and1.5, so we applied 1.25. Fine root turnover for pine (0.65),shrubs (0.56), and graminoids (1.27) were measured byFiner and Laine [1998] in our 30-year-old drained siteduring 1991–1993. These values were also applied to the22- and 55-year-old drained sites based on rather similarhydrology and species composition. For pine and shrubs,litterfall was estimated only for the <2 mm fraction, whoseannual mortality is higher than that of thicker roots [Pers-son, 1980; Laiho and Finer, 1996]. The nutrient content offine root litterfall was estimated assuming the same elementconcentration for dying roots as was measured for our livingfine roots. According to Bernard [1974] and Bernard andHankinson [1979], the minimum belowground biomass andnutrient content in a standing crop of sedges occurs in lateJune (our sampling time), the time when maximum trans-location of nutrients has occurred for growth abovegroundand when many old rhizomes die. We measured the nutrientconcentrations of dead fine roots in our sites, and they did

22 - 4 LAIHO ET AL.: PLANT-MEDIATED OM AND NUTRIENT CYCLING

not suggest the occurrence of further translocation fromsedge roots before senescence. Neither did notable trans-location seem to take place from tree and shrub roots beforesenescence [see also the work of Nambiar, 1987].

2.5. Nutrient Fluxes

[24] We used the nutrient return in annual litterfall as themeasure of the annual cycle of nutrients through vegetation.With perennial plants, such as trees and shrubs, that retain amajor biomass and nutrient reserve over winter, we face aconceptual problem of timing. The nutrients returned to soilin litterfall in a certain year may actually have been taken upin earlier years. We assumed that this would be of nosignificant consequence in our analysis, where the focus ison longer-term trends and most annual variation is averagedout on purpose.[25] The annual changes in nutrient pools in ground

vegetation biomass (shrubs, graminoids, mosses) were cal-culated as the difference of their (annual maximum) bio-mass nutrient pools between two points of time in thechronosequence, divided by the number of years betweenthe time points. For the trees, the annual changes werecalculated as differences between the reconstructed annualnutrient contents of the stands using the dendrochronologydata. Thus the tree stands have a higher estimation fre-quency than ground vegetation.

[26] Total nutrient uptake from soil per year was esti-mated by adding the nutrients annually cycled throughvegetation to the annual changes in the plant biomassnutrient pools. Moss litter was excluded from the totaluptake estimates, however, because (1) moss turnover wasnot measured in our sites and thus moss litter estimatesinvolved the greatest uncertainty in our analysis and (2)difficulties in separating moss litter from ‘‘peat proper.’’ Thetotal uptake was further partitioned to aboveground andbelowground components to estimate the allocation ofnutrient uptake to aboveground and belowground biomass.A separate total uptake estimate was calculated for themosses.

3. Results

3.1. Plant Biomass and Litterfall Dynamics AfterWater-Level Drawdown

[27] In the undrained sites, the total (aboveground andbelowground) plant biomass was about 1.7 kg m�2, twothirds of which was found in the ground vegetation(Figure 1). Within 50 years following water-level draw-down, the total biomass had increased sevenfold, and morethan 90% of it was tree biomass.[28] A closer examination of the vascular ground vegeta-

tion biomass (Figure 2) showed that the shrub and sedgebiomasses were originally about equal. However, while thegraminoid biomass steadily declined over time, shrub bio-mass increased dramatically at first, peaking 10–20 yearsafter water-level drawdown. After that, mire shrubs declinedwhile forest shrubs gained dominance.[29] Estimated total annual litterfall from vascular plants

slightly increased over time after water-level drawdown,from ca. 550 g m�2 yr�1 in the undrained state and duringtwo decades following water-level drawdown, to ca. 600 g

Figure 1. (top) Aboveground and (bottom) belowgroundplant biomass (dry mass, g m�2) in the drainage successionchronosequence. FR, fine roots (diameter � 10 mm).

Figure 2. Aboveground biomass of vascular groundvegetation (dry mass, g m�2) by species groups: grami-noids, mostly C. lasiocarpa Ehrh. (before water-leveldrawdown) and E. vaginatum L. (after water-level draw-down); mire shrubs, B. nana L., Empetrum nigrum L., L.palustre L., Vaccinium oxycoccos L., Vaccinium uliginosumL.; forest shrubs, V. myrtillus L., V. vitis-idaea L.

LAIHO ET AL.: PLANT-MEDIATED OM AND NUTRIENT CYCLING 22 - 5

m�2 yr�1 when 30–50 years had passed (Figure 3). Thetotal annual litterfall estimate including moss litter peakedsoon after water-level drawdown at 960 g m�2 yr�1, andthereafter ranged from ca. 600 to 850 g m�2 yr�1.[30] Changes in the vegetation composition were clearly

reflected in the litterfall composition (Figure 3). Onundrained sites, about 80% of the annual abovegroundlitterfall was estimated to derive from mosses and sedges.Belowground, an estimated 80% of litter derived fromsedges. Within 20 years after water-level drawdown, how-ever, shrubs and trees had become the major litter source,producing 90% of the total vascular plant litter, and 75% ofthe total litter estimate including moss litter. Over time, thecomposition of litterfall from arboreal plants changed grad-ually: the proportions of slowly decomposing components,such as woody debris and cones, increased.

3.2. Plant Biomass Nutrient Pools and NutrientCycling Through Vegetation

[31] The patterns of change in the nutrient pools in thevegetation standing stock generally followed the corre-sponding changes in biomass. Accordingly, aboveground,arboreal biomass nutrient pools increased rapidly afterwater-level drawdown, and maintained a rather consistentincrease over the 55-year study period (Figure 4). Grami-noids showed a steady decline in the biomass nutrientpools, while for mosses, N and K showed slightly differentinitial patterns. Belowground, water-level drawdown ini-tiated a decline in graminoid fine root nutrient pools(Figure 4). Tree and shrub fine root nutrient pools showeda fluctuation with two periods of increase and an interven-ing recession during the third decade following water-leveldrawdown.[32] Before water-level drawdown, most of the annual

nutrient cycling (measured as nutrient return in litterfall)

Figure 3. Annual (top) aboveground and (bottom) below-ground litterfall (dry mass, g m�2) by litter type. Leaves,shrub leaves + birch leaves; other nonw., unidentifiednonwoody tree litter (male flowers, bark fragments etc.);FWD, fine woody debris (twigs and branches). Below-ground litter is fine root litter only.

Figure 4. Total (left) N and (right) K pools in plant biomass (g m�2 in dry mass). Trees aboveground andcoarse root values are from the dendrochronological growth analysis of the study site drained 55 yearsbefore measurements. Ground vegetation and fine root (FR) values are from the chronosequence data.

22 - 6 LAIHO ET AL.: PLANT-MEDIATED OM AND NUTRIENT CYCLING

was accounted for by graminoids (Figure 5) and mosses(Figure 6), which shed a considerable part of their biomassand nutrients each year. Arboreal plants accounted for lessthan 25% of the annual N cycle, and less than 10% of theannual K cycle through vegetation. After two decades,however, arboreal plants had a major role in the annualnutrient cycle. They accounted for 90–98% of the annual Nand K cycles through vascular plants, and 60–90% of thetotal annual cycles including mosses.[33] After the initial peak, the total annual N uptake by

vascular plants that was allocated to the aboveground partsremained permanently on a higher level than before water-level drawdown (Figure 7). The total annual N uptakeallocated to the belowground parts, in contrast, remainedfairly constant at first, and even seemed to temporarilydecrease after some decades.[34] Comparing the uptake of K by vascular plants

allocated aboveground and belowground gives us an inter-esting picture of the total K dynamics. The rapid decline insedge fine roots after water-level drawdown was enough tocause a decrease even in the total K uptake (Figure 7). Lateron, the highs and lows of aboveground versus belowground

uptakes more or less cancelled each other out, and the totaluptake remained reduced for as long as some decades.

4. Discussion

4.1. Peatland Forest Succession: Changes in the OMand Nutrient Fluxes

[35] The succession induced by water-level drawdownthat we have described may be called a ‘‘peatland forestsuccession.’’ Species adapted to wet conditions declinedwhile shrubs and trees proliferated. Along with the forestsuccession, the OM and nutrient fluxes shifted from groundvegetation dominance to tree dominance in about 20 yearsin our sites, which were originally very sparsely stockedwith trees.[36] Simultaneously, with the change in growth form

dominance, the quantity and tissue type composition ofannual litterfall, as well as the quantity and allocation oftotal annual nutrient uptake, changed. On one hand, therewas a shift from annual cycling of OM and nutrientsthrough the ground vegetation to a long-term cycle throughthe cumulatively growing tree stand, but simultaneously, the

Figure 5. Amounts of (left) N and (right) K cycled annually through vascular plant biomass (g m�2 indry mass) measured as the total nutrient content in annual litterfall.

Figure 6. Estimated total annual (left) N and (right) K uptake by mosses and its components, i.e.,changes in the N and K pools of moss biomass, and annual N and K cycle through mosses.

LAIHO ET AL.: PLANT-MEDIATED OM AND NUTRIENT CYCLING 22 - 7

volume of the annual OM and nutrient cycles increased aswell. These changes may affect the OM and nutrientbalances of peatlands in several ways.[37] Graminoid and herbaceous litters, which were the

major vascular plant litter types in undrained sedge-pinefens, as in many other minerotrophic mire types, decomposefaster than foliage litter from shrubs and trees [Hobbie,1996; Szumigalski and Bayley, 1996], which became themajor litter types after water-level drawdown. Woodymaterials, branches, and cones, decompose especiallyslowly [Taylor et al., 1991; Hobbie, 1996]. Both Sphagnummosses and forest species that increase after water-leveldrawdown, such as P. screberi and Hylocomium splendens(Hedw.) B.S.G., generally decompose slowly [Hobbie,1996; Karsisto et al., 1996]. With the forest succession,the ample moss and woody litter input, decreasing soiltemperature [Minkkinen et al., 1999] and increasing acidityof the surface soil [Laine et al., 1995a, 1995b] may lead tosurface accumulation of OM [Vompersky et al., 1992; Laihoand Laine, 1994b].[38] When evaluating the changes in nutrient cycles, it is

clearly essential to include all vegetation layers, and bothaboveground and belowground parts of vascular plants.Nutrient uptake in vascular plants showed partly contrasting

response patterns to water-level drawdown abovegroundand belowground (Figure 8). These patterns were largelycaused by the changes in species and growth form compo-sition, and tree stand development [e.g., Helmisaari, 1995].Initially, they may also reflect changes in the shoot:rootratios with changing soil moisture conditions [Weltzin et al.,2000].[39] The trends for both N and K may be attributed to the

vegetation dynamics (Figures 1 and 3) [also Laine et al.,1995b]. The uptake of both elements into the abovegroundbiomass of vascular plants increased soon after drainage,due to a buildup of both the relatively nutrient-rich foliageand the accumulating woody biomass of shrubs and trees.The N uptake allocated to belowground biomass (Figure 7)more or less directly reflected the patterns of change in thetotal belowground biomass and litterfall (Figures 1 and 3).The pattern of K uptake was strikingly different. Thedisappearance of sedges, which cycle a lot of K throughtheir fine roots (Figure 5), led to a decrease in the allocationof K uptake to belowground biomass. The increasing shruband tree roots did not reverse this decrease until after adecade. The second period of decrease in belowground Kallocation was largely caused by the decrease of shrub andbirch roots during the time of the major species change frommire shrubs to forest shrubs (Figure 2).[40] The total uptake by vascular plants of N, which is

abundant in the soil (Figures 4 and 8), increased andremained at a higher level than in the pristine state. Onemight end up drawing the same conclusion for K ifobservations were restricted to the aboveground biomassonly. Including belowground biomass reveals, however, that

Figure 7. Estimated total annual (top) N and (bottom) Kuptake by plants, and its allocation aboveground andbelowground.

Figure 8. Total (top) N and (bottom) K concentrations (mgg�1 dry mass) and amounts per unit area (g m�2) in a 0–30cm peat layer in the chronosequence material of Laiho andLaine [1994a, 1995]. Only tall-sedge pine fen sites areincluded, and values have been calculated for the 30-cmlayer that forms the major rooting zone in these sites [Laihoand Finer, 1996]. The data have been smoothed with arobust locally weighted regression, where the local flex ofthe curve was restricted by including 80% of the data pointsin the running window.

22 - 8 LAIHO ET AL.: PLANT-MEDIATED OM AND NUTRIENT CYCLING

the total uptake of K did not change essentially, or ratherdecreased, after drainage. The soil pool of K is small(Figure 8), not much higher than the plant biomass pool(Figure 4), thus total uptake obviously cannot increase:when it increases in one vegetation component, it simulta-neously decreases in another (Figure 7). The total uptake ofK reached the predrainage level only after about 40 yearsfollowing drainage. At this stage, the tree stand was alreadywell developed and could thus itself affect the elementbalance of the site through the increased interception of(dry) deposition [Schauffler et al., 1996]. This interceptionmay be crucial for the K regime of forested peatland sites[Laiho et al., 1999].[41] Despite the changing nutrient requirements of the

vegetation, the nutrient pools in the top 30 cm of peat (themajor rooting zone, Laiho and Finer [1996]) have beenfound to remain rather constant over several decades afterwater-level drawdown, in the absence of other disturbances(Figure 8) [Laiho and Laine, 1994a, 1995; Westman andLaiho, 2003]. Subsidence of the mire surface and compac-tion of the surface peat following water-level drawdown[Rothwell et al., 1996; Minkkinen and Laine, 1998] com-pensate for the decrease observed in base cation concen-trations [Laiho and Laine, 1995; Laiho et al., 1999]. Thuson a nutritional basis, the forest succession may proceed andforest ecosystem functioning may be sustained.[42] The soil pool of N is so large that vegetation uptake

may alter it only slowly. Notable leaching of the largelyorganic soil N has not been observed from drained peatlandsites [Sallantaus, 1992]. Thus the stability of the soil N poolmay be expected. The stability of the small, largely water-soluble soil K pool, however, is interesting as it proves thehighly efficient retention of K in the nutrient cycle [Stoneand Kszystyniak, 1977; Damman, 1978; Miller et al., 1979].Net output of K from the soil, likely through leaching[Kaunisto, 1997], seems to take place only during the periodof lowered total plant uptake immediately following drain-age (Figure 7), when the K concentration (mg g�1) in thesoil decreases (Figure 8). This net output is obviouslymediated by the disturbance in the biological cycle. How-ever, we cannot deduce from our material whether it is acause for or a consequence of the lowered total uptake byplants.

4.2. Is Peatland Forest Succession Feasible in theChanging Climate?

[43] Tree and shrub remains observed in peat stratigraphicstudies suggest that forest successions have been initiated inpeatlands during the Holocene, caused by changes either inlocal conditions or in the climate leading to lowered waterlevels [e.g., Bridge et al., 1990].[44] Ditching causes relatively rapid lowering of the

water level [Hillman, 1992; Paavilainen and Paivanen,1995]. To what extent can we assume the responses of agradual climate change to follow those induced by artifi-cial drainage? Just one exceptionally dry summer has beenfound to cause a similar change in the water level [Alm etal., 1999] as ditching during the first decade (Table 1, site3), when there has not yet been such profound changein evapotranspiration that the tree stand later causes. For

tree growth, a deep water level during the growing season,especially in summer, is much more essential than thedormant season or early summer water levels [Paivanen,1984]. Thus even if wintertime rainfall increased, creatinghigh late fall to early summer water levels, persistent lowsummer water levels might induce release growth in thetree stand. Tree stand productivity may reach its potentialmaximum when the water level remains below 30–40 cmduring 60% (pine) to 80–90% (spruce) of the growingseason [Vompersky and Sirin, 1997]. However, sufficientlylong dry periods are more easily experienced on drainedsites, where after intermittent flooding events the ‘‘excess’’water is drained via the ditch network, unlike in undrainedsites where lateral water movement is usually slow [Hill-man, 1992; Mugasha et al., 1993; Verry, 1997; Vomperskyand Sirin, 1997].[45] With several consecutive dry summers, a forest

succession might begin more or less similarly as afterartificial drainage. How rapidly trees may respond to thereduced water level depends on tree species, size, and age[Seppala, 1969; Macdonald and Yin, 1999]. A responsemay usually be seen already during the growing seasonfollowing water-level drawdown, first in fine roots andneedles, then in radial growth [Seppala, 1969; Mugasha etal., 1993]. It may take 5–30 years for the release growth toreach its maximum, depending on tree species and size/age,and site nutrient regime [Seppala, 1969]. Once the waterlevel has been down long enough for a tree stand to becomewell established, the stand itself has a major effect on thewater balance and water level of the peatland throughgreatly increased interception of rainfall, and evapotranspi-ration [Heikurainen and Paivanen, 1970; Paivanen, 1980].If intervening wet summers occurred frequently enough,however, they might lead to the deterioration, even thedeath, of the tree stand. This might leave mire dwarf shrubsas the dominant vegetation, as is typical of early stages ofthe succession (Figure 2).

4.3. Sensitivity of Our Estimates

[46] We want to emphasize that the focus in interpretingour results should be on the pattern of change rather than theactual values. Due to the higher-frequency measurements,the biomass and nutrient estimates of trees may be consid-ered the most reliable.[47] Moss and fine root litterfall were the components of

OM and nutrient fluxes for which we had the least intensivemeasurements. They are difficult to measure, and there arerelatively little literature data to be used either. However, netprimary production and biomass generally have a closecorrelation within physiognomic species groups [e.g.,Moore et al., 2002] (Table 2). Consequently, we estimatedmoss and fine root litterfall by applying turnover ratios tothe measured biomass data, accounting for the hydrologicalstatus (undrained versus drained).[48] Concerning moss litter, equation (1), which leads to a

maximum turnover of 0.66, may be considered to produceminimum estimates for our fen sites, as higher turnoverrates have been observed in more nutrient-rich sites than thebog sites from which equation (1) was derived. Moore et al.[2002] reported a turnover as high as 1.4 for a poor fen. Still

LAIHO ET AL.: PLANT-MEDIATED OM AND NUTRIENT CYCLING 22 - 9

assuming the turnover:biomass relation to follow equation(1) in the drainage succession, applying this higher turnoverrate would result in about doubled amounts of moss litter,which may be considered the maximum estimates. Thepattern of change would remain the same, however, andthe higher estimates would not change our conclusions butwould further emphasize the role of mosses.[49] Based on biomass, sedge roots are the major below-

ground components in OM and nutrient cycling in thepristine state, and tree and shrub roots following water-level drawdown. Fine root turnover of graminoids andherbs, related to late summer biomass (our biomass meas-urement time) seems to generally vary between 1 and 1.5[e.g., Bernard, 1974; Bernard and Hankinson, 1979; Finerand Laine, 1998]. The values that we applied fall in themiddle of this range. The turnover of tree and shrub rootswas measured in our 30-year-old drained site. We see noreason to assume that the turnover rates would have beenessentially different at the 22- and 55-year-old drainedsites.

4.4. Conclusions

[50] (1) During their Holocene-long persistence, borealpeatlands have developed an inherent autogenic change-ability, and react fast to changing hydrological conditions. Itis important to account for the changes in plant-mediatedOM and nutrient fluxes when evaluating ecosystemresponse to changing environmental conditions.[51] (2) The main adaptation mechanism to long-term

alteration of water level is change in species compositionand relative abundance. Long-term water-level drawdowninduces a decline in graminoids and proliferation of shrubsand/or trees. Mosses retain their functional importance afteradaptation at the community level.[52] (3) Consequently, the tissue type composition of

litterfall changes. The litter material of dried-out sites islargely slowly decomposable, which will counteract theenhancing effect of water-level drawdown on decomposi-tion processes.[53] (4) Nutrient uptake by plants shows partly contrast-

ing response patterns aboveground and belowground, andbetween elements abundant or sparse in the soil. On a soilnutritional basis, however, the developing peatland forestecosystem may be sustained.

[54] Acknowledgments. The assistance of Veli-Matti Komulainen,Jouni Meronen, Kari Minkkinen, Mikko Tiirola, Luo Jun, Leena Finer,Seija Repo, Kirsti Derome, Matti Siipola, Mikko Kukkola, and laboratorystaff members of the Finnish Forest Research Institute in Joensuu, Rova-niemi, and Salla at various stages of the study is gratefully acknowledged.Line Rochefort, Jukka Alm, and the referees made valuable comments onthe manuscript. This study was funded by the Academy of Finland and theFinnish Ministry of Agriculture and Forestry.

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�������������������������R. Laiho, J. Laine, and H. Vasander, Peatland Ecology Group,

Department of Forest Ecology, University of Helsinki, P.O. Box 27(Latokartanonkaari 7), FIN-00014 Helsinki, Finland. (raija.laiho@helsinki.fi; harri.vasander@helsinki.fi; jukka.k.laine@helsinki.fi)T. Penttila, Finnish Forest Research Institute, Vantaa Research Centre, P.O.

Box 18 (Jokiniemenkuja 1), FIN-01301 Vantaa, Finland. (timo.penttila@metla.fi)

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