Scand. J. For. Res. 16: 37 ± 43, 2001

Microbial and Plant Community Structure Across a PrimarySuccession Gradient

TAINA PENNANEN1, RAUNI STROÈ MMER2, ANNAMARI MARKKOLA3 andHANNU FRITZE1

1Finnish Forest Research Institute, P.O. Box 18, FI-01301 Vantaa, Finland, 2Department of Ecological and En×ironmental

Sciences, Uni×ersity of Helsinki, FI-15140 Lahti, Finland, and 3Department of Biology, Uni×ersity of Oulu, P.O. Box

3000, FI-90014 Oulu, Finland

Pennanen, T.1, StroÈ mmer, R.2 Markkola, A.3 and Fritze, H.1, (1Finnish Forest Research

Institute, P.O. Box 18, FI-01301 Vantaa, Finland, 2Department of Ecological and Environ-

mental Sciences, University of Helsinki, FI-15140 Lahti, Finland, and 3Department of

Biology, University of Oulu, P.O. Box 3000, FI-90014 Oulu, Finland). Microbial and plant

community structure across a primary succession gradient. Received February 23, 2000.

Accepted June 7, 2000. Scand. J. For. Res. 16: 37 ± 43, 2001.

The formation of the organic layer within Scandinavian forest soil started about 10000 yr ago,

following the retreat of the continental ice sheet. Since then the land has been slowly rising in

northern Europe and uplift still occurs on the coast of the Bothnian Bay at a rate of about

0.6 ± 0.9 m per 100 yr. Four, 300 m long, successional gradients were studied from the shoreline

to a Scots pine (Pinus syl×estris) stand with a fully developed humus layer of a few

centimetres’ thickness. The plantless shoreline was followed by small foredunes and dunes,

characterized by Agrostis stolonifera and Leymus arenarius, respectively, and the de¯ ation

basin characterized by lichens and sparse Festuca o×ina and Deschampsia ¯ exuosa. The study

sites situated in Scots pine stands of about 25 and 40 yr age were characterized by sparse

dwarf shrubs, lichens and bryophytes. The amount of organic matter in soil increased along

the gradient. When the microbial biomass, estimated as indicative phospholipid fatty acids

(PLFAs), was calculated on the organic matter basis, the total microbial biomass as well the

amount of bacterial PLFAs decreased from the earlier stages of succession towards the pine

forest. The ratio of fungal to bacterial PLFAs increased along the succession. The bacterial

community structure in the shore soil was different to that in the dune soil or in forested

zones. On the plantless shore the microbial community was almost completely described by

PLFAs indicative of Gram-negative bacteria. In addition to these bacterial PLFAs, dunes were

characterized by PLFAs indicative of actinomycetes. Thus, the fungal part of the microbial

community seemed to respond most to the accumulation of organic matter and increasing C:Nratio, while the bacterial biomass and bacterial community structure seemed also to re¯ ect the

composition of the vegetation and the varying quality of the organic matter. Key words: forest

soil, microbial community structure, plants, PLFA, succession.

INTRODUCTION

Microbial and plant communities are functionally

dependent on each other, owing to root exudates and

mineralization of nutrients from plant litter by soil

microbes, and their responses to primary soil forma-

tion are expected to be parallel (Ohtonen & VaÈ re

1998, Ohtonen et al. 1999, Aikio et al. 2000). The

formation of the organic layer within Scandinavian

forest soil started about 10000 yr ago, following the

retreat of the continental ice sheet. Since then the

land has been slowly rising in northern Europe and

the uplift still occurs in certain areas. On the coast of

the most northern part of the Baltic Sea, Bothnian

Bay, the uplift continues at a rate of about 0.6 ± 0.9 m

per 100 yr. Hailuoto, the largest island in Bothnian

Bay, has a ¯ at topography and thus a rapid shoreline

retreat, of several hundred metres in a century

(Alestalo 1979). The shorelines, therefore, provide an

unique opportunity to study the primary formation

of the soil organic layer which is controlled by vege-

tation and microorganisms. Further, the main soil-

forming process in this area is podzolization, due to a

strong impact of acidic exudates and products of

plant roots and mycorrhizal fungi (van Breemen et al.

2000) and low buffering capacity of the sandy soil. In

this area, podzolization normally takes 340 and 1000

yr to become visible (Starr 1991). This study aimed to

describe these early successional stages of above-

ground vegetation and below-ground microbial com-

munities, and relate the changes in microbial

community to the accumulation of soil organic mat-

ter (OM).

© 2001 Taylor & Francis. ISSN 0282-7581

T. Pennanen et al.38 Scand. J. For. Res. 16 (2001)

MATERIALS AND METHODS

Soil sampling

The study area is located near Marjaniemi, on the

western coast of the Hailuoto island in Finland

(65°02Æ N, 24°35Æ E). The six study sites (named as

shore, foredune, dune, basin, young forest and forest)

were selected according to the vegetation zones seen

along the succession gradient in the August 1994.

This transect was about 300 m long and started at the

shoreline and ended at a Scots pine (Pinus syl×estris)

stand. Samples were collected from the surface soil at

the depth of 1 ± 2 cm using a soil corer 7.2 cm in

diameter. The Scots pine stand was characterized by

a fully developed humus layer a few centimetres

thick. The humus layer was thinner and patchy in the

young forest and basin. The dune, foredune and

shoreline samples mainly consisted of mineral soil,

which was sand throughout the gradient. Fourteen

soil cores were collected from each zone from within

an area of 3 × 12 m adjacent to the transect. The soil

cores were combined and samples were sieved (2 mm

mesh) before analysis. To obtain more replicate sam-

ples, three additional transects were sampled close to

the main transect in August 1995. The samples were

taken from the same vegetation zones along the

gradient as described above. Five replicate cores for

each vegetation zone were taken from all three addi-

tional transects, and the ® ve cores were combined to

form one sample per zone and transect. Thus, four

replicate samples (n¾4) were obtained per vegetation

zone.

Vegetation analyses

All four transects showed a distinct visual similarity,

and therefore the vegetation succession was described

using only the main gradient. Vegetation cover analy-

ses were done on a percentage scale using 1 m2

squares, ® ve for each of the six vegetation zones.

Chemical and microbiological analyses

Soil samples were stored at »4°C before phospho-

lipid fatty acid (PLFA) analysis was performed. Hu-

mus dry weight was determined by drying duplicate

subsamples at 105°C overnight and OM content by

heating dried subsamples at 550°C for 4 h. Total

amounts of carbon and nitrogen were determined

using dry combustion (Leco CHN-1000) on air-dried

humus. pHH2O (1:1.7 v:v) and dissolved organic car-

bon (DOC) analyses were done using undried soil

which had been stored in a freezer (¼18°C). DOC

was measured from the ® ltered (0.45 mm) water ex-

tract using a Shimadzu TOC-5000 total organic car-

bon analyser. pH, C:N ratio and DOC measurements

were done only for samples from the main gradient

line (n¾1). The results are expressed on an OM

basis.

The phospholipid extraction and analysis of

PLFAs was done as described previously by

FrostegaÊ rd et al. (1993) with some modi® cations

(Pennanen et al. 1999). To summarize this procedure:

0.5 g fresh weight of humus was extracted with

chloroform:methanol:citrate buffer mixture (1:2:0.8)

and the lipids were separated into neutral lipids,

glycolipids and phospholipids on a silicic acid

column. The phospholipids were subjected to a mild

alkaline methanolysis and the fatty acid methyl esters

were detected by gas chromatography. Fatty acids

were designated in terms of total number of carbon

atoms: the number of double bonds, followed by the

position of the double bond from the methyl end of

the molecule. The pre® xes i and a indicate iso- and

anteiso-branching, br indicates unknown branching

and cy indicates cyclopropane fatty acid. Me refers to

the position of methyl group from the carboxyl end

of the chain. The sum of PLFAs considered to be

predominantly of bacterial origin (i15:0, a15:0, 15:0,

i16:0, 16:1v9, 16:1v7t, i17:0, a17:0, 17:0, cy17:0,

18:1v7 and cy19:0) was chosen as an index of the

bacterial biomass (FrostegaÊ rd & BaÊ aÊ th 1996). The

quantity of PLFA 18:2v6,9 was used as an index of

fungal biomass, as it has been suggested to correlate

with the amount of ergosterol (FrostegaÊ rd & BaÊ aÊ th

1996), a sterol found only in fungi. However, the

amount of ergosterol is also known to vary among

different fungal species (MuÈ ller et al. 1994). The ratio

of fungal and bacterial PLFAs was used as an index

of the ratio of fungal:bacterial biomass in the soil.

The Gram-negative bacteria usually have more mo-

nounsaturated or cyclic fatty acids (Wilkinson 1988),

Gram-positive bacteria have more iso-, anteiso- or

otherwise branched fatty acids (O’ Leary & Wilkinson

1988) and actinomycetes often have a methyl group

in the 10th carbon atom from the carboxyl end of the

chain (Kroppenstedt 1985). Because the distribution

of PLFAs across the taxonomic groups may not

always follow the generalizations presented above

(Haack et al. 1994) and most of the individual

PLFAs are common in all microbial cells, only the

relative abundances of these different bacterial

groups were discussed.

Succession of microbial and plant communitiesScand. J. For. Res. 16 (2001) 39

RESULTS

The vegetation zones from the shore to the forest

were clearly distinguishable from each other (Table

1). The plantless shoreline was followed by small

foredunes and dunes, dominated by Agrostis

stolonifera and Leymus arenarius, respectively. The

de¯ ation basin behind the dunes was characterized by

Cladonia lichens and sparse Festuca o×ina and

Deschampsia ¯ exuosa. The oldest part of the de¯ ation

basin contained a 25-yr-old Scots pine forest, referred

to here as a young forest, and consisted of patchily

distributed Empetrum nigrum, Vaccinium uliginosum

and Salix repens in the ® eld layer and Polytrichum

juniperum and abundant hepatics in the ground layer.

Table 1. Vegetation analysis of the different succession zones presented as co×erage percentages

De¯ ation basin Young forest ForestForedune Dune

Vascular plants

70Agrostis stolonifera

3Calluna ×ulgaris

»Carex brunnescens

2C. nigra

77 »Deschampsia ¯ exuosa

»Drosera rotundifolia

2822Empetrum nigrum

8 »Festuca o×ina 6

»Hieracium umbellatum » 1

0.5Honkenya peploides 1

0.5Leontodon autumnalis

26 3Leymus arenarius 7

1 »Rumex acetosella »4Salix repens

Sonchus ar×ensis »Tanacetum ×ulgare 1

Vaccimiun myrtillus »227V. uliginosum

3V. ×itis-idaea

Bryophytes

1Dicranum sp.

0.5 2Pleurozium schreberi

»Pohlia nutans »8Polytrichum juniperinum

P. strictum 11

» 52Hepaticeae

Lichens

216Cladina arbuscula

10 18C. rangiferina

4 »C. syl×atica

» 3 »Cladina sp.

1Cladonia botrytes

11C. chlorophaea

5 7C. cornuta

» 1C. crispata

6C. deformis 2

81C. gracilis

C. uncialis 2

»C. sulphurea

»» 4Cetraria ericetorum

C. islandica 1

2 »Ceratodon purpureus

»Stereocaulon sp.

» The species was present and the coverage percentage was less than 0.5%.

T. Pennanen et al.40 Scand. J. For. Res. 16 (2001)

Fig. 1. (A) Soil organic matter content, (B) pH, (C) C:N ratio, and (D) amount of DOC along the succession gradient. Bars

(A) indicate the standard error for four replicate transects.

The Scots pine forest consisted of 40-yr-old stands

and was characterized by dwarf shrubs (mainly Em-

petrum nigrum ) and by Cladina lichens (mainly C.

arbuscula and C. rangiferina ) and bryophytes (mainly

Polytrichum strictum ) on the ground layer.

The OM accumulated in the soil along the gradi-

ent, reaching 6% of the sample dry weight in the

forest (Fig. 1A). The acidity of soil (Fig. 1B) and C:Nratio (Fig. 1C) increased throughout the gradient,

while DOC remained relatively stable except for a

decrease in the forest (Fig. 1D). There was an in-

crease in the microbial biomass along the gradient

when biomass was calculated per dry matter content

(data not shown). However, when calculated on an

OM basis, the total microbial biomass decreased

from the ® rst stages of succession towards the forest

(Fig. 2A).

The index of the bacterial biomass measured as

PLFAs of bacterial origin (Fig. 2C) was highest in

the shore and foredune. On the dry dunes the amount

of bacterial PLFAs was lower and also remained low

at the older stages of succession. The amount of

fungal PLFA was highest in the de¯ ation basin (Fig.

2B). The ratio of fungal to bacterial PLFAs (Fig. 2D)

increased along the succession gradient. The micro-

bial community on the plantless shore was almost

completely described by monounsaturated PLFAs,

and the relative proportion of most of these PLFAs

decreased with the progressing succession (Fig. 3A,

B). The cyclic PLFAs (Fig. 3C) showed a variable or

increasing trend along the gradient. All of the methy-

lated PLFAs (Fig. 3D) and a few branched PLFAs

(i15:0, a17:0, i17:0, data not shown) appeared to

re¯ ect the distinct vegetation zones instead of contin-

uous trends along the gradient. The other determined

PLFAs, e.g. saturated straight-chain PLFAs 15:0,

16:0, 17:0, 18:0, common in all kind of microorgan-

isms, did not change with respect to the successional

gradient (data not shown).

DISCUSSION

The decrease in the microbial biomass along the

succession was in agreement with the results from a

study on the chronosequence of reclamation sites

(Insam & Domsch 1988). These authors suggested

that a decrease in the ratio of microbial C to organic

C (Cmic:Corg) is an indication of the decreasing

availability of carbon for microorganisms. Carbon

availability depends largely on the quality of organic

compounds, which further depends on the composi-

tion of plant community. Thus, carbon availability is

not only an age-dependent factor, even though the

effect of time and change in the quality of plant litter

cannot be separated in this study (see also Ohtonen et

al. 1999). Haslam et al. (1998) showed that the in-

Succession of microbial and plant communitiesScand. J. For. Res. 16 (2001) 41

crease in the age and stage of decomposing OM is

associated with increasing relative intensity of the

alkyl- and methyl-C signal of the [13C]NMR (nuclear

magnetic resonance) spectra. Preston (1996) related

the increasing relative intensity of these signals to the

loss of easily metabolized carbohydrates and accumu-

lation of plant biopolymers. In the present study the

quality of the OM was described by measuring the

amount of DOC and C:N ratio in the soil OM. DOC

remained relatively stable throughout the gradient

Fig. 2. Amount of total (A) microbial, (B) fungal, and (C) bacterial PLFAs expressed as nmol PLFAs g¼1 OM, and (D)

the ratio of fungal to bacterial PLFAs. Bars indicate the standard error for four replicate transects.

Fig. 3. The relative abundance of PLFAs common in (A ± C) Gram-negative bacteria and (D) actinomycetes along the

succession gradient. Bars indicate the standard error for four replicate transects.

T. Pennanen et al.42 Scand. J. For. Res. 16 (2001)

except for a decrease in the forest (Fig. 1D). The

C:N ratio of the soil increased along the gradient

(Fig. 1C) and this trend was also noticed in the

older stages of succession in Hailuoto (Aikio et al.

2000). A decrease in soil pH has been connected

several times to a decrease in the availability of

carbon (Persson et al. 1989, BaÊ aÊ th et al. 1995, Mer-

ilaÈ & Ohtonen 1997). According to this hypothesis,

increasing acidity along the gradient (Fig. 1B) may

be related to the lower availability of carbon. Thus,

the change in vegetation along the chronosequence,

especially the appearance of dwarf shrubs and trees

in the older vegetation zones, may have altered the

availability of carbon to the decomposing mi-

cro¯ ora. However, in the older stages of the succes-

sion with lichen-dominated vegetation on the island

of Hailuoto, Aikio et al. (2000) found that the soil

respiration rate was high. This was suggested to

re¯ ect the increased C:N ratio of the OM and lack

of nutrients for microorganisms, causing the loss of

carbon from the ecosystem through microbial respi-

ration. The humus layer was found to become thin-

ner and nutrient concentrations per unit area lower

at these later stages of primary succession.

The amount of the PLFAs of bacterial origin

(Fig. 2C) was highest in the shore and foredunes,

which are subjected to ¯ uctuating sea levels. On the

dry dunes this index of bacterial biomass was lower

and also remained low at the older stages of succes-

sion. PLFA analysis gave an estimate of the fungal

biomass, and the amount of the fungal indicator

was highest in the de¯ ation basin (Fig. 2B), possibly

due to the PLFA 18:2v6,9 present in lichen residues

which were dif® cult to separate from the extremely

poorly developed organic layer in that zone. The

ratio of fungal to bacterial PLFAs increased along

the succession (Fig. 2D). It is probable that this

change in the microbial community structure was

mainly due to the appearance of mycorrhizal eri-

coids and Scots pine (Table 1). Soil moisture has

been suggested to be the main factor affecting the

growth of the trees in the northern part of Both-

nian Bay coast, and not the age of the podzolic

pro® le (Starr 1988). Thus, mycorrhizal symbiosis

improving the water supply may be of extreme im-

portance to the plants in this zone, and the mycor-

rhiza contribute to the increasing proportion of the

fungal biomass. Such a trend towards fungal domi-

nance in the microbial community with time was

found by Ohtonen et al. (1999), in a primary suc-

cessional chronosequence on a glacier forefront in

Alaska. They showed that the vegetation determined

the accumulation of OM in soil and also the size of

the microbial community. In addition, Bardgett et

al. (1999) suggested that as the amount of plant

litter and the complexity of the plant ± soil system

increase, the growth of fungi increases in relation to

the growth of bacteria. Both Bardgett et al. (1999)

and Wardle et al. (1995) came to this conclusion by

measuring short-term trends (9 months and 3 yr,

respectively) in the terrestrial model communities.

This ® eld study covered a time span of hundreds of

years and was in agreement with the results of these

experiments.

The microbial community on the plantless shore

was almost completely described by monounsatu-

rated PLFAs indicative of Gram-negative bacteria

(Wilkinson 1988), and the relative proportion of

these PLFAs decreased with the progressing succes-

sion (Fig. 3A, B). However, this generalization of

the Gram-negative bacterial community must be

taken as tentative because the relative amount of

the PLFA 16:1v5 (Fig. 3B), indicative of arbuscular

mycorrhizal fungi and also of Gram-negative bacte-

ria responding to easily available carbon in soil

(FrostegaÊ rd et al. 1996), did not decrease along the

gradient. In addition, the cyclic PLFAs (Fig. 3C),

common in Gram-negative bacteria (Wilkinson

1988), showed variable or increasing trends. The

plant composition in the successional stages was

better re¯ ected by individual PLFAs common in

Gram-positive bacteria than the PLFAs or PLFA-

derived biomass indicators mentioned above. For

example, methylated PLFAs (Fig. 3D), indicative of

actinomycetes (Kroppenstedt 1985), and other

branched PLFAs (i15:0, a17:0, i17:0; data not

shown) common in Gram-positive bacteria (O’Leary

& Wilkinson 1988) appeared to re¯ ect the distinct

vegetation zones instead of continuous trends along

the gradient.

From the data, it can be stated that the changes

in vegetation along the primary succession gradient

were re¯ ected in the overall microbial community

structure. In particular, the Gram-positive bacteria

seemed to re¯ ect the changing composition of the

vegetation in different zones. A predominant group

of Gram-negative bacteria decreased with the pro-

gressing succession and accumulation of carbon.

The ratio of fungal to bacterial PLFAs increased

along the succession, indicating a change towards

more fungal dominance in the microbial commu-

nity.

Succession of microbial and plant communitiesScand. J. For. Res. 16 (2001) 43

ACKNOWLEDGEMENTS

The study was supported by The Academy of Fin-

land. We thank Andrew Rebeiro-Hargrave for revis-

ing the English.

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