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Carbon,nitrogenandphosphorusdynamicsofantmounds(Formicarufagroup)inmanagedborealforestsofdifferentsuccessionalstages

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Carbon, nitrogen and phosphorus dynamics of ant mounds(Formica rufa group) in managed boreal forests of differentsuccessional stages

J. Kilpelainen a,*, L. Finer a, P. Niemela b, T. Domisch a, S. Neuvonen a,M. Ohashi a, A.C. Risch c, L. Sundstrom d

a Finnish Forest Research Institute, Joensuu Research Unit, P.O. Box 68, FI-80101 Joensuu, FinlandbUniversity of Joensuu, Faculty of Forestry, P.O. Box 111, FI-80101 Joensuu, FinlandcSwiss Federal Institute for Forest, Snow and Landscape Research, Community Ecology, Zuercherstrasse 111, 8903 Birmensdorf, SwitzerlanddUniversity of Helsinki, Department of Biological and Environmental Sciences, Ecology and Evolutionary Biology Unit, P.O. Box 65,

FI-00014 Helsinki, Finland

1. Introduction

Wood ants (Formica rufa group) are a prominent feature in the

boreal forests of Eurasia. Ants build their mounds from forest

litter and tree resin droplets, and they use honeydew and

insects and small invertebrates as their food source. On

average three mounds per hectare occur in the most common

forest site types in Finland (Rosengren et al., 1979; Domisch

a p p l i e d s o i l e c o l o g y 3 6 ( 2 0 0 7 ) 1 5 6 – 1 6 3

a r t i c l e i n f o

Article history:

Received 29 September 2006

Received in revised form

22 January 2007

Accepted 24 January 2007

Keywords:

Boreal forest

Formica rufa group

Carbon

Nitrogen

Phosphorus

Bulk density

a b s t r a c t

Wood ants (Formica rufa group) are ubiquitous in European boreal forests and their large

long-lived mound nests, which mainly consist of forest litter and resin, accumulate carbon

(C) and nutrients. The C and nutrient dynamics of wood ant mounds in response to forest

succession have received minor attention in boreal forests. We aimed to study whether the

C, nitrogen (N) and phosphorus (P) concentrations and the bulk density of ant mounds differ

from those of the surrounding forest soil, to estimate the C, N and P pools in ant mounds,

and to test whether the concentrations and pools change with forest age. Norway spruce

(Picea abies (L.) Karst.) stands on medium-fertile sites in 5-, 30-, 60- and 100-year stand age

classes were studied in eastern Finland. Carbon and P concentrations in the above-ground

mound material were higher than those in the surrounding organic layer. The C, N and

extractable P concentrations were higher in the soil under the ant mounds than in the

surrounding mineral soil (0–21 cm). The low bulk densities in the ant mounds and the soil

below them could be a result of the porous structure of ant mounds and the soil-mixing

activities of the ants. The C/N ratios were higher in the mounds than in the organic layer.

Carbon concentrations in the ant mounds increased slightly with stand age. Carbon, N and P

pools in the ant mounds increased considerably with stand age. Carbon, N and P pools in ant

mounds were <1% of those in the surrounding forest soil. Nevertheless, the above- and

belowground parts of the ant mounds contained more C, N and P per sampled area than the

surrounding forest soil. Wood ants therefore increase the spatial heterogeneity in C and

nutrient distribution at the ecosystem level.

# 2007 Elsevier B.V. All rights reserved.

* Corresponding author. Tel.: +358 10 211 3177; fax: +358 10 211 3001.E-mail address: jouni.kilpelainen@metla.fi (J. Kilpelainen).

avai lable at www.sc iencedi rec t .com

journal homepage: www.e lsev ier .com/ locate /apsoi l

0929-1393/$ – see front matter # 2007 Elsevier B.V. All rights reserved.doi:10.1016/j.apsoil.2007.01.005

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et al., 2005; Kilpelainen et al., 2005). Most of the boreal forests

in Fennoscandia are managed and forest succession starts

from clear-cutting. This can be destructive for ant colonies

because it removes food resources and changes the micro-

climatic conditions (Rosengren et al., 1979). Ants often

abandon their mounds within a few years after clear-cutting,

but new mounds are soon established and the composition of

wood ant species may change during forest succession (e.g.

Punttila, 1996). Ant mounds remain active in the same

locations for several years, sometimes even for decades.

The results of studies in temperate and alpine forests in

Europe indicate that ants can play a significant role in nutrient

cycling and aggregation (e.g. Frouz et al., 1997) but, so far, their

contribution to element pools and fluxes have not been

studied in boreal forests (see Laakso and Setala, 1998). Studies

on active ant mounds in temperate, hemi-boreal and

subalpine areas (Zakharov et al., 1981; Lenoir et al., 2001;

Frouz et al., 2005; Risch et al., 2005) report only carbon (C),

nitrogen (N), phosphorus (P) concentrations and/or bulk

densities of the above-ground mound material, and they do

not investigate the effects of forest succession on element

concentrations or pools.

The C and nutrient concentrations in ant mounds can be

higher than those in the surrounding soil (Zakharov et al.,

1981; Lenoir et al., 2001; Risch et al., 2005) due to the

extensive flow of prey (Stradling, 1978), honeydew (Roseng-

ren and Sundstrom, 1991) and mound constructing material

(Rosengren and Sundstrom, 1987) into the mound, and the

high number of nitrogen-fixing bacteria (Frouz et al., 1997)

and other soil microbes (Laakso and Setala, 1998) living in

the mounds. Obviously the type of material used for

construction also affects the chemical properties of ant

mounds. Nutrient concentrations in the organic layer of

forest soils decrease during forest succession (Tamminen,

1991) and, if the nutrient concentrations of the mound

material are primarily dependent on the surrounding litter,

the nutrient concentrations in ant mounds could thus also

decrease during succession. The total C and nutrient pools

in ant mounds; however, are likely to increase during forest

succession because ant mound and ant densities are higher

in older stands (Sorvari and Hakkarainen, 2005), and

the ants have had more time to build larger mounds. There

are no estimates of the contribution of ant mounds to

the total element pools in boreal forest soil (cf. Risch et al.,

2005).

Bulk density could indicate ant colony vitality since in

recently clear-cut sites severely depleted colonies may not be

able to keep their mounds in good condition and aerated,

which could result in on average higher bulk densities of

mounds in clear-cut sites than in mature sites. Dense and less

active mounds can decompose faster than dry and porous

active mounds (Lenoir et al., 2001). Thus forest management,

ant colony vitality, bulk density, decomposition and C and

nutrient concentrations and pools of ant mounds could be

related.

The aim of the study was to determine whether the C, N

and P concentrations and the bulk density of active ant

mounds differ from those of the surrounding forest soil, and

whether such differences relate to forest age. We also assessed

and compared the C, N and P pools in active ant mounds in

forests of different age.

2. Material and methods

2.1. Study sites

The study was carried out in four replicate 5-, 30-, 60- and 100-

year-old stands (2.3–11.3 ha) growing on medium-fertile

(Myrtillus type according to the Finnish site type classification

by Cajander, 1949) sites in eastern Finland (298520E, 638040N,

170 m a.s.l.). The 16 stands were managed as Norway spruce

(Picea abies (L.) Karst.) stands. Although only Norway spruce

was planted in the 5-, 30-, and 60-year-old stands, deciduous

trees and Scots pine (Pinus sylvestris L.) are numerous in the

early successional stages (Table 1). The 100-year-old stands

were naturally regenerated because at that time planting was

rare. The stands were managed according to normal practices

including thinning at appropriate times. The soil type in the

sites was haplic podzol (FAO-Unesco, 1990) on glacial till and

the organic layer was on the average 8 cm thick. The stands

contained an average of 3.8 active ant mounds ha�1 (Table 2).

Table 1 – Mean tree number (haS1), height (cm) and stem volume (m3 haS1) and their standard errors (in parentheses) inthe different stand age classes (n = 4)

Age(years)

Piceaabies

Pinussylvestris

Betulaspp.

Populastremula

Alnusincana

Sorbusaucuparia

Total

5 Trees 1727 (104) 1610 (1129) 13397 (3363) 192 (120) – 8547 (4415) 25,473 (7490)

Height 52 (3) 37 (4) 65 (7) 66 (25) – 82 (3) –

30 Trees 1258 (228) 95 (87) 149 (49) 8 (5) 249 (55) 25 (25) 1,784 (289)

Volume 128 (10) 11 (10) 19 (10) 0 (0) 4 (2) 0 (0) 163 (17)

60 Trees 760 (147) 128 (103) 134 (75) 69 (66) 52 (49) – 1,143 (363)

Volume 192 (51) 23 (16) 7 (5) 7 (6) 0 (0) – 229 (36)

100 Trees 708 (232) 103 (65) 97 (39) 9 (7) 31 (24) – 949 (193)

Volume 237 (34) 65 (39) 21 (13) 0 (0) 1 (1) – 325 (24)

The measured trees in seedling stands were �20 cm tall, and in the other stands �4 cm thick at breast height. Height describes the stand

structure in the 5-year stand age class.

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All the ant mounds in the 16 stands were inventoried and their

heights and diameters measured during summer 2003. After

the inventory, we sorted the active ant mounds within stand

age classes by volume and selected sample mounds randomly

from certain fractiles: 60, 70, 80 and 90% in the 5-year; 45, 60, 75

and 90% in the 30- and 60-year; 30, 50, 70 and 90% in the 100-

year age class. The volume range of the sample mounds was

63–2353 dm3. One to three ant mounds were sampled in each

stand. Most of the ant mounds were inhabited by F. aquilonia

Yarr., but F. polyctena Forst. inhabited one of the ant mounds in

the 30-year stand age class and F. rufa L. one mound each in the

5- and 30-year age classes.

The ant mounds had a moist surface layer, a dryer and

looser interior, and a transition from mixed organic/mineral to

mineral soil belowground. Three core samples were taken

from the aboveground parts of each ant mound with a

stainless steel corer with a diameter of 14 cm: one sample was

taken at the centre of the mound, one at the edge of the mound

and one in between the centre and edge (Fig. 1). The depths of

the core samples were measured from the exposed ant

mound, the lowest point of the sample being the level of

the uppermost mineral soil around the ant mound. One below-

ground soil sample was taken under each of the three above-

ground ant mound samples to a depth of 21 cm with a

cylindrical sampler with a diameter of 72 mm and length

49 mm.

Four samples were taken from the forest soil at points

3 m from the ant mound edge in north, west, south and east

directions (Fig. 1). We assumed that ant mounds did not

have any significant effect on the nutrient concentrations in

forest soil at this distance (Karhu and Neuvonen, 1998). The

organic layer was sampled with a stainless steel borer with a

diameter of 14 cm, and the depth of the layer was measured

to an accuracy of 0.5 cm. A mineral soil sample (E horizon

and the upper part of B horizon) was taken below the

sampled organic layer in the same way as the soil sampling

under the ant mounds. The four forest soil samples were

combined by layer for the nutrient analyses.

2.3. Analyses and calculations

Stones, cones, dead branches, etc. with a diameter >2 cm

were separated from the samples and their mass and

volume subtracted from the corresponding values for the

samples. The samples were dried to constant weight at

40 8C. The samples taken below the ant mounds and from

the mineral soil were sieved through a 2-mm sieve, and both

fractions were weighed. Nutrients were determined on the

<2 mm fraction. The samples from the ant mounds and

organic layer were milled before analysis. The total C and N

concentrations in all the samples dried at 40 8C were

determined with a LECO CHN-1000 analyzer. The samples

from the ant mounds and organic layer were wet-digested in

HNO3–H2O2 and their total P concentration (dry matter

basis, i.e. dried at 105 8C) determined by ICP-AES (inductively

coupled plasma-atomic emission spectrometry). The sam-

ples taken below the ant mounds and from the mineral

soil were extracted with ammonium acetate (pH 4.65) and

the extractable P concentration (dry matter basis) was

determined by the molybdate-hydrazine method (Halonen

et al., 1983) on a spectrophotometer (Perkin-Elmer Lambda

11).

Ant mound volumes (dm3) were calculated using the

equation of a half ellipsoid. The C and nutrient pools in the

ant mounds were calculated by multiplying the average

nutrient concentrations of the sampled ant mounds in each

stand by the area-based ant mound masses.

2.4. Statistical analyses

Linear mixed models (SPSS 14.0.1 for Windows) and

Bonferroni multiple comparisons were used to test for

significant differences in C and nutrient concentrations, C/N

ratios and bulk density between the fixed factors, stand age

classes and sample loci, and their interaction. Forest stand

was used as a random factor. Among the sample loci, (1) ant

mounds versus soil organic layer and (2) soil under ant

mound versus mineral soil, were tested separately. The

same analysis was performed for the C, N and P pools m�2.

To reduce heteroscedasticity, P concentrations and C/N

ratios of the organic layer and ant mounds were ln(x + 1)

transformed before analysis. The C and P concentrations of

mineral soil and soil under ant mounds were ln(x + 1)

transformed, and N concentrations of the same samples

ln(ln(x + 1)) transformed. One-way ANOVA and Bonferroni

multiple comparisons were applied to compare the C, N and

P pools ha�1 of ant mounds between stand age classes.

Nutrient pools were ln(x + 1) transformed to retain normal

distribution and equal variances between stand age classes.

Table 2 – Mean numbers and volumes of active antmounds in the individual stand age classes

Age (years) Number (ha�1) Volume (dm3)

5 2.5 (1.0) 147 (37)

30 3.2 (0.9) 237 (36)

60 5.4 (1.6) 417 (41)

100 4.1 (0.7) 1062 (86)

Standard errors are in parentheses.

Fig. 1 – Sampling design: (1) above-ground mound, (2)

surrounding organic layer, (3) soil under mound and (4)

surrounding mineral soil. The organic layer and mineral

soil were sampled in four positions around each ant

mound.

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Mean nutrient pools of the above-ground parts of active ant

mounds are presented per mound base areas and per

hectare. The results were considered statistically significant

when the significance level was a < 0.05.

3. Results

3.1. C, N and P concentrations, bulk density

Both the C and P concentrations were higher in the above-

ground ant mound material than in the surrounding organic

layer (F1,28 = 128.1, p < 0.001 and F1,28 = 74.2, p < 0.001, respec-

tively), but the total N concentrations did not differ signifi-

cantly (Fig. 2). The C concentrations in above-ground ant

mound and surrounding organic layer were higher in the 100-

year than in the 30-year stand age class (F3,13 = 4.7, p = 0.021).

Similarly, the concentrations of C, N and extractable P in the

soil below the ant mounds were significantly higher than

those in the surrounding mineral soil (F1,28 = 86.4, p < 0.001;

F1,28 = 81.6, p < 0.001; F1,28 = 111.2, p < 0.001, respectively)

(Fig. 2).

In the 30- and 60-year stand age classes, the bulk density of

the ant mound material was lower than that in the

Fig. 2 – Mean C, N and P concentrations (dry matter basis) and bulk density and their standard errors in ant mounds and

surrounding organic layer (left) in different stand age classes (n = 4), and the same for C, N and extractable P concentrations

and bulk density in soil under ant mounds and surrounding mineral soil (right). Only the data indexed with different

lowercases and capitals differ significantly ( p < 0.05) between sample loci and between age classes, respectively. Note the

different y-scales.

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surrounding organic layer (F3,28 = 6.0, p = 0.003, Bonferroni’s

p < 0.017) (Fig. 2). In the 60- and 100-year stand age classes, the

soil under the ant mounds had a lower bulk density than the

surrounding mineral soil (F3,28 = 3.1, p = 0.044, Bonferroni’s

p < 0.004) (Fig. 2).

3.2. C, N and P pools

The above-ground parts of the ant mounds contained more C

(F1,28 = 175.1, p < 0.001), N (F1,28 = 109.5, p < 0.001) and P/m2

(F1,28 = 114.2, p < 0.001) than the surrounding organic layer

(Fig. 3). In the 100-year stand age class the above-ground parts

of the ant mounds had more C/m2 than in the 5- and 30-year

age classes (F3,28 = 7.4, p = 0.001, Bonferroni’s p < 0.004). The

soil under the ant mounds had more C (F1,28 = 223.5, p < 0.001)

and N/m2 (F1,28 = 203.9, p < 0.001) than the surrounding

mineral soil.

On a hectare basis, the C (F3,12 = 5.9, p = 0.010, Bonferroni’s

p = 0.016), N (F3,12 = 4.9, p = 0.019, Bonferroni’s p = 0.023) and P

(F3,12 = 5.1, p = 0.017, Bonferroni’s p = 0.022) pools in the ant

mounds were higher in the 100-year age class than those in the

5-year class, and the C pool in the 100-year age class was

higher than that in the 30-year age class (Bonferroni’s

p = 0.042) (Table 3). The C, N and P pools in the organic layer

and in the mineral soil were many times higher than the pools

in the above-ground part of the ant mounds or the soil under

the ant mounds (Table 3).

3.3. C/N ratio

The C/N ratio was higher in the above-ground parts of the ant

mounds than in the organic layer (F1,28 = 84.0, p < 0.001) (Fig. 4).

The C/N ratio did not differ between the soil under the ant

mounds and the surrounding mineral soil.

4. Discussion

In this study, the C, N and P concentrations in the above-

ground parts of the ant mounds were similar to those reported

for ant mounds in a coniferous forest in the Moscow region,

Russia (Zakharov et al., 1981). The C and N concentrations and

the C/N ratios were almost similar to those in the ant mounds

in subalpine coniferous forests in Switzerland (Risch et al.,

2005). In the Swiss subalpine forests both the C and N

concentrations were higher in the ant mounds than in the

surrounding forest soil, but the C/N ratios showed no

difference (Risch et al., 2005). As was the case in this study,

Fig. 3 – Mean C, N and P pools and their standard errors in ant mounds and surrounding organic layer (left), and C and N

pools in soil under ant mound and surrounding mineral soil down to 21 cm depth (right) in different stand age classes

(n = 4). Only the data indexed with different lowercases and capitals differ significantly ( p < 0.05) between sample loci and

between age classes, respectively.

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the C/N ratio was higher in the ant mounds than in the organic

layer in boreal coniferous and mixed forests in central Sweden

(Lenoir et al., 2001), which implies that the material in the ant

mounds was less decomposed than that in the surrounding

organic layer. Low C/nutrient ratios usually indicate a fast

decomposition rate (Berg and McClaugherty, 2003) but, for

instance, N mineralization from decomposing litter can be

associated with initially higher C/N ratios in mature forests

than in clear-cut areas (Berg and Ekbohm, 1983).

The ant mound material is selectively collected from the

forest floor by ants, and in coniferous stands primarily

consists of conifer needles. Norway spruce needle litter,

which is common mound-building material, usually has lower

N and P concentrations (Berg and Tamm, 1991; Johansson,

1995; Lundmark-Thelin and Johansson, 1997; Berg et al., 2000)

than the ant mound material (Zakharov et al., 1981; Lenoir

et al., 2001; Frouz et al., 2005; Risch et al., 2005), and therefore

the presence of other ant mound material explains the higher

nutrient concentrations. Ant mounds contain relatively more

resin particles (Lenoir et al., 1999) and food remains than the

surrounding organic layer. The soil microbe (Frouz et al., 1997;

Laakso and Setala, 1998) and root composition (Farji-Brener,

2000; Ohashi etal., inpress) of ant moundsalsodiffer fromthose

in the surrounding soil, and they may have an impact on the C

and nutrient concentrations. In a temperate forest in the Czech

Republic, a higher N fixing bacterial assemblage was found in

the ant mounds compared to the surrounding organic layer

(Frouz et al., 1997). However, this was probably not the case in

our study because the N concentrations were similar in the ant

mounds and in the surrounding organic layer. Laakso and

Setala (1998) found a larger soil animal biomass, suggesting a

higher amount of resources, in ant mounds than in the

surrounding forest soil in Finland, but no differences in the N

and P concentrations between the surface layers of ant mounds

and the surrounding litter layer. Fine root N and P concentra-

tions were higher in ant mounds than in the surrounding

organic layer (Ohashi et al., in press). Ants themselves have also

been reported to affect litter quality (Stadler et al., 2006).

Furthermore, the organic layer comprises the whole decom-

position continuum from litter to humus, while the material in

the ant mounds is less decomposed and younger.

The higher C, N and extractable P concentrations in the soil

under the ant mounds compared to the surrounding mineral

soil might be explained by the input of organic material caused

Table 3 – Mean C, N and P pools (kg haS1) in ant mounds, soil under ant mounds (0–21 cm), organic layer and mineral soil(0–21 cm) in the different stand age classes (n = 4)

Age (years) Sample locus C N P

5 Above-ground mound 25a (8) 0.5a (0.2) 0.04a (0.01)

Soil under mound 11 (3) 0.5 (0.2) –

Organic layer 32,745 (5703) 1034 (183) 53 (9)

Mineral soil 24,644 (2836) 1170 (138) –

30 Above-ground mound 26a (6) 0.7 (0.1) 0.05 (0.01)

Soil under mound 14 (3) 0.7 (0.2) –

Organic layer 25,074 (3438) 929 (153) 46 (3)

Mineral soil 23,499 (3452) 1179 (98) –

60 Above-ground mound 93 (49) 2.1 (1.1) 0.15 (0.09)

Soil under mound 42 (24) 2.0 (1.2) –

Organic layer 29,579 (4594) 933 (137) 43 (7)

Mineral soil 31,946 (7736) 1380 (337) –

100 Above-ground mound 180b (63) 3.7b (1.3) 0.25b (0.09)

Soil under mound 53 (10) 2.6 (0.5) –

Organic layer 27,526 (2397) 817 (83) 38 (3)

Mineral soil 21,741 (5440) 997 (142) –

Standard errors are in parentheses. Only the data indexed with different letters differ significantly ( p < 0.05) between stand age classes, only

pools in ant mounds were tested.

Fig. 4 – Mean C/N ratios and their standard errors of ant mounds and surrounding organic layer, and soil under ant mounds

and surrounding mineral soil in different stand age classes (n = 4). Only the data indexed with different lowercases and

capitals differ significantly ( p < 0.05) between sample loci and between age classes, respectively.

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by the mixing activity of the ants, whilst nutrient leaching by

percolation water is highly unlikely in the dry conditions of ant

mounds. Also in Denmark, higher C, N (Kristiansen and

Amelung, 2001) and P (Kristiansen et al., 2001) concentrations

were found under the abandoned ant mounds than in the

surrounding soil.

Our results did not support the hypothesis that the nutrient

concentrations would decrease in ant mounds simultaneously

with the decreases in the surrounding soil during forest

succession. In our study, nutrient concentrations in the

organic layer did not decrease during forest succession, which

was the opposite of earlier findings in boreal forests in

southern Finland (Tamminen, 1991). Actually the C concen-

trations in the ant mounds increased slightly along with

increasing stand age.

The bulk densities of the ant mounds in subalpine forests

in Switzerland (Risch et al., 2005) were similar to our values.

The bulk densities in the above- and below-ground parts of

the ant mounds were lower than in the surrounding soil.

The interior of the mounds of the Formica rufa group is

known to have a porous structure with ant tunnels and

chambers, and the mineral soil under the mounds is also

mixed with organic matter by the ants, resulting in a lower

bulk density. There were no significant stand-age related

differences in the bulk densities of the ant mounds although

the bulk density seemed to be highest in the seedling

stands. This might indicate reduced ant activity in the

recently clear-cut sites, which leads to accelerated decom-

position.

The contribution of ant mounds to the total C, N and P pools

in the forest soil was small (<1%), and smaller than that in the

subalpine forests in Switzerland where it was 0.6–5% of the C

and N pools in organic layer depending on the forest type

(Risch et al., 2005). The pools in ant mounds may seem

negligible when extrapolated to the ecosystem level. However,

ant mounds were shown to increase the spatial heterogeneity

in the distribution of C, N and P in forest soil, and this might

also affect e.g. nutrient availability to the trees. The results of

this study also show that more nutrients are accumulated in

ant mounds along with forest succession.

Acknowledgements

We acknowledge the technical help of Ms. Laura Ikonen, Mr.

Teuvo Vauhkala, Ms. Anita Pussinen, Ms. Seija Repo, Ms. Anki

Geddala, Dr. Sirpa Piirainen and Ms. Maini Mononen. We

thank Mr. Pekka Punttila for identification of the ant species

and Mr. Jaakko Heinonen for statistical advice. Thanks go to

Dr. John Derome for revising the text. We appreciate the

comments of two anonymous reviewers. The Academy of

Finland (project 200870) financed the study.

r e f e r e n c e s

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