Vegetation transition following drainage in a high-latitude
hyper-oceanic ecosystem
Anna Maria Fosaa, Erla Olsen, William Simonsen, Magnus Gaard & Heidi Hansen
Abstract
Questions: How does draining affect the composi-
tion of vegetation? Are certain functional groups
favoured? Can soil parameters explain these differ-
ences?
Location: Central Faroe Islands, treeless islands in
the northern boreal vegetation zone. Since 1987, an
area of 21 km2 at 100–200ma.s.l. was drained in
order to provide water for hydro-electric produc-
tion.
Method: Vegetation and soil of a drained area and a
control, undrained neighbouring area of approxi-
mately the same size were sampled in 2007. Six sites
were sampled in each area. The vegetation was
classified with cluster analysis.
Results: Four plant communities were defined in the
area: Calluna vulgaris–Empetrum nigrum–Vaccinium
myrtillus heath, Scirpus cespitosus–Eriophorum an-
gustifolium blanket mire, Carex bigelowii–
Racomitrium lanuginosum moss-heath, Narthecium
ossifragum–Carex panacea mire. Heath was more
extensively distributed within, and was the domi-
nant community of the drained area, whereas moss-
heath was more extensive in the undrained area.
Blanket mire and mire had approximately the same
distribution in both areas. For the blanket mire,
species composition indicated drier conditions in the
drained than in the undrained area. The drained
area had higher frequencies of woody species and
lichens, grasses had finer roots and available soil
phosphate was considerably higher, whereas the
undrained area had higher frequencies of grasses
and sedges.
Conclusion: The dominant plant communities were
different in the two areas, which indicated that the
blanket mire was drying in the drained area. Higher
concentration of soil phosphate in the drained area
also indicated increased decomposition of organic
soils owing to desiccation.
Keywords: Drainage; Fine roots; Functional groups;
Plant communities; Soil decomposition.
Nomenclature: Jo´hansen (2000); Smith (1978).
Introduction
Wetland covers 1.5% of the Faroe Islands.
Some of these wetlands have been partly drained in
order to increase grazing area, access peat and, in
recent times, develop hydropower. Only limited in-
formation is available from the Faroe Islands about
the impact of drainage on this ecosystem. Studies
from neighbouring countries have shown that drai-
nage has an impact on vegetation as well as on
mineralization. Plant species diversity is generally
lower in drained areas (Pfandenhauser & Grootjans
1999). For example, Grootjans et al. (2005) found
an overall loss (12–45%) of plant species after 26 yr
of drainage, although some species were persistent.
At the community level, severely drained areas lost
almost all wetland species. During the first 5 yr of
drainage, rate of nitrogen-mineralization was very
high (Grootjans et al. 1985) and grasses expanded
vigorously.
Some functional groups may be favoured while
others might decline as a result of drainage. Such
species can serve as useful indicators for manage-
ment and restoration (Hobbs 1997). Species
composition of mire systems in Fennoscandia is dif-
ferent from that of systems in central Europe or the
UK, but use of functional groups allows compar-
ison. For example, drainage commonly promotes
woody vegetation in mires across Europe (Akers &
Allcorn 2006; Ma¨lson et al. 2008), although fa-
voured species are different.
In drained blanket mires, Sphagnum mosses
may decrease (Yeloff et al. 2006) and heather (Cal-
luna vulgaris) may become more abundant or
dominant. Lichens, mostly Cladonia species, may
form conspicuous areas in the heath. On dry hum-
mock crests, the moss Hypnum jutlandicum typically
occurs under the Calluna, and Racomitrium lanugi-
Fosaa, A.M. (corresponding author; anmarfos@ngs.
fo), Olsen, E. (erlao@ngs.fo), Simonsen, W. (williams
@ngs.fo),Gaard, M. (magnusg@kallnet.fo) &Hansen,
H. (Heidi_hansen81@hotmail.com): Faroese Museum
of Natural History, V.U. Hammershaimbsgta 13,
FO–100 To´rshavn, Faroe Islands.
Applied Vegetation Science 13: 249–256, 2010
DOI: 10.1111/j.1654-109X.2009.01066.x
& 2009 International Association for Vegetation Science

nosum commonly forms hummocks on the bog sur-
face (Ellis & Tallis 2000, 2001), especially where
there has been past cutting or disturbance (Cooper
et al. 2001).
The response of a mire ecosystem to drainage is
complex and the effects of which may resemble that
of global warming (Heathwaite 1993; Laine et al.
1995) because drainage may result in a release of soil
carbon as CO2 (Moore & Knowels 1989; Komulai-
nen et al. 1999) and in changes to soil nutrient
dynamics. Soil nutrients are usually patchily dis-
tributed and plant roots may respond by
proliferating within nutrient-rich patches that can
be triggered by mainly phosphate, nitrate and am-
monium (Hodge 2004). Plant uptake of phosphate,
especially in nutrient-poor soils, is greater in plants
colonized by symbiotic arbuscular mycorrhizal fun-
gi (Smith & Read 1997).
In this study, the effect of drainage on the flor-
istic composition of plant communities and
functional types (grasses, sedges, mosses, herbs, li-
chens and woody species) was investigated by
comparing an area that has been drained for about
20 yr and a neighbouring undrained area. Soil nu-
trients as well as plant roots and mycorrhiza were
also studied, in order to see if these parameters could
explain the differences between the drained and un-
drained areas.
Material and Methods
Study area
The study area is in the northern and central
part of the Faroe Islands (Fig. 1). The northernmost
part of the area (21 km2) has been drained since 1987
for hydropower production. All large rivers in the
area have been dammed and the water diverted
through tunnels. In addition, areas between the riv-
ers are drained by ditches. Thus, water from the area
is diverted to a reservoir in the north. An undrained
area south of the drained area was used as control.
The two areas are adjacent on a slope that ends
in a fjord. There is no indication of any appreciable
hydrological connection between them. It was as-
sumed that the vegetation in the undrained and
drained areas was similar before drainage as they
have the same aspect and altitude. Grazing should
also be similar, as no significant difference was seen
Fig. 1. The location of the Faroe Islands and the drained and undrained areas on the west side of the island Eysturoy.
250 Fosaa, A. M. et al.

in the number of sheep in the two areas. The Faroe
Islands have been treeless since the last Ice Age (Jo´-
hansen 1985).
The climate in the islands is extremely oceanic
(Crawford 2000). The warmest months are Jul and
Aug with a mean of 111C (lowland) and the coldest
is Feb with a mean of 41C (lowland). The precipita-
tion reflects the topography of the islands such that
the coastal areas typically receive 1000mmyr–1, in-
creasing to more than 3000mmyr–1 in the central
parts (Cappelen & Laursen 1998).
Plant communities
Vegetation was sampled at six sites each in the
drained and undrained areas, distributed through-
out each area. In order to ensure homogeneous
sampling, each site had two transects that started
about 10m from a river. In the drained area, one
transect was above the intake to a tunnel and one
was below it. In the undrained area, two transects
were similarly placed below and above planned in-
takes. A pair of 10-m long transects at each site were
placed in relatively similar, homogeneous vegeta-
tion. Along each transect, eight mesoplots (each
0.25m2) were placed at regular intervals. These were
subdivided into 25 (0.01m2) microplots. The pre-
sence/absence of each plant species was noted for
each microplot, which yielded a frequency value for
each species for each mesoplot. One site thus con-
tained 16 mesoplots. Thus 192 mesoplots and 4800
microplots were sampled.
Soil
Four soil cores (6 cm wide and 8 cm deep) were
sampled at each site. The soil samples were frozen at
 181C the same day they were sampled. The soil
was sampled in the microplots containing the most
Agrostis capillaris because this grass is one of the
most common plant species in the Faroes, found at
both high and low altitude (Fosaa 2004). In addi-
tion, A. capillaris is suitable for measuring
arbuscular mycorrhizal colonization because the
colonization pattern in its roots is affected by soil
nutrients (Olsen 2006).
Soil moisture was measured in all 25 microplots
of each mesoplot from which soil was sampled. Vo-
lumetric moisture content was measured with a
ThetaMeter type HH1 and ThetaProbe TypeML2X
(Delta-T Devices, Ltd, Cambridge, UK). In the
same mesoplots, three measurements of soil tem-
perature were acquired using a digital thermometer
(Jenway model 2152, Essex, UK). After the field
season, the soil samples were thawed and roots
picked out of the soil for measuring arbuscular
mycorrhizal colonisation. The soil samples were
left to air-dry at room temperature and then
they were oven-dried at 651C for 4 hr. The soil sam-
ples were analysed for pH and phosphate, using the
Reflectoquant system from Merck (http://www.
merck-chemicals.com). The plant roots were ex-
amined for mycorrhiza and whether they were fine
or coarse. A typical fine root was between 0.1mm
and 0.15mm in diameter, but a diameter up to
0.2mm was considered as a fine root if the stele was
poorly developed.
Data analysis
Plant communities
The data were analysed using the computer
programme MVSP (Kovac 1986–1999). All the vas-
cular plant species were included in addition to moss
species. Sphagnum, Polytrichum and liverworts spe-
cies were pooled into three groups. Agglomerative
hierarchical techniques were used to classify the ve-
getation. Minimum variance and squared Euclidean
distance were adopted to calculate the variance be-
tween pairs. The analysis included 192 sample plots
and 74 species of vascular plants, lichens and bryo-
phytes. Four significantly different communities
were identified using an appropriate cut level. These
communities were found to be statistically different
(Po0.01) using a t-test based on scores from the first
axis in a detrended correspondence analysis (DCA;
Hill & Gauch 1980). Syntaxonomic sorting of tables
was performed with the method of Braun-Blanquet
(1932), Mu¨eller-Dombois & Ellenberg (1974) and
Westhoff & van der Maarel (1978) to classify the
vegetation types. The frequency and constancy of
species in the mesoplots were used to determine the
communities. The constancy of species was categor-
ized from I to V (V5 81–100%, IV5 61–80%,
III5 41–60%, II5 21–40 and I5 1–20%). Fre-
quency is the mean percentage occurrence of a
species in mesoplots. The character species used
to define a community were differential species,
together with species unique for the community with
constancy from V to II. The plant communities are
named after one plant species or as a combination of
two or tree species of high constancy that distinguish
one community from the others.
Functional types
All the plant species recorded were assigned to
one of the following functional groups: grasses,
Vegetation transition following drainage 251

herbs, lichens, mosses, sedges or woody plants. The
‘‘mean accumulated frequency’’ of each functional
group for each site was computed by adding the fre-
quencies of all the species within the group in each
mesoplot and calculating the mean from all 16 me-
soplots at the site. The diversity of each functional
type was calculated for each site using the Shannon–
Wiener index (H), calculated by the formula:
H ¼ Spi ln pi
where pi here is computed as the frequency of the i
th
species in proportion to the sum of all frequencies
within the functional type. Species richness (S) was
defined as the number of species within each func-
tional type in each area.
Results
Plant communities
Four significantly (Po10 4) different plant
communities were defined (Table 1). The dominant
plant community in the drained area was the C. vul-
garis–Empetrum nigrum-Vaccinium myrtillus heath,
described before by, for example Bo¨cher (1940). In
the undrained area, the Carex bigelowii-R. lanugi-
nosum moss-heath was the dominant community.
This community has been described by Hobbs &
Averis (1991) as an equivalent of the C. bigelowii-
R. lanuginosum association (Rodwell 1991–1995).
The two other communities were found in approxi-
mately equal numbers of plots in each area. These
two communities were the Scirpus cespitosus–
Eriophorum angustifolium blanket mire and the
Narthecium ossifragum–Carex panicea mire. The
first of these communities has been described by
Hobbs & Averis 1991 as an equivalent of the British
Scirpus–Eriophorum blanket bog (Rodwell 1991–
1995). The second community is equivalent to the
British N. ossifragum–Sphagnum papillosum mire
(Rodwell 1991–1995).
A Fisher exact test revealed that the heath
community was significantly (Po10 4) more fre-
quent in the drained area, while the moss-heath
community was significantly (Po10 4) more fre-
quent in the undrained area. The blanket mire and
the mire communities were not significantly differ-
ent between the two areas (Table 2).
Functional types
The mean accumulated frequencies of the six
functional types were compared between undrained
and drained sites using a t-test. Significantly
(Po10 6) higher mean accumulated frequencies of
woody plants and lichens were found in the drained
Table 1. Details on the four communities in the study
area. For each species, abundance and constancy of spe-
cies are given. The communities are: (A) Calluna vulgaris–
Empetrum nigrum–Vaccinium myrtillus heath, (B) Scirpus
cespitosus–Eriophorum angustifolium blanket mire, (C)
Carex bigelowii–Racomitrium lanuginosum moss-heath
and (D) Narthecium ossifragum–Carex panicea mire. Spe-
cies in bold are indicator species for the communities.
Roman numbers from I to V indicate the constancy of
each species in the communities. Only taxa with a con-
stancy class 4I in at least one community are shown.
Additional species with constancy less than III:Campylium
stellatum, Campylopus atrovirens, Carex binervis, Carex
demissa, Carex echinata, Carex nigra, Alchemilla alpina,
Alchemilla filicaulis, Anthoxanthum odoratum, Aulacom-
nium turgidum, Blechnum spicant, Cetraria islandica,
Cladonia arbuscula, Cladonia uncialis, Cornus suecica, Cte-
nidium molluscum, Dactylis glomerata, Deschampsia
flexuosa, Dicranella species, Dicranum bonjeanii, Dicranum
scoparium, Dicranum spp., Epilobium alsinifolium, Erica
cinerea, Eriophorum vaginatum, Euphrasia scottica, Eu-
phrasia spp., Festuca rubra, Festuca vivipara, Huperzia
selago, Hypericum pulchrum, Hypnum cupressiform, Juncus
bulbosus, Juncus triglumis, Listera cordata, Luzula multi-
flora, Luzula sylvatica, Pinguicula vulgaris, Plantago
maritima, Pleurozium schreberi, Polygala serpyllifolia,
Polygala vulgaris, Polytrichum spp., Pyrola minor, Racomi-
trium canescens, Racomitrium spp. Rhytidiadelphus
squarrosus, Selaginella selaginoides, Thymus praecox ssp.
arcticus, Vaccinium uliginosum, Viola palustris, Viola
riviniana.
A B C D
Cover total (%) 100 100 100 100
Maximum height of herbs (cm) 21 20 13 17
Number of species 58 59 43 50
Number of plots 58 45 56 35
% Plot with each community 31 22 29 18
Calluna vulgaris 18/V 3/II 1/I 3/I
Empetrum nigrum 7/IV 4/III 8/III 3/I
Vaccinium myrtillus 8/III 1/III 0 1/I
Eriophorum angustifolium 4/II 8/III 1/I 5/I
Scirpus cespitosus 1/I 8/III 0 5/I
Carex bigelowii 1/I 1/I 5/IV 1/I
Racomitrium lanuginosum 4/III 0 25/V 23/II
Narthecium ossifragum 6/IV 4/III 1/I 18/II
Carex panicea 2/III 5/IV 6/V 7/II
Potentilla erecta 15/V 13/V 13/V 8/II
Agrostis canina 2/IV 5/IV 3/V 4/II
Agrostis capillaris 5/IV 7/IV 9/V 4/II
Nardus stricta 3/IV 4/III 1/IV 5/II
Dactylorhiza maculata 2/IV 2/IV 1/III 1/I
Hylocomium splendens 6/V 1/II 1/I 1/I
Rhytidiadelphus loreus 3/IV 1/II 1/I 1/I
Carex pilulifera 1/III 1/I 2/III 1/I
Juncus squarrosus 3/III 5/III 1/I 4/I
Diphasiastrum alpinum 1/I 1/I 3/III 1/I
Sphagnum spp. 1/I 5/II 0 1/I
Cladonia portentosa 3/II 1/I 1/I 1/I
Galium saxatile 1/I 1/I 2/II 1/I
252 Fosaa, A. M. et al.

area, whereas the frequencies of grasses and sedges
were significantly (Po0.0005) higher in the un-
drained area (Fig. 2). The frequencies of herbs and
mosses were not significantly different between the
two areas. A t-test showed a higher (Po0.05) di-
versity of woody plants and mosses in the drained
area, whereas diversity of grasses and sedges was
found to be higher (Po0.05) in the undrained area.
The diversity of herbs or lichens did not differ be-
tween the two areas. For grasses, sedges, herbs and
woody plants, the patterns of frequency and Shan-
non–Wiener diversity were similar (Fig. 3), indicating
that a functional type with a high frequency had a
high diversity. The pattern was different for mosses
and lichens, for which high frequency was not ne-
cessarily associated with high diversity.
Using mean accumulated frequency, herbs were
the most frequent functional type for the whole area
(32%), as well as separately for the undrained (30%)
and drained (33%) areas. The most conspicuous
difference between drained and undrained sites was
for woody plants with 27% and 12% frequency, re-
spectively.
Soil
There was more available phosphate in the
drained than in the undrained area (on average
12.2 ppm compared with 7.3 ppm Po0.001), and the
fraction of fine roots was higher in the drained area
than in the undrained area (on average 64.7% com-
pared with 55.5% Po0.01). Mycorrhizal para-
meters, soil pH and soil moisture did not show a
response to drainage.
Discussion
Plant communities of the drained and un-
drained areas were found to differ substantially, the
most conspicuous of which was the dominance of
the C. vulgaris–E. nigrum–V. myrtillus heath in the
drained area and the dominance of C. bigelowii–
R. lauginosum moss heath in the undrained area.
The three main heath communities in the Faroe Is-
lands are usually defined by increasing moisture.
The C. vulgaris–E. cinerea community is the driest
heath. With increasing availability of moisture, the
C. vulgaris–E. cinerea community is replaced by the
C. vulgaris community, and in turn by the Empe-
trum–Vaccinium community. In these wet heaths, N.
ossifragum and Juncus sqarrosus are frequent (Bo¨-
cher 1940).
The study area was dominated by a relatively
wet heath community that was rich in wetland spe-
cies such as N. ossifragum, Juncus squarrosus, and
E. angustifolium. In the drained part of the heath,
the frequency of Eriophorum vaginatum was sig-
nificantly higher. In contrast, this species was found
to increase in rewetted areas in restored peatland in
southern Finland (Komulainen et al. 1999), perhaps
owing to changes in competition. R. lanuginosum
also had higher frequencies in the drained area; this
species which is considered to be an indicator of a
drying mire (Ellis & Tallis 2000). The C. bigelowii-
R. lanuginosum moss-heath, which is the dominant
plant community in the undrained area, has fewer
wetland moss species, but R. lanuginosum is fre-
quent. In the Faroe Islands, this community is
described as an alpine vegetation type (Bo¨cher 1940;
Hobbs & Averis 1991) and is thus more frequent in
wind-swept areas compared with the dwarf shrub
community dominant in the drained area.
Table 2. Percentage distribution of the four plant commu-
nities in the drained and undrained area. (A) Calluna
vulgaris–Empetrum nigrum–Vaccinium myrtillus heath,
(B) Scirpus cespitosus–Eriophorum angustifolium blanket
mire, (C)Carex bigelowii–Racomitrium lanuginosummoss-
heath and (D) Narthecium ossifragum–Carex panicea
mire.
A B C D
Drained area 47 19 17 18
Undrained area 19 21 42 19
P value Po10 4 ns Po10 4 ns
0
5
10
15
20
25
30
35
Grasses Lichens Sedges Woody
Functional types
Fr
eq
ue
nc
y
Drained area
Undrained area
Fig. 2. Frequencies (mean and standard error) of the
functional types: grasses, lichens, sedges and woody spe-
cies that showed a significant difference (all Po0.0001)
between the drained and undrained area.
Vegetation transition following drainage 253

nutrient uptake but might benefit for other reasons
such as pathogen defence (Newsham et al. 1995).
Thus the finer plant roots may be explained as a di-
rect response to phosphate-richer soils.
Peatlands are important sinks for carbon and
nutrients such as phosphorus and one of the con-
sequences of drying of peatlands is increased
phosphate leaching (Zak et al. 2008). The near dou-
ble amount of phosphate in soils from the drained
area, an effect that was observed for all plant com-
munities, indicates that the drained soils had started
decomposing.
Acknowledgements. This study was financed by the Faroe
Islands electricity company SEV and the Faroese Museum
of Natural History.
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