Vegetation shifts towards wetter site conditions on oceanic ombrotrophic bogs in southwestern Sweden
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Journal of Vegetation Science 18: 595-604, 2007
© IAVS; Opulus Press Uppsala.
- Vegetation shifts towards wetter site conditions on oceanic ombrotrophic bogs - 595
Vegetation shifts towards wetter site conditions
on oceanic ombrotrophic bogs in southwestern Sweden
Gunnarsson, Urban1* & Flodin, Lars-Åke2
1Department of Plant Ecology, EBC, Uppsala University, Villavägen 14, SE-752 36 Uppsala, Sweden;
2County Administration Board of Halland, 301 86 Halmstad, Sweden;
*Corresponding author; Fax +46 18553419; E-mail urban.gunnarsson@ebc.uu.se
Abstract Introduction
Question: Is ombrotrophic bog vegetation in an oceanic re-
gion of southwestern Sweden changing in the same direction One of the major challenges in vegetation monitoring
over a five year period (1999 - 2004) as northwest European is to disentangle short-term vegetation fluctuations from
bogs in the last 50 years, i.e. towards drier and more eutrophic
long-term vegetation succession (Økland & Eilertsen
vegetation?
Location: The province of Halland, southwestern Sweden. 1996; Økland et al. 2004). Probably the only way to dis-
Methods: Changes in species composition were monitored in tinguish the nature of the vegetation change is to follow
750 permanently marked plots in 25 ombrotrophic bogs from the vegetation over a long time period in permanent plots
1999 to 2004. Changes in species occurrences and richness (Austin 1981). Fluctuations in the vegetation depend on
were analysed and a multivariate statistical method (DCA) short term (annual) variation in the local climate and/or
was used to analyse vegetation changes. other external factors. Meanwhile, persistent trends in
Results: The species composition changed towards wetter vegetation depend on long-lasting directional changes
rather than drier conditions, which is unlike the general pattern in the climate and/or in other external or internal factors.
of vegetation change on bogs in northwestern Europe. Species
One way to differentiate long- versus short-term trends
typical of wetter site conditions including most Sphagnum
species increased in abundance on the bogs until 2004. The is to analyse the persistence of species between years
total number of species per plot increased, mostly due to the (Økland 1994, 1995; Nordbakken 2000). Changes in
increased species richness of Sphagnum species. Nitrogen- highly dynamic species may be considered as natural
demanding (eutrophic) species increased in occurrence. fluctuations, meanwhile changes in highly persistent spe-
Conclusions: Ombrotrophic bog vegetation in an oceanic re- cies may be considered as part of directional vegetation
gion in Sweden became wetter and was resilient to short-term trends. A complementary strategy is to monitor climatic
climatic shifts, after three years of below normal precipitation conditions so that changes caused by climatic conditions
followed by several years with normal precipitation levels. can be distinguished from other deterministic trends in
Shifts towards more nitrogen demanding species were rapid
the vegetation. Ultimately, the identification of short-term
in this region where the deposition levels have been high for
several decades. fluctuations versus long-term trends may be important
to develop cost effective conservation regimes for the
long-term management of ecosystems.
Keywords: Eutrophication; Monitoring; Species richness;
During the last century, several models of succes-
Sphagnum; Vascular plant; Vegetation change. sion for ombrotrophic bogs have been proposed, but
these models generally do not consider the dynamics of
particular species or vegetation types due to shorter term
Nomenclature: Karlsson (1997) for vascular plants; Söder- climatic variations. One of the most influential models
ström & Hedenäs (1998) for bryophytes; Moberg et al. (1995) has been the classic hydrosere of bog succession in which
for lichens. open bogs stage would eventually be replaced with a
forest stage. This hydrosere model have been criticised
by several authors who argued that open ombrotrophic
bogs could be an alternate climax stage (Walker 1970;
Klinger et al. 1990; Klinger 1996). When looking at
stratigraphical studies on open peatlands of northwestern
Europe, it is clear that many of these peatlands have been
covered with trees during warm and dry periods (e.g.
Lundqvist 1955; Godwin 1975). A change to colder and
wetter climate has later turned these ecosystems back to596 Gunnarsson, U. & Flodin, L.-Å.
open bogs. Such patterns show that climate may drive a with drying up, reduced Sphagnum-cover was observed
variety of stable stages to occur over time and in regions in southeast Norway (Nordbakken 2001) and in southern
with different climate and local environmental conditions Sweden (Malmer & Wallén 1999); in both these areas,
(Glaser & Janssens 1986; Davis & Wilkinson 2004). non-peat-producing vegetation types increased in cover.
Establishment of species typical of drier conditions (e.g., In mid-Sweden, no decrease in Sphagnum-cover was
trees and dwarf shrubs) might occur during exceptionally observed, but the tree cover increased (Gunnarsson et
dry years, but little is known about for how long they al. 2000). All three processes influencing the vegetation
might persist if the conditions become wetter again. change including eutrophication, acidification and drying
Three recent vegetation trends have been observed up, are closely interrelated and their exact causes are thus
in field studies in ombrotrophic bogs in western Europe difficult to disentangle.
including changes associated with eutrophication, acidifi- In this study, we investigated vegetation changes
cation and drying. In the eutrophication process, nitrogen during a five year period (1999-2004) on bogs in the
demanding species increase their abundances often dis- province of Halland, southwestern Sweden. The objec-
placing slower growing species with higher nitrogen use tives were to describe patterns of change in vegetation
efficiency (Hogg et al. 1995; Risager 1997; Heijmans et and in species richness during the period, with particular
al. 2001; Gunnarsson et al. 2002; Tomassen et al. 2003). emphasis on detecting local differences in the changes
In response to eutrophication, the nitrogen limited species within the region. We tested if the species composition
e.g. Betula pubescens, Calluna vulgaris, Eriophorum was changing towards wetter or drier conditions. Based
angustifolium, Molinia caerulea, Pinus sylvestris and on the knowledge of past vegetation changes in the re-
Sphagnum fallax, increase in abundance (Hogg et al. gion, we expected that the vegetation would have changed
1995; Risager 1997; Nordbakken 2001; Gunnarsson et towards drier, more tree-covered and more nitrogen-rich
al. 2002; Tomassen et al. 2003; Limpens et al. 2003), conditions. Lastly, we discussed if the observed changes
while species adapted to low nitrogen levels decrease, were short-term fluctuations or if these changes repre-
e.g. Drosera species, Scheuchzeria palustris, Sphagnum sented a directional trend.
balticum and S. tenellum (Risager 1997; Gunnarsson et
al. 2002; Wiedermann et al. 2007). All ombrotrophic
bogs are by nature acidic, thus only marginal effects Material and Methods
of acidification have been observed in ombrotrophic
ecosystems (Hogg et al. 1995; Gunnarsson et al. 2000, The regional climate and the selection of sites
2002). The drying process is associated with increased
tree dominance along with associated shade-tolerant The province of Halland has the most oceanic cli-
dwarf-shrub and bryophyte vegetation (Nordbakken 2001; mate in Sweden, with an average annual precipitation
Gunnarsson et al. 2002). In response to drying, an increased of 1051 mm, as measured during the period 1961-1990
tree cover has been observed at several sites over north at the weather station Torup (56º57' N, 13º04' E, 150
Europe and North America (Risager 1997; Gunnarsson m a.s.l.) and a mean annual temperature 6.1 ºC (July
& Rydin 1998; Frankl & Schmeidl 2000; Gunnarsson et mean 16.2 ºC and January mean 1.0 ºC; Anon. 2004).
al. 2002; Linderholm & Leine 2004; Freléchoux et al. The highest amounts of precipitation in Halland occur
2004; Lachance et al. 2005) including Pinus sylvestris and east of the escarpments about 20 to 30 km east from the
Betula pubescens in southwestern Sweden. A potential coast and it is in this area where most bogs are situated.
ecological driver related to the drying of these wetlands The oceanic climate is reflected in the vegetation of the
is the drainage of the local region (Linderholm & Leine bogs in that many species that in other parts of the country
2004). grow only in fens, grow on ombrotrophic bogs in this
Both eutrophication and drying up were observed on region, e.g. Erica tetralix, Eriophorum angustifolium,
the bog Åkhultmyren in southern Sweden, which was Myrica gale, Narthecium ossifragum and Sphagnum
studied to monitor changes between 1956 and 1997. The papillosum (Malmer 1962). The high precipitation in
environment in 1997 was probably drier, more nitrogen Halland is accompanied by large amounts of nitrogen and
rich and more shade-tolerant than in 1956 as indicated sulphur, with wet deposition of > 9 kg.ha–1.yr–1 and > 5
by the vegetation. Tall plant species increased and short kg.ha–1.yr–1, respectively. The wet deposition is among
plant species decreased in abundance over that time the highest measured in Scandinavia (Lövblad et al. 1994)
interval (Gunnarsson et al. 2002). On ombrotrophic and the total deposition (including dry deposition and
bogs in Denmark, drying was observed, so that hum- nitrogen fixation) exceeds the long-term critical load
mock vegetation (dwarf shrubs and hummock mosses) for ombrotrophic bog ecosystems (Grennfelt & Törnelöf
increased in areal cover and at the same time the Sphag- 1992).
num-cover decreased (Risager 1998). Also in association We investigated 25 randomly selected ombrotrophic- Vegetation shifts towards wetter site conditions on oceanic ombrotrophic bogs - 597
bogs among the 86 highest ranked (the most valuable,
Class I objects) bogs according to the national survey
of Swedish wetlands (the Wetland Inventory of Sweden;
VMI) in the province of Halland (Forslund & Rundlöf
1984). Of the investigated bogs, 18 were classified as
plateau formed bogs, 3 were flat to weakly raised bogs
and 4 were domed bogs (Forslund & Rundlöf 1984; Table
1). The bogs in the study were situated in the eastern,
high altitude parts of the province (Fig. 1). For each bog,
the area and the degree of man-made hydrologic influ-
ence was recorded during the VMI-inventory (Forslund
& Rundlöf 1984). The man-made hydrologic influence
was graded along a 0-4 scale, with: 0 = no influence; 3 =
weak local influence and 4 = strong local influence (Anon.
1983). Bogs with grade 1 or 2 were not encountered.
Location of plots for vegetation and tree recording
Fig. 1. Map of the surveyed ombrotrophic bogs in Halland,
On open parts of the study bogs, we placed selec- south-western Sweden. See Table 1 for site names and further
tively one 50 m long transect and we recorded trees in information about the bogs.
three circular 100-m2 plots. The transects and the tree
plots were permanently marked and placed so they could
easily be relocated. Along each transect, 30 vegetation Species with broad niches could not be classified along
plots (0.5 m × 0.5 m) were randomly placed and scored this gradient.
for presence of all species in subplots on a scale from 0
(absent from all subplots) to 4 (present in all four sub-
plots). The subplots were the four quadrants (0.25 m ×
0.25 m) of the plot. A total of 750 vegetation plots were Table 1. Ombrotrophic bog characteristics including bog type,
sampled on the 25 bogs. In the tree plots, we counted all size and hydrologic influence category (data from Forslund &
established trees taller than 0.2 m. In 1999, we did not Rundlöf 1984). The man-made hydrologic influence is graded
sample tree plots on the sites at Yamossen, Storemosse as: 0, no influence; 3, weak local influence and 4, strong local
Kba and Lyngmosse. Tree density was calculated as influence (Anon. 1983).
number of established trees per ha. The field investiga- Bog Bog Area Hydrologic
tions were performed between August and September (Nr, name) type (ha) influence
both in 1999 and 2004.
1. Oxhult Plateau 33 4
2. Killeberg Flat to weakly raised 68 3
Investigated taxa 3. Bastamossen Plateau 193 3
4. Getamossen Domed 157 3
5. Asperamsmossen Plateau 562 4
All vascular plant species, Sphagnum-species, mosses 6. Risömossen Plateau 485 3
and lichens were identified directly in the field (a few 7. Skameltamossen Plateau 386 3
specimens were collected for later identification under 8. Store Jönsmosse Plateau 121 3
9. Snokamossen Plateau 256 4
a dissecting microscope). Small liverworts of the genera 10. Ivåsabäcken Plateau 27 3
Cephalozia, Cephaloziella and Cladipodiella often grow 11. Västra Davidsmosse Plateau 127 3
intermingled with Sphagnum-shoots and can sometimes 12. Östra Davidsmosse Plateau 191 3
13. Skärkeån Plateau 37 3
be overlooked. We identified the larger Cephalozia con- 14. Storemosse Domed 297 3
nivens and C. macrostachya at the species level, while 15. Klintamossen Plateau 90 3
other taxa (including C. lotliesbergi and C. lunulifolia) 16. Flymossen Plateau 53 4
17. Ugnshult Plateau 34 3
were treated as a group (Cephalozia spp.). The small 18. Sutaremossen Domed 46 3
species of the genus Cephaloziella were not recorded. 19. Tjuvömosse Flat to weakly raised 70 3
The lichens were merged into two genera Cladonia spp. 20. Björnåsen Plateau 35 0
21. Tjärnemossen Plateau 19 0
and Cladina spp. Species were grouped according to 22. Fläskabackarna Flat to weakly raised 64 3
their main habitat type along a wetness gradient (forests, 23. Yamossen Plateau 49 3
hummocks, lawns or carpets) according to the species 24. Storemosse Kba. Plateau 81 3
25. Lyngmosse Domed 93 3
descriptions in Albinsson (1997) and Rydin et al. (1999).598 Gunnarsson, U. & Flodin, L.-Å.
Data analysis Results
To test for differences in species occurrences be- Floristic changes
tween 1999 and 2004 on the bogs, we used Wilcoxonʼs
signed rank test for paired observations (procedure Overall, 63 taxa were observed in the study bogs,
NPAR1WAY in SAS, Anon. 2004), with the difference among them 19 vascular plants, 11 Sphagnum species,
in number of subplots per bog (transect) as a response 15 mosses, 16 liverworts and 2 lichens (Table 2). Three
variable. Bogs with absences of a species during both occasional species were only found in 1999 (Dicranella
years were excluded from these analyses. Uncommon cerviculata, Drosera anglica/intermedia and Splachnum
species, with less than 10 subplot occurrences at both ampullaceum). Significant increase in occurrence was
inventories were omitted from the analysis. Changes in observed for ten taxa over the 5-year interval including
species richness were measured as the change in number Cephalozia spp., Drosera rotundifolia, Erica tetralix,
of recorded species per plot and the significance of the Eriophorum vaginatum, Myrica gale, Odontoschisma
changes were tested for each bog and for all bogs together sphagni, Sphagnum austinii, S. fallax, S. magellanicum
with Wilcoxonʼs signed rank test for paired observations. and S. rubellum; five taxa decreased: Barbilophozia at-
To detect overall changes in species composition in all tenuate, Empetrum nigrum, Lophozia silvicola, Pinus
bogs, a DCA ordination analysis (Hill & Gauch 1980) sylvestris and Rubus chamaemorus. Vascular plant
was performed using the program CANOCO, version 4.5 species showed big changes in mean abundance. Tree
(ter Braak & Šmilauer 2002). Species that were known species were mostly represented by small seedlings, with
to be highly dynamic (e.g., Drosera rotundifolia, Rubus high recruitment and mortality rates. Sphagnum species
chamaemorus, Picea abies and Pinus sylvestris; Nord- increased in frequency, except S. papillosum and S.
bakken 2000) were not included in the DCA analysis. tenellum. Other mosses did not change much, but some
Vegetation plots with less than five species were omitted liverworts had high turnover rates.
from the ordination analysis, because they contain little
information, their relation to other plots is weak and Changes in tree occurrence
thus they tend to behave like outliers. Median frequency
down-weighing of rare species was used in the ordina- The total density of established trees recorded in the
tion (Eilertsen et al. 1990; Økland 1990). To interpret the tree plots on the bogs did not change significantly (Table
ordination axes we used Spearmanʼs rank correlations 3). In contrast to the observed increase of Picea abies in
between species scores and (1) Ellenbergʼs species indi- the vegetation plots, which mainly refers to smaller-sized
cator values for light, moisture and pH (Hill et al. 1999), seedlings, the density of established P. abies decreased
with Albinssonʼs (1997) corrections for the region, and significantly in the tree plots (Table 3). The density of
(2) classification of species according to their positions Pinus sylvestris decreased significantly in the vegetation
along the poor - rich gradient and the peat productivity plots (Table 2), meanwhile no significant change was
gradient according to Økland (1989). Ellenbergʼs indi- observed for P. sylvestris in the tree plots (Table 3).
cator values are empirical values developed for central
European plants. The values are ordered from low (1) to Species richness changes
high (9), where the value of 1 is given to species growing
in low light, moisture or pH and the value 9 for species The mean species richness per 0.5 m × 0.5 m plot
growing in high light, moisture and pH conditions (cf. increased when looking at all bogs with a mean change
Ellenberg et al. 1991; Hill et al. 1999). In order to test of 0.3 species per plot, from 10.5 species in 1999 to 10.8
if there were significant changes along the ordination species in 2004 (Table 4). The increase was mainly due
axes, we used Wilcoxonʼs signed rank test for paired to the mean increase of Sphagnum species per plot (Table
observations. 4). Two bogs had decreased species richness over this
time interval (Snokamossen and Björnåsen, Table 4),
while five bogs showed a significant increase in species
richness (Table 4). Sphagnum species increased in all
bogs except for Snokamossen, while the other species
groups experienced more sporadic changes in species
richness (Table 4). The pattern of change in species rich-
ness could neither be attributed to local differentiation
within Halland nor to bog type or degree of hydrological
disturbance (Table 1).- Vegetation shifts towards wetter site conditions on oceanic ombrotrophic bogs - 599
Vegetation changes Table 2. Relative frequencies (%) of species (total number of subplots
relative to the total number of subplots investigated each year) in the
25 investigated ombrotrophic bogs of Halland and the change (%) in
Species typical of wetter habitat types, lawns and
species frequencies 1999-2004. Significance of the change in species
carpets increased in occurrence (Fig. 2), while species frequencies was tested with Wilcoxonʼs signed rank test for paired
typical of forests and hummocks had more divergent observations. n.p. = statistical test not performed; n.s. = not significant
patterns of change (Fig. 2). Thus, the wetter habitat types (P ≥ 0.05); * = P < 0.05; ** = P < 0.01; *** = P < 0.001.
changed more than others over the 5 year period, based Species Freq. Freq. Freq. P-
on median changes of species occurrences (Kruskal 1999 2004 change value
Wallisʼ H = 7.95, P < 0.05, df = 3). Vascular plants
Andromeda polifolia 36.7 33.1 -10 n.s.
The first DCA axis explained 9 % of the total variation Betula pubescens 1.93 1.47 -24 n.s.
in the species data (eigenvalue = 0.23, gradient length = Calluna vulgaris 68.0 71.3 +5 n.s.
Drosera anglica/intermedia 0.033 0 - n.p.
3.2). Axis 1 is interpreted as a combined open mire - forest Drosera rotundifolia 8.20 14.4 +76 **
gradient and a hummock - hollow gradient, where open Empetrum nigrum 34.3 30.6 -11 *
Erica tetralix 70.5 73.2 +4 ***
hollow species were located in the left part of the ordina- Eriophorum angustifolium 0.433 0.667 +53 n.s.
tion diagram (Fig. 3b) and forest species were located in Eriophorum vaginatum 86.6 88.3 +2 *
Myrica gale 13.8 15.3 +11 **
the right part. The idea that Axis 1 is related to a vegeta- Picea abies 1.20 1.80 +50 n.s.
tion gradient was supported by the significant negative Pinus sylvestris 7.00 3.13 -55 ***
Rhynchospora alba 2.17 3.40 +57 n.s.
correlations between axis 1 and Ellenbergʼs values for Rubus chamaemorus 26.0 20.9 -20 *
Trichophorum caespitosum 5.07 6.00 +18 n.s.
light and moisture (rSpearman = –0.75, P < 0.001 and rSpear- Vaccinium myrtillus 0.733 0.87 +18 n.s.
man = -0.64, P < 0.001, respectively). Axis 2 explained V. oxycoccus 70.0 71.9 +3 n.s.
V. uliginosum 0.367 0.400 +9 n.s.
an additional 7 % of the total variation (eigenvalue = V. vitis-idaea 0.90 10.1 +19 n.s.
0.16, gradient length = 2.5) and was positively correlated Sphagnum species
with the peat-productivity gradient (rSpearman = 0.86, P < Sphagnum austinii 2.60 3.40 +30 *
S. balticum 0.87 1.03 +19 n.s.
0.001) and with the poor - rich gradient (rSpearman = 0.52, S. capillifolium 0.300 0.367 +22 n.s.
P < 0.001). High peat producing species were found in S. cuspidatum 3.13 3.30 +5 n.s.
S. fallax 1.37 2.43 +78 *
the upper half of the graph (e.g. S. balticum, S. cuspi- S. fuscum 2.27 2.67 +17 n.s.
datum, S. fuscum, S. papillosum and S. rubellum) low S. magellanicum 45.0 51.7 +12 ***
S. papillosum 1.90 1.80 -4 n.s.
peat producing species were situated in the lower half S. rubellum 36.3 44.1 +22 ***
of the diagram (e.g. S. tenellum, Cladopodiella fluitans S. tenellum 10.1 10.0 -1 n.s.
and the lichen species). Mosses
Aulacomnium palustre 1.80 2.00 +11 n.s.
Neither bogs from different parts of Halland, nor the Campylopus introflexus 0.033 0.033 0 n.p.
different bog types could be separated in the ordination Ceratodon purpureus 0.067 0.033 -50 n.p.
Dicranella cerviculata 0.030 0 - n.p.
diagram (Fig. 3a, Table 1). When testing if the vege- Dicranum bergeri 0.900 0.567 -37 n.s.
tation plots in the ombrotrophic bogs moved along the D. polysetum 0.200 0.067 -67 n.p.
D. scoparium 3.03 3.23 +7 n.s.
ordination axes, we found a significant decrease (on Hypnum jutlandicum 18.0 18.0 -0.1 n.s.
average 0.056 SD units) along DCA axis 1 (Wilcoxonʼs S Leucobryum glaucum 4.50 4.70 +5 n.s.
Pleurozium schreberi 13.2 14.2 +8 n.s.
= – 45661, n = 743, P < 0.001), but no significant change Pohlia nutans 0.067 0.100 +50 n.p.
along DCA axis 2 (Wilcoxonʼs S = 5389, n = 743, P = Polytrichastrum formosum 0.100 0.033 -67 n.p.
Polytrichum strictum 2.60 2.50 -5 n.s.
0.35). In the ordination space (Fig. 3a), most centroids Racomitrium lanuginosum 0.567 0.633 +12 n.s.
Splachnum ampullaceum 0.030 0 - n.p.
Liverworts
Barbilophozia attenuata 0.600 0.200 -67 **
Table 3. Median density of established trees taller than 0.2 m Bazzania trilobata 0.030 0.030 0 n.p.
Calypogeia integristipul 0.100 0.067 -33 n.p.
(per ha) found in the tree plots on the investigated ombrotrophic C. neesiana 1.60 2.33 +46 n.s.
bogs in 1999 versus 2004 (n = 22; first and third quartiles in C. sphagnicola 4.30 4.63 +9 n.s.
parentheses) and the percent change of the median number. Cephalozia connivens 5.83 5.30 -9 n.s.
C. macrostachya 0.200 0.333 +67 n.s.
Differences in tree occurrences were tested with Wilcoxonʼs Calypogeia spp. 10.0 13.5 +35 *
signed rank test for paired observations on the change for Cladopodiella fluitans 2.10 4.07 +94 n.s.
Gymnocolea inflata 0.53 0.70 +31 n.s.
each bog. Kurzia pauciflora 1.00 1.60 +60 n.s.
Lophozia silvicola 2.80 1.50 -48 **
Species 1999 2004 % change Mylia anomala 1.60 1.40 -13 n.s.
Odontoschisma denudatum 0.233 0.067 -71 n.p.
Betula pubescens 615 (300, 930) 533 (467, 733) -13 n.s. O. sphagni 57.9 65.0 +12 ***
Picea abies 270 (30, 500) 100 (0, 267) -62 * Ptilidium ciliare 0.033 0.067 +100 n.p.
Pinus sylvestris 1950 (1270, 2770) 2067 (1300, 2867) 6 n.s.
Total trees 2915 (2140, 3540) 2717 (2033, 3533) -7 n.s. Lichens
Cladonia spp. 11.1 12.2 +11 n.s.
n.s. = not significant (P ≥ 0.05); * = P < 0.05. Cladina spp. 18.8 19.0 +1 n.s.600 Gunnarsson, U. & Flodin, L.-Å.
Fig. 2. Change in species subplot frequencies during the period 1999 through 2004, in ombrotrophic bogs of Halland, Sweden.
The species are sorted according to their main habitat type along a gradient from typical forest species, via hummock and lawn to
carpet species according to Albinsson (1997) and Rydin et al. (1999). Uncommon species with < 10 occurrences during both years
and species that did not differentiate along the gradient were not included in the analysis. Black bars indicate that species changed
significantly in frequency over the 5 year study period according to Table 2.
moved to the left, indicating a vegetation change towards Flymosse, and Fläskabackarna (Fig. 3a). The vegetation
wetter and more open site conditions. The centroids did shift along the ordination axes was neither correlated
not move or moved to the right for five bogs including with the spatial distribution of the bogs in Halland nor
Bastamossen, Asperamsmossen, Östra Davidsmosse, with their degree of hydrological disturbance.
Table 4. Species richness of various species types in 1999 versus 2004 and the change in richness, all measured as mean number
of species per plot. Species richness and richness change are given for the total number of species and for the number of vascular
plants, Sphagnum, mosses and liverworts separately. The changes in diversity were tested with Wilcoxonʼs signed rank test for paired
observations on the change for each plot.
Total # of species # vascular plants # Sphagnum species # mosses # liverworts
Bog (No, name) 1999 2004 Change 1999 2004 Change 1999 2004 Change 1999 2004 Change 1999 2004 Change
1. Oxhult 9.0 9.3 0.3 n.s. 4.6 4.6 0 1.3 1.4 0.1 n.s. 0.6 0.8 0.2 n.s. 1.4 1.6 0.2 n.s.
2. Killeberg 10.6 11.2 0.6 n.s. 5.6 5.5 – 0.1 n.s. 0.8 1.2 0.4** 0.5 0.6 0.1 n.s. 2.4 2.8 0.4 n.s.
3. Bastamossen 10.3 11.6 1.3* 5.4 5.7 0.3 n.s. 0.9 1.1 0.2 n.s. 0.7 1.4 0.7** 2.1 2.2 0.1 n.s.
4. Getamossen 10.4 10.5 0.1 n.s. 6.0 6.0 0 1.1 1.6 0.5* 1.3 1.3 0 n.s. 1.3 1.2 – 0.1 n.s.
5. Asperamsmossen 11.0 11.5 0.5 n.s. 6.8 6.5 0.3 n.s. 1.5 1.9 0.4*** 0.6 0.8 0.2 n.s. 1.6 1.7 0.1 n.s.
6. Risömossen 10.4 10.7 0.3 n.s. 5.9 6.4 0.5* 0.9 0.9 0 n.s. 1.4 1.3 – 0.1 n.s. 1.8 1.8 0
7. Skameltamossen 11.2 10.6 – 0.6 n.s. 5.7 5.5 – 0.2 n.s. 1.8 2.1 0.3* 0.4 0.4 0. 2.2 1.7 – 0.5**
8. Store Jönsmosse 10.1 11.1 1.0* 5.1 5.2 0.1 n.s. 1.6 2.2 0.6** 1.6 1.6 0 1.5 1.7 0.2 n.s.
9. Snokamossen 9.2 8.5 – 0.7* 6.4 6.1 – 0.3 n.s. 1.2 1.1 – 0.1 n.s. 0.6 0.4 – 0.2 n.s. 0.8 0.8 0
10. Ivåsabäcken 10.8 10.2 – 0.6 n.s. 5.3 5.3 0 1.8 1.9 0.1 n.s. 0.8 0.7 – 0.1 n.s. 1.6 1.7 0.1 n.s.
11. V. Davidsmosse 9.6 10.3 0.7 n.s. 6.0 6.3 0.3 n.s. 1.2 1.2 0 1.0 1.3 0.3 n.s. 1.2 1.2 0.
12. Ö Davidsmosse 10.0 11.9 1.9*** 5.8 6.2 0.4 n.s. 2.0 2.5 0.5*** 0.4 0.5 0.1 n.s. 1.5 2.1 0.6**
13. Skärkeån 11.4 11.0 – 0.4 n.s. 5.3 5.2 – 0.1 n.s. 1.6 1.6 0 0.8 0.4 – 0.4* 2.9 3.0 0.1 n.s.
14. Storemosse 9.6 9.8 0.2 n.s. 4.3 4.3 0 0.9 0.9 0 1.2 1.0 – 0.2 n.s. 2.0 2.2 0.2 n.s.
15. Klintamossen 10.9 11.6 0.7* 6.3 6.3 0 2.7 2.8 0.1 n.s. 0.3 0.1 0.2 n.s. 1.7 2.3 0.6***
16. Flymossen 10.9 11.5 0.6 n.s. 5.6 5.5 – 0.1 n.s. 1.5 1.6 0.1 n.s. 1.6 1.8 0.2 n.s. 1.5 1.8 0.3 n.s.
17. Ugnshult 9.5 9.9 0.4 n.s. 5.1 5.3 0.2 n.s. 1.8 1.6 0.2 n.s. 0.6 0.9 0.3 n.s. 1.4 1.5 0.1 n.s.
18. Sutaremossen 12.6 12.2 – 0.4 n.s. 6.8 6.6 – 0.2 n.s. 2.1 2.3 0.2 n.s. 0.8 1.0 0.1 n.s. 1.8 1.5 0.3 n.s.
19. Tjuvömosse 10.4 10.4 0 6.1 5.9 – 0.2 n.s. 2.1 2.3 0.2 n.s. 0.2 0.2 0 1.5 1.6 0.1 n.s.
20. Björnåsen 12.9 11.8 – 1.1* 6.6 5.8 – 0.8*** 2.2 2.3 0.1 n.s. 1.2 1.2 0. 2.4 2.1 – 0.3 n.s.
21. Tjärnemossen 11.1 11.0 – 0.1 n.s. 5.9 5.9 0 2.4 2.5 0.1 n.s. 0.5 0.3 – 0.2 n.s. 1.7 2.1 0.4 n.s.
22. Fläskabackarna 11.1 11.7 0.6 n.s. 6.0 6.2 0.2 n.s. 2.5 2.6 0.1 n.s. 0.2 0.3 0.1 n.s. 1.8 1.8 0
23. Yamossen 10.2 10.8 0.6* 4.9 5.4 0.5** 1.4 1.8 0.4** 1.0 0.9 – 0.1 n.s. 1.8 1.8 0
24. Storemosse Kba. 9.2 9.6 0.4 n.s. 5.5 5.8 0.3 n.s. 0.9 1.1 0.2 n.s. 1.3 1.5 0.2 n.s. 0.8 0.8 0
25. Lyngmosse 11.1 10.8 – 0.3 n.s. 6.3 6.1 – 0.2 n.s. 1.7 2.0 0.3** 0.8 0.7 – 0.1 n.s. 1.5 1.6 0.1 n.s.
Total 10.5 10.8 0.3 ** 5.7 5.7 0 n.s. 1.6 1.8 0.2 *** 0.8 0.8 0 n.s. 1.7 1.8 0.1 n.s.
n.s. = not significant (P ≥ 0.05); * = P < 0.05; ** = P < 0.01; *** = P < 0.001.- Vegetation shifts towards wetter site conditions on oceanic ombrotrophic bogs - 601
Discussion
Short-term shifts in vegetation composition can occur
in ombrotrophic bogs in response to normal levels of
precipitation following a drought. We observed increased
relative frequencies of species typical for wetter condi-
tions (carpets and lawns), most notably the Sphagnum
species (Tables 2 and 4). We had expected a drying trend
in the vegetation over the 5-year time interval based on
previously reported vegetation changes on ombrotrophic
bogs in a nearby region (Risager 1997; Gunnarsson et al.
2002; Linderholm & Leine 2004) and in northwestern
Europe in general (Gunnarsson & Rydin 1998; Frankl &
Schmeidl 2000). On the other hand, this increase in wet
species of bogs in southwestern Sweden coincided with
an increased bryophyte abundance in Norwegian forests
found during the same period (1996-2002; Økland et al.
2004).
Changed climatic conditions?
The driving force behind the observed vegetation
development in our study bogs is likely the change in
regional climate towards wetter conditions during the
last five-year period. When looking at the amount of
precipitation during the vegetation periods (May-Sep-
tember) 1995-2004, it is clear that the years 1995 and
1997 were dry, 1998, 1999 and 2004 were wet and 1996
and 2000-2003 were near the 30-year average (Fig.
4). We have no direct measurements of changes in the
groundwater table on the bogs, however, measurements
of the groundwater tables in wells in the area (Anon.
2006), which we know are correlated to the groundwater
table of bogs (Gunnarsson 1994), show that the growing
seasons 1995 and 1997 had lower and 1996 had much
lower groundwater table levels than normal, while the
Fig. 3. DCA ordination of the two first ordination axes (DCA
Axes 1 and 2) showing: (a) centroids of the plots from each bog
(numbered according to Table 1) and their movements in the
ordination space from 1999 (start of the line connected to the
circle) to 2004 (circle); (b) the species ordination. Abbreviations
for the species include the first letters of the genera and species
names, with full names given in Table 2. In order to improve
the readability of the graph, uncommon species are not shown
(i.e., species with < 10 occurrences during both years).
Fig. 4. Precipitation (mm) during the growing season (May
through September), 1995 through 2004 at the meteorological
station Torup, Halland. The reference line shows the 30-year
average during the vegetation periods 1961-1990 (data from
SMHI 1995-2004).602 Gunnarsson, U. & Flodin, L.-Å.
years 1998 and 1999 experienced a higher groundwater Eutrophication
table level than normal. The years 2000-2003 had normal
groundwater table levels and in 2004 slightly higher than Even during this relatively short investigation, we
normal (Anon. 2006). Thus the dry years 1995-1997 found signs of increased eutrophication. Species typical
might have had an effect on the vegetation during the of fens or more nutrient-rich habitats (more eutrophic
first inventory, reducing the abundance of Sphagnum. habitats) increased in occurrence, e.g. Sphagnum fallax,
The condition of the Sphagnum species recovered to a Eriophorum angustifolium (not significantly), Myrica
more typical condition after precipitation levels became gale and Erica tetralix. Increased occurrences of S. fallax
more normal. on ombrotrophic bogs have been observed in many parts
During the period from 1990 through 2002, precipi- of West and Central Europe, probably as a consequence
tation in Sweden has increased by 11 % compared to of increased nitrogen deposition (Limpens et al. 2003).
the period 1901-1990, mainly because the winters have Such strong increase as observed for S. fallax and other
become milder and warmer (Lindström & Alexanders- nitrogen demanding species on bogs of southwestern
son 2004). Changed climatic condition may affect the Sweden, during such a short period, should arouse at-
groundwater table dynamics of bogs and also have had tention of the conservation agencies. This development
effects on the length of the growing season (Walther & might have been the result of a long-term nitrogen depo-
Linderholm 2006). Bryophytes are especially affected sition level far over the critical load for ombrotrophic
by growing season because they can be photosynthetic- bogs (Grennfelt & Thörnelöf 1992).
ally active during periods without frost, i.e. early spring
and late autumn far outside of the growing period of Fluctuations or directional change?
vascular plants. However, during the study period, these
changes were overridden by the effects of a few years Some of the bog species are highly dynamic in small
with exceptionally dry weather. Nevertheless, climatic plots (16 cm × 16 cm) e.g. Drosera species and Rubus
change will have an important long-term effect on the chamaemorus, while other species have higher persist-
future ombrotrophic vegetation in the region. ence, including Eriophorum vaginatum, Erica tetralix,
There might be a complementary explanation for Trichophorum caespitosum, most Sphagnum species
the observed change in species composition in favour and most liverworts (Nordbakken 2000). Changes in the
of vegetation that is associated with wetter conditions in dynamic species (Drosera rotundifolia, Rubus chamae-
2004 rather than in 1999. The drainage systems (ditches) morus and seedlings of Pinus sylvestris; Table 2) can
surrounding bogs have been the main factors responsible mostly be considered as being caused by inter-annual
for the relatively high degree of hydrologic disturbances population size variation, meanwhile the significant
on the bogs (Table 1). In the past, the drainage systems changes in the highly persistent group (in this study
(ditches) have probably had an important effect by lower- Eriophorum vaginatum, Myrica gale, Sphagnum austinii,
ing the groundwater table of the bogs and for increasing S. fallax, S. magellanicum, S. rubellum, Cephalozia spp.
the drying process (Freléchoux et al. 2000; Gunnarsson and Odontoschisma sphagni) are more likely to reflect
et al. 2002; Linderholm & Leine 2004). Most of the directional changes in basic environmental conditions,
ditches in this area were created during the early 1900s, i.e. site wetness and eutrophication status. As most spe-
some during the 1970s and the early 1980s (Hånell 1990) cies that showed significant changes in frequency over
in order to enhance forest growth on the peatlands and time in Halland belong to this highly persistent group, the
adjacent swamp-forests. In the mid-1980s, the digging observed changes probably reflect a long-term vegeta-
of new ditches was prohibited and economic incentives tion shift. This idea is further supported by the relatively
for the maintenance of ditches were reduced. However, uniform pattern of vegetation change all over Halland.
now after several years without management, the ditches Our results indicate that a series of dry years may
may have become filled in with vegetation and no longer have prolonged effects on the vegetation composition.
operating as effective drainage systems. This in-filling However, the vegetation responded to the normal pre-
will increase the water table and peat productivity of bogs cipitation levels, which shows that bogs in this region
by creating wetter growth conditions. For most plants are resilient to short-term climatic shifts. Further east
growing on an ombrotrophic bog, the level and dynamics in Sweden, in areas with less precipitation but similar
of the groundwater table are essential (Rydin 1986; Økland temperature regimes, resilience after short-term climatic
1990). If the filling in of drainage systems currently is one shifts may be reduced. Reduced precipitation together
of the driving forces for the vegetation development on with an increased evapotranspiration might be why we
ombrotrophic bog ecosystems in this region, the wetter currently observe that open bogs become more tree cov-
bog vegetation may be developed in Sphagnum-dominated ered in southeastern Sweden. Furthermore, it seems that
peatlands in southwestern Sweden. the shift between open treeless bogs and bogs with trees- Vegetation shifts towards wetter site conditions on oceanic ombrotrophic bogs - 603
is sensitive to climate change. Monitoring programmes Mountains, Switzerland. Ann. For. Sci. 61: 309-318.
are needed also in less oceanic regions of Sweden to Godwin, H. 1975. History of the British flora. 2nd. ed. Cam-
determine if changes in tree dominance are also occurring bridge University Press, Cambridge, UK.
elsewhere. A continuation of this ongoing monitoring Glaser, P.H. & Janssens J.A. Raised bogs in eastern North
America: transitions in landforms and gross stratigraphy.
program is essential to document short- versus long-term
Can. J. Bot. 64: 395-415.
changes in bogs, and their relationship to shifts in climate Grennfelt, P. & Thörnelöf, E. 1992. Critical loads for nitrogen
and environment. – a workshop report. Nord 1992: 41.
Gunnarsson, U. 1994. Pine population dynamics on two raised
bogs in eastern Sweden. M.Sc. Thesis, Department of Plant
Acknowledgements. This project was supported by the county Ecology, Uppsala University, Uppsala, SE.
administrative board of Halland (the regional monitoring pro- Gunnarsson, U. & Rydin, H. 1998. Demography and recruit-
gramme) and by Formas (project Nr: 21.5/2003-0603). We ment of Scots pine on a raised bog in eastern Sweden and
thank Rune H. Økland, Beth A. Middleton and Ingvar Backéus relationships to microhabitat differentiation. Wetlands
for comments on the manuscript and Charlotte Sweeney for 18: 133-141.
correcting the language. Gunnarsson, U. & Rydin, H. 2000. Nitrogen fertilisation
reduces Sphagnum production in bog communities. New
Phytol. 147: 527-537.
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