Lindbergia 38: 30–38, 2015
ISSN 2001-5909
Accepted 10 November 2015
Porella obtusata: distribution, ecology and threats at the west
coast of Norway, the northern fringe of its European distribution
Kristian Hassel, John Inge Johnsen, John Bjarne Jordal and Asbjørn Knutsen
K. Hassel (kristian.hassel@ntnu.no), Univ. Museum, The Natural History Dept, Norwegian Univ. of Science and Technology, NO-7491
Trondheim, Norway. – J. I. Johnsen, Fylkesmannen i Rogaland, Box 59, NO-4001 Stavanger, Norway. – J. B. Jordal, Auragt. 3, NO-6600
Sunndalsøra, Norway. – A. Knutsen, Grønåsvågen 28, NO-5440 Mosterhamn, Norway.
Porella obtusata is a liverwort with a southwestern distribution in Europe. In Norway it is confined to areas with
oceanic climate along the west coast, where it also reaches its northern limit. Typically it is growing on sunlit, base
rich rocks at sea level in southwestern Norway, a scarce habitat in these coasts dominated by granitic bedrock. There
are some old records, but investigations after year 2000 have yielded much new information. Porella obtusata is a
rare species in Norway, and the new information shows that its habitat is threatened by exploitation and shrub
encroachment of the shoreline.
The liverwort Porella obtusata (Tayl.) Trev. has status as
a threatened species in Norway, and is evaluated as endangered (EN) in the Norwegian red list (Hassel et al.
2010). The size of the Norwegian population is estimated to be about 2000 individuals, and no subpopulations have more than 250 individuals, combined with an
ongoing decline in number of individuals (Hassel et al.
2010). The reasons for decline are twofold; first there is
encroachment of areas earlier used for livestock grazing
and second, human exploitation of the species habitat by
e.g. boathouses, harbors, roads etc. Thus, the rarity and
decline of P. obtusata in Norway does not seem to result
from natural fluctuations of populations at the edge of
their distribution.
The traditional view was that P. obtusata had a wide
Palearctic distribution, reported from two main areas,
East Palearctic (China and Japan), and West Palearctic
(Europe, North Africa and Asia Minor; Gökler 1998,
Damsholt 2002). However, the Asian taxon Porella obtusata var. macroloba (Steph.) S.Hatt. et. M.X.Zhang
has proven not to be conspecific with the European P.
obtusata (Hentschel et al. 2007), and is better treated as
Porella macroloba (Steph.) S.Hatt. et. Inoue. In the West
Palearctic P. obtusata s.s. is recorded from most of the
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org/licenses/by/4.0/>.
30
states bordering the Mediterranean Sea (Ros et al. 2007).
In addition the distribution includes Macaronesia and
the Atlantic coast of the Iberian Peninsula, France, Great
Britain and Ireland, and with the northern limit at the
southwestern coast of Norway (Paton 1999, Damsholt
2002).
The wide Mediterranean distribution of P. obtusata
may indicate survival during the last glacial maximum in
several refugial areas like Asia Minor, Italy, Spain, and one
or more of the Macaronesian Islands (Taberlet et al. 1998,
Ansell et al. 2011, Hutsemékers et al. 2011). Based on
current distribution the source populations for northward
dispersal after the last glaciation were most likely situated
on the southwestern European mainland or Macaronesia
(Kyrkjeeide et al. 2014). Whether the northward dispersal resulted from spores or vegetative fragmentation is not
known, but today spore production in the Atlantic region
seems to be restricted to Macaronesia (Paton 1999, Damsholt 2002, Fontinha et al. 2010).
The first record of P. obtusata in Norway was made
in 1889. Early knowledge on the species was compiled
by Jørgensen (1934), as P. thuja. In total only five localities were mentioned; Sogn og Fjordane, Askvoll, Alden
(1889); Sogn og Fjordane, Flora, Kinn (1898, very sparse,
no specimens known – needs confirmation); Rogaland,
Kvitsøy, Ystabø (1912); Hordaland, Austevoll, the islands
of Møkster and Litlakalsøy (1919). Few records have been
added after Jørgensen (1934) until the year 2000, but
© 2015 The Authors. This is an Open Access article.
thereafter the species has been searched for more systematically, especially in the period 2008–2015.
In its typical form, P. obtusata is easily separated from
other Scandinavian Porella species by its underleaves twice
as wide as the stem, broadly rounded leaf lobes with lobules about as wide as the underleaves, and the lack of acrid
taste (Paton 1999, Damsholt 2002, Atherton et al. 2010).
It is the only Scandinavian species of the genus that mainly is found on sea cliffs. In the field the flat glossy patches,
yellowish green with a secondary brownish pigmentation
are characteristic (Fig. 1). Porella platyphylla (L.) Pfeiff.
and P. cordaeana (Huebener) Moore are not glossy, but
dull green, normally without brown pigmentation. Porella
arboris-vitae (With.) Grolle is glossy, but has the characteristic strong acrid taste; a modification of P. arboris-vitae
(syn. P. laevigata var. thuja Nees) with strong acrid taste,
was recorded (Arnell 1956) with a range from Vest Agder
to Sogn og Fjordane, similar in range to P. obtusata. These
plants were described as having an acrid taste and can
therefore not belong to P. obtusata, although they mimic
that species.
The aim of the current study is to compile information
on the distribution and ecology of P. obtusata at the northern limit of its distribution in Europe. We also evaluate the
status of the populations and current threats to populations of P. obtusata in Norway.
Material and methods
Collections of Porella obtusata in Norwegian herbaria
were examined by the first author. Both old and new
collections and field notes are available at “Artskart”
(Artsdatabanken and GBIF 2015). During recent years,
the species has been systematically searched for in areas
where it could be expected to grow. This field work has
been conducted in important areas for nature management and partly on contract for the management authorities. In particular we have searched for P. obtusata
in areas with calcareous or other base-rich bedrocks
close to the sea in the oceanic southwestern districts of
the counties Rogaland and Hordaland. Geographic coordinates of all registered populations were taken with
handheld GPS with an accuracy of ± 10 m. Notes on
habitat, substrate and altitude were made at each locality. Associated species were registered at some localities.
Collections from new localities are deposited in public
herbaria, mainly TRH.
Figure 1. Porella obtusata growing in yellowish green glossy patches with a characteristic secondary brownish pigmentation. (Photo:
J. B. Jordal).
31
Nomenclature of bryophytes, lichens and vascular
plants follows Artsnavnebasen (Artsdatabanken 2015).
Results
Distribution
Porella obtusata is reported from five new municipalities:
Rennesøy in Rogaland, Bømlo and Stord in Hordaland
and Selje and Bremanger in Sogn og Fjordane (Fig. 2a).
We have also found many new localities close to earlier
known localities, and got hold of much more information
about the occurrence of P. obtusata at the old localities
reported by Jørgensen (1934), especially in the southern
part of the distribution area. Based on current knowledge
Kvitsøy (Rogaland) and Bømlo (Hordaland) have most
localities, the largest populations, and are the core areas
for P. obtusata in Norway (Table 1).
Rogaland district
Porella obtusata has for a long time been known from Kvitsøy County, and here it is now recorded from many localities. In the Kvitsøy archipelago P. obtusata occurs both on
the main island and many of the smaller islands (Fig. 2c;
Jordal and Johnsen 2009). All old records labelled “Rennesøy” belong to Kvitsøy (Ystabø). The species is widespread and rather common in the Kvitsøy archipelago,
which is in contrast to all neighboring areas that also seem
to have suitable habitats. Rennesøy and Finnøy are two islands with rather large areas of suitable habitat, but intensive field investigations have resulted in only two records
of P. obtusata.
Hordaland district
Field investigations at potential localities of Porella
obtusata in Bømlo, Stord and Austevoll, shows that it
Figure 2. Distribution of Porella obtusata in (A) Norway, (B) Bømlo County and (C) Kvitsøy County (based on data from Artsdatabanken and GBIF 2015, Jordal and Johnsen 2009).
32
Table 1. Known localities (>500 m apart) of P. obtusata with information on population size. Shrub encroachment, is a general threat that applies to most localities of P. obtusata and
therefore not specified under threats.
Locality
County
District
Altitude m a.s.l. No. of records
Year
Population size
Threats
medium
habitat loss
Litla Kalsøy, near the harbour
Austevoll
Hordaland
1–19
7
1919–2008
Litla Kalsøy, northeastern part
Austevoll
Hordaland
2-3
3
2008
small
unknown
Møkster south
Austevoll
Hordaland
1–5
9
1919–2008
small
habitat loss
Mosterhamn
Bømlo
Hordaland
1–10
49
1967–2014
large and important
habitat loss
Mosterhamn, Hestavikjo
Bømlo
Hordaland
-
1
1967
unknown
destroyed
Bømlahamn
Bømlo
Hordaland
1–5
14
2008–2014
medium
habitat loss
Eidesvika
Bømlo
Hordaland
2–5
2
2013–2014
small
habitat loss
Holmen
Bømlo
Hordaland
5
1
2011
small
unknown
Litle Trettholmen
Bømlo
Hordaland
5
1
2009
small
unknown
Store Nappaholmen
Bømlo
Hordaland
2–5
1
2014
medium to large
unknown
Spyssøya, Vedneset
Bømlo
Hordaland
1–5
5
2008–2014
small
habitat loss
Spyssøya, west of Klubbaskeia
Bømlo
Hordaland
2-4
5
2008–2014
small
unknown
Steinsvåg
Bømlo
Hordaland
4
1
2011
small
unknown
Vestre Vika
Bømlo
Hordaland
1–10
6
2008–2014
small
habitat loss
Vorland: Rundøya
Bømlo
Hordaland
3-5
1
2013
small
unknown
Espevær
Bømlo
Hordaland
2-5
1
2015
small
unknown
Nautøya with islands near by
Stord
Hordaland
2-5
7
1995–2015
medium
unknown
Storsøy
Stord
Hordaland
-
1
1972
unknown
unknown
Kvitsøy mainland with nearby islands
Kvitsøy
Rogaland
1–8
79
1912–2013
large and very important
habitat loss
Bussholmen
Kvitsøy
Rogaland
3
1
2008
small
unknown
Hestholmen
Kvitsøy
Rogaland
-
3
2011
small
unknown
Higgelen
Kvitsøy
Rogaland
5
1
2011
small
unknown
Håland
Kvitsøy
Rogaland
2
1
2009
small
unknown
Krågøy
Kvitsøy
Rogaland
3
1
2011
small
unknown
Sandholmen and Lyngholmen
Kvitsøy
Rogaland
1–3
4
2008–2011
small
unknown
Sandholmen, western
Kvitsøy
Rogaland
-
2
2011
small
unknown
Sandøya
Kvitsøy
Rogaland
1–3
10
2008
medium
unknown
Ternøy
Kvitsøy
Rogaland
6
1
2008
small
unknown
33
Ertensøy, eastern part
Rennesøy
Rogaland
1–10
3
1987–2015
small
unknown
Sørbø church
Rennesøy
Rogaland
20
1
1985
small
destroyed
Alden
Askvoll
Sogn og Fjordane
0–300
9
1889
unknown
unknown
Atløyna: Brurastakken south
Askvoll
Sogn og Fjordane
-
1
2003
small
unknown
unknown
Ytre Grotle, Storholmen
Bremanger
Sogn og Fjordane
4
2
2014
small
Kinn, sydsiden nær stranden
Flora
Sogn og Fjordane
<10
1
1898
small (old info. 1898)
unknown
Geitnesvika, between Drage and Indre Fure
Selje
Sogn og Fjordane
5
2
1967–2014
small
unknown
is restricted to areas with base-rich bedrock, although
other substrates have been investigated (Jordal and
Gaarder 2009, Jordal 2010, Fadnes 2013, Flynn and
Gaarder 2013). Most records are from Bømlo (Table
1, Fig. 2b) and this area seems together with Kvitsøy
in Rogaland district to be the main area for P. obtusata
in Norway.
Sogn og Fjordane district
In this region we have not searched systematically for P.
obtusata. Altogether five localities are known. There are
two old localities, Alden Island in Askvoll County and
Kinn Island in Flora County, which have not been revisited. The three more recent localities are from Selje
County between Drage and Indre Fure on the Stadtlandet peninsula, discovered in 1967 and refound in 2014,
from Atløyna island, Brurastakken sør in Askvoll County,
discovered in 2003, and from Ytre Grotle in Bremanger
County, discovered in 2014 (Table 1). The localities in
Selje are the northernmost known for P. obtusata in Europe.
Habitat and substrate
Porella obtusata is usually found growing in thick patches or small carpets on sunlit calcareous rocks or other
base-rich rocks close to the sea (Fig. 3). The substrates
of P. obtusata along the Norwegian coast are different
base-rich bedrocks like greenstone, marble and limestone. In Bømlo the species seems to have preference
for greenstone and is not very frequent in the areas with
limestone and marble. The Kvitsøy area is also rich in
greenstone. The tendency of the rock to create fissures
and small cavities may be of higher significance to the
species than the kind of rock itself. Exposure varies between east/southeast and northwest, but most localities
are southerly to westerly exposed. All new sites are below
20 m a.s.l., and most records are below 6–7 m a.s.l. At
Alden island in Askvoll, Jørgensen (1934) cites B. Kaalaas 1889 on P. obtusata growing “up to 300 m”. This is
very different from all other known Norwegian localities and should be confirmed. In Kvitsøy and Bømlo P.
obtusata often grows in small fissures with trickling water, or protected against the wind and sun in small cavities made by erosion of the rocks. Sometimes it is found
Figure 3. Porella obtusata is typically growing in small patches on sunlit base rich rocks close to the sea, like here at Bømlo, Hordaland.
(Photo: K. Hassel).
34
slightly sheltered by small bushes like Juniperus communis or graminoids. This shelter seems to be important
during dry windy periods like the winter 2013/14 where
many of the exposed patches got dried out and color
changed to brown, whereas the patches below J. communis looked green and healthy.
Associated species
Among mosses and liverworts many associated species
have been noted. In Kvitsøy associated bryophytes noted
were e.g. Pterogonium gracile, Bryum alpinum, B. capillare,
Trichostomum brachydontium, Hypnum jutlandicum, Frullania tamarisci and Homalothecium sericeum. Other less
frequent species were e.g. Hedwigia stellata, Polytrichum
piliferum, Hypnum lacunosum, Scapania compacta, Riccia
sorocarpa, Frullania fragilifolia, Mnium hornum, Dicranum scoparium, Tortula subulata, Gymnostomum aeruginosum, Grimmia pulvinata and Schistidium maritimum. In
Bømlo Ptycomitrium polyphyllum, Tortella bambergeri and
T. tortuosa were also noted.
Also some lichens were observed to co-occur with P. obtusata, one of the most remarkable was the rare Leptogium
britannicum at some of the localities in Kvitsøy, Bømlo and
Austevoll. The ecological demands of this species seem very
much to resemble those of P. obtusata. Other lichens recorded together with P. obtusata were e.g. Parmotrema crinitum
and different Collema spp. and Scytininum spp. and further
Parmelia saxatilis, Dermatocarpon spp., Enchylium tenax, Nephroma laevigatum, Peltigera collina, and Cladonia spp. (e.g.
C. portentosa, C. furcata, C. strepsilis and C. subcervicornis).
Commonly associated vascular plants in Bømlo and
Kvitsøy were e.g. Asplenium adianthumnigrum, A. rutamuraria, Sagina subulata, Geranium columbinum, G. sanguineum, Viola tricolor, Hylotelephium maximum, Rhodiola
rosea, Aira praecox, Sedum anglicum, S. acre, Armeria maritima, Plantago maritima and P. lanceolata.
Threats
Two of the 34 localities were destroyed due to fertilization by use of wet manure, and building of houses. At
Figure 4. At least eight localities of P. obtusata are threatened by habitat loss due to construction work, like here at Litla Kalsøy in
Austevoll, Hordaland. (Photo J. B. Jordal).
35
Figure 5. Shrub encroachment, due to reduced grazing by livestock, is a general threat that applies to most localities of P. obtusata, like
here at Bømlo, Hordaland. (Photo J. B. Jordal).
least eight localities are threatened by habitat loss due to
construction work (Fig. 4, Table 1).
Shrub encroachment, due to reduced grazing by livestock, is a general threat that applies to all localities (Fig.
5). This also applies to the few localities in protected areas.
In Kvitsøy, Rogaland, one locality lies within Higgelen nature reserve (created to protect seabirds), and five of the
localities (Ternøya, Sandøya, Sandholmen, Lyngholmen,
Bussholmen) lie in the Higgelen-Eime bird protection
area, which offers only weak protection, mainly prohibition of hunting.
Discussion
Distribution and ecology
The Norwegian distribution of Porella obtusata is confined
to the southwestern coast, an area which is characterized
by an oceanic climate with mild winters and cool sum-
36
mers (Størmer 1969). We show that the main areas for P.
obtusata in Norway, with most localities and the largest
populations, are found in the southern part of the distribution area. Most populations are found close to sea level,
which seems to be an effect of latitude as it is recorded
from higher altitudes further south, e.g. from sea level
to 650 m a.s.l. in the British Isles (Blockeel et al. 2014),
370–980 m a.s.l. and 900–1850 m a.s.l. in Portugal and
Madeira (Fontinha et al. 2010, Hespanhol et al. 2010).
Porella obtusata belongs to an oceanic southern-temperate
element sensu Hill and Preston (1998). They include in
this element 18 species from the British Isles, six of which
are also known from the Norwegian southwestern coast,
i.e. Glyphomitrium daviesii, Ptycomitrium polyphyllum,
Ulota calvescens, Lophocolea fragrans, Porella obtusata and
Saccogyna viticulosa. The only species that may grow together with P. obtusata is Ptycomitrium polyphyllum, which
is also the only other species with relatively large local populations. Large local populations of P. polyphyllum can be
explained by efficient dispersal, as P. polyphyllum is commonly found with sporophytes. This is also reflected in a
more even distribution along the Norwegian southwestern coast. All the other species of this oceanic southerntemperate element occupy other habitats and have small
and scattered distributions.
The restriction of P. obtusata to low altitudes is an important factor contributing to its evaluation as a threatened species in Norway. Low altitude areas are under
high pressure by human exploitation. Impacts of land-use
changes on habitats during the last 50 years are by far the
most important reason for putting species at risk of dying out in Norway, especially physical disturbances (i.e.
house building, roads and other infrastructure) and landuse changes associated with farming and forestry (Kålås
et al. 2010).
In earlier times human exploitation probably had a
positive influence on the populations of P. obtusata. The
agricultural practice of the first half of the 20th century
with high densities of livestock and a high grassing pressure kept the vegetation open and favorable for a weak
competitor like P. obtusata (Austrheim et al. 2011). Thus
both change in agricultural practice and increased exploitation of lowland areas leads to loss of P. obtusata habitat.
The habitat preference of P. obtusata is rather narrow
and limited to base-rich bedrocks. The available areas
for population establishment is highly restricted along
the acidic bedrock dominating the southwestern coast of
Norway (Norges Gologiske Undersøkelser 2015). Sexual
reproduction by spores is not known in the North Atlantic part of the distribution area, and only sterile and
female plants are observed (Jørgensen 1934, Paton 1999,
Damsholt 2002). Spore production in the Atlantic region
is only reported from Madeira, where it seems to be common (Fontinha et al. 2010). Thus dispersal and establishment in possible habitats along the Norwegian coast,
probably rely mainly on vegetative diaspores. The current
distribution of P. obtusata in Norway could be hypothesized to have originated from a few long-distance dispersal
events by wind leading to establishment of isolated populations along the Norwegian west coast. From these populations local dispersal could take place. The success of local
dispersal would depend on distance to available habitats
and potential dispersal vectors. The reason for finding the
large populations at Bømlo and Kvitsøy is probably due
to old human settlements in the region (Alsaker 1987),
and large sheep populations that has been moved from the
main islands and out to smaller island during the season to
utilize available resources. Sheep are known to be effective
vectors for local dispersal of bryophyte fragments (Pauliuk
et al. 2011), and with the help of human boat transport
between islands, the dispersal could be very effective.
In a historic perspective it is likely that P. obtusata has
had more numerous and larger populations in its main
distribution area (Kvitsøy and Bømlo) up to the mid-20th
century. Up to this time the landscape was kept open due
to high utilization by sheep grazing. Later the grazing pressure of unimproved land has decreased by as much as 50%
in western Norway (Austrheim et al. 2011). During the
second half of the 20th century managed grazing with the
use of fertilizers to increase grass production are of growing importance (Asner et al. 2004, Austrheim et al. 2011).
This lead to shrub encroachment in abandoned areas with
subsequent increased competition from vascular plants.
Formerly grazed Calluna vulgaris heath areas and grazed
pastures in upper parts of the sea shore are transformed by
tall herbs, bushes and trees, like Juniperus communis, Rosa
spp., Betula pubescens, Sorbus aucuparia, Salix spp. and
others. This reduces the light conditions and the possible
habitat where P. obtusata can survive. In the more intensively used grassland areas a more dense grass vegetation
develop leading to increased competition from vascular
plants due to fertilization.
In addition increased pressure on lowland areas due to
a larger human population and demands of roads, marinas, buildings etc. lead to direct loss of available habitats.
In Kvitsøy filling with rock masses from underwater tunnel construction also may become a threat, if the plans
become realized. Stone and gravel may be used for different purposes like building roads, e.g. to connect different
islands. The coastal areas are popular in many ways and
there is an increasing human population and pressure to
exploit it.
Climate change can offer increasing problems. In the
winters 2012–2014 most of the distribution area had
periods with very dry conditions without a protecting
snow cover. After this, it was observed that a large part
of the populations in Bømlo were strongly reduced due
to drought.
Red list status
The species is listed as endangered (EN) in Norway (Hassel et al. 2010), and this is not changed in the proposal
for the 2015 Norwegian red list, based on the above mentioned knowledge on the current population and the actual threats. The species is not known in Sweden, Finland,
Denmark or Germany. In other European countries P. obtusata is listed as vulnerable (VU) in Serbia, near threatened (NT) in Canary Island and Italy, and data deficient
(DD) in San Marino and Bulgaria (Hodgetts 2014).
State of knowledge
The investigations of the last years have given a more thorough picture of the distribution and ecology in the most
important areas in Rogaland and Hordaland. In Sogn og
Fjordane the situation is more uncertain. We know that
the distribution of suitable rocks for the species is limited,
but we still have work to do here. There is also a possibility that the species could occur further north, e.g. in the
southern part of Møre og Romsdal. However, areas with
37
suitable rocks are scarce here. The areas with appropriate
rocks near the sea in the right climate are as a whole very
limited in Norway. Therefore it is not expected that the
number of known localities with P. obtusata will rise much
in the future.
Acknowledgements – Norwegian Biodiversity Information Centre
(Artsdatabanken) have financed part of the work through the
project “Liverworts of western Norway”. We thank bryologists
for sharing their records of Porella obtusata on Artsobservasjoner
(<www.artsobservasjoner.no/>). We thank Lars Söderström for
useful comments on the manuscript and Marc Daverdin for
making the maps.
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