Oecologia (2011) 166:769–782
DOI 10.1007/s00442-011-1911-6
C O M M U N I T Y E C O L O G Y - O RI G I N A L P A P E R
Balancing positive and negative plant interactions: how mosses
structure vascular plant communities
Jemma L. Gornall · Sarah J. Woodin ·
Ingibjorg S. Jónsdóttir · René van der Wal
Received: 17 July 2009 / Accepted: 6 January 2011 / Published online: 30 January 2011
© Springer-Verlag 2011
Abstract Our understanding of positive and negative
plant interactions is primarily based on vascular plants, as
is the prediction that facilitative eVects dominate in harsh
environments. It remains unclear whether this understanding is also applicable to moss–vascular plant interactions,
which are likely to be inXuential in low-temperature environments with extensive moss ground cover such as boreal
forest and arctic tundra. In a Weld experiment in high-arctic
tundra, we investigated positive and negative impacts of the
moss layer on vascular plants. Ramets of the shrub Salix
polaris, herb Bistorta vivipara, grass Alopecurus borealis
and rush Luzula confusa were transplanted into plots
manipulated to contain bare soil, shallow moss (3 cm) and
deep moss (6 cm) and harvested after three growing seasons. The moss layer had both positive and negative
impacts upon vascular plant growth, the relative extent of
which varied among vascular plant species. Deep moss
Communicated by Bryan Foster.
J. L. Gornall · S. J. Woodin
School of Biological Sciences,
University of Aberdeen, Aberdeen AB24 3UU, UK
J. L. Gornall · I. S. Jónsdóttir
UNIS, PB 156, 9171 Longyearbyen, Norway
I. S. Jónsdóttir
Institute of Biology, University of Iceland,
101 Reykjavik, Iceland
R. van der Wal (&)
Aberdeen Centre for Environmental Sustainability,
School of Biological Sciences, University of Aberdeen,
Aberdeen AB24 3UU, UK
e-mail: [email protected]
cover reduced soil temperature and nitrogen availability,
and this was reXected in reduced graminoid productivity.
Shrub and herb biomass were greatest in shallow moss,
where soil moisture also appeared to be highest. The relative importance of the mechanisms by which moss may
inXuence vascular plants, through eVects on soil temperature, moisture and nitrogen availability, was investigated
in a phytotron growth experiment. Soil temperature, and
not nutrient availability, determined Alopecurus growth,
whereas Salix only responded to increased temperature if
soil nitrogen was also increased. We propose a conceptual
model showing the relative importance of positive and negative inXuences of the moss mat on vascular plants along a
gradient of moss depth and illustrate species-speciWc outcomes. Our Wndings suggest that, through their strong inXuence on the soil environment, mat-forming mosses structure
the composition of vascular plant communities. Thus, for
plant interaction theory to be widely applicable to extreme
environments such as the Arctic, growth forms other than
vascular plants should be considered.
Keywords Competition · Facilitation · High-arctic ·
Nutrient availability · Soil temperature
Introduction
The study of plant–plant interactions provides fertile
ground for both conceptual debate and concrete tools for
interpreting patterns of plant distribution and abundance.
Interactions between plants run along a continuum from
competition to facilitation, also termed negative and positive plant interactions, the sum of which determines the net
outcome (Bertness and Callaway 1994; Brooker and
Callaghan 1998). Competition is now widely acknowledged
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as a powerful force structuring plant communities, and
perceived as most important in relatively productive ecosystems (Weldon and Slauson 1986). More recently, facilitation has been identiWed as a potentially equally important
process notably structuring ecosystems of low productivity
(Callaway et al. 2002; Brooker et al. 2008). The balance
between competition and facilitation has been investigated
in neighbour removal experiments along environmental
gradients, which have repeatedly revealed that facilitative
components predominate in severe environments such as
arctic and alpine ecosystems, salt marsh and desert
(Brooker et al. 2008). Our understanding of plant–plant
interactions is, however, almost exclusively based on vascular plants. Whilst this may be appropriate for large parts
of the world’s ecosystems, in a range of extreme environments for which facilitation has been identiWed as a strong
structuring force, vascular plants grow in otherwise mossor lichen-dominated vegetation. The Arctic is a key example of this (Matveyeva and Chernov 2000), and here, as
well as in other northern ecosystems, mosses inXuence both
microclimate and soil processes to such an extent that they
may be considered engineers of these plant communities
(Malmer et al. 2003; Gornall et al. 2007). Here we report
on experimental Wndings from a high-arctic Weld study in
which we investigate positive and negative impacts of the
moss layer on the performance of vascular plants.
Mosses are the dominant ground cover in a range of ecosystems, and an increasing number of studies have explored
vascular plant impacts on mosses. Vascular plants may beneWt mosses by generating a favourable microclimate, as has
been suggested for both grassland and woodland (Ingerpuu
et al. 2005; Startsev et al. 2007). More frequent, however,
are accounts of negative relationships between vascular
plant and moss biomass, reXecting suppressed moss growth
through leaf litter deposition and shading, despite mosses
having a generally low light compensation point (Bergamini et al. 2001; Malmer et al. 2003, Van der Wal et al.
2005; Startsev et al. 2008). The presence of palatable vascular plants, notably graminoids, may also draw in vertebrate herbivores that, through trampling and defecation,
suppress mosses in the ground layer (Van der Wal et al.
2004). Although these eVects could be described as competition for light and apparent competition, respectively, the
nature of such interactions is highly asymmetric, as the suppression of mosses may not inXuence resource availability
to the vascular plants. Hence, the validity of applying these
concepts, derived from vascular plant competition studies,
to interactions between vascular plants and mosses may be
questionable.
The few neighbour removal studies that have explored
the potential for mosses to inXuence vascular plants point
towards moss ground cover being an inXuential rather than
a suppressed ecosystem component. Removal of Sphagnum
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Oecologia (2011) 166:769–782
spp. increased the aboveground biomass of Betula nana
(Hobbie et al. 1999) and Narthecium ossifragum (Malmer
et al. 2003). Likewise, Betula pubescence, Picea abies and
particularly Pinus sylvestris beneWtted from the removal of
the moss layer (Wardle et al. 2008). Findings from such
experimental manipulations are in line with the notion that
the moss layer can prevent early seedling establishment by
denying access to the soil required for rooting or smothering seedlings (Hörnberg et al. 1997; Sedia and Ehrenfeld
2003; Freestone 2006). However, these studies also indicate the positive eVect the moss layer can have on vascular
plants, for instance by fostering seed retention and germination, whilst other studies point at enhancing nutrient availability and soil moisture during periods of drought (e.g.
Sohlberg and Bliss 1987).
Given that mosses can have both negative and positive
inXuences on vascular plants, it remains to be determined
whether their net outcome changes along stress gradients
in ways predicted by competition/facilitation theory.
Counter to prediction, a moss-removal experiment in
northern Sweden established that the negative eVect of
moss on vascular plants was greatest on the least productive of 30 lake islands studied, and no evidence for a netpositive eVect of mosses on the trees was found (Wardle
et al. 2008). Another such apparent contradiction of theory
was observed in a detailed study on the rare Californian
forb Delphinium uliginosum growing almost exclusively
in moss mats on inhospitable serpentine soil, with the
facilitative eVects of the moss being constant rather than
strongest at the harsher end of the environmental gradient
studied (Freestone 2006). It thus remains to be seen
whether the balance between positive and negative plant
interactions in extreme environments leans towards a
facilitative eVect of moss on vascular plants as predicted
by competition theory.
We investigate this question in the Arctic, where mosses
make up much of the plant biomass. Because of their prevalence in these typically severe environments, previous studies have aimed to determine experimentally the inXuence of
the moss layer on vascular plants. We are aware of four
studies, among which three—all conducted in Alaskan tundra—involve partial removal of the moss layer (Fetcher
1985; Hobbie et al. 1999; Bret-Harte et al. 2004). Their
Wndings point at either a suppressive or a lack of inXuence
on vascular plants, but are diYcult to interpret as the potentially inXuential decaying and/or dead layer of mosses
remained in place. The fourth study, conducted in higharctic Canada, does involve complete removal of the moss
layer. Whilst the environmental conditions under which
Sohlberg and Bliss (1987) conducted their Weld trials were
severe, both net positive and negative impacts of the moss
layer on vascular plants were demonstrated for two diVerent plant species. This clearly raises the question of
Oecologia (2011) 166:769–782
whether relationships between vascular plants and mosses
are species speciWc, even in highly severe environments.
Despite the paucity of moss removal studies, a plethora
of potential mechanisms that could drive moss–vascular
plant interactions have been identiWed. Notably, deeper
moss is found to reduce soil temperature (Oechel and Van
Cleve 1986; Van der Wal and Brooker 2004), which in turn
limited nitrogen mineralization (Gornall et al. 2007). Furthermore, evidence suggests that aerially deposited nitrogen
(including that derived from litter) is sequestered by the
moss layer, and thus unavailable to vascular plants
(Jónsdóttir et al. 1995; Malmer et al. 2003; Curtis et al.
2005). Conversely, mosses provide a habitat for N-Wxing
cyanobacteria that play a critical role in the N cycle of arctic environments (Solheim et al. 1996). Mosses may prevent frost damage to roots (Sohlberg and Bliss 1987;
Olofsson et al. 2001; Startsev et al. 2007) and also provide
shelter, a factor often cited as the mechanism behind positive plant interactions (Callaghan and Emanuelsson 1985;
Carlsson and Callaghan 1991). Their growth form both aids
moisture retention and enhances boundary layer resistance
(Sveinbjornsson and Oechel 1992), reducing the desiccating impacts of strong arctic winds. Thus, the potential
exists for both positive and negative eVects of the moss
layer on vascular plants in arctic ecosystems.
In an arctic Weld manipulation experiment, we aimed to:
(1) determine the extent of positive and negative impacts of
mosses on higher plants, (2) investigate whether the
impacts of the moss layer are consistent among vascular
plant growth forms, and (3) identify potential mechanisms
by which the moss layer aVects the performance of vascular
plants. The Weld experiment was set up in high-arctic tundra
in which moss was either removed or replaced by either a
shallow moss layer (3 cm) or a deep moss layer (6 cm).
Vascular plants were transplanted into the treatment plots
and left for three growing seasons. Species used were the
dwarf shrub Salix polaris, the herb Bistorta vivipara, the
grass Alopecurus borealis and the wood rush Luzula confusa. The importance of the mechanisms by which moss
may inXuence vascular plants, speciWcally through eVects
on soil temperature, moisture and nitrogen availability, was
investigated in a controlled environment growth experiment.
Materials and methods
Field experiment
Study site
To investigate the relative extent of positive and negative
impacts of mosses on higher plants, a Weld experiment was
771
initiated in Adventdalen, high-arctic Spitsbergen (78°10⬘N,
16°07⬘E). Being in the central-west of Svalbard, climatic
conditions in this valley system are relatively mild. During the course of our Weld experimental study (2002–
2004), the yearly average temperature was ¡5°C, with a
monthly maximum of 6°C in July and minimum of ¡15°C
in January and February (data from the nearby Longyearbyen weather station). Precipitation is low (yearly average
185 mm) and it largely falls as snow. At our study site,
located on the outermost part of an alluvial fan, winter
snow depth is low and variable (10–15 cm) compared to
surrounding areas due to strong winds that pass down the
valley; it is also one of the Wrst snow-free areas within the
region. During the experimental period, soil thaw started
after the site became snow-free in May, and reached a
maximum depth of thaw of around 50 cm in August
(Gornall et al. 2007). Soils had a shallow organic horizon
(3–5 cm), a mildly acidic pH (5.9), and started oV wet at
snowmelt, but because of low summer precipitation (4–
27 mm per month) gradually dried out during the growing
season (early June to late July). Vegetation at this mesic
site is dwarf shrub-grass heath, in which dominant vascular plant species include Salix polaris and Alopecurus
borealis, with some Luzula confusa and Bistorta vivipara.
Vascular plant biomass ranged from 67 to 103 g m¡2
aboveground and 290–360 g m¡2 belowground (Gornall,
unpublished data). The site has continuous moss cover
dominated by Tomentypnum nitens and Sanionia uncinata. Both moss species are common, widespread and
locally abundant (i.e. >50% cover) in Svalbard (Frisvoll
and Elvebakk 1996) and frequently grow together in mesic
habitats. Whereas S. uncinata is ubiquitous, T. nitens
occurs under a slightly narrower set of moisture conditions ranging from mesic tundra to the drier parts of Svalbard’s mire communities (Elvebakk 1994; Vanderpuye
et al. 2002). Total live moss biomass at our site ranged
from 169 to 304 g m¡2, and the depth of the moss layer
ranged from 2 to 6.5 cm.
Field experimental setup and protocol
Our Weld experimental setup was a fully randomized
blocked design laid out within an area of 1,500 m2. Blocks
(n = 7) were selected for homogeneity in both microtopography and plant species composition, and were at least
3 m apart. A single block contained three plots (each
75 £ 75 cm) which were placed at least 30 cm apart, and
each plot was randomly assigned to one of the three treatments: no, shallow (3 cm) or deep (6 cm) moss layer. These
treatments, created to determine the inXuence of the moss
layer on vascular plant growth, were put in place in July
2002 when the turf was stripped from all experimental plots
using a soil knife, thereby removing the whole vegetation
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layer. The “no moss layer” plots were left bare, whilst the
shallow and deep moss layer plots were covered with nine
25 £ 25 cm moss mats that were placed out at natural shoot
density. For these treatments, moss was collected from
source material that just exceeded the target depth and then
trimmed to 3 or 6 cm by removing lower sections of the
turfs. The selected turfs were dominated by the moss Tomentypnum nitens. Vascular plants were weeded out of the
moss mats once in place.
Although acknowledging the importance of early life
history processes, these were outside our research focus.
Instead, we conducted experiments with ramets of Salix
polaris, Bistorta vivipara, Alopecurus borealis and Luzula confusa. These were collected from an area adjacent
to the Weld experimental site in early July 2002. Plants
were carefully removed from the soil to limit root damage and kept damp until replanted. Species-speciWc standards were set to minimise variation due to diVering
growth stages. All shoots were non-Xowering and of the
following lengths: Salix 9 cm, Bistorta 6 cm, Luzula
8 cm and Alopecurus 8 cm. In the cases of Luzula and
Alopecurus, young shoots (only two dead leaves) were
used to reduce the chance of Xowering. Any rhizomes
were removed.
All target vascular plant species were planted in the
central 25 £ 25 cm of each plot, at a distance of 10 cm
from each other to minimise interference. In the case of
plant death, target plants were also planted in the outer
25 cm of each plot. Single ramets were planted for Salix
and Bistorta, whereas two ramets were planted for
Alopecurus and Luzula in order to maintain replication
in the event of Xowering. In total, each plot contained
twelve individual plants. Before planting, each ramet
was weighed and tagged with a label loosely attached
with string around the main shoot or rhizome. A slit was
made through the moss mat, where present, and into the
soil, after which the plant’s roots were placed in the soil
and watered. Planting was carried out in the second
week of July 2002.
In August 2004 (after 26 months), all transplants were
harvested from the experimental plots by taking a core of
soil around the individual to ensure the collection of all
roots. Plants were transported back to the lab, where excess
soil was carefully removed. The total fresh weight of each
plant was recorded. Individuals of Salix were split into
leaves, shoot and root, Bistorta was split into live leaves,
rhizome and roots, and Alopecurus and Luzula were split
into live leaves and belowground parts, which included
both roots and rhizome. Once separated, the plant parts
were dried in an oven at 70°C for 48 h and weighed. To calculate the initial plant dry weight, the Wnal fresh-weight-todry-weight ratio was multiplied by the initial fresh weights
of each plant.
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Soil characteristics
Measurements of soil characteristics in the experimental
plots were taken in the last week of July 2005. Five measurements of soil temperature were taken randomly in each
plot. A temperature probe (model 612–849, RS, Corby,
UK, 0.1°C accuracy) was inserted into the soil to a depth of
2 cm and left to equilibrate. To determine soil nitrogen content, Wve soil cores were taken from each plot and stored at
2°C. To extract NH4+ and NO3, 10 g of fresh soil were
shaken with 25 ml 1 M KCl for 30 min on an orbital shaker.
The resulting suspension was Wltered through Whatman no.
42 paper and the concentrations of NH4+–N and NO3¡–N in
the extracts were determined by autoanalyser procedures
(Bran and Luebbe continuous Xow AA3, Delavan, WI,
USA). Soil moisture content was determined on 1 g subsamples of each core, Wve per plot. Samples were dried at
105°C for 48 h and percentage soil moisture was calculated
as (wet mass ¡ dry mass)/dry mass £ 100.
Phytotron
Experimental setup
To elucidate the relative importance of possible moss-mediated eVects on vascular plant growth, we set up a controlled environment experiment in which two of the
vascular plant species also used in the Weld experiment
were exposed to diVerent levels of soil temperature, moisture and nutrient availability.
Ramets of Salix and Alopecurus were collected from the
Weld site in September 2003, covered in wet moss, and
transported to a cold room and stored in the dark at 2°C
until February 2004. Before starting the experiment, all
ramets were individually planted in pots with a sand/vermiculite mix (90%/10%) and placed in a phytotron at 5°C
and an irradiance of 500 mol photons m¡2 s¡1 for
4 weeks. During this time, the plants were watered twice a
week with 20 ml of 10% Long Ashton Solution. After the
4 weeks, all plants had established roots and leaves. The
plants were then removed from the pots and their roots
were carefully washed. Excess water was removed and
each plant was weighed. Thereafter the ramets were individually potted in plastic containers (diameter 5 cm, depth
6 cm) Wlled with a fresh sand/vermiculite mix (90%/10%)
and placed back into the phytotron.
Phytotron experimental treatments and protocol
The individually potted ramets of Salix and Alopecurus
were exposed to eight diVerent treatment combinations for
2 months (25th March to 25th May 2004). These arose
from a full factorial crossing of two levels of each soil
Oecologia (2011) 166:769–782
temperature, moisture and nutrient availability. We used
four replicates per treatment combination and thus the total
number of pots was 2 species £ 8 treatment combinations £ 4
replicates = 64.
The two levels of soil temperature were created by submerging the plant containers in tanks of water maintained at
temperatures of 8.1 (§0.04 SE) and 4.2 (§0.05 SE)°C.
These temperatures were similar to the soil temperature
conditions in the Weld experiment with no moss and moss
of 6 cm depth, respectively (see “Results”). A layer of
polystyrene was placed over each tank to minimise temperature Xuctuations. In these polystyrene sheets, 6 cm diameter holes were created through which plastic containers
containing experimental plants were hung. Each tank contained 32 plants. Soil temperature was monitored daily
throughout the experiment using thermometers. Plants were
placed in the tanks randomly and their positions were
moved once a week to avoid the complication of potential
edge eVects.
The two levels of nitrogen availability were created by
varying the molarity of ammonium nitrate added to Long
Ashton solution. The solution was added daily to the experimental plants. As soil was replaced with sterile sand and
vermiculite, it was assumed that no microbial mineralization was occurring and thus all plant-available nitrogen was
derived from the added source. Gross mineralization estimates for arctic soils are between 1 and 6 g N g dry
soil¡1 day¡1 (Shaw and Harte 2001). The amount of arctic
soil (not sand) held by a container was determined and the
amounts of ammonium nitrate required to achieve
2 g N g¡1 day¡1 (low treatment) and 6 g N g¡1 d¡1 (high
treatment) were calculated.
The two moisture levels were created by adding diVerent amounts of nutrient solution to the plant pots. The low
moisture treatment received 5 ml of solution per day
whilst the high moisture treatment received 10 ml per day.
The nutrient solution was Long Ashton at 15% in pots
receiving 5 ml and 7.5% in pots receiving 10 ml. Soil
moisture in the pots was measured gravimetrically on an
additional set of pots without plants at the end of the
experiment. Average soil moisture of the low watering
treatment was 22% (§3% SE) and that of the high watering treatment was 40% (§4%). These moisture contents
are lower than those found in the Weld plots, reXecting the
diVerent water-holding capacities of sand and soil; in
practice, this meant that the high watering treatment was
well saturated.
Plants were harvested after 2 months by lifting them out
of their pots; sand was carefully removed from the roots
and the Wnal fresh biomass recorded. Individuals of Salix
were separated into leaves, shoot and root biomass. Individuals of Alopecurus were separated into live leaves, rhizome
and root. For comparison with Weld data, root and rhizome
773
data were pooled to obtain a single value for belowground
biomass. All samples were dried at 70°C for 48 h and
reweighed.
Statistical analysis
Data were analysed in SAS for Windows v.9.1 using linear
mixed models. Models applied to the Weld data were Wtted
by the method of residual maximum likelihood (REML)
using the PROC MIXED procedure, and included “experimental block” as a random eVect. Denominator degrees of
freedom were estimated using Satterthwaite’s approximation (Littell et al. 1996). Data were log-transformed where
appropriate. To investigate the inXuence of a moss layer on
the total biomass and the biomasses of separate parts of
Salix, Bistorta, Alopecurus and Luzula, plant data were
averaged to give one value per plot for each species. Analysis was conducted with estimated initial dry weight which
was Wtted as a Wxed eVect before the eVect of treatment and
run for each species separately. Field measurements of soil
temperature, moisture and nitrogen availability were also
averaged so that a single value per plot for each parameter
was used in the analysis.
To statistically compare patterns of response between
species groups (graminoids vs. non-graminoids)—and
thereafter also species-speciWc responses within either
group—to our moss mat manipulations, total biomass values were standardized [by calculating Z-scores as (Xi ¡ )/
, with Xi being the individual total biomass value of species i in a given plot, whilst and are the species-speciWc
mean and standard deviation, respectively], and analysed
with linear mixed models, thereby including “block” (7 levels) and “plot” (21 levels) as random terms to take account
of the Weld experimental design.
Linear mixed modelling was also used to analyse data
from the phytotron experiment, with soil temperature,
moisture and nitrogen availability and their interactions as
categorical Wxed eVects in the model and block as a random
eVect. DiVerences between individual treatments in both
Weld and phytotron experiments were inspected with post
hoc contrasts within the appropriate model structure.
Results
EVects of the moss layer on the plant growth—Weld
experiment
Dwarf shrub: Salix polaris
Total live biomass of Salix at the end of the experiment
diVered signiWcantly among treatments (F2,11 = 6.27,
P < 0.01), with plants from shallow moss mats having
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Oecologia (2011) 166:769–782
Fig. 1 Vascular plant species
performance in response to
experimental manipulations of
moss mat: a Salix polaris,
b Bistorta vivipara,
c Alopecurus borealis and
d Luzula confusa. Bars represent
the average biomass of each
plant component §SE (n = 7).
Bars with diVerent letters within
a component type are signiWcantly diVerent (P < 0.05)
greater biomass than those from both bare soil and deep
moss (Fig. 1a). This diVerence arose primarily from greater
accumulation of shoot biomass, the plant’s main storage
organ (F2,11 = 5.78, P < 0.05). Leaf and root biomass were
also slightly, but not signiWcantly, greater in plants from the
shallow moss mat treatment.
Herb: Bistorta vivipara
As with Salix, biomass of Bistorta was on average greatest
when grown in shallow moss plots (F2,11 = 3.99, P < 0.05),
suggesting that both the absence of moss and deep moss
had a negative eVect on productivity (Fig. 1b). However,
only the diVerence between shallow and deep moss plots
was statistically signiWcant. Again, negative eVects of the
deep moss layer resulted in a lower biomass of the plant’s
main storage organ, the rhizome (F2,12 = 3.97, P < 0.05).
Unlike Salix, the leaf biomass of Bistorta was also signiWcantly less in plants grown in deep moss compared
with the other treatments (F2,12 = 5.67, P < 0.05). There
were no signiWcant diVerences in root biomass among
treatments.
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Grass: Alopecurus borealis
Alopecurus plants grown in deep moss were signiWcantly
smaller (F2,10 = 4.55, P < 0.05) than those grown without
moss or with shallow cover (Fig. 1c). In particular, the root
growth of Alopecurus was impeded in deeper moss
(F2,12 = 3.71, P < 0.05). No diVerences were found in leaf
biomass although patterns followed the same trend, with
lower values for plants grown in deep moss.
Rush: Luzula confusa
Total biomass of Luzula productivity was reduced,
although only marginally signiWcantly, when grown in deep
moss (F2,11 = 3.39, P = 0.06; Fig 1d). Both the deep and
shallow moss layer did, however, signiWcantly reduce
belowground biomass compared to plants from bare plots
(F2,12 = 5.35, P < 0.05). Aboveground biomass did not
diVer signiWcantly among treatments.
Thus, it would appear that the four species of vascular
plants respond to the moss layer in two ways. The presence of a moss layer hampered the growth of the two
Oecologia (2011) 166:769–782
775
Fig. 2 EVects of moss depth
manipulations on a soil
temperature, b soil moisture,
c soil ammonium availability,
and d soil nitrate availability.
Data are plot averages (§1 SE,
n = 7) taken in July. Bars with
diVerent letters diVer signiWcantly (P < 0.05)
graminoids, with even shallow moss causing a slight
reduction in biomass. The dwarf shrub and herb, however,
achieved its highest biomass in shallow moss, suggesting
that some degree of moss cover facilitates growth. Formal
analysis of standardized total biomass data (required to
meaningfully compare the four species) conWrmed this
duality of response, as shown by the highly signiWcant
treatment £ species group (graminoid vs. non-graminoid)
interaction term (F2,59 = 5.20, P < 0.01). Subsequent analyses revealed no further diVerences in treatment response
between species within either group (treatment £ species
terms F2,18 = 2.44, P > 0.1 and F2,18 = 0.08, P > 0.9 for
within graminoids and within non-graminoids, respectively).
Soil characteristics
Soil temperature was highly sensitive to moss cover
(F2,12 = 201.2, P < 0.001; Fig. 2a). Plots with no moss
cover had the highest soil temperature of around 8°C. The
addition of shallow moss lowered temperatures by approximately 3°C and deep moss by 4°C. Soil nitrogen availability followed a similar pattern, with less nitrate in soil under
deep moss (F2,12 = 4.81, P < 0.05; Fig. 2d), and a similar,
although non-signiWcant, trend for ammonium (Fig. 2c).
Interestingly, soil moisture content appeared to be highest
under shallow moss cover, although diVerences among
treatments were only marginally signiWcant (F2,12 = 3.59,
P = 0.06; Fig. 2b).
EVects of the soil environment on plant growth—phytotron
experiment
Grass: Alopecurus borealis
Total biomass, leaf biomass and root biomass of Alopecurus were all greatly enhanced by the 4°C increase in soil
temperature (Table 1; Fig. 3a). In fact, total biomass in pots
incubated at 8°C was nearly 3.5 times that of plants grown
at 4°C. Leaf biomass also showed a positive response to
increased nutrient input, but seemingly at the expense of
root biomass, as total plant biomass remained uninXuenced
(Table 1; Fig. 3e). Change in soil moisture had no eVect on
any of the growth parameters measured (Table 1; Fig. 3c).
There were no signiWcant interactions between any of the
treatments applied.
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Oecologia (2011) 166:769–782
Fig. 3 Total, leaf and root dry
weight biomass of Alopecurus
borealis and Salix polaris in
response to soil temperature (a,
b), moisture (c, d) and nutrient
(e, f) treatments applied in a fully factorial design to individuals.
Data are main eVect means
(§1 SE, n = 4). SigniWcance is
indicated as ***P < 0.001,
**P < 0.01, *P < 0.05, 9P < 0.1
Dwarf shrub: Salix polaris
There were no signiWcant main eVects of either temperature or nitrogen on the biomass of Salix (Table 1). However, root biomass was signiWcantly lower in the high
moisture treatment than in the low moisture treatment
(Table 1; Fig. 3d). The most striking treatment eVect on
Salix growth was the interactive eVect of soil temperature
123
and nitrogen. When Salix was grown in a low-nutrient
environment, higher soil temperature caused lower biomass than in any other treatment combination (Table 1;
Fig. 4). Only in a nutrient-rich environment did Salix beneWt from growing in warmer soil. This pattern was apparent for leaf biomass but was very strong for root biomass,
whilst no signiWcant eVect on shoot biomass was observed
(Table 1).
Oecologia (2011) 166:769–782
777
Table 1 Summary of fully factorial GLMM analyses of biomass
components of Alopecurus borealis (a) and Salix polaris (b) resulting
from the phytotron experiment in which plants were subject to two
levels of each temperature, moisture and nutrient treatment, each of
which was applied for 2 months (n = 4 per treatment combination)
Source
Total
Leaf
biomass biomass
Below
ground
biomass
a) Grass Alopecurus borealis
Temperature
11.64** 18.97*** 9.06**
Nutrient addition
0.03
8.15*
Moisture
0.49
0.06
0.48
Temperature £ nutrients
0.04
1.43
0.01
Temperature £ moisture
0.04
2.61
0.15
Nutrients £ moisture
1.69
0.38
2.00
Temperature £ nutrients £ moisture
2.03
0.17
2.02
Source
0.25
Total
Leaf
Shoot
Root
biomass biomass biomass biomass
b) Dwarf shrub Salix polaris
3.159
Temperature
0.50
0.44
0.50
Nutrient addition
2.14
1.48
0.01
1.36
Moisture
0.37
1.34
0.03
5.14*
Temperature £ nutrients
9.90**
5.36**
2.79
7.11**
Temperature £ moisture
1.47
1.92
0.28
0.88
Nutrients £ moisture
1.58
1.34
1.62
0.24
Temperature £ nutrients
£ moisture
0.01
0.35
0.19
0.17
Figures are F values (df 1,24), with signiWcance levels indicated by a
superscript (*** P < 0.001, ** P < 0.01, * P < 0.05, 9 P < 0.1); signiWcant values in bold
120
Low temp/ low N
High temp/ low N
Low temp/ high N
High temp/ high N
Biomass (mg)
100
80
60
40
20
0
Total
Leaves
Shoot
Root
Plant component
Fig. 4 Total, leaf, shoot and root biomasses of Salix polaris after
2 months of treatment in the phytotron experiment. Data are averages
(§1 SE; n = 8) and show the signiWcant temperature £ nutrient interaction
Discussion
Our study clearly shows that mosses can play an important
role in inXuencing the growth of vascular plants. Moreover,
the presence of a moss layer had both positive and negative
impacts upon vascular plant growth, the relative extent of
which depended upon moss depth. Analysis of standardised
data as well as visual inspection of Fig. 1 brings out two
distinct patterns of response, with graminoids (Alopecurus
borealis and Luzula confusa) generally being negatively
aVected by deep moss, whilst the shrub (Salix polaris) and
the herb (Bistora viviparum) tended to beneWt from a shallow moss layer. The observed eVects of moss on the soil
environment (Weld experiment) and plant responses to soil
variables (phytotron experiment) together shed some light
on potential mechanisms driving these responses (see
below for detail). Collectively, our Wndings suggest that
mat-forming mosses structure the composition of vascular
plant communities.
Moss layer depth is strongly inXuenced by ecological
factors such as time since last disturbance, levels of nitrogen deposition and vertebrate grazing pressure, as well as
environmental factors. Greater soil moisture, for example,
generally allows the development of deeper moss, but with
increasing depth the moss starts to inXuence soil condition
factors itself; it is this process and how it inXuences vascular plants within moss mats that we are interested in here.
We created treatments with diVerent moss depths in an otherwise relatively homogeneous environment. The combination of environmental conditions that favour a deeper or
thinner moss layer with the eVects of the moss layer itself
on the soil environment may result in even more pronounced eVects on vascular plant growth than are demonstrated in this study.
Negative impacts of mosses
The negative eVects of the deep moss layer were most notable for plant parts that are mainly located in the organic soil
horizon: belowground biomass of graminoids and storage
organs of the herb (but not of the shrub). Moss-induced
changes to the soil environment may therefore be driving
the negative impact of mosses on vascular plants. Indeed, a
negative relationship between moss depth and graminoid
biomass has been established in several tundra studies
(Jónsdóttir 1991; Olofsson et al. 2001; Olofsson et al. 2004;
Van der Wal et al. 2004) and, similarly to our study, Hobbie et al. (1999) found that Sphagnum negatively aVected
the performance of the shrub Betula nana and concluded
that the moss’ ability to keep the soil cold and damp probably hindered nutrient cycling in the soil.
In our Weld plots, the presence of a moss layer signiWcantly reduced soil temperature, and deeper moss reduced
123
778
nitrogen availability. Gornall et al. (2007) also showed that
an increase in moss depth reduced plant-available nitrogen
and microbial biomass and respiration, and that deeper
moss substantially delayed soil thaw, thereby shortening
the growing season for vascular plants. Soil temperature
measurements in that study, running at exactly the same
time and location as the one currently reported, were based
on temperature loggers with external probes providing
readings every 6 h for a period of 14 months; this demonstrated clearly that the soil underneath moss mats of 6 cm
depth was signiWcantly cooler and displayed reduced diurnal variability and signiWcantly fewer days above 0°C than
soil underneath moss of 3 cm depth. There were diYculties
with employing temperature sensors in plots without a
moss mat (both in terms of direct radiation to the sensor and
damage from both polar foxes and reindeer to the cabling).
Therefore, we did not instrument plots without moss cover
and thus relied on spot measures taken in July. Lacking the
insulating cover of the moss, however, we would expect
bare soil to heat and cool more quickly, diurnally and seasonally, than shallow moss, and to be subject to greater
temperature extremes while annual average temperatures
might be similar. This is supported by soil temperature
measurements (taken at 10 cm depth) from two contrasting
habitats at a nearby site (Endalen). Here, mean July soil
temperatures were higher and diurnal Xuctuations greater in
sparsely vegetated plots (i.e. with areas of bare soil) than in
soils underneath a continuous moss mat of 2–5 cm depth
with vascular plants growing interspersed, while annual
mean soil temperatures in the two habitats were similar
(Jónsdóttir, unpublished data). Likewise, Coulson et al.
(1993) demonstrated the suppressive eVect of vegetation
cover on soil mean growing season temperatures and diurnal Xuctuations at 3 cm depth when comparing a sparsely
vegetated fellWeld site and a site with a continuous moss
mat of 5–10 cm depth near Ny-Ålesund, Svalbard.
The moss layer also limits the nutrient uptake of vascular
plants by eVectively intercepting aerially deposited nutrients (Pearce et al. 2003; Curtis et al. 2005), including nitrogen derived from vascular plant litter, such that vascular
plants growing in deeper moss rely on soil-derived nitrogen
sources (Malmer et al. 2003). Nutrient availability to vascular plants is further compromised by the slow release of
nutrients from moss litter through decomposition (Cornelissen et al. 2007). Thus, it is clear that deep moss reduces soil
temperature and nutrient availability, which are often limiting to vascular plant growth in arctic systems (Jonasson
et al. 1996; Brooker and Van der Wal 2003; Chapin et al.
2002), creating an unfavourable soil environment that is
limiting to belowground productivity.
It is also possible that vascular plants compete directly
with mosses for light. Investigations of competition
between moss species often identify light as a limiting
123
Oecologia (2011) 166:769–782
factor to shoot growth at the top of the moss layer (Okland
2000), which is where leaves of Salix and Bistorta grow.
Mosses have lower light compensation and saturation
levels than do vascular plants (Longton 1992), and so competition for light is likely to aVect vascular plants before
self-shading within the moss layer suppresses moss growth
(Okland and Okland 1996). Salix grows prostrate amongst
the moss with little protrusion of the leaves above the moss
layer. Likewise, new leaves of Bistorta grow up through the
moss layer, so the deeper the moss, the more the growing
shoots will be shaded during development, becoming etiolated and likely of lower biomass (Chapin and Shaver
1996). For both species, shoots in deeper moss also have to
grow longer to ensure that leaves obtain light eVectively (as
rooting is mostly at or near the moss–soil interface).
Negative eVects of mosses on the shrub Salix and the herb
Bistorta are therefore possibly imparted through shading.
Light competition with moss is less important for grasses,
the leaves of which protrude well above the moss layer.
This is another mechanism by which the moss layer may
diVerentially inXuence vascular plant growth forms.
Positive impacts of mosses
A shallow layer of moss positively aVected the shrub and
herb, and graminoid biomass was no less in shallow moss
than in the absence of moss. This suggests that there may
be some facilitation oVsetting the negative impacts moss
has via its inXuence on the soil environment and through
shading. Positive interactions between vascular plants are
numerous and important in harsh environments (Carlsson
and Callaghan 1991; Brooker et al. 2008). However, relatively few studies (see below) have identiWed the positive
eVect of mosses on vascular plants.
Mosses may ameliorate environmental conditions for
vascular plants by increasing water availability. Mosses
have evolved structural properties that limit water loss
(Proctor 1979), which enables the moss layer to retain
moisture from snowmelt or precipitation, possibly maintaining the water status of vascular plants during periods of
drought. Whilst this may be especially important at the
beginning of the growing season when desiccating winds
are strongest (Carlsson and Callaghan 1991), large parts of
the Arctic have low precipitation and hence a moss mat
may also increase water availability to shallow-rooted
plants and seedlings throughout later periods of drought, as
has been observed in polar semi-desert vegetation (Sohlberg and Bliss 1987). In addition, the relatively smooth surface of a moss mat maximises boundary layer resistance
(Rice et al. 2001), reducing turbulence and thus evapotranspiration from the leaves of vascular plants growing
amongst the moss. The gravimetric soil moisture measurements determined upon completion of the Weld experiment
Oecologia (2011) 166:769–782
revealed only a marginally signiWcant diVerence (P = 0.06)
among treatments, which we interpret as a lack of statistical
power rather than an absence of treatment diVerences. In
line with our Wndings, permanent instrumentation of neighbouring plots revealed that soil underneath 3-cm deep moss
had a consistently greater volumetric water content (of
around 0.45 m3 m¡3 during mid-summer) than soil underneath 6-cm deep moss (0.32 m3 m¡3; Gornall et al. 2007).
Unfortunately, it was not practicable to instrument plots
without moss, and hence we have to rely for the current
study on the pattern of soil moisture presented in Fig. 2. In
the Weld experiment, patterns of the relatively shallow-rooting (see Brooker and Van der Wal 2003) dwarf shrub and
herb biomass reXected those of soil moisture, suggesting
that moss inXuences the water relations—and thus productivity—of vascular plants, not just in polar semi-desert vegetation, but also in tundra heath.
Mosses may also ameliorate autumn and winter severity by providing an insulating layer which can buVer soil
against low temperatures and protect vascular plants
from frost damage (Gornall et al. 2007; Startsev et al.
2007). Maintaining warmer soils in winter may aVect
over-winter nutrient dynamics, which, in turn, can have
an impact upon growing season productivity (Chapin
et al. 2002). Given the strong insulating properties of a
deep snow pack, the inXuence of the moss layer on soil
nutrient cycling and hence vascular plants is likely to be
greatest under conditions of shallow or no snow, and is
hence of increasing importance throughout northern
climes as the period of snow lie has rapidly shortened
(ACIA 2005).
Potential mechanisms
Field data show that soil temperature and nutrient availability are both negatively related to moss depth and, in contrast, shallow moss depth marginally increases soil
moisture. The phytotron experiment was designed to further investigate the negative impacts of mosses in terms of
their eVect on soil temperature and nutrient availability and
their positive impacts in terms of moisture retention. Both
soil temperature and nutrient availability were important
determinants of the growth of Salix polaris and Alopecurus
borealis. However, in this study, soil moisture appeared to
have few signiWcant eVects on plant growth.
In an incubation experiment, Brooker and Van der Wal
(2003) found that increasing soil temperature enhanced the
growth of arctic vascular plants, particularly grasses,
thereby suggesting that increased shoot biomass was a
response to temperature-driven changes in soil nutrient status. Indeed, nitrogen is commonly suggested to be the more
important limiting factor in arctic ecosystems (Chapin et al.
2002; Jonasson et al. 1999; Richardson et al. 2002). In our
779
study, however, soil temperature was far more important
than nutrient availability in determining the biomass of
Alopecurus. Dormann et al. (2004) also showed that Luzula
confusa was more responsive to increased temperature than
nutrients, suggesting that these high-arctic graminoids may
be more temperature than nutrient limited. Indeed, the
between-year variation in plant biomass in central Spitsbergen was found to be strongly and positively related to the
between-year variation in summer temperature (Van der
Wal and Hessen 2009), in contrast to Wndings from the
lower Arctic (Chapin and Shaver 1985). Likewise, the
observation that the growth of Cassiope tetragona
responded more to temperature at high-arctic latitudes and
more to nutrients in the subarctic (Havstrom et al. 1993)
suggests that high-arctic plants are very temperature sensitive.
Low soil temperatures may reduce growth through
eVects on root production. Starr et al. (2004) found that levels of the growth inhibitor ABA (abscisic acid) were
increased in root tissue of arctic sedges grown at low soil
temperatures. With impeded root growth, nutrient and
moisture absorption is hampered, which can cause an overall reduction in growth. Thus, it is possible that the eVect of
the moss layer on soil temperature may have directly
caused the reduced growth of graminoids observed in the
Weld experiment.
In the phytotron experiment, Salix biomass only
increased with soil temperature when receiving the high
nitrogen treatment, demonstrating that soil temperature
increases alone are insuYcient to enhance the productivity
of this dwarf shrub. In fact, the combination of high temperature and low nutrient supply reduced biomass, possibly
because root respiration and turnover increase with temperature (Forbes et al. 1997). Thus, should a moss layer be disturbed, for example by the activity of herbivores, Salix will
only respond to the resultant increase in soil temperature as
far as any associated increase in nutrient availability will
allow (Gornall et al. 2009).
The short timescale over which the phytotron study was
carried out limits its usefulness in interpreting Weld results.
Arctic plants have been shown to be highly seasonal in their
response to environmental variables. For example, Chapin
and Shaver (1996) found that only early-season growth of
Eriophorum vaginatum and Ledum palustre was constrained by temperature; late-season growth, on the other
hand, was largely determined by nutrient availability. Also,
many arctic species rely on stored reserves of nitrogen and
carbon (e.g., Shaver and Chapin 1991), particularly for
early growth when snow has melted but the soil is still frozen. Therefore, although plants were incubated for 2 weeks
before the start of the experiment, the results may have
been confounded by carry-over eVects from the previous
year. Thus it is important to acknowledge that, whilst our
123
phytotron experiment provides interesting results, caution
must be taken with the translation of the Wndings to plants
in the Weld.
The experiment, however, does provide evidence suggesting that soil temperature and nutrient availability are
both important determinants of the growth of arctic vascular plants, supporting the argument that moss-mediated
eVects on the soil environment may be responsible for the
vascular plant growth responses observed in the Weld.
Moisture appeared to be less important for growth in this
study, but the positive eVects of mosses in retaining moisture may be particularly important in Weld conditions where
strong winds exacerbate moisture limitation. Vascular plant
growth responses to other important properties of the moss
layer, such as shelter from the wind, buVering winter severity and shading in the moss canopy need to be investigated
to give a more complete picture of the mechanisms
involved.
Experimental approach
The inXuence of mosses on the performance of vascular
plants is clearly characterised by both positive and negative components. Predictions regarding the relative
importance of the positive and negative interaction components are currently hindered by a shortage of experimental manipulation studies and, in those studies which
have been undertaken, by the lack of description of the
moss mat and the actual manipulation employed. For
instance, “moss removal” has been implemented as the
removal of just green moss (Hobbie et al. 1999), green
moss and any attached brown tissue (Bret-Harte et al.
2004), or the whole moss mat including dead moss underneath (this study), whilst in most other studies the exact
nature of the manipulations remain unclear. Perhaps as a
result, some of the “incomplete” moss removal studies
Wnd little eVect on vascular plant performance (Fetcher
1985) or report on patterns that are diYcult to interpret
due to relatively strong side eVects such as increased
nutrient availability deriving from remaining dead moss
basal stems (Bret-Harte et al. 2004). Whilst recognising
that every Weld manipulation will inevitably generate
undesirable side eVects, we suggest that for a comprehensive evaluation of moss impacts on vascular plants, total
removal of the moss mat is required (where possible, i.e.
it is not possible in deep peat where moss is the substrate
vascular plants grow in) and, ideally, moss replacement
treatments should also be included. Critical to any experiment is also a description of the moss mat in terms of its
depth, biomass and species composition. The omission of
the speciWc depth of the moss mat is, in our opinion, the
single most constraining factor preventing generalisation,
as outlined below.
123
Oecologia (2011) 166:769–782
Extent of moss layer impact
on a vascular plant species
780
Positive effects
+
0
↓
Net effect
Spe
cie
sX
-
Ne
ga
tive
eff
ec
ts
Increasing depth of the moss layer
Fig. 5 Conceptual model adapted from Brooker and Callaghan
(1998) depicting the positive and negative impacts of the moss layer
on the performance of vascular plants. The net eVect of the moss layer
on species X is drawn in as an example. For this species, a shallow
moss layer is beneWcial (e.g. not too cold in winter, but still warming
up readily in summer) whilst at intermediate moss depth, positive and
negative eVects cancel each other out. At greater moss depths (beyond
the small arrow), negative eVects outweigh positive eVects on the performance of this species
Synthesis
The relative importance of positive and negative interactions between vascular plants is predicted to change along
gradients of environmental severity (Brooker and Callaghan 1998). Whilst there are too few moss manipulations to
determine the applicability of this model to moss–vascular
plant interactions, we propose that the balance between
positive and negative interactions is in the Wrst instance
inXuenced by the depth of the moss layer (Fig. 5). In a shallow moss layer, positive eVects such as provision of shelter
and moisture retention outweigh the negative eVects of the
moss. As moss depth increases the negative eVects of
mosses on the soil environment, and for some plant species
through shading, become more important. Once a certain
depth of moss is reached, the positive eVects of the moss do
not increase further, and thus as the moss layer gets even
deeper it has an increasingly negative inXuence on vascular
plant productivity. Hence, an increase in moss depth may
be viewed as an increase in environmental severity as experienced by a vascular plant, with gradual decreases in soil
temperature, soil nutrient availability and length of the
growing season.
We suggest that the relative extent of the positive and
negative impacts of mosses will diVer between vascular
plant species, depending on a wide range of factors, including
Oecologia (2011) 166:769–782
plant size, rooting depth, allocation pattern and intrinsic
growth rate, hence structuring the vascular plant community. Our study thus highlights that, for plant interaction
theory to be widely applicable to extreme environments
such as the Arctic, growth forms other than vascular plants
need to be considered.
Acknowledgments We are grateful to Hera Sengers and AnneMette Pedersen for invaluable help with Weld and laboratory work, to
UNIS for the logistic support provided, and to referees for insightful
comments and textual changes. This work was funded by NERC
(NER/S/A/2001/05958) and permission for the Weld experiments was
kindly granted by the Governor of Svalbard.
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