Oecologia

DOI 10.1007/s00442-011-1911-6

COMMUNITY ECOLOGY - ORIGINAL PAPER

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© Springer-Verlag 2011

Abstract Our understanding of positive and negativeplant interactions is primarily based on vascular plants, asis the prediction that facilitative eVects dominate in harshenvironments. It remains unclear whether this understand-ing is also applicable to moss–vascular plant interactions,which are likely to be inXuential in low-temperature envi-ronments with extensive moss ground cover such as borealforest and arctic tundra. In a Weld experiment in high-arctictundra, we investigated positive and negative impacts of themoss layer on vascular plants. Ramets of the shrub Salixpolaris, herb Bistorta vivipara, grass Alopecurus borealisand rush Luzula confusa were transplanted into plotsmanipulated to contain bare soil, shallow moss (3 cm) anddeep moss (6 cm) and harvested after three growing sea-sons. The moss layer had both positive and negativeimpacts upon vascular plant growth, the relative extent ofwhich varied among vascular plant species. Deep moss

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 rela-tive importance of the mechanisms by which moss mayinXuence vascular plants, through eVects on soil tempera-ture, moisture and nitrogen availability, was investigatedin a phytotron growth experiment. Soil temperature, andnot nutrient availability, determined Alopecurus growth,whereas Salix only responded to increased temperature ifsoil nitrogen was also increased. We propose a conceptualmodel showing the relative importance of positive and neg-ative inXuences of the moss mat on vascular plants along agradient of moss depth and illustrate species-speciWc out-comes. Our Wndings suggest that, through their strong inXu-ence on the soil environment, mat-forming mosses structurethe composition of vascular plant communities. Thus, forplant interaction theory to be widely applicable to extremeenvironments such as the Arctic, growth forms other thanvascular plants should be considered.

Keywords Competition · Facilitation · High-arctic · Nutrient availability · Soil temperature

Introduction

The study of plant–plant interactions provides fertileground for both conceptual debate and concrete tools forinterpreting patterns of plant distribution and abundance.Interactions between plants run along a continuum fromcompetition to facilitation, also termed negative and posi-tive plant interactions, the sum of which determines the netoutcome (Bertness and Callaway 1994; Brooker andCallaghan 1998). Competition is now widely acknowledged

Communicated by Bryan Foster.

J. L. Gornall · S. J. WoodinSchool of Biological Sciences, University of Aberdeen, Aberdeen AB24 3UU, UK

J. L. Gornall · I. S. JónsdóttirUNIS, PB 156, 9171 Longyearbyen, Norway

I. S. JónsdóttirInstitute 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, UKe-mail: r.vanderwal@abdn.ac.uk

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as a powerful force structuring plant communities, andperceived as most important in relatively productive eco-systems (Weldon and Slauson 1986). More recently, facili-tation has been identiWed as a potentially equally importantprocess notably structuring ecosystems of low productivity(Callaway et al. 2002; Brooker et al. 2008). The balancebetween competition and facilitation has been investigatedin neighbour removal experiments along environmentalgradients, which have repeatedly revealed that facilitativecomponents predominate in severe environments such asarctic and alpine ecosystems, salt marsh and desert(Brooker et al. 2008). Our understanding of plant–plantinteractions is, however, almost exclusively based on vas-cular plants. Whilst this may be appropriate for large partsof the world’s ecosystems, in a range of extreme environ-ments for which facilitation has been identiWed as a strongstructuring force, vascular plants grow in otherwise moss-or lichen-dominated vegetation. The Arctic is a key exam-ple of this (Matveyeva and Chernov 2000), and here, aswell as in other northern ecosystems, mosses inXuence bothmicroclimate and soil processes to such an extent that theymay be considered engineers of these plant communities(Malmer et al. 2003; Gornall et al. 2007). Here we reporton experimental Wndings from a high-arctic Weld study inwhich we investigate positive and negative impacts of themoss layer on the performance of vascular plants.

Mosses are the dominant ground cover in a range of eco-systems, and an increasing number of studies have exploredvascular plant impacts on mosses. Vascular plants may ben-eWt mosses by generating a favourable microclimate, as hasbeen suggested for both grassland and woodland (Ingerpuuet al. 2005; Startsev et al. 2007). More frequent, however,are accounts of negative relationships between vascularplant and moss biomass, reXecting suppressed moss growththrough leaf litter deposition and shading, despite mosseshaving a generally low light compensation point (Berga-mini et al. 2001; Malmer et al. 2003, Van der Wal et al.2005; Startsev et al. 2008). The presence of palatable vas-cular plants, notably graminoids, may also draw in verte-brate 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 compe-tition for light and apparent competition, respectively, thenature of such interactions is highly asymmetric, as the sup-pression of mosses may not inXuence resource availabilityto the vascular plants. Hence, the validity of applying theseconcepts, derived from vascular plant competition studies,to interactions between vascular plants and mosses may bequestionable.

The few neighbour removal studies that have exploredthe potential for mosses to inXuence vascular plants pointtowards moss ground cover being an inXuential rather thana suppressed ecosystem component. Removal of Sphagnum

spp. increased the aboveground biomass of Betula nana(Hobbie et al. 1999) and Narthecium ossifragum (Malmeret al. 2003). Likewise, Betula pubescence, Picea abies andparticularly Pinus sylvestris beneWtted from the removal ofthe moss layer (Wardle et al. 2008). Findings from suchexperimental manipulations are in line with the notion thatthe moss layer can prevent early seedling establishment bydenying access to the soil required for rooting or smother-ing seedlings (Hörnberg et al. 1997; Sedia and Ehrenfeld2003; Freestone 2006). However, these studies also indi-cate the positive eVect the moss layer can have on vascularplants, for instance by fostering seed retention and germina-tion, whilst other studies point at enhancing nutrient avail-ability and soil moisture during periods of drought (e.g.Sohlberg and Bliss 1987).

Given that mosses can have both negative and positiveinXuences on vascular plants, it remains to be determinedwhether their net outcome changes along stress gradientsin ways predicted by competition/facilitation theory.Counter to prediction, a moss-removal experiment innorthern Sweden established that the negative eVect ofmoss on vascular plants was greatest on the least produc-tive of 30 lake islands studied, and no evidence for a net-positive eVect of mosses on the trees was found (Wardleet al. 2008). Another such apparent contradiction of theorywas observed in a detailed study on the rare Californianforb Delphinium uliginosum growing almost exclusivelyin moss mats on inhospitable serpentine soil, with thefacilitative eVects of the moss being constant rather thanstrongest at the harsher end of the environmental gradientstudied (Freestone 2006). It thus remains to be seenwhether the balance between positive and negative plantinteractions in extreme environments leans towards afacilitative eVect of moss on vascular plants as predictedby competition theory.

We investigate this question in the Arctic, where mossesmake up much of the plant biomass. Because of their preva-lence in these typically severe environments, previous stud-ies have aimed to determine experimentally the inXuence ofthe moss layer on vascular plants. We are aware of fourstudies, among which three—all conducted in Alaskan tun-dra—involve partial removal of the moss layer (Fetcher1985; Hobbie et al. 1999; Bret-Harte et al. 2004). TheirWndings point at either a suppressive or a lack of inXuenceon vascular plants, but are diYcult to interpret as the poten-tially inXuential decaying and/or dead layer of mossesremained in place. The fourth study, conducted in high-arctic Canada, does involve complete removal of the mosslayer. Whilst the environmental conditions under whichSohlberg and Bliss (1987) conducted their Weld trials weresevere, both net positive and negative impacts of the mosslayer on vascular plants were demonstrated for two diVer-ent plant species. This clearly raises the question of

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whether relationships between vascular plants and mossesare species speciWc, even in highly severe environments.

Despite the paucity of moss removal studies, a plethoraof potential mechanisms that could drive moss–vascularplant interactions have been identiWed. Notably, deepermoss is found to reduce soil temperature (Oechel and VanCleve 1986; Van der Wal and Brooker 2004), which in turnlimited nitrogen mineralization (Gornall et al. 2007). Fur-thermore, evidence suggests that aerially deposited nitrogen(including that derived from litter) is sequestered by themoss 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-Wxingcyanobacteria that play a critical role in the N cycle of arc-tic environments (Solheim et al. 1996). Mosses may pre-vent frost damage to roots (Sohlberg and Bliss 1987;Olofsson et al. 2001; Startsev et al. 2007) and also provideshelter, a factor often cited as the mechanism behind posi-tive plant interactions (Callaghan and Emanuelsson 1985;Carlsson and Callaghan 1991). Their growth form both aidsmoisture retention and enhances boundary layer resistance(Sveinbjornsson and Oechel 1992), reducing the desiccat-ing impacts of strong arctic winds. Thus, the potentialexists for both positive and negative eVects of the mosslayer on vascular plants in arctic ecosystems.

In an arctic Weld manipulation experiment, we aimed to:(1) determine the extent of positive and negative impacts ofmosses on higher plants, (2) investigate whether theimpacts of the moss layer are consistent among vascularplant growth forms, and (3) identify potential mechanismsby which the moss layer aVects the performance of vascularplants. The Weld experiment was set up in high-arctic tundrain which moss was either removed or replaced by either ashallow moss layer (3 cm) or a deep moss layer (6 cm).Vascular plants were transplanted into the treatment plotsand left for three growing seasons. Species used were thedwarf shrub Salix polaris, the herb Bistorta vivipara, thegrass Alopecurus borealis and the wood rush Luzula con-fusa. The importance of the mechanisms by which mossmay inXuence vascular plants, speciWcally through eVectson soil temperature, moisture and nitrogen availability, wasinvestigated in a controlled environment growth experi-ment.

Materials and methods

Field experiment

Study site

To investigate the relative extent of positive and negativeimpacts of mosses on higher plants, a Weld experiment was

initiated in Adventdalen, high-arctic Spitsbergen (78°10�N,16°07�E). Being in the central-west of Svalbard, climaticconditions in this valley system are relatively mild. Dur-ing the course of our Weld experimental study (2002–2004), the yearly average temperature was ¡5°C, with amonthly maximum of 6°C in July and minimum of ¡15°Cin January and February (data from the nearby Longyear-byen weather station). Precipitation is low (yearly average185 mm) and it largely falls as snow. At our study site,located on the outermost part of an alluvial fan, wintersnow depth is low and variable (10–15 cm) compared tosurrounding areas due to strong winds that pass down thevalley; it is also one of the Wrst snow-free areas within theregion. During the experimental period, soil thaw startedafter the site became snow-free in May, and reached amaximum 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 atsnowmelt, but because of low summer precipitation (4–27 mm per month) gradually dried out during the growingseason (early June to late July). Vegetation at this mesicsite is dwarf shrub-grass heath, in which dominant vascu-lar plant species include Salix polaris and Alopecurusborealis, 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 coverdominated by Tomentypnum nitens and Sanionia unci-nata. Both moss species are common, widespread andlocally abundant (i.e. >50% cover) in Svalbard (Frisvolland Elvebakk 1996) and frequently grow together in mesichabitats. Whereas S. uncinata is ubiquitous, T. nitensoccurs under a slightly narrower set of moisture condi-tions ranging from mesic tundra to the drier parts of Sval-bard’s mire communities (Elvebakk 1994; Vanderpuyeet al. 2002). Total live moss biomass at our site rangedfrom 169 to 304 g m¡2, and the depth of the moss layerranged from 2 to 6.5 cm.

Field experimental setup and protocol

Our Weld experimental setup was a fully randomizedblocked design laid out within an area of 1,500 m2. Blocks(n = 7) were selected for homogeneity in both microto-pography and plant species composition, and were at least3 m apart. A single block contained three plots (each75 £ 75 cm) which were placed at least 30 cm apart, andeach plot was randomly assigned to one of the three treat-ments: no, shallow (3 cm) or deep (6 cm) moss layer. Thesetreatments, created to determine the inXuence of the mosslayer on vascular plant growth, were put in place in July2002 when the turf was stripped from all experimental plotsusing a soil knife, thereby removing the whole vegetation

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layer. The “no moss layer” plots were left bare, whilst theshallow and deep moss layer plots were covered with nine25 £ 25 cm moss mats that were placed out at natural shootdensity. For these treatments, moss was collected fromsource material that just exceeded the target depth and thentrimmed to 3 or 6 cm by removing lower sections of theturfs. The selected turfs were dominated by the moss Tom-entypnum nitens. Vascular plants were weeded out of themoss mats once in place.

Although acknowledging the importance of early lifehistory processes, these were outside our research focus.Instead, we conducted experiments with ramets of Salixpolaris, Bistorta vivipara, Alopecurus borealis and Luz-ula confusa. These were collected from an area adjacentto the Weld experimental site in early July 2002. Plantswere carefully removed from the soil to limit root dam-age and kept damp until replanted. Species-speciWc stan-dards were set to minimise variation due to diVeringgrowth stages. All shoots were non-Xowering and of thefollowing lengths: Salix 9 cm, Bistorta 6 cm, Luzula8 cm and Alopecurus 8 cm. In the cases of Luzula andAlopecurus, young shoots (only two dead leaves) wereused to reduce the chance of Xowering. Any rhizomeswere removed.

All target vascular plant species were planted in thecentral 25 £ 25 cm of each plot, at a distance of 10 cmfrom each other to minimise interference. In the case ofplant death, target plants were also planted in the outer25 cm of each plot. Single ramets were planted for Salixand Bistorta, whereas two ramets were planted forAlopecurus and Luzula in order to maintain replicationin the event of Xowering. In total, each plot containedtwelve individual plants. Before planting, each rametwas weighed and tagged with a label loosely attachedwith string around the main shoot or rhizome. A slit wasmade through the moss mat, where present, and into thesoil, after which the plant’s roots were placed in the soiland watered. Planting was carried out in the secondweek of July 2002.

In August 2004 (after 26 months), all transplants wereharvested from the experimental plots by taking a core ofsoil around the individual to ensure the collection of allroots. Plants were transported back to the lab, where excesssoil was carefully removed. The total fresh weight of eachplant was recorded. Individuals of Salix were split intoleaves, shoot and root, Bistorta was split into live leaves,rhizome and roots, and Alopecurus and Luzula were splitinto live leaves and belowground parts, which includedboth roots and rhizome. Once separated, the plant partswere dried in an oven at 70°C for 48 h and weighed. To cal-culate the initial plant dry weight, the Wnal fresh-weight-to-dry-weight ratio was multiplied by the initial fresh weightsof each plant.

Soil characteristics

Measurements of soil characteristics in the experimentalplots were taken in the last week of July 2005. Five mea-surements of soil temperature were taken randomly in eachplot. A temperature probe (model 612–849, RS, Corby,UK, 0.1°C accuracy) was inserted into the soil to a depth of2 cm and left to equilibrate. To determine soil nitrogen con-tent, Wve soil cores were taken from each plot and stored at2°C. To extract NH4

+ and NO3, 10 g of fresh soil wereshaken 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 subs-amples of each core, Wve per plot. Samples were dried at105°C for 48 h and percentage soil moisture was calculatedas (wet mass ¡ dry mass)/dry mass £ 100.

Phytotron

Experimental setup

To elucidate the relative importance of possible moss-med-iated eVects on vascular plant growth, we set up a con-trolled environment experiment in which two of thevascular plant species also used in the Weld experimentwere exposed to diVerent levels of soil temperature, mois-ture and nutrient availability.

Ramets of Salix and Alopecurus were collected from theWeld site in September 2003, covered in wet moss, andtransported to a cold room and stored in the dark at 2°Cuntil February 2004. Before starting the experiment, allramets were individually planted in pots with a sand/ver-miculite mix (90%/10%) and placed in a phytotron at 5°Cand an irradiance of 500 �mol photons m¡2 s¡1 for4 weeks. During this time, the plants were watered twice aweek with 20 ml of 10% Long Ashton Solution. After the4 weeks, all plants had established roots and leaves. Theplants were then removed from the pots and their rootswere carefully washed. Excess water was removed andeach plant was weighed. Thereafter the ramets were indi-vidually potted in plastic containers (diameter 5 cm, depth6 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 Alopecuruswere exposed to eight diVerent treatment combinations for2 months (25th March to 25th May 2004). These arosefrom a full factorial crossing of two levels of each soil

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temperature, moisture and nutrient availability. We usedfour replicates per treatment combination and thus the totalnumber of pots was 2 species £ 8 treatment combinations £ 4replicates = 64.

The two levels of soil temperature were created by sub-merging the plant containers in tanks of water maintained attemperatures of 8.1 (§0.04 SE) and 4.2 (§0.05 SE)°C.These temperatures were similar to the soil temperatureconditions in the Weld experiment with no moss and mossof 6 cm depth, respectively (see “Results”). A layer ofpolystyrene was placed over each tank to minimise temper-ature Xuctuations. In these polystyrene sheets, 6 cm diame-ter holes were created through which plastic containerscontaining experimental plants were hung. Each tank con-tained 32 plants. Soil temperature was monitored dailythroughout the experiment using thermometers. Plants wereplaced in the tanks randomly and their positions weremoved once a week to avoid the complication of potentialedge eVects.

The two levels of nitrogen availability were created byvarying the molarity of ammonium nitrate added to LongAshton solution. The solution was added daily to the exper-imental plants. As soil was replaced with sterile sand andvermiculite, it was assumed that no microbial mineraliza-tion was occurring and thus all plant-available nitrogen wasderived from the added source. Gross mineralization esti-mates for arctic soils are between 1 and 6 �g N g drysoil¡1 day¡1 (Shaw and Harte 2001). The amount of arcticsoil (not sand) held by a container was determined and theamounts of ammonium nitrate required to achieve2 �g N g¡1 day¡1 (low treatment) and 6 �g N g¡1 d¡1 (hightreatment) were calculated.

The two moisture levels were created by adding diVer-ent amounts of nutrient solution to the plant pots. The lowmoisture treatment received 5 ml of solution per daywhilst the high moisture treatment received 10 ml per day.The nutrient solution was Long Ashton at 15% in potsreceiving 5 ml and 7.5% in pots receiving 10 ml. Soilmoisture in the pots was measured gravimetrically on anadditional set of pots without plants at the end of theexperiment. Average soil moisture of the low wateringtreatment was 22% (§3% SE) and that of the high water-ing treatment was 40% (§4%). These moisture contentsare lower than those found in the Weld plots, reXecting thediVerent water-holding capacities of sand and soil; inpractice, this meant that the high watering treatment waswell saturated.

Plants were harvested after 2 months by lifting them outof their pots; sand was carefully removed from the rootsand the Wnal fresh biomass recorded. Individuals of Salixwere separated into leaves, shoot and root biomass. Individ-uals of Alopecurus were separated into live leaves, rhizomeand root. For comparison with Weld data, root and rhizome

data were pooled to obtain a single value for belowgroundbiomass. All samples were dried at 70°C for 48 h andreweighed.

Statistical analysis

Data were analysed in SAS for Windows v.9.1 using linearmixed models. Models applied to the Weld data were Wttedby the method of residual maximum likelihood (REML)using the PROC MIXED procedure, and included “experi-mental block” as a random eVect. Denominator degrees offreedom were estimated using Satterthwaite’s approxima-tion (Littell et al. 1996). Data were log-transformed whereappropriate. To investigate the inXuence of a moss layer onthe total biomass and the biomasses of separate parts ofSalix, Bistorta, Alopecurus and Luzula, plant data wereaveraged to give one value per plot for each species. Analy-sis was conducted with estimated initial dry weight whichwas Wtted as a Wxed eVect before the eVect of treatment andrun for each species separately. Field measurements of soiltemperature, moisture and nitrogen availability were alsoaveraged so that a single value per plot for each parameterwas used in the analysis.

To statistically compare patterns of response betweenspecies groups (graminoids vs. non-graminoids)—andthereafter also species-speciWc responses within eithergroup—to our moss mat manipulations, total biomass val-ues were standardized [by calculating Z-scores as (Xi ¡ �)/�, with Xi being the individual total biomass value of spe-cies i in a given plot, whilst � and � are the species-speciWcmean and standard deviation, respectively], and analysedwith linear mixed models, thereby including “block” (7 lev-els) and “plot” (21 levels) as random terms to take accountof the Weld experimental design.

Linear mixed modelling was also used to analyse datafrom the phytotron experiment, with soil temperature,moisture and nitrogen availability and their interactions ascategorical Wxed eVects in the model and block as a randomeVect. DiVerences between individual treatments in bothWeld and phytotron experiments were inspected with posthoc 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 experimentdiVered signiWcantly among treatments (F2,11 = 6.27,P < 0.01), with plants from shallow moss mats having

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greater biomass than those from both bare soil and deepmoss (Fig. 1a). This diVerence arose primarily from greateraccumulation of shoot biomass, the plant’s main storageorgan (F2,11 = 5.78, P < 0.05). Leaf and root biomass werealso slightly, but not signiWcantly, greater in plants from theshallow moss mat treatment.

Herb: Bistorta vivipara

As with Salix, biomass of Bistorta was on average greatestwhen grown in shallow moss plots (F2,11 = 3.99, P < 0.05),suggesting that both the absence of moss and deep mosshad a negative eVect on productivity (Fig. 1b). However,only the diVerence between shallow and deep moss plotswas statistically signiWcant. Again, negative eVects of thedeep moss layer resulted in a lower biomass of the plant’smain storage organ, the rhizome (F2,12 = 3.97, P < 0.05).Unlike Salix, the leaf biomass of Bistorta was also sig-niWcantly less in plants grown in deep moss comparedwith the other treatments (F2,12 = 5.67, P < 0.05). Therewere no signiWcant diVerences in root biomass amongtreatments.

Grass: Alopecurus borealis

Alopecurus plants grown in deep moss were signiWcantlysmaller (F2,10 = 4.55, P < 0.05) than those grown withoutmoss or with shallow cover (Fig. 1c). In particular, the rootgrowth of Alopecurus was impeded in deeper moss(F2,12 = 3.71, P < 0.05). No diVerences were found in leafbiomass although patterns followed the same trend, withlower values for plants grown in deep moss.

Rush: Luzula confusa

Total biomass of Luzula productivity was reduced,although only marginally signiWcantly, when grown in deepmoss (F2,11 = 3.39, P = 0.06; Fig 1d). Both the deep andshallow moss layer did, however, signiWcantly reducebelowground biomass compared to plants from bare plots(F2,12 = 5.35, P < 0.05). Aboveground biomass did notdiVer signiWcantly among treatments.

Thus, it would appear that the four species of vascularplants respond to the moss layer in two ways. The pres-ence of a moss layer hampered the growth of the two

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 signiW-cantly diVerent (P < 0.05)

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graminoids, with even shallow moss causing a slightreduction in biomass. The dwarf shrub and herb, however,achieved its highest biomass in shallow moss, suggestingthat some degree of moss cover facilitates growth. Formalanalysis of standardized total biomass data (required tomeaningfully compare the four species) conWrmed thisduality of response, as shown by the highly signiWcanttreatment £ species group (graminoid vs. non-graminoid)interaction term (F2,59 = 5.20, P < 0.01). Subsequent anal-yses revealed no further diVerences in treatment responsebetween species within either group (treatment £ speciesterms F2,18 = 2.44, P > 0.1 and F2,18 = 0.08, P > 0.9 forwithin graminoids and within non-graminoids, respec-tively).

Soil characteristics

Soil temperature was highly sensitive to moss cover(F2,12 = 201.2, P < 0.001; Fig. 2a). Plots with no mosscover had the highest soil temperature of around 8°C. Theaddition of shallow moss lowered temperatures by approxi-mately 3°C and deep moss by 4°C. Soil nitrogen availabil-ity 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 highestunder shallow moss cover, although diVerences amongtreatments 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 Alopecu-rus were all greatly enhanced by the 4°C increase in soiltemperature (Table 1; Fig. 3a). In fact, total biomass in potsincubated at 8°C was nearly 3.5 times that of plants grownat 4°C. Leaf biomass also showed a positive response toincreased nutrient input, but seemingly at the expense ofroot biomass, as total plant biomass remained uninXuenced(Table 1; Fig. 3e). Change in soil moisture had no eVect onany of the growth parameters measured (Table 1; Fig. 3c).There were no signiWcant interactions between any of thetreatments applied.

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 signiW-cantly (P < 0.05)

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Dwarf shrub: Salix polaris

There were no signiWcant main eVects of either tempera-ture or nitrogen on the biomass of Salix (Table 1). How-ever, root biomass was signiWcantly lower in the highmoisture treatment than in the low moisture treatment(Table 1; Fig. 3d). The most striking treatment eVect onSalix growth was the interactive eVect of soil temperature

and nitrogen. When Salix was grown in a low-nutrientenvironment, higher soil temperature caused lower bio-mass than in any other treatment combination (Table 1;Fig. 4). Only in a nutrient-rich environment did Salix ben-eWt from growing in warmer soil. This pattern was appar-ent for leaf biomass but was very strong for root biomass,whilst no signiWcant eVect on shoot biomass was observed(Table 1).

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 ful-ly 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

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Discussion

Our study clearly shows that mosses can play an importantrole in inXuencing the growth of vascular plants. Moreover,the presence of a moss layer had both positive and negativeimpacts upon vascular plant growth, the relative extent ofwhich depended upon moss depth. Analysis of standardiseddata as well as visual inspection of Fig. 1 brings out twodistinct patterns of response, with graminoids (Alopecurusborealis and Luzula confusa) generally being negativelyaVected by deep moss, whilst the shrub (Salix polaris) andthe herb (Bistora viviparum) tended to beneWt from a shal-low moss layer. The observed eVects of moss on the soilenvironment (Weld experiment) and plant responses to soilvariables (phytotron experiment) together shed some lighton potential mechanisms driving these responses (seebelow for detail). Collectively, our Wndings suggest thatmat-forming mosses structure the composition of vascularplant communities.

Moss layer depth is strongly inXuenced by ecologicalfactors such as time since last disturbance, levels of nitro-gen deposition and vertebrate grazing pressure, as well asenvironmental factors. Greater soil moisture, for example,generally allows the development of deeper moss, but withincreasing depth the moss starts to inXuence soil conditionfactors itself; it is this process and how it inXuences vascu-lar plants within moss mats that we are interested in here.We created treatments with diVerent moss depths in an oth-erwise relatively homogeneous environment. The combina-tion of environmental conditions that favour a deeper orthinner moss layer with the eVects of the moss layer itselfon the soil environment may result in even more pro-nounced eVects on vascular plant growth than are demon-strated in this study.

Negative impacts of mosses

The negative eVects of the deep moss layer were most nota-ble for plant parts that are mainly located in the organic soilhorizon: belowground biomass of graminoids and storageorgans of the herb (but not of the shrub). Moss-inducedchanges to the soil environment may therefore be drivingthe negative impact of mosses on vascular plants. Indeed, anegative relationship between moss depth and graminoidbiomass 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, Hob-bie et al. (1999) found that Sphagnum negatively aVectedthe performance of the shrub Betula nana and concludedthat the moss’ ability to keep the soil cold and damp proba-bly hindered nutrient cycling in the soil.

In our Weld plots, the presence of a moss layer signiW-cantly reduced soil temperature, and deeper moss reduced

Table 1 Summary of fully factorial GLMM analyses of biomasscomponents of Alopecurus borealis (a) and Salix polaris (b) resultingfrom the phytotron experiment in which plants were subject to twolevels of each temperature, moisture and nutrient treatment, each ofwhich was applied for 2 months (n = 4 per treatment combination)

Figures are F values (df 1,24), with signiWcance levels indicated by asuperscript (*** P < 0.001, ** P < 0.01, * P < 0.05, 9 P < 0.1); signiW-cant values in bold

Source Total biomass

Leaf biomass

Belowground biomass

a) Grass Alopecurus borealis

Temperature 11.64** 18.97*** 9.06**

Nutrient addition 0.03 8.15* 0.25

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 Total biomass

Leaf biomass

Shoot biomass

Root biomass

b) Dwarf shrub Salix polaris

Temperature 0.50 0.44 0.50 3.159

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

Fig. 4 Total, leaf, shoot and root biomasses of Salix polaris after2 months of treatment in the phytotron experiment. Data are averages(§1 SE; n = 8) and show the signiWcant temperature £ nutrient inter-action

0

20

40

60

80

100

120

Total Leaves Shoot Root

Plant component

Bio

mas

s (m

g)

Low temp/ low N

High temp/ low N

Low temp/ high N

High temp/ high N

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nitrogen availability. Gornall et al. (2007) also showed thatan increase in moss depth reduced plant-available nitrogenand microbial biomass and respiration, and that deepermoss substantially delayed soil thaw, thereby shorteningthe growing season for vascular plants. Soil temperaturemeasurements in that study, running at exactly the sametime and location as the one currently reported, were basedon temperature loggers with external probes providingreadings every 6 h for a period of 14 months; this demon-strated clearly that the soil underneath moss mats of 6 cmdepth was signiWcantly cooler and displayed reduced diur-nal variability and signiWcantly fewer days above 0°C thansoil underneath moss of 3 cm depth. There were diYcultieswith employing temperature sensors in plots without amoss mat (both in terms of direct radiation to the sensor anddamage from both polar foxes and reindeer to the cabling).Therefore, we did not instrument plots without moss coverand thus relied on spot measures taken in July. Lacking theinsulating cover of the moss, however, we would expectbare soil to heat and cool more quickly, diurnally and sea-sonally, than shallow moss, and to be subject to greatertemperature extremes while annual average temperaturesmight be similar. This is supported by soil temperaturemeasurements (taken at 10 cm depth) from two contrastinghabitats at a nearby site (Endalen). Here, mean July soiltemperatures were higher and diurnal Xuctuations greater insparsely vegetated plots (i.e. with areas of bare soil) than insoils underneath a continuous moss mat of 2–5 cm depthwith vascular plants growing interspersed, while annualmean soil temperatures in the two habitats were similar(Jónsdóttir, unpublished data). Likewise, Coulson et al.(1993) demonstrated the suppressive eVect of vegetationcover on soil mean growing season temperatures and diur-nal Xuctuations at 3 cm depth when comparing a sparselyvegetated fellWeld site and a site with a continuous mossmat of 5–10 cm depth near Ny-Ålesund, Svalbard.

The moss layer also limits the nutrient uptake of vascularplants by eVectively intercepting aerially deposited nutri-ents (Pearce et al. 2003; Curtis et al. 2005), including nitro-gen derived from vascular plant litter, such that vascularplants growing in deeper moss rely on soil-derived nitrogensources (Malmer et al. 2003). Nutrient availability to vascu-lar plants is further compromised by the slow release ofnutrients from moss litter through decomposition (Cornelis-sen et al. 2007). Thus, it is clear that deep moss reduces soiltemperature and nutrient availability, which are often limit-ing to vascular plant growth in arctic systems (Jonassonet al. 1996; Brooker and Van der Wal 2003; Chapin et al.2002), creating an unfavourable soil environment that islimiting to belowground productivity.

It is also possible that vascular plants compete directlywith mosses for light. Investigations of competitionbetween moss species often identify light as a limiting

factor to shoot growth at the top of the moss layer (Okland2000), which is where leaves of Salix and Bistorta grow.Mosses have lower light compensation and saturationlevels than do vascular plants (Longton 1992), and so com-petition for light is likely to aVect vascular plants beforeself-shading within the moss layer suppresses moss growth(Okland and Okland 1996). Salix grows prostrate amongstthe moss with little protrusion of the leaves above the mosslayer. Likewise, new leaves of Bistorta grow up through themoss layer, so the deeper the moss, the more the growingshoots will be shaded during development, becoming etio-lated and likely of lower biomass (Chapin and Shaver1996). For both species, shoots in deeper moss also have togrow longer to ensure that leaves obtain light eVectively (asrooting is mostly at or near the moss–soil interface).Negative eVects of mosses on the shrub Salix and the herbBistorta 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 maydiVerentially inXuence vascular plant growth forms.

Positive impacts of mosses

A shallow layer of moss positively aVected the shrub andherb, and graminoid biomass was no less in shallow mossthan in the absence of moss. This suggests that there maybe some facilitation oVsetting the negative impacts mosshas via its inXuence on the soil environment and throughshading. Positive interactions between vascular plants arenumerous and important in harsh environments (Carlssonand Callaghan 1991; Brooker et al. 2008). However, rela-tively few studies (see below) have identiWed the positiveeVect of mosses on vascular plants.

Mosses may ameliorate environmental conditions forvascular plants by increasing water availability. Mosseshave evolved structural properties that limit water loss(Proctor 1979), which enables the moss layer to retainmoisture from snowmelt or precipitation, possibly main-taining the water status of vascular plants during periods ofdrought. Whilst this may be especially important at thebeginning of the growing season when desiccating windsare strongest (Carlsson and Callaghan 1991), large parts ofthe Arctic have low precipitation and hence a moss matmay also increase water availability to shallow-rootedplants and seedlings throughout later periods of drought, ashas been observed in polar semi-desert vegetation (Sohl-berg and Bliss 1987). In addition, the relatively smooth sur-face of a moss mat maximises boundary layer resistance(Rice et al. 2001), reducing turbulence and thus evapotrans-piration from the leaves of vascular plants growingamongst the moss. The gravimetric soil moisture measure-ments determined upon completion of the Weld experiment

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revealed only a marginally signiWcant diVerence (P = 0.06)among treatments, which we interpret as a lack of statisticalpower rather than an absence of treatment diVerences. Inline with our Wndings, permanent instrumentation of neigh-bouring plots revealed that soil underneath 3-cm deep mosshad a consistently greater volumetric water content (ofaround 0.45 m3 m¡3 during mid-summer) than soil under-neath 6-cm deep moss (0.32 m3 m¡3; Gornall et al. 2007).Unfortunately, it was not practicable to instrument plotswithout moss, and hence we have to rely for the currentstudy on the pattern of soil moisture presented in Fig. 2. Inthe Weld experiment, patterns of the relatively shallow-root-ing (see Brooker and Van der Wal 2003) dwarf shrub andherb biomass reXected those of soil moisture, suggestingthat moss inXuences the water relations—and thus produc-tivity—of vascular plants, not just in polar semi-desert veg-etation, but also in tundra heath.

Mosses may also ameliorate autumn and winter sever-ity by providing an insulating layer which can buVer soilagainst low temperatures and protect vascular plantsfrom frost damage (Gornall et al. 2007; Startsev et al.2007). Maintaining warmer soils in winter may aVectover-winter nutrient dynamics, which, in turn, can havean impact upon growing season productivity (Chapinet al. 2002). Given the strong insulating properties of adeep snow pack, the inXuence of the moss layer on soilnutrient cycling and hence vascular plants is likely to begreatest under conditions of shallow or no snow, and ishence of increasing importance throughout northernclimes as the period of snow lie has rapidly shortened(ACIA 2005).

Potential mechanisms

Field data show that soil temperature and nutrient availabil-ity are both negatively related to moss depth and, in con-trast, shallow moss depth marginally increases soilmoisture. The phytotron experiment was designed to fur-ther investigate the negative impacts of mosses in terms oftheir eVect on soil temperature and nutrient availability andtheir positive impacts in terms of moisture retention. Bothsoil temperature and nutrient availability were importantdeterminants of the growth of Salix polaris and Alopecurusborealis. However, in this study, soil moisture appeared tohave few signiWcant eVects on plant growth.

In an incubation experiment, Brooker and Van der Wal(2003) found that increasing soil temperature enhanced thegrowth of arctic vascular plants, particularly grasses,thereby suggesting that increased shoot biomass was aresponse to temperature-driven changes in soil nutrient sta-tus. Indeed, nitrogen is commonly suggested to be the moreimportant limiting factor in arctic ecosystems (Chapin et al.2002; Jonasson et al. 1999; Richardson et al. 2002). In our

study, however, soil temperature was far more importantthan nutrient availability in determining the biomass ofAlopecurus. Dormann et al. (2004) also showed that Luzulaconfusa was more responsive to increased temperature thannutrients, suggesting that these high-arctic graminoids maybe more temperature than nutrient limited. Indeed, thebetween-year variation in plant biomass in central Spitsber-gen was found to be strongly and positively related to thebetween-year variation in summer temperature (Van derWal and Hessen 2009), in contrast to Wndings from thelower Arctic (Chapin and Shaver 1985). Likewise, theobservation that the growth of Cassiope tetragonaresponded more to temperature at high-arctic latitudes andmore to nutrients in the subarctic (Havstrom et al. 1993)suggests that high-arctic plants are very temperature sensi-tive.

Low soil temperatures may reduce growth througheVects on root production. Starr et al. (2004) found that lev-els of the growth inhibitor ABA (abscisic acid) wereincreased in root tissue of arctic sedges grown at low soiltemperatures. With impeded root growth, nutrient andmoisture absorption is hampered, which can cause an over-all reduction in growth. Thus, it is possible that the eVect ofthe moss layer on soil temperature may have directlycaused the reduced growth of graminoids observed in theWeld experiment.

In the phytotron experiment, Salix biomass onlyincreased with soil temperature when receiving the highnitrogen treatment, demonstrating that soil temperatureincreases alone are insuYcient to enhance the productivityof this dwarf shrub. In fact, the combination of high tem-perature and low nutrient supply reduced biomass, possiblybecause root respiration and turnover increase with temper-ature (Forbes et al. 1997). Thus, should a moss layer be dis-turbed, for example by the activity of herbivores, Salix willonly respond to the resultant increase in soil temperature asfar as any associated increase in nutrient availability willallow (Gornall et al. 2009).

The short timescale over which the phytotron study wascarried out limits its usefulness in interpreting Weld results.Arctic plants have been shown to be highly seasonal in theirresponse to environmental variables. For example, Chapinand Shaver (1996) found that only early-season growth ofEriophorum vaginatum and Ledum palustre was con-strained by temperature; late-season growth, on the otherhand, was largely determined by nutrient availability. Also,many arctic species rely on stored reserves of nitrogen andcarbon (e.g., Shaver and Chapin 1991), particularly forearly growth when snow has melted but the soil is still fro-zen. Therefore, although plants were incubated for 2 weeksbefore the start of the experiment, the results may havebeen confounded by carry-over eVects from the previousyear. Thus it is important to acknowledge that, whilst our

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phytotron experiment provides interesting results, cautionmust be taken with the translation of the Wndings to plantsin the Weld.

The experiment, however, does provide evidence sug-gesting that soil temperature and nutrient availability areboth important determinants of the growth of arctic vascu-lar plants, supporting the argument that moss-mediatedeVects on the soil environment may be responsible for thevascular plant growth responses observed in the Weld.Moisture appeared to be less important for growth in thisstudy, but the positive eVects of mosses in retaining mois-ture may be particularly important in Weld conditions wherestrong winds exacerbate moisture limitation. Vascular plantgrowth responses to other important properties of the mosslayer, such as shelter from the wind, buVering winter sever-ity and shading in the moss canopy need to be investigatedto give a more complete picture of the mechanismsinvolved.

Experimental approach

The inXuence of mosses on the performance of vascularplants is clearly characterised by both positive and nega-tive components. Predictions regarding the relativeimportance of the positive and negative interaction com-ponents are currently hindered by a shortage of experi-mental manipulation studies and, in those studies whichhave been undertaken, by the lack of description of themoss mat and the actual manipulation employed. Forinstance, “moss removal” has been implemented as theremoval of just green moss (Hobbie et al. 1999), greenmoss and any attached brown tissue (Bret-Harte et al.2004), or the whole moss mat including dead moss under-neath (this study), whilst in most other studies the exactnature of the manipulations remain unclear. Perhaps as aresult, some of the “incomplete” moss removal studiesWnd little eVect on vascular plant performance (Fetcher1985) or report on patterns that are diYcult to interpretdue to relatively strong side eVects such as increasednutrient availability deriving from remaining dead mossbasal stems (Bret-Harte et al. 2004). Whilst recognisingthat every Weld manipulation will inevitably generateundesirable side eVects, we suggest that for a comprehen-sive evaluation of moss impacts on vascular plants, totalremoval of the moss mat is required (where possible, i.e.it is not possible in deep peat where moss is the substratevascular plants grow in) and, ideally, moss replacementtreatments should also be included. Critical to any experi-ment is also a description of the moss mat in terms of itsdepth, biomass and species composition. The omission ofthe speciWc depth of the moss mat is, in our opinion, thesingle most constraining factor preventing generalisation,as outlined below.

Synthesis

The relative importance of positive and negative interac-tions between vascular plants is predicted to change alonggradients of environmental severity (Brooker and Calla-ghan 1998). Whilst there are too few moss manipulations todetermine the applicability of this model to moss–vascularplant interactions, we propose that the balance betweenpositive and negative interactions is in the Wrst instanceinXuenced by the depth of the moss layer (Fig. 5). In a shal-low moss layer, positive eVects such as provision of shelterand moisture retention outweigh the negative eVects of themoss. As moss depth increases the negative eVects ofmosses on the soil environment, and for some plant speciesthrough shading, become more important. Once a certaindepth of moss is reached, the positive eVects of the moss donot increase further, and thus as the moss layer gets evendeeper it has an increasingly negative inXuence on vascularplant productivity. Hence, an increase in moss depth maybe viewed as an increase in environmental severity as expe-rienced by a vascular plant, with gradual decreases in soiltemperature, soil nutrient availability and length of thegrowing season.

We suggest that the relative extent of the positive andnegative impacts of mosses will diVer between vascularplant species, depending on a wide range of factors, including

Fig. 5 Conceptual model adapted from Brooker and Callaghan(1998) depicting the positive and negative impacts of the moss layeron the performance of vascular plants. The net eVect of the moss layeron species X is drawn in as an example. For this species, a shallowmoss layer is beneWcial (e.g. not too cold in winter, but still warmingup readily in summer) whilst at intermediate moss depth, positive andnegative eVects cancel each other out. At greater moss depths (beyondthe small arrow), negative eVects outweigh positive eVects on the per-formance of this species

Negative effects

Net effect

Increasing depth of the moss layer

Ext

ent o

f mos

s la

yer

impa

ct

on a

vas

cula

r pl

ant s

peci

es

0

-

+

Species X

↓

Negative effects

Positive effects

0

-

+

Species X

↓

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Oecologia

plant size, rooting depth, allocation pattern and intrinsicgrowth rate, hence structuring the vascular plant commu-nity. Our study thus highlights that, for plant interactiontheory to be widely applicable to extreme environmentssuch as the Arctic, growth forms other than vascular plantsneed to be considered.

Acknowledgments We are grateful to Hera Sengers and Anne-Mette Pedersen for invaluable help with Weld and laboratory work, toUNIS for the logistic support provided, and to referees for insightfulcomments and textual changes. This work was funded by NERC(NER/S/A/2001/05958) and permission for the Weld experiments waskindly granted by the Governor of Svalbard.

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