Gastropod grazing shapes the vertical distribution of epiphytic lichens in forest canopies

Authors


*Correspondence author. E-mail: johan.asplund@umb.no

Summary

1. Species composition and biomass of epiphytic lichens varies along a complex environmental gradient from the ground to the top of the forest canopy. It is not known if this gradient, considered to be shaped by succession (age of bark surface) and climatic factors, is also influenced by invertebrate grazing.

2. To investigate the grazing hypothesis, the natural height ranges of four old-forest Lobaria species on tree trunks were quantified. These large foliose epiphytes with different secondary chemistry were transplanted onto Fraxinus excelsior trunks 0.5, 3 and 6 m above-ground in five broad-leaved deciduous forests in southern Norway. After 4.5 months of exposure to natural climbing gastropods, grazing was quantified.

3. Grazing pressure strongly increased with increasing proximity to the ground. At all heights, gastropods clearly preferred Lobaria scrobiculata followed by L. amplissima. Lobaria pulmonaria, highest in carbon-based secondary compounds (CBSCs), and L. virens, nearly deficient in CBSCs, were both much less grazed. Therefore, CBSCs cannot explain the preferences. According to existing literature the stictic acid complex, present in L. pulmonaria and L. scrobiculata, represents a herbivore defence when occurring in quantities as high as those in L. pulmonaria. The identity of the strong defence in the CBSC-deficient L. virens is unknown.

4. Gastropods’ preference for these epiphytes mirrors the distribution of the lichens in nature. The highly palatable L. scrobiculata occurs mainly in localities with low gastropod abundance such as boreal forests and on bark with slightly lower pH. Lobaria amplissima occurs in gastropod-rich localities, but above the lower parts of the trunk. Lobaria pulmonaria and L. virens frequently grow down to a level of < 1 m above the ground.

5.Synthesis. Our results suggest that climbing gastropods play a role in determining the lower distribution limit of epiphytic lichens along a vertical canopy gradient and influence the spatial pattern of susceptible lichen species. By grazing lichens in a species-specific way, gastropods can shape epiphytic communities in broad-leaved deciduous forests.

Introduction

Large epiphytic lichens commonly envelop the tree bark and provide hiding places for invertebrates that represent important fodder for foraging birds (Pettersson et al. 1995). Species composition and biomass of lichens vary along a vertical gradient in forest canopies (e.g. Hale 1952; McCune 1993; Fritz 2009). Patterns in vertical distribution have mainly been related to microclimatic conditions (e.g. McCune et al. 1997; Campbell & Coxson 2001) or bark chemistry (Kermit & Gauslaa 2001; Fritz, Caldiz & Brunet 2009). Biotic factors such as competition from bryophytes (Scheidegger, Frey & Zoller 1995) or invertebrate grazing may also play a role, but have so far been little studied as driving forces in successions shaping lichen-dominated epiphytic communities. Nevertheless, lichen-feeding snails (e.g. Cepaea hortensis, Cochlodina laminata, Helicigona lapicida) frequently climb trees (Boycott 1934; von Proschwitz 1994; Gauslaa et al. 2006). The large slug Lehmannia marginata, which predominantly feeds on lichens, can even climb to the top of trees in humid weather (Andersson et al. 1980). However, the abundance of lichen-feeding gastropods is presumably highest close to the tree base.

Palatability for generalist gastropods depends on the lichen’s chemical defence (Gauslaa 2005), which varies much among species. Thereby, gastropods should have the potential to shape lichen-dominated communities. Apart from a few studies, the knowledge of gastropods’ preferences for different epiphytic lichens is scarce. Gauslaa (2008) found that gastropods discriminate between the old-forest lichens Pseudocyphellaria crocata and Lobaria pulmonaria. Other studies found that a natural gastropod community grazes intensively on certain saxicolous limestone lichen species and avoids other co-occurring species (Fröberg, Baur & Baur 1993; Baur, Baur & Fröberg 1994). For the old-forest lichen L. pulmonaria gastropod grazing may particularly limit the establishment phase because juvenile thalli are more poorly defended against herbivores than mature ones in terms of lichen compounds (Asplund & Gauslaa 2007, 2008). Moderate grazing on mature thalli, however, does not necessarily affect their subsequent growth rate (Gauslaa et al. 2006).

This study aimed to quantify the upper and lower distribution limits on tree trunks of epiphytic members of the lichen genus Lobaria: L. scrobiculata (purely cyanobacterial), L. amplissima (green algal with external cephalodia of cyanobacteria), L. pulmonaria and L. virens (green algal with internal cephalodia). This genus has given the name to a species rich but declining old-forest lichen community Lobarion (e.g. Rose 1988) mainly inhabited by cyanobacterial members of the lichenized fungal order Peltigerales. The selected lichen species have been shown to differ in composition of lichen compounds (carbon-based secondary compounds, CBSCs; Elix & Tønsberg 2006). We aimed to quantify the individual compounds in all species, as the content of specific compounds is an important factor determining the susceptibility to gastropod grazing (Gauslaa 2008). Another objective was to assess the grazing pressure on these four lichens along a vertical gradient on tree trunks, and to relate the grazing level to the observed height distributions. To do so, we tested the gastropods’ preference of the four Lobaria species in a field cafeteria experiment using transplanted thalli at three different heights along a vertical gradient on trunks of Fraxinus excelsior at five sites. Little is known about gastropods’ preference in nature for different epiphytic lichens and the vertical feeding range of these lichenivorous invertebrates. Lichens are often replaced by bryophytes on lower parts of tree trunks. We hypothesized that gastropods may play a role in shaping this lower distribution limit of the studied epiphytic lichens. Thereby, this study provides a novel approach to the understanding of lichenivorous gastropods’ influence on the studied lichen community and the distribution of Lobaria species on tree and stand scales.

Materials and methods

Lobaria amplissima, L. pulmonaria and L. virens were sampled from mixed and rich populations in a Fraxinus-dominated open broad-leaved deciduous stand in Støle (59°59′ N, 05°29′ E, 60 m a.s.l., Tysnes, Hordaland, W Norway) on 6 April 2008. Lobaria scrobiculata was sampled on trunks of Salix caprea in open Picea abies forests in Horka (64°26 ′N, 11°47 ′E, 30 m a.s.l., Overhalla, Nord-Trøndelag, W Norway). The lichens were stored air-dried in the refrigerator until the start of the experiment. Lobaria virens is phytogeographically restricted to temperate Atlantic habitats (Fig. 1). Lobaria amplissima is also a coastal species but extends further north, while L. pulmonaria and L. scrobiculata have wider distributions.

Figure 1.

 Distribution map of the four study species in Norway according to the Norwegian lichen data base (Timdal 2009). The dots show all entered herbarium and check-list records.

Transplantation sites

The lichens were transplanted to five sites in the hemiboreal region in SE Norway: Askehagen (one site, 59°40′ N, 10°46′ E, 100 m a.s.l.), Bekkevoll (two sites 100 m apart, 59°43′ N, 10°43′ E, 20 m a.s.l.) and Pollevann (two sites 100 m apart, 59°44′ N, 10°45′ E, 10 m a.s.l.). Fraxinus excelsior is the dominating tree species at all sites. Askehagen has deep soils and is relatively flat with Filipendula ulmaria dominating the field layer. The two sites at Bekkevoll have calcareous soils with an abundant cover of e.g. Hepatica nobilis and Viola mirabilis. The terrain at the northern site at Bekkevoll is rocky and uneven, which in combination with calcareous soils forms a good habitat for snails (Waldén 1981). The southern site at Bekkevoll is on a ledge (< 10 m wide) beneath a high vertical east-facing wall and above a smaller east-facing wall. The northern site at Pollevann is also rocky with only a thin layer of soil covered with Primula veris, Geranium robertianum and Polygonatum odoratum. At the southern site at Pollevann, the terrain is flat and falls slightly towards a small lake. The field layer is dominated by F. ulmaria.

Transplantation experiment

Air-dry lichens were weighed in the lab (± 0.1 mg). The air-dry weights were converted to oven-dry weights (70 °C; dry matter) by using the ratio between air-dry and oven-dry weights obtained from additional, sacrificed thalli. Subsequently, the thalli were fully hydrated and photographed with a Nikon D70s digital SLR equipped with a Tamron AF SP 90/2,8 Di Macro 1:1 lens. When photographed, each thallus was placed on graph paper and covered with a piece of glass to flatten the lichen. Before transplantation, two random thalli of each species were tied with a thin white flax thread onto a nylon mesh (15 × 20 cm). The thalli were randomly placed in two rows. The transplantation meshes were then stapled onto trunks of F. excelsior at three heights (0.5, 3 and 6 m above the ground) using plastic Plas-staples (Takkurat®; Dr. Gold & Co. KG, Nürnberg, Germany). These height levels were selected because they cover the normal vertical distribution range of studied species (see Results). In total, five random trees in each of the five sites were used. The lichens were transplanted on 25 April 2008 and harvested on 9–11 September 2008 and were thus exposed on the trunks for a period of 137–139 days. Various snail species (Cepaea hortensis, Cochlodina laminata, Columella edentula, Helicigona lapicida and Vertigo pusilla), two slugs (Arion fuscus and Lehmannia marginata) and one single lepidopteran larva (Dahlica sp.) were seen on the transplanted lichens during occasional visits. The vast majority of grazing marks were interpreted as gastropod grazing due the typical radula marks. In a handful of cases, the grazing marks were probably caused by lepidopteran larva.

Post-transplantation measurements

At the end of the exposure period, all thalli were weighed and photographed as described above. Images were georeferenced against the graph paper used as background when the photos were taken. Thallus and grazing mark areas were measured by polygon delineation in arcgis™ version 9.2 (ESRI®, Redlands, CA, USA).

Quantification of CBSCs

Carbon-based secondary compounds were measured in six thalli of each species. The lichens were ground to powder and extracted with acetone for 20 min. The extraction was repeated four times. The combined extracts were evaporated to dryness and dissolved in 1.5–5 mL acetone. The extracted compounds were quantified by HPLC using an ODS Hypersil column, 60 × 4.6 mm using 0.25% orthophosphoric acid and 1.5% tetrahydrofuran in Millipore (Millipore, Billerica, MA, USA) water (A) and 100% methanol (B) as mobile phases at 2 mL min−1, and UV detection at 245 nm. The run started with 30% B which was increased to 70% after 15 min. After a further 15 min 100% B was reached and held for 5 min after which the amount decreased to 30% in 1 min. A 10-min post-run of 30% B was performed. Extracts of L. scrobiculata were run on a 250 × 4.6 mm column, with a flow of 1 mL min−1, to improve the separation of usnic acid and metascrobiculin. Compound identification was based on retention times, online UV-spectra and co-chromatography of commercial standards of usnic acid (Sigma, St Louis, MO, USA), atranorin (Apin Chemicals, Abingdon, UK) and norstictic acid (Gaia Chemical Corporation, Gaylordsville, CT, USA). Standards of stictic acid were kindly given to us by Prof. Harrie Sipman (Botanical Museum, Berlin, Germany). The compounds, except metascrobiculin and pseudocyphellarin, were quantified against response curves of the above-mentioned standards. Metascrobiculin and pseudocyphellarin were quantified on a relative scale using the response curves of usnic acid and atranorin respectively. We present only the sum of stictic acid, norstictic acid and their derivatives in L. pulmonaria and L. scrobiculata. A detailed list of all derivatives found in these two species is given by Nybakken et al. (2007) and Nybakken, Johansson & Palmqvist (2009).

Quantification of height limits of naturally occurring Lobaria species

Trees with L. pulmonaria and at least one additional Lobaria species were recorded during a field work period lasting for 24 days. Sampling trees were > 15 m apart. The visited area comprised a long coastal section from eastern to western parts of S Norway, including elevations from 40 to 600 m a.s.l. Most trees were located in temperate broad-leaved deciduous forests, but some were deciduous trees within boreal conifer stands. The heights above the ground for the lowermost and uppermost thallus were measured with measuring tape or a scale hypsometer (Suunto Inc., Vantaa, Finland) for each species separately.

Quantification of the radiation climate

Hemispherical digital photographs (Englund, O’Brien & Clark 2000; Hardy et al. 2004) were used to quantify the radiation climate. Photos were taken at each transplantation mesh with a Nikon Coolpix 950 camera with a Nikon Fisheye Converter (FC-E8). Images were analysed by hemiview 2.1(Delta-T Devices, Burwell, Cambridge, UK) to estimate the direct radiation reaching each thallus throughout 1 year. The direct radiation varied between heights and localities (Table 1; Height: F = 4.56, < 0.05; Locality: F = 30.7, < 0.001; Height × Locality: F = 1.42 NS, two-way anova).

Table 1.   Mean direct radiation (kmol m−2 year−1; ± 1 SE, n = 5) in transplantation sites at each locality
 Height above ground (m)
Locality0.53.06.0
Askehagen0.98 ± 0.111.73 ± 0.221.67 ± 0.19
Bekkevoll N1.26 ± 0.241.44 ± 0.141.56 ± 0.11
Bekkevoll S0.95 ± 0.291.03 ± 0.0960.72 ± 0.18
Pollevann N1.86 ± 0.0652.03 ± 0.202.06 ± 0.28
Pollevann S2.21 ± 0.262.52 ± 0.203.04 ± 0.25
Average1.45 ± 0.131.75 ± 0.131.81 ± 0.18

Statistical analyses

All statistical analyses were performed on the mean value of every lichen species from each mesh. Preference was quantified by dividing the area consumed for a given lichen species with the total consumption within that mesh (Lockwood 1998). In this case only the thalli from the meshes on which grazing occurred were used (70 of 75 meshes). When testing the difference in preference, the anova’s assumption of independence was violated as the consumption of one thallus is dependent on the consumption of the neighbouring thalli. Instead, we used a modified Hotelling’s T2 test followed by a pair wise comparison as described by Lockwood (1998). Hotelling’s T2 was calculated using octave 3.0.3 (http://www.octave.org), and other statistical analyses were performed with minitab® 15 (Minitab Inc., State College, PA, USA).

Results

The amount of grazing marks differed between studied Lobaria species. The gastropods preferred L. scrobiculata, in which as much as 65 ± 3.2% of the total grazing per mesh occurred, followed by L. amplissima (28 ± 3.1%), L. pulmonaria (4.6 ± 1.1%) and L. virens (2.9 ± 0.9%). These differences were statistically significant (T2 = 15.1; d.f.=67; < 0.01; Hotelling’s T2) apart from the difference between L. pulmonaria and L. virens.

Grazing significantly decreased with height above-ground for all species apart from L. virens (Fig. 2). Furthermore, grazing differed significantly among most studied forest stands (Table 2, Fig. 3). Grazing was very low at Askehagen, where grazing at 3 m only occurred on two out of five trees, and grazing marks at 6 m were absent from all trees and all lichen species. At a local scale, grazing intensity varied substantially, as shown by the statistically significant contrasts between the two neighbouring stands at Bekkevoll and Pollevann respectively. Microclimate, measured as direct radiation, did not influence grazing pressure (Table 2).

Figure 2.

 Grazing on four Lobaria species transplanted for 137–139 days to trunks of Fraxinus excelsior at three different heights above-ground. Mean ± 1 SE, = 25.

Table 2.   Two-way anovas using direct radiation as covariate for area grazed (%) of each of the four Lobaria species
 d.f.L. amplissimaL. pulmonariaL. scrobiculataL. virens
MSFMSFMSFMSF
  1. Values are significant at the **1% and ***0.1% levels.

  2. †log (y + 1) transformed.

Locality412.324.52***5.5711.34***8999.428.64***1.977.46***
Height221.442.49***3.777.68**12626.020.41***0.662.50
Locality × Height80.861.700.0580.12679.81.540.220.81
Direct radiation10.811.620.130.26172.00.390.100.76
Error590.50 0.49 440.9 0.26 
Total74        
Figure 3.

 Mean total grazing of all four Lobaria species combined at each locality. Mean ± 1 SE, = 15. Bars not sharing the same letter are significantly different at a 5% level.

The studied lichen species exhibited different types of grazing marks. On L. scrobiculata meshes the herbivores fed on the upper cortex and the underlying photobiont layer. The white medulla was left untouched except under extreme grazing pressure on a few transplant meshes. Grazing marks on L. pulmonaria were often restricted to the upper and lower cortices. In such cases, gastropods avoided the photobiont layer and the medulla. The grazing marks on this species were minute and were probably made by Columella spp. or juvenile Clausiliidae e.g. Cochlodina laminata. In contrast, the gastropods often grazed through all layers in L. amplissima simultaneously. The latter type of grazing marks was also characteristic for the few L. virens thalli that were grazed.

Carbon-based secondary compounds varied qualitatively and quantitatively among the four species (Table 3). Lobaria pulmonaria had the highest levels of CBSCs (in total 6.9%; ranked according to decreasing concentration: constictic, peristictic, stictic, cryptostictic, norstictic and methyl norstictic acids) followed by L. scrobiculata (in total 2.5%; stictic acid, usnic acid, peristictic acid, metascrobiculin, constictic acid, norstictic acid, cryptostictic acid, methylnorstictic acid and unknown derivatives). Lobaria amplissima had substantially lower concentrations of CBSCs (in total 0.8%; metascrobiculin, pseudocyphellarin, atranorin), whereas L. virens only contained traces of acetone-extractable CBSCs (0.004% atranorin).

Table 3.   Concentration (mg g−1) of carbon-based secondary compounds in Lobaria spp. Mean ± SE, = 6
 L. amplissimaL. pulmonariaL. scrobiculataL. virens
Depsides
 Atranorin0.06 ± 0.020.04 ± 0.008
 Metascrobiculin5.9 ± 0.53.0 ± 0.8
 Pseudocyphellarin2.2 ± 0.4
Depsidones
 Stictic acid and its derivatives68.8 ± 4.416.2 ± 2.1
Dibenzofuranes
 Usnic acid5.5 ± 1.0
 Total8.2 ± 0.968.8 ± 4.424.8 ± 2.70.04 ± 0.008

The lower and the upper distribution limit observed in natural lichen sites in S Norway varied among the studied species (Fig. 4, Table 4). Lobaria pulmonaria was the most common species in the study area, exhibiting the widest total height range (0–11 m). Lobaria virens had the lowest mean lower height limit on studied trunks, whereas the lower distribution limit of L. amplissima was restricted to significantly higher parts of the tree trunk (Fig. 4). The highest L. amplissima thallus seen grew 11 m above the ground. Lobaria scrobiculata, which was found only on Populus tremula, Salix caprea, Sorbus aucuparia and Quercus (mainly Q. petraea), did not share host trees with L. amplissima and L. virens, except for one single Quercus trunk. Lobaria scrobiculata tended to occur in the more oligotrophic sites where total tree height was slightly lower (Fig. 4). Furthermore, trees with L. scrobiculata generally had a lower gastropod grazing pressure as evidenced by few grazing marks on lichens of these trees.

Figure 4.

 Mean upper (triangle down) and lower (triangle up) distribution limits of four Lobaria species on tree trunks in southern Norway, as well as the mean heights of their host trees (bar). Bars ± 1 SE.

Table 4.   One-way anova with blocks, locality, for upper and lower limit on tree trunks for natural populations of Lobaria amplissima, L. pulmonaria, L. scrobiculata and L. virens
 d.f.Lower limitUpper limit
MSFMSF
  1. Values are significant at the ***0.1% level.

Species31.059.24***1.198.15***
Block200.23 0.44 
Error870.11 0.15 
Total110    

Discussion

Natural populations of gastropods showed much stronger preferences for some Lobaria species than for others. Assuming a functional relationship between gastropod grazing (Fig. 2) and lichen abundance, our results suggest the following ranking of lichens according to falling abundance in gastropod-rich forests: Lobaria virens ≥ L. pulmonaria > L. amplissima > L. scrobiculata. Lobaria virens is the species that is most restricted phytogeographically to temperate Atlantic habitats (Degelius 1935; Fig. 1) characterized by the richest and most diverse gastropod fauna (Kerney & Cameron 1979). In such an environment, it grows successfully even in gastropod-rich basal parts of the trunk (Fig. 4). These patterns are consistent with a leading position for L. virens in a kind of arms race with gastropods, evidenced by its high grazing resistance. Lobaria pulmonaria is the most widespread Lobaria species. It often co-occurs with L. virens in gastropod-rich sites, but extends further eastward and northward into colder and/or drier zones with fewer gastropods. Compared to L. virens, it often has got grazing marks in broad-leaved deciduous forest (Gauslaa et al. 2006), and can be susceptible to gastropod grazing (Asplund & Gauslaa 2008). Lobaria scrobiculata, is also a widespread lichen despite the fact that it faced devastating levels of grazing (Fig. 2). Such a species can hardly sustain viable populations in sites with high abundance of lichenivorous gastropods. In Scandinavia L. scrobiculata has now its main distribution in the boreal zone. In southernmost nemoral/boreonemoral parts of Norway, where the abundance of tree-climbing lichenivorous gastropods is high, L. scrobiculata is the least common among the four Lobaria species and is often restricted to trees with low gastropod abundance such as Quercus spp. and Sorbus aucuparia. In addition, Gauslaa (1985) showed that L. scrobiculata can colonize trees with slightly lower bark pH than other Lobaria species. As gastropods avoid acidic litter and substrates at a local scale (von Proschwitz 1994; Gärdenfors, Waldén & Wäreborn 1995; Solhøy et al. 2002), L. scrobiculata can be considered as a species escaping grazing by having a wider ecological niche with respect to abiotic factors than that of lichen-feeding gastropods. Recently, L. scrobiculata has strongly declined in southern Sweden, tentatively ascribed to increased air pollution (Hallingbäck & Martinsson 1987; Hallingbäck 1989). Likewise, the highly palatable cyanobacterial lichen Pseudocyphellaria crocata has become extinct in its southern gastropod-rich temperate localities but is still viable in boreal rain forests in west-central Norway (Gauslaa 2008). Such declining patterns in the south may have been strengthened by gastropods grazing and/or been triggered by climate change-induced prolongation of the gastropod grazing season due to milder and shorter winters.

Our results show that grazing can be avoided or reduced at higher positions on the tree trunk (Fig. 2). If we exclude L. scrobiculata because of its slightly different habitat requirements (discussed above), there is a close relationship between the species-specific lower distribution limit on the trunk (Fig. 4) and the species-specific level of grazing damage (Fig. 2). The grazing-resistant specialist L. virens grows down to the ground, whereas the grazing-susceptible L. amplissima avoids the lowest part of the trunk. Considering the grazing results alone, a high position should be the best one for all species. However, old-forest cyanolichens such as Lobaria commonly avoid the upper canopy (e.g. McCune et al. 1997), as shown for studied species (Fig. 4). They are susceptible to high light levels (Gauslaa & Solhaug 1996), they need high humidity (Sillett & Antoine 2004) and barks with pH >5.0 (Gauslaa 1985; Fritz, Caldiz & Brunet 2009), implying that the abiotic environment is more hostile in the upper canopy. Furthermore, old-forest lichens are assumed to be limited by poor dispersal due to heavy symbiotic diaspores (Sillett et al. 2000; Walser et al. 2001; Walser 2004) resulting in a predominantly downward dispersal. Therefore, sparce colonization of the upper trunk is expected. Available data (Fig. 2) suggest that the lower distribution limits of grazing-susceptible lichens on tree trunks often are pushed upwards by gastropod grazing, whereas the upper distribution limit is more likely shaped by abiotic factors. Such trends support the view that epiphyte succession in tree canopies is a kind of allogenic succession (Stone 1989).

The relationship between the lichen-specific palatability in the preference study (Fig. 2) and the concentration of CBSCs in studied lichens (Table 3) is complex. Carbon-based secondary compounds in lichens cannot explain the observed preferences. Lobaria virens, exhibiting the lowest palatability, contains no lichen compounds apart from traces of atranorin. So far, the mechanisms behind the low palatability of L. virens are not known. However, recent field and laboratory experiments have shown that acetone-extractable CBSCs provide L. pulmonaria with a strong herbivore defence (Gauslaa 2005; Asplund & Gauslaa 2008). The concentrations of these compounds are substantially lower in the highly palatable L. scrobiculata. Apparently, these substances are only effective in high concentrations. As stictic acid is absent from L. virens and L. amplissima, their chemical defence in terms of CBSCs differs from that of L pulmonaria and L. scrobiculata. We do not know the efficiency of the individual substances in L. amplissima as feeding deterrents. However, pseudocyphellarin is chemically related to nephroarctin and phenarctin (Elix & Lajide 1984), which deters grazing on Nephroma arcticum (Asplund & Gauslaa 2009).

Lobaria scrobiculata is a purely cyanobacterial lichen, while the other species have both green-algal and cyanobacterial photobionts. Pseudocyphellaria crocata, another purely cyanobacterial member of the Lobariaceae, was also clearly preferred over L. pulmonaria in a recent field transplantation study (Gauslaa 2008). Furthermore, slugs consistently preferred the cephalodia on the tripartite N. arcticum (Asplund & Gauslaa 2009). This was explained by the lack of CBSCs in the cephalodia and not a higher preference for cyanobacteria per se. However, cyanolichens and/or cyanomorphs are not always more preferred than green-algal lichens (James & Henssen 1976). As lichen compounds are produced by the fungal partner, it is the lichen symbiosis as such, rather than an individual lichen biont, which plays a role in determining the herbivore defence.

The order of preference presented here is consistent with unpublished field observations. However, grazing on L. pulmonaria was lower than expected on the basis of field observations. This species is the most common Lobarion species, and occurs frequently in places where the three other species are lacking. Apparently, gastropods did not substantially feed on L. pulmonaria in the presence of the more palatable species. In an earlier grazing study at Bekkevoll with no species choices we found high levels of grazing on L. pulmonaria (Asplund & Gauslaa 2008). Grazing on L. virens is seldom observed in nature. Therefore, we expected a much greater difference between these two species. However, giving the herbivores many choices decreases the probability of finding significant differences in preference (Raffa, Havill & Nordheim 2002). As the preferences for L. virens and L. pulmonaria were very low compared to the other species, the chance to find significant differences between them is low. Some transplanted L. virens thalli showed localized damage similar to that occurring at high light levels and presumably induced by the higher light levels in some transplant sites than in the source sites. Grazing was often restricted to these damaged parts, suggesting that decreased viability increases the palatability of L. virens. It is well documented that gastropods prefer senescent rather than fresh plant material due to the lower levels of secondary compounds in old tissues (e.g. Richter 1979; Speiser & Rowell-Rahier 1991; Speiser 2001). Damage was only observed on L. virens. Therefore, species-specific transplantation damage has hardly affected the observed preferences.

We have shown that the grazing pressure on epiphytic lichens varies at a local landscape scale even among fairly similar broad-leaved deciduous forests (Fig. 3). In addition, snails can show an aggregated pattern on a more local scale due to small-scale differences in micro-climate, habitat heterogeneity or a patchy distribution of food (Ledergerber et al. 1997; Kleewein 1999). Spatial variability in grazing pressure is presumably one of the factors causing a patchy distribution of lichens in localized microhabitats. Asplund & Gauslaa (2008) found a higher grazing pressure under more shading canopies. However, in the current study the light gradient was too narrow to detect interactions between microclimate and gastropod activity (Tables 1 and 2).

To our knowledge, this study is the first successful attempt to quantify interactions between herbivores, photosynthetic epiphytes and the height within forest canopies. Gastropods have earlier been found to limit the upper distribution limit of intertidal macroalgae (Underwood 1980). Furthermore, slugs are assumed to have excluded the herb Arnica montana from lower elevations in the Harz mountains (Bruelheide & Scheidel 1999). Our results are consistent with the hypothesis inferring a role of gastropods in exterminating L. amplissima and L. scrobiculata from the lowest parts of tree trunks (Fig. 4).

In conclusion, closely related old-forest lichen species can use different strategies and defence compounds for reducing grazing damage. As the herbivore defence varies considerably among studied lichens, lichenivorous gastropods likely play a significant role in shaping the lichen community along a vertical gradient in forest canopies and influence the spatial pattern of susceptible lichen species.

Ancillary