Gastropod diversity in aspen stands in coastal northern Sweden

Forest Ecology and Management 175 (2003) 403±412

Gastropod diversity in aspen stands in coastal northern Sweden Otso Suominena,*, Lars Edeniusb, GoÈran Ericssonb, Victor Resco de Diosb a

b

Department of Ecology and Environmental Science, UmeaÊ University, SE-90187 UmeaÊ, Sweden Department of Animal Ecology, Swedish University of Agricultural Sciences, SE-90183 UmeaÊ, Sweden Received 12 June 2001; received in revised form 7 January 2002; accepted 15 April 2002

Abstract Forest management has actively reduced proportion of deciduous trees in Fennoscandian managed forests. Several species of cryptogams and invertebrates depend on deciduous trees, among which aspen (Populus tremula L.) is especially important. The occurrence and abundance of several terrestrial gastropod species are linked to aspen leaf litter. However, the impact of forestry practices on the gastropod communities is largely unknown. We examined difference in species richness, diversity (H0 ), and species composition in the gastropod fauna in aspen stands in a 400 km2 managed boreal forest area in northern Sweden by collecting them with masonite boards from 20 selected stands. We contrasted isolated (>500 m from the neighboring stand) and aggregated (<300 m from neighboring stand) stands, stands close to arable land (<50 m) and stands in the forest, and used locations outside the aspen stands in the surrounding forest as controls. Stand sizes ranged from 100 to 3000 m2. Gastropod species richness and diversity were high in aspen stands near arable land and in aggregated stands in the forest and low in controls. Both diversity and assembly composition of the gastropods in isolated aspen stands in the forest interior were intermediate between, and did not differ signi®cantly from, controls and other aspen stands. Species diversity and richness increased with stand area up to an area of 700 m2. We conclude that aspen stands in this managed boreal forest landscape are important for gastropods, even for species that are not strict habitat specialist connected to aspen. The total amount and the distribution of aspen in the landscape also may be important, since many species can be lost if area and connectivity of aspen stands drop to low levels. The positive species±area relationship suggests that future forestry practice should favor stands at least 500 m2 (0.05 ha) preferably in connection to other stands. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Boreal forest; Biodiversity; Gastropods; Aspen; Leaf litter

1. Introduction The amount of mixed forests has declined in Fennoscandia during the last century, mostly due to changed land use practices including abandonment of marginal farmland, cessation of forest livestock grazing * Corresponding author. Present address: Section of Ecology, Department of Biology, University of Turku, FIN-20014 Turku, Finland. Tel.: ‡358-2-333-6357; fax: ‡358-2-333-5765. E-mail address: [email protected] (O. Suominen).

and by forestry (e.g. HaÈmet-Ahti, 1983; Zackrisson, 1985). Deciduous species were long considered a problem in forest management and measures were taken to reduce their abundance, for example, by cleaning, girdling and chemical treatment. Of the deciduous trees, European aspen (Populus tremula L.) is of high importance for biodiversity (AndreÂn and Angelstam, 1993; Samuelsson and IngeloÈg, 1996; Hazell, 1999; Ericsson et al., 2001). In Sweden, aspen normally occurs spatially aggregated in stands (`clones') (Johansson, 1996). Today, in lieu of ®re disturbance

0378-1127/02/$ ± see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 8 - 1 1 2 7 ( 0 2 ) 0 0 1 4 2 - 1

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most aspen regeneration in Fennoscandia is asexual through suckering (BaÈrring, 1988; Linder et al., 1997). Aspen is an important substrate for several species' feeding, growth and nesting (e.g. Worrell, 1995; Samuelsson and IngeloÈg, 1996; Stenberg, 1996; Hazell, 1999; Uliczka and Angelstam, 1999). For example, Samuelsson and IngeloÈg (1996) found that aspen is host for at least 130 different red-listed invertebrates, fungi, lichens and mosses. The majority of the studies on the signi®cance of aspen on biodiversity have concentrated on the species living on, often coarse old or dead, aspens (e.g. HelioÈvaara and VaÈisaÈnen, 1984; Siitonen and Martikainen, 1994; Linder et al., 1997), while studies on the importance of aspen leaf litter for ground-living organisms are few (but see, e.g. Koivula et al., 1999). Microclimate and other physical habitat characters are usually considered as the most important factors determining the local distribution of terrestrial gastropods (Boag, 1985; South, 1992). The importance of food resources appears to be less conclusive. Some consider it negligible, since most species are food generalists (e.g. South, 1992). Others consider both physical habitat and food resources are important determinants of gastropod distribution (e.g. Chang and Emlen, 1993). For gastropods in the boreal forests of Fennoscandia, aspen has been considered as the most important tree species due to its high quantity and quality leaf litter, which provides both good physical conditions and food resources (Karlin, 1961) and because its leaves have a high concentration of calcium needed by gastropods for shell formation (Valovirta, 1968; WaÈreborn, 1979). Aspen leaf litter also has indirect effects on soil biota by raising pH of soil and litter, which has been found to increase both the abundance and species richness of land snails (Valovirta, 1968; GaÈrdenfors, 1992). In addition to boreal forests being poor in calcium but rich in acidic soils, anthropogenic acidi®cation has made the situation for snails even worse. Consequently, it has been suggested that snails would be a suitable indicator group for monitoring changes in forest litter and soil due to acid rain (GaÈrdenfors, 1987; WaldeÂn et al., 1991; WaÈreborn, 1992). Uotila (1988) classi®ed the leaf litter of tree species in southern Finland into four categories according to how `good' they are for gastropods. In the highest quality class were aspen, linden (Tilia cordata Mill.), and hazel (Corylus avellana L.)

with their calcium-rich leaves. Of these three species, aspen is the only one that is relatively common through out the boreal zone in Fennoscandia. Despite the presumed ecological signi®cance of aspen for gastropod species, and with modern forestry practices affecting the amount and distribution of deciduous trees, the relationships between aspen and boreal gastropod abundance and occurrence are largely unknown. We sequentially test three hypotheses and predictions to address the role of aspen stands for gastropod biodiversity. (1) Aspen (leaf litter) is important for gastropods, i.e. their species richness and composition differ between stands with and without aspen. We predict the species richness to be higher in aspen stands than in the surrounding mixed conifer dominated forests. (2) The distribution and abundance of aspen stands in the landscape have an impact on gastropod species richness, and these differences among aspen stands depend on the size of aspen stand and on the distance to neighboring aspen stands. We predict gastropod fauna to be richer in large or aggregated stands than in small or isolated stands, and that we most likely ®nd aspen-related specialist species in large or aggregated stands. (3) Gastropod species richness will depend upon the type of area in conjunction to aspen stands. We predict to ®nd both forest species and gastropod species typical to grasslands and cultural habitats in aspen stands near arable land, thus producing higher species richness. 2. Materials and methods 2.1. Study area The 400 km2 (20  20 km2) study area is located in VaÈsterbotten, northern Sweden (648120 N, 208450 E) close to the Baltic coast in the middle boreal zone (Ahti et al., 1968). The area is dominated by managed forests (75%) of different ages, with arable land (15%), lakes and mires (10%) creating a mosaic landscape. The forests are dominated by Scots pine, Norway spruce, and birches (Betula pendula L. and B. pubescens L.), whereas dwarf-shrubs (Vaccinium myrtillus L., V. vitis-idaea L., and Calluna vulgaris L.) dominate the ®eld layer. Other tree species are aspen, rowan (Sorbus aucuparia L.), bird cherry (Prunus

O. Suominen et al. / Forest Ecology and Management 175 (2003) 403±412

padus L.), gray alder (Alnus incana (L.) Moench), and some willows (Salix sp. L.). Linden and hazel, which provide excellent leaf litter for gastropods (Uotila, 1988), do not grow naturally this far north. The climate is continental with comparatively short thermal summers (1 June±10 September). Annual precipitation ranges 600±700 mm. The ground is generally snow-covered from the ®rst week of November to the last week of April (SNA, 1995). 2.2. Sampling of gastropods We performed the gastropod survey during the summer and autumn of 2000. In a previous study, we used three methods to locate all aspen stands in the area (Ericsson et al., 2001) and found altogether 189 aspen stands (mean size 786 m2, S.D. 1060 m2, range 100±9000 m2). For this study, each stand was later classi®ed regarding location (forest/close to farmland) and distance from other aspen stands (isolated/aggregated). Among those stands, we randomly selected ®ve replicates of ®ve stand types (`treatments'): (1) isolated (distance to closest neighboring stand over 500 m) aspen stands in the forest, (2) aggregated (distance to closest neighboring stand under 300 m) aspen stands in the forest, (3) isolated aspen stands close to arable land (less than 50 m from the ®eld edge), (4) aggregated aspen stands close to arable land, and (5) control (Table 1). The controls were evenly distributed over the entire 20  20 km2 area but only placed in the mature forest. As a rule they were never placed closer than 500 m to the nearest aspenstand or piece of farmland. Stand sizes ranged from 100 to 3000 m2. We tried to select a similar range of stand sizes for all stand categories so that we could study the effect of stand size as a continuous variable. There were small stands in all categories but for the isolated stands in the forest we could not ®nd any large stands (1000 m2). Gastropods were collected using 50  40 cm2 masonite squares (boards) placed with the rough side down. The boards were distributed in a uniform grid, where the boards were placed at 5 m distance from each other. In order to keep the minimum distance between boards, the number of boards varied in different stands. If the stand area was 100 m2 four boards were used, eight boards if the stand area was 100±200 m2, and 12 boards if the stand area was larger

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Table 1 Stand categories used in this study Abbreviation

Stand type

Cont

Forest site 500 m from nearest aspen stand or arable land Isolated aspen stands in forest Aggregated aspen stands in forest Isolated aspen stands close to arable land Aggregated aspen stands close to arable land

IsFo AgFo IsAr AgAr

than 200 m2. Eight boards were used in the ®ve control stands. In total, 248 boards were used. The boards were checked for gastropods four times (1/7, 30/7, 2/9, 1/10). All slugs and snails found under or on the boards were immediately placed in ethanol (70%), and were later counted and identi®ed to species with the help of the manuals of Hutri and Mattila (1991) and Kerney et al. (1979). 2.3. Data analyses Gastropods from all the sampling dates were pooled for the analyses, and gastropods from all boards from each site were pooled. The majority of the individuals were collected in September and October. Species richness, diversity (Shannon's index, H0 ), 0 and evenness (Shannon's J0 , …H 0 =Hmax †) (Shannon and Weaver, 1949) were calculated for each stand and corresponding control plot. Species richness was standardized by rarefaction (Simberloff, 1972). We used rarefaction estimation counted for each stand for sample size of ®ve individuals to be able to produce an estimate also for the sites with low number of individuals caught. The use of rarefaction corrects the bias in species richness due to differences in sample size (different number of boards and low abundance in poor habitats). Differences between stand types (`treatments') were tested with a one-way ANOVA. We used Tukey's a posteriori test for pair-wise tests between levels of signi®cant treatment effects. The relation between species richness (rarefaction estimations for sample size of 10 and 20 individuals), diversity (H0 ) and evenness (J0 ) to the size of aspen stand were tested by regressing these variables against stand size (m2). Species composition of gastropod assemblage was analyzed by ordination with detrended correspon-

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dence analysis (DCA) (CANOCO, ter Braak, 1988, 1990). We used detrending by segments and nonlinear rescaling of axes, and the species that were present in less than ®ve sites were omitted from the analysis, thus ``down-weighing'' of rare species was not necessary. Since in our data the ®rst axis was so clearly linked to difference among treatments, we could test differences among treatments along the ®rst axis alone (onedimensional ordination space) using ANOVA. Differences in the abundance (individuals/board) of all gastropods and that of four most common species (present in >10 sites) among treatments were tested utilizing the Kruskall±Wallis test (too many 0-abundance sites for parametric tests) combined with Dunn's multiple comparison a posteriori test. The

species tested were Discus ruderatus (Ferussac), Arion subfuscus (Draparnaud), Euconulus fulvus (MuÈller), and Limax tenellus (MuÈller). 3. Results In total, we caught 664 slug and snail individuals belonging to 10 species. Rarefaction estimation of gastropod species richness, corrected for similar sample size of 5, differed among treatments (Fig. 1a). Controls had lower species richness than aspen stands, and they differed signi®cantly from aggregated stands in forest and both stand types near arable land. Species richness of isolated stands in the forest did not differ

Fig. 1. Differences among stand types in: (a) species richness estimation for a sample size of ®ve individuals (rarefaction), and (b) Shannon's diversity (H0 ) and evenness (J0 ) indices. Abbreviations for stand categories, i.e. treatments as in Table 1. ANOVA for differences among treatments in species richness: F4; 19 ˆ 4:114; P ˆ 0:0087; for H0 : F4; 20 ˆ 7:293; P ˆ 0:0009; for J0 : F1; 18 ˆ 1:532, ns. Results of Tukey's a posteriori test …P < 0:05† indicated with letters in the ®gure when ANOVA indicated statistical signi®cance.

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statistically signi®cantly from other stand types or controls (Fig. 1a). Differences among treatments in species diversity H0 were similar to those in species richness. Aggregated aspen stands and isolated stands near arable land had the highest diversity and differed from the controls. Gastropod diversity of isolated aspen stands in the forest did not differ signi®cantly from other types of aspen stands or controls (Fig. 1b). 0 Evenness J0 …H 0 =Hmax † of gastropod fauna did not differ among the studied stand types (Fig. 1b). Rarefaction estimates for sample sizes of 10 and 20 individuals as well as H0 had a good ®t to one phase

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exponential association curve (R2: 0.36, 0.67 and 0.43, respectively, Fig. 2). The richness/diversity values increased with stand area until a threshold of about 700±1000 m2, after which they leveled. Noteworthy is that while there were no linear relationships of richness or diversity to stand area when analyzing the whole data, there was a positive linear relationship of these variables to stand area when the analyses were restricted to stands <1000 m2 (analyses not presented here). The use of rarefaction should in itself correct for the effect of different sample sizes in the estimation of species richness. Excluding stands with less than 10

Fig. 2. Relationships between stand size and (a) species richness (rarefaction estimation for sample sizes of 10 or 20 individuals, and (b) Shannon's diversity (H0 ) and evenness (J0 ) indexes. One phase exponential association curve shown when the ®t was reasonably good …R2 > 0:35†.

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Fig. 3. Joint plot of unconstrained DCA ordination for the gastropod species composition. Abbreviations for stand categories, i.e. treatments as in Table 1. Gastropod species are: Colu: C. lubrica, Diru: D. ruderatus, Vipe: V. pellucida, Arsu: A. subfuscus, Neha: N. hammonis, Nepe: N. petronella, Lite: L. tenellus, Eufu: E. fulvus, Arar: A. arbustorum.

Fig. 4. Differences among stand types in the abundance of gastropod species. Abbreviations for stand categories, i.e. treatments as in Table 1. Kruskall±Wallis test for differences among treatments for D. ruderatus: P ˆ 0:0116; for all other species ns: results of Dunn's a posteriori test …P < 0:05† indicated with letters in the ®gure when Kruskall±Wallis test indicated statistical signi®cance.

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individuals leave stands with only four boards out from the analyses. Since all stands larger than 200 m2 had 12 boards, nearly the whole species richness range covered emanates from stands with the same number of boards. Thus, the higher richness and diversity in larger stands are not just simply resulting from more boards being used in larger stands. DCA ordination of the gastropod assemblages in aspen stands followed the pattern of species richness and diversity (Fig. 3). The composition of the gastropod assemblages of control stands was separated without overlap from both aggregated aspen stand types and isolated stands near arable land on the ®rst ordination axis, and the composition of gastropod assemblage in these three aspen stand types was similar to each others. The site scores of isolated stands in forests fell between controls and other aspen stands on the ®rst ordination axis and overlapped with both of them. The close linkage ofthis separation ofstandtypestothe ®rstaxisallowed us to test the difference among site scores on this axis with ANOVA (F4; 19 ˆ 7:664; P ˆ 0:0007, Tukey: controls vs. aggregated forest stands P < 0:001, controls vs. isolated stands near arable land P < 0:01, controls vs. aggregated stands near arable land P < 0:01, all other comparisons ns). Analyses suggest that A. subfuscus, E. fulvus and Nesovitrea petronella (Pfeiffer) were more common in controls than in aspen stands, while higher abundance of D. ruderatus, Cochlicopa lubrica (MuÈller), L. tenellus, Arianta arbustorum (L.), Vitrina pellucida (MuÈller), N. hammonis (StroÈm) was found in the typical aspen stands. Of the four most common gastropod species tested, only the abundance of D. ruderatus differed signi®cantly among treatments, and was higher in aggregated forest stands and isolated stands close to arable land than in controls (Fig. 4). Of the less common species V. pellucida, A. arbustorum, C. lubrica, and Columella edentula (Draparnaud) were found only from aspen stands but not control sites, while N. hammonis and N. petronella were present also in controls sites. 4. Discussion Our results veri®ed that aspen litter is important for many terrestrial gastropods in boreal Fennoscandia. We observed higher richness and diversity of gastropods in aspen stands than at sites in mixed forest

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(conifer dominated with a mix of birches). The majority of the species we found are more or less ubiquitous in mixed boreal forests, which makes the strong positive effect of aspen somewhat surprising, and underlines the importance of aspen for gastropod biodiversity. Since we only compared aspen stands to the mixed forest, we do not know whether some other deciduous tree species could have similar impact on gastropods as aspen. On the other hand, it has been argued that aspen litter is better for gastropods than the leaf litter of any other deciduous tree species growing in northern Sweden. For example birches, which are the most common deciduous trees in northern Fennoscandia, produce leaf litter that is classi®ed by Uotila (1988) to same category as spruce needle litter as habitat for gastropods. Our second hypothesis was connected to distribution and abundance of aspen at the landscape scale. We predicted that isolation and small size of aspen stands would reduce gastropod diversity and richness. This was true for small stands (<700 m2), and while isolated stands inside the forest did not differ statistically signi®cantly from the rest of the aspen stands they were not separated from the controls either as the rest of the aspen stands were. Our result, that isolated aspen stands do not necessarily increase gastropod diversity in the same way as more connected stands in the forest, could have an in¯uence on the discussion of how forestry and moose browsing might affect species richness through their effects on the amount and distribution of aspen in landscape. It seems that even if forest management practice leaves small scattered aspen stands in the forest, there can still be a substantial negative effect on aspen-related fauna like the gastropods. Due to their poor dispersal capacity (Paul, 1978), gastropods might be especially sensitive to isolation of suitable habitats. However, for stands growing near arable land, isolation from other aspen stands did not have any effect. This might be partly due to other deciduous trees being abundant in the forest edges close to arable land having, to some extent, similar (leaf litter) properties to aspen and thus weakening the `island' nature of aspen stands. For example, according to Valovirta (1968) species composition and abundance of gastropods in Salix caprea (L.) leaf litter resemble that of aspen leaf litter, and S. caprea is common in ®eld edges. It is also possible that there are some chemical and physical

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differences in the soil and litter layer between ®eld edges and forest interior because of the impact of agricultural practices (more calcium and nutrients). The third hypothesis was that in the aspen stands close to arable land there would be both forest species and species belonging to gastropod assemblage of cultural and farmland habitats. Due to this supposed in¯ux of species from the ®eld edge, species richness was expected to be higher in stands close to arable land compared to similar stands in the forest interior. Our data did not support this hypothesis. Richness and diversity of gastropods in aggregated stands within the forest was as high as that of stands near farmland. V. pellucida was the only species, which might have been somewhat more common near farmland, but even it is a `forest species'. One factor that could have a negative effect on gastropods, and effectiveness of boards as sampling method, is that humidity is lower near forest edge and maximum temperatures higher than inside the forest. Cardboards or masonite squares have been frequently used for collecting terrestrial gastropods, since they are an easy way to collect large samples, but this method has been criticized for biased sampling (e.g. McCoy, 1999). It is best for large species that move actively on litter surface but underestimates small species, and species that live deeper in the litter or climb actively on vegetation (Matveinen, 1996; McCoy and Nudds, 1997; Hawkins et al., 1998). According to McCoy (1999), boards are not appropriate for estimating true abundance or detailed assembly composition, but they can be used to estimate species richness and other community metrics. We aimed to test whether the gastropod assemblages of different kinds of aspen stands differ from each other and from the surrounding forest. To do that we did not need detailed information on species composition and densities. Thus, the board method should be adequate for us. Even if we most likely missed some species and the relative abundance among species might not be properly estimated, our main results about the differences among stand types should be valid. 5. Conclusions Our results showed that the spatial arrangement of aspen stands may affect gastropods associated with

aspen. In this `aspen-poor' landscape aggregated aspen stands, regardless of whether they occur close to farmland or in the forest, promote diversity of gastropods compared to the surrounding forest. Moreover, isolated aspen stands close to farmland showed higher diversity than controls without aspen. Isolated stands in the forest interior, and small stands in general, seemed to lack parts of the rich gastropod fauna connected to aspen stands. This suggests that gastropods may be considered as indicators of habitat connectivity, i.e. for de®ning thresholds in amount of suitable habitat in the landscape. The idea of this concept is that population persistence is proportional to the amount suitable habitat in the landscape above a certain limit, whereas the relationship turns nonlinear below this cut-off point, i.e. the spatial con®guration of habitat becomes important (AndreÂn, 1994). Actions to promote regeneration of aspen in the interior of forests would probably bene®t gastropods by increasing the effective habitat area. However, such actions could be counteracted by the tendency of moose to browse intensively on isolated aspen ramets in forest (Edenius et al., 2002). It has been shown that moose browsing has a negative effect on gastropod abundance in boreal forests, and that impact has been attributed to reduced amount of deciduous trees in browsed forests (Suominen, 1999). Thus the active forest management measures to reduce the abundance of aspen in the past and current high browsing pressure may jeopardize such restoration actions. Acknowledgements We are grateful to David SundstroÈm and Peter NaÈslund for providing data on aspen stands, and to Eric Andersson for assistance in the ®eld work. This study was ®nanced by the Swedish Council for Forestry and Agricultural Research (funding to LE and GE). References Ahti, T., HaÈmet-Ahti, L., Jalas, J., 1968. Vegetation zones and their sections in northwestern Europe. Ann. Bot. Fenn. 5, 169±211. AndreÂn, H., 1994. Effects of habitat fragmentation on birds and mammals in landscapes with suitable habitat: a review. Oikos 71, 355±366.

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