Long-term effects of riparian clear-cutting -richer land snail ...

Department of Studies in Biology and Environmental Science (BMG)S-90187 Umeå University, SwedenDegree Thesis in Biology 30 ECTSSwedish Master’s Level2004-03-02Long-term effects of riparian clear-cutting-richer land snail communities inregenerating forestsLotta StrömSupervisor: Mats DynesiusLandscape Ecology GroupUminova Science ParkDepartment of Ecology and Environmental ScienceUmeå UniversitySE-901 87 Umeå, SWEDEN

IntroductionSince more than half of Sweden’s forests have been clear-cut after 1950, the interestin what happens to those ecosystems has increased. To maintain biological diversityand preserve threatened species it’s important to find out more about the ecologicalconsequences of clear-cutting. In Fennoscandia, the amount of information abouteffects of forestry on invertebrates have accumulated (Niemelä 1997), but few studieshave been published on litter-dwelling land snails.All snails belong to the phylum Mollusca and class Gastropoda which contains 60 000species. Primitively Gastropoda are marine animals (Barker 2001) but two subclassescontain groups that have independently invaded land, Pulmonata and Prosobranchia,together containing about 30 000 species (Abbot 1989, Kerney 1979). Pulmonates arefreshwater snails and land snails with a lung, four tentacles and a set of male andfemale sex organs in each individual (Abbot 1989). They represent the vast majorityof land snails and are the subclass that all land snails in Sweden belong to. Theterrestrial snails evolved from clades with coiled shells, but in some groups the shellshave been reduced (Barker 2001). Species lacking a shell are called slugs and are notincluded in this study. Most prosobranch snails are marine, but some of them live onland (with main occurrence in Central America and the islands of the Caribbean). Themost noticeable difference to the pulmonates is the operculum, a “door” on the tail,which they can close the shell with (Kerney 1979). They don’t have a lung, only twotentacles and usually separate sexes (Abbot 1989). A total of 113 land mollusc species(of which 21 are slugs) have been recorded in Sweden (Falkner et al. 2001). Themollusc fauna is poorer further north in the country. In the county of Jämtland, whichis situated in the middle of Sweden, 45 land snail species have been recorded duringan inventory of different habitats (Andersson 1996). In the riparian forests along smallstreams, partly in the same area, Göthner (2002) found 21 species. The county ofJämtland has calcareous areas, so the number of species is probably higher than forother areas at this latitude.In boreal forests land snail habitats include litter as well as trees. Most small specieslive permanently near the soil surface, but some of them sometimes climb up in thevegetation (Kerney et al. 1979). The litter dwelling species are decomposers,predominantly eating higher plant material but they are also specialised on asecondary substrate e.g. living plants, fungi or animal material (Mason 1970a). Therole of snails as decomposers in a beech forest has been evaluated by Mason (1970b).The snails only ingested a small part of the annual litter production, suggesting themto be of little direct importance in litter decomposition. However, they physically andchemically alter the litter, with a low assimilation and high faeces production, andhence promote fungal and microbial growth and decomposition. (Mason 1970b,Dallinger et al. 2001). The feeding fragments left of coarse plant material also providemore rapid access by microorganisms (Dallinger et al. 2001). Calcium is vital to thesnails for the shell construction. They are not dependent on lime in mineral form, butuse calcium present in organic compounds in vegetation and litter (Wäreborn 1969).Calcium is also important for the egg production; more calcium in combination withhigher pH can increase the snail’s reproductive output (Wäreborn 1979). Howeverdifferent species have varied requirements on calcium content in the food (Wäreborn1982). Several studies show that density and species richness correlates with amountof calcium and with the pH in the litter samples (Burch 1955, Waldén 1981,3

Gärdenfors 1992, Hotopp 2002). Close positive correlations have also been foundbetween the calcium content, pH and base saturation of the litter (Waldén et al. 1991),which makes it difficult to sort out which of the variables that are effecting the snailabundance most (Gärdenfors et al. 1996). Because of the requirement of calcium, soilacidification has a negative effect on snails. The calcium is leached from the litter byacid rain, which causes a decrease in snail density (Waldén et al. 1991, Wäreborn1992, von Proschwitz 2001). This is a problem especially in nutrient poor coniferousforests and in the southern part of Sweden where the deposition is high. This problemhas also given rise to a new one. Most passerines depend on an intake of calcium inaddition to the normal food for eggshell production and skeletal growth and they usesnail shells as the main calcium source. A large proportion of Great tits (Parusmajor), in forests with nutrient-poor soils in the Netherlands, produce eggs withdefective shells as a result of calcium deficiency (Graveland 1996 and Gravland andvanderWal 1996). Similar increases in eggshell defects have also been observed inGermany and Sweden (Gravland and vanderWal 1996). Considering the gastropodscentral position in terrestrial food chains they probably have a strong impact also onthe turnover of trace elements (Dallinger et al. 2001). The gastropods possess anexceptional affinity for certain trace elements such as copper, zinc, cadmium and lead.They may accelerate the metal cycling in the soil ecosystem, but also act asbiomineralization sinks and “buffers” for certain metals (Dallinger et al. 2001).Clear-cutting causes large changes in forest ecosystems affecting both abiotic andbiotic processes. The most evident changes occur at the ground surface, which bothobtain more light, larger temperature fluctuations and changed wind and moistureconditions. The ground water level elevates, but the top layer (a few cm thick) of thesoil dries out. The ground also gets compacted by heavy logging equipment(Lundmark 88, Fisher and Binkley 2000). After clear-cutting the pH of the humusincreases due to the ceased ion exchange that follows tree removal. On a longer timescale,the decomposition of logging residues also increases the pH (Lundmark 88,Fisher and Binkley 2000). A change in plant community structure with moredeciduous trees and herbs influences the litter quality.Snails are a group of invertebrates well known for their poor mobility, so they shouldnot be able to easily escape the changed local environment. Environmental factors,such as moisture, sun exposure, vegetation, litter calcium content and pH influencethe occurrence of land snails (Wäreborn 1969, 1982). Their wet skin and method ofmoving, with loss of water through mucus, makes snails restricted in their habitat bythe need to avoid drying up (Kerney et al. 1979). Except to finde moist places, theonly defence they have against dry conditions is to become inactive and retreat insidethe shell. Due to moisture conditions and partly also temperature, at least small snailsare more active at night than during the day (Boag 1985). Many species showaggregation behaviours (Mason 1970b, Baur and Baur 1988). Those might beexplained by an attraction of moist and local concentrations of food or by signals fromother snails, such as pheromones in mucus trails (Baur and Baur 1988).A common view is that the poor dispersal ability of land snails (Baur and Baur 1990),together with their dependence of stable microclimate, makes them sensitive tologging (Waldén 1998). von Proschwits (1998) argues that the need of a stabileenvironment makes many species good indicators of forest continuity.4

To test these hypotheses I compared the snail fauna in matched pairs of old forestsand young forests, regenerating after clear-cutting about 40-60 years ago. The studyplots were situated in the boreal region, in riparian forests along small streams.Materials and methodsStudy areaThe study was performed in ten localities in the northern part of Sweden, in thecounties of Västernorrland, Västerbotten and Norrbotten from 63° 42′to 65° 67′N(Fig. 1). The landscape has a bedrock that is mostly composed of acidic gneisses andgranites and the vegetation is dominated of Vaccinium myrtillus–type. The matureforest consists mainly of Norway spruce (Picea abies) and Scots pine (Pinussylvestris) with some deciduous elements such as birch (Betula pubescens and B.pendula), aspen (Populus tremula) and rowan (Sorbus aucuparia). In the riparianzones deciduous trees are more common and also include grey alder (Alnus incana)and willow (Salix sp.). The field layer has more deciduous shrubs, herbs andgraminoids than the upland areas.1. Herrbergsliden2. Långrumpskogen3. Blaikfjället södra4. Stenbitshöjden5. Brånaberget6. Lillberget7. Blaikfjället norra8. Buberget9. Hällbergsträsk10. SpjutbergetFigure 1. The ten localities in northern Sweden.6

ProductiveTheCompletelyA▪NoSimilar▪▪ ▪ ▪ ▪1mforest land, i.e. 3 trunk volume (above stump height) potentiallyproduced per ha and year, including the bark.stream must not have been directly affected by draining operations.≥In the surrounding landscape of each old forest plot a matched young forest plot werechosen by the criteria:clear-cut 40-60 years ago (no trees left).stream similar in size (catchment area) and slope to the old forest stream.drainage operationsto the old forest plot in topography, amount of rocks and boulders andproductivity.In the middle of each study plot a transect of 20 x 5-m was established across thestream (Figure 3). The transect was used in the snail sampling method, but also toestimate some of the environmental variables.5 m= sample20 mFigure 3. Transect across thestream with examples of how thelitter samples were collected.Environmental variablesThe slope from the stream to the edge of the transect was measured on both sidesusing a clinometer. The basal area of trees was measured from the middle of thetransect, using a relascope, for Norway spruce, Scots pine and deciduous treesrespectively. The following variables were estimated by eye, as percent cover of thetransect area (100 m 2 ): wet, moist, mesic and dry ground (Hägglund and Lundmark1981), boulders, deciduous and conifer shrubs less than 2 m high, herbs, graminoids,dwarf shrubs, peat mosses (Sphagnum spp), and other bryophytes. The aspect of slopewas taken from maps. The other environmental variables were collected by Dynesiuset al (2001). Altitude (m. a. s. l.) taken from maps, longitudinal slope (m/50m)measured using level and rod and a measuring tape and average channel width (m)measured at five places along a 50-m stretch. The transect used in this study wasplaced in the middle of each 50-m stretch.Snail samplingThe sampling method used to collect land snails, was a modified version of the onedescribed by Valovirta (1996). It is a litter based semiquantitative method, where landsnails are picked out from sieved litter. The litter was collected between July 14 th andAug 6 th 2002. The two plots of a pair were always sampled on the same day, theyoung forest plot first and the old about 3 hours later. Each sampling took less thenone hour. The 20 x 5-m transect was established across the stream in the middle ofeach study plot. Each transect was divided into 20 sample squares of 2 x 2.5 m (Fig8

3). From every other square four litter samples, each of 1.25 dm 2 , were collectedtotalling 0.5 m 2 from the entire transect. The samples were subjectively chosen amongmoister places, e.g. close to boulders, tree trunks or in cavities, to get as large numberof snails as possible. The litter from each 2 x 2.5 square was then sieved in 50 secondstrough a 4-mm mesh and mixed with the other 9 samples from the whole transect.Snail extractionAfter drying the litter in newspaper boxes at room temperature, the volume wasmeasured. From each site 1,5 l of the dried litter was passed trough a set of sieves(0.5, 1.0, 2.0, 4.0 mm), dividing it into four fractions. The snails were extracted fromeach fraction by hand under a magnifying glass (2x) and then identified to speciesunder a dissecting microscope. The empty shells (i.e. those that had no living snails atthe field sampling) were included in the study. The nomenclature follows Falkner etal. (2001). All young specimens of Coclicopa and Discus were treated as C. lubricaand D. ruderatus respectively, since they were the only adult species recorded of thesegenera. The three young specimens of Collumella were treated as C. aspera, as it wasthe only adult species of the genus in that site. The other young specimens that couldnot be satisfactorily identified (Vertiginidae, Vertigo and Euconulus) were notincluded in the analyses of the abundance of individual species. In analyses of numberof species, they were included as a species when there were no other individual in thegroup that could be identified to species. Euconulus praticola is grouped together withE. trochiformis under the name E. praticola.Litter pHThe litter pH was determined using the method of Karltun (Internet publication).From each plot, the dried litter fractions, from which the snails had been extracted,were mixed together. Two samples of 2g litter each where taken from each plot. Thesamples were put in 50-ml tubes and distilled water was added up to the 25-ml mark.After shaking the closed tubes, they were left over night. Next morning the sampleswere analysed with a calibrated pH-meter, Mettler toledo MX300. The samples weremeasured two times each to detect possible electrode drift. The average pH values ofthe two measurements of the two samples were calculated and used in the analyses.Data analysisMost analyses were performed using the statistical package SPSS (Norušis 1999).Abundance and species richnessIn the sampling method a specific litter amount from each plot is generally used, butin this study the total volume of sieved litter was higher in the old forests of all plotpairs. If the reason for this were that the litter from the old forest plots passed thesieve more easily due to the litter composition, it would be better to use an area-basedmethod. Therefore, besides the 1.5-l sub-samples, abundance and species richnesswere calculated for the total sample of each plot. The calculated abundance of theentire sample was calculated by multiplying the sub-sample snail density with totalvolume litter. To estimate species richness I started with a computer re-samplingprocedure in a spreadsheet. Individuals were randomly drawn from the distribution ofspecies in the 1.5-l sub-sample. The number of species after sampling e.g. 10, 20, 50and 80 individuals were noted and average values from 200 simulations werecalculated. With the logarithmic values of individuals as independent and average9

number of species as dependent variable in an extrapolated linear regression, thespecies richness of the total sample could be calculated.To detect significant differences between the forest types in snail abundance andspecies richness t-tests were performed. Logarithmic values of abundance were usedin the analysis to meet the assumptions of a normal distribution.Environmental variablesThe environmental variables that are potentially affected by clear-cutting werecompared between stand age with a non-parametric test; Wilcoxon paired test. Meanvalues and standard deviation were calculated for all variables. For pH, two pairs ofplots were excluded from this analysis. The young forest plot in Långrumpskogen hadmineral soil in the litter sample, which probably affected the pH value and would alsoaffect this result. Hällbergsträsk was excluded because of the bad pair matchregarding pH conditions. The old forest plot was placed on a more calcareous groundthan the young forest plot and would in a comparison between plots bias the result. Tosee if snail abundance or species richness correlated with any of the environmentalvariables, Spearman’s rank correlation tests were performed. The pH value from theyoung forest plot in Långrumpskogen was excluded in this analysis of the samereason as in the paired test. The old forest plot in Hällbergsträsk was used, as pH wascompared with snail variables and not other pH values.Species compositionThe change in species composition between old and young forests was analysed withpartial Redundancy Analysis (pRDA) in the statistical package CANOCO (ter Braakand Prentice 1988). This ordination method assumes linear relationships over theordination axes. Old or young forest was used as the constraining environmentalvariable assuming either a positive or negative species response to the variable. Thestudy design with paired plots made it possible to set the plot identities as covariables.This eliminates the variance between pairs and was done by making adummy variable for each plot. To test if species composition was significantlydifferent between old and young forests a permutation test was made in the pRDA.The null hypothesis of no influence of the constrained axes on the speciescomposition was evaluated with 2000 permutations. The test was performed both withspecimens found in 1.5l and the calculated number of specimens in the total samples.SpeciesThe difference in abundance of individual species between old and young plots wastested using Wilcoxon´s non-parametric paired test. The mean values and standarddeviations of each species were calculated. In these analyses number of replicatesvaried among species. Only localities where the specific species had been recorded inat least one of the plots were included in the analyses.With the pRDA analysis it is possible to see for each species the bias in occurrencetowards old or young forests. Each species get a position along the axis of theconstraining environmental variable, where a strong positive or negative valuerepresents a combination of abundance and frequency that is higher in one of theforest types and a value around zero indicates no larger difference.10

ResultsCommunity levelIn total 1 896 shells belonging to 16 snail species were found in the 20 1.5l littersamples. The number of species per sample ranged from 2 to 14 and the abundanceranged from 6 to 344 individuals. The old forest plots had a total number of 556 shellsof 14 species in 1,5-l litter, with a range of 6 to 139 shells and 2 to 10 species. Theyoung plots had 1 340 shells of 15 species. The number of shells ranged from 25 to344 and the number of species from 3 to14 (Table 2).AbundanceIn the 1.5-l litter samples young forest plots had more individuals in 9 of 10 pairs(Table 2). The average abundance was more then twice as high in the young plots,134 shells in the young plots and 55.6 in the old plots. The total volumes of sievedlitter from each transect varied a lot. The largest difference in a pair was 1.2 l and itwas always the old forests that had the larger volume. Still the use of calculatednumbers of individuals in the total sample did not change the qualitative result much.The exception was Herrbergsliden that obtained a slightly higher abundance in the oldforest plot than in the young, compared to the number in 1.5 l that was higher in theyoung forest plot. In the paired tests of difference between old and young forest plots,significantly higher snail abundance was found in the young forests, both for the 1.5-lsub-sample and for the calculated abundance in the total sample (Table 3).Table 2. Litter volume of the total sample, snail abundance and number of species in each of the plots.O = old forest plots, Y = young forest plots.Volume littertotal sample(l)Abundance1.5 lAbundancetotal sample aNo. ofspecies1.5 lNo. ofspeciestotal sample aLocalities O Y O Y O Y O Y O YHerrbergsliden 3.60 2.85 119 149 286 283 10 9 11.2 9.39Långrumpskogen 3.20 2.90 6 25 12.8 48.3 3 3 3.28 3.00Blaikfjället södra 2.75 1.55 25 104 45.8 107 5 11 5.33 10.8Stenbitshöjden 2.20 1.95 21 157 30.8 204 4 11 4.46 11.7Brånaberget 3.05 2.40 96 225 195 360 9 11 11.0 12.4Lillberget 2.90 2.05 139 61 269 83.4 10 7 11.1 7.79Blaikfjället norra 2.65 1.75 31 344 54.8 401 6 14 5.62 15.1Buberget 3.75 2.60 28 56 70.0 97.1 7 8 7.75 8.22Hällbergsträsk 3.90 3.80 79 107 205 271 8 11 9.38 13.3Spjutberget 3.85 2.95 12 112 30.8 220 2 11 2.30 12.1a Calculated from the data from 1.5 l and the total amount of litter in the sample.Species richnessA significant difference between old and young forest plots was found in the speciesrichness in 1.5 l, while the calculated number of species in the entire sample was closeto significant, p = 0.055 (Table 3). In 7 out of the 10 pairs, higher species richnesswas found in the young plots (1.5-l samples, table 2). Of the other three pairs, one hadthe same number of species and two pairs had higher richness in the old forest plots.In the calculated species richness the pair with three species each, Långrumpskogen,got a slightly higher number of species in the old forest plot than in the young. Inaverage there was about 3 more species in the young forest plots, 9.6 species in theyoung plots and 6.4 species in the old plots.11

Environmental variablesSome of the measured environmental variables that may be affected by clear-cuttingdiffered significantly between old and young forests (Table 3). Basal area of spruce,total basal area of all trees and coverage of Sphagnum were all higher in the old plots.Both pH and coverage of graminoids were significantly higher in the young plots.Table 3. Means and standard deviation of snail abundance, species richness and environmentalvariables (potentially affected by clear-cut) in old and young plots. Bold text indicates statisticallysignificant difference between forest types (Snail abundance and species richness paired t-test,environmental variables Wilcoxon´s paired test). N = 10 except for pH where N = 8.Snail and environmental bOld forest plots Young forest plotsvariables Mean S. D. Mean S. D. PAbundance 1.5 l 55.6 48.3 134 93.4 0.009Ab. Calculated a 120 107 208 122 0.033Species number 1.5 l 6.40 2.88 9.60 3.03 0.036Sp. number Calculated a 7.14 3.39 10.4 3.44 0.055Litter pH 4.71 0.21 4.99 0.23 0.025Dry ground (%) 1.00 3.16 1.00 3.16 1.000Mesic ground (%) 55.0 30.2 60.0 27.6 0.511Moist ground (%) 22.0 24.7 24.0 16.1 0.812Wet ground (%) 22.0 23.9 15.0 22.1 0.446Spruce (m 2 /ha) 14.5 7.85 5.20 3.05 0.007Pine (m 2 /ha) 2.00 2.84 1.15 1.75 0.598Deciduous trees (m 2 /ha) 3.40 2.90 4.15 3.87 0.767Trees total (m 2 /ha) 19.9 7.52 10.5 4.93 0.005Conifer shrubs (%) 2.70 3.43 4.00 5.68 0.750Deciduous shrubs (%) 10.0 8.50 17.0 21.4 0.478Herbs (%) 24.0 15.8 32.0 20.4 0.113Graminoids (%) 23.0 18.7 38.0 20.3 0.050Dwarf shrubs (%) 31.5 21.1 26.5 19.6 0.258Sphagnum spp. (%) 33.0 25.9 12.5 15.9 0.046Other bryophytes (%) 27.5 26.6 40.0 22.1 0.140a Calculated from the data from 1.5 l and the total amount of litter in the sample.b Percentages are cover of the 100 m 2 transect and the trees are given in basal area.Correlations between environmental variables and the snail variables showed a fewstatistically significant results but these had high correlation coefficients (Table 4,figure 4). In the old forest snail parameters had positive correlations with percentagemoist ground. Calculated abundance also correlated with pH in the old plots but not asstrongly. The snail abundance in 1.5 l were almost significantly correlated with pH, p= 0.0537. The young forest plots had significant positive correlations between all foursnail parameters and pH. The abundance variables were strongest correlated with pH.The calculated number of species was negatively correlated with basal area of spruce.Species compositionThe partial redundancy analysis (pRDA) revealed a significant (p = 0.027) differencein species composition between old and young forests. When the specimens found inthe 1.5-l sub-sample was used the forest types could account for 20.4 % of thedifference in the data. When the calculated number of specimens in the total sampleswas used the stand age could explain 11.2 % of the difference (p = 0.044).12

Table 4. Spearman’s rank correlation coefficients between environmental variables and the snailparameters. Figures in bold indicate statistically significant correlations, ** p < 0.01 and * 0.01 < p >0.05. O = old forest plots, Y = young forest plots. N = 10 except for pH in young forests where N = 9.Abundance1.5 lAbundancetotal sample aNo. of Species1.5 lNo. of Speciestotal sample aEnvironmental b variables O Y O Y O Y O YLitter pH 0.62 0.95** 0.65* 0.93** 0.57 0.68* 0.58 0.70*Lateral slope (degrees) -0.61 0.27 -0.51 0.10 -0.54 0.10 -0.52 -0.02Boulders (%) -0.17 0.16 -0.05 0.33 -0.17 0.01 -0.15 0.15Dry ground (%) -0.06 -0.41 0.06 -0.29 0.06 -0.31 0.06 -0.29Mesic ground (%) -0.30 0.26 -0.21 0.24 -0.35 0.12 -0.35 0.16Moist ground (%) 0.94** -0.47 0.91** -0.47 0.89** -0.39 0.89** -0.43Wet ground (%) -0.35 0.14 -0.44 0.05 -0.31 0.04 -0.31 -0.01Spruce (m 2 /ha) 0.13 -0.34 0.05 -0.46 0.20 -0.48 0.24 -0.64*Pine (m 2 /ha) -0.53 -0.32 -0.37 -0.29 -0.45 -0.35 -0.45 -0.26Deciduous trees (m 2 /ha) -0.12 0.16 -0.07 0.19 -0.09 -0.12 -0.12 -0.05Trees total (m 2 /ha) 0.03 -0.19 0.02 -0.27 0.11 -0.54 0.14 -0.55Conifer shrubs (%) -0.09 0.25 0.03 0.05 0.00 0.15 -0.03 0.12Deciduous shrubs (%) 0.08 0.25 0.15 0.28 0.11 0.62 0.11 0.58Herbs (%) 0.27 0.42 0.21 0.31 0.28 0.08 0.31 -0.01Graminoids (%) 0.06 -0.42 0.01 -0.48 -0.03 -0.18 -0.08 -0.12Dwarf shrubs (%) -0.02 0.17 0.05 0.33 -0.04 0.43 -0.02 0.46Sphagnum spp. (%) -0.05 -0.01 -0.25 0.01 -0.08 0.18 -0.13 0.16Other bryophytes (%) -0.04 -0.31 0.09 -0.42 -0.05 -0.12 -0.04 -0.24a Calculated from the data from 1.5 l and the total amount of litter in the sample.b Percentages are cover of the 100 m 2 transect and the trees are given in basal area.20.015.010.05.00.04 4.5 5 5.5Litter pH50040030020010004 4.5 5 5.5Litter pHOldYoungReg. OldReg. Young20.015.010.05.00.00 20 40 60 80Moist ground cover %50040030020010000 20 40 60 80Moist ground cover %20.015.010.05.00.00 10 20 30Basal area of spruceFigure 4. Selected correlations between snail andenvironmental variables. Only data calculated for thetotal sample was used. Statistically significantcorrelations are shown with a linear regression (excel).Left plots: correlations between calculated speciesrichness and pH (R 2 = 0.56 young), percentage moistground (R 2 = 0.53 old) and with basal area of spruce (R 2 =0.50 young). Right plots: correlations between calculatedabundance and pH (R 2 = 0.22 old, 0.90 young) and withpercentage moist ground (R 2 = 0.73 old).13

Species levelOf the 16 species found, 14 species had a higher abundance in the young forest plotsin the 1.5-l sub-samples. There were 6 species with statistically significant differencesin abundance between old and young forest plots, all of them were more abundant inthe young plots: Cochlicopa lubrica, Euconulus fulvus, Nesovitrea hammonis,Punctum pygmaeum, Vitrina pellucida and Zoogenetes harpa (Table 5). Mostsignificant (p = 0.007) was the difference of E. fulvus, which were found in all 20plots and was also the most abundant species in both old and young forests(13.6/33.8). In the same test of E. fulvus, but with abundance calculated for the totalsample, the difference was still significant but not as strongly (p = 0.028). Mostevident was the difference of C. lubrica, which had a ratio of 31.5 (Table 5). P.pygmaeum had almost three times higher mean abundance in the young forest plots(ratio 2.78) but the standard deviation was very large. Four species were found in atleast one of the pairs in all ten localities; Discus ruderatus, Euconulus fulvus,Nesovitrea hammonis and Punctum pygmaeum. Two species were only found in theyoung forests, Vertigo lilljeborgi and Zoogenetes harpa, and one species Clauciliacruciata were only found in two old forests. Vertigo ronnebyensis was also foundwith more individuals in the old plots.Table 5. The mean species abundance in 1.5-l litter samples from old and young plots. F = frequency,number of plots where the different species were encountered. Bold names and letters indicate specieswith statistically significant differences (Wilcoxon´s test).Old forest plotsYoung forest plotsSpeciesMean aMean aRatioF (Range) S. D. F (Range) S. D. Y/O p bClausilia cruciata 2 2.00 (1-3) 1.41 0 0 (0-0) 0 0Cochlicopa lubrica 2 0.22 (0-1) 0.44 7 7.00 (0-28) 11.1 31.5 0.036Columella aspera 4 2.38 (0-12) 4.24 6 4.13 (0-12) 5.00 1.74 0.206Columella edentula 4 3.57 (0-16) 5.88 5 8.14 (0-36) 12.6 2.28 0.735Discus ruderatus 5 3.10 (0-12) 4.65 8 7.80 (0-23) 6.55 2.52 0.113Euconulus praticola 2 0.71 (0-4) 1.50 7 1.86 (1-4) 1.21 2.60 0.084Euconulus fulvus 10 13.6 (3-36) 10.3 10 33.8 (11-52) 14.0 2.49 0.007Nesovitrea hammonis 9 6.10 (0-20) 6.14 10 13.0 (1-22) 6.09 2.13 0.032Nesovitrea petronella 7 8.44 (0-28) 9.29 9 12.0 (2-28) 8.73 1.42 0.407Punctum pygmaeum 6 10.3 (0-33) 12.7 10 28.6 (1-68) 26.8 2.78 0.047Vertigo lilljeborgi 0 0 (0-0) 1 3.00 (3-3)Vertigo modesta arctica 1 0.50 (0-1) 0.71 2 6.50 (3-10) 4.95 13.0Vertigo ronnebyensis 7 5.00 (0-13) 4.87 4 2.75 (0-13) 4.53 0.55 0.325Vertigo substriata 1 3.60 (0-18) 8.05 5 7.80 (1-22) 8.35 2.17 0.345Vitrina pellucida 4 1.00 (0-3) 1.10 6 6.50 (3-11) 2.66 6.50 0.026Zoogenetes harpa 0 0 (0-0) 0 6 4.50 (2-8) 2.59 0.027a Means are calculated from the pairs where the species have been recorded in at least one of the plots.b The p values of species found in fewer than 3 plot pairs are not displayed. Remaining values have Nbetween 5 and 10.Species scores from the pRDA confirmed that the two species, Vertigo ronnebyensisand Claucilia cruciata, were the only species on the negative side of the axis (higherabundance combined with frequency in old forests) while the rest were on the positive(higher values in young forests). The species with scores around zero had about thesame presence in both forest types (Figure 5). Most favoured by young forests werethe same species that had significant differences in the paired test, but also Discusruderatus had high scores.14

0.800.600.400.200.00-0.20* * * * * *1.5lTotal sample-0.40E. fulvusZ. harpaV. pellucidaC. lubricaD. ruderatusP. pygmaeumN. hammonisV. modestaE. praticolaV. lilljeborgiC. edentulaV. substriataC. asperaN. petronellaV. ronnebyensisC. cruciataFigure 5. Species scores from the partial redundancy analysis (pRDA). The scores are calculated bothfrom specimens found in the 1.5-l sub-samples and from the calculated number of specimens in thetotal sample. Species that have significant differences in abundance between the stand ages(Wilcoxon´s paired test, table 5) are denoted *.DiscussionThe results showed a difference in abundance, species numbers and speciescomposition of litter dwelling land snails between old and regenerating small streamforests. According to my hypothesis most species had higher abundance in youngforests, which indicates that populations do grow after the initial reduction after clearcutting.However, different species reacted differently. Why are land snails moreabundant and species rich in young forests?Community levelAbundanceMoisture, sun exposure, vegetation, calcium content and pH influence the occurrenceof land snails (Wäreborn 1969, 1982). The abundance in the young plots correlatedvery well with litter pH (r = 0.93, calculated abundance in the total sample), whichshows that pH is important for the abundance in these areas (Table 4, figure 4).However, it is not clear to what extent pH has a positive effect on snails. Severalstudies show that density and species richness correlates both with amount of calciumand with the litter pH (Burch 1955, Waldén 1981, Gärdenfors 1992, Hotopp 2002).Close positive correlations have also been found between the calcium content, pH andbase saturation of the litter (Waldén et al. 1991), which makes it difficult to sort outwhich of the variables that are effecting the snail abundance most (Gärdenfors et al.1996). In some studies pH have been used as a substitute for the complex of calcium,pH and base saturation (Waldén 1981). Wäreborn (1979) has studied pH and calcium15

effects on the reproduction separately. An increase of pH increased the snailreproduction, but addition of calcium had a larger positive influence.The old plots also had a significant correlation between calculated abundance in totalsample and pH (r = 0.65), but a stronger correlation with cover of moist ground (r =0.91). As the snail abundance is dependent of litter pH (Figure 4), and pH (but notcover of moist ground) significantly differs between the stand ages (Table 3), it islikely that this difference is the main cause for the difference in snail abundancebetween old and young forests. A more accurate measuring method is used of litterpH compared to the visual estimations of ground moisture, but it is clear that both ofthese variables are very important in boreal regionsSpecies richnessThe results for species richness were very similar to those for abundance patterns.Species richness in young forest plots correlated significantly with litter pH (r = 0.7)and in the old forest plots with moist ground (r = 0.89). Species richness in the oldforests did not correlate significantly with pH but the trend was positive (Table 4,figure 4). Young forests have in general high pH due to decomposition of loggingresidues, more herbs and deciduous trees and shrubs, which also is good food for mostsnails. They also have fewer trees, especially spruce, which decrease the soil pH withion exchange as they grow and the litter pH with the dropped needles. In this study theold forests had both significantly higher basal area of trees in total and of spruce(Table 3). The calculated species richness in young forests had a significant negativecorrelation with the basal area of spruce (r = -0.64). In a regression the basal area ofspruce accounted for 50 % of the variation in species richness in young forests.Other environmental variablesAnother significant difference between the forest ages was cover of graminoids. Theyincrease in disturbed areas and produce a lot of root-litter that aerates the top soil layer(Lundmark 1986:1) and this might create a better habitat for the snails. However, thisis not supported by my results. Even though there was variations in graminoid cover,there were no correlations to the snail variables. Litter from deciduous trees isimportant to gastropods. In the boreal forest landscape correlations between snails anddeciduous trees have been recorded (Göthner 2002) and aspen is especially importantto snail species richness and diversity (Suominen 2002). I did not find any significantdifferences or correlations in deciduous trees, probably because of the low numbers inall plots. It may also be difficult to assess the amount of deciduous leaves in the litterfrom the measurements of basal area and estimates of cover of shrubs.Species levelThe species Columella aspera and Vertigo ronnebyensis are described in the literatureas preferring old spruce or mixed forests and comparatively low litter pH (VonProschwitz 1993). They are both associated with Vaccinium spp., on which they areoften found feeding. Hylander et al. (in press) found a significant decrease of C.aspera initially after clear cutting, but I found more specimens of C. aspera in theyoung plots (19/33). In this study the only species with higher abundance in old forestplots was Vertigo ronnebyensis and Claucilia cruciata.C. cruciata was only recorded in two old forests and only with four specimens, whichmakes it impossible to evaluate the long term clear-cutting effects on that species.16

պ*պ*պ*andHylander et al. (in press) found a significant decrease initially after clear-cut (Table 6)as they found it on 5 sites before but not on any sites after. In Sweden C. cruciata isconfined to the northern part and is often found in old, moist and shady spruce ormixed forests (Ehnström and Waldén 1986). V. ronnebyensis was found in more oldforest plots (7/4) and with almost twice as many specimens in the old forest plots(40/22). However no significant difference in abundance was detected, because two ofthe eight pairs had the opposite result. From the pRDA V. ronnebyensis got a negativevalue, which shows that the combination of abundance and frequency indicates apreference of the old forests (Figure 5). According to the Swedish Red List,(Gärdenfors 2002) Vertigo ronnebyensis is classified as nearly threatened (NT) due toits habitat choice of old spruce-forests and possible sensitivity to clear-cutting.Hylander et al. (in press) did not find any significant difference in abundance of V.ronnebyensis before and initially after clear-cutting (Table 6) but a significantdecrease in number of sites where it was encountered, from 13 to 5.Zoogenetes harpa is another species that often is found climbing in Vaccinium-shrubs(Gärdenfors et al. 1988). Waldén (1998) argued that forest management probablyreduces Z. harpa abundance but he did not find enough specimens to support that. Inthis study, Z. harpa instead showed a significant difference in abundance as 27individuals were found in 6 young forest plots and no one in the old. This result alsogave a clear positive score in the pRDA, indicating young forests as a quality habitatto Z. harpa. Although most of the individuals could have been climbing in theVaccinium-shrubs, there was no significant difference in dwarf shrub-cover betweenthe old and young forest plots. Hylander et al. (in press) did not find enoughspecimens to evaluate Z. harpa.SpeciesInitialresponseLong-termresponseClausilia cruciata ռ*Vertigo ronnebyensisEuconulus praticolaPunctum pygmaeumVertigo lilljeborgiVertigo substriataVitrina pellucidaCochlicopa lubricaColumella aspera ռ*Columella edentula ռ**Discus ruderatus ռ**Euconulus fulvus ռ* պ**Nesovitrea hammonis ռ* պ*Nesovitrea petronella ռ**Zoogenetes harpaVertigo modesta arctica Not foundպ ռTable 6. Summary of species abundancedifferences. From Hylander et al. (in press)an increased abundance initially afterclear-cutting is denoted պ, a decreasedand no difference պռ. Higher abundance inthe young forest plots compared to the oldis denoted lower abundance (longtermresponse). The tests of differenceswas performed with Wilcoxon singed ranktest and the levels of significance are ** p< 0.01 and * 0.01 < p > 0.05. N =10, inHylander et al. N =15.ռ պ ռCochlicopa lubrica had the highest ratio of all species (31.5, table 5) with thesignificant higher abundance in the young plots. It was found in 2 old forest plots andin 7 young forest plots. Two years after clear-cutting it had decreased, but notsignificantly (Table 6) and was found in one site less (7/8). Vitrina pellucida had alsoa high ratio (6.50). It both increased in abundance two years after clear-cut and had asignificantly higher abundance in the young plots. The initial effects on Euconulusfulvus and Nesovitrea hammonis were significant decreases (Hylander et al. in press),but they also showed a significantly higher abundance in my young plots (Table 6).17ռ պ*

They had both high frequencies, E. fulvus was found in all plots in both studies. Someother species that showed a significant initial decrease was e.g. Columella edentula,Discus ruderatus and Nesovitrea petronella. In this study they all had higherabundance in the young forest plots, but they did not differ significantly. Punctumpygmaeum also had significantly higher abundance in my young plots but it wasrather indifferent initially after clear-cutting as it showed an increase in abundance butwas encountered in 2 sites less then before (14 to 12).Many of the snail-species decreased in abundance initially after clear-cuts (Hylanderet al. in press). If this is a general pattern it seems like most of them survive or recoloniseand later increase their populations compared to before the clear-cutting. Sodespite the problem with surviving the first environment change of clear cut, myresult show that snails are more abundant in the regenerating forest, with theexceptions of V. ronnebyensis and probably C. cruciata.Scale issuesIn a review article about invertebrates and boreal forest management, Niemelä (1997)concluded that on a small scale, managed forests are not necessarily less species richthan old growth forests but that certain specialist species of old growth forests still donot survive in the managed forests. Intensive large-scale forestry seems to decreasediversity at landscape and bio-geographical scales. More species were found in theyoung plots in 7 of 10 pairs, which shows that on the scale of this study, youngriparian forests are more species rich than old forests. This result is not possible toextrapolate to a larger landscape scale due to the small samples. Even though a certainspecies was not found in the sample it can be present at the site, especially when thenumbers are low.In boreal riparian forests there are often large differences in abundance of individualland snail species and the variations are positively correlated with severalenvironmental variables, such as pH, deciduous trees and moist ground (Hylander etal. unpublished). Thus, most species are favoured by a good quality habitat, and insuch sites even the most specialised species can build up a viable population(Hylander et al. unpublished). In my study number of species seems to depend on thenumber of specimens (Figure 6) and the curves of old and young plots are similar. Ifall snails from the different forest types are summarised there are totally only onespecies more found in the young forest plots (14/15). Still for species like Vertigoronnebyensis (Hylander et al. unpublished, Waldén 1998) it may be important thatareas of old spruce forests remains, even though it was found in some regeneratingforests.20.0Species15.010.05.00.00 200 400 600SpecimensOldYoungLog. OldLog. YoungFigure 6. The relation betweennumber of species and number ofspecimens in all 20 plots.18

Survival, dispersal and population reboundAccording to Hylander et al. (in press) abundance and number of snail species wereunaffected in wet clear-cut sites where the bryophytes survived in a high proportionand protected the underlying litter from direct sunlight. The wet forests acted asrefugia. The land snails have aggregation behaviours to e.g. moisture and temperature(Mason 1970, Baur and Baur 1988) and riparian forests should have a higher numberof moist micro-refugia compared with non-riparian forests, increasing the survival ofthe snails. Another factor that increases the survival of land snails is buffer stripsalong the streams at clear-cut sites. Buffers of 10-m help the survival, but are toonarrow to retain an unaffected snail community (Hylander et al. in press). Thedifference in moisture may be one of the reasons for different opinions about if landsnails in general are sensitive to clear-cutting.Shikov (1984) also argues that micro refugia permits survival of small isolatedpopulations while a secondary deciduous forest with more favourable conditions forthe land molluscs establishes. Shikov (1984) suggests that clear-cutting of primaryconiferous forests have little effect on the diversity of the mollusc-fauna. The densityof most molluscan species increases, but a few species adapted to life in the litterlayers of coniferous forests are reduced in abundance. Unfortunately no data ispresented in the paperStrayer et al. (1986) claims that distance to older forests is important. They suggestedthat in their study the gastropod communities rapidly recovered from forestdisturbance because of the small areas of disturbed sites and a rapid recovery of thevegetation, which would help a re-colonisation. They found no clear relationshipbetween time elapsed since disturbance (2->60 years) and snail density, speciesrichness or composition of gastropod community, only depressed densities andspecies richness in stands

OnTheTheMostThey▪ ▪ ▪Vertigo▪ ▪ ▪since the samples in a pair where collected the same day. When the litter quality is ofvarious kinds and an area based method is wanted, number of species and specimensshould be extracted from the total amount of sieved litter. I used 1,5-l litter from eachplot, so the number of snails from the old forest plots may be underestimated. Tryingto avoid this problem the abundance and species richness was extrapolated to the totalsample for each plot. However, there was no qualitative difference between the resultsof extracted and extrapolated values.The abundance of all species in the total sample was calculated by multiplying thesub-sample snail density with total volume litter. The same method was used tocalculate abundance of each species in the total samples, and these values were usedin the pRDA (Figure 5). The results from the 1.5-l sub-samples tend to indicate a toohigh abundance in young forests compared to old forests, as the total litter sample inold forests was higher. When the calculated values of the total samples are used allspecies show a lower abundance in young forests, which should be a more area basedresult. However, it is not completely correct. Only Euconulus fulvus was found in allplots. When a species was not found in a sub-sample, the number of specimensremained zero in the total sample and the abundance was underestimated. Since thereare more zeroes from old forest plots than from young (96/64), the result may stillhave a slight bias towards higher abundances in the young forest. Summarising thecalculated abundance of each individual species in the old forest plots and comparingit with calculated total abundance of all land snails in the old forest plots, 6.0 % (68.3specimens) are missing due to the zeroes. In the young forest plots 6.2 % (120.5specimens) are missing. The pRDA scores are based on both abundance andfrequency and with more individuals included some species would also appear inmore plots. If mean species richness is compared between the 1.5-l sub-samples andthe calculated total samples, old forest plots would increase with 11.6 % (0.74species) and young plots with 8.3 % (0.8 species). Thus, the error is probably notaffecting the species scores much, especially not the more abundant species.Conclusionsthe scale of this study regenerating riparian forests, 40 to 60 years after clearcutting,have a higher abundance of land nails and are more species rich than oldforests of at least 150 years.higher abundance and species richness are mainly associated with the higherlitter pH in young forests.species composition is different between the old and young forests.species are favoured by the habitat quality in regenerating riparian forests.have a higher abundance in the regenerating forests than in the old, eventhough many of them decrease initially after clear-cutting of riparian forests(Hylander et al. in press).ronnebyensis is more abundant in old riparian forests, but not with asignificant difference.Future studiesWith a higher number of pairs the result would probably give more straight answersabout how the different species react to clear-cutting. More species would showsignificant differences between stand ages, but it would also be clear which speciesthat are indifferent. To understand the mechanisms that create the results experiments20

have to be performed. How is pH and calcium affecting snails separately, and how isground moisture affecting the snail community?It would be interesting to do a similar study in non-riparian areas, to find out to whatextent land snails re-establish after clear-cutting there? What does the non-riparianspecies composition look like after clear-cutting compared to matching old areas andthese riparian clear-cuts? Are other environmental variables important there? Are theinitial survival smaller due to less moist micro-refugia and how do this affect the reestablishmentduring a longer time period?To know how the size of clear-cuts and environmental variables affects the reestablishmentof snails and other litter dwelling invertebrates, would be an importanthelp to maintain biological diversity and preserve threatened species.AcknowledgementsFirst I would like to thank my supervisor Mats Dynesius for guiding me in my writingand in my train of thoughts, for ideas, patience and support. I want to thank KristofferHylander who also has acted as a supervisor, for helping me with statistics andreferences and having the time and patience to answer all kinds of questions. EvaHylander, for introducing me to these beautiful animals and checking my attempt ofidentification. My dear friend Kicki Lindström, who gave me invaluable support.Thank you for sorting situations out on hot days and in hungry moments, keeping myspirits up and making things happen! Gunnar Öhlund for taking time to read mytheses and contributing with comments. I also want to thank everyone in theLandscape Ecology Group for answers, ideas and a really nice time.Thank you Emilia Scherlund and Anna Lagerström, a big hug.Literature citedAbbot, R. T. 1989. Compendium of land shells- a color guide to more than 2,000 of the world’sterrestrial shells. American Malacologist, Inc. Melbourne.Andersson, A. 1996. Landmollusker i jämtländska nyckelbiotoper. Skogsstyrelsen. Rapport 3.Barker, G. M. 2001. Gastropods on land: Phylogeny, Diversity and Adaptive Morphology. Pages 1-146in G. M. Baker, editor. The biology of terrestrial molluscs. CABI Publisher, Oxon, UnitedKingdom.Baur, A. and B. Baur. 1988. Individual Movement Patterns of the Minute Land Snail Punctumpygmaeum. The Veliger. 30(4): 372-376.Baur, B. and J. Bengtsson. 1987. Colonizing ability in land snails on Baltic uplift archipelagos. Journalof Biogeography. 14:329-341.Boag, D. A. 1985. Microdistribution of three genera of small terrestrial snails (Stylomatophora:Pulmonata). - Can. J. Zool. 63: 1089-1095.Burch, J. B. 1955. Some ecological factors of the soil affecting the distribution and abundance of landsnails in eastern Virginia. The Nautilus. 69(2): 62- 69.21

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