VENUS 77 (1–4): 15–26, 2019 DOI: http://doi.org/10.18941/venus.77.1-4_15Influence of Deforestation on Land Snail Fauna©The Malacological Society of Japan15

The Influence of Deforestation on the Land Snail Fauna of Kuromatsunai District, Southwestern Hokkaido, Japan

Yuta Morii1,2* 1Department of Forest Science, Research Faculty of Agriculture, Hokkaido University, Kita-9-Jo, Nishi-9-Chome, Sapporo, Hokkaido 0608589, Japan 2Ecology Group, School of Agriculture and Environment, Massey University, Private Bag 11-222, Palmerston North 4410, New Zealand

Abstract: Forest clear-cutting and the subsequent habitat fragmentation causes catastrophic damage to plant and animal communities as well as the entire forest ecosystem. Many studies have suggested that forest disturbance decreases the biomass and species richness in forests. However, the long-term influence of deforestation on local-scale patterns of diversity is poorly understood. I investigated the land snail fauna in the soil of primary and secondary forests and compared the number of individuals and species richness in each. Two primary forests and two secondary forests were surveyed in Kuromatsunai District (Hokkaido, Japan), and they were all dominated by the Japanese beech tree, Fagus crenata. Six soil blocks (50 cm × 50 cm) were sampled from each forest, and all the land snails from each soil block were collected and identified. The number of individuals and species richness were subsequently compared between the primary and secondary forests. A significantly larger number of individuals and significantly greater species richness were recorded in the primary forests (generalized linear mixed models (GLMMs), likelihood ratio tests, P < 0.05). The diversity of the land snail fauna in one of the two primary forests, Utasai Forest, was particularly high, with an average of 239.2 individuals and 7.2 species per soil block. In contrast, only 12.3 individuals and 4.8 species, on average, were detected in the two secondary forests. In addition, the number of individuals of smaller species (2.0 mm or less) was significantly lower in the secondary forests, but that of the larger species (greater than 2.0 mm) was not. I also estimated the age of the two secondary forests using an increment borer and found that both secondary forests were cleared approximately 100–150 years ago. My results imply that the deforestation of more than 100 years ago continues to impact the land snail fauna, thus affecting the soil fauna of the forest.

Keywords: Terrestrial molluscs, primary forest, intact forest, clearance, clear-felling, Japanese beech tree, Kuromatsunai Depression

Introduction

Forest clear-cutting and the subsequent habitat fragmentation is one of the most radical landscape changes in forest ecosystems, and it decreases the biomass and species richness in forests (Turner, 1996; Siira-Pietikainen, 2001; Hylander et al., 2004; Lindo & Visser, 2004; Palviainen et al., 2005; Ewers & Didham, 2006; Chiba et al., 2009). However, the long-term influences of deforestation and local extinction on local-scale patterns of diversity are relatively poorly understood (Taylor et al., 2003; Watters et al., 2005; Ewers & Didham, 2006; Graham et al., 2006; Kappes, 2006; Chiba et al., 2009; Ström et al., 2009), as most studies of historical land use have been conducted less than 100 years after a change to the landscape (Ewers & Didham, 2006).

* Corresponding author: [email protected] 16 Y. Morii

Several studies suggest that there is a time lag between environmental change and a species-level effect (Hanski & Ovaskainen, 2002; Chiba et al., 2006) and that different species and habitats vary in their responses to habitat destruction and human intervention (Lindo & Visser, 2004; Gotmark et al., 2008). Terrestrial molluscs are particularly useful taxa for investigating spatial patterns of species diversity and population density as well as the influence of historical land use on these factors because of their habitat preferences, low mobility, high population density and high species richness (Chiba et al., 2006; Gotmark et al., 2006; Kappes, 2006). Moreover, land snails are likely able to persist in smaller habitat patches than other taxa due to their small body sizes and restricted mobility (Kerney & Cameron, 1979; Baur & Baur, 1993). Globally, the total area of primary forests continues to decline, so their value is increasing (Gibson et al., 2011; Morales-Hidalgo et al., 2015). Hokkaido Island, Japan, has been undergoing deforestation since 1869, and almost all primary forests around Kuromatsunai District in southwestern Hokkaido were heavily logged until 1923 (Saito, 2012), but some forests remain intact. Kuromatsunai District is well known as the northernmost distribution limit of the Japanese beech tree, Fagus crenata (Saito, 2006, 2015; Matsui et al., 2012; Kitamura et al., 2015), and the primary forests in this area were mainly dominated by this species (Saito, 2006, 2012, 2015). In particular, Utasai Forest is known as the largest patch of primary forest in this area (92 ha) and was therefore designated a natural monument of Japan in 1928 and named “Utasai buna north limit” (Saito, 2006, 2012, 2015). However, the snail fauna of the primary forests in Kuromatsunai District has not been surveyed. I investigated the land snail fauna in the soil of the primary and secondary forests in this region and here discuss the legacy effect of historical forest disturbances.

Material and Methods

Study sites Two primary forests, Utasai Forest (42.65396°N, 140.32848°E, Alt. 80 m; Site ID: Pri1) and Shiroikawa Forest (42.70482°N, 140.39041°E, Alt. 180 m; Site ID: Pri2), and two secondary forests, Soibetsu Forest (42.68711°N, 140.26756°E, Alt. 50 m; Site ID: Sec1) and Shimo- choposhinai Forest (42.69782°N, 140.35242°E, Alt. 210 m; Site ID: Sec2) were surveyed. All are in Kuromatsunai District (Hokkaido, Japan) and are dominated by F. crenata (Figure 1). The land spaces of two primary forests, Pri1 and Pri2 are 92 ha and 20 ha, respectively (Saito, 2006, 2012, 2015), but there are younger forests adjacent to these forest patches. Other two research sites, Sec1

Fig. 1. Research sites in Kuromatsunai District, Hokkaido, Japan. Two primary forests, (A) Utasai Forest (Site ID: Pri1) and (B) Shiroikawa Forest (Pri2), and two secondary forests, (C) Soibetsu Forest (Sec1) and (D) Shimo-choposhinai Forest (Sec2). Influence of Deforestation on Land Snail Fauna 17 and Sec2 are also one part of larger forest area in Kuromatsunai District. I tried to choose unbiased geologic feature among research sites; the geological base of the Pri1 and Sec1 is igneous rock, Pri2 and Sec2 is sedimentary rock (Geological Survey of Japan & AIST, 2017).

Quantitative investigation of the snail fauna Six soil blocks (50 cm × 50 cm), which were separated by at least 5.0 m, were collected from each forest from 5 October to 8 November 2015, and all the land snails in each block were sorted by hand and identified. Both live snails and empty shells still at least partly covered by the periostracum were collected and used for the analyses, and the numbers of individuals and species within each soil block (i.e., densities of individuals (DI) and species (DS)) from the primary and secondary forests were subsequently compared. The differences between the DI and DS of the primary and secondary forests were analysed using generalized linear mixed models (GLMMs) with a Poisson distribution and a log link using R version 3.3.2 software (R Development Core Team, 2016). The DI or DS was the response variable, and the forest type (primary or secondary) and site were the fixed- and random-effect explanatory variables, respectively. The influence of forest type on the DI and DS were statistically examined using likelihood ratio tests (LRTs), in which the deviance of the full model was compared with that of a model lacking the explanatory variable of forest type. The shell diameter of each species was based on the descriptions in Azuma (1995). The micro-snail species (those with shell diameters of 2.0 mm or less based on the descriptions in Azuma (1995)), were highly abundant in terms of the total number of individuals collected in this research, and these species seem to occur more in primary forests than secondary forests (Figure 2). To test this hypothesis, I also analysed the differences in the DI and DS of micro-snails, those with shell diameters of 2.0 mm or less (DI-small and DS-small), and larger snails of greater than 2.0 mm (DI-large and DS-large), between primary and secondary forests (Appendix Table S1). Moreover, I also analysed DI and DI-small without

Fig. 2. (A) Proportions of each category of snails based on shell diameter to total number of individuals and species. (B) Proportions of four study sites to total number of individuals for each species. Numerals written with species name are consistent with the species numbers in Table 1. 18 Y. Morii

Carychium pessimum, because C. pessimum was much more abundant in one of the two primary forests, Pri1 (68.2% of total individuals collected from Pri1).

Age estimation of secondary forests Woody materials were sampled in winter from 20 to 24 February 2016 to estimate the age of the two secondary forests. Age estimation of beech trees in this study basically followed Matsui et al. (2012). The girth of all beech trees was measured within a 30-m × 30-m quadrat in each forest, and core samples were collected from 16 of these beech trees in each forest using an increment borer. The number of annual growth rings was counted under a stereoscopic microscope. Linear regressions between girth and the number of annual growth rings were conducted for each secondary forest and were used to estimate the ages of all other beech trees in the quadrats. I did not conduct this age estimation for two primary forests to avoid an adverse environmental impact to the invaluable forests. The snow depth was randomly measured ten times within the 30-m × 30-m quadrats of Sec1 and Sec2 on 20 and 23 February 2016, respectively, to determine the average snow depth for each forest. Girth data and the core samples were collected at a height of 30 cm above the snow depth. To estimate the age at which the beech trees reach a height of 30 cm above the snow depth, the following regression was used:

Age (years) = 0.210 × Height (cm) – 3.031 (r2 = 0.770, P = 0.002; Matsui et al., 2012)

This equation was developed using several juvenile beech trees in the forest near Sec2 in Kuromatsunai District (Matsui et al., 2012).

Results

Quantitative investigation of the snail fauna In total, 17 snail species from 10 families were detected, and significantly higher DI and DS were recorded in the primary forests than in the secondary forests (GLMMs, LRTs, χ2 = 4.3688 and 3.9041, df = 4 and 4, and P = 0.037 and 0.048, respectively). This tendency was consistent with the result of analysis using DI without C. pessimum (GLMMs, LRTs, χ2 = 5.2871, df = 4, and P = 0.021). The density of land snails in one of the two primary forests, Pri1, was particularly high; on average, 239.2 individuals (including 163.2 individuals of C. pessimum) and 7.2 species were collected per 50-cm × 50-cm soil block at this site (Table 1; Figure 3). In contrast, only 12.3 individuals and 4.8 species per soil block, on average, were detected in the two secondary forests (Table 1; Figure 3). 88.2% of the total number of individuals collected in this research represented micro-snail species with shell diameters of 2.0 mm or less, although only five species were included in this category (Figure 2A). These smaller species (i.e., C. pessimum, Palaina pusilla paucicostata, Columella edentula, Punctum atomus and Discoconulus sinapidium) tended to be less abundant in the secondary forests (DI-small and DI-small without C. pessimum, GLMMs, LRTs, χ2 = 6.9057 and 13.052, df = 4 and 4, and P < 0.001 for both, respectively; Table 1; Appendix Table S1, Figure 2B), although DS-small was not significantly different (GLMMs, LRTs, χ2 = 2.4610, df = 4, and P = 0.117; Table 1; Appendix Table S1). 127.9 individuals and 3.7 species were collected, on average, per 50-cm × 50-cm soil block in the two primary forests, but only 3.7 individuals and 1.6 species were detected in the two secondary forests. In contrast, DI-large and DS-large did not differ significantly between the primary and secondary forests (GLMMs, LRTs, χ2 = 0.3078 and 0.0130, df = 4 and 4, and P = 0.579 and 0.909, respectively; Table 1; Appendix Table S1). Influence of Deforestation on Land Snail Fauna 19 ) 2 ( S e c 5 9 3 5 2 7 8 9 2 2 ...... 2 0 0 0 0 0 0 0 0 0

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a 0 0 3 0 1 6 6 0 8 0 4 0 5 0 7 6 5 3 ...... – 1 0 0 1 1 5 5 4 6 6 2 2 6 3 l a n d 0 3 3 9 . 1 9 t h e d i a m e t r w i t h n o f S . E S h e l a n d E . ) editha

i l u s t r a o n gainesi )

) ( S . s n a i l o f ( S . E ) t a x o n m y C o l r e d ( B r a d y b e n i ) v i d u a l s Ainohelix Ezohelix ( ( ) :

a n d s p e c i i n d n u m b e r 1 9 5 o f o f ( s p e c i M . M e a n e r t i g n d a

H e l i c n d a D i p l o m a t n d e P o m a t i p s d e Z o n i t d a e E l o b i d a e H e l i c a r o n d C i o n e l d a D i s c d a e C a m e n i d Hemipoma hakodadiense Palaina pusilla paucicostata Palaina japonica bensoni Blanfordia Retinella radiatula radiata Retinella radiatula Carychium pessimum Carychium Columella edentula Trochochlamys labilis Trochochlamys Punctum atomus Discoconulus sinapidium Nipponochlamys hokkaidonis Nipponochlamys Cochlicopa lubrica Cochlicopa Parakaliella affinis Parakaliella Discus pauper Trochochlamys borealis Trochochlamys Euhadra brandtii brandtii brandtii Euhadra Karaftohelix Karaftohelix n u m b e r n u m b e r

...... S n a i l 6 7 0 1 0 2 0 3 0 5 0 4 0 6 1 2 0 7 0 8 0 9 1 3 1 0 1 4 1 1 5 1 1 A z u m a

a m i l y a m i l y a m i l y a m i l y a m i l y a m i l y V a m i l y a m i l y a m i l y a m i l y o t a l o t a l Table 1. Table * F F F F F F T F F F F T 20 Y. Morii

Fig. 3. Number of individuals (A) and species (B) within a quadrat at each site. Significant differences in both the number of individuals and the number of species (GLMMs, LRTs, P < 0.05) were detected between the primary and secondary forests.

Estimated ages of secondary forests The average snow depths of the two secondary forests, Sec1 and Sec2, were 102.5 and 112.3 cm, respectively. Therefore, the ages at which the beech trees reached a height of 30 cm above the snow depth were estimated to be 24.794 and 26.852 years at Sec1 and Sec2, respectively. Positive correlations were found between the girth and the number of annual growth rings in both secondary forests (Figure 4A-a, 4B-a), and the linear regressions were estimated as follows:

Fig. 4. The estimated ages of the two secondary forests, (A) Soibetsu Forest (Sec1) and (B) Shimo-choposhinai Forest (Sec2). Linear regressions between the girth at a height of 30 cm above the snow depth (cm) and the number of annual growth rings based on increment borer samples in Sec1 (A-a) and Sec2 (B-a). Estimated age distributions of all Japanese beech trees in 30-m × 30-m quadrats in Sec1 (A-b) and Sec2 (B-b). Influence of Deforestation on Land Snail Fauna 21

Age (Sec1; years) = 0.620 × Girth (Sec1; cm) + 49.714 (r2 = 0.661, P < 0.001) Age (Sec2; years) = 0.282 × Girth (Sec2; cm) + 67.570 (r2 = 0.297, P = 0.029)

Histograms of the estimated tree ages (Figure 4A-b, 4B-b) indicated that forest Sec1 is approximately 30 years older than forest Sec2. The distribution of tree ages in Sec1 were relatively wide, from 60 to 130 years (Figure 4A-b), and the oldest tree was estimated to be 145.8 years old (Appendix Table S2). In contrast, most trees in Sec2 were of similar age, 70 to 80 years, and the oldest was 113.1 years old (Appendix Table S2). Therefore, both secondary forests were likely to have been cleared more than 100 years ago.

Discussion

The DI, DI without C. pessimum and DS were clearly higher in primary forests than secondary forests. Specifically, the diversity of the land snail fauna in Pri1 was particularly high, but the reasons for this are unclear. In contrast, an average of only 12.3 individuals and 4.8 species were detected in the two secondary forests because the abundance and diversity of smaller species (2.0 mm or less; DI-small) tended to decrease in the secondary forests. These results might imply that smaller snail species are likely to be more influenced by forest disturbances in the boreal forests of Kuromatsunai District than larger species, although the reasons for this are still unclear. Many studies have mentioned that the habitats in natural forests are more variable than those of secondary forests (Shimada et al., 1991; Touyama & Nakagoshi, 1994a), so smaller snail species in this study might have not been able to establish in secondary forests because of the habitat differences between primary and secondary forests. However, the total number of species did not seem to differ among sites (11, 13, 13 and 8 species in Pri1, Pri2, Sec1 and Sec2, respectively; Table 1), and the total number of species in Pri1 was lower than that in Sec1. I only focused on diversity at a small scale in the soil (50-cm × 50-cm quadrat), so larger or rarer species were difficult to detect in this study. Therefore, further investigation is needed with wide-range qualitative sampling to compare the total species number between primary and secondary forests. However, it could also be possible that this result was consistent with the intermediate disturbance hypothesis (Connell, 1978), under which species richness or diversity is higher when disturbances are intermediate on the scales of frequency and intensity. Many empirical studies have shown this pattern in forest ecosystems (e.g., Aoki et al., 1977; Terayama, 1982; Kondoh & Kitazawa, 1984; Touyama & Nakagoshi, 1994a, b), but it is still unclear whether this study also supports this hypothesis or not. The results suggest that both secondary forests were cleared more than 100 years ago, and they are consistent with results from the secondary forest near Sec2 (Matsui et al., 2012), although Sec1 was estimated to be a few decades older than the other secondary forests. The maximum diameters of the trees in both secondary forests were estimated to be approximately 50 cm (beech tree IDs: Sec1-27 and Sec2-34; Appendix Table S2), but this is less than half the maximum diameter of the trees in the two primary forests (138 cm and approximately 100 cm diameter at breast height (DBH) in Pri1 and Pri2, respectively) (Saito, 2015). The tree ages of the other two primary forests near the research sites in this study, called Shimo-choposhinai-P2 Forest (42.70999°N, 140.36401°E, Alt. 310 m; Matsui et al., 2012) and Karibayama Forest (42.59826°N, 139.99137°E, Alt. 480 m; Kitamura et al., 2007) were estimated to be as high as 266 and 301 years respectively, and the maximum DBHs in each forest were 56.6 and 78.2 cm (Kitamura et al., 2007; Matsui et al., 2012). Thus, the ages of the two primary forests in this study, Pri1 and Pri2, seem to be at least equivalent to those of Shimo-choposhinai-P2 and Karibayama Forests (more than 250 years old). These historical differences among the research forests may have caused the differences in DI and DS, especially between the primary and secondary forests, although more site and sample 22 Y. Morii replicates are needed to further address this point. My results showed a clear correlation between forest age and the densities of land snails (DI and DS). Specifically, they might imply that the impact of past deforestation on the land snail fauna has persisted for more than 100 years and affects the soil fauna, as has been suggested by several previous studies (e.g., Douglas et al., 2013). In particular, smaller snails (2.0 mm or less) appear to be more strongly impacted than larger species (larger than 2.0 mm). However, it is possible that recent disturbances less than 100 years ago affect the differences of DI and DS between primary and secondary forests, because the treatment and land-use of each forest can be different; for instance, the Pri1 forest (Utasai Forest) has been more protected as a natural monument of Japan than the other research sites (Saito, 2006, 2012, 2015). Many other environmental and biological factors besides forest history (e.g., leaf litter depth, pH and other soil chemical traits, the plant community, forest size and forest isolation and fragmentation) also likely affect the spatial patterns of DI and DS (Ewers & Didham, 2006; Graham et al., 2006; Chiba et al., 2009; Hylander, 2011), but these factors were not assessed in this study. Future research should concentrate on understanding the spatial and temporal patterns of soil fauna diversity and the influence of deforestation.

Acknowledgements

I am grateful to Hitoshi Saito, Kanna Tatsuta, Eishi Fujito and Kazu Ichikawa for their technical support and helpful suggestions. I also appreciate Hirotaka Katahira, Yusaku Ohkubo, Megumi Ohya and Yuta Inoue for collecting samples; Futoshi Nakamura, Satoshi Chiba, Hideaki Shibata, Yu Fukasawa and Yasuhiro Kuwahara for scientific discussions; and Masayuki Imoto for obtaining the necessary permissions. This study was financially supported by the “Kuromatsunai Biodiversity Conservation Research Grant” from Kuromatsunai, Hokkaido, Japan and the “Japan Society of Soil Zoology (JSSZ) Early Career Grant 2016”.

References

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i n d s p e c i s p e c i i n d s n a i l s n a i l 1 9 5 ( o f o f s p e c i s n a i l s n a i l s n a i l s n a i l M . g e g e e r t i g n d a S n a i l C i o n e l d a D i s c d a e B r a d y b e n i D i p l o m a t n d e P o m a t i p s d e E l o b i d a e Z o n i t d a e H e l i c a r o n d H e l i c n d a l a r s m a l s m a l l a r Karaftohelix Nipponochlamys hokkaidonis Nipponochlamys affinis Parakaliella borealis Trochochlamys labilis Trochochlamys lubrica Cochlicopa Discus pauper brandtii brandtii Euhadra Karaftohelix Discoconulus sinapidium Palaina pusilla paucicostata Palaina japonica bensoni Blanfordia pessimum Carychium radiata Retinella radiatula Columella edentula Punctum atomus Hemipoma hakodadiense n u m b e r n u m b e r

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1 7 0 9 1 0 1 1 2 1 3 1 4 1 5 1 6 0 2 0 3 0 4 0 5 0 6 0 7 0 0 1 a m i l y a m i l y a m i l y a m i l y a m i l y a m i l y a m i l y a m i l y V a m i l y a m i l y o t a l o t a l * Appendix Table S1. Table Appendix N o . T T N o . N o . N o . F F F F F F F F F F Influence of Deforestation on Land Snail Fauna 25

Appendix Table S2. Data set of beech trees in two secondary forests. Beech tree Girth No. of Estimated Sec1-53 98.2 – 110.6 ID (cm) tree rings tree age Sec1-54 45.7 – 78.1 (years) Sec1-55 18.3 29 61.1 Soibetsu Forest (Site ID: Sec1) Sec1-56 52.7 74 82.4 Sec1-1 105.0 92 114.8 Sec1-57 104.5 – 114.5 Sec1-2 74.2 57 95.7 Shimo-choposhinai Forest (Sec2) Sec1-3 96.3 – 109.4 Sec2-1 119.6 – 101.3 Sec1-4 37.8 – 73.2 Sec2-2 125.1 – 102.9 Sec1-5 34.5 49 71.1 Sec2-3 148.2 – 109.4 Sec1-6 45.0 – 77.6 Sec2-4 87.6 – 92.3 Sec1-7 25.2 – 65.3 Sec2-5 99.8 – 95.8 Sec1-8 78.1 – 98.1 Sec2-6 17.1 – 72.4 Sec1-9 47.1 – 78.9 Sec2-7 88.7 46 92.6 Sec1-10 73.7 – 95.4 Sec2-8 110.8 62 98.9 Sec1-11 70.6 – 93.5 Sec2-9 10.9 – 70.6 Sec1-12 89.5 – 105.2 Sec2-10 6.8 – 69.5 Sec1-13 29.8 – 68.2 Sec2-11 15.2 – 71.9 Sec1-14 122.5 94 125.7 Sec2-12 26.0 – 74.9 Sec1-15 37.0 – 72.7 Sec2-13 21.7 – 73.7 Sec1-16 113.0 – 119.8 Sec2-14 119.7 – 101.4 Sec1-17 151.0 – 143.3 Sec2-15 32.4 53 76.7 Sec1-18 23.9 – 64.5 Sec2-16 15.9 – 72.1 Sec1-19 118.9 – 123.4 Sec2-17 28.2 – 75.5 Sec1-20 51.5 – 81.6 Sec2-18 17.8 – 72.6 Sec1-21 119.2 82 123.6 Sec2-19 124.2 – 102.6 Sec1-22 27.4 18 66.7 Sec2-20 32.2 – 76.7 Sec1-23 79.0 – 98.7 Sec2-21 30.2 56 76.1 Sec1-24 64.7 – 89.8 Sec2-22 27.9 – 75.4 Sec1-25 136.5 – 134.4 Sec2-23 119.0 – 101.2 Sec1-26 114.9 – 121.0 Sec2-24 65.0 63 85.9 Sec1-27 155.0 – 145.8 Sec2-25 38.2 35 78.4 Sec1-28 84.5 – 102.1 Sec2-26 130.1 – 104.3 Sec1-29 73.9 – 95.5 Sec2-27 47.7 64 81.0 Sec1-30 88.3 85 104.5 Sec2-28 37.4 – 78.1 Sec1-31 87.1 – 103.7 Sec2-29 45.0 38 80.3 Sec1-32 47.0 53 78.9 Sec2-30 17.1 – 72.4 Sec1-33 28.8 41 67.6 Sec2-31 60.3 33 84.6 Sec1-34 126.2 – 128.0 Sec2-32 39.0 – 78.6 Sec1-35 56.7 73 84.9 Sec2-33 96.9 – 94.9 Sec1-36 26.3 – 66.0 Sec2-34 161.2 – 113.1 Sec1-37 99.4 – 111.4 Sec2-35 18.4 – 72.8 Sec1-38 85.7 – 102.9 Sec2-36 151.8 – 110.4 Sec1-39 19.8 – 62.0 Sec2-37 15.8 – 72.0 Sec1-40 105.0 – 114.8 Sec2-38 43.3 – 79.8 Sec1-41 13.6 – 58.1 Sec2-39 21.7 – 73.7 Sec1-42 94.2 – 108.1 Sec2-40 117.0 – 100.6 Sec1-43 67.1 92 91.3 Sec2-41 17.3 – 72.5 Sec1-44 83.7 – 101.6 Sec2-42 81.5 72 90.6 Sec1-45 118.4 – 123.1 Sec2-43 102.3 77 96.5 Sec1-46 47.5 34 79.2 Sec2-44 77.8 79 89.5 Sec1-47 100.5 – 112.0 Sec2-45 87.2 – 92.2 Sec1-48 69.9 89 93.1 Sec2-46 41.4 68 79.3 Sec1-49 107.0 – 116.1 Sec2-47 68.9 62 87.0 Sec1-50 33.1 52 70.2 Sec2-48 37.1 57 78.0 Sec1-51 79.4 – 98.9 Sec2-49 126.1 84 103.2 Sec1-52 28.8 – 67.6 Sec2-50 47.0 – 80.8 26 Y. Morii

北海道・黒松内低地帯の原生林と二次林における陸産貝類相の比較

森井悠太

要 約

森林の皆伐は森林生態系へ壊滅的な損害を与えうる。皆伐によって森林生態系の生物量や種多様性が著 しく減少することが知られている。しかしながら，皆伐の長期的な影響を評価した研究は少ない。本研究 では，原生林と二次林の林床土壌中の陸産貝類相を定量的に調査し，過去の皆伐の影響の評価を試みた。 北海道，黒松内低地帯に位置するブナの優占する原生林と二次林をそれぞれ 2 箇所ずつ調査地とし，調査 地それぞれの林床に 50-cm × 50-cm の区画をそれぞれ 6 箇所，林床に設置した。リター層中の陸産貝類 を目視で摘出したのち，双眼実体顕微鏡を用いて種を同定した。原生林と二次林との間で種密度と個体密 度を比較したところ，種密度・個体密度共に二次林よりも原生林において有意に高い値が示された。原生 林 2 箇所のうちのひとつ，歌才ブナ林では特に陸産貝類相の多様性が高く，50-cm × 50-cm の区画で平均 239.2 個体・7.2 種もの陸産貝類が採集された。一方，二次林では 2 箇所の平均で 12.3 個体・4.8 種を記録 するのみであった。その中でも，殻長 2.0 mm 以下の微小貝の個体密度が二次林において有意に低かった。 加えて，成長錐を用いて二次林の樹齢を推定した結果から，調査対象とした 2 箇所の二次林はいずれも 100～150 年前に伐採されたことが示された。これらの結果は，森林伐採が 100 年以上にも渡って林床の 陸産貝類相に影響を与えることを示している可能性がある。