Bird Community and Vegetation Structure in a Forest with a High Density of Sika Deer

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1 Jpn. J. Ornithol. 48: , 2000 Bird Community and Vegetation Structure in a Forest with a High Density of Sika Deer Teruaki HINO Kansai Research Center, Forestry and Forest Products Research Institute, Momoyama, Fushimi, Kyoto , Japan Bird community and vegetation structure were surveyed in a forest, dominated by Fagus crenata, Acer shirasawanum and Picea jezoensis, with a high density of sika deer Cervus nippon in Ohdaigahara, central Japan, during the breeding season from 1994 to The average density and number of species were 409 pairs/100 ha and 20 species, respectively, and were stable throughout the five years. The most dominant species was Parus ater, followed by Parus major, Sitta europaea, Troglodytes troglodytes and Tarsiger cyanarus. Birds nesting in undergrowth were absent and those in low-canopy were low in density and number of species. This result was related to the small amount of foliage for their nesting sites, which were overgrazed by deer. Key Words: Bird community, Overgrazing, Sika deer, Vegetation structure Ohdaigahara has one of the largest forests of Fagus crenata and has the southernmost forest of Picea jezoensis in Japan. This area is also known for a sika deer Cervus nippon population of high density, estimated to be 30-40/km2 (Yokoyama et al. 1995). These deer have caused serious damage to vegetation by browsing and trunk-barking. That is, Sasa nipponica has decreased calm height and biomass (Yokoyama & Shibata 1998, Hino et al. unpublished), standing trees have become dead (Sekine & Sato 1992) and regeneration has been prevented in many tree species (Hino et al. unpublished). The same problems have been reported in many other forests in Japan (Kaji et al. 1988, Takatsuki & Gorai 1994) Bird community structure has often been related to vegetation structure in forests (MacArthur & MacArthur 1966, James & Wamer 1982, Hino 1985, Fujimaki 1986). However, we have never known how a bird community is effected by vegetation change as a result of grazing by herbivorous mammals. In this study, I report the results of a five-year census of birds with a survey of vegetation in a forest in Ohdaigahara. I also examine the effect of vegetation overgrazing by deer on bird community structure through comparison with other studies in ungrazed forests. STUDY AREA AND METHODS Ohdaigahara is located at an altitude of m in the eastern part (136*E, 34* N) of the Kii Peninsula, central Japan. The census site of 12 ha (300 m *400 m) was on a gentle slope (c.1550 m in altitude) of the southeastern part of Ohdaigahara. The trail for the bird census and vegetation survey was marked with poles at 50-m intervals lengthwise and crosswise.

2 198 Teruaki HINO [Jpn.J.Ornithol.Vol.48 No.3 Fig. 1. Geographic locations of the forests used for comparison. 1: Ohdaigahara (this study), 2: Chichibu-Tama (Uramoto 1961), 3: Okususobana (Mitsuishi 1970), 4: Tomakomai (Fujimaki 1986), 5: Ashoro (Fujimaki 1988), 6: Nakagawa (Hino & Nakano 1992). I took the census of bird population with the territory mapping method, walking along the trail five to ten times each year in the morning from April to June in All birds seen or heard within 50 m on either side of the census trail were recorded on a scale map. The number of censuses seemed large enough to determine the bird population accurately because the vegetation coverage was so sparse in the low-canopy and undergrowth layers that I could identify and locate the birds easily. The territory overlapping the boundary of the census site was treated as 0.5. I surveyed the vegetation in the census site in July I set up the sample plots (10 m * 10 m) at each point marked with a pole (35 points in total). In each plot, I recorded the species, number, and DBH (diameter at breast height) for every tree above 1.5 m in height. Basal area was calculated as the sum of cross sectional areas of trunks determined from the DBH. In each plot, I recorded the four categories of foliage volume (0: none, 1: 1-33%, 2: 34-66%, 3: %) by eye in each height layer (0-0.5 m, m, m, 3-6 m, 6-9 m, 9-12 m, m, 15m-). The vertical distribution of foliage coverage was described with the average of category values multiplied by 30% in each layer. I also quantified the

3 February 2000] Bird Community and Sika Deer 199 main species of herbaceous plants roughly based on foliage volume. I selected, for comparison, five studies of the bird communities done in forests that appeared not to be effected by deer's overgrazing (Fig 1). The following points were considered in selecting the studies (Appendix): (1) a territory mapping method or a line transact method with a territory mapping had to be used for bird census; (2) the number of censuses had to be large enough to determine the bird population; (3) the census area had to be more than 10 ha; (4) the forests had to be natural; (5) coniferous trees were not a major component of the forests; (6) the forest area had to be large as not to be influenced by surrounding habitats. RESULTS 1) Vegetation In this study site, standing live trees were 1091/ha in density and 45.2 m2/ha in basal area (Table 1). The most dominant tree species was Fagus crenata, followed by Piece jezoeasis and Acer shirasawanum in the basal area. The density of small trees (DBH<10 cm), dominated by Symplocos coreana, Pourthiaea villosa and Fagus crenata, was low because there were very few of the smallest trees (DBH<5 cm) (Table 2). Standing dead trees were 271/ha in density and 8.7 m2/ha in basal area. The ratios of dead trees to live trees were 0.25 in density and 0.19 in the basal area. The few small trees and many dead trees were outstanding Table 1. Numbers in sample-plots (0.35 ha) and density in DBH-class, basal area and dominance of each tree species.

4 200 Teruaki HINO [Jpn.J.Ornithol.Vol.48 No.3 Table 2. Vegetation characteristics in a forest of Ohdaigahra and the ungrazed forests. 1: no data 2: The value calculated based on the description "most of standing dead trees were <14 cm DBH" in the text. 3: The values calculated with our unpublished data. Foliage (%) Fig. 2. Vertical distributions of foliage coverage in the overgrazed forests in Mt.Ohdaigahara (shaded) and the ungrazed forests in Hokkaido (unshaded; Hino & Nakano 1992).

5 February 2000] Bird Community and Sika Deer 201 Table 3. Number of breeding pairs of birds/100 ha a forest of Ohdaigahara. Nesting sites were categorized as follows; HC-high canopy, LC-low conopy, T-tree hole, G-ground, P-parasitic characteristcs in this study site (Table 2). Foliage coverage was more sparse in every height-layer than that of an ungrazed forest (Fig. 2). The difference in coverage between the forests increased with decreasing height. The small amounts of foliage in the low and high canopies were related to the few small trees and the abundant dead trees, respectively. The undergrowth layer, mainly composed of Skimmia japonica (51%) and Sasa nipponica (36%), was low in both height and foliage volume.

6 202 Teruaki HINO [Jpn.J.Ornithol.Vol.48 No.3 Table 4. The numbers of species and pairs of birds according to nesting sites in a forest of Ohdaigahra and the ungrazed forests. Nesting sites were categorized as follows; HC-high canopy, LC-low conopy, T-tree hole, U-undergrowth, G-ground, P-parasitic 2) Bird community During the study, 23 species bred in total and 18 species bred every year (Table 3). The average density and number of species were 409 pairs/ha and 20 species, respectively, all of which differed little between years. The density of each species was also stable throughout the five years. Both the density and the species richness were lower than those of the ungrazed forests (Table 4). The most dominant species was Parus ater (29%), followed by Parus major, Sitta europaea, Troglodytes troglodytes and Tarsiger cyanurus (8-10%). Among major family groups, the present community lacked the birds of Emberizidae, Fringillidae and Columbidae and had few birds of Sylviidae and Muscicapidae. Based on nesting sites, none of the undergrowth-nesting birds were found in any years (Table 4). The birds nesting in low canopy (1.5 m - 6 m in height) were lower in both density and species richness than those in the other forests. However, the birds nesting in tree-holes and on the grounds were no less abundant. DISCUSSION The characteristics of vegetation in this study site were (1) few small trees,

7 February 2000] Bird Community and Sika Deer 203 (2) low and sparse undergrowth, and (3) abundant dead trees. All of these have resulted from browsing and trunk-barking by sika deer. The few small trees show that regeneration has been prevented because deer have browsed seedlings and saplings excessively (Hino et al. unpublished). Also, overgrazing by deer has decreased the height and biomass of Sasa nipponica (Yokoyama & Shibata 1998) but has increased, instead, the density of Skimmia japonica which deer do not eat. Frequent trunk-barking by deer has caused the large number of dead trees. Sekine & Sato (1992) showed that 69% of tree species were barked in the forests in Ohdaigahara. The standard density and numbers of species of breeding birds were about 500 pairs/ha and 30 species, respectively, in large forests dominated by broad-leaved deciduous trees in Japan (Hino 1990, 1993). Both values were low in my study site because the undergrowth-nesting birds were absent and the low-canopy-nesting birds were low in density and species richness. The small number of birds will be related to the small amount of foliage for their nesting sites (Hino 1985), which were overgrazed by deer. However, some species which were absent in my study site were thought to be breeding in steep areas in Ohdaigahara as their songs could be heard during the breeding seasons. These birds were Cettia diphone nesting in undergrowth, and Turdus dauma, Turdus cardis and Sphenurus sieboldii in low canopy as well as Ficedula narcissina in tree-holes. The steep area has a dense bush of Sasamorpha borealis owing to low density of deer. This fact suggests the idea that some bird species have disappeared through vegetation change as a result of deer's overgrazing. On the other hand, the birds nesting in tree-holes and on the ground were not affeced by a high density of deer. The many standing dead trees may provide nesting sites for woodpeckers and other hole-nesting birds (Matsuoka & Takada 1999). The fallen trees, which are also abundant in Ohdaigahara, may be used as shelters by ground nesters. The effect of browsing and trunk-barking by sika deer on vegetation has been a serious problem in many natural forests in Japan (Kaji et al. 1988, Sekine & Sato 1992, Takatsuki & Gorai 1994). The present study suggests that the highly dense population of deer has not only damaged the vegetation but has also decreased the diversity and abundance in the bird communities in the forests. Proper controls of the deer populations are urgently needed for the restoration of biological diversity in the forests. I am grateful to Y.Higuchi, Y.Fujita and N.Kumashiro for setting up the study site, and also to the staff of the Ohdaigahara Visitor's Center, and S.and M.Tagaito for their kind cooperation in the fieldwork. LITERATURE CITED Fujimaki, Y., Breeding bird community in a deciduous broad-leaved forest in southern Hokkaido, Japan. Jap. J. Ornithol. 35: Fujimaki, Y., Breeding bird community of a Quercus mongolica forest in eastern Hokkaido, Japan. Jap. J. Ornithol. 37: Hino, T., Relationships between bird community and habitat structure in shelterbelts of Hokkaido, Japan. Oecologia 65: Hino, T., Palaearctic deciduous forests and their bird communities: comparisons

8 204 Teruaki HINO [Jpn.J.Ornithol.Vol.48 No.3 between east Asia and west-central Europe. Keast, A. (ed.) Biogeography and ecology of forest bird communities: SPB Academic Pub, Hague. Hino, T., Bird fauna and its distribution in Hokkaido. Higashi, S., Osawa, A. and Kanagawa, K. (eds.) Biodiversity and ecology in the northernmost Japan: Hokkaido Univ. Press, Sapporo. Hino, T. & Nakano, S., Breeding bird community of a deciduous broad-leaved forests in northern Hokkaido, Japan. Res. Bull. Hokkaido Univ. For. 49: (in Japanese with English summary). James, F. C. & Wamer, N. O., Relationships between temperate forest bird communities and vegetation structure. Ecology 63: Kaji, K., Koizumi, T. & Ohtaishi, N., Effects of resource limitation on the physical and reproductive condition of sika deer on Nakanoshima Island, Hokkaido. Acta Theriol. 33: MacArthur, R. H. & MacArthur, J. W., On bird species diversity. Ecology 42: Matsuoka, S. & Takada, Y., The role of snags in the life of woodpeckers and snag management in a forest: a review. Jpn. J. Ornithol. 47: (in Japanese with English summary). Mitsuishi, H., Ecological study of the bird community in Okususobana natural garden Nagano Prefecture. Bull. Inst. Nat. Edc. in Shiga Heights 9: (in Japanese). Sekine, T. & Sato, H., Tree barking by sika deer, Cervus nippon Temminck, on Mt.Odaigahara. Jap. J. Ecol. 42: (in Japanese with English summary). Takatsuki, S. & Gorai, T., Effect of sika deer in the regeneration on Fagus crenata forest on Kinkazan Island, northern Japan. Ecol. Res. 9: Uramoto, M., Ecological study of the bird community of the broad-leaved deciduous forest of central Japan. J. Yamashina Inst. Ornithol. 3: Yokoyama, S., Koizumi, T. & Shibata, E., Food habits of sika deer as assessed by fecal analysis in Mt. Ohdaigahara, central Japan. J. For. Res. 1: Yokoyama, S. & Shibata, E., The effects of sika-deer browsing on the biomass and morphology of a dwarf bamboo, Sasa nipponica, in Mt. Ohdaigahara, central Japan. For. Ecol. Manag. 103: (Received 10 Septmber 1999; Accepted 4 December 1999) Appendix. Census methods in the studies used for comparison. 1: M Territory mapping; L+M Line transect with territory mapping 2: - not described 3: The census was taken in two breeding seasons 4: The census area was calculated as the transect route length multiplied by the indentification distance for each species

9 (Breeding Bird Community and Vegetation Structure in a Forest with a High Density of Sika Deer. 48: )