Kazuhito Ishimatsu, Keitaro Ito, Yasunori Mitani 景観生態学 17（2）31 - 41 ．2012 総 説 Developing urban green spaces for biodiversity: a review Kazuhito Ishimatsu 1*・Keitaro Ito 1・Yasunori Mitani 2 1Department of Civil Engineering, Kyushu Institute of Technology, 1-1 Sensui-cho, Tobata-ku, Kitakyushu-shi, Fukuoka, 804-8550, Japan 2Department of Electrical Engineering and Electronics, Kyushu Institute of Technology, 1-1 Sensui-cho, Tobata-ku, Kitakyushu-shi, Fukuoka, 804-8550, Japan 生物多様性保全を目的とした都市緑化 石松 一仁 1*・伊東 啓太郎 1・三谷 康範 2 1 九州工業大学大学院工学府建設社会工学系 〒 804-8550 福岡県北九州市戸畑区仙水町 1-1 2 九州工業大学大学院工学府電気電子工学系 〒 804-8550 福岡県北九州市戸畑区仙水町 1-1 Abstract: Urban green spaces, which are important for human and wildlife in the urban area, have changed into man-made spaces. Increased urbanisation has had and continues to have a negative impact on urban green spaces, and affects the urban microclimate as represented by‘ Urban Heat Island (UHI)’ phenomena. Especially, habitat fragmentation can be extreme within urban ecosystems, and fragments of natural vegetation may be too small or even too isolated to support some species. Therefore, the networks, which offer habitats and corridors that help conserve biodiversity, are also important to maintain the ecological services of a sustainable urban landscape, as well as total green area. However, there are rarely enough open spaces due to urban densification. This is why rooftops of buildings, which had not previously been regarded as spaces for planting for vegetation, have been utilised as a type of open space, and so green roofing has become one of the gradually developing fields of urban ecological engineering. This present paper aims to explore the potential of rooftops as habitats of urban wildlife comparing cases in Japan and UK, and pave the way for a preservation of urban biodiversity under restricted urban environmental conditions. As a result, it was suggested that Japanese green roof industries consider the concept of brown/biodiverse roofs instead of extensive roofs with sedum or lawn. At the same time, intensive roofs should be efficiently installed to provide‘ stepping stone’ in urban areas not having enough open space. If possible, however, to create habitats on the ground is more preferable than on the rooftop due to accessibility. Finally, if impervious spaces (e.g., concrete and asphalt) are replaced with pervious spaces (e.g., green area or brownfield), it is significant for not only urban biodiversity but also UHI. Key Words: Urban area, Green space, Biodiversity, Green roof, Brown/biodiverse roof 要旨：生物多様性だけでなく我々の生活の豊かさを保つために極めて重要な都市緑地は，人工的な空間に置き換えられている．その 最大の原因である急速な都市化は，都市緑地に悪影響を及ぼし続け，ヒートアイランド現象に代表される局所的な気候変動まで引き 起こしている．さらに，都市域において野生動植物の生息地の断片化・孤立化は深刻であり，この変化に適応できない生物種は絶滅 に追いやられている．そのため，単に緑地の総面積を増やすだけでなく，残存する緑地間のネットワーク性強化が，弱体化した生態 系サービスを復元する有効な手法であると考えられる．しかしながら，都市の過密化により緑地を創出するためのスペースはほとん ど残されていない．以前は緑化空間として見なされていなかった建物の屋上に近年注目が集まり，景観生態学・緑化工学分野におい て屋上緑化技術は次第に発展してきている．本総説は，我が国と英国の屋上緑化の事例を比較しながら今後の屋上空間の可能性につ いて調査すると同時に，生物にとって厳しい条件下で生物多様性を保全するための緑化手法を提案することを目的とした．その結果， 我が国のセダムやシバによる単一植栽による粗放型屋上緑化は，生態学的価値が低いだけでなく期待されている室内熱環境改善効果 も大きくないため，英国で普及しているブラウンルーフに切り替えた方が，生物多様性保全や室内熱環境の観点から，より有益であ * 連絡先：[email protected] 受付：2012 年 10 月 14 日／受理：2012 年 10 月 29 日 － 31 － Developing urban green spaces for biodiversity ることが示唆された．さらに，集約型屋上緑化は残存する緑地間に飛び石のように配置することで，ネットワーク性を強化すること ができると考えられた．しかしながら，屋上空間に到達できない生物種が存在する等の理由から，屋上と地上の緑地を同等に評価す ることは難しい．また，地上を覆っているアスファルトやコンクリートなどの不透水層をブラウンフィールドのような透水層に切り 替えることができれば，生物多様性保全とヒートアイランド現象緩和の両方に大きく貢献できると考えられた． キーワード：都市域，緑地，生物多様性，屋上緑化，ブラウンルーフ Introduction wildlife (Attwell 2000). In the past two decades, landscape ecology has become Urban green spaces can be important elements one of the most rapidly developing ecological fields contributing to urban sustainability (Esbah et al. 2009) worldwide (Kong et al. 2007), because increased urbanisation and ecosystem services, because they can contribute to the has had and continues to have a negative impact on urban reduction of various types of pollution, the improvement green spaces (Kong and Nakagoshi 2006), and affects the of microclimate conditions, the absorption of stormwaters, urban microclimate (Georgi and Dimitriou 2010) as and the prevention of flooding (Georgi and Dimitriou represented by ‘Urban Heat Island (UHI)’ phenomena. 2010). In particular, in cases where urban green space Unfortunately, urban green spaces, which are important for includes not only grass but also trees, the contributions to human and wildlife in the urban area, have largely been the above functions of urban green spaces increase. For changed into man-made spaces. In other words, urban green example, in terms of the improvement of the microclimate spaces are increasingly heavily managed and used by people in urban spaces, deciduous trees offer shade during the for recreation, such that the potential for biodiversity is summer and as Georgi and Dimitriou (2010) note, the reduced. In addition, open spaces are increasingly covered suitable selection of the right species can enhance cooling with hard surfaces rather than left green spaces. Thus, the through evapotranspiration reducing the temperature by absence of green spaces is characteristic of most up to 3.1 degrees C. Evapotranspiration creates pockets of contemporary cities globally (Georgi and Dimitriou 2010). In lower temperature in an urban environment, known as the addition, loss and isolation of habitats due to urbanisation ‘phenomenon of oases’. Conversely, it permits the sun to threaten biodiversity and warrant limits on development shine through the branches during the winter. (Kong et al. 2010). If these areas become separated from one Furthermore, the spaces are an effective way of another by barriers such as large expanses of buildings and protecting significant features and scenic views, as well as other human structures, species extinction may occur (Esbah providing buffers for differing land uses and valuable et al. 2009), because the majority of the remnant urban spaces for urban wildlife (Esbah et al. 2009). They shelter wildlife is located in small fragments of indigenous the native flora and fauna and help maintain native vegetation that have been set aside during development (Rudd biodiversity (Esbah et al. 2009), and offer important et al. 2002). These fragments are commonly subject to high harbours for remnant urban wildlife (Kong et al. 2010). levels of disturbance, due to the nature of human activities Some species cope with, and survive in, urban areas better undertaken within or adjacent to these areas (Fernandez- than others (Wood and Pullin 2002); for example, Sattler Juricic 2000; Marzluff and Ewing 2001). Therefore the et al. (2010) note that urban areas host many arthropod networks, which offer habitats and corridors that help species and cannot be regarded as species-poor conserve biodiversity (Kong et al. 2010), are also important environments. Green spaces can obviously provide to maintain the ecological components of a sustainable urban vegetation and‘ unsealed’ surfaces, and ameliorate the landscape (Sandström et al. 2006), as well as total green area. detrimental effects of urbanisation on species assemblages Because of this, rapid urbanisation makes people more aware by preserving or creating habitat, or by maintaining of urban green space, and there is an increasing realisation corridors for movement through the urban matrix that it is difficult to live without some contact with nature (González-García et al. 2009; Smith et al. 2006). Although (Kong and Nakagoshi 2006). urban areas are among the most modified and complex of However, there are rarely enough open spaces due to landscape, they still maintain a significant diversity of urban densification. This is why rooftops of buildings, － 32 － Kazuhito Ishimatsu, Keitaro Ito, Yasunori Mitani which had not previously been regarded as spaces for substrate base, offering between 25 and 100 mm deep root planting for vegetation, have been utilised as a type of zones due to restrictions on weight loading on the open space, and so green roofing has become one of the building’s structure. However, because of the thin gradually developing fields of urban ecological substrate layer, the extensive roof environment is a harsh engineering. Even so, regarding objectives for installing one for plant growth; limited water availability, wide green roofing there are big differences between countries temperature fluctuations, and high exposure to wind and due to various backgrounds. Especially, it should be noted solar radiation create high stress. As a result, a relatively that the UK has moved brownfield’s ecosystems to small range of plant species is normally used for extensive rooftops of buildings for enhancing urban biodiversity green roofs. Sedum is a common and very suitable plant over the last decade. This paper aims to explore the for use on extensive green roofing (Castleton et al. 2010; potential of rooftops as habitats of urban wildlife Oberndorfer et al. 2007). While ensuring some degree of comparing cases in Japan and UK, and pave the way for a success under stressful growing conditions, widespread preservation of urban biodiversity under restricted urban use of sedum-only green roofs also has the disadvantages environmental conditions. typical of any ecological system with limited species diversity. Misunderstanding functions of green roofs in Japan According to Ministry of Land, Infrastructure, Transport and Tourism (2009), the total green roof area in Green roofs are mainly divided into two types: intensive Japan