Sustainable development of regional cities calls for viable regional-spatial strategies that strengthen the network of cities and their hinterlands. At present, the significance of urban-rural interactions has been well established for sustainable city development. Cities cannot be self-regulating without maintaining stable links with the surrounding areas from which they draw food and other bio-resource materials and into which they release their wastes. Recently, localized food production-consumption system strategies have received considerable attention in Europe, North America and Japan as a means of improving sustainability.

Agricultural activities play a vital role in maintaining the sustainability and conservation of urban and rural environments. In response to population growth, urban development, and climate change, Asian megacities and agricultural producers are facing complex challenges; bio-resources management has become a prominent area of research as well as an important policy issue in agricultural areas near large cities. The changing nature of the relationship between urban and rural land uses has deep consequences both for human quality of life and for the environment.

Most Asian megacities are located in rich alluvial deltas that have long served as the "rice-baskets" of their respective regions. The study was carried out in China's national capital region, Tianjin, which is in the northeast part of the North China Plain and located on a low-lying alluvial plain along the mouth of the Hai River (38°34'N-40°15'N and 116°43'E-118°04'E). This area has not only experienced rapid industrialization and urbanization (including the famous Jing-Jin-Tang Industrial Belt), but it also has played a pioneering role as an urban development model for other Chinese cities. Due to the rising demand for water caused by industrial and urban development and upstream irrigation construction, water has become a pronounced factor limiting development of the city.

This study attempts to identify and analyze the emerging form of the urban-rural relationship through examining the role of agriculture in the Asian megacity's countryside. The main objectives of the study were (1) to identify the local and regional food flow and the extent to which the region's food consumption depends on local production and to assess the relationships between urban, suburban, and rural areas in the region (Chapter 2); (2) to examine spatial-temporal and functional changes from 1993 to 2009 in terms of agricultural land use and food production in both suburban and rural areas of the city (Chapter 3); and (3) to explore whether the dependence of local consumption on local production could be improved based on the current trend of agricultural land use changes, in particular changes in paddy fields (Chapter 4). Furthermore, suggestions for how to promote a sustainable material cycle between urban and rural areas are discussed in Chapter 5. The main approaches were food flow analysis, landscape analysis by satellite image interpretation, and semi-structured and key informant interviews with local people.

A better understanding of food flows and the extent to which a region's food consumption depends on its local production is important to understand future sustainable cities. Chapter 2 examines food self-sufficiency and food flow in Tianjin, divided into three areas (urban, suburban, and rural), as a case study to gain a better understanding of local and regional food flow. The main objectives were to evaluate the city's capacity for self-sufficiency within its administrative boundary by calculating a self-sufficiency ratio using statistical data at both the city and local scales, to estimate potential food flow at the city scale from data derived from field surveys of local markets and statistics provided by local government authorities, and to observe real flows of the two primary types of arable crops (cereal grains and vegetables) from semi-structured household interviews at the local scale. The results show that Tianjin has experienced a rapid increase in self-sufficiency capacity since the 1980s, and it appears to have the capacity to support the citizens within its borders. By 2007, the self-sufficiency ratio of major foods reached more than 140%, and in the case of milk, the ratio was 360%; the only category in which the agricultural sector did not meet demand was cereal crops. Even with these high ratios, wholesalers, retailers and consumers have has chosen to use and exchange products from distant areas, utilizing regional food flow. In the urban area, local fresh vegetables (produced primarily in suburban areas) currently account for only 33% of the vegetable market, whereas the other 67% is supplied by other regions of China, and all cereal products consumed by the urban population are supplied by other regions. At the local scale, rural areas have had a surplus in all food categories studied since the 1980s, and suburban areas have a history of high self-sufficiency for vegetables, poultry, and fish. However, a low degree of locally sourced product flow was observed. Most local cereal crops were consumed in agricultural production areas, but 70% of vegetables produced in rural areas flowed outside of the region and did not support local consumption in Tianjin.

Local agricultural production is the main driver of self-sufficiency and food flow. Agricultural processes are connected with the increased urbanization that has accompanied the urban regional development in Tianjin. An accurate understanding of changes in agricultural land use and their driving factors has important implications for assessing food security and policy making. In Chapter 3, Landsat images are used to evaluate agricultural land use changes at the landscape scale in suburban and rural areas in Tianjin from 1993 to 2009, based on two case studies. The main driving factors of the changes were investigated at the farm scale through a survey of local farmers. The image classification results showed an obvious loss of agricultural land since the early 1990s in suburban areas, similar to what has occurred in other peri-urban areas in developing countries. Two other notable trends in land use, which differ from those of other peri-urban areas, were observed: (1) land was converted from the production of traditional perishable food to the production of staple foods, in this case, from vegetables and fruits to maize; and (2) there was a shift from staple production to fiber production, in this case, from paddy rice and maize to cotton. In rural areas, a similar shift from staple food to fiber production was noted during the study period, in this case, from rice and maize to cotton. Two other results, which differ from those of suburban areas, were found: (1) a relatively small amount of land had been converted from the production of staple foods to fresh food, in this case, from maize to vegetables; and (2) recently, some land has shifted back from fiber production to the production of staple foods, in this case, from cotton to rice. Through semi-structured household interviews, a scarcity of water was noted as an important problem for agriculture in Tianjin-this was found to be a primary factor for agricultural land use change both in suburban and rural areas. Beyond the limitation of clean water, suburban farmers had to adjust their agricultural activities by adapting to an insufficient amount of labor and the use of treated wastewater from urban areas for irrigation. Rural farmers' activities, however, were still driven by market mechanisms and government policies.

Changes in local production have accounted for the trend of food self-sufficiency levels and contributed to the current regional and local pattern of food flow. Specialized vegetable production in rural areas generated an active flow from the rural areas of Tianjin to other Chinese regions, whereas the decreased production of paddy rice was only able to support consumption in the production areas themselves, so there was no local surplus to flow locally. Chapter 4 explores whether it would be possible to enlarge the production area for local consumption, in this case of paddy rice, to enhance food security and develop a more sustainable urban-rural system. Further understanding of the dynamics of the traditional rice-dominated agriculture area was undertaken both at the county and village level in rural areas through a comparative study of the planting area of recent annual crops, precipitation variability, and water flow. The manner in which the involved stakeholders attempt to cope with and adapt to a constantly changing environment was also determined through semi-structured and key informant interviews. With limited water flow from the outside and in response to recent historic precipitation variability, local people adjusted to both year-to-year variability and extreme events through collective and individual actions in the short term by changing planting areas and switching crops. Annual rainfall and water flow records show that a serious and continuous drought occurred during 1999-2003. The drought compelled farmers to shift paddy fields to field crops, and some irrigation and drainage systems were also destroyed. As they switched to less water-intensive crops (e.g. cotton), the farmers appeared to be making a relatively permanent adaptation to water stress, but in so doing, many have become more vulnerable through worsening pest and disease management and a low level of self-sufficiency. Although most villagers were willing to replant paddy rice, shifting land use back to paddy cropping to adapt to the more recent increased precipitation has not been common because of damaged irrigation and drainage systems and a lack of willingness of community leaders to maintain the systems as they previously did. Only farmers in a few villages, which maintained their properties well and had leaders who seemed aware of the importance of traditional crops, succeeded in switching back to paddy cropping again. The possibility of enlarging the paddy rice production area will most likely be highly dependent on local institutions in initiating and supporting adaptations and participating in collective irrigation regimes.

In conclusion, the food flow and self-sufficiency analysis indicated that Tianjin needs to recognize the high level of productivity of its local area and reduce its dependence on the capacity of other regions to supply its food needs. A better relationship between urban and rural areas through local food production and consumption needs to be re-established. Water is currently the largest barrier to increasing food production in the city. In order to improve self-sufficiency and establish a localized food flow, it is necessary to enhance the resilience and adaptive capacity of local agricultural communities and associated stakeholders to cope better with not only the constraints but also the opportunities of climate variability and urbanization. It is also important to give policy support to localized food production and consumption as well as addressing people's interest in local products for the future sustainability of the urban-rural system in Tianjin.

都市の持続可能性を高めるうえで、食料フローのローカライゼーション(地産地消)による域内食料自給率の向上は、食料安全保障の観点のみならず、輸送等にかかるエネルギー・二酸化炭素排出の削減を通じた低炭素社会実現の観点からもきわめて重要である。都市を取りまく農村は、食料供給をはじめとして、さまざまな生態系サービスを都市に対して提供することから、都市の後背地としての農村は都市環境の健全化をはかるうえで重要な役割を果たすことが期待されている。とりわけ、拡大を続けるアジアの大都市においては、周辺農村との食料・資源・エネルギー循環系の再構築を通じた、持続可能な都市圏形成と都市環境の健全化が喫緊の課題となっている。

本研究は、中国・天津市を対象として、都市における食料消費の域内依存度および、農業的土地利用と食料生産の時間的・空間的変化を明らかにしたうえで、食料フローのローカライゼーションを促進するための方策を提示することを目的とする。研究対象である天津市は渤海湾に面する沖積低地に位置し、水田耕作の卓越する広大な農業地域を後背地に有する一方で、沿岸部の経済拠点として近年急速な都市化・工業化が進行していることから、食料フローの観点からアジア型の持続的都市圏形成のあり方を検討するうえで最適な事例都市といえる。研究は、天津市を「都市」、「郊外」、「農村」の3区域に区分したうえで、以下の手順によって進めた。(1)食料自給率および食料フロー解析により、食料の域内依存度を把握した。(2)農業的土地利用変化の空間解析により、域内依存度変化の要因を明らかにした。(3)基幹作物である米に着目し、域内依存度向上にかかる制限要因を抽出するとともに、改善の方策を検討した。

統計資料に基づき天津市における推定食料自給率を算出した結果、1980年代以降、統計データの入手できた2007年まで、穀物以外の農産物の食料自給率は急速に向上していることが確認された。一方、食料生産区域(「郊外」および「農村」)において半構造化面接による農家調査を行い、実際の食料フローを解析した結果、生産された食料は食料生産区域内で消費される割合が高く、穀物でその傾向が顕著であった。とくに米については、生産量の96%が食料生産区域内で消費されており、「都市」へのフローはきわめて低いことが明らかになった。また、推定食料自給率の比較的高かった野菜についても、域内でのフローは低く、域外へのフローも一部で確認されたが、これは、生産量の季節変動が大きいこと、生産品目が限られていること等が原因として考えられた。以上から、天津市の「都市」における食料消費は域外からの供給によって維持されており、域内の「郊外」「農村」との関係が脆弱であることが示唆された。

「郊外」および「農村」において、推定食料自給率が顕著に変化した地域を対象に、複数時期の衛星画像(LANDSAT TM/ETM画像:1993年、2001年、2009年)を用いて過去約20年間の農業的土地利用の変化を解析した結果、特徴的な作目の変化が抽出された。「郊外」では、都市的土地利用への変化が顕著であったほか、作目については野菜→穀物、穀物→繊維作物(綿花)への変化が特徴的であった。「農村」ではとくに穀物→繊維作物への変化が大きく、穀物の栽培面積は約60%減少したことが分かった。半構造化面接による農家調査の結果、上記の作目変化の動機としては、収益の向上よりもむしろ水資源の枯渇であることが明らかになった。この傾向は、基幹作物である米から耐乾性・耐塩性の強い綿花への作目変化においてとくに明瞭であったことから、天津市においては水資源の安定供給が、食料自給と食料フローの改善に向けた重要課題であることが示唆された。

過去20年間の降水量変動と作目変化の関係を解析した結果、1999年~2002年に生じた干ばつ期を契機として米から綿花への転換が進んだことが明らかになり、農家は短期間の降水量変動に対応して作目選択を行っていることが確認された。半構造面接による農家調査およびキーインフォーマントインタビューの結果、自家消費用食料の確保や病害虫・土壌劣化の防止等の理由から、農家は綿花から米への再転換を強く望んでいるものの、米栽培に不可欠な灌漑設備がすでに破壊されており、修復には多額の費用と共同管理が必要なため、米への転換が進んでいないことが分かった。これに対し、干ばつ後に綿花から米への再転換が成功した一部の集落では、集落のリーダーが基幹作物の重要性を強く認識しており、干ばつ期間も灌漑施設が適切に維持管理されていたことが明らかになった。以上から、天津市において、基幹作物である米の生産を回復させるためには、灌漑施設の修復と共同管理に代表されるような、ローカルな農地管理制度の再構築が不可欠であることが示唆された。

以上、本研究の結果、天津市においては推定食料自給率は向上しているものの、食料フローからみた「都市」「郊外」「農村」の関係は脆弱であること、作目の変化は市場メカニズムや農業政策よりもむしろ降水量変動にもともなう水資源の枯渇によって引き起こされていること、食料の域内依存度向上のためには、ローカルな農地管理制度の再編を通じたレジリエンス強化が必要であることを明らかにした。

本研究は、食料フローと空間情報に関する信頼性の高いデータに基づき、中国大都市圏における食料生産・消費の実態を明らかにするとともに、食料自給の観点から都市の持続可能性のあり方を論じた実証的研究として高く評価できる。都市圏の境界や空間スケールの適切な設定については今後さらに精査していく必要があるものの、他都市へのモデルとなる研究の枠組みを提供しており、審査委員一同は、博士(農学)の学位を与えるに十分値する論文であると判断した。