旱区约占全球陆地面积的41%，支撑着全球约40%的人口，包含全球约33%的生物多样性热点地区。由于缺水和长期干旱，旱区生态系统非常脆弱，极易受到气候变化和人类活动的干扰。随着社会经济的快速发展，旱区近年来经历了快速的城市扩展过程，并且未来仍将继续。这一过程给区域自然生境、生物多样性以及粮食生产和淡水供给等生态系统服务带来严重威胁。量化旱区城市扩展过程并评估其对区域生态系统服务的影响对于旱区可持续发展具有重要意义，已经成为地理学和生态学研究的热点。但是，目前有效模拟旱区城市扩展过程以及评估其对生态系统服务影响的研究仍有待进一步完善。为此，本文基于“过程-趋势-影响”的研究思路，以中国北方旱区的呼和浩特-包头-鄂尔多斯-榆林（呼包鄂榆）城市群为案例研究区，开展了旱区城市扩展过程对生态系统服务影响的研究，目的在于认识旱区城市扩展过程及其驱动机制，模拟旱区城市扩展趋势，揭示旱区城市扩展过程对生态系统服务的影响，为促进旱区城市可持续发展提供帮助。

主要工作包括以下四个方面。

（1）恢复了区域近四十年的城市土地数据，并揭示了城市扩展过程的时空格局和驱动机制。以Google Earth Engine平台为支撑，通过目视解译获取了区域1980-2017年城市土地数据。在此基础上，分析了区域城市土地扩展的数量和空间格局，并利用随机森林模型量化了城市扩展过程中区位因素的重要性。

（2）改进了城市扩展模型，并模拟了5种本地化共享社会经济路径下区域未来三十年的城市扩展过程。基于数据同化的思想，利用集合卡尔曼滤波方法改进了LUSD-urban（Land Use Scenario Dynamics-urban）模型，然后结合本地化共享社会经济路径（Shared Socioeconomic Pathways, SSPs）情景，模拟了5种本地化SSPs情景下呼包鄂榆城市群2017-2050年城市扩展过程。

（3）结合InVEST（Integrated Valuation of Environmental Services and Tradeoffs）模型和改进的LUSD-urban模型多尺度地评价了区域城市扩展过程对自然生境质量的影响。利用InVEST模型在全区、不同规模等级城市和单个城市尺度上评价了区域1990-2017年城市扩展过程对自然生境质量的影响。在此基础上，预估了5种本地化SSPs情景下区域2017-2050年城市扩展过程对自然生境质量的潜在影响。

（4）综合多种生态系统服务模型和改进的LUSD-urban模型预估了5种情景下城市扩展过程对区域五种关键生态系统服务的潜在影响。利用生态系统服务模型和相关性分析评估了1990-2017年城市扩展过程对区域自然生境质量、粮食供给、水源涵养、空气质量调节和景观美学服务的影响。在此基础上，预估了5种本地化SSPs情景下区域2017-2050年城市扩展过程对多种生态系统服务共同损失的潜在影响。

主要发现包括以下四个方面。

（1）区域近四十年来经历了快速的城市扩展过程。区域城市土地面积由1980年的81.57 km2增加到2017年的1231.33 km2，增加了1149.76 km2，年均增长率为7.61%。城市扩展过程以边缘型为主，城市景观越来越破碎，形状越来越复杂。

（2）在本地化SSPs情景下，区域未来三十年城市土地将加速扩展。在5种本地化SSPs情景下，呼包鄂榆城市群2017-2050年城市土地面积将平均增加1362.38 km2，年均增长量为41.28 km2，相较于1980-2017年将增加32.86%。从不同城市规模等级来看，小城市的城市土地面积增速最快，2017-2050年城市土地年均增长量将为18.08 km2，相较于1980-2017年将增加79.13%。在所有城市中，子洲的城市土地面积增速最快，2017-2050年城市土地年均增长量将为0.85km2，相较于1980-2017年将增加9倍。

（3）区域未来城市扩展过程将会加剧自然生境质量退化。在本地化SSPs情景下，呼包鄂榆城市群2017-2050年城市扩展过程将导致自然生境质量平均下降0.50%，相较于1990-2017年下降程度将增加28.21%。对于不同城市规模等级的城市，小城市扩展过程引起的自然生境质量下降将增长最快，2017-2050年将导致自然生境质量平均下降0.28%，将比1990-2017年增加88.30%。在所有城市中，子洲城市扩展过程导致的自然生境质量下降将增长最快，2017-2050年自然生境质量将平均下降0.53%，将是1990-2017年下降的3.86倍。

（4）区域未来城市扩展过程将会导致多种生态系统服务共同损失，且共同损失的程度在加剧。呼包鄂榆城市群2017-2050年城市扩展过程将导致粮食供给、水源涵养、空气质量调节、自然生境质量和景观美学服务分别下降1.12-1.76%、0.63-1.04%、0.43-0.72%、0.31-0.69%和0.27-0.44%。自然生境质量与空气质量调节、粮食供给与水源涵养、水源涵养与景观美学、粮食供给与景观美学以及水源涵养与空气质量调节这5对生态系统服务将呈现共同损失特征，且共同损失的程度在增强。其中，自然生境质量与空气质量调节服务共同退化程度将最严重，它们之间的正相关关系最强，2017和2050年的相关系数都超过0.85。粮食供给与水源涵养服务共同退化程度加剧将最快，相较于1990-2017年，共同退化程度将增加1.87倍。

因此，建议呼包鄂榆城市群应加强城市土地格局优化、提升城市土地集约化利用水平，保护重要生态系统服务。对于不同规模等级的城市要采取针对性措施，其中大城市要确保城市发展规模与区域生态环境承载能力相适应，中等城市要依托城市群交通网络建设高效型城市，小城市要因地制宜发展绿色生态型城市，进而促进区域城市可持续发展。

Drylands cover 41% of the global land surface, support approximately 40% of the global population, and contain around one-third of biodiversity hotspots. Dryland ecosystems are fragile and vulnerable to climate change and human activities due to water scarcity and long-term arid environment. With the rapid socioeconomic development, drylands have experienced a fast urban expansion in recent years and will continue to do so in the future. This process poses serious threats to regional natural habitat, biodiversity, and ecosystem services. It is of great significance to quantify the characteristics of urban expansion and assess its impacts on ecosystem services for a sustainable development of drylands, which has become a hot topic in geography and ecology. However, previous studies on the simulation of urban expansion and the assessment of the impacts of urban expansion on ecosystem services in drylands still need to be improved. Therefore, based on the research framework of "process-trend-impact", this dissertation takes Hohhot-Baotou-Erdos-Yulin (HBOY) urban agglomeration in the drylands of northern China as a case study to assess the impacts of urban expansion on ecosystem services. This dissertation aims to understand the characteristics and driving mechanism of urban expansion, simulate its process, and investigate its impacts on ecosystem services. The results can contribute to promoting urban sustainable development in drylands.

The main studies of the dissertation include the following four aspects.

(1) Urban land data in the past 40 years were produced, and the spatial-temporal pattern and driving mechanism of urban expansion were analyzed. Based on the Google Earth Engine platform, urban land data in the HBOY urban agglomeration from 1980 to 2017 were obtained by visual interpretation. On this basis, the quantity and spatial pattern of urban expansion were analyzed. Then, the influence of location factors on urban expansion was quantified by a random forest model.

(2) The urban expansions in the next 30 years under the five localized shared socioeconomic pathways (SSPs) were simulated after improving an urban expansion model. Based on the concept of data assimilation, the LUSD-urban (land use scenario dynamics-urban) model was improved by employing the ensemble Kalman filter method. Urban expansions from 2017 to 2050 in the HBOY under different scenarios were simulated by combining the improved LUSD-urban and the localized SSPs.

(3) The impacts of urban expansion on natural habitat quality were evaluated on multiple scales by coupling the InVEST (integrated valuation of environmental services and tradeoffs) model and the improved LUSD-urban model. The impacts of urban expansion on natural habitat quality from 1990 to 2017 were evaluated by the InVEST model at the scales of the whole region, cities of different sizes, and city. Then, the potential impacts of urban expansion on natural habitat quality under the five localized SSPs from 2017 to 2050 were estimated.

(4) The potential impacts of urban expansion on five key ecosystem services (ESs) under the five localized SSPs were estimated by integrating multiple ES models and the improved LUSD-urban model. Using the ES models and correlation analysis, the impacts of urban expansion on natural habitat quality, food production, water retention, air quality regulation, and landscape aesthetic services from 1990 to 2017 were evaluated. On this basis, the potential impacts of urban expansion on the concurrent losses of multiple ecosystem services under the five localized SSPs from 2017 to 2050 were estimated.

Main findings can be summarized as the following four points.

(1) The HBOY urban agglomeration experienced a rapid urban expansion in the past 40 years. The area of urban land increased from 81.57 km2 to 1231.33 km2 during 1980-2017, with an average annual growth rate of 7.61%. Meanwhile, edge expansion was the primary mode of urban expansion in the HBOY urban agglomeration during 1980-2017. The urban landscape was becoming more fragmented and complex over time.

 (2) Under the localized SSPs, urban land will expaned accelerately in the next 30 years. Under the five localized SSPs, the area of urban land will increase by 1362.38 km2 on average from 2017 to 2050, with an annual growth of 41.28 km2, a 32.9% increase in annual growth comparing to that in 1980-2017. Among the different types of cities, the growth of urban land area in small cities will be the fastest, with an annual growth of 18.08 km2, or a 79.1% increase comparing to that in 1980-2017. Among all cities, the urban land growth of Zizhou city will be the fastest, with an annual growth of 0.85 km2, which will be a 9-fold increase in annual growth during 1980-2017.

(3) Urban expansion in the future will aggravate the degradation of natural habitat quality. Under the five localized SSPs, urban expansion will lead to an average decline of 0.50% in natural habitat quality during 2017-2050 in the HBOY urban agglomeration, which will be a 28.2% increase comparing to the decline in 1990-2017. Among the different types of cities, the decline in natural habitat quality caused by urban expansion in small cities will be the fastest. From 2017 to 2050, the average decline in natural habitat quality will be 0.28% in small cities, which represents an 88.3% increase comparing to that during 1990-2017. Among all cities, the decline in natural habitat quality caused by urban expansion in Zizhou city will be the fastest, with an average decline of 0.53% from 2017 to 2050, which will be 3.86 times comparing to that during 1990-2017.

 (4) Future urban expansion will lead to concurrent losses of multiple ecosystem services, and such losses will be further intensified over time. With the expansion of urban land in the HBOY urban agglomeration from 2017 to 2050, food production, water retention, natural habitat quality, air quality regulation, and landscape aesthetic will reduce by 1.12-1.76%, 0.63-1.04%, 0.43-0.72%, 0.31-0.69% and 0.27-0.44%, respectively. Natural habitat quality and air quality regulation, food production and water retention, water retention and landscape aesthetic, food production and landscape aesthetic and water retention and air quality regulation will exhibit concurrent losses, and the intensity will become increasingly severe. Among them, the concurrent loss degree between natural habitat quality and air quality regulation will be the greatest, and the correlation coefficient between them in 2017 and 2050 will be above 0.85. The growth in the concurrent loss degree between food production and water retention will be the fastest, increasing by 1.87 times comparing to that in 1990-2017.

Therefore, the HBOY urban agglomeration should strengthen the optimization of urban land patterns, enhance the intensive use of urban land, and protect important ESs. Targeted measures should be taken for different types of cities. Large cities should ensure that their urban sizes are compatible with the carrying capacity of their regional environment. Medium cities should build efficient cities by relying on the transportation network of the urban agglomeration. Small cities should develop green ecological cities according to local conditions.