1 Introduction

Since about the year 2000, more than half of the world’s population lives in urban or highly urbanized areas (Swanson, 2007). The World Bank estimates that by 2030 the built-up area of industrialized countries will have expanded to some 500,000 square kilometers (Angel et al., 2005*). So it can be emphasized that an important part of research concerning sustainable development should focus on these areas. From a traditional perspective, “urban areas” are generally interpreted as cities or towns with their specific urban form (Lynch, 1954*), but numerous studies during the last decades suggest that urban areas have grown far beyond the edges of core city areas and agglomerations, this growth manifesting itself as urban sprawl, sometimes resulting in hybrid peri- or semi-urban landscapes. Suburbs and exurbs are rapidly expanding (Pickett and Cadenasso, 2008*), with negative impacts on the environment (Baker et al., 2002*; Doughty and Hammond, 2004*; Hasse and Lathrop, 2003*; Nuissl et al., 2008*; Pauchard et al., 2006*; Perry and Nawaz, 2008*; Shaw et al., 1998*; Syphard et al., 2007*).

When systematically browsing scientific literature on subjects related to incompletely urbanized areas, three main research topics can be identified. The topic covered by the majority of research papers is about the environmental and ecological impacts of land transformations from rural to urban, or about the vicinity of urban development to natural resources. There is also much literature to be found on the economic (e.g. land value (Dwyer and Childs, 2004*; Mori, 1998*)) and social (e.g. segregation in the suburbs (Galster et al., 2001*)) impacts, making the entire research topic around sprawl, the urban fringe (and its dynamics) and semi-urban areas a sustainable development issue in its full right. The effects of the urbanization processes or the effects of the presence of urban and semi-urban areas on the environment have been studied in numerous disciplines: urban planning (Cadieux, 2008*; Frenkel, 2004b*; Hümmeler, 1998*), geography (Antrop, 2004*; Bourne, 1996*; Hagoort et al., 2002*), sociology (Bialasiewicz, 2006; Bryant, 1995*; Hite, 1998*), ecology (Cornelis and Hermy, 2004*; Gaston et al., 2005a*; Shaw et al., 1998) and hydrology (Hasse and Lathrop, 2003*; Perry and Nawaz, 2008*). As a result of these studies, a rich but inconsistent vocabulary emerges to describe the region that is characterized by the expansion of urban fabric, the latter being the driving force of pressures to the environment. Overall, these papers focus on complexity, fragmentation, and heterogeneous land use (Carrión-Flores and Irwin, 2004*; Gallent et al., 2004*; Galster et al., 2001*; Tacoli, 1998a*,b*). A second group of papers covers the analytical background of the landscape properties: research on indicators, scale issues, and method to measure sprawl, urban landscapes, and fringe dynamics. A third and smaller group of papers covers the actual descriptions, definitions, frameworks, concepts and policies around sprawl, urban fringes and semi-urban areas.

More often than not, the land use policy storylines remain within the classical dichotomy of urban versus rural areas: a result of separated traditions in science and in policies. Rural policies in the European Union for example, are still strongly linked to ‘core’ agricultural areas or to rural areas little transformed by urban and industrial development. Other sectors and activities in rural areas such as nature conservation are still relatively remote from urban conditions, both geographically and conceptually. Compared to rural discourses and policies, urban policies are based on completely different traditions and theories, with guiding disciplines such as architecture, civil engineering, urban planning, and transport science (Pickett and Cadenasso, 2008).

Because of disciplinary clefts between these different traditions and approaches, a standard definition of these semi-urban areas does not exist. Going more deeply into focal concepts of semi-urban areas, many authors also claim that there is a lack of a clear definition of closely linked phenomena: what exactly is sprawl, what is the urban fringe and what more generally are semi-urban landscapes (Adell, 1999*; Antrop and Van Eetvelde, 2000*; Fulton et al., 2001*; Galster et al., 2001*; Masuda and Garvin, 2008; Schuyler, 1986; Tacoli, 1998a*; Theobald, 2001*; Wolman et al., 2005*; Yang and Hillier, 2007*). With growing popularity of holistic, integrated (landscape) research (Antrop and Van Eetvelde, 2000*; Musacchio and Wu, 2004*), and with growing pressure on developing sustainability policies for complex environments, the need for a general framework for semi-urban areas, including definitions, measurement and interpretation tools, becomes inevitable.

The first specific objective of this paper is to make a synthesis of major descriptors of the semi-urban condition. A second specific objective is to give a brief but comprehensive overview of the research topics related to semi-urban areas, in which the concepts from the first objective could be applied, or why they should be developed. In the first part of this paper, a short explanation about the methods, used to study the literature dealing in some way with semi-urban areas, is given, followed by a brief overview of some of the research programs, study topics and study regions often cited. The second part will then attempt to distill the main theories and concepts found in the literature and link them to actual research on semi-urban areas, ecological impacts, and sustainable land use planning.

Although it is difficult to create a clear-cut structure in a review with a such broad perspective, framework and somewhat theoretical and philosophical approach, the paper is organized in the following structure:

• The preceding text is the introduction: it gives the main background, the ‘reason for reviewing’ and the specific angles and viewpoints which form the baseline of the study.
• The next Section 2, ‘Reviewing semi-urban areas’, can be read as a “materials and methods” chapter. It explains the methods used conducting the review study. Since the “materials” in such a study mainly consist of (all kinds of) other publications, all of which can be found in the reference list, this chapter also gives a brief overview of a possible classification and summary (yet without interpretation) of the “where and what” of the studies, conducted in the literature that was consulted as sources for this study. In a later phase of the review, this information can be useful to give a – be it brief – practical state of the art of research on semi-urban areas and its parallel research topics.
• Sections 3 and 4, coping with definitions, theories, and research issues concerning semi-urban areas, should be read as the “results and discussion” part of the study. In these chapters, all information distilled during the reviewing process is identified, interpreted, linked to other information and explained. Because of the specific character (review) of the study, these sections, evidently, are much more than a sober summary of ‘results’, as is done when publishing field or lab experiments, for example. Both sections will already be more conclusive and interpretative and the reader may note that the ‘line’ between these sections and the last chapter, the conclusions, is not as “pure” as in other scientific papers.
• The last Section 5 contains the general conclusions, as a summary drawn from the discussion and results in the preceding chapters.