The Rise and Decline of Settlement Sites and Traditional Rural Architecture on Therasia Island and Their Reciprocal Interaction with the Environment

Abstract

This study reconstructs the recent history of Therasia by examining the interplay between traditional settlements and the island’s natural and agricultural landscape. Through an interdisciplinary approach that integrates spatial and architectural analysis, historical records, and NDVI-based vegetation mapping, this research highlights the significant impact of agrarian structures like dry-stone walls and terraces on the environment. The unique dry-stone wall and terrace system is shown to have been crucial for both spatial organization and the survival of Therasia’s inhabitants. This study also addresses the challenges of dating these structures, employing a relative dating methodology to establish a timeline of habitation from the late medieval period to the present. This research reconstructs the island’s recent history, from early seasonal settlements to the eventual decline of rural architecture in the 20th century. The findings indicate that settlement patterns were closely linked to environmental and geomorphological factors, with the evolution of these sites driven by agricultural practices and broader socio-economic changes. This research not only reconstructs Therasia’s past, but also proposes a novel methodological framework combining remote sensing with traditional historical analysis, offering new insights into the relationship between human settlements and the environment in the Aegean region.

1. Introduction—The Unique Case of Therasia

Therasia, part of the larger Santorini (Thera) island group in the south of the Cyclades, has a unique volcanic environment with special architectonic features [1,2,3] (Figure 1). It is also associated with an unexplored and unique environment, which is the result of the volcanic landscape. The rural economy and activity of Therasia has changed considerably in the last century, with only a small percentage of the island now cultivated. Significant population decline, internal migration, and changes in agricultural technology over the last century have led to the abandonment of both traditional rural structures and settlement sites [3,4]. Today, the landscape of Therasia is covered by building remains from the past. Apart from the three modern villages of Manolas, Potamos, and Riva, there are dozens of abandoned rupestrian settlements compounds, while uncultivated terraces, destroyed paths, semi-destroyed dry-stone walls, ruined mills, half-buried threshing floors, etc., characterize the deserted rural landscape. The island can therefore be considered a closed “isolated” system with a preserved natural environment.

Until recently, the island of Therasia was terra incognita for the scientific community. The vast majority of publications come from one source: the THALES project (2012–2015), “Island Cultures in Diachronic Perspective: The Case of Therasia”. This project was a multidisciplinary study with a primary focus on the island’s history. The project encompassed a wide range of research areas, including history and archaeology, archaeological surface survey, geology, ecosystem, settlement patterns and architecture, as well as social anthropology. The findings were collated in four edited volumes (Therasia I–IV) [5,6,7,8]. This paper presents part of the research results from the more recent Arch2Plant project (2021–2024): “A Diachronic Perspective of Rural Architecture, Settlement Sites, and Plant Biodiversity on Therasia Island”. The research presented in this paper was conducted as part of the Arch2Plant project. Its aim was to study rural architecture, plant biodiversity, and human activities on the island of Therasia over time. The project sought to explore the relationship between the human-made environment and natural vegetation, from prehistory to the present, through interdisciplinary methods including archaeology, architectural analysis, ethnography, and molecular plant identification.

Most of the Cycladic villages and towns, which emerged between the late medieval times and the middle of the 20th century, have, as their main characteristics, a dense, interwoven construction, narrow access and communication roads, similar types of small houses, often consisting of two floors, and a general sense of spatial economy that seems to dominate the settlement organization [9]. Santorini and Therasia share two main types of settlement plans: linear settlements, typically built on the edge of the caldera and rupestrian settlements, built in the ravines [2,10,11]. All of Therasia’s settlements (and most of Thera’s) follow the above characteristics, but also have some features that render them unique. All of them were built on volcanic soil and are the only rupestrian settlements in Greece. In contrast to Thera, where touristic development has overwhelmed the landscape and erased all micro-settlement sites [12,13], the remains of Therasia’s settlements can be found deserted and abandoned throughout the island, though some are still in use as pens and rural storages.

Therasia, as a closed system, has preserved a large part of the natural and cultural landscape, resulting in features that make its study particularly interesting. A survey between 2013 and 2014 has indicated the existence of more than 80 rupestrian compounds, each of which could contain from a few to a dozen cave buildings, with the village of Agrilia, an abandoned rupestrian village spread over two ravines, being the most prominent [4,14]. In addition, a direct link between the layout of the settlements and the intra-site cultivations (gardens, cisterns, etc.) with unique characteristics (architectural, structural, and social), creates a rare rural built environment [15].

There are also a large number of buildings and structures in the countryside, examples of traditional rural architecture, with features not commonly found on other islands. Specifically, 75% of the island is covered with terraces [3], an exceptionally large area that is the largest in the Aegean [16] (Figure 2). Moreover, the traditional path/road network is over 130 km long compared to 16 km of the modern road. These two features together make up a unique man-made environment, which covers the whole of Therasia, creating a network of rural structures and the context in which all the island’s activities take place. The main element and tool of this context is the dry-stone walls, the construction type used in the vast majority of traditional buildings on the island. Terraces with dry-stone walls over 4 m high have been recorded, and dry-stone walls forming roads over 6 m wide have been found in the island’s path network [3]. These two characteristics are considered to be exceptionally abundant given the size of the island, dwarfing the humble rupestrian structures and indicating the great importance of rural architecture.

Dry-stone walls, being the most prevalent and widespread structural feature in the countryside of Therasia, have been recognized as a means of enhancing biodiversity for over four decades [17]. These constructions are considered to promote biodiversity, facilitating the creation of ecosystem linkages, corridors, and platforms, and supporting the endemic flora and fauna [18,19]. The impact of dry-stone walls on biodiversity in the Aegean has been confirmed by a number of studies. These include research conducted on the Dodecanese by the authors of [16], on Kythera by the authors of [20], and on Andros, Naxos, and Milos by the authors of [21].

The investigation of settlement trajectories through time, particularly the analysis of the factors that led to their abandonment, has been a prominent area of research for over half a century. The intricate nature of the subject and the necessity for a multilevel approach were first articulated in the 1960s by Hélène Bibicou-Antoniadis, who emphasized the importance of integrating historical and archaeological studies [22]. The field of archaeology has thus far provided excellent examples of multidisciplinary and diachronic approaches to the study of settlement patterns. Notable projects including the Boeotia Survey Project [23], the Laconia Survey Project [24], and the Pylos Regional Archaeological Project [25] were conducted in complement to archaeological fieldwork with historical studies and geological analysis.

While studies of terraced spaces and settlement patterns have been conducted globally, the Mediterranean region has received the greatest attention, particularly in recent years. This is due to the complex interplay between human activities, environmental management, and long-term landscape evolution. In the Mediterranean basin, terraced landscapes are particularly significant, yet they remain among the least understood agrarian features despite their widespread presence [26,27]. Numerous studies have examined the historical development of these terraces, such as in the Ebro Valley [28] and the Basque Country, where the progressive expansion of terrace agriculture was linked to vineyard production over the last 1300 years. Similarly, in other parts of the Mediterranean, terrace construction peaked in the Middle Ages, with settlement and agrarian practices profoundly shaping the landscape [26,29,30]. In the Cyclades, research on islands like Naxos and Keos has employed archaeological, ethnoarchaeological, and historical methods to characterize the development of rural landscapes and settlement patterns, emphasizing shifts in habitation and land use over time [31,32,33]. These studies highlight the complex relationship between terraced agrarian systems and settlement locations, a topic that remains underexplored in many Mediterranean regions [34]. By situating the investigation of Therasia within this broader context, this study aims to contribute to understanding the rise and decline of settlement sites and rural architecture in the Aegean.

Over the past two decades, the study of abandoned settlements has emerged as a key area of research to gain a deeper understanding of the transformation of Greek agrarian societies. The advent of new technologies and tools has brought the landscape to the fore in this field of study. Software programs, such as Geographic Information Systems (G.I.S.s) facilitate the organization of spatial data [35], while others like Agisoft’s Photoscan/Metashape enable the rapid documentation of land and buildings in great detail [36]. Remote sensing techniques offer valuable tools that can provide interpretations that are otherwise impossible [37].

A thorough understanding of the history of Therasia, particularly its settlement patterns and rural architecture, is essential for contextualizing the island’s development through time. While previous studies have touched on the general settlement trends within the Santorini complex, there remain some gaps in understanding the specific environmental, geomorphological, and socio-economic factors that influenced the establishment, rise, and decline of settlements on Therasia [4,38]. There is limited research into when the first habitats and settlements were established during the early modern period. In addition, the intricate network of dry-stone walls and terraces, which served not only agricultural but also spatial organization purposes, has not been fully explored in terms of its long-term impact on the landscape. Furthermore, little attention has been paid to the gradual abandonment of these structures, especially during the 19th and 20th centuries, which is important for a better understanding of the history of the landscape in Therasia. These gaps highlight the need for a more detailed investigation of the spatial and architectural evolution of Therasia, motivating this study to reconstruct the island’s historical timeline and provide insights into its rural transformation. By addressing these unanswered questions, this research aims to contribute not only to the understanding of Therasia’s history, but also to the broader study of settlement patterns in the Santorini complex.

The principal objective of this study was to reconstruct the trajectory of Therasia through the last few centuries, seeking to address the following questions:

• Are there rules governing the selection of habitats and settlement locations in Therasia?
• How do environmental and geomorphological factors impact the habitation and the selection of habitation sites over the last few centuries?
• Are there any research tools that can depict the two-way relationship between the vegetation and the traditional rural architecture and the settlement and habitat sites?
• Are there any gaps that can be filled in the history of Therasia over the last two centuries by studying the island’s countryside?

This work represents the first investigation into the role of terraces and dry-stone walls on the vegetation of Therasia, exploring the composition, organization, and impact of these structures on the island’s wild flora. The aim of this research was threefold: (i) to investigate the abandoned settlement sites and terraces of Therasia, and to explore their interconnections; (ii) the unique research approach of the interactions among rural architecture, settlement placement, and environmental and geomorphological factors, and (iii) an in-depth understanding of the relationship between the island’s rural architectural heritage and the natural environment. The results are expected to contribute to a comprehensive understanding of the changing pattern of inhabited sites, and to shed new light on the recent past of Therasia.

2. Materials and Methods

In the context of the Arch2Plant project, seven field trips were conducted in Therasia between 2021 and 2023. The objective was to survey the land and document and study the abandoned settlements and rural architecture of the island. The survey methods incorporated both traditional and technological approaches for the collection of architectural and topographic data. More specifically, the traditional land surveys were conducted using a total station, and they were supplemented with aerial reconnaissance, which was performed by a small DJI Mavic 3 uncrewed aerial vehicle (UAV). The use of the UAV facilitated the process of surveying and documentation, enabling a more rapid and systematic collection of imagery, including areas on the steep cliffs of the caldera, in ravines with difficult access, and regions where vegetation posed challenges.

The spatial study of Therasia’s rural architecture and settlement sites was conducted using comprehensive data sheets for each site, which were utilized to create maps, plans, architectural drawings, constructional details, and three-dimensional depictions of the man-made and natural environments. To facilitate comparison, aerial photographs of Therasia from the 1940s, 1960s, and 1980s, along with maps from the Hellenic Military Geographical Service, were employed as reference points in conjunction with the modern ones. Maps depicting rural architecture and numerous orthophoto maps of all the settlement sites were developed using design programs, such as Autodesk AutoCAD and Adobe Photoshop, along with the geophysical, slope, and visibility maps of the Geographic Information System (G.I.S.). Additionally, high-definition satellite images, processed appropriately, were employed to develop Vegetation Density, True Color, False Color, and Land Cover maps. The development of these maps allowed for the systematic and structured collection, management, and spatial analysis of substantial amounts of data with the objective of elucidating the interrelationship between the built and the natural environment of the island.

Therasia’s past was approached using the relative dating method within its historical context. The objective of this study was to construct a sequence of terrace abandonment, thereby establishing a relative timeframe for the inception of deterioration in rural architecture in the countryside of Therasia. This approach encompasses an extensive field survey focusing on abandoned settlements and rural architecture, an investigation of scarce written sources, visual sources, such as aerial photographs and old maps, toponyms (based on the work of Boufalis [39,40]), oral interviews with local informants, and the utilization of remote sensing tools including NDVI maps.

2.1. Abandoned Settlement Sites

Based on the results of the THALES project (2012–2015) and the initial survey conducted, nineteen possible settlement sites have been selected. For each settlement site, detailed data sheets were created with information regarding (a) orientation and coordinates, (b) morphology, (c) uses and possible dating information, (d) description, (e) urban, architectural, and construction elements, and (f) environmental and biodiversity data. The data sheets were completed with photographs, maps, and sketches.

The main challenge was to identify the settlement sites through the application of appropriate selection criteria. In the context of the THALES program, the sole criterion for identifying a settlement as a habitat was the number of rupestrian buildings. It was assumed that every site comprising twelve or more rupestrian caves was a settlement site [4]. However, interviews with the locals demonstrated that the assumption was wrong. Many sites, although they had many rupestrian buildings, were used solely as livestock facilities or were extended farmsteads. New research on Therasian house typologies at the semi-abandoned village of Agrilia [15,41,42] provided the theoretical tool to re-examine all of the sites. According to Daniil, a “Residential Complex consists of a one-room or two-room residential building, above ground or rupestrian with a courtyard. Buildings of auxiliary facilities can be included in the complex”. Given the challenging conditions and limited communal facilities in all the sites that are in the countryside (and not in a village), access to water was of paramount importance. Thereby, the inclusion of a “sterna”, a rainwater cistern and the main water supply structure in the island’s old villages, to the above definition was used as a tool to delineate the fundamental residential unit of a micro-settlement in Therasia.

A significant portion of the research was dedicated to the study of the rupestrian buildings present at each site, involving a detailed examination of their architecture characteristics and construction features. Over two hundred such buildings were studied and measured drawn. These data were then used to establish distinct typologies and determine whether these structures were utilized for residential, auxiliary, or husbandry purposes. This analysis has been invaluable in the identification of the specific Residential Complexes (RCs) present at each site.

The term “hamlet” is defined in the Oxford English Dictionary [43] as “a group of houses or a small village in the country; especially a village without a church, included in the parish belonging to another village or town”. This definition of a hamlet, with a population range extending from 10 to 250 residents [44], allows the definition of a Therasian micro-settlement. Given that the 19th century demographic data of the island [45] indicated an average family size of four members, a Therasian hamlet or micro-settlement site would comprise a minimum of three RCs.

2.2. Rural Architecture

On an island level, a series of detailed maps depicting the terraces, the dry-stone walls, the paths and the roads, the threshing floors, the windmills, and the wells were developed. These maps allowed for a comprehensive understanding of rural architectural features on Therasia, and they were the link between the abandoned settlement sites and the natural environment.

The island was scanned using a variety of remote sensing tools, including high-resolution satellite photographs, aerial photographs from UAVs, and traditional surveying methods (in situ photographs, sketches, and notes). Every dry-stone wall on the island was recorded and documented, excluding those present in the three modern villages (Manolas, Potamos, and Riva) and the semi-abandoned village of Agrilia, which had been previously thoroughly studied [15,41,42]. The rate of deterioration of the Therasian dry-stone walls was ascertained through the analysis of data and photographs collected from the THALES project (conducted over a decade ago), along with aerial photographs provided by the Hellenic Military Geographical Service from the 1940s, 1960s, and 1980s, and interviews with elder locals. A map was developed to illustrate the current state of the dry-stone walls on the island. This is discussed in further detail in the Results section below. Dry-stone wall condition was recorded on a four-point scale: (i) walls in good condition; (ii) walls that had collapsed in places; (iii) walls where over half of the walling had been collapsed; and (iv) walls with almost nothing standing.

The aforementioned ethnographic research was based on semi-structured interviews, which are an appropriate approach for ethnographic studies [46]. In particular, more than 40 interviews were conducted, which is a notable number given that the total population of Therasia is 249, according to the most recent census data [47]. The informants were of diverse gender and age groups, ranging from 50 to 100 years old. All of the informants were or had been farmers, and they had memories of ancestors who had lived on the island from the late 19th century onwards.

At an architectural scale, one hundred dry-stone walls were selected to study their features. Each wall was measured, drawn, and its unique relation with the surrounding vegetation and biodiversity was documented (Figure 3). The selection process was based on the following criteria: (a) the dry-stone walls must have architectural or structural significance, (b) they must be in good condition, and (c) they should be located across the entire island. Additionally, special attention was given to (d) the dry-stone walls along the paths and (e) those in the abandoned micro-settlements.

The role of the extensive road/path network has also been fully explored in the frame of the Arch2Plant survey, which complemented that of the THALES project by studying twenty-nine characteristic paths. This resulted in the production of the typology and a deeper understanding of the employed architectural forms and constructional techniques.

2.3. Toponymical Research

The toponyms of an area reflect the way space is marked at the time of recording. At the same time, however, they also constitute a projection into the past, a living source of information about the historical trajectory of the place. The toponyms of the Aegean islands are considered remarkably resilient over time and have remained in use, often unchanged from at least the period of Ottoman rule (16th–19th century) to the present day [48]. The work presented herein was based on the recording of the toponyms in Therasia, which was conducted by Boufalis [39,40] between 2009 and 2014, resulting in the documentation of more than 280 toponyms. Following the example of Boufalis, a complementary ethnographic research study was carried out focusing on the toponyms of settlement sites and those related to rural activities (micro-toponyms) or plants (phytonyms). By selecting specific categories of toponyms and placing them in different map contexts (vegetation map and dry-stone wall condition map), the information was organized in such a way that spatial and land use conclusions could be drawn.

2.4. Vegetation Density Maps

To understand the interrelation between the rural architecture, the flora associated with it, and the environment of Therasia, the Normalized Difference Vegetation Index (NDVI) was employed. Using satellite images, such as those from Sentinel II, NDVI produces maps with a color range from red to blue/black, where red indicates a high density of vegetation and blue/black indicates low or no vegetation. As such, the NDVI quantifies vegetation by measuring the differential absorption and reflection of light by plants, i.e., the reflection of near-infrared light and the absorption of red light [49,50]. The tool has been employed in numerous studies to facilitate the investigation of rural environments and settlement sites [51,52,53].

3. Results

3.1. Abandoned Micro-Settlement and Habitat Sites

Therasia Island is one of the three segments of the Santorini volcanic cone that remained above sea level after the last large catastrophic eruption of the Santorini volcano in the 2nd millennium BC [54], also known as the “Minoan” eruption [55]. Geomorphologically, the island is divided into three distinct zones: (a) the east, characterized by the steep cliffs of the caldera with a short flat area at the top; (b) the middle, shaped by seventeen large ravines and many smaller ones, fifteen of which run east–west; and (c) the plain to the west and northwest. The settlements are distributed into two main areas: at the brow of the caldera, facing east, and along the cliffs of the ravines. The buildings, dating from the late medieval period to the first half of the 20th century, are, in the vast majority of cases, rupestrian, dug into the volcanic soil (Figure 4).

A comprehensive architectural study was conducted in conjunction with an investigation of environmental and geomorphological factors. Of the nineteen sites initially studied, only nine met the criteria to be considered micro-settlements. However, four additional habitat sites were studied in depth due to their unique characteristics, bringing the total number of sites to thirteen (Figure 2). The objective was to devise, through spatial analysis, the “typical Therasian settlement”, which would encapsulate the decision-making processes of the islanders when selecting new habitation sites.

The settlements are categorized into two broad types based on their typology. The first type comprises settlements located in the ravines, usually with rupestrian buildings on both sides of the cliffs (Figure 5a). All these sites have an east–west orientation. The second type consists of linear settlements where all buildings are oriented towards an open space, either at the brow of the caldera, looking east toward Thera, or west toward the plain (Figure 5b). These sites have a north–south orientation. Of the thirteen sites studied, seven belong to the first category, while the remaining six fall into the second category. The settlements are categorized according to their location. Their specific characteristics and the number of Residential Complexes (RCs) are presented on (

Table 1. Abandoned settlements and selected habitat sites.

Sites at Ravines				Sites Looking at Caldera/West Plain
	Name	RCs	Notes		Name	RCs	Notes
1	North of Potamos	3	Due to their close proximity to Potamos, many of the caves are still in use for husbandry purposes. The ravines have a similar morphology.	8	Peristeries	20	This is the largest micro-settlement. Some of the caves are still used for husbandry purposes. Many roads and paths have collapsed, making the lower caves inaccessible. It is located to the north of Manolas.
2	South of Potamos	3		9	Vigla	3	It is a known micro-settlement identified from local interviews. The name, of Venetian origin, indicates the presence of a lookout.
3	Trachylas	15	Situated 50 m to the southeast of modern Potamos, many caves are still used for husbandry purposes. There is a windmill to the south.	10	Ano Kera	10	Ano and Kato Kera were considered a single settlement (Kera), separated by 150 m. Between them, on a lower terrace, are numerous rupestrian caves used for husbandry purposes. This is the most important micro-settlement on the island.
4	Spentanes	3	Although it has three RCs, the arrangement of the caves is sufficiently dispersed to warrant further investigation.	11	Kato Kera	7
5	Volia	8	The most remote of the settlements. It features several excellent RCs and a distinctive large dry-stone wall at the torrent bed, likely constructed to manage the torrential stream.	12	Evangeli-smos	2	It features (a) the largest rupestrian building on the island (30 m in length), (b) a church, and (c) a canava (winery). The habitat site is oriented towards the western plain.
6	NW of Agios Ioannis	3	It is one of the few habitat sites where the ravine is oriented to the north. However, it is too dispersed to be considered a micro-settlement.	13	Katoikies (Apostolaki)	2	It may have originally comprised one or two large farmsteads. The habitat site is oriented towards the western plain.
7	Agios Georgios	2	Although it has only two RCs, the density of the rupestrian caves and the two levels of habitation make it a probable micro-settlement.

). The sites marked in gray, although they do not meet the criteria for a settlement, are included due to their unique features (Figure 2).

The use of vegetation maps (NDVI) proved to be an unexpected, yet invaluable, tool in the research of the “typical Therasian settlement” in a ravine. Two vegetation maps were developed: one during spring, when the vegetation is lush, and one during early autumn, when the flora is almost entirely dried out. By studying the ravines of the island, and specifically the locations of the settlements, it was found that the vegetation on the southern cliffs is denser than that on the northern cliffs (Figure 6).

To explore this further, the focus turned to the wind patterns of Therasia. Information was gathered through informal conversations with local informants, and bibliographical research [56,57,58] indicated that the strongest and most influential wind on the island is from the northwest. Studying the settlements in the ravines under the light of these findings revealed two important spatial choices. The majority of the rupestrian houses (RCs) are built on the northern cliffs of the ravines at a ratio of approximately 2.5:1 compared to those on the southern cliffs. Additionally, the NDVI maps revealed that the majority of the gardens are located on the southern part of the ravines (Figure 7).

Regarding the spatial planning of the settlements in a ravine, the selected locations were usually the narrowest parts of the ravines. This is evident in the majority of the habitat sites and the smaller micro-settlements. The placement of the settlements is typically in close proximity to a large, flat, cultivated area, most often the western plain of the island. These sites also needed to be steep enough to accommodate one, two, or more floors of rupestrian buildings.

The dimensions of the ravines accommodating habitat sites and settlements are also standardized. The width of the ravines in smaller sites ranges from 30 to 50 m, while the length of the settlement is generally no more than 100 m. In the largest micro-settlements, these dimensions are approximately doubled (Figure 8).

The habitat sites facing the western plain never reached the level of micro-settlements. The exposure to strong northwest winds may be a contributing factor. On the other side of the island, the sites facing east towards the caldera evolved into the most prominent settlements, including the modern head village of Therasia, Manolas. The eastern sites are well-protected from the strong northwest winds, since they are situated on the east side of the cliff. The NDVI map corroborates this, confirming the well-protected settlements in the east and the exposed ones in the west.

Regarding the settlement plan, all the RCs are positioned in juxtaposition, giving the distinctive linear configuration. The terraces are elongated in a north–south direction and short in an east–west direction, with widths ranging from two to fifteen meters. The typical pattern from east to west is (i) rupestrian building(s), (ii) courtyard, (iii) road, and (iv) garden. In some cases, due to space limitations, the courtyard or the garden may be absent. The total length of the settlements is approximately 100 m in the smaller sites, whereas Peristeries and Kera (both Ano and Kato) measure over 300 m in length (Figure 8).

3.2. Rural Architecture

The island’s unique topography is the result of the Therasian practice of dry-stone wall construction and terrace formation. To better explore its significance, all the dry-stone walls and terraces were mapped and their condition was recorded (Figure 9). This challenging and time-consuming process was necessary to better understand their impact on the landscape of the island. Part of the terraces are the traditional road/path network of the island. The path/road system was developed either parallel to the terraces or transversely to them. In both cases, the road is located on the edge of the terrace, utilizing the existing dry-stone wall. The major roads of Therasia had both sides built with dry-stone walls, some of which are notable for their width reaching five to six meters (Figure 10).

Therasia is an anhydrous island with no rivers or lakes, while the two to three wells that exist on the island have brackish water, which is unsuitable for either drinking or irrigation. The only source of potable water was the precipitation that occurred during the cold months [56], which flowed through the torrents in the ravines. By building dry-stone walls at the torrent beds, the Therasians created channels that directed and distributed the torrent water to their homes and fields. They utilized the morphology of the land, and by creating suitable dry-stone walls and small terraces, they developed a simple, yet effective, irrigation system that fully exploited the scarce rainwater. The most impressive dry-stone walls of this kind are over two meters in height and are found in the settlements of the ravines.

There are two main types of dry-stone walls based on their structure: those forming the traditional paths and those making up the terraces. This architectural study further emphasizes the significance of the road network by highlighting the distinct construction of the dry-stone walls used for the paths. The extensive and comprehensive measure drawing of these walls revealed that they were built differently from the majority of the terrace dry-stone walls. The construction technology followed specific criteria: (a) the stones used for the roads are larger; (b) while the footing stones of terraces are typically larger than those in the upper rows, the stones in road construction are often relatively uniform in size along the length of the wall, or smaller stones are placed in the lower rows with larger ones on top; (c) although rare, roads are sometimes demarcated by top stones, a feature absent in terrace walls; and (d) in many cases, two or more construction phases can be identified in the dry-stone walls of the roads or in the dry-stone walls of the micro-settlements.

3.3. Environment

The study of Vegetation Density maps in Therasia provides valuable insights into ways in which rural architecture creates micro-environments and ecological niches for the flora. Notably, this is the first known application of NDVI in examining abandoned settlement settings and the impact of rural architecture on the local flora of Therasia.

Vegetation maps provided useful insights into the relationship between the environment, particularly plant diversity, and rural architecture. The NDVI maps depicted the density of vegetation on the island, highlighting the importance of rural architecture, especially terraces and dry-stone walls (Figure 11a). The results were examined and compared with the map of the dry-stone walls from Therasia (Figure 11b). Surprisingly, the two maps (Figure 11c) have a lot in common.

It is striking that each dry-stone wall is a prominent feature on the NDVI map, which indicates its importance (Figure 11c). Each dry-stone wall, especially those of the paths, is highlighted with a warmer color than its surroundings, indicating a greater density of flora on and around it (Figure 11d). The presence of flora was studied on one hundred dry-stone walls of terraces and paths, and we found that it was richer than that found in their surroundings, especially in the driest seasons (Figure 11a). In this case, rural architecture, and in particular terraces and paths, promote the presence of vegetation. The vegetation includes halophytic species, including Artemisia arborescens, Atriplex halimus, and Lycium schweinfurthii and phryganic species, such as Helichrysum italicum, Calicotome villosa, Corydothymus capitatus, along with species of Poaceae and Fabaceae families. The hundreds of kilometers of dry-stone walls create multiple microenvironments with specific flora that would not grow otherwise.

The traditional system of roads/paths, constructed with dry-stone walls, functions as a vegetation channel, establishing ecological connectivity. This is more evident in the arable land in the west plain of the island. There, the cultivated and plowed fields are depicted as blue or dark in color, while the roads are green-yellow or even red color (Figure 11a,d), indicating a higher density of vegetation. In contrast, the modern road, whether asphalt or soil, is shown as black in color, defining a clear barrier in the land through the absence of vegetation (Figure 11d)

Rural architecture impacts the vegetation of the island in more than one way. The dry-stone walls of the terraces and paths organize, demarcate, and enhance the flora. This is most evident in the paths where the different colors on either side of the wall indicate that the density of the vegetation is affected, even on a small scale, in a field or terrace. This is even more pronounced in the paths with an east–west direction. In all cases, it can be observed that the warmer color is always to the north, probably due to environmental factors, such as the strong north and northwest winds (Figure 11d).

3.4. An Integrated Approach to Rural Architectural Analysis and Toponym Research

In order to examine the settlement patterns of Therasia in recent history, this study employed an integrated approach, combining rural architectural analysis with toponym research.

The terrace dry-stone wall condition map (Figure 9) provided a comprehensive representation of the current state of rural architecture on the island. The west plain and areas around Manolas, the major village of Therasia, have terraces in good condition, corresponding to the fields that are still cultivated today. The walls in the ravines in the north and center of the island are in medium condition, while in the south part of the island, almost entirely, the agricultural landscape is in poor condition. Furthermore, in two areas neighboring the pumice quarries, one in the north and one in the south, the walls are almost completely collapsed.

The oldest available high-resolution aerial photographs are from 1975 and 1983. By monitoring the evolution of the fields and identifying those that were abandoned between these dates, it is possible to ascertain the approximate time of abandonment. A comparison of the walls in 1975 and 1983 with those recorded in this study (2022) allows for the relative calculation of the wall degradation extent (Figure 12). In the southern part of Therasia, where rural architecture is in a particularly deteriorated state, observations indicate that over approximately 40–50 years, the walls have deteriorated by one rank according to the dry-stone wall condition map (Figure 9). Archival research conducted in the Municipality of Therasia lends further support to the aforementioned hypothesis. A list of each field and its owner from the cultivated areas in the 1950s indicates that the majority of the southern Therasia region had already been abandoned. A more comprehensive understanding of the island’s evolution could be gained through an examination of the approximately 45-year span of abandonment.

Regarding rural architecture, while there is a paucity of visual documentation from the mid-18th to the mid-20th century, a number of written sources attest that Therasia was fully cultivated during the mid-19th century [45,59]. This assertion is further corroborated by interviews with elder locals. From the mid-20th century onward, the condition of the landscape underwent a notable transformation, as evidenced by the gradual abandonment of fields observed in aerial photographs taken in 1960, 1975, and 1983. This trend culminates in the satellite images of 2016, which depict the majority of the terraces as deserted. In general, it can be assumed that most of the Therasian countryside was used for farming until the mid-twentieth century.

The mapping of micro-toponyms of the fields and phytonyms enabled the identification of resilient locations over time. The present study builds on the work of Boufalis [39,40] by incorporating newly discovered toponyms related to settlement sites, such as Katoikies and Spentanes. The analytical study of these toponyms yielded valuable insights into rural architecture, cultivated fields, and flora. More than one-third of, or 112, toponyms, refer to fields, wells, mills, and terraces, indicating the enormous importance that cultivation played in the daily lives of the Therasians. Of these 112 toponyms, the vast majority—102—refer to fields. A significant number of the fields in question indicate ownership, such as Nikiforou Field (translated as the Field of Nikiforos) and Ag. Gianniou Field (field belonging to the church of St John). Additionally, several of the fields indicate the geomorphology of the area in question, with Potamida being a notable example, as it indicates that the field is situated at the bottom of a ravine. Furthermore, there are 24 phytonomic place names, with the most prominent being Agrilia village (translated as the wild olive tree). Place names of this type have a strong presence in island regions, with the predominant element being the need to denote the function indicated by their names, rather than the naming of the place itself [48].

Over half of the toponyms correspond to active, cultivated areas, which indicates a direct relationship between the toponyms and the cultivation of the land. It can be reasonably deduced that a field is more likely to have a name if it is cultivated. A comparable relationship exists between the toponyms and the condition of the fields. Approximately two-thirds of the names are located in areas where the dry-stone walls are in good condition, while approximately one-fifth are in areas where the terraces are in medium condition. The greater the condition of a field, the more likely it is to have a toponym (Figure 13). These observations indicate that if there is a memory of a place, it is more likely to have been abandoned more recently.

4. Discussion

The findings of this study demonstrate a reciprocal relationship between the traditional settlements and the natural and agricultural landscape of Therasia. Rural architecture, including dry-stone structures and terraces, has exerted a direct influence on the surrounding environment and the distribution of vegetation. Furthermore, by evaluating the extent of deterioration in rural architecture and integrating ethnographic insights with historical data, this study endeavors to document snapshots of Therasia’s recent past.

Although it is well established that rural architecture influences and supports aspects of biodiversity [18,60,61], this is the first time that this phenomenon has been demonstrated in an NDVI map. The application of Vegetation Density (NDVI) maps, in this context, proved invaluable for identifying vegetation presence and expansion. Although there is a considerable corpus of studies employing NDVI in the investigation of rural environments, the majority of these studies are situated within the field of archaeology, with the primary objective of identifying buried structures or settlement sites. It is notable that none of the aforementioned research projects employed this tool to document environmental features that affect vegetation, such as the impact of strong winds or the interaction between geomorphology and the rural built landscape. A further innovation of this study is the scale and scope employed. The majority of research studies focus on a relatively narrow area or feature, such as farmlands, a village, a well, or roads [62,63,64]. In the present study, the island is considered as a whole, encompassing all its micro-environmental zones. Moreover, the research question of this study, “Are there rules governing the selection of habitats and settlement locations in Therasia?”, necessitated the identification of universal conditions in Therasia that could link the built environment with the geomorphological factors influencing habitation. The combination of NDVI with a comprehensive architectural and settlement planning investigation, coupled with an ethnographic study and a toponymic analysis, has elevated the significance of the natural environment and the specific characteristics of vegetation in the anthropogenic countryside of Therasia.

The process of dating and studying the historical development of agricultural regions characterized by the presence of terraces and dry-stone walls is notoriously challenging [65]. The application of architectural criteria alone renders the absolute dating of rupestrian settlements on Therasia an exceptionally challenging endeavor [38]. The distinctive morphology of these buildings, with walls, floors, and ceilings carved from volcanic rock, frequently results in the complete loss of architectural features following a period of abandonment, leaving only bare caves. Direct dating via archaeological research was not selected due to the time-consuming bureaucratic procedures, high costs, and uncertain success rates, even when utilizing established methods, such as luminescence dating [66].

This study employed a relative dating approach to examine the traditional built environment, integrating multilevel and interdisciplinary methodologies. While such approaches are well-established in Mediterranean research [28,67,68], they offer a comprehensive understanding of a location, allowing for the establishment of a sequence of events [69]. Moreover, the efficacy of analogous methodologies in research conducted on Keos provided further justification for the selection of this approach [32]. By examining the approximately 45-year span of abandonment and documenting the evolution of Therasia’s development over the past two centuries, a more comprehensive understanding of the island’s evolution was gained.

Environmental and geomorphological conditions have played a major role in shaping the morphology, typology, and selection of settlement sites in Therasia. Previous researchers, such as Palyvou [2] and Boufalis [70], have explored the built heritage of Therasia. For instance, Palyvou suggests that the rupestrian settlements in ravines were particularly favored by the lower class because they allowed for construction without wood, a scarce resource on the islands, and enabled efficient rainwater collection. These settlements were also strategically located near agricultural fields in the western plain and southern regions. Boufalis, building on Roberts’ work [71], reaffirms these criteria and further explores factors like defense against piracy in the 18th and early 19th centuries, thermal comfort in rupestrian buildings, and the ownership status of these buildings.

This study addresses the dearth of detailed analysis on the environmental and geomorphological factors that were pivotal in the selection of settlement sites within the Santorini complex. By focusing on how these factors influenced settlement choices in Therasia, the research establishes a foundation for investigating potential settlement sites across Santorini. Therasians sculpted the island with the aid of dry-stone walls and terraces. These two components formed a system, a double interdependent tool that, though humble, was in fact a highly powerful and influential tool that it is considered crucial to the survival of the people and the smooth functioning of society and the economy [3].

The uniqueness of the dry-stone wall–terrace system, in comparison to other Aegean islands, lies in its dual functionality: it served not only as a means of creating arable land, but also as a tool for the spatial organization of Therasia. Specifically, this spatial organization was “drawn” on the island by the extensive path/road system, which was formed by dry-stone walls (Figure 2). The island’s roads and terraces were constructed using the same dry-stone walls, and they developed concurrently. The spatial organization of the island through its road and path network was a complex process, influenced not only by geomorphological conditions, but also by socio-economic factors. The dense network of paths and roads, which connect each settlement, field, and property on the island, represents a significant achievement, particularly given the limited size of the island. Despite the paucity of studies on traditional road and path systems in the Mediterranean, some have underscored their importance [32,67]. These networks functioned as public spaces with multiple uses and were sites of intense social interaction, particularly during the cultivation of vineyards [72,73], as evidenced on Therasia.

This study aims to reconstruct the recent history of Therasia, building upon historical and economic insights. Uniquely, it employs spatial and architectural analyses to examine the island’s past. Constructing a timeline of the Therasian countryside over the past two and a half centuries requires considering several factors: the condition of dry-stone walls and terraces, which indicate abandonment; the creation, rise, decline, and abandonment of settlements; and the premise that places with lingering memory were likely abandoned more recently. The following section provides a historical overview of Therasia’s settlements and rural architecture, drawing on a variety of sources from the late medieval period to the present.

The oldest micro-settlement is believed to be that of Volia, situated in the southern region of the island. Archaeological surface surveys in the area have revealed finds and evidence of intensive activity during the post-Byzantine to late modern period [74,75]. In addition to the preceding findings, the initial seasonal settlements are believed to have been established in Therasia during the 16th and 17th centuries. This is supported by the evidence that the first settlements outside the castles of Santorini were established during this period [10,11]. It is likely that the island had a minor settlement and a number of seasonal habitat sites in the western plain, situated in the areas currently occupied by Potamos and Agrilia (Figure 14a). This is supported by the following: (a) archaeological evidence indicating that the settlement of Volia already existed at the time [75], suggesting the island was inhabited; (b) the decentralization of Santorini’s population in the 16th and 17th centuries, which led to the abandonment of castles and the creation of seasonal habitat sites in the countryside [10]; and (c) the control of much of Therasia, particularly the fertile western region, by the Santorinian elite of Oia [45]. Given this evidence, and considering that seasonal habitat sites were common on Therasia in the 18th and 19th centuries [74], it can be reasonably inferred that Santorinian field workers began their activities in western Therasia during the 16th or 17th centuries. This interpretation differs from that proposed by Boufalis, who posits that this could not have been a permanent settlement. He advances the argument that Therasia, following the example of other islands, did not have permanent settlements until the late 18th or early 19th century [38].

By the midpoint of the 18th century, the island was fully terraced and cultivated, as evidenced by the sketch of the Ukrainian monk Vasilij Grigorovic Barskij, dated 1745 (Figure 15a), which depicted Therasia as he sailed near the western coast of the island. The presence of rural architecture is indicated by the organization of the fields by paths and dry-stone walls, as well as the prominence of an east–west road, which is likely the same route traversing the Volia settlement (Figure 14b). It was not until approximately another half-century had elapsed that the village of Manolas was recorded on Oliver’s map (Figure 15b). The village is depicted in a linear fashion similar to the settlements of Fira, Oia, and others in the caldera of Santorini [76]. It can be reasonably assumed that the settlements of Manolas at the brow of the caldera, Volia at the south, and a series of habitat sites might already have existed in Therasia by that time (Figure 14c).

In the initial decades of the 19th century, a number of settlements were established, including Potamos, Agrilia, and Kera (Figure 14d). It seems probable that additional, smaller habitation sites were also developed. This proliferation of settlements and habitation sites was accompanied by demographic growth due to the Treaty of Küçük Kaynarca (1774) and the Greek War of Independence (1821–1832). The conversion of seasonal to year-round settlement sites is closely related to the rise of viticulture, which requires more time in the fields than cereal farming [77].

By 1875, Volia was abandoned, with its residents relocating to Agrilia. This migration is preserved in the oral tradition with stories of the transfer of sacred images from the church of one settlement to another, as the ethnographic study revealed. At the same time, the population of the villages of Potamos and Agrilia increased and additional habitat sites were established on the island (Figure 14e). This growth derived from the intensification of mining activity on Therasia, and the construction of two substantial quarries driven by the growing demand for Santorinian Earth for the construction of the Suez Canal (1859–1869) [78]. As indicated by the dry-stone wall condition map (Figure 9), it is probable that this period saw the initial abandonment of terraces in the northern and southern regions of the island. These areas were amongst the most remote on the island and were characterized by steep slopes. Interestingly, these two areas border Therasia’s quarries.

Over the subsequent five decades, Therasia’s countryside remained a highly active region, with two significant alterations occurring in the island’s settlement pattern (Figure 14f). Firstly, the population of Manolas, situated in close proximity to the port of Korfos, experienced a doubling in size as a consequence of the decline in mining operations [79]. A significant proportion of Therasian residents relocated from the countryside to the mining town of Lavrio or pursued careers in shipping as crew members on ships owned by Santorini shipowners. This enabled them to maintain their families on Therasia while working on ships based in Oia [4].

Secondly, during the initial three decades of the 20th century, a number of smaller settlements, including Trachylas, Spendandes, South Potamos, and North Potamos, underwent a process of urbanization, resulting in the formation of micro-settlements. In addition, other sites, including Agios Georgios, Evangelismos, Katoikies, and Agios Ioannis, underwent a transition from seasonal to permanent habitation. This transformation was prompted by a resurgence in Santorini wine production and the subsequent increase in wages, which provided landless farmers with the means to purchase land. It seems probable that a considerable number of these farmers purchased the fields they had previously cultivated, thereby reinvigorating the countryside, albeit for a period of only two decades. During this period, agrarian buildings such as stables and warehouses were converted into farmsteads [38]. The majority of toponyms associated with cultivated fields, indicating ownership, likely date from this period, marking the transition from landless laborers to landowners. Additionally, a significant number of dry-stone walls in settlements and along the traditional road network exhibit evidence of multiple construction phases, suggesting that repairs were made during the interwar period.

The 20th century marked a period of decline for Therasia. Sites that had been inhabited a few decades earlier were now uninhabited. The initial indications of decline were observed in the southern fields, which were abandoned by 1975 (Figure 14g). Subsequently, the phenomenon extended to the northern and central regions of the island, as well as the western plain, on numerous occasions. The only villages to survive were those of Manolas and Potamos, along with the two small ports of Riva and Korfos, which are now utilized exclusively for tourism. The terraces and dry-stone walls in the countryside began to crumble, remaining unrepaired, and the knowledge of their construction diminished as the older generation passed away. The causes of this decline can be attributed to a combination of environmental, historical, and social factors. The volcanic eruptions that occurred in 1925, 1939, and 1950, in addition to the disastrous earthquake that took place in 1956, had a considerable impact on the island’s environment. Furthermore, the unfavorable living conditions and subsequent urbanization, internal migration, and changes in agricultural technology that occurred in Greece during World War II and the subsequent decades contributed to the decline [4].

5. Conclusions

This study successfully reconstructs the recent history of Therasia’s settlements, demonstrating the significant interplay between rural architecture, environmental factors, and settlement patterns. The research met its objectives by answering key questions regarding the selection of settlement locations, environmental influences, and the interaction between vegetation and traditional rural architecture. The use of Normalized Difference Vegetation Index (NDVI) maps proved invaluable in identifying the environmental impact of dry-stone walls and terraces, revealing their crucial role in supporting biodiversity and shaping vegetation patterns. This is the first time NDVI analysis has been used to illustrate the environmental influence of rural structures in a Mediterranean context.

The results showed that environmental and geomorphological factors, particularly wind patterns and proximity to agricultural fields, played a decisive role in the selection and evolution of settlement sites. The unique dry-stone wall–terrace system was not only vital for creating arable land, but also for organizing space across the island. The spatial organization of Therasia’s paths, roads, and terraces through dry-stone walls highlights a sophisticated landscape management strategy, demonstrating the importance of rural architecture in both environmental and social contexts.

This study’s novel integration of NDVI analysis with architectural and historical investigation offers a new methodological framework that can be applied to similar regions. By exploring the abandonment of terraces and settlements, and analyzing their spatial and environmental relationships, this research contributes to a deeper understanding of the long-term interaction between human activity and the environment. It also fills critical gaps in the history of Therasia, such as the gradual abandonment of rural structures in the 19th and 20th centuries.

Future research should expand on these findings by applying remote sensing techniques across other Cycladic islands and regions with similar ecological and architectural features. This will provide comparative insights and further illuminate the intricate relationships between human settlements, rural architecture, and environmental factors in Mediterranean landscapes.