Garden on the Great Wall—A Sustainable Solution for the Plants on the Top Surface of the Great Wall Heritage Site

Abstract

The Great Wall is a world-renowned cultural heritage site and a national key protected cultural relic in China. The plants on the top surface of the Great Wall heritage site are one of the factors leading to the occurrence and continuous deterioration of the Great Wall. Therefore, there is ongoing academic controversy on how to scientifically dispose of the plants on the top surface of the Great Wall heritage site. Taking Beijing Dazhuangke Great Wall as an example, the preliminary field research was completed. This paper further describes the key technology for the disposal of remaining roots after removing the ground part of the plants from the top surface of the Great Wall heritage site and clarifies the basic idea of using the original covered soil on the top surface of the Great Wall heritage site, back-filling the original covered soil stripped via reparation of the layers and the regenerated plants in a seed bank to create ‘soft capping’ protection. This study provides a basic framework for ‘soft capping’ on the top surface of the Great Wall heritage site and construction of the cultural landscape of the ‘Garden on the Great Wall’.

1. Introduction

The Great Wall of China is the oldest military defense work in the world and was used by the Central Plains regime in ancient China to block the invasion of northern nomadic tribes, such as the Huns, Turks and Mongols [1]. The Great Wall is distributed across 15 provinces and regions, including Beijing, Tianjin, Hebei, Shanxi, etc., spanning the north, northwest and northeast of China, with a total length of 21,196 km [2,3]. In 1961, the State Council of China announced the Great Wall to be one of the first key cultural relics under national protection, and it was added to The Catalog of World Cultural and Natural Heritage in 1987 (No. 438). In the 1980s, for the purpose of display, restoration work restored much of the Great Wall to its original state, including at the Badaling Great Wall and the Mutianyu Great Wall. In this process, the plants on the top surface of the Great Wall (hereinafter referred to as wall-top plants) were all removed (including from the top surface of the wall, watchtowers, enemy broadcasting stations and other outbuildings). With the broadening of cultural heritage protection and improvement in the protection technology promoted by the International Council on Monuments and Sites (ICOMOS), those restoring the Great Wall gradually realized that ‘the process of unplanned removal of the wall-top plants also destroys the Great Wall itself and changes the original state of cultural relics’; ‘the authenticity, integrity and the historical style of the Great Wall heritage should be protected’; and wall-top plants may play a positive role in heritage protection [4,5,6,7,8,9]. Thus, in connection with the restoration of the Great Wall body, the technical system of plant disposal on the top surface has been a topic of discussion among researchers [10,11,12,13,14,15].

Hadrian’s Wall, built in the early 2nd century AD, is one of the typical heritage masonry sites in Europe [16]. It was originally used as the northern frontier of the Roman Empire of Great Britain, connecting the east and west coasts, with a total length of about 169 km in present day [17]. Additionally, it was an excellent case of cultural heritage conservation and restoration. In the 1830s and 1840s, John Clayton presided over the reconstruction work of one of the sections of Hadrian’s Wall, now known as the Clayton Wall. First, workers removed the waste heap along the wall and protected the scattered original Great Wall stones by means of a dry barrier. Second, after the restoration of the main wall, a layer of crushed stones was used to level the top surface of the wall, followed by installation of turf to prevent erosion from precipitation and freeze–thaw repetition [18]. The restoration method of the Clayton Wall gained attention because people wanted to know how to interpret and address the relationship between architectural heritage and plants in the area within the process of restoration or conservation and how to protect the top surface after restoration.

Initially, people regarded architectural heritage and plants in the area as an integrated entity, and they preserved plants in the heritage area for esthetic purposes. In the 1970s, some scholars believed that there was a relationship, direct or indirect, between the architectural damage of ancient architectural heritage and plants in the area and that retaining plants in the area accelerated destruction; they believed that plants should be removed to prevent the continuous damage and protect the heritage with a ‘hard cover’, mainly cement mortar, after restoration. This approach is also found in the ‘stylistic restoration’ of architectural heritage proposed by the French architect Eugène Emmanuel Viollet-le-Duc (1814–1897) [19]. ‘Hard cover’ protection was used in the restoration of the Roman Colosseum in Italy. In the 1990s, based on observation, some British scholars argued that ‘hard cover’ protection of ancient architectural heritage was not only ineffective but could even accelerate the process of decay [19]. In fact, as early as the 1930s, when the principle of ‘conserve as found’ was followed, the restoration and protection methods of the Clayton Wall from the 1830s resurfaced once again, and prolonged exploration and discussions were carried out on issues related to protecting the top surface of architectural heritage [18]. In the 1980s, British scholars, taking into account the regional climate, proposed and began exploring the ‘soft capping’ protection method for the top surface of architectural heritage. Until 1993, the publication of scientific observations regarding the use of ‘soft capping’ and the successful application in the military fort on St. Mary’s Island and the Peninsula monastery made this protection technology widespread. At the same time, studies on ecology and landscape ecology provided evidence supporting the acceptance of this technique [20]. The protection technology has since been improved for several purposes, including improving the survival rate and ecological stability of plants within the ‘soft capping’. For example, in Scotland, a double layer of turf has been employed for protection. Additionally, in Nordic countries, such as Denmark and Sweden, the ‘soft capping’ soil is supplemented with native plant seeds instead of using commercial turf or exotic plants [19].

In the restoration of the Great Wall in China, which has been continuing since the 20th century, various methods of wall-top plant disposal have been explored, such as original restoration or applying ‘hard cover’ protection after complete removal of wall-top plants. The former does not align with the fundamental positioning of the Great Wall heritage (‘the Great Wall is composed of two forms: ancient architecture form and ancient relic form, mainly in the form of ancient relic’), while the effect of the latter has been widely questioned and discussed by scholars and the public [19,21,22].

Researchers who advocate ‘stylistic restoration’ argue that wall-top plants do not originally belong to the Great Wall itself, and the splitting effect of root growth of some trees and shrubs has caused irreversible and continuous damage to the brick-and-stone structure. In their opinion, all wall-top plants should be completely removed, and the Great Wall should be restored to its historical appearance. In contrast, researchers who advocate ‘preservation in situ’ argue that nearly 400 years (from the mid-17th century to the present) have passed since the end of the last military-defense-focused restoration of the Great Wall. Various complex factors, such as plant growth, have led to the deterioration of the building materials, structural damage and even collapse. According to the principles of ‘minimum intervention’ and ‘preservation in situ’, caution should be exercised when removing wall-top plants. In addition, some researchers believe that wall-top plants are an integral part of the cultural landscape of the Great Wall [22,23]. The obstructive effect of wall-top plants’ branches and leaves on rainfall and run-off can reduce erosion intensity, offering some protection to the top surface of the Great Wall [14,15].

In summary, European and Chinese scholars have discussed numerous challenging topics regarding the protection of masonry in architectural heritage. In this paper, based on many basic research works and results [11,12], we discuss the overall research framework of wall-top plants and the key technologies involved in protective restoration. These include the distribution and growth of wall-top plants, the types of architectural damage caused directly by wall-top plants and how to remove the roots of wall-top plants, which have already caused or pose a risk of damage to the Great Wall (focusing on the principles and methods). Moreover, taking Beijing Great Wall as an example, we examine what scientific evidence is needed for the ‘soft capping’ protection technology on the top surface of the Great Wall.

2. Overview of the Great Wall of China

2.1. The Beijing Great Wall and Its Construction Method

The Beijing area is located in the transition zone of China’s semi-humid and semi-arid regions and the transition zone between the North China Plain and the Mongolian Plateau. It has a continental monsoon climate, characterized by strong winds in spring, autumn and winter. Winters are cold, summers are hot, and rain and heat occur in the same period. The total annual precipitation was 492.5 mm in 2022. The area has diverse native plants, with a forest coverage of 44.8%. The predominant regional vegetation type is warm-temperate deciduous broad-leaved forest. In the Ming Dynasty (1368–1644), the area was a mixed zone of agricultural people and northern nomadic people and was an important military area and the most concentrated distribution area of the Great Wall since ancient times. The Great Wall in Beijing consists of two major systems: the east–west system and the northeast–southwest system. These systems form a relatively continuous and complete semi-circular shape, with a total length of 527.65 km [2,24]. Beijing city, the capital of the Ming Dynasty, was repeatedly invaded by northern Mongol tribal armies, and so, the Great Wall defense system and ancillary facilities have a high degree of integrity. The majority of the sections are made of brick and stone, with a high level of construction quality and craftsmanship [1,3] (Figure 1). The steps in the construction of the Beijing Dazhuangke Great Wall are as follows: the loose soil and rubble along the ridge line were cleared, and foundation stones were laid, forming the base of the wall; the core of the wall was then mixed with crushed bricks and stones, while the inner and outer Yan wall were constructed with stones wrapped outside for reinforcement; the top surface of the wall was then leveled and covered with square bricks; and then, the Yu wall and Duo wall were constructed (the Chinese name is Zhi-Die). The width of the wall’s top surface varies between 2 m and 5 m [11,25] (Figure 2).

2.2. Species and Distribution Characteristics of Wall-Top Plants

The top surface of the Ming Great Wall in Beijing area was initially covered with square bricks. However, due to the natural conditions of the location of the Great Wall, especially the lack of maintenance and repair after the Great Wall lost its military defense function, the auxiliary architecture, such as watchtowers and enemy broadcasting stations, subsequently collapsed over the next 400 years. The bricks on the top surface of the wall were gradually broken, and their roughness increased for various reasons, such as freeze–thaw repetition. During the windy seasons of winter and spring, sand-carrying winds deposit sand and soil between the Yu walls and Duo walls. Wind and animals (such as birds) carry native plant seeds here, which are naturally distributed on both sides of the Great Wall. As a result, wall-top plants can germinate and grow (Figure 3). After investigating the current plant situation on the top surface of the Dazhuangke Great Wall in 2021 and 2022, we referred to reference books such as Flora of China and Flora of Beijing, confirming a total of 36 plant species (10 tree species, 11 shrub species and 15 herbaceous species) belonging to 32 genera and 20 families [11]. Among them are twenty-five plants native to the Beijing area, including one nationally protected plant species, three provincially protected plant species and two invasive species. There are 22 plant species on the top surface of the Great Wall, which bear ornamental flowers and fruits [11]. In addition, during fieldwork, it was observed that wall-top plants have undergone changes to gradually adapt to the strong wind, intense sunlight, high evaporation rate and anenvironment in which the soil is relatively thinner and less fertile. For example, their root systems have a larger growth area, mainly distributed in the soil cover zone, and those plants, which originally had deep and straight roots, show no obvious tap root formation [11].

Due to the unique structural features of the Great Wall, the roots of wall-top plants are typically concentrated at the junctions of Yan walls, Duo walls and the top surface of the wall. The further exacerbation of cracks caused by the growth of plants has a serious effect on various structural components of the Great Wall (Yu wall, Duo wall, top surface of the wall, and inner and outer Yan wall), leading to typical architectural damage to the Great Wall [11] (Figure 4).

2.3. Architectural Damage to the Great Wall Caused by the Growth of Wall-Top Plants

Wall-top plants grow in relatively small spaces and in a barren soil environment, absorbing water and nutrients mainly from their root systems. These systems expand in area, and the root diameter increases. The ‘cracking’ effect of the root system gradually damages the originally rigid brick-and-stone structure of the Great Wall, which is an important factor in architectural damage. The types of damage caused by the growth of wall-top plants mainly include loosening, tilting and hollowing [26].

Loosening: The rigid connection system between the square bricks, auxiliary structures and wall core is broken and eventually collapses due to destruction of the roots of wall-top plants and the horizontal spread on the top surface of the wall (this type of damage can also be induced by erosion and the freeze–thaw of precipitation and run-off on the top surface of the wall).

Tilting: Parts of the Great Wall, such as the Yu wall and Duo wall foundation stones, are subjected to lateral pressure caused by the growth of plant roots, resulting in deformation, displacement and tilting of the wall. Initially, this was mainly manifested in the splitting of Yu walls and Duo walls. Later, with precipitation and the roots of wall-top plants going deep into the wall core along the gap, displacement and deformation of the wall intensified.

Hollowing: The inner or outer Yan walls protrude outwards and deviate from the original outer edge of the Great Wall due to the roots of wall-top plants penetrating into the wall core below the top surface (this form of damage can also be caused by water leakage from precipitation on the top surface of the Great Wall).

The location of wall-top plant growth and root distribution on the top surface causing each type of damage are different. Wall-top plants that cause loosening are mainly distributed near the center line of the Great Wall. The roots of these plants primarily spread horizontally on the top surface before penetrating deep into the wall core. During their growth, they damage the rigid connection system of the wall core, resulting in the gradual breaking of the bricks on the top surface of the wall (Figure 5a). Wall-top plants that cause tilting mainly grow in triangular sandy areas formed by the Yu walls, Duo walls and the top surface of the wall. The roots of these plants primarily spread horizontally along the corners of the Yu walls and Duo walls and penetrate into the junction area between the Yu walls, Duo walls and the top surface of the wall at specific locations, such as brick gaps or cavities. The splitting and compression effects of the roots cause the Yu walls and Duo walls as a whole to deviate from the center line of the Great Wall (Figure 5b). Wall-top plants that cause hollowing are located between the Yan walls and the wall core. During growth, the roots of these plants push the inner and outer Yan walls away from the original location, leading to the destruction and collapse of their structural system (Figure 5c).

3. Disposal Results of Wall-Top Plants

3.1. Quantitative Evaluation of Wall-Top Plants

Professional technical personnel carry out comprehensive investigations of damage to the Great Wall using methods such as ground-penetrating radar, acoustic waves monitoring and infrared thermal imaging. Based on identification of the type of damage, an analysis is conducted of the causes and development of the damage [13]. If the types of damage described above are caused by wall-top plants, a quantitative evaluation based on previous research findings is conducted using the analytic hierarchy process method combined with the Delphi method. This evaluation is performed on three constraint-level indices (C), namely growth characteristics, the destructive impact and application value, as well as 13 standard-level indices (P), such as damage impact on the near wall (this index system can be sustainably adjusted and optimized) [27,28,29,30,31] (Figure 6). For wall-top plants, the evaluation value is calculated on a species/type basis. After the evaluation, a list of wall-top plants is compiled, specifying whether they should be retained, removed from the aboveground part or removed during repair. During the removal of wall-top plants, which have caused architectural damage, the wall-top plants, which have not yet caused architectural damage to become an integral part of the cultural landscape of the Great Wall, are preserved. Taking plant disposal on the top surface of the Beijing Dazhuangke Great Wall as an example, a quantitative evaluation using the aforementioned analytic hierarchy process method was conducted on 45 species/types of wall-top plants. We recommend that 18 species/types of wall-top plants should be retained (Grade I), 20 species/types should be removed from the aboveground part (Grade II), and 7 species/types should be removed during repair (Grade III) [12].

Based on the quantitative evaluation results, subsequent technical measures can be carried out, such as manual stripping of the remaining root systems during the repair of architectural damage and disposal of the remaining root systems after mechanical treatment of the aboveground parts of wall-top plants (Figure 6).

3.2. The Disposal Framework of the Remaining Root Systems of Wall-Top Plants

Within the technical system for wall-top plant disposal, the disposal of remaining root systems of trees and shrubs removed from the aboveground part and removed during repair is the core of the entire technical system. If this step is omitted or not properly executed, it may directly result in irreversible secondary damage, contradicting the principles of ‘preservation in situ’ and ‘minimum intervention’ in cultural heritage conservation and restoration (Figure 7). In order to address this aspect, the following technical recommendations are proposed for further research and discussion:

• Wall-top plants in areas of architectural damage should be removed, and all remaining root systems within the Great Wall’s structure should be thoroughly cleared during the repair of architectural damage. If cracks or cavities exist in the cleared root system area, injection grouting should be used to reinforce areas with diameters below 1 cm, while areas with diameters above 1 cm should be back-filled with lime soil and compacted.
• For wall-top plants, which are not located in areas of architectural damage but pose a threat to the structural safety of the Great Wall, the aboveground parts should be removed, but the remaining root systems should not be cleared. During the seasonal maintenance period in the summer and autumn, the decayed parts of remaining root systems and possible sprout tillers should be cleared until the plant and roots have completely died off (the method of injection grouting and lime soil filling for cavities within the Great Wall’s structure formed by the original root system is the same as mentioned above).
• The remaining root systems of dead wall-top plants, which are not located in areas of architectural damage, should be gradually cleared as they decay. During the seasonal maintenance period in the spring and autumn, the decayed portions of root systems should be cleared, and the cavities should be filled with lime soil and compacted (the technical requirements are the same as those mentioned above).

4. Discussion of the ‘Soft Capping’ Protection System

The original covered soil of the top surface of the Great Wall is retained as a growing space for preserved plants. The original covered soil and retained plants originally created a stable ecosystem on the top surface of the Great Wall. After the restoration of the Great Wall, the system will play its role in protecting the top surface. Previous studies have suggested that the plant cover formed by root systems of herbaceous plants on the surface of the brick-and-stone architecture, which helps consolidate the shallow soil layer (0–30 cm), can weaken the intensity of natural rainfall erosion, buffer temperature and humidity changes on the top surface and reduce soil permeability via transpiration. Thus, wall-top plants have positive effects on the protection of the top surface of the Great Wall [10,20,32,33]. The original covered soil and retained wall-top plants are integral components of ‘soft capping’ and the main body of the ‘Garden on the Great Wall’.

Following restoration, the top surface of the Great Wall is still in an open environment with strong winds, bright sunshine and high evaporation rates, and the risk of material deterioration and architectural damage remains high. The selection of cover type and cover construction is therefore particularly important. Considering the experimental research and environmental characteristics of the Great Wall heritage, the protection method of using ‘soft capping’ will be adopted [4,5,6,7,22]. Based on studies conducted on the plant diversity and regenerative characteristics of the seed bank of the original covered soil on the top surface of the Great Wall, we recommended specific engineering practices to technical personnel, following comparative experiments and a series of observations. Before repairing architectural damage to the Great Wall, the original covered soil should be stripped in two layers—the surface layer and the deep layer—in the area where growing plants will be cleared. After the repair, a protective layer, a root blocking layer and a sacrificial layer should be installed in the stripped area, followed by back-filling the deep and surface layers of the original covered soil. The regenerative plants from the seed bank on the surface of the original covered soil will form ‘soft capping’ (the results of research on this engineering work will be published later). The original covered soil and regenerative plants on the top surface of the Great Wall are integral parts of ‘soft capping’ and the main components of the Garden on the Great Wall.

It must be noted that although the Garden on the Great Wall is formed by a combination of the Great Wall itself and ‘soft capping’ (original and regenerative) plants on the top surface of the Great Wall, the formation and continuous maintenance of the Garden on the Great Wall center around protecting the Great Wall itself. This aligns with the basic international requirements for cultural heritage protection [10,34,35]. In other words, the Garden on the Great Wall is composed of the Great Wall itself—repaired due to architectural damage—and wall-top plants, which do not pose a threat to the safety of the Great Wall structure in its current state. However, plant growth is a dynamic process, and therefore, this balanced state may vary. Hence, during the daily maintenance of the Garden on the Great Wall, professional technicians should conduct regular inspections and monitoring of the structural safety of the Great Wall. If any potential safety hazard is discovered, the aforementioned balanced state may be considered lost, and the key technical process of repairing the architectural damage to the Great Wall, removing plants and disposing of remaining root systems, should be repeated (Figure 8).

5. Implementation Recommendations

The aim of the removal or preservation of wall-top plants and the construction of a ‘soft capping’ protection system is to achieve the conservation of the Great Wall.

• The structure of the Great Wall of Beijing, the climate and climate change of the natural surroundings, the micro-environment on the top surface, have certain unique characteristics. These are fundamental factors for the existence of plants on the top surface. These factors also mean that the disposal technology for this type of plant does not have universal characteristics. The interpretation and management of the relationship between the types of damage affecting the Great Wall in Beijing and the plants on its top surface should therefore be carried out by professional technical personnel.
• Before making a decision on whether to remove or preserve wall-top plants, a thorough on-site investigation should be conducted to accurately identify the types and spatial distribution characteristics of wall-top plants. Based on comprehensive research by experts in Great Wall architectural damage, quantitative evaluation methods should be used to determine the specific plant species and categories to be removed or preserved.
• Only wall-top plants, which are a direct cause of architectural damage, should be removed. Along with removal of the plants, thorough restoration of the Great Wall’s architectural damage should be carried out. The technology for disposal of the remaining root systems of wall-top plants also requires more practical testing and further research.
• It is theoretically feasible to protect the Great Wall using the original soil cover from the top surface and reserved plants, back-filling of the original soil cover and regeneration of wall-top plants from seed banks to form ‘soft capping’; however, testing and practical verification are still required. The key technical links involved in this process need to be supported by scientific evidence. The protection mechanism of this ‘soft capping’ technology also needs to be further developed.
• The construction of the Garden on the Great Wall and the Great Wall cultural landscape takes the Great Wall itself as a subject and a carrier, with the preserved and regenerated wall-top plants as a key component. It also includes the natural plants and the ecosystem in the Great Heritage Environment area where the Great Wall is located.