Species characteristics and cultural value of stone wall trees in the urban area of Macao

The stone walls remaining in the highly urbanized Macao area provide a special habitat for plants, repair the fragmentation of the habitat caused by urbanization, and enhance the urban biodiversity. The main object of this survey are stone wall trees in Macao. The species composition, frequency of occurrence and distribution were ascertained, and the feasibility of establishing stone wall tree landscape trail systems was discussed. The results showed that there were 96 stone wall trees in Macao. Among them, 47.9% of the total in the historical center of Macao. In addition, research and analysis on the species composition, life form, frequency and similarity of the associated plants of stone wall trees were analyzed. The survey found that there were 101 species of companion plants, and herbaceous plants had the greatest growth advantage. Most of the companion species were selective and incidental to the stone wall habitat; the similarity of the companion plants in different habitats was less than 0.25, showing that the stone wall was conducive to species diversity. The results of this research aim to explore planning strategies for holistic conservation of stone wall landscape, and provide a theoretical basis for studying the biodiversity of special habitats in Macao.

www.nature.com/scientificreports/ et al. 8 further studied the plant communities' differences due to the slope direction in the north and south section of Jingzhou Ancient City Wall. Wang 9 explored the propagation mechanism of vascular plants on the Ming Dynasty wall of Nanjing. In recent years, some researches on wall biodiversity and city habitat in Hong Kong, Macao, Pearl River Delta, Zhejiang, Chongqing, have been in an increasing trend. In terms of wall habitat, Wu 10 proposed suggestions for plant configuration on the wall by comparing the plant diversity and spatial structure of urban plant walls with the ones of rural plant walls; Li et al. 11 analyzed the reproduction characteristics of the stone wall plants, and compared the differences in species composition between cities in adjacent climate zones. The study of wall habitat began to focus on the connection between wall plants and surrounding ecological environment. Researches on wall plants in Hong Kong, Macao and the Pearl River Delta mostly focused on plant diversity. Zhang et al. 12 studied the flora and life form of wall plants in Macao. The results showed that wall plants were most herbaceous species, and had the most distribution types of tropical Asia 13 . Xie et al. 14 investigated and analyzed the drought tolerance, ornamental, and application of wall plants in the Pearl River Delta. The researches focused on the existing wall plants species and classified analysis.
As stone wall trees less distribute in the world and the traditional stone masonry technology has been lost, stone wall trees are becoming rarer. There are few studies on stone wall trees in Macao. This study is expected to acquaint the special components of urban forestry in Macao, and to provide some inspirations for the conservation of stone wall trees.

Materials and methods
Study area. This study was conducted in Macao (22°06′39″ ~ 22°13′06″N, 113°31′45″ ~ 113°35′43″E), which locates on the coast of the South Sea, connects with Zhuhai city in the north, and connects Hong Kong in the east (Fig. 1). It is sub-tropical maritime climate with mean annual temperature of 14.6 °C and mean annual precipitation of more than 2000 mm 15  Analytical methods. We inspected the distribution of stone wall trees in Macao, and there were a total of 31 samples. According to the Flora of China and The Flora of Macao, the species was classified and identified. The main species identifiers were Professors Xing Fuwu and Qin Xinsheng, both of whom have conducted long-term field surveys in Hong Kong, Macao and Guangdong and are familiar with the species in the region. The study used excel and GIS for data analysis and statistical integration.
1. Proportion of occurrence frequency of plants = Number of occurrence of a certain specie/total number of stone wall trees × 100% 2. Jaccard Similarity Coefficient   (Fig. 2). Based on the frequency of occurrence of various tree species, the frequency was concentrated in the range of 1-5%. Among them, Ficus microcarpa had the highest frequency, reaching 58 times, with a frequency of 60.4% (Fig. 3). This tree species is robust, adaptable and fast growing, which is the main population of Ficus 19 . Stone wall trees in the historic center of Macao. The historic center of Macao, covering an area of about 2.8 km 2 , is the heartland of Macao's historical and cultural heritage, which plays a significant role in the cultural heritage around the world 18 . The historic center of Macao provides valuable historical and cultural resources that enable Macao to transform into a world tourism center 20 .
A total of 14 plots were located in the historic Center of Macao ( Fig. 4), with 45 stone wall trees, accounting for 47.9% of the total number of trees in the survey. Among them, Jardim Luís de Camões has the largest number of 9 stone wall trees. The park, built in the mid-eighteenth century, is one of the oldest gardens in Macao and has the largest number of old trees in Macao. The park had provided good time and environmental conditions for the growth of stone wall trees.
According to Decree No. 56/84/M of the Macao Special Administrative Region Government Printing Department, immovable property that represents the creation of man, or the development of nature or technology and has cultural significance is considered tangible cultural property. The occurrence of the stone wall tree was inextricably linked to ancient wall-building techniques of that time, which was of great significance for the study of the technological development and ecological landscape of the historic center of Macao. The concept of "historic urban landscape" was proposed by Zhang Song 20 , who argued that cities were organisms in continuous evolution, emphasizing respect for the interrelationship between natural and man-made environments. The stone wall trees in the historic center of Macao have been associated with the local culture and ecology tightly and should be preserved as important urban landscape.
Symbiotic relationship between tree and stone walls. As shown in the table below (Table 2), it was found that most of the stone wall trees had root systems that were not only superficially attached to the wall but also extended to the top or bottom of the wall. In particular, Ficus spp. whose strong root system could closely mosaic with the wall, thus forming a strong symbiosis.
Stone walls can imitate the traditional nature-accommodating features to permit spontaneous establishment of a diverse plant assemblage. Besides vegetative diversities in terms of species composition, growth form and biomass structure, stone walls can support a mass collection of urban wildlife and provide various ecosystem service. It is highly recommended that modern urban design be created to embrace stone wall landscape as an integral part of naturalistic or ecological design.
Vision for the establishment of the stone wall tree trail system in the historic of Macao. The traditional street environment in the Macao Peninsula is a kind of distinctive urban landscape, which can high- www.nature.com/scientificreports/ light the specificity and value of the urban context. The combination of the stone wall trees and walls, together with the traditional streets, form a spatial urban landscape. Starting from the location of the stone wall tree landscape, the dots and lines are prospective to promote the establishment of a comprehensive stone wall tree landscape trail system (Fig. 5), so that the public can make use of the existing biological resources to have a better understanding of the land on which they live. Since 2012, the Macao Government has been implementing the "Strolling along Macao Street" project, which aims at studying and exploring the history and culture of the streets of Macao through an in-depth cultural tourism route and promoting it to different levels of society. The establishment of the stone wall tree trail system can rely on this project to raise the public's awareness of the protection and cultural identity of the stone wall tree landscape through a variety of ways. For example, route design competition, photography competition and exhibition, recruitment of "Stonewall Tree Protection Ambassadors" and other forms of participation, so that the public could complete the "role change" in the high degree of such participation-from "onlookers" to "bystanders".   www.nature.com/scientificreports/ There were 85 species with a frequency of 1-5 times, accounting for 84.2% of total species. A total of 11 species appeared 11-15 times, accounting for 4.0% (Fig. 6). There were a total of 4 species that appeared more Table 2. The relationship between the root system of the stone wall tree and the wall. (photo was taken by Professor Qin Xinsheng). Life form composition. Herbaceous plants with 37 species, accounting the percentage of 52.3% (Fig. 7), were dominant in the associated plant species because the seeds of herbaceous plants are lighter and can be propagated to the wall surface by wind force.

Similarity analysis of the associated plants in Macao.
In order to compare the similarity of associated plant species in different environment, the surveyed sample sites for this study were divided into three categories: motorized lanes, non-motorized lanes, and park habitats (Table 3). According to Jaccard's similarity principle, S j  www.nature.com/scientificreports/ is extremely dissimilar when it is 0.00-0.25, and the analysis showed that the similarity of companion plant species in all three habitats was extremely dissimilar. Therefore, it indicated that the companion plants in different habitats had obvious diversity and uniqueness.

Discussion
Differences in species composition of stone wall plants between Macao and cities in adjacent climate zones. The growth of stone wall trees was compared between several cities whose climate conditions are similar to Macao. The life form and species composition of the trees on the stone walls in Macao, Hong Kong, Nanjing and Chongqing show regular changes as the climate zone moves southward 21 . Hong Kong, similar to the climate and environment of the Macao region, also existed extensive stone wall trees. Similar to the survey of the stone wall trees in Macao, Ficus microcarpa was also the dominant species of stone wall trees 2 . The reason why it can become widespread group of stone wall trees was closely related to its root vitality. Ficus microcarpa could maintain the state of "dry death" in the environment of long-term water shortage 22 . When the rain falls, the aerial roots that seem to be dry and dead for a long time could grow new roots, reflecting a strong vitality. Therefore, compared with other plants, Ficus microcarpa may have better adaptability to the barren and dryness on the stone wall.
According to a survey of 289 stone walls in Chongqing 11 , Ficus virens was dominant tree species of this city. Ficus virens has a long history of urban greening in Chongqing, which enables it to become an absolute dominant tree species in Chongqing. Compared with Ficus microcarpa, Ficus virens as a kind of deciduous tree is more adaptable to the climate conditions with less precipitation in Chongqing area and is more tolerant to the winter environmental stress in the area.
The dominant tree species in Nanjing was Broussonetia papyrifera, which was also the common tree species in Hong Kong, and Chongqing. Broussonetia papyrifera with fast-growing characteristics, whose fruits are succulent and large in quantity, are easy to spread seeds through birds 23 .
The dominant stone wall tree species in various regions were closely related to local climate characteristics, natural species resources, characteristics of tree species, and human cultivation history 24 . The combination of different factors had created differences in the dominant stone wall tree species between cities.

Ecological benefits of stone wall trees in Macao.
Most of the stone wall tree groups were Ficus species 25 , which were tall in shape and dense in crown. The most frequent occurrence of Ficus microcarpa among the Ficus was 60.4%. It shows that Ficus microcarpa is well adapted to the special growing environment of the stone wall and can absorb the nutrients inside the stone wall through its strong root system, thus ensuring its growth in the difficult environment. Ficus microcarpa are excellent habitats for many birds and insects, giving  Stone wall trees should be integrally protected together with the wall. The stone walls together with their vegetative companions constitute a unique urban ecology in Macao. The fortunate combination of abiotic and biotic factors, in an inordinately harsh compact city environment, on an apparently inhospitable habitat, has allowed abundant vegetative. Considering the stressful habitat conditions, the resulting of plant diversity is rare surprising, comprising a various range of species and life forms that range from lichen-moss to herbs, shrubs and trees. The rich diversity and pleasant landscape quality of wall vegetation provide essential environmental and visual amenities. The mural vegetation has thrived spontaneously owing to a century or more of minimal human disturbance. Thus stone wall plants and walls are supposed to be protected as a combination to maximize their ecological benefit and aesthetic value. Measures could be taken to impede the decline in the natural supply of water-nutrient, seeds that could allow plant establishment.

Conclusions
In the twenty-first century, the development of new technology revolution has brought tremendous changes to the economic structure, people's living concepts, lifestyles and cultural practices. While enjoying the convenience and comfort brought by high technology, cities also impart people a sense of loneliness and depression. Many start to attach importance to the urban environment which is humane, historical and cultural. This research clarified in detail stone wall trees' important ecological and cultural value in Macao by analyzing their species characteristics and distribution locations. The combined landscape formed by the stone wall tree and walls is an important element in the study of Macao, as the landscape is closely connected with the adjacent space and the emotions of the residents through time, therefore it is necessary to explore rational and scientific conservation strategies for the stonewall landscape in order to optimize the urban ecosystem.