Integration of the Natural World Heritage conservation and development of buffer zone agroforestry: from scientometrics insights and implications for the Karst World Heritage

Abstract

Currently, global warming, invasive alien species and tourism development pose major threats to Natural World Heritage sites, and the strengthening of conservation methods and management programs in buffer zones is urgent for the sustainable development of sites. Agroforestry, as a nature-based solution, not only promotes the buffer zone to effectively maintain the Outstanding Universal Value of the heritage site, but also balances the relationship between the conservation of the heritage site and the economic development of the buffer zone, and effectively enhances the protection of the buffer zone in response to various threats. However, there is currently a lack of comprehensive understanding of research trends, research focuses, and recent developments, regarding the conservation of natural heritage sites and buffer zone agroforestry development. Therefore, this study utilized the Scopus database along with Excel and Bibliometrix software to conduct a bibliometric analysis and explore and visualize popular research topics and foreword issues related to Natural World Heritage conservation and buffer zone agroforestry development. We discuss the current publication trends and quantity, keyword and abstract word frequencies, thematic maps and evolutions, journal productivity, national publication outputs, collaboration relationships, and contributing institutions. The research shows that the relationship between biodiversity conservation and agroforestry has been the focus of research in the field since 1992, and the development of agroforestry has contributed to biodiversity conservation in heritage sites. However, there have been fewer studies addressing the geomorphic and aesthetic value of agroforestry conservation, and little attention has been paid to buffer zone development. Based on this, this manuscript suggests that in the future, emphasis should be placed on the linkage between the development of buffer zones and heritage conservation, especially in the Karst World Heritage Sites with ecologically fragile, and that the role of agroforestry development in buffer zones should be strengthened in the promotion of ecological balance and sustainable development of the sites, to provide scientific references for the conservation and development of other geoheritage sites around the globe.

Introduction

As a crucial component of World Heritage, natural heritage encompasses more than 369 million hectares of land and sea, accounting for approximately 8% of the total global protected area coverage [1]. These areas represent some of the most important natural landscapes on Earth, retaining the most intact and pristine natural features, providing critical habitat for endangered species, and documenting natural landscapes of outstanding scientific and aesthetic value in the evolution of the planet [2]. These heritage sites have a profound impact not only on current societies, but also on future generations’ knowledge and understanding of the world. As of August 2024, there are a total of 271 Natural World Heritage (NWH) sites, 40 of which are Cultural and Natural World Heritage [3]. Since the signing of the UNESCO World Heritage Convention in 1972, which incorporated the protection of cultural and natural heritage of outstanding universal value into an international treaty [4], the United Nations and its member states have implemented stringent measures to ensure their conservation. Despite these efforts, natural heritage sites continue to face increasing pressure. According to the 2020 World Heritage Outlook 3 assessment report indicates that nearly all categories of threats are present at NWH sites [1, 5]. These sites not only endure a range of negative impacts from climate change [6] but also face significant pressures from human activities [7, 8], including agricultural expansion, infrastructure development, poaching, and the spread of non-native species [9]. Consequently, in the face of this global situation, States Parties and relevant organizations are stepping up their efforts to study the conservation of NWH, with a view to better coordinating the conservation of heritage with the development of the regions in which it is located.

As the field of heritage conservation research continues to advance, the goals of heritage conservation extend beyond nature conservation to include human communities within core and buffer zones [10, 11]. The establishment of protected areas and the imposition of strict legal restrictions can sometimes lead to unintended negative spillover effects in adjacent areas, such as increased human activities outside the protected zones [12,13,14]. Existing research confirms that establishing protected areas can reduce forest loss within a zone, but forest loss shifts to the buffer zone [15, 16]. The buffer zone is an area established around a designated heritage site to ensure its effective protection. It includes the direct vicinity of the heritage site, significant landscapes, and other areas or features that are functionally crucial for the heritage site and its conservation [4]. As the ecological barrier for the core area of the heritage site, the buffer zone not only acts as an ecological filter [17], effectively preventing external threats to the heritage site but also, bears the responsibility for regional development, providing the material basis for the survival and development of local communities and residents. Studies have been conducted specifically on buffer zones, [94] employed SWOT-AHP analysis in the management planning of buffer zones, incorporating communities and interest groups to balance conservation and development goals [18]. Therefore, understanding how protected areas interact with buffer zones is crucial for enhancing heritage conservation, maintaining the integrity of the surrounding environment, and achieving global conservation objectives [11, 14].

The management and development approaches of buffer zones significantly influence the effectiveness of heritage conservation; thus, finding solutions that balance heritage conservation with regional economic development is essential. In this context, agroforestry (AF) is an effective nature-based solution [19], as a cost-effective long-term strategy [20,21,22]; it addresses both social challenges (such as rural development, poverty reduction, and food security) and ecosystem needs (such as biodiversity, soil fertility, and climate regulation) [23, 24], which positively impacts the ecological conservation of natural heritage and the socioeconomic development of buffer zone. AF is a system based on woody plants and consisting of crops and/or livestock farming that can provide resources and ecosystem services similar to those of forests [25, 26], effectively mitigate the frequency of deforestation [27], and contribute to the conservation of natural forests. Land use in AF systems within the buffer zone of heritage sites is the result of balanced economic and ecological adaptation, weighing the risks posed by economic pressures and climate change surrounding these sites [28]. The Western Ghats have established large-scale coffee AF plantations in the buffer zones, preserving the rich biodiversity of the heritage site, such as communities of butterflies, birds, bats, etc [29,30,31]. , while also generating economic benefits for the region. In addition, the development of community forestry, plantations, and homegardens in buffer zones, among other approaches [32,33,34], are all manifestations of the development of AF at heritage sites by local conditions. Consequently, the development of both conservation and development industries in buffer zones is a prerequisite for the sustainable conservation of heritage sites, and AF is one of these eco-industrial models that combine ecological conservation and community development (Fig. 1).

Fig. 1

Schematic diagram of the relationship between buffer zone agroforestry and heritage conservation

In promoting heritage conservation and sustainable development, AF in buffer zones is exceptional. With minimal disruption to the natural environment [35], AF provides substantial ecosystem services and biodiversity protection, and offers cultural, supportive, and provisioning services to surrounding communities, protecting the natural environment to the greatest extent while actively contributing to human well-being. Currently, research on AF development and heritage conservation in buffer zones focuses on biodiversity conservation, for example, in the Tehuacán-Cuicatlán Valley and the Western Ghats, where a large number of studies have been carried out on how AF contributes to regional biodiversity conservation [36,37,38], focusing on the relationship between biodiversity and habitat and AF development [39, 40] (Table 1). However, the importance of geological elements, which support and provide active processes, landscapes, and substrate materials for these interactions, should not be overlooked, and recognizing the significance of geodiversity is essential for sustaining biodiversity and the enduring development of humanity [41]. The Karst World Heritage Site, as one of the representatives of the geoheritage among the NWH sites [42], is one of the most fragile sites in the world where soluble rock landscapes are combined with hydrogeological formations, compared to the criteria based on (ix) and (x). However, little attention has been paid to whether AF practices can provide conservation research on karst ecosystems and overall environmental vulnerability in the NWH site by altering landscape connectivity and habitat condition enhancement. In conjunction with existing research, only a few researchers have conducted studies on the conservation of the Karst Heritage site in conjunction with AF development, focusing on the perspectives of integrity conservation [43] and value conservation [44], while Yang et al. (2021) have examined the role of AF in maintaining biodiversity in karst regions, starting from a rural landscape perspective [45]. Based on the above analysis, we found that the exploration and understanding of the relationship between AF and its relationship to geoheritage conservation based on geomorphological and aesthetic values is still largely lacking. It is necessary to excavate literature information on the role of AF in the conservation and development of NWH sites, to reveal the mechanisms of these interactions and inspire the conservation and management of Karst World Heritage sites.

Table 1 Research priorities for Natural World Heritage conservation in different regions and countries

According to the description above, we should think from a future perspective about the current issues faced by NWH sites, understand the context, causes, and intentions behind the threats they face, and identify short-term, medium-term, and long-term responsibilities to formulate and implement sustainable conservation strategies [46]. This study aims to conduct a quantitative and comprehensive review of the NWH conservation and development of buffer zone AF, so as to provide scientific reference for the sustainable conservation and development of karst heritage sites and other similar geoheritage sites. The findings are presented in the following ways:

• The annual number of articles and citations for publications were determined.

• Research hotspots, thematic evolution, and trend patterns were identified.

• The prominent journals, geographic distribution, and publishing organizations in the research field were identified.

• Summarize key findings of the heritage conservation and development of AF in buffer zone research and the implications for future research directions in Karst World Heritage sites.

The remainder of the paper is structured as follows: The “Research design” section describes the research methods and tools used, as well as the process for obtaining the literature. The “Analysis and results” section describes the trends in annual publications and citations, keyword analysis, influential journals and institutions, thematic maps and trends, and national publication numbers and collaborations, and provides an interpretation of these findings. The “Key findings” section summarizes the contributions of current research on NWH conservation and AF development in buffer zones. The “Future research directions” section suggests enhancing research on karst heritage sites in this field. Finally, the “Limitations and Conclusion” section summarizes the entire paper and discusses its limitations.

Research design

Literature retrieval, acquisition, and screening criteria

To ensure the quality of the relevant literature and minimize the possibility of errors, this study used the Scopus database as the primary source, and the analysis focused on research hotspots related to NWH conservation and the development of buffer zone AF. To cover as many NWH sites as possible, the search terms used were “Natural World Heritage” or “National Park” or “Protected Area” or “Reserve” and “buffer zone” and “Agroforestry” or “Silvopastoral” or “Agrosilvopastoral”. Additionally, the retrieved articles were manually screened based on the following criteria: (1) the study area or research theme must be the NWH site or the Cultural and Natural World Heritage site; (2) the research content must be related to heritage conservation and AF; (3) non-English articles were excluded; (4) articles for which the full text was not accessible; and (5) duplicate articles were excluded. This process resulted in the final selection of 131 articles (Fig. 2).

Fig. 2

Process of the literature search and screening

Bibliometric tools

This literature review employs bibliometric analysis methods to rigorously analyze a large amount of scientific data, helping to reveal the research trends and knowledge structure in the field, as well as commonly used research methods and techniques, etc., for exploring new research topics [47]. By utilizing scientific mapping, it examines the relationships among research components, thereby presenting the bibliometrics and knowledge structure of the field [48, 49].

Bibliometric analysis was conducted through the Bibliometrix analysis toolkit (v4.3.0), an open-source scientific mapping analysis tool developed in RStudio, that was designed for comprehensive multistep analysis of bibliometric data [50]. Compared to other bibliometric tools, Bibliometric excels in various aspects of analysis [51], and is able to handle the complete process from data import, and preprocessing to analysis and visualization, while allowing the user to write custom scripts based on specific needs, resulting in more flexible and refined analysis. Using this software, we analyzed the annual publication number, annual citations, word clouds, clusters, and collaboration networks related to NWH site conservation and the development of buffer zone AF. This allows us to understand specific thematic areas and the scientific output of different countries on this topic.

Analysis and results

Research on NWH conservation and the development of buffer zone AFs has garnered attention from researchers in related fields, leading to publications in various countries, organizations, and journals. Table 2 reveals that 131 publications were sourced from 75 journals, with an annual publication growth rate of 6.72%, and an International Co-Authorship rate of 41.22%. Based on the data provided in Table 2, Section Annual number of publications and trends to The affiliations that matter most will offer a detailed analysis of the number of publications, average annual citations, keywords, research themes, journals, and institutions, thus helping researchers understand the dynamic development and research progress within this field.

Table 2 Preliminary information from the analysis

Annual number of publications and trends

Figure 3 illustrates the evolution in the number of studies on NWH conservation and buffer zone AF development. It can be divided into three phases, with an overall fluctuating growth trend. The first stage is 1992–2005, with a small number of publications, and more years with zero annual publications, indicating that this stage is still in the embryonic stage. Early studies exploring and analyzing the concept of agroforestry and species combination design in AF provided the basis for subsequent research on the multiple benefits, such as ecological, social and economic, that different AF species and types can provide to heritage site conservation and development [52,53,54,55,56], which became the dominant study in this phase.

The second stage, from 2006 to 2017, shows a wave of growth in the number of publications, with 2014 recording the highest number at 7 papers. At this stage, research began to address the relationship between a species and AF, such as habitat impacts on honey pots, Crested Guinea-fowl (windfall guinea fowl) [57, 58], utilization and sustainable management of trees and shrubs [59,60,61,62]. At the same time, the relationship between AF and species diversity has received a great deal of attention, especially in Mexico [63,64,65,66].

In the third stage, from 2018 to 2023, it can be clearly seen that the number of articles has grown rapidly during this period, with annual publications of 7 or more, in the rapid growth stage. The research themes have become more diverse and abundant, including addressing climate change, carbon storage, and other topics that have gradually attracted the attention of researchers, and buffer zone management and farmers’ groups, which are gradually being studied more intensively [38, 67, 68].

Fig. 3

Annual scientific production

Average annual citations

Figure 4 illustrates the annual average citations for studies on NWH conservation and buffer zone AF development, based on a bibliometric analysis. The trend shows fluctuations, with the highest average annual citations reaching 4 in 2007, followed by 3.5 in 2001, 3.1 in 2010, and 2.7 in 2020. Each of these four high citation points corresponds to a high posting year in Fig. 3. Based on the average annual citations, it is concluded that although the number of articles published in the budding phase (1992–2005) was low, the impact and attention of articles in this phase were high. Early studies exploring and analyzing the concept and species assemblages in AF provided fundamental support for subsequent research on the various benefits that different AF species and types for heritage conservation and development, such as economic, social, and ecological benefits. This has been referenced and cited in subsequent studies, playing an important role. The second stage (2006–2017) had the highest annual citation. It is worth noting that the research themes in this stage are targeted, particularly in the field of biodiversity conservation, which not only affects subsequent research in this field, but also provides a rich theoretical framework and diverse research methods for the third stage of research. In addition, since the convening of the 42nd World Heritage Conference and the launch of the third round of Periodic Reporting in 2018, the emphasis on the theme of World Heritage and sustainable development has promoted the rapid growth of the third phase of literature, and to some extent, the increase in the citation volume of articles in the second and first phases.

Fig. 4

Annual average citations

Keyword analysis

Keywords, top authors, and country relations

Figure 5, a three-field plot, displays the relationships among the top authors, keywords, and countries with the authors “casas a” and “moreno-calles ai” having the highest publication output, and the richest research topics, focusing on the themes “biodiversity conservation”, “Tehuacán Valley,” “agroforestry system,” and “biocultural diversity”, etc., with corresponding contributions from the following authors: “vallejo m” and “blancas j”. Although they all focus on the popular research theme of the relationship between AF systems and the conservation of species diversity in heritage sites, each author also explores different research directions. “blancas j” focuses on AF types such as homegarden, “casas a” focuses on “domestication” and “columnar cacti”. Additionally, the plot clearly shows that the number of authors directly associated with keywords such as “protected areas”, “buffer zone”, and “conservation”, etc., is relatively small.

From the linking of keywords to countries, it is evident that Mexico and India have the broadest scope of research. Mexico focuses on themes such as “biodiversity conservation”, “tehuacán valley”, “biological diversity”, etc. As the Tehuacán-Cuicatlán Valley is a dual heritage site with a long history of AF development and planting experience, Mexico has conducted extensive research on this heritage site. While India focuses more on “agroforestry” and “western ghats”. The Western Ghats, as a hotspot of species diversity in India, occupies an important position in heritage conservation and AF development, and thus receives high attention. Germany and the UK have a more balanced coverage of research themes and do not emphasize the concentration of research on a particular topic.

In the overall analysis, the themes of “buffer zone”, “protected area”, and “conservation” were emphasized in national research, while themes such as “biodiversity conservation” and “agroforestry systems” were emphasized by the authors, reflecting the inconsistency between the concerns of the country and the author. In contrast, the country focus is more specific, such as the “tehuacán valley” which is only of interest to Mexico, the “western ghats” of India, and the “bennin” of Benin, while the authors’ research themes are broader and more diverse.

Fig. 5

Three-field plot of authors, affiliations, and keywords

Comparative analysis of word frequency of keywords and abstracts

To understand the main concerns of researchers at this stage, an analysis of keywords provided by authors in articles was performed. Figure 6 highlights the frequency of keywords used in publications, with the size of each keyword determined by its frequency of occurrence in the data center. From the word cloud, it can be seen that the most frequent keywords are “agroforestry (20)”, “biodiversity conservation (12)”, “conservation (9)”, “western ghats (8)” and “buffer zone (7)”, reflecting the fact that most of the current research in this area focuses on AF and biodiversity conservation and that buffer zones have also attracted some attention, but they still need to be strengthened. In addition, “natural world heritage (2)”, “livelihood capital (2)”, and “land use change (2)” are less frequent, but relevant themes that deserve to be analyzed in depth by researchers.

Fig. 6

Keywords WordCloud

Compared to the keywords provided by the authors, the word cloud analysis of abstracts was richer and better reflected the research themes related to world heritage conservation and buffer zone AF. By filtering the combinations of word lengths in the N-Grams option, Trigrams were chosen to analyze three consecutive words as one entity and were able to capture more complex semantic relationships than Unigrams and Bigrams.

As illustrated in Fig. 7 below, words related to the NWH site, such as “world heritage site (12)”, “world natural heritage (11)”, “western ghats india (7)”, “kerinci seblat national (8)”, and “pendjari biosphere reserve (6)” frequently appeared. Additionally, words related to AF development such as “agroforestry planting patterns (10)” and “agroforestry systems afs (9)”, are directly visible, and “buffer zone agroforestry (4)”, although less frequent, has also received some attention. In addition, some richer research themes were presented, such as “traditional agroforestry systems (5)”, “heritage site integrity (5)”, and “site integrity protection (6)”, have begun to explore the impact of AF on the conservation of the integrity of heritage sites. The concept of heritage conservation has been enriched by niche themes, which are no longer limited to the study of heritage areas, but also to the integrity of heritage sites, outstanding universal value, and the protection of landscapes, which are equally important.

Fig. 7

Abstract WordCloud

Abstract co-occurrence analysis

To better understand and identify interpretable relationships, we generated a co-occurrence network based on the abstracts provided by the authors. As shown in Fig. 8, the most prominent clusters are shown in blue, with the central term “agroforestry system”, including “biodiversity conservation”, “national park”, “species richness”, etc., revealing the close relationship between AF systems and biodiversity, and that these themes occupy an important position in AF systems. Secondly, there are many small nodes, which indicate that the current agroforestry-related research topics are the most numerous and the relationships are the most complex, revealing the diversity and complexity of the topics and research directions of the research field centred on the “agroforestry system”. The small nodes also represent more specific research topics, such as “coffee farms”, “climate change”, “trees dynamics”, etc.

In red is the second cluster, with “buffer zone” as the main keyword, is directly linked to “heritage site”, and at the same time derives themes such as “natural heritage”, “world heritage”, “economic development” and so on. This cluster is relatively small, but it is a specific theme, and the nodes in the cluster are basically directly related to the NWH, indicating a highly concentrated research area. Overall, the co-occurrence analysis of the abstracts shows the interconnections between AF and natural heritage sites. They are interrelated, interact, and influence each other on various research themes such as “local people,” “national parks,” and “buffer zones”, etc.

Fig. 8

Co-occurrence network (the color of the nodes represents the clusters linked by the keywords. The keywords and their relationships imply that each cluster is associated with a specific research topic.)

Journal productivity and influence

We conducted a comprehensive journal analysis to identify those who have made significant contributions to research on NWH conservation and the development of buffer zone AF. The leading journal is Agroforestry Systems (20), Journal of Ethnobiology and Ethnomedicine (7), Forests (4), Sustainability (4), Land, Agriculture, Ecosystem and Environment, Ambio, etc., and 7 other journals contributed 3 articles, while the rest mostly contributed 1 or 2 articles. From the productivity situation of journals, it can be seen that there is a lack of specialized journals that accept the participation of AF in heritage conservation and management, except for Agroforestry Systems as a publication of agroforestry-related research. Yet fewer publications focusing on AF development in an interdisciplinary nature are protected at NWH sites, resulting in a small number of publications in journals with comprehensive themes such as Plos One and Scientific Reports (Fig. 9).

Fig. 9

Journal productivity

Thematic map analysis

Figure 10 displays the thematic map of the study, which categorizes the research topics or issues related to the NWH conservation and the development of buffer zone AF, according to two axes–“Relevance degree” and “Developmental degree”, and is divided into the following four categories:

Niche themes: “cocoa mapping”, “geographically weighted regression”, “land use”, “human-wildlife conflict”, “local livelihoods”, etc. Belonging to the niche or emerging themes within this field of study, the large distribution and dense arrangement of themes in this quadrant. In one way, it also indicates the diversity of research themes at different stages of development.

Motor themes: This quadrant represents the core themes, that are by high relevance and developmental, “biodiversity conservation”, “protected areas”, and “agroforestry systems” are the main focus areas, extensively studied and widely discussed. This quadrant contains the largest distribution of themes, closely aligning with the study’s main research topics. It indicates that these studies are actively evolving and driving progress in the field. This quadrant has the most themes distributed and is closest to the main research topics of this paper, indicating that these studies are actively developing and driving progress in the field.

Emerging or declining themes: This quadrant encompasses belongs to the developing themes, e.g. “benin”, “Nepal”, “pendjari biosphere reserve” are among the identified natural heritage site names or countries, indicating that targeted natural heritage site research is currently being carried out. Additionally, themes such as “sustainability” and “natural world heritage” are still evolving and require ongoing attention.

Basic themes: “community forestry”, “agroforestry”, “conservation”, “buffer zone”, “protected area”, etc. are the foundational themes within this field, which will be expected to continue developing and likely generate an array of more related themes in the future.

Fig. 10

Thematic map

Trend analysis of research themes

Figure 11 shows the trends of the research themes from 1992 to 2023. In this figure, various themes or keywords are listed along the vertical axis, while the horizontal axis represents the years. To gain a comprehensive understanding of our research trends, we included all years [69]. From 1992 to 1998, attention was paid to the exploration of the relationship between heritage sites and AF development in Nepal, Sri Lanka and Indonesia. Since the 20th century, there has been a gradual diversification of research themes and sustained interest in them, with the most popular thematic trends being “forestry”, “ecosystem”, “biodiversity” and “agroforestry”, which appear most frequently. Due to the threats posed to heritage sites by climate change, the topics of “sustainability” in 2020, “climate change” and “tropical” in 2022 are receiving attention from researchers.

Fig. 11

Thematic evolution and map (the horizontal lines and dots next to each theme or keyword represent the frequency of discussion or relevance of the topic in specific years. Longer lines indicate more frequent discussion of the theme in the literature of that year; larger dots may suggest greater relevance or importance of the topic in that specific year)

Number of national publications and collaborative maps

Figure 12 shows the number of national publications of studies related to NWH conservation and the development of buffer zone AF. India ranks first with 23 publications, focusing particularly on the Western Ghats, a biodiversity hotspot in the region. The challenge of balancing the conservation of the site’s Outstanding Universal Value with regional economic development has attracted significant attention from researchers. Mexico, with 19 publications, ranks second, focusing on the Tehuacán-Cuicatlán Valley, a Natural and Cultural World Heritage site, where AF has been developed for thousands of years, hence the substantial number of articles written on this theme. Next is the United States with 11 publications, which has made substantial contributions to natural heritage conservation and AF development. China ranks fourth with 10 publications, and although it has a relatively large number of NWH, research on heritage conservation and AF development has received more attention due to the enormous demographic development pressures facing the country.

From a regional perspective, North America, Asia, and Western Europe have the more related research. In North America and Asia, the abundance of NWH, such as the Tropical Rainforest Heritage of Sumatra, Mount Everest, and other popular research subjects. Thus there is a greater abundance of relevant research involving AF in maintaining biodiversity and community development. The research advantages of Western Europe mainly stem from its developed scientific research infrastructure and high-level education talent reserves, providing advantageous opportunities for heritage conservation and AF development. In contrast, research in Oceania and Africa is more sparse, with only a few countries giving attention to this research theme.

Fig. 12

Country scientific production (based on first unit countries)

Figure 13 illustrates the global collaboration pathways in research on the theme of NWH conservation and the development of buffer zone AF. The red lines in the figure indicate research collaboration networks between countries, with the thickness of the lines reflecting the closeness of the partnership. As shown in Fig. 13, the United States has the closest connections with South Africa, the United Kingdom, and Benin. Secondly, there are also solid partnerships between the UK and France, Indonesia and Switzerland. In addition, India has been active in global collaborations and has frequently collaborated in research with several countries, such as Kenya, Japan and Malawi. Of these, India’s partnership with France is the strongest and has the thickest connecting lines. However, China and Australia have hardly engaged in collaborative research with other countries. From the perspective of global cooperation, North and South America have closer ties with Western Europe and Africa, but less cooperation with Asia and Oceania. In future research, cooperation between these regions can be further strengthened to promote research on the conservation of NWH sites and sustainable development of AF in buffer zones.

Fig. 13

Country collaboration map (analysis based on all author countries provided in the article) Note: the number of publications is indicated by the intensity of the blue color; the thickness of the red line indicates the intensity of the collaboration based on frequency

The affiliations that matter most

Table 3 shows that “UNIVERSIDAD NACIONAL AUTÓNOMA DE MÉXICO” has the highest number of publications between 1992 and 2023, with 13 papers. “GUIZHOU NORMAL UNIVERSITY” has published 5 papers, while both “CENTRO DE INVESTIGACIONES EN ECOSISTEMAS” and “UNIVERSITY OF PARAKOU” have published 3 papers. Six institutions have each published 2 papers, and the remaining institutions have each published 1 paper. As a whole, the number of articles published by higher education institutions is larger than that of research institutions, indicating that higher education institutions pay high attention to the conservation of NWH and the development of AF in the buffer zone, and have a strong motivation for scientific research output.

Table 3 Most relevant affiliations

Key findings and future research directions: implications for Karst World Heritage site

Key findings

Viewpoint 1: Current research on the NWH conservation and the development of buffer zone AF is largely focused on biodiversity conservation.

From a conservation perspective, buffer zone AF, as a human-modified landscape, has been proven to have the potential to support biodiversity protection [34], and it is evident from the high frequency of the keyword ‘biodiversity’ in Figs. 5 and 6. Buffer zone AF can provide food, habitat, and growth environments for cherished and endangered species within protected areas. Firstly, AF has a similar system composition to forests, which, for forest-dwelling species, reduces edge disturbances and increases habitat availability [38, 70]. For example, coffee plantations surrounding the Western Ghats in India serve as corridors between forest fragments, providing supplementary habitats for the Palamedes butterfly [29]. In Mexico, AF systems provide important resources for birds, such as shelter, food, and so on, which promotes bird management and conservation [71]. Secondly, Secondly, there is a strong positive correlation between canopy cover and species diversity—the higher the canopy cover, the greater the number of taxa in the landscape [38]. A typical example is the multi-layered and diverse tree structure of home gardens in the tropics, which utilize tall canopy trees to provide suitable environments for shade-loving and moisture-loving species. At the same time, plant management within AF systems has facilitated vegetation succession and the introduction of new cultivated species and varieties, thereby enhancing both species and genetic diversity within these systems [70]. For instance, community-managed AF (plantations mixed with crops) supports the preservation of the genetic structure and diversity of Euterpe edulis [61].

Viewpoint 2: AF in the buffer zone plays a crucial role in eliminating threats to NWH sites.

Under the current combined pressures of human activity and climate change, one-third of the world’s protected areas are experiencing significant human stress, with many NWH sites located in regions of higher human pressure [8]. The most direct and evident impact is habitat loss due to agricultural expansion, which directly threatens protected areas’ biodiversity [72,73,74]. Relying solely on the protected area itself is insufficient to address biodiversity conservation, and the buffer zone AF plays a crucial role in this effort, as thoroughly discussed in Viewpoint 1. Second, under the influence of global climate change, extreme droughts, and heavy rainfall events have become more frequent. On the one hand, AF provides shelter for the soil by reducing the leaching effect of heavy rains and decreasing soil erosion, and on the other hand, it creates a microclimate under the forest canopy, which reduces plant transpiration and increases air and soil humidity [75]. This mitigates the risks faced by core areas due to climate change, thereby maintaining the stability of the heritage site and surrounding ecosystems. For instance, farmers near the Sundarbans in Bangladesh consider AF systems to be one of the best strategies for coping with the environmental, climatic, and weather pressures caused by climate change [75].

Third, the buffer zone AF development provides local communities with forest products such as firewood, fruits, and grains, thereby reducing the pressure on forest resources within protected areas [33, 53, 66]. In the Sundarbans, local farmers are encouraged to engage in AF practices, such as silvofisheries, in coastal mangrove areas, to maximize the use of regional resources and reduce reliance on mangroves, thereby alleviating the pressure on these vital ecosystems [76]. Fourth, AF plays a crucial role in mitigating human-wildlife conflicts, easing the tension between regional development and environmental protection. Villages and farmers in buffer zones face risks, such as crop damage, human-elephant conflicts in Tanzania, human-tiger conflicts in Nepal, etc. Through AF management, on the one hand, it reduces the frequency of human-wildlife encounters, which reduces the risk of human deaths and the number of wildlife hunted to a certain extent, and on the other hand, it lessens the extent of crop damage [77,78,79].

Viewpoint 3: The management of NWH conservation is increasingly focusing on the interests and demands of farmers in buffer zone communities, creating opportunities for the adoption of AF.

Due to the protection of the Outstanding Universal Value of heritage sites by national governments and UNESCO, boundaries and strict conservation regulations are typically established. However, the formulation of these policies often overlooks the impact on local economies, leading to conflicts between local farmers and protected areas [80, 81]. Before the establishment of protected areas, these communities tended to rely on natural resources for their livelihood, characterized by intensive agriculture and unregulated use of forest resources [82, 83]. If the relationship between the developmental demands of farmers in the surrounding areas and heritage conservation is not properly addressed, the natural environment of the heritage site will be difficult to maintain. Therefore, it is essential to collaboratively address these issues. First, a variety of research methods and models should be adopted to understand the perspectives and developmental demands of farmers in buffer zones. Based on questionnaires, semi-structured interviews, and data collection, etc., insights into the buffer zone farmers’ views on heritage conservation, their involvement, and agricultural development, managers should encourage locals to better utilize nature rather than through restrictive, prohibitive, or isolative management practices, by considering local people’s interests and livelihood activities and implementing sustainable planning and management [84,85,86]. [95] used a structural equation model to explore farmers’ conservation attitudes, awareness, perceived behavioural control, etc., analyzing the interplay among variables to propose policies that coordinate ecological protection and economic development around protected areas [86].

Second, local communities are adapting and utilizing the regional landscape of forests and agricultural spaces through suitable business models. For example, in the Tropical Rainforest Heritage of Sumatra, the Kerinci National Park has leveraged an Arabica Coffee Plantation project, which has not only increased local income but also reduced deforestation within the national park, achieving the goals of community welfare, sustainable agriculture, and forest conservation [82]. Based on the above analysis, the development of buffer zone AF, can not only satisfy the private interests of local farmers by providing diversified income sources, but also modulate the landscape of heritage sites, and increase the overall forest cover through the intentional integration of trees within farmland. AF is a sustainable land use method that not only helps maintain the ecological environment of heritage sites but is also crucial for achieving sustainable development in buffer zones.

Viewpoint 4: Buffer zone AF provides diversified socio-economic benefits to local communities and farmers.

As a typical sustainable production method, AF can not only preserve the original ecosystem service function but also improve the yield and quality of AF products and increase their value [86]. This approach meets the economic, social, and ecological needs of farmers in the buffer zone of heritage sites and serves as a strong driver of sustainable livelihoods. Compared to other forms of agriculture, farmers can derive greater benefits from crop-tree combinations. Firstly, multipurpose and multi-species trees enhance the profitability of farmland, by providing farmers with fruits, firewood, and wild foods for sale, which directly boosts household income. Additionally, livelihood sources also include undergrowth husbandries such as cattle, ducks, and chickens, with a large portion of income coming from milk, eggs, duck eggs, etc. [75]. Farmers enhance their family’s economic gains by diversifying the AF products they develop. Secondly, AF practices also bring about improvement in human capital in the region, particularly in impoverished developing countries. On the one hand, as family economic income increases, more attention is paid to the children’s education; on the other hand, women are actively involved in the management and labor of AF, which enhances their social status [75]. Thirdly, AF systems are low-cost, require less labor, and are easier to manage, creating various employment opportunities, such as seasonal laborers and hourly workers, for harvesting, transporting, sorting, and so on. Finally, utilizing the unique natural resources of heritage sites to develop eco-tourism can help alleviate conflicts and restrictions faced by local farmers in the use of heritage resources [80]. Rwenzori, Uganda strengthens the link between local people and natural resources through ecotourism, sustainable planning, and management of the region’s complex, beautiful, and ever-changing AF, strengthening its tourism appeal and increasing economic revenues [83].

Future research directions: implications for Karst World Heritage sites

• 1. Based on the consideration of Viewpoint 1, future efforts should focus on enhancing the research on the relationships between AF practices in buffer zones and the geomorphological values (vii) and aesthetic values (viii) of NWH sites. Similarly, the Outstanding Universal Values of Karst World Heritage sites contain vii and viii, and intensifying the research on the interactions between AF and Karst Heritage sites will help address this gap.

  Based on the analysis above, buffer zone AF has carried out more studies on conservation and development in response to heritage sites based on criteria (ix) and (x), with a focus on biodiversity conservation, as detailed in Section Four, Viewpoint 1. In contrast, there is a lack of studies on how the development of buffer zone AF affects geological heritage and natural phenomena, with a total of 63 heritage sites satisfying geomorphological values (vii) and aesthetic values (viii), only a few articles touch upon the relationship between buffer zone AF development and karst heritage conservation [44, 87]. However, [96] argued that biodiversity-related heritage sites may not primarily face issues of biological recovery, but when considering inclusion under criteria (vii) and (viii) of the World Heritage list, the issue of biological restoration needs to be taken seriously [88]. This is because once exceptional natural landscapes are impacted by ecological imbalances, environmental restoration becomes extremely difficult. Currently, research on the geomorphological and aesthetic value of Karst Heritage Sites focuses more on popularization and tourism, on the one hand, landscape displays, offer the public an intuitive understanding of Earth’s history and processes [89], and on the other hand, it contributes to local economic growth. AF activities directly impact the ecological and aesthetic values of the Karst Heritage site, maintaining their geodiversity while also considering the agricultural development and ecological conditions of the entire buffer zone, which is a way to promote the sustainable development of the heritage site.

• 2. Emphasize the inherent advantages of developing AF in the buffer zone of the Karst World Heritage site, to enhance the effectiveness of the buffer zone in protecting the site.

  The buffer zone of karst heritage sites is characterized by strong human activities [90], due to strict protection regulations in the core areas, almost all activities take place within the buffer zone. The buffer zone not only supported the livelihoods and development of the heritage site communities and residents but also mitigated the negative ecological and environmental impacts from the periphery of the heritage site, playing a crucial role in economic and ecological buffering. The ecological environment in the buffer zone of karst heritage sites is fragile, and there is a sharp contradiction between people and land, which will lead to the dilemma of “one side of the land cannot support one side of the people” [91]. If the buffer zone fails to undergo effective development, it will exacerbate the local community’s plunder of natural resources, thereby resulting in the degradation of the heritage site’s ecological environment and the loss of its Outstanding Universal Value. Scholars have already studied this issue, proposing that emphasizing the development of tourism in the buffer zones, can stimulate regional economic growth and alleviate the pressure on natural resource utilization [10, 92]. However, the negative impacts of tourism cannot be overlooked. In this context, AF development plays a crucial role in the sustainable management of buffer zones. Through the development of AF in the buffer zone of the Karst World Heritage site, the plundering of natural resources and ecological damage will be reduced, and the ecological resilience of the buffer zone will be enhanced. At the same time, it provides sustainable livelihoods for farmers in the buffer zone and eases the human-land relationship, thus realizing a win-win situation between ecological protection of heritage sites and community economic development. Therefore, in the face of the research on the conservation and sustainable development of Karst World Heritage, we can realize a win-win situation for regional development and heritage conservation by developing buffer zone AF to enhance the stability and resilience of karst ecosystems.

• 3. Strengthen ecological and economic research between the development of buffer zone AF and the Karst World Heritage site.

  As a complex and heterogeneous environment that integrates biological and physical processes on and below the surface [88], karst has unique geomorphological value. Among the existing 271 NWH sites, a total of 95 are designated as geological heritage sites, with 33 of them linked to karst. Compared to other NWH sites, the ecosystem of Karst World Heritage sites is more fragile, and their environments and resources (soils, biota, water, etc.) are highly vulnerable to damage, with long-term recovery being difficult [88, 93]. The buffer zone, as a transition zone for human activities and an important area for the effective conservation of heritage sites, faces enormous ecological pressure, and local farmers are often dependent on local natural resources for their livelihoods [43]. If not managed scientifically, it could easily lead to ecological deterioration in the karst region, which in turn threatens the outstanding universal value of the heritage site. At present, there have been studies on the coordination and regulation of the integrity protection of karst heritage sites and the development of AF in buffer zones, and the results show that the development of AF in the buffer zone can reduce the sensitivity of the karst area to human activities, protect the karst microhabitat, and maintain the original ecological environment of the area [43]. At the same time, strengthening the role of AF in economic income generation and land use can contribute to economic development and improve the tension in man-land relationship in karst buffer zones; for example, [97] demonstrated that by comparing different AF planting patterns within buffer zones of karst heritage site, and chose the most suitable models based on environmental costs, livelihood capital, and other aspects, thereby promoting the sustainable development of Karst Heritage site [44]. Therefore, by strengthening the ecological and economic impacts of AF development in the buffer zone, providing diversified livelihood choices for local farmers, reducing the interference and destruction of the karst environment and over-dependence on natural resources caused by human activities. Simultaneously reducing ecological threats to the core area and maintaining the ecological balance of the heritage site.

Limitation

Despite its contribution to the field, this paper recognizes some limitations. It is based only on the Scopus database, excluding publications from other databases. We recognize that the inclusion of other databases such as Web of Science and Google Scholar would increase the number of literature screened, but it may also increase the rate of errors in software analysis results. Furthermore, considering the readability of publications, this paper only selected articles predominantly in English, which to some extent resulted in the exclusion of non-English literature relevant to the research topic. Additionally, due to word recognition problems with the analysis software, variations in the singular/plural forms and spellings of nouns and adjectives have led to repetitive frequency counts for certain terms describing the same concept. For instance, in Figs. 5 and 6, and 7, “protected area” and “protected areas,” “forest” and “forestry,” “natural world heritage” and “world natural heritage”, etc. Ultimately, while the bibliometric tool is best suited for extensive data analysis, the relatively limited quantity of research literature in this study may result in less precise thematic mapping and trend analysis in the field. Nonetheless, they still offer valuable insights into bridging the gap between NWH conservation and buffer zone AF development.

Conclusion

This manuscript uses the Bibliometrix literature analysis package in R language to conduct a systematic bibliometric analysis of research on the conservation of NWH and the development of AF in buffer zones from 1992 to 2023. The analysis covers a wide range of aspects such as the number of publications, citations, research themes, keywords, the number of national publications and collaborations, and the publishing organizations. The research found that despite the gradual increase in the number of research publications in this field, there is still an imbalance in the research on AF development in buffer zones, especially when it comes to the preservation of geomorphological and aesthetic values, which has received less attention. The research hotspots are mainly focused on the subject areas of “agroforestry system”, “biodiversity conservation”, “buffer zone”, and “national parks”, etc., which reflects the high interest in reporting this field in the journals Agroforestry Systems and Journal of Ethnobiology and Ethnomedicine. At the national level, India, Mexico and the United States have published the most literature on the subject, with UNIVERSIDAD NACIONAL AUTÓNOMA DE MÉXICO in Mexico being the main research institution. These countries and institutions have led to a boom in related research due to the high priority given to heritage conservation and AF development.

In summary, through the analysis of research hotspots, this manuscript recognizes the unique contribution of AF development in buffer zones in heritage conservation, including the provision of habitat for biodiversity conservation, meeting the socio-economic needs of farmers in heritage sites, and mitigating threats to NWH sites. These findings provide important insights into the conservation of Karst World Heritage sites. In particular, in the face of the enormous conservation and development pressure on the buffer zone of Karst World Heritage sites, the development of AF can not only alleviate the regional development pressure, but also enhance the protection of geological features and aesthetic values. Future research should pay more attention to the role of AF development in the buffer zone in these aspects to further promote the sustainable development of heritage sites.