1. Introduction
A seed orchard is defined as a special plantation of assumed superior genotypes established for the production of seed crops. Typically, a seed orchard contains clones or seedlings from selected trees that are isolated to reduce pollination from outside sources, grown on even ground and widely spaced to facilitate cone harvesting, and managed for an early, easily accessible, and abundant seed crop [
1]. After World War II, establishing clonal seed orchards began immediately. For instance, clonal seed orchards of Scots pine (
Pinus sylvestris L.) and Norway spruce (
Picea abies (L.) Karst.) were established in Sweden in the 1950s. In Denmark, a Hybrid larch (
Larix x eurolepis A. Henry) clonal seed orchard was established in 1946. Scots pine, Larch (
Larix spp.), Black pine (
Pinus nigra Arnold.), and spruce (
Picea spp.) clonal seed orchards were established in Hungary in 1951. In New Zealand, a Radiata pine (
Pinus radiata D. Don) clonal seed orchard was established in 1958. In the United States, Loblolly pine (
Pinus taeda L.), Slash pine (
Pinus elliotti Engelm.), and Shortleaf pine (
Pinus echinata Mill.) clonal seed orchards were established. In Canada, a Douglas fir (
Pseudotsuga taxifolia Carr.) clonal seed orchard was established in 1966, and in South Korea, the first clonal seed orchards of Japanese larch (
Larix kaempferi (Lamb.) Carriére) and Korean pine (
Pinus koraiensis (Sieb. et Zucc.)) were established in 1968 and 1969 [
1]. However, the first book on seed orchards was published in 1922 [
2].
In spite of many biological and environmental factors, there are two main types of seed orchards, named according to the way of propagation methods: clonal seed orchard and seedling seed orchard. The clonal seed orchard is a seed orchard raised from selected clones propagated by grafting, cutting, air-layering, tissue culture, or other methods of vegetative propagation. The seedling seed orchard, also called seed plantation, is a seed orchard raised from seedlings produced from selected parents through open- or control-pollination [
1]. So, seed orchards are subjected to both vegetative and generative prorogation at the beginning of their establishment, and various publications due to many steps of orchards from selection of mother trees to crop harvest (i.e., seedling/graft prorogation for establishment, management practices such as fertilization, protection, soil treatments, tending). Many research papers have been published in national and international journals, and proceedings and books have addressed various aspects of seed orchards for a century. Directions of the trend of published papers may change based on the balance of supply and demand of wood and seed production. The balance can be getting the importance of seed orchard research on some forest tree species, such as fast-growing Pacific hardwood trees (i.e.,
Acacia spp.,
Eucalyptus spp., and
Cunninghamia spp.), which have been used widely as natural and exotic in plantation forestry. Therefore, it can also be effective in a number of published papers and their present and future trends in seed orchards, which are improved seed sources for forest plantations. The papers can be analyzed to reveal trends and to guide future research to improve the design and management of seed orchards. One of the ways to summarize these publications is through bibliometric analysis. This is a predictive tool to indicate the likely research direction in the field of seed orchards. Bibliometric analysis measures the contributions of scientific articles to a field or specific topic. It reflects current scientific developments and can be used to predict the next scientific model. Bibliometric studies that summarize and analyze the current state of seed orchard studies are needed for research planning. A bibliometric analysis is a quantitative or inventorial analysis of a specific topic related to the keywords of a field based on a literature survey. It is also a research method for assessing the productivity of authors, countries, and institutions by revealing their global distribution, collaboration, knowledge structures development trends [
3,
4], depth topics, citations, citation times, and keyword co-occurrence based on Web of Science (WoS) database. While many bibliometric studies have been carried out in different scientific fields such as medicine, computing, geography, environmental science, social science, water management, and animal sciences for different purposes (e.g., [
3,
4,
5,
6,
7,
8,
9,
10]), limited studies have been carried out in forestry (e.g., [
11,
12]). In addition, published papers related to seed orchards have not yet been subjected to bibliometric analysis.
In this study, the annual numbers, citation trends, directions, authors and their countries, and keywords of seed orchard publications were subjected to bibliometric analysis based on 1018 publications in the WoS database between 1980 and 2022 to outline their contributions to the future directions and planning of potential studies on seed orchards and to examine bibliometric inventory of published papers on seed orchards to reflect on the development in this area and determine future research opportunities.
2. Materials and Methods
The bibliometric analysis had many steps, from the selection of keywords to analysis criteria (i.e., author, citations, country). The data for the bibliometric analysis were extracted from the well-known academic database present in the Science Citation Index, Science Citation Index Expanded, and citation index databases of Web of Science (WoS), considering that it provides comprehensive citation information in various academic disciplines [
13]. The database is accessible and easy to search for researchers. It also has an English language advantage compared to national databases and other databases. The search criteria topic/keyword “seed orchard*” was chosen based on the purposes and topic of the present study to access publications about seed gardens from the Web of Science Core Collection (update: 14 March 2023). The keyword criteria “seed orchard*” was searched based on titles, abstracts, and author keywords in published papers of the database. The “*” sign was used to reach a more extensive literature database by expanding the scope of relevant keywords. The WoS category was chosen to limit the research to the field of “forestry”. In order to make the analysis more general, only articles written in English were selected from the database. No time limit was set to carry out a complete literature review from the past to the present. Following the selection process, the related data were retained in the “Full Record and Cite References” format using the “Tab-delimited” configuration for subsequent analysis. The searching and screening processes are given in a flow chart in
Figure 1. The recorded data were converted into Excel data, and deficiencies were checked. According to the defined criteria, 1058 studies were identified for the 42 years between 1980 and 2022. Two researchers independently searched and reviewed the identified documents. As a result of these reviews, studies that were found to be unrelated to the research topic were removed from the created database. In total, 92.83% (945) of the 1018 publications were articles.
A total of 1018 publications were included in the analysis based on the most prolific contributors, references, countries, and keywords in the paper, while it was also extended in some studies by adding institutes and journals based on the purposes (e.g., [
11]). In the next step, the publications were converted to a data file format suitable for CiteSpace and VOSviewer and subsequently analyzed. The study focused on previously published studies on seed orchards in the field of forestry. As this study involved a bibliometric analysis of existing published studies and did not involve human or animal subjects, no ethical review was required.
The choice of appropriate software was determined by evaluating the features offered by the software and the adaptability of the network to accommodate these features. CiteSpace was used, as it is a popular software tool for in-depth bibliometric analysis (CiteSpace Advanced 6.1 R6 update 8 January 2023, software available at
https://citespace.podia.com/) (15 January 2023). In the next step, VOSviewer 1.6.9 software was applied as a tool that provided a visual representation of the data for network analysis [
14,
15].
In this study, published studies on seed orchards were examined in detail and visualized according to the following aspects using bibliometric analysis and descriptive statistics: (1) the growth trends of publications on seed orchards over time and the main characteristics of these trends; (2) countries/regions and researchers; and (3) current research topics and analysis of keywords and co-citation documents on these topics, as other studies carried out bibliometric analysis for different purposes in large fields of science (i.e., [
8,
9,
11,
12,
16,
17]). Descriptive statistics were calculated and obtained using an Excel worksheet.
Visualization maps consisting of links and nodes representing analytical elements such as authors, countries, journals, references, and keywords were created. A link between two nodes represents a joint relationship, such as cooperation between authors, institutions, or countries [
18]. The size and linkage numbers of a node reflected the total frequency of the co-occurrence of an item, while the thickness of a node and the color of its ring indicated the time periods of co-occurrence for that item. Three structural indicators were used to measure network integrity. Firstly, the modularity Q index was used to examine the sub-structuring of the network in smaller groups. Secondly, the average silhouette score was used to evaluate the homogeneity and quality of the generated clusters. Finally, centrality values were used to determine the effectiveness of communication between nodes within the network [
19,
20]. A purple color around a node represented high centrality (greater than 0.1). The centrality value varied between 0 and 1.
In addition to these structural metrics, the node analysis also considered temporal metrics. Another important metric is the research impact of citation bursts. It is commonly used to detect sudden changes in the literature [
21]. A citation burst is a phenomenon that identifies keywords, authors, institutions, or publications that show significant changes in the literature during a specific period or timeline. A timeline view provides an overview of how a cluster has evolved over time and highlights the persistence of a particular trend [
22]. Cluster analysis is employed to extract significant patterns and relationships from large and complex data sets. This powerful statistical method enables publications to be grouped on the basis of the similarity of their authors, keywords, or topics. As a result, the connections between research fields and important trends in the scientific literature can be illustrated more clearly. In these cluster analyses, log-likelihood ratio (LLR) tests were employed for the analysis of node clusters with the aim of extracting noun phrases from the article titles in CiteSpace. The correlation between annual numbers of publications and citations was estimated by Pearson correlation.
4. Discussion
Bibliometric analysis helps to better visualize the organization and dynamics of scientific domains, to better understand a particular scientific field, and to provide predictions regarding future trends [
29]. Several parameters, such as leading countries, organizations, and journals, and the contributions of various authors, citations, and keywords, have been analyzed with the help of bibliometric mapping [
7,
8,
9,
12,
30,
31]. Bibliometric analysis has been used for different purposes in various scientific fields (i.e., [
3,
11,
12,
20,
21,
32,
33]). However, this analysis has not been carried out for seed orchards. The present study could be accepted as the first investigation of seed orchards in the field of forestry using bibliometric analysis. Therefore, it was not possible to compare our results with other bibliometric studies of publications based on seed orchard research.
The sustained increase in the number of papers demonstrates the effort invested in seed orchard research since the 1980s (
Figure 2). This increase could also reflect the greater number of established orchards that have reached seed-bearing age over the recent decades. The readership of these publications, as evidenced by the number of citations, also steadily increased over the last forty years. The increase in the number of citations was greater than the increase in the number of published papers (
Figure 2). This was an expected result, as one published paper can cite many papers. The average annual growth rate of the publications was approximately 4%. However, when calculating the citation growth rate, the initial citation value was zero, which led to division by zero in the growth rate formula, resulting in an infinite value. When calculated for the 40 years from 1982, when the citation value was 6, to 2022, there was an annual increase of 13.13% in the number of citations in the relevant field. These numbers could be related to the establishment years of the orchards [
1], the number of orchards, the sizes of the orchards, or the ability of authors (i.e., budget and number of members) to give accurate data to researchers for publications.
There were 31 years between the first publishing (1981) and last publishing (2012) years of the ten most prolific authors. Some authors had similar centrality values despite large differences in their numbers of published papers (
Table 1). The top five authors with the strongest citation bursts (
Table 2) were also among the ten most prolific authors (
Table 1). As presented in
Table 2, the periods of the strongest citation bursts of these five authors (
Table 2) ranged from 3 (Lstibůrek) to 11 (Lindgren) years. The authors generally had citation bursts a few years after first publishing (
Table 2). However, self-citation and the availability of papers (i.e., open access) by researchers were not taken into consideration in the analysis. The results of the most prolific contributors emphasized the importance of innovative methods, new ideas, and theoretical framework for the lifespans of the papers, such as [
25,
34], and provided a guide for future studies (
Table 1 and
Table 2).
Cluster analysis helps us understand the main features of science mapping [
35].
Figure 3a shows that the most massive clusters accorded well with the productivity and citation bursts of the authors (
Table 1 and
Table 2). A total of 1018 academic papers were divided into seven clusters (
Table 3 and
Figure 3b). The clusters were designated on the basis of the strengths of the silhouette scores, which ranged from 0 to 1, and clusters with low silhouette scores can’t be shown in
Table 3 or
Figure 3b. “Douglas-fir seed orchard”, which was the youngest, and “reproductive phenology” had larger cluster sizes (36 and 21) than the others (
Table 3 and
Figure 3b). They may be correlated with the basic field of seed orchards or characteristics of the species (i.e., the sizes of natural and exotic distribution areas and commercial importance of the species). Basic fields have been refreshed by researchers. Fields such as reproductive phenology (
Table 3 and
Figure 4), which was a main topic in the seed orchard field, can be changed by many abiotic and biotic factors such as species, climate, and altitude. They may also be related to the basic steps in the establishment and management of seed orchards. Therefore, some fields need to be examined continually. Future studies can be expected in basic fields of seed orchards, such as reproductive phenology.
Figure 4 provides a timeline visualization of the co-authorship networks of the authors and keywords. The authors El-Kassaby and Lindgren had larger and longer clusters (
Figure 4). Close collaboration between authors can be determined based on the density of the lines [
12] and the number of sub-clusters.
The network consisted of 76 nodes and 206 linkages, where each node represented a country. However, some countries are not visible in
Figure 5 because of their low centrality values. Canada, the USA, and Sweden were the most productive and central countries in the field of seed orchards, while Sweden had the most centrality, with a value of 0.45, which meant that it was a linkage country for other countries (
Table 4). The home countries of the authors were not taken into consideration in the analysis. International bibliometric databases were generally focused on papers written in English. This gave an advantage to countries whose mother tongue was English. The number of papers might also be related to the amount of forest area and the importance given to forestry in each particular country, as well as the number of forestry organizations such as faculties and research institutes in each country. In addition, scientific rules, such as those of WoS papers and Ph.D. theses (i.e., [
36,
37]), and publishing project results supported by foundations in the database could have important impacts on the number of papers and the centrality of countries. The centrality of countries could change in the future.
Canada, the USA, and Sweden were the countries that made the greatest contributions to this field based on their higher numbers of papers (
Table 4).
Of the strongest citation bursts, those of Canada and the USA were the longest at 14 and 16 years, while those of Sweden were the shortest at 3 years, with a strength value of 6.13 (
Table 5). Increases in the number of strong citation bursts in China, Brazil, and Germany showed that these countries have mainly focused on seed orchard studies in recent years, based on WoS papers (
Table 5). This could be due to innovation and the scope of the published papers. It is interesting that China, Brazil, and Germany had higher numbers of citations recently than in the early years. This shows that these countries have mainly focused on seed orchard studies in recent years, based on WoS papers (
Table 5). The results indicated that the strength value, depending on the citation bursts of countries, could change in the future.
Some fields of seed orchards, such as “fertility variation” and “genetic variation”, have been activated/refreshed based on clusters of countries (
Table 6 and
Figure 6). The results also had linkages to the results of the keyword analysis (
Table 7 and
Figure 7). The results of the keywords analysis (
Figure 8) accorded well with keywords such as “fertility variation” in recent papers (i.e., [
38,
39,
40]). It is expected that researchers publishing work on seed orchards will use different keywords to describe their work in future publications. They may include phrases such as ”climate change” if this forms a key consideration in their research. We found that some keywords are interchangeable, for example, Scots pine/
Pinus sylvestris, flowering/strobili, and pine/
Pinus. If only one of the interchangeable words is entered into a search, some publications that use the other keyword could be overlooked.
The analysis databases were generally focused on papers written in English, and this resulted in a bias against inclusion in the databases for non-English-language publications. However, this could be rectified by carrying out a national analysis of publications written in the mother tongues of non-English-speaking countries, and it could be balanced by a national analysis.
Citation burst detection reflected dynamic changes in keywords over time in this field, which reflected an explosion of information that has attracted researchers’ attention [
41]. The number of citations for some keywords increased over a short period, such as climate change and management, allowing us to detect some exciting trends (
Table 8). “Cone”, which was a main seed orchard crop, had the highest strength value (5.80) and the longest citation burst period at 16 years, while some keywords (i.e., pollination and growth), which began in the 1990s, had long lifespans (
Table 8). “Climate change” and “management” were active keywords at the time of the analysis (
Table 8). They could be related to a current problem and a basic field of seed orchards. However, their fates cannot be estimated in the future. The timeline of the keywords in the clusters helped to study the periodic visibility of keywords. “Mating pattern”, “Swedish forestry”, “fertility variation”, and “outbreeding depression”, together with others, were active and past keywords according to the timeline and clusters (
Figure 8). These results accorded well with the results of the cluster map of the cited references (
Figure 9b). However, they could not be stable. For instance, the “threatened Pacific sandalwood” keyword could be related to fast-growing Pacific tree species, which have been used widely in plantation forestry harvested by seed crops from improved seed sources (i.e., seed orchards) in the whole world, such as
Acacia spp.,
Eucalyptus spp.,
Cryptomeria spp., and
Cunninghamia ssp. to supply forest product demand. It also accorded well with the active keyword “
Eucalyptus niten” in
Figure 8 and recent papers in seed orchards of these species (i.e., [
28,
42]). However, they could be checked periodically to give future directions in the seed orchard field.
The 1018 published studies on seed orchards were cited 23275 times. They had 1358 nodes and 4726 linkages (
Figure 9a). A cluster map of the cited references is shown with the largest clusters (
Figure 9b). They accorded well with the results in
Table 9 and
Table 10. The top ten cited articles were generally co-authored papers that included new and innovative methods. Kang had three papers in the top ten cited articles (
Table 9). One of these papers included innovative methods and had the highest centrality (0.23) despite not being published in a WoS database journal. This indicated the importance of innovative methods and ideas in the papers. It also accorded well with Lindgren’s paper, which had the highest strength value (11.42) and the longest citation burst (
Table 10).