3.1 Main information and publication trend analysis
The basic bibliometric details on Economics of Forest Carbon Sequestration (EFCS), obtained from biblioshiny application, are presented in Table 1. There are 1,439 publications from 503 sources within the period 2001-2021. Average citations per article and average citations per year per document are 28.57 and 3.733 respectively. The total number of authors is 5,258 and references is 38,388. Authors per Documents index (3.65) is calculated as the total number of authors divided by the total documents. The Co-Authors per Document index (4.47) is the average number of co-authors per document. This index considers the author appearances whereas “authors per documents” counts an author only once, even if he is the author of more than one documents. That is why Authors per Document ≤ Co-Authors per Document. The Author Collaboration Index (3.97) is obtained as the ratio of the Authors of Multi-Authored Documents and Total Multi-Authored Documents (Elango and Rajendran 2012). In a sense, the Collaboration Index (CI) is a co-author per documents index obtained by only taking the multi-authored document set (www.bibliometrix.org).
The evolution of annual scientific production is shown in Fig. 2. On average 68.52 peer-reviewed articles on EFCS (SD = 48.82; Median = 69) have been produced each year during (2001-2021) where the minimum number of articles counts for 12 and maximum for 169 in total 1,439 publications. During 2001-2014 (567 total documents), published articles are less than a hundred per year with an average publication of 40.5 per year (SD = 26.37; Median = 30.5; Minimum = 12; Maximum = 89). From 2015-2021 (872 total articles), the number of publications has crossed a hundred documents per year with an average publication of 124.57 per year (SD = 30.70; Median = 114). In 2016 and 2017, the number of articles has slightly fallen to 94 and 92 respectively otherwise there is a rapid increase in publications in the following years. The year 2020 has been the most productive in terms of article publications (169).
Table 1
Main information about bibliometric data of the study
Description | Results |
Timespan | 2001:2021 |
Sources (Journals, Books, etc) | 503 |
Documents | 1439 |
Average citations per document | 28.57 |
Average citations per year per doc | 3.733 |
References | 38388 |
DOCUMENT TYPES | |
article | 1439 |
AUTHORS | |
Authors | 5258 |
Author Appearances | 6427 |
Authors of single-authored documents | 131 |
Authors of multi-authored documents | 5127 |
AUTHORS COLLABORATION | |
Single-authored documents | 142 |
Authors per Document | 3.65 |
Co-Authors per Documents | 4.47 |
Collaboration Index | 3.97 |
3.2 Number of publications in the top 10 categories of disciplines and SDGs
The number of publications has been assigned to the top 10 subject categories and Sustainable Development Goals within Fig. 3 in panels (a) and (b), respectively. In panel (a), the top 3 subject categories are Environmental Sciences (510 publications), Agricultural & Veterinary Sciences (422), and Biological Sciences (311) for obvious rationales as forests have ever been an interesting and significant part of natural sciences disciplines. But in recent times, techno-economic analysis of forest biomass for carbon capture and sequestration (CCS) is catching the attention of environmental scientists because the biomass use and CCS combination results in net negative carbon emission (Yang et al. 2021). The techno-economic assessment under the current field of study helps to discover the technological efficient and economically viable method for climate change mitigation. Such kinds of studies are carried out in the disciplines like Engineering (266 publications) and Economics (233 publications). Studies in Human Society and Earth Sciences also have significant contributions in this field followed by Built Environment & Design, Chemical Sciences, and Technology.
Panel (b) shows that SDG13: Climate Action has the largest number of documents (622 publications) that aims to “take urgent action to combat climate change and its impacts” (www.globalgoals.org). Such a huge number of publications under this category proves the importance of forests for combating climate change. 396 documents have been produced under SDG 7: Affordable and Clean Energy which main aim is to “ensure access to affordable, reliable, sustainable and modern energy for all” (www.globalgoals.org). SDG 15: Life on Land has 262 documents that aim to “protect, restore, and promote sustainable use of terrestrial ecosystems, sustainably manage forests, combat desertification, and halt biodiversity loss” (www.globalgoals.org). SDG 1: No Poverty; SDG 12: Responsible Consumption and Production; SDG 2: Zero Hunger have also significantly contributed in this field followed by SDG 11, SDG 10, SDG 6, and SDG 8.
3.3 Documents bibliographic coupling analysis and top-cited articles
A network map of documents bibliographic coupling based on citations is generated through VOSviewer software in Fig. 4, showing the most influential works of the highest citation. Of 1,439 publications, 908 meet the threshold while keeping the minimum number of citations to 5. Out of which 100 documents with the greatest total link strength are selected for the study, where the whole combined total link strength is 2785. According to Kessler (1963), bibliographic coupling occurs when two articles refer to common references. Two scholarly works are said to be more intellectually co-related when they have a large number of references in common indicated by a higher numerical value of link strength in this study. The size and colour of circles differ according to the referred common number of citations among the selected documents.
Cluster analysis determines cluster of objects that belongs to similar subject areas. In Fig. 4, five clusters are found based on subject relatedness representing five economic dimensions of studies under EFCS. The first cluster is the largest one having 28 items (red nodes) followed by 23 items in the second cluster (green nodes), 20 items in the third cluster (blue nodes), 11 items in the fourth cluster (yellow nodes), and 6 items in the fifth and smallest cluster (purple nodes). The assigned cluster names with their corresponding studies of highest citations along with their research fronts are shown in Table 2. Only those studies are included in each cluster that are relevant to the cluster themes. Cluster 1 is made of the studies related to the economics of forest carbon sequestration (FCS) mainly contributed by Corbera (2007) – 240 citations, Van Kooten (2004) – 101 citations, Mckenney (2004) – 56 citations, Stainback (2002) – 53 citations, Boyland (2006) – 35 citations, Slaney (2009) – 9 citations, and Lintunen (2016) – 8 citations. Cluster 2 includes topics confronting to the economic assessment of FCS mainly conducted by Liu (2014) – 63 citations, Triviño (2015) – 55 citations, Rodríguez-veiga (2017) – 42 citations, De Koning (2005) – 33 citations, Pelletier (2012) – 27 citations and Chu (2019) – 23 citations. Cluster 3 corresponds to the socio-economic scenario of FCS, main studies are carried out by Swetnam (2010) – 188 citations, Drescher (2016) – 140 citations, Samii (2014a, 2014b) – 68, 25 citations, Bluffstone (2013) – 39 citations, and Djanibekov (2014) – 23 citations. The fourth cluster is about the environmental and economic impact of land-use systems, studies are mainly performed by Popp (2017) – 320 citations, Daioglou (2019) – 57 citations, and Brandão (2010) – 31 citations. The fifth cluster is based on the economic viability of FCS, main works are done by Jerath (2016) – 53 citations and Thompson (2014) – 49 citations.
Table 2
Summary of total 5 clusters of Documents bibliographic coupling
Clusters | Studies with the highest citations | TC | Research fronts |
1. Economics of forest carbon sequestration (FCS) | Corbera (2007) DOI: 10.1111/j.1467-7660.2007.00425.x | 188 | Equity and legitimacy of markets for ecosystem services |
Van Kooten (2004) DOI: 10.1016/j.envsci.2004.05.006 | 115 | How costely are carbon offsets? |
Mckenney (2004) DOI: 10.1016/j.forpol.2004.03.010 | 66 | Cost estimates for carbon sequestration |
Stainback (2002) DOI: 10.1078/1104-6899-00006 | 61 | Economic analysis of slash pine FCS |
Boyland (2006) DOI: 10.1139/x06-094 | 35 | The economics of using forests for carbon storage |
Slaney (2009) DOI: 10.1016/j.forpol.2009.07.009 | 15 | The economics of carbon sequestration through pest management |
Lintunen (2016) DOI: 10.1016/j.jfe.2016.05.001 | 8 | Economics of forests and climate change |
2. Economic assessment of FCS | Liu (2014) DOI: 10.1007/s10980-014-0081-4 | 63 | Grain to Green Program for FCS |
Triviño (2015) DOI: 10.1016/j.ecoser.2015.02.003 | 55 | Managing boreal forest with economic investment for FCS |
Rodríguez-veiga (2017) DOI: 10.1007/s40725-017-0052-5 | 42 | Quantification of forest carbon stocks |
De Koning (2005) DOI: 10.1579/0044-7447-34.3.224 | 33 | Assessment of ecological and economic potential of FCS |
Pelletier (2012) DOI: 10.1016/j.forpol.2010.05.005 | 27 | Economic assessment of forest carbon stocks uncertainties |
Chu (2019) DOI: 10.1016/j.jclepro.2018.12.296 | 23 | Economic assessment of FCS |
3. Socio-economic scenario of FCS | Swetnam (2010) DOI: 10.1016/j.jenvman.2010.09.007 | 196 | The socio-economic scenario of land cover change |
Drescher (2016) DOI: 10.1098/rstb.2015.0275 | 122 | Socio-economic functions of tropical land use |
Samii (2014a, 2014b) DOI: 10.4073/csr.2014.11, DOI: 10.4073/csr.2014.10 | 27, 12 | Effects of FCS on poverty in low- and middle-income countries |
Bluffstone (2013) DOI: 10.1016/j.ecolecon.2012.12.004 | 38 | REDD+ for low-income countries |
Djanibekov (2014) DOI: 10.1007/s10640-014-9843-3 | 22 | Stochastic economic assessment of afforestation |
4. Environmental and economic impact of land-use systems | Popp (2017) DOI: 10.1016/i.gloenvcha.2016.10.002 | 305 | Land-use futures in the shared socio-economic pathways |
Daioglou (2019) DOI: 10.1016/j.gloenvcha.2018.11.012 | 59 | Integrated assessment of biomass supply and demand |
Brandão (2010) DOI: 10.3390/su2123747 | 34 | Environmental and economic impacts of multifunctional land-use systems |
5. Economic viability of FCS | Jerath (2016) DOI: 10.1016/j.envsci.2016.09.005 | 51 | Role of economic, policy, and ecological factors in carbon stock estimation |
Thompson (2014) DOI: 10.1016/j.ecoser.2014.03.004 | 46 | The economic viability of blue carbon |
TC: Total Citations |
Out of the most cited articles on the economics of forest carbon sequestration, the top 10 studies shorted by relevance as per the theme of the current study have been selected and shown in Table 3 along with their digital object identifier (DOI) number. The article POPP A (2017) entitled “Land-use futures in the shared socio-economic pathways” is the highly cited document with 305 citations followed by RICHARDS KR (2004)’s “A review of forest carbon sequestration cost studies: A dozen years of research” with 252 citations, SWETNAM R (2010)’s “Mapping socio-economic scenarios of land cover change: A GIS method to enable ecosystem service modelling” with 196 citations, CORBERA E (2007)’s “The Equity and Legitimacy of Markets for Ecosystem Services” with 188 citations, and RHODES JS (2005)’s “Engineering economic analysis of biomass IGCC with carbon capture and storage” with 136 citations. The remaining five significant studies are DRESCHER J (2016), VAN KOOTEN (2004), BARR CM (2012), GOLUB A (2009), KURGANOVA I (2014).
Table 3
Top 10 highly cited research articles sorted by relevance
Paper | Title | TC |
POPP A, 2017 DOI: 10.1016/i.gloenvcha.2016.10.002 | Land-use futures in the shared socio-economic pathways | 305 |
RICHARDS KR, 2004 DOI: 10.1023/B:CLIM.0000018503.10080.89 | A review of forest carbon sequestration cost studies: A dozen years of research | 252 |
SWETNAM R, 2010 DOI: 10.1016/j.jenvman.2010.09.007 | Mapping socio-economic scenarios of land cover change: A GIS method to enable ecosystem service modelling | 196 |
CORBERA E, 2007 DOI: 10.1111/j.1467-7660.2007.00425.x | The equity and legitimacy of markets for ecosystem services | 188 |
RHODES JS, 2005 DOI: 10.1016/J.BIOMBIOE.2005.06.007 | Engineering economic analysis of biomass IGCC with carbon capture and storage | 136 |
DRESCHER J, 2016 DOI: 10.1098/rstb.2015.0275 | Ecological and socio-economic functions across tropical land-use systems after rainforest conversion | 122 |
VAN KOOTEN, 2004 DOI: 10.1016/j.envsci.2004.05.006 | How costly are carbon offsets? A meta-analysis of carbon forest sinks | 115 |
BARR CM, 2012 DOI: 10.1016/J.BIOCON.2012.03.020 | The political economy of reforestation and forest restoration in Asia-Pacific: Critical issues for REDD+ | 97 |
GOLUB A, 2009 DOI: 10.1016/j.reseneeco.2009.04.007 | The opportunity cost of land use and the global potential for greenhouse gas mitigation in agriculture and forestry | 89 |
KURGANOVA I, 2014 DOI: 10.1111/gcb.12379 | Carbon cost of collective farming collapse in Russia | 77 |
TC: Total Citations |
3.4 Highly cited and most productive authors
The VOSviewer citation map of the most influential authors extracted from the Dimensions bibliometric database is represented in Fig. 5. From the 5,250 total authors, the top 100 highly cited authors have been selected for the analysis, out of which 34 researchers met the threshold according to the relatedness of their number of co-authored publications. The graph is demonstrating 6 clusters and 116 total co-authorship links. The most cited author in each cluster representing the name of that cluster. For instance, cluster 1 is represented by Hertel, Dietrich having 567 citations. In the same way, cluster 2 is represented by Obersteiner, Michael securing 744 citations followed by Kriegler, Elmer with 402 citations, Popp, Alexander with 525 citations, Calvin, Katherine V with 1,305 citations, and Faaij, André P C with 644 citations. It is remarkable to observe from the map that no clusters have any overlap, evidencing relative independence of research groups under EFCS.
Table 4 reveals the list of the top 10 most active authors shorted by the number of documents produced by them. The table provides information about the author’s citations, affiliations, and the title of one of the articles produced by the respective author which is closely related to the research agenda of the present study along with their DOIs. The highest documents are published by André P C Faaij (11 documents), followed by Pete, Smith (9 documents), Bren L Sohngen (8 documents), Michael Obersteiner and Timo Pukkala (7 documents), Mohamed Pourkashanian and Daniel W Mckenney (6 documents), Jiamen Ge and Indu K Murthy (5 documents), and Gerrit Cornelis Van Kooten (4 documents).
Table 4
Top 10 most productive authors
Authors | Doc | TC | Title of one of the studies produced by the corresponding author with DOI | Affiliations |
André P C Faaij | 11 | 644 | Forestry projects under the Clean Development Mechanism? DOI: 10.1023/A:1026370624352 | University of Groningen, Netherlands |
Pete Smith | 9 | 779 | The Technological and economic prospects for CO2 utilization and removal DOI: 10.1038/S41586-019-1681-6 | University of Aberdeen, United Kingdom |
Brent L Sohngen | 8 | 338 | The opportunity cost of land use and the global potential for greenhouse gas mitigation in agriculture and forestry DOI: 10.1016/J.RESENEECO.2009.04.007 | The Ohio State University, United States |
Michael Obersteiner | 7 | 744 | Site identification for carbon sequestration in Latin America: A grid-based economic approach DOI: 10.1016/j.forpol.2004.12.003 | International Institute for Applied Systems Analysis, Austria |
Timo Pukkala | 7 | 106 | At what carbon price forest cutting should stop? DOI: 10.1007/S11676-020-01101-1 | University of Eastern Finland, Finland |
Mohamed Pourkashanian | 6 | 1,332 | Techno-economic and environmental assessment of BECCS in fuel generation for FT-Fuel, BIOSNG and OME X DOI: 10.1016/J.RSER.2009.06.001 | University of Sheffield, United Kingdom |
Daniel W Mckenney | 6 | 141 | Cost estimates for carbon sequestration from fast growing poplar plantations in Canada DOI: 10.1016/j.forpol.2004.03.010 | Natural Resources Canada, Canada |
Jiamen Ge | 5 | 40 | Carbon sinks and output of China’s forestry sector: An ecological economic development perspective DOI: 10.1016/j.scitotenv.2018.11.219 | Xiamen University, China |
Indu K Murthy | 5 | 49 | Carbon forestry economic mitigation potential in India, by land classification DOI: 10.1007/s11027-006-9063-4 | Indian Institute of Science Bangalore, India |
Gerrit Cornelis Van Kooten | 4 | 199 | Economics of Forest Ecosystem Carbon Sinks: A Review DOI: 10.1561/101.00000006 | University of Victoria, Canada |
Doc: Number of documents, TC: Total citations |
3.5 Citations analysis of Core Journals and Bradford’s Law
From the analysis of subject categories and cluster analysis of articles, it is observed that studies on EFCS have multidisciplinary features and cover a wide range of research themes that have been published in a variety of journals with various research orientations. The source-citation analysis of core journals has been performed to understand the significance and collaboration of each specific journal in creating the knowledge on EFCS. Journal density visualization map based on documents-weights is prepared through VOSviewer shown in Fig. 7. Of the 503 sources, producing 1,439 documents, 65 meet the thresholds keeping the minimum number of documents of a source to 5 and the minimum number of citations of a source to 5. The largest set of connected items are consist of 63 items. The colour schemes of the source density visualization map is of Rainbow type varying from red, yellow, green to blue colour where red corresponds to the highest document density and blue corresponds to the lowest document density. In other words, the colour of each node in the density visualization plat depends on the density of that node.
In this way, the density visualization pattern helps to understand the whole structure of the map by concentrating on the most relevant areas in the map. Hence, we can trace the most productive journals instinctively on the map. From Fig. 6, Forest policy and economics, Forest ecology and management, Forests, Journal of cleaner production and Journal of environmental management represent the core and most productive five sources in the current field of study. The top 10 leading journals with their frequencies, citations, H index, and start year are shown in Table 5. The frequencies of the top 5 journals are 50, 41, 34, 32, and 29 respectively. Forest ecology and management has the highest citation count (1782) and The science of the total environment has the highest H index (244), both journals have started in the year 2002 and 2016 respectively. On the other side, Forests has the lowest citation (312) and lowest H index (44), which started in the year 2010.
Table 5
List of top 10 most productive journals
Sources | Frequency | Citations | H index | Start year |
FOREST POLICY AND ECONOMICS | 50 | 955 | 68 | 2001 |
FOREST ECOLOGY AND MANAGEMENT | 41 | 1782 | 176 | 2002 |
FORESTS | 34 | 312 | 44 | 2010 |
JOURNAL OF CLEANER PRODUCTION | 32 | 722 | 200 | 2009 |
JOURNAL OF ENVIRONMENTAL MANAGEMENT | 29 | 1235 | 179 | 2003 |
THE SCIENCE OF THE TOTAL ENVIRONMENT | 29 | 830 | 244 | 2016 |
APPLIED ENERGY | 23 | 590 | 212 | 2013 |
MITIGATION AND ADAPTATION STRATEGIES FOR GLOBAL CHANGE | 22 | 406 | 71 | 2001 |
ENVIRONMENTAL SCIENCE AND POLICY | 19 | 978 | 115 | 2003 |
ECOLOGICAL ECONOMICS | 18 | 828 | 202 | 2001 |
Table 6
Zone wise distribution of journals
Zones | Total Journals (503) | Journal Percentage | Total articles (1,439) | Article Percentage |
Zone 1 | 21 | 4.17% | 486 | 33.77% |
Zone 2 | 94 | 18.68% | 480 | 33.35% |
Zone 3 | 388 | 77.14% | 473 | 32.87% |
Bradford’s law is a distribution pattern discovered by Samuel C. Bradford (1934) to measure the exponentially diminishing returns of reference searches in scholarly journals. Bradford claims that for a given subject field “there are a few very productive periodicals, a larger number of more moderate producers, and a still larger number of constantly diminishing productivity” (Nash-Stewart et al. 2012). Under this law, if scientific journals of a subject are arranged into three groups by the number of articles, each occupying about 1/3rd of total articles, then the proportion of the number of journals in each group will be 1:n:n2 (Table 6). These three groups are formally known as Zone 1 or core, Zone 2 or middle part, and Zone 3 or tail. Fig. 7, derived from Biblioshiny app, highlights the Zone 1 or “core journals” that are most frequently cited and the researchers in this field are likely to have more interest in these journals. Zone 2 consists of the journals with average citations. Zone 3 reflects a long tail like picture of those journals that are considered as of marginal importance and very rarely cited (Potter 2010). Table 6 shows that Zone 1 includes 21 “core journals” which is 4.17% of the total 503 journals containing 486 articles. Zone 2 have 94 journals (18.68% of total journals) with 480 articles. Zone 3 have the largest number of less cited journals covering 77.14% of all journals in this discipline, containing 473 articles. From the table, it is clear that each Zone has comprised of one-third of the total articles.
3. 6 Co-authorship analysis of countries and institutions
Regional collaboration, degree of communication, and research hotspots could be identified from the network visualization map of co-authorship occurrences among countries. The overlay visualization map of country co-authorship based on documents-weights in the field of EFCS is represented in Fig. 8. Keeping the minimum number of documents and citations of a country to 5, of the 99 countries 51 meet the thresholds. For each of the 51 countries, the total strength of the co-authorship with other countries is analysed. The size of the frames represents the influential countries according to the number of occurrences of documents. The distance between two nodes reflects the connection of their co-authorship link, and the thickness of the networking lines shows the strength of the link. The colour gradient is shown in the lower right corner of the map indicating the publication year of the author. The colour of the frame of a country is decided by the number of scores where blue represents the lowest score (the oldest or pioneer countries) and yellow represents the highest score (the latest or emerging countries).
The global distribution of documents published on EFCS is illustrated on the world map in Fig. 9 where the white spaces are representing no data. In the map, the colour gradient varies from red (highest publication) to green (lowest publication). Table 6 shows the top 10 countries of highest scientific production with their citations and total link strength (TLS) values. From the overlay map (Fig. 8), world map (Fig. 9) and Table 7, it is understood that the United States (US) is a pioneer country in this field with the highest document production (336), citations (13315) and TLS (264). With such great scores, the US becomes the research centre in the area of EFCS followed by China, which is the central research spot in Asia with 197 documents production and 3329 citations. The US is strongly linked with China having 31 link strengths. Link strength between US and UK is 30 and it is equally connected with Canada, Netherlands, and Germany having 15 link strengths. China is strongly linked with the US, Canada, and Germany. The other main pioneering countries with blue, purple frames are Germany, Netherlands, Canada, Norway, South Africa, Mexico, and Columbia. Iran, Philippines, Pakistan, Belgium, Singapore, Portugal and Greece are the latest emerging countries in this field represented in yellow frames. Out of which Belgium has created strong links with pioneering countries like the US, UK, Netherlands, and Canada in a very short period.
Table 7 Top 10 most productive countries
Country
|
Documents
|
Citations
|
TLS
|
United States
|
336
|
13315
|
264
|
China
|
197
|
3329
|
121
|
United Kingdom
|
136
|
6021
|
172
|
Australia
|
99
|
4806
|
138
|
Germany
|
99
|
4364
|
171
|
Canada
|
86
|
3239
|
90
|
Netherlands
|
60
|
3615
|
109
|
India
|
58
|
1127
|
20
|
Finland
|
51
|
1461
|
59
|
Spain
|
48
|
2832
|
64
|
TLS: total link strength
The overlay network map of co-authorship occurrences in institutes based on documents- weights are shown in Fig. 10. Of the 1240 institutes, 133 meet the threshold. Out of which, 100 items are selected for the scientific collaboration analysis of institutes. The top five institutes with the highest documents production are the University of Florida – 22 documents (U. S.), University of Chinese Academy of Sciences – 20 documents (China), Wageningen University & Research – 20 documents (Netherlands), University of British Columbia – 19 documents (Canada), and University of Eastern Finland – 17 documents (Finland). The overlay visualization illustrates that Wageningen University & Research, University of Florida, University of British Columbia, University of Göttingen, and University of East Anglia are the main pioneering organizations whereas Federal University of Rio De J, National University of Singapore, University of Tokyo, Harvard University and Kyoto University are the latest emerging institutes in this field. Among the newest institutes, Harvard University is growing rapidly and has created a good link strength with all kinds of institutes ranging from oldest to the newest.
3.7 Title’s Keyword analysis and thematic evolution
The title’s keyword analysis analyses the keywords that have been occurring in the title of the articles published in the concerned field. Such kind of investigation is required to establish the research trend, ascertain the research gaps in the studies on EFCS, and discover the interesting fields and themes that need special attention of researchers. Word Cloud is an instrument for representing the word’s frequency in terms of occurrences in a text body. This approach visualizes a bunch of words that appear in different font sizes depending on how frequently they have occurred in the source text. This lucrative and effective technique of keyword analysis brings forth an instant result of trendy research fronts.
To create the Word Cloud of keywords, R-Studio based bibliometrix tool- ‘Biblioshiny app’ is used (Fig. 11). In the Biblioshiny app, under graphical parameters, the field is selected as ‘Titles’, N-grams is selected as ‘Bigrams’, and the number of words is selected as ‘70’. The word occurrence measure is ‘Frequency’, shape- ‘Cardiod’, font type- ‘Verdana’, and text colour is ‘Random Dark’. At last, the Ellipticity and Padding values are selected as 0.65 and 1 respectively. The Treemap of the top 20 most frequent keywords with their frequency and percentage of occurrences are illustrated in Fig. 12. The Word Cloud and Treemap both figures altogether depict that ‘carbon sequestration’ is the most used keyword with 201 frequency and 21% of occurrence followed by ‘climate change’, ‘ecosystem services’, ‘carbon stocks’, ‘carbon capture’, ‘forest carbon’, ‘forest management’, and ‘carbon storage’ that have usually occurred 9-4% of the times in the title of the articles, indicating the most highlighted, trendy, and significant part of the field. But at the same time, keywords like ‘techno economic’, ‘economic analysis’, ‘economic assessment’, ‘socio-economic’, and ‘sequestration potential’ have retained comparatively less percentage of occurrences that is about 3-2%. This analysis infers a huge research gap in the economic analysis of forest carbon sequestration where themes like carbon sequestration, climate change, and carbon capture are studied in the majority and the economic aspects of this field are lacking attention. However, in recent times, the trend of themes has shuffled as delineated by Sankey diagram of the thematic evolution in Fig.13.
The thematic evolution technique identifies the changing paths of studies, evolutionary relationships as well as structures, contexts, and strength of emerging themes that appear over time. This method plays a very important role in portraying the degree and direction of field development and also in forecasting the trends of the field (Cobo et al. 2011). The Sankey diagram (Fig. 13) displaying thematic evolution is a kind of flow chart where the width of the arrow is proportional to the quantity flow. Each node in Fig. 13 corresponds to a topic and the width of the node is proportional to the frequency of keyword occurred under the theme. A thick node characterizes the relevance of that theme. Three-time zones (2001-2013; 2014-2017; 2018-2021) are wired to illustrate the temporal movement between research topics. This Sankey graph has clearly shown the evolution and extinction of themes related to the current field of study over time. During 2001-2013, almost one decade, the themes like forest carbon sequestration and greenhouse gas emissions were the major part of the study which came on top during 2014-2017. In the time slice (2001-2013), there was no sign of the economic aspects of FCS (forest carbon sequestration) that further emerged for the first time during 2014-2017 but was not on the list of priority. The ‘economic analysis’ which is the main focus of this study came on top in recent times during 2018-2021, signifying the awareness of researchers in bridging up the research gaps that were identified by the Word Cloud and Treemap analysis of keywords.