1. Introduction
Currently, there is no uniform standard for the definition of hollow trees, but woody plants with more than one hole (≥2 cm in diameter) in the trunk or branches are generally considered to be hollow trees [1]. The formation of hollow trees is the result of a combined effect of biotic and abiotic factors [2]. Trees are susceptible to stress from various external factors and damage to the surrounding environment during the growth process, which creates opportunities for the microorganisms to invade the plant, resulting in structural changes to the wood, and eventually becomes flimsy, fragile, and rotten, etc., thus promoting the development of hollowing to their main trunk [3]. The decay of standing wood, as a slow process, is the result of the synergistic action of a variety of bacteria and fungi [4,5]. Specifically, decay fungus decomposes wood cell walls and cellulose and lignin in wood by infecting standing trees, leading to the decay of tree trunks, heartwood, sapwood, or the entire trunk, affecting tree growth and even causing widespread tree death [4,6,7].
In addition, the decay or hollowing of the main trunk of the tree will inevitably lead to the decline of the mechanical strength of the tree [8,9]. Once the hollow tree encounters extreme weather, such as strong winds or sandstorms, it will cause lodging, branch breakage, and other hazards [10,11]. Existing studies have mainly focused on the significance of hollow trees in forest ecosystems for biodiversity conservation [12,13,14,15,16], which can provide a temporary or permanent habitat for many tree-hole nesting animals for reproducing, wintering, feeding, and escaping from predators [17]. It has been suggested that the number of hollow trees directly influences the diversity and abundance of nesting fauna in tree cavities [12,18]. However, for the tree itself, the decay of standing trees will have a profound effect on water transport, the mechanical structure, and community health. During the early to late stages of trunk decay, the wood properties of the tree change dramatically, and their use-value and physicochemical properties decline significantly [4,6,19], which can lead to the breakage or death of standing trees, thereby promoting the formation of gaps, and influencing vegetation succession and composition [20,21]. Previous studies have shown that the heartwood gradually decomposes as trunk decay degree increases, and together with the predatory sucking of nutrients by parasitic plants, will have a significant impact on the tree’s biomass and available trunk volume [22]. In addition, the increasing amount of decaying wood leads to a decline in the carbon sequestration capacities of forests [21,23] and even disrupts the carbon balance of forest ecosystems, ultimately threatening forest ecosystem functions, services, and sustainability.
Scientometric is a method for the quantitative evaluation of scientific activity based on bibliometrics and science statistics. Scientific knowledge mapping is an image that takes the field of knowledge as the object of study and shows its development processes and structural relationships. It shows both the connections, structures, and interactions between units of knowledge and the nourishment of new knowledge within a large body of knowledge [24]. It uses mathematical, statistical, and bibliographic methods to quantify the characteristics of research in a particular field, including the number of written publications, countries, institutions, journals, authors, and keywords [25], to identify the sources used, to demonstrate the depth of research, to place work in context, and to acknowledge the work of other scholars. Many researchers have used this approach extensively to conduct qualitative and quantitative analyses of research patterns and trends in a particular field [26,27]. As scientometrics allow for a comprehensive and rapid description of a specific academic field, it is one of the most effective methods for building bibliometric networks [28]. It involves collecting all the literature in a particular field over a specific period and reviewing it. Its evaluation process is powerfully objective, authoritative, simple, and reproducible [29].
In summary, the studies related to tree trunk hollowing and decay in forest ecosystems have received a great deal of attention from scholars in many countries and have become one of the most important research hotspots in forest ecosystem management and conservation. In the face of the vast amount of literature in this field, it is necessary to gain a comprehensive understanding of the current research progress, research hotspots, and developments of tree decay research in the forest ecosystem. Using a systematic and accurate bibliometric approach with a combination of quantitative and qualitative visual dissection can provide a more accurate and comprehensive review of relevant research in this field. Therefore, this study comprehensively reviewed and summarized the scientific literature related to forest tree trunk decay and hollowing obtained from the CNKI (China national knowledge infrastructure) and Web of Science core collection databases during the period 2002–2021 by using CiteSpace (v.5.8.R3 Philadelphia, PA, United States) knowledge mapping software and bibliometric analysis. The aim is to present a clear and objective overview of the research, research developments and processes, and trends in this research area and to identify and distill the latest research hotspots and future trends that will not only provide scientific reference and inspiration for future research on hollow rot trees but also contribute to the conservation and sustainable management of forest ecosystems.
2. Data Sources and Research Methods
To ensure the comprehensiveness of the relevant literature, the data in this paper were extracted from the Science Citation Index Expanded (referred to as SCI) of the internationally recognized authoritative database Web of Science (WoS) Core Collection TM and CNKI (China’s national knowledge infrastructure), the data retrieval data was on 16 December 2021, and the subject terms in Figure 1 were searched and analyzed (with “or” relationships between the subjects). In this study, relevant disciplines, such as ecology, environment, and forestry, were screened according to the subject classification of the search database and titles that differ significantly from these subject types were eliminated. Abstracts, keywords, and other relevant content were read to screen, and the 2205 pieces of literature needed were obtained (search period 2002–2021). This paper uses CiteSpace (v.5.8.R3) analysis software to analyze the current status of research, hotspots, and trends in forest tree decay and trunk-hollowing rots, as well as the main research countries, institutions, and co-cited authors. CiteSpace was developed by Dr. Chaomei Chen from Drexel University, USA, and it is widely used for text analysis, mining, and visualization of bibliographic databases [24]. It is mainly based on co-citation analysis theory and pathfinding network algorithms to carry out quantitative modeling of relevant literature in the research field and explore the evolutionary paths and development frontiers of the research field through a series of visual maps [30].
3. Results and Analysis
3.1. Temporal Analysis of the Literatures Published
The number of publications over time reflects the development pace of the research topic. Internationally, the overall trend in the number of publications on trunk decay and hollow rot internationally is experiencing a rapid growth phase (Figure 2).
The amount of relevant literature was less than 40 articles per year on average prior to 2002, and they were mainly focused on the basic questions and developing basic concepts, which was at the initial stage. The number of articles published between 2002 and 2010 gradually increased but with little fluctuation. During this time, a large number of empirical studies were published based on previous studies that continue to provide empirical and sophisticated research methods. The number of papers produced in 2011 was more than double that of the initial stage, with the number surpassing 100; the research interest in the study of hollow rot in forest tree trunks continued to grow internationally. A significant increase in literature output of up to 180 articles by 2020 represents a boom in the field during this period. In contrast, the literature in Chinese shows a slow-growth trend, with an average of fewer than 25 articles from 2002 to 2021, and the average annual number of Chinese publications on CNKI is quite a few compared to the WoS database.
3.2. Research Hotspots and Trends
The CiteSpace Timezone map provides a visual representation of the research frontiers in different time zones and their derivative relationships, allowing for reasonable predictions for future developments [19]. Keywords provide a high-level overview of the hotspots of research in the literature and are the core of the literature. The term “burst” refers to the detection of words that have a high rate of change in frequency over a certain period of time from a large number of subject words by examining the word frequency. The keywords with high emergence are those with a high rate of frequency change over a certain period of time, reflecting the hotspots of research and indicating the frontiers and trends of research. The temporal evolution of the hot spot keywords for forest trunk hollow rot was plotted (Figure 3), with a node size proportional to frequency; the color of the plot from red to yellow represents the progression through time, and the color on the annual ring of the node corresponds to the time chromatogram. CiteSpace settings: Node Types; Keyword; Selection Criteria: Top 10%; and default settings are used for the rest. The production of high frequency and emergent keyword information tables are shown in (Table 1).
Exploration phase (2002–2004): As the research on forests continued, trunk decay and cavity began to enter the research field of scholars. The terms “tree”, “decay”, and “dead wood” were the most frequent keywords in this phase, indicating that the relationship between tree decay and mortality was the core of this research.
Developmental phase (2005–2011): A large number of emergent and high-frequency terms appeared in this phase, indicating that decay and cavity trees have become a research hotspot, with related ecological concepts and new measurement techniques and emerging methods, and expanding the research fields. The following keywords, as “dynamics”, “forest”, “diversity”, “growth”, “decomposition”, “wood”, “coarse woody debris”, “conservation”, “community”, “management”, and “cavitation”, have the highest frequency of over 100 times. Studies involving the keywords “forest”, “decay”, and “decomposition” refer to the direct causes of wood “decay” and the necessary conditions required for “forest” and “decay”. There are also factors influencing decay, etc. [31,32,33]. Studies using the keywords “tree” and “cavitation” refer to hollow tree formation and influencing factors [12,34,35]. By exploring the keywords, scholars were mainly concerned with the mechanisms of hollow rot formation in forest stands and its prevention and management. The study received increasing attention after 2007, when “wood decay” appeared with the strongest change in frequency [36]. In addition, “climate change”, “vegetation”, “xylem”, and “hydraulic conductivity” all burst above 6, with representative studies and publications appearing in which scholars considered the relationship between vegetation and different environments [12,35] and explored ways to counteract tree decay and hollowing.
Expansion phase (2012–2021): As research continued to deepen and refine, new technologies and theories emerged with new breakthroughs. “phylogeny”, “decay rate”, “X-ray microtomography”, “decomposition rate”, etc., are current research hotspots in this field. The direction of research has gradually become more nuanced from a macroscopic perspective, and as technology has developed, many scholars have begun to use these tools for the quantitative detection and assessment of trunk decay and cavity. For instance, the non-destructive diagnosis and health assessment of old and valuable trees, detection of trunk cavities using ground-penetrating radar offset imaging [37,38,39,40,41], etc. Additionally, some scholars are concerned with the study of “old and ancient trees”, “red pine”, “poplar”, etc. For example, quantitative detection of trunk decay of live standing red pine in the Xiaoxing’an Mountains and its relationship with site conditions [42] and the decay of poplar desert riparian forests under water and salt stress [10,11,43], etc. The aforementioned study is of great theoretical value, broadening the breadth and depth of this research, and represents a new breakthrough in the study of trunk decay and cavity.
The keyword emergent mapping clearly shows the beginning and end time of the keywords with an emergent nature and can accurately reflect the change and duration of the keywords in each time period. As can be seen from Figure 4, the three keywords with the highest intensity of emergence are “phylogeny” (9.51), “X-ray microtomography” (7.63), and “wood decay” (7.18). With “wood decay” starting in 2018 and ending in 2021, indicating that research on tree trunk decay has become very active in recent years, and it can be assumed that research on these topics will continue to increase in the future. From 2012 to 2021, the research trends became increasingly microscopic, and specific research methods have been derived for different research purposes, with instruments for internal decay of standing trees and the health of old and ancient trees being successfully developed and widely used in natural forests and urban street tree management. Dozens of techniques have been applied to detect a heart rot inside trees, such as stress wave detection, radiographic detection, nuclear magnetic resonance detection, infrared spectroscopy, resistance detection, radar detection, and ultrasonic detection [44,45].
3.3. Research Themes
Figure 5 explores the common themes in similar literature and generates a literature co-citation clustering timeline graph showing the top 16 clustered themes. The color of the clusters changes over time; CiteSpace settings: Node Types: Cited Reference; Selection Criteria: Top 10%; links: Cosine; Other default settings are used. The module value Q and the average profile value S can be obtained to determine the mapping effect; when the Modularity Q is 0.4–0.8, the mapping is considered to meet the requirements, and the obtained mapping has a Q value of 0.7841, which is in line with the basic requirements; Silhouette (Each cluster is represented by a silhouette, which is based on a comparison of its closeness and separation. This silhouette shows which objects lie well within their clusters and which objects are just between clusters. The entire clusters are displayed by combining the silhouettes into a single graph so that the relative quality of the clusters and the profile of the data configuration can be appreciated), an indicator of similarity within clusters, is a decimal number between 0 and 1, the larger the value, the higher the similarity. The S-values in Table 2 are all close to 1, then the research themes within each cluster are clear.
From Figure 5 and Table 2, it can be seen that the four themes with cluster sizes above 100 are “Eastern Canada”, “hydraulic vulnerability segmentation”, “deadwood management”, and “hydraulic safety”, with “Eastern Canada” referring to research focusing on tree species in Eastern Canada, for instance, boreal balsam fir (Abies balsamea (L.) Mill.) forests of Quebec, in Eastern Canada, Eastern Canadian black spruce (Picea mariana (Mill.) Britton, Sterns & Poggenb), sugar maples, and so on. The word “deadwood management” has the highest S-value. Deadwood management has received much attention from researchers, as it is critical to the processes, structural complexity, and biodiversity of forest ecosystems [46], and therefore research on deadwood management contributes to maintaining the carbon balance of forest ecosystems and the sustainable health of various biological resources. As the changes in the hydraulic characteristics of tree trunks caused by the external environment have attracted the attention of scholars [47,48,49], the subject of “hydraulic vulnerability segmentation”, and “hydraulic safety” has been studied more often.
Compared to related studies around the world, most studies have focused on sustainable management of forest ecosystems, conservation of plant and animal diversity, and safety assessment of old and ancient trees and urban street trees [21,45,50,51,52], while less research on trunk-hollowing rot has been less extensive and in-depth, focusing mainly on the quantitative detection of decay and physicochemical properties of wood [9,19,53,54,55,56]. In addition, the clusters “lignin-modifying enzymes”, “wood decomposition”, “decay resistance”, and “material resistance” were also studied in the context of trunk-hollowing rot. As a result, the study and management of dead-standing wood due to tree decay have become a research hotspot of great interest.
3.4. Analysis of the Main Forces of the Study
3.4.1. Main Countries (or Regions) Active in This Filed
Figure 6 shows the top 10 countries in terms of the number of articles published in this field. The size of the node is proportional to the number of articles published; node centrality is the number of arbitrary shortest paths through a node in its network and is a measure of the size of the node’s connectivity role in the overall network. The outer circle marked with a purple circle indicates that the node has a large centrality (not less than 0.1) and is a key node in the graph. The research content corresponding to the emergent characterization node has a high rate of frequency change in a short period of time, and the corresponding chromosphere turns bright red. CiteSpace settings: node types; country; selection criteria: top 50; pruning: pathfinder; other default settings are used.
The analysis of the literature published in different countries reflects, to some extent, the importance and influence of that country in the field. As can be seen from Figure 6 and Table 3, the US is ranked top in the number of articles published in this research area (722), with a centrality of 0.62. These two indicators are both higher than those of other countries, and the US has laid the foundation and provided prior reference and scientific basis for other countries to carry out research in this area. Apart from the United States, research conducted to date in this area has been concentrated in France (193), Canada (191), Germany (164), Finland (163), China (159), Australia (116), and Sweden (116). China ranked sixth with 159 publications while ranking third with a higher centrality (0.05), which also reflects the high importance China attaches to the promotion of ecological civilization and forest conservation and its high academic status internationally. The scope of research has expanded from regional to global scale. This field of study is closely linked in a diamond shape between countries.
3.4.2. Analysis of Key Research Institutions
Analyzing the distribution of research institutions can help to understand the level of academic support and recognition of the field, thus facilitating collaboration between institutions. The node size is proportional to the number of articles; CiteSpace settings: node types: institution; selection criteria: top 50; other default settings are used (Figure 7). The United States accounted for three of the top 10 publishers, with 141 articles from the US Forest Serv, 62 from Oregon State Univ, and 62 from Univ of Minnesota; Finland accounted for two, 82 at Univ Helsinki and 43 at Finnish Forest Res Inst; Swedish Univ Agr Sci was second with 108 articles; Chinese Acad Sci was third (107); French National Institute for Agricultural Research (INRA) was fifth (66); Univ of British Columbia was eighth with 48 articles, and Australian Natl Univ was ninth with 43 articles. There are only two institutions with a centrality above 0.2: the Chinese Acad Sci is 0.22, and the INRA is 0.28. There are a large number of publishers, with US Forest Serv in the first place and dominating the list. The complex and tight connections between the institutions indicate close cooperation between them.
3.4.3. Key Authors and Author Co-Citation Information
The author’s co-citation time zone map reflects the key authors of relative research in this field at different periods. The combined author co-citation frequency, burstiness, and half-life can effectively express the academic status of the author in the field. For clarity, only authors with a co-citation frequency greater than 150 are mapped, anonymous authors are not analyzed, and recurring authors are shown only once, with node size proportional to co-citation frequency. Citespace settings: node types: cited author; selection criteria: top 50; other default settings are used (Figure 8).
As can be seen from Table 4, Harmon ME ranks as a leading author with a total citation frequency of 326, and he discovered that the carbon concentration of woody debris varies with tree species, decay class, and tissue type [57]; Tyree MT and Sperry JS revealed whether xylem cavitation and deformation in forest fires and leaf abscission protect stems from cavitation during seasonal drought [58,59]; Hacke UG sampled roots and stems from a selection of conifers and found that increased resistance to burrows did not significantly affect the resistance in tracheids [60], Choat B and his team quantified the vulnerability of roots and stems of five tree species to drought-induced cavitation [61]. Boddy L examined the inhibitory and stimulatory effects of wood decay fungal communities by exploring the effects of secretions from undefined beech and partially decayed beech on the expansion rates of 16 late-stage decay fungi [62]; Cochard H, on the other hand, estimated the moisture content of the heartwood by using electrical resistance tomography (ERT) in situ [63]; Brodribb TJ found that stem hydraulic supply was related to leaf photosynthetic capacity [64]. Collectively, these key authors were early contributors to this area of research, and their contributions were relatively large; all of these results provide a scientific reference for the in-depth study in this field.
4. Discussion
The analysis of the keywords reveals, firstly, a shift in research from a macro to a micro level. Previous studies have focused on the relationship between tree cavity type, abundance, frequency of occurrence, and biodiversity [12,13,14,15,16], mainly in eucalyptus forests, tropical rainforests, and temperate forests in North America and Europe, and are still at a qualitative or semi-quantitative level. Some studies have dealt with the mechanisms by which natural phenomena such as wind and fire affect the formation of tree hollows [12,65], but there was a lack of systematic research.
There is growing interest in future research on detection techniques in this area, and significant efforts have been made in developing robust non-destructive testing techniques (NDT: Non-destructive testing methods) by locating and quantifying wood decay, and defects are used to measure the physical condition of the trees within the urban forest to promote public safety and property protection. These methods are of special value to the urban forest managers and arborists responsible for the general safety of city residents, roadway transportation, and utility corridors [66]. This would enable predicting the intrinsic wood properties of individual trees and assessing wood quality at the stand and forest scale [67,68]. Among the various tree decay detection techniques, three main detection devices, including electrical resistance techniques [45,53,54], acoustic techniques, and radar detection [55,69], are widely used for assessing the mechanical properties of trees and detecting internal defects [44,45,52,67,70]. Therefore, how to accurately quantify the degree of internal decay of trees is a hot and difficult scientific issue for scholars.
The CiteSpace software tool can help quickly and easily grasp cutting-edge directions and hot issues in the field of forest air decay, identify the knowledge base and key literature in the research, and identify the leading researchers and major research institutions in the field. The results of the mapping and tables reveal an uneven contribution of research in the field from various countries, institutions, and scholars. Some countries and scholars are particularly prominent in their research findings and contributions to the field, but others are less so. What are the reasons for this result? Is it related to the importance attached by countries to the management of the internal health of forest tree trunks or the limitations of detection tools? Is there a need for greater encouragement of research in this area in some regions or countries, depending on the ecological context? What about including articles published in other languages, such as French, Spanish, and German. Given the context of research and regional focus (North America, particularly French Canada-Quebec, Europe), it would be interesting to explore articles published in these languages as well. Does the research skewed or biased toward English-speaking Anglo researchers? Several questions are worthy of exploring in the future. CiteSpace is still being updated, and more and more algorithms and features are being added to the software [71]. In the future, research on forest decay and cavity trees will be increasingly important for forest ecosystem health and management, and CiteSpace can be used to keep an eye on breakthroughs and transitions in this area, presenting a richer and clearer map of progress.
5. Conclusions
The collection and collation of decay and cavity tree studies in the WoS database show that the United States has been the most influential country in this field. Since many important Chinese studies have been published in Chinese, limiting the literature analysis to English-language-based databases may produce partial or biased conclusions. Therefore, the analysis of the 2205 decay and cavity tree-related research papers reviewed for this study included those from WoS (English) and those from CNKI (Chinese).
This study explores brief research patterns in the field based on a bibliometric analysis of these papers, highlighting the fact that research on decay and cavity of trees varies in type and size. The focus of the research is on addressing decay and cavity tree to improve ecological resilience. Although tree decay and cavity studies currently represent a relatively low proportion of the overall ecological research, the use of urban forests and trees as nature-based solutions have been gaining attention from researchers based on keyword analysis. The geographical distribution and research methods for decay and cavity tree studies are diverse, with a clear regional advantage in Europe and the USA. Most studies use quantitative methods to address environmental issues. Forests and trees have a major role to play in addressing environmental challenges such as stormwater management, air pollution reduction, and urban heat mitigation, and research on decay and cavity tree is widely recognized, but there is still a need to optimize and improve research methods for greater protection of forests and trees.
Conceptualization, T.A.; methodology, W.J. and T.A.; writing—original draft preparation, W.J. and T.A.; writing—review and editing, Ü.H., Z.W. and M.W.; funding acquisition, T.A. All authors have read and agreed to the published version of the manuscript.
The data presented in this study are available upon reasonable request from the corresponding author.
The authors declare no conflict of interest.
Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.
Enlarge this image.Figure 2. The number of domestic and international published literature on tree decay and stem cavitation from 2002 to 2021.
The information of high frequency and burst keywords.
| Year | Keywords | ||
|---|---|---|---|
| Burst | Frequency | ||
| 2002–2011 | pattern (4.61) | pattern (127) | dynamics (203) |
| resistance (3.56) | resistance (66) | forest (188) | |
| climate change (6.48) | climate change (59) | diversity (187) | |
| species richness (3.24) | species richness (55) | tree (152) | |
| log (3.17) | log (53) | decay (145) | |
| stem (4.95) | stem (50) | growth (145) | |
| cavitation resistance (4.45) | cavitation resistance (45) | decomposition (126) | |
| xylem cavitation (3.58) | xylem cavitation (38) | wood (118) | |
| organic matter (3.53) | organic matter (36) | coarse woody debris (114) | |
| inhabiting fungi (3.17) | inhabiting fungi (36) | conservation (113) | |
| response (3.74) | response (34) | community (109) | |
| wood decay (7.18) | wood decay (31) | management (102) | |
| density (5.43) | density (31) | cavitation (100) | |
| mechanism (3.9) | mechanism (29) | deadwood (94) | |
| vegetation (6.45) | vegetation (29) | vulnerability (92) | |
| temperature (4.01) | temperature (28) | ecology (75) | |
| xylem (6.48) | xylem (25) | fungi (67) | |
| tree mortality (4.35) | tree mortality (25) | nitrogen (62) | |
| hydraulic conductivity (6.58) | hydraulic conductivity (20) | ecosystem (42) | |
| old and valuable trees (25) | ancient dry lily (21) | ||
| salvage and protection (20) | ancient trees (17) | ||
| protection (14) | rare and ancient trees (11) | ||
| tree hollow (9) | hollow tree (7) | ||
| stress waves (5) | dry base decay (5) | ||
| non-destructive testing (5) | |||
| 2012–2021 | phylogeny (9.51) | phylogeny (24) | decomposition rate (18) |
| decay rate (3.4) | decay rate (23) | basidiomycota (12) | |
| X-ray microtomography (7.63) | X-ray microtomography (19) | annosum (12) | |
| trait (4.73) | trait (17) | conductivity (10) | |
| drought-induced embolism (4.78) | drought-induced embolism (14) | conifer (10) | |
| stress-induced cavitation (3.62) | stress-induced cavitation (13) | xylem vulnerability (8) | |
| temperate (5.33) | temperate (13) | fagus sylvatica (8) | |
| beech forest (4.67) | beech forest (13) | carbon dioxide (7) | |
| vulnerability curve (3.27) | vulnerability curve (8) | decaying wood (7) | |
| meta-analysis (3.59) | meta-analysis (7) | red pine (3) | |
| rare and ancient trees (3.1) | rare and ancient trees (11) | ground-penetrating radar (3) | |
| rejuvenation (8) | poplar (3) | ||
| tree hole repair (4) | trunk hollow (3) | ||
| protection advice (3) | health assessment (3) | ||
Clustering information of the co-cited reference.
| Rank | Theme | Size | Silhouette |
|---|---|---|---|
| #0 | Eastern Canada | 134 | 0.821 |
| #1 | hydraulic vulnerability segmentation | 119 | 0.946 |
| #2 | deadwood management | 115 | 0.958 |
| #3 | hydraulic safety | 103 | 0.867 |
| #4 | hydraulic trait | 82 | 0.946 |
| #5 | lignin-modifying enzymes | 73 | 0.949 |
| #6 | wood decomposition | 73 | 0.917 |
| #7 | old-growth forest | 57 | 0.958 |
| #8 | transcriptome signature | 24 | 0.993 |
| #9 | decay resistance | 21 | 1 |
| #10 | hydraulic conductivity recovery | 21 | 0.978 |
| #11 | material resistance | 17 | 0.999 |
| #12 | species-specific effect | 13 | 0.987 |
| #13 | new species | 13 | 1 |
| #14 | single species decay | 12 | 0.992 |
| #15 | inventory standardization | 12 | 1 |
Top 10 countries on the number of published papers.
| Country | Publications | Centrality | Burst |
|---|---|---|---|
| USA | 722 | 0.62 | |
| France | 193 | 0.16 | |
| Canada | 191 | 0.01 | 12.54 |
| Germany | 164 | 0.04 | |
| Finland | 163 | 0.01 | |
| Peoples R China | 159 | 0.05 | |
| Australia | 116 | 0.02 | |
| Sweden | 116 | 0.03 | 7.59 |
| Japan | 78 | 0.01 | |
| Spain | 72 | 0.01 |
The index information of the crucial author.
| Rank | Author | Co-cited Frequency | Burst | Half-Life |
|---|---|---|---|---|
| 1 | Harmon ME | 326 | 10.5 | |
| 2 | Tyree MT | 277 | 11.5 | |
| 3 | Sperry JS | 268 | 3.15 | 11.5 |
| 4 | Hacke UG | 229 | 11.5 | |
| 5 | Boddy L | 205 | 13.5 | |
| 6 | Cochard H | 204 | 12.5 | |
| 7 | Choat B | 181 | 10.5 | |
| 8 | Brodribb TJ | 158 | 12.5 |
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; Halik, Ümüt 1
; Wei, Zhicheng 1 ; Maierhaba Wumaier 1 1 College of Ecology and Environment, Xinjiang University, Urumqi 830046, China; Ministry of Education Key Laboratory of Oasis Ecology, Xinjiang University, Urumqi 830046, China