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Review

Forest Fragmentation and Forest Mortality—An In-Depth Systematic Review

by
Debebe Dana Feleha
1,2,
Luiza Tymińska-Czabańska
1,* and
Paweł Netzel
1
1
Department of Forest Resources Management, Faculty of Forestry, University of Agriculture in Krakow, Al.29 Listopada 46, 31-425 Krakow, Poland
2
Department of Natural Resource Management, College of Agriculture, Wolaita Sodo University, Wolaita Sodo P.O. Box 138, Ethiopia
*
Author to whom correspondence should be addressed.
Forests 2025, 16(4), 565; https://doi.org/10.3390/f16040565
Submission received: 4 February 2025 / Revised: 9 March 2025 / Accepted: 13 March 2025 / Published: 24 March 2025
(This article belongs to the Section Forest Ecology and Management)
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Abstract

:
In recent decades, forest fragmentation has been shown to directly increase forest mortality by increasing stress, damaging habitats, and heightening vulnerability to disturbances. It also disrupts local climates and ecological processes across various regions. Therefore, we aim to summarize the literature on forest fragmentation and forest mortality. The Web of Science Core Collection (WoSCC) database was searched using the PRISMA 2020 framework. We searched for publications from 1990 to 2023 and included research articles that reported on fragmentation and mortality. Out of the 159 articles found, we selected 119 research articles for systematic review. Our review documents that most studies on forest fragmentation and forest mortality tend to be relatively short-term, focused on a local or regional scale, and based on ground survey data. We identified articles from 35 countries and major hotspots for research on forest fragmentation and mortality. The results identified that the most underrepresented biomes are Mediterranean forests, woodlands and shrubs, boreal forests, and tropical and subtropical dry broadleaf forests. The longer the time horizon of the studies, the more neutral and positive effects of forest fragmentation are reported. These positive effects are more likely to be reported for temperate biomes and studies using field measurements. The study highlighted the importance of adopting a global perspective and integrating diverse methodologies to advance our understanding of forest fragmentation and mortality. Based on our findings, we recommend that future research on forest fragmentation and mortality should have a consistent geographic distribution, use varied methodologies, and perform the efficient integration of existing data types to improve the comparability and reliability of the results.

1. Introduction

Forests are an important part of the Earth’s system and enable the maintenance of the ecological balance and integrity of ecosystems [1]. They provide key ecosystem services to societies and contribute to cultural aspects of human well-being [2,3,4]. However, in the last few decades, forest ecosystems have been extremely exposed to climate change, anthropogenic pressure due to land-use change, and human disturbances [5]. As a result, many of the world’s forested areas are experiencing fragmentation, which is a direct and major cause of biodiversity loss [6], the degradation of ecosystems [7], and forest mortality [8].
Numerous studies have indicated that the currently observed accelerated rate of forest mortality is greatly triggered by large-scale forest fragmentation [8,9,10]. This can be explained by the fact that edge effects caused by forest fragmentation lead to sudden changes in temperature, relative humidity, and soil moisture, which can exceed the physiological tolerance of trees, leading to forest disturbance and mortality [11]. Forest edge areas are characterized by higher temperatures, higher light levels, and lower relative humidity [12,13,14]. This makes forest edges more vulnerable to drought stress than forest interiors [15]. In addition, fragmented forest edges are exposed to increased wind speed and turbulence, often resulting in increased wind throw [16] and damage to forest structure [17,18]. However, the results of studies on the effects of forest fragmentation on forest mortality are inconclusive and show strong spatial variation.
Studies report that increased fragmentation, particularly in tropical forests, leads to higher tree mortality and greater ecosystem degradation [19,20]. However, recent studies in temperate forests reported that newly formed edges do not show increased mortality following edge creation. This suggests that forest fragmentation, in some cases, reduces the overall forest mortality rate [21,22]. Simultaneously, a major increase in aboveground carbon stocks within 5 m of the edge of European temperate forests has been observed [23]. Moreover, climate change may have different impacts on the dynamics of forest fragmentation and forest mortality in specific regions. For instance, the decline in the forest fragmentation index in northern Eurasia from 2000 to 2020 [24] might be attributed to the expansion of forests induced by climate warming at high latitudes [25]. This expansion has led to the transformation of small patches into larger ones, thereby reducing forest fragmentation. On the contrary, the increased frequency of fires, linked to climate change, in regions such as Canada, far-east Russia, the Brazilian Amazon, tropical Africa, and coastal Australia has resulted in significant forest losses and heightened levels of forest fragmentation [26,27]. These different processes suggest that interactions between forest fragmentation and forest mortality are complex and may depend on different factors related to the geographical region and local conditions.
We have categorized the effects of forest fragmentation on forest mortality based on the direction of the effects and their interpretations, as follows:
  • Negative effects: this relationship indicates that, as the degree of fragmentation increases, so does forest mortality, leading to adverse ecological consequences.
  • Positive effects: this relationship indicates that, as the degree of fragmentation increases, forest mortality decreases.
  • Neutral effects: neutral effects of forest fragmentation on forest mortality refer to scenarios where changes in fragmentation levels do not result in a measurable increase or decrease in mortality.
Despite significant progress in forest fragmentation research, a comprehensive review or meta-analysis of forest fragmentation and forest mortality on a global scale is still lacking. This gap represents a critical limitation in our understanding of the broader impacts of forest fragmentation. Consequently, there is an absence of a consistent global framework that quantifies the relationship between forest fragmentation and forest mortality, as well as a lack of a synthesized understanding of how these processes vary across different regions. The existing approaches to assessing forest fragmentation and forest mortality rely on different metrics and definitions. Therefore, there is a need for a standardized framework to ensure consistency and comparability in assessing these processes. The lack of a comprehensive perspective hinders our ability to assess the true global impact of forest fragmentation on forest mortality. Furthermore, the lack of a holistic understanding limits our ability to develop effective conservation strategies and policies that can mitigate the negative impacts of fragmentation while enhancing the resilience of forest ecosystems.
A systematic review of the latest research on forest fragmentation and its relationship to forest mortality is urgently needed. Such an effort would not only fill the existing knowledge gap, but also provide a basis for understanding the role of forest fragmentation in shaping the future of forests.
Our aim is to synthesize the literature on forest fragmentation and forest mortality. We also aim to provide new insights into forest fragmentation and mortality from geographical, temporal, research approach, and bibliographical perspectives. We analyzed and evaluated relevant publications to answer the following questions:
  • o What is the geographical distribution of research and are there hotspots of research on forest fragmentation and forest mortality?
  • o What ecological, geographical, and temporal contexts have been used to study forest fragmentation and forest mortality?
  • o What data sources have been used in these studies?
  • o What approaches to the definitions of forest fragmentation and the determination of forest mortality have been adopted in these studies?
  • o What are the publishing trends over time related to the research topics?
  • o What effects of forest fragmentation were reported and how were they linked with forest mortality?
  • o How is international cooperation within this research area developing?
  • o What are the publication trends in this area?

2. Methods

2.1. Study Design

We surveyed and analyzed studies combining forest fragmentation and forest mortality using the PRISMA 2020 framework [28]. We conducted a comprehensive search of the Web of Science Core Collection (WoSCC) database to identify relevant studies by using the search expression (“forest fragmentation” AND “tree mortality” OR “forest mortality” OR “forest decline” OR “tree decline”) and then validated the results by comparing them in Scopus. The last search was performed on 11 January 2023 to avoid bias, we excluded all proceedings, review articles, book chapters, and editorials from the list of publications obtained from the dataset. We also limited the search to a specific publication timeframe, specifically from 1990 to 2023, to understand the full scope of forest fragmentation and its impacts on mortality, based on three decades of diverse and evolving research efforts. The records were then assessed independently by three reviewers (the authors). Potential studies were assessed for eligibility by reading the title, abstract, and text and removing any studies that did not directly address the issue of forest fragmentation and forest mortality (Figure 1).
After completing this preprocessing work, 29 papers were eliminated from the downloaded dataset. Ultimately, we selected a final set of 119 research papers for the systematic review. To facilitate the study of these selected papers, we used the Zotero reference manager [29].

2.2. Data Collection and Analyses

The data included in the review was collected by one reviewer and then checked and discussed by all three reviewers. We gathered information on the year of publication, geographical distribution, and scales of the study, the study period, the data type used, and the bibliometric data (Table 1). Information about the data collection process was also collected, depending on its availability in the individual studies. Additional information was extracted from the study area description, title, and abstract to identify specific study sites and countries for the analysis of the geographical distribution. The geographical names of the study areas in the analyzed articles were used to automatically assign geographical coordinates to them. We performed geo-coding in R studio software version 2024.12.1-563, published 10 June 2024 using the ggmap package. Using the study location, we searched for the biome of each study and categorized all studies into their biome classes, using a map of global biomes [30]. Additionally, we assigned a country to each specific study location. To illustrate the geographical distribution of the studies, we created a map of the specific study locations and a map of study countries using a mapping program [31]. We used the information on the length of the studies included in the methodology to assess the temporal variation of the research. Using this information, we categorized the studies. The first group consisted of studies with duration of less than 5 years, which were categorized as short-term studies. Medium-term studies, conducted over a period of 5–20 years, comprised the second group. Finally, long-term studies lasting more than 20 years were included in the third group. To identify the sources of data used in research, we extracted information on the data sources used in each study. Data sources were categorized as ground data, remote sensing data, or combined ground and remote sensing data. To assess the geographical scope of the research, we used a five-point scale, as follows: local, regional, national, continental, and global.
Various approaches have been used to assess and quantify forest fragmentation [32]. An overview of the types of approaches used to define and measure forest fragmentation is important, particularly in the context of effective conservation, land management, and forest management. Therefore, to extract information on the approaches used to determine forest fragmentation, we validated and analyzed our final set of articles in depth. Then, based on the metrics and data used, we proposed a four-group classification (Table 1). This contributes to clarifying the conceptual boundaries of fragmentation, avoiding the misinterpretation of the concept, and identifying common themes and variations across studies.
The geometry-based approach is a highly effective method for quantifying forest fragmentation because it uses patch geometry metrics such as patch density, total edge distance, perimeter-to-area ratio, and shape indices [33,34,35,36,37]. It contributes standardized and scalable metrics, which support the modeling and simulation of fragmentation scenarios. However, the ecology-based approach is a critical method for understanding forest fragmentation because it focuses on the ecological consequences of habitat loss and the spatial distribution of habitat elements, thereby providing a deeper understanding of how fragmentation affects ecosystems [11,38,39,40,41]. It emphasizes species-specific responses to fragmentation and highlights the importance of functional connectivity, which is essential for maintaining ecological processes. The remote sensing approach uses satellite and aerial imagery, along with advanced remote sensing techniques, to analyze vegetation indices and assess forest fragmentation [42,43,44]. This approach is justified by its ability to provide comprehensive, large-scale, and cost-effective monitoring of forest ecosystems. By enabling the mapping of fragmentation patterns at regional or global scales, the approach provides a unique perspective that is difficult to achieve with ground-based surveys alone. In addition, it provides temporal data to track changes in forest fragmentation over time, which is critical for understanding the dynamics of fragmentation. The other mixed approach, which integrates remote sensing, geometric, and ecological approaches, is justified by its ability to provide a holistic and multi-dimensional understanding of forest fragmentation. Each component of this approach contributes unique strengths, ensuring a comprehensive analysis of the complex processes driving fragmentation and its ecological consequences [12,34].
Similarly, in the case of forest mortality, authors identify the occurrence of mortality using different metrics and datasets. We have proposed two general categories of approaches to determining forest mortality and have assigned an approach type to each study in our database (Table 2).
The quantitative approach is grounded in the systematic collection and analysis of numerical data, utilizing statistical methods to objectively measure forest mortality. Employing metrics such as mortality rates, tree density, and biomass loss ensures precision and reproducibility when assessing forest health. Its reliance on empirical data allows for the identification of patterns and trends. Its ability to process large datasets enables the development of predictive models that are invaluable for forecasting future mortality under different scenarios, contributing significantly to evidence-based decision making and enabling policy makers and forest managers to design targeted interventions and mitigation strategies. On the other hand, the qualitative approach focuses on descriptive and observational methods to assess forest mortality, providing a deeper, context-specific understanding of the underlying causes and processes. This contributes detailed information that quantitative methods overlook, by providing rich, contextual insights that enhance the interpretation of quantitative data, ensuring that management strategies are not only data-driven, but also environmentally and socially informed.
There are various effects of forest fragmentation on forest mortality [20,22]. We grouped the articles into three categories based on their overall assessment of the fragmentation process: positive, negative, and neutral effect. We evaluated these effects based on researchers’ findings regarding the effect of forest fragmentation on forest mortality (Table 3). A negative effect of fragmentation is identified when an increase in fragmentation leads to an increase in mortality. Conversely, a positive effect of forest fragmentation on forest mortality is observed when an increase in fragmentation leads to a decrease in mortality. A neutral effect of fragmentation refers to a scenario where there is no relationship between fragmentation and mortality, meaning that fragmentation neither increases nor decreases mortality.
To gain new insights into forest mortality and fragmentation from the selected papers we analyzed bibliometric data. Online system VOSviewer (10.1007/s11192-009-0146-3), a tool that is freely available and commonly used for the visualization of mapping [45] was used for bibliometric analysis. To analyze the primary clusters of research topics associated with forest fragmentation and forest mortality, we conducted a though examination of titles and abstracts using keyword analysis. To achieve this, we utilized full counting options for key word analysis, applying a minimum threshold of at least 5 occurrences for a keyword to be considered significant. This ensured that we focused on the most prevalent and prominent research topics within the dataset.
The strength of international research collaboration was assessed using the Total Link Strength (TLS), a metric assigned by VOSviewer during the mapping of research activities among selected countries. The TLS value is directly proportional to the level of international research collaboration, with a higher value indicating a greater degree of collaboration. In our analyses, we established a minimum threshold of 3 documents for a country to be considered. Using the TLS in the VOSviewer program we examined cooperation between countries involved in research on forest fragmentation and forest mortality. Countries were automatically assigned to one of the color-coded clusters based on the strength of collaboration as determined by the TLS measure. Countries with zero TLS were excluded.
To identify publishing trends and assess the contributions of journals, we employed bibliographic coupling to create a map of published sources. In our analysis, we considered a minimum threshold of 3 document sources to ensure an adequate sample size for meaningful insights. These parameters enabled us to effectively identify patterns of collaboration and highlight the contributions of specific journals in the research landscape.

3. Results

From the identified articles in the WoSCC, 119 articles exploring forest fragmentation and mortality were selected for systematic review (Figure 1). Over the past decades, we have identified a general increasing trend in the number of studies in the area of forest fragmentation and forest mortality (Figure 2b). The overall number of studies per year stayed relatively low until the early 2010s. Since then, the number of studies per year has increased rapidly (Figure 2b). The importance of the increasing trend in research on forest fragmentation and mortality highlights its relevance for addressing global environmental challenges and informing conservation strategies. It implies the need for continued and expanded research efforts to understand better and mitigate the impacts of these critical ecological problems.
The most popular journals in which the research was published were Forest Ecology and Management (23 papers), Ecological Applications (6 papers), Ecology (6 papers), Global Change Biology (5 papers), and Journal of Ecology (5 papers) (Figure 3). In recent years, there has been an increase in the number of studies on fragmentation and mortality in journals publishing in the field of remote sensing, e.g., Remote Sensing of Environment, and those closely related to the modeling of ecological processes, e.g., Ecological Modeling or Carbon Management. This emphasizes the importance of high-impact journals in advancing research on forest fragmentation and mortality, while also highlighting the growing role of remote sensing and modeling in this field.
We identified articles from 35 countries (Figure 4). Brazil, the United States, Canada, and Australia account for more than 60% of the studies. However, other countries also stand out in terms of the number of publications, such as China, Mexico, Chile, and Finland. Surprisingly, we found relatively few studies in European countries, even though European forests are characterized by relatively high levels of forest fragmentation. We found one study for Russia, Poland, and Spain. Similarly, we identified three papers for Africa, namely South Africa, Madagascar, and Ethiopia. The blank spots on the map—where few or no studies were identified—may indicate gaps in our knowledge of region-specific drivers of mortality due to forest fragmentation. The uneven allocation of research efforts and the focus on underrepresented areas are crucial subjects in the review. There is a pressing necessity to enhance global collaboration and financial support for research in these underrepresented regions.
The Total Link Strength (TLS) revealed that the strongest cooperation is between the USA and Brazil (TLS = 17), the USA and Australia (TLS = 10), Australia and Brazil (TLS = 10), and Brazil and the UK (TLS = 8). The colors of the clusters related to international collaboration help to visually distinguish and identify the different groups of co-authorship in the analyzed dataset. A higher level of co-authorship between the countries in the network studied can be seen in the individual clusters (Figure 5). For example, a specific group (the yellow cluster) was distinguished for the collaboration between authors from European countries (Spain, France, Portugal, Switzerland, and Belgium) and African countries (Kenya, Ethiopia, and Nigeria).
We identified also two major hotspots for research on forest fragmentation and mortality, including the tropical forests of the Amazon and the temperate forests of North America (Figure 6).
The imbalance in the geographical distribution of studies has implications for the representation of biomes (Figure 7). Based on the coordinates of the study locations, we found that studies in the tropical and subtropical moist deciduous forest biomes accounted for almost half of all locations. In the temperate forest zone, the temperate deciduous and mixed forest biomes and the temperate coniferous forest biome are fairly well represented. The most underrepresented biomes are Mediterranean forest, woodlands and shrubs, boreal forest, and tropical and subtropical dry broadleaf forests. This imbalance indicates that current research may not adequately capture the ecological diversity and dynamics of these underrepresented biomes. Addressing this imbalance is critical to ensuring comprehensive and equitable representation of global biomes in scientific studies.
We found that most studies used direct field observations at local and regional scales (Figure 2b), which are usually accurate but can be limited in spatial scale. Currently, remote sensing is highly applicable for detecting broad patterns of forest fragmentation change and mortality; this method is already being used at all spatial scales. This emphasize the need to evaluate methodological approaches in studies, noting that while direct field observations provide accurate localized data, they are limited in spatial scope. In contrast, remote sensing effectively detects large-scale patterns of forest fragmentation and mortality across multiple spatial scales. This suggests that integration of both methods is essential for a comprehensive understanding of large-scale ecological change. This integration could improve the effectiveness of monitoring and conservation efforts.
On the basis of the articles analyzed, we noticed that short and medium-term surveys were the most common, while surveys covering a period of more than 25 years were the least common (Figure 8a).
During the review analysis, out of 119 articles, 107 research articles reported a negative effect of forest fragmentation, indicating that an increase in forest fragmentation leads to an increase in forest mortality. Eleven articles reported positive effects of forest fragmentation, which are often rare, localized, and context-specific scenarios where increased fragmentation leads to a reduction in forest mortality. One research article reported a neutral effect of forest fragmentation. In this case the changes in fragmentation do not lead to measurable increases or decreases in mortality, highlighting the lack of relationship between fragmentation and mortality (Figure 8b). To compare the five categories of data, a ribbon chart was used to show the relative values in each category and the flow of data (Figure 9). We found that, despite the fact that the majority of studies on forest fragmentation have assessed this process negatively (Figure 8b and Figure 9), this assessment can be influenced by both the type of data used and the time horizon of the study. Almost all short-term studies indicate a negative effect of fragmentation, while neutral and positive effects become more frequent as we extend the scale of the studies (Figure 9). We also found that studies based on ground measurement data are more likely to report a negative effect, while combined and remote sensing data are almost exclusively positive.
In the studies analyzed, we found different approaches used to determine forest fragmentation. We classified these approaches into four different categories, namely the geometry-based approach (GBA), the ecology-based approach (EBA), the mixed approach (MIX) and the remote sensing-based approach (RBA) (Table 1). Approaches to determine forest mortality fall into two categories: quantitative and qualitative (Table 2). The most commonly used approach to determine forest fragmentation in studies was GBA, followed by EBA (Table 4). The use of MIX was the third most common method, while RBA was the least used. In terms of approaches to determine forest mortality, the most commonly used approach was quantitative (Table 4).
During the systematic review, we found that, out of 119 articles, 107 research articles reported a negative effect of forest fragmentation, thereby indicating that an increase in forest fragmentation leads to an increase in forest mortality. Eleven articles reported positive effects of forest fragmentation, comprising often rare, localized, and context-specific scenarios where increased fragmentation led to a reduction in forest mortality. One research article reported a neutral effect of forest fragmentation. In this case, the changes in fragmentation did not lead to measurable increases or decreases in mortality, highlighting the lack of relationship between fragmentation and mortality (Figure 8b). To compare the five categories of data, a ribbon chart was used to show the relative values in each category and the flow of data (Figure 9). We found that, despite the fact that the majority of studies on forest fragmentation have assessed this process negatively (Figure 8b and Figure 9), this assessment can be influenced by both the type of data used and the time horizon of the study. Almost all short-term studies indicate a negative effect of fragmentation, while neutral and positive effects become more frequent as we extend the scale of the studies (Figure 9). We also found that studies based on ground measurement data are more likely to report a negative effect, while combined and remote sensing data are almost exclusively positive.
In the studies analyzed, we found different approaches used to determine forest fragmentation. We classified these approaches into four different categories, namely the geometry-based approach (GBA), the ecology-based approach (EBA), the mixed approach (MIX) and the remote sensing-based approach (RBA) (Table 1). Approaches to determine forest mortality fall into two categories: quantitative and qualitative (Table 2). The most commonly used approach to determine forest fragmentation in studies was GBA, followed by EBA (Table 4). The use of MIX was the third most common method, while RBA was the least used. In terms of approaches to determine forest mortality, the most commonly used approach was quantitative (Table 4).
Using keyword frequency over time, we assess trends in the main topics of research. Using bibliographic data, we analyzed the occurrence of individual keywords using an index of the average year of publication (Figure 10). We found that research on forest fragmentation at the beginning of the 21st century focused on tropical forest dieback (keyword mortality), habitat fragmentation (keyword habitat fragmentation), edge effects (keyword edge effects), and forest structure (keyword vegetation structure) and clear-cutting (Figure 10, purple color). In later years, research on fragmentation began to focus on tree mortality (keyword tree mortality), but in the context of disturbances (keyword disturbance), patterns of this process (keyword patterns), and their dynamics (keyword dynamics), but still focused on tropical Amazon forests (keyword amazon and tropicalforest) (Figure 10, sea green color). However, recent research has studied forest mortality due to fragmentation in the context of climate change (keyword climate change), particularly drought and fire (keyword drought, fire), land-use change (land-use), and deforestation (keyword deforestation) (Figure 10, yellow color). What is important is that research is carried out in the fields of conservation (keyword conservation), biodiversity (keyword biodiversity), and carbon storage (keyword carbon stocks). For the first time, studies concern boreal forests (keyword boreal forest), which means that this problem is becoming important for forests other than tropical forests. Analysis of keyword frequency over time is essential to identify evolving trends and priorities in forest fragmentation research.

4. Discussion

We found that research on forest fragmentation and forest mortality has increased significantly in recent years, confirming the growing importance of this issue to the scientific community. We documented that most studies tended to be relatively short-term, focused on a local or regional scale, and based on ground survey data. Promisingly, research using remote sensing data is becoming increasingly important, enabling analysis at larger scales. Importantly, our work documents that research on forest fragmentation and mortality is mainly concentrated in North and South America, indicating a major imbalance in and underrepresentation of research in regions other than the tropical rainforests of the Amazon and the temperate forests of North America. Another interesting aspect is the evolution of research topics over time regarding forest fragmentation and forest mortality. While, at the turn of the 20th and 21st centuries, the main concern was the overexploitation of the Amazon rainforest in the context of forest fragmentation and mortality, research in this area is now increasingly being approached from the perspective of the effects of climate change.
The temporal distribution of studies reflects the development of research on forest fragmentation and forest mortality and the importance of this issue. The significant increase in the number of surveys after 2010 can be attributed to several factors. Firstly, the spread of new technologies, particularly satellite and LIDAR, has opened up the possibility of greater access to data that was often difficult to access or not economically feasible. Secondly, the development of research methods, including machine learning and artificial intelligence, makes it possible to apply groups of research methods such as Random Forest [46], Support Vector Machines [47], Classification and Regression Trees or Artificial Neural Networks (ANNs) to analysis [48]. Finally, the growing problem of forest mortality caused by the fragmentation of forest ecosystems as a result of climate change [49] and anthropressure is one of the greatest challenges facing the scientific community, making this issue topical and being studied in various contexts [6,7].
To the best of our knowledge, a comprehensive review of the global distribution of studies on forest fragmentation and mortality has not yet been undertaken, despite the important role of geography in shaping ecological theories [5,50]. Evaluating the geographical distribution of research is key to our understanding of the complex process being analyzed. It allows the identification of patterns, trends and commonalities across regions, providing a broader view of the phenomenon under study. Therefore, analysis of geographical distribution helps to identify hotspots or regions with high levels of forest fragmentation and mortality. A well-distributed literature also improves our understanding of global trends in the fragmentation-mortality process and our ability to identify areas at risk in the future. This information is crucial for prioritizing research efforts and targeting resource allocation and partnerships.
Our detailed review of the literature shows that the countries with the highest number of studies are Brazil, the USA, Canada, and Australia (Figure 3). We have also documented the most extensive cooperation between these four countries (Figure 4). Several factors contribute to the difference in study representation around the world. In general, there is an over-representation in countries with a higher level of gross national income, while countries with a lower level of gross national income tend to be under-represented. This has also been observed in other fields, including studies of land use change [5] and conservation [51]. In ecology studies, for instance, 90% of the survey sites were located in the wealthiest 30% of countries [50].Our research is partly in line with this theory. The large number of studies conducted in South American countries is related to the scientific interest in the Amazon rainforest as one of the largest forest areas in the world. Due to a number of factors, including higher deforestation rates, land-use change, and population growth, forest fragmentation was high in tropical regions [52]. In the 20th century there has been a rapid increase in the area of tropical forests that have experienced fragmentation [19,20]. Thus, forest fragmentation has been identified as one of the major threats to tropical forests and associated biodiversity loss [53,54]. This has made the assessment of forest fragmentation an important issue in modern ecology of tropical forests [55] and may be the direct reason for the high number of studies in this location. The importance of this issue is confirmed by both our analysis of the geographical distribution of studies (Figure 3) and hotspots (Figure 6) and our results of keyword analysis (Figure 10).
However, currently, regions with temperate climates have also faced disturbances [56] that cause enhanced forest fragmentation and forest mortality. As our data show, this issue has been analyzed quite frequently in the temperate forest zone of North America. In this region, extreme events such as fire, drought and wind are particularly important, which is reflected in the research topics [51,57,58]. But only a few studies exist for the temperate forest zone of Europe (Figure 6 and Figure 8). Temperate forests are almost 1.5 times more fragmented than tropical forests [51]. However, comparing patterns of mortality due to fragmentation between biomes is inaccurate due to differences in land use history, particularly the time since the edge was created [51]. Forests in the European temperate forest biome have experienced centuries of deforestation, forest alteration and fragmentation. Many forest edges have existed for decades. Studies of newly created edges in this region have shown a large increase in the number of trees remaining in the immediate aftermath of edge creation, with no associated increase in mortality [21]. Individual tree characteristics, such as height, drought tolerance and rooting depth, may explain why fragmentation cause mortality in some forests but not others, as sudden edge formation may expose previously intact forests to disturbances. This complex set of factors, especially the history of land use, may explain the low mortality rate due to fragmentation and consequently the low number of surveys in European forests.
A lack of research was identified in studies of specific biomes (Figure 9), highlighting a critical research gap that should be addressed in the future. This finding underscores the importance of focusing on these biomes to better understand the interaction between fragmentation and mortality.
In particular, the forests of the Boreal biome show a significant underestimation, especially concerning the occupied forest area in this biome. Although the boreal forest is a relatively homogeneous complex with relatively low fragmentation, the forests of this zone are currently highly exposed to disturbances related to climate change [59]. This problem has already been partially identified and reflected in the keyword analysis (Figure 10). Further development of research in this area is crucial for boreal forests in the context of the mitigation capacity of forests and the sustainability of these ecosystems.
Our review analysis revealed that most of forest fragmentation and mortality studies were conducted by using field data (Figure 10). Field data provide accurate information. However, ground measurements are costly and time-consuming. Therefore, they are most often spatially limited or restricted to measurements on sample plots. Now, technological developments and the increasing availability of remote sensing data, including high resolution multispectral images and ALS data have opened up new opportunities for exploring natural resources and the environment [60]. Nevertheless, remote sensing has some limitations due to its spectral resolution, the influence of atmospheric conditions and the difficulty of validation [43,58]. The problem of limited data availability is largely overcome by using the vast amount of information derived from satellite data to inventory forests and study their dynamics. The ability to continuously monitor forests over large areas also opens up the possibility to verify existing knowledge on patterns of forest fragmentation and forest mortality, as evidenced by an increasing number of studies using this type of data (Figure 2a). Furthermore, the combination of remote sensing data and field data can provide comprehensive assessments of forest fragmentation and forest mortality [61,62]. Integration of these datasets may be promising for analyses of complex patterns of forest fragmentation and forest mortality patterns [57,63,64,65].
The source of the data largely determines the type of approach used to determine forest fragmentation, which is crucial for developing the research methodology and identifying potential benefits and limitations. For instance, GBA method is important to quantify the physical structure of fragmented forest and identify different types of pattern of fragmentation. The GBA method is easy to interpret and allows comparison of forest fragmentation across landscapes through the use of consistent geometric metrics [36]. However, it ignores fragment quality and ecological processes such as edge effects and microhabitat heterogeneity [44]. This approach is biased by assumptions about the scale of analysis and pixel size [50] and shape complexity [41]. In turn, EBA method assesses the transition between core forests, edge forest and patches to understand changes in forest structure, biomass and species composition [44]. It helps to understand the consequence of forest fragmentation on biodiversity, species vulnerability and overall ecosystem resilience. However, it can be complex and require significant data on ecological processes. The EBA is sometimes biased due to assumptions of a critical fragmentation threshold [66], variation in defining to forest fragmentation [24], lack of experimental manipulation [67], and influence of edge, core area and shape [44].
The rarest, but promising in our view, is the RBA approach used to measure forest fragmentation. RBA approach allows the analysis of forest fragmentation on a large spatial scale [44], enabling the generation of an objective and quantitative measure of forest fragmentation [41,44]. However, challenges related to data resolution and scale, compatibility of metrics need to be addressed [68]. Researcher needs also to narrow the gap between the integration of remote sensing data with ecological field studies [41] and advances in higher spatial resolution, such as using multisensor data fusion techniques and exploring the potential of machine learning algorithms for automated analysis of remote sensing data will increase the efficiency of assessing forest fragmentation and improve the accuracy with which subtle changes in landscape patterns are detected [68]. This gap is already partly covered by the MIX approach, which involves integrating quantitative data with data on ecological processes [69]. Such an approach can be very useful for decision-making regarding conservation and biodiversity protection strategies [12,34], providing a more holistic understanding of forest fragmentation and mortality [44]. However, it can be challenging due to the compatibility of ecological data, and geometric metrics, especially when dealing with diverse ecological communities [49,70,71,72]. Therefore, improving the integration of ecological and geometric measures while standardizing and comparing results across studies is needed [73,74]. This can be achieved by developing protocols for combining disparate data sets and methods, exploring advanced spatial analysis techniques, and promoting interdisciplinary collaboration between ecologists, landscape architects, and spatial analysts [73,74].
Our findings suggest that current knowledge is mainly based on medium- and short-term studies that point to the negative effects of fragmentation on mortality (Figure 8a and Figure 9). However, we have shown that, the longer the time horizon of the research, the more neutral and positive effects are reported (Figure 9). This means that the time scale of the studies may influence the assessment of this process. Interestingly, positive effects were more frequently reported for temperate biomes’ forest mortality [75]. Researchers also reported positive effects of fragmentation on ecological processes, including soil nutrient enrichment, without an increase in forest mortality. This pattern may be related to specific conditions at the temperate forest edges, namely the edge effect caused by fragmentation, and suggests that the edges of temperate forests are less threatened by wind and less sensitive to increased temperatures and water stress. Furthermore, increased radiation can release the most limiting biogeochemical constraints of temperate forests, namely temperature and light [76], and improve soil properties [75]. Increased productivity is therefore linked to increased light availability, which affects canopy architecture and can increase forest leaf area index, stimulating productivity [77]. It can also increase tree succession [78], soil carbon stocks [79,80], ponderosa pine regeneration [81], and species abundance and richness [82], as well as growth and biomass along forest edges [22].
However, it is important to note that the impacts of climate change on forest ecosystems are different at different geographical scales. On the one hand, climate change can lead to fragmentation, which can cause disturbance and mortality in many areas. On the other hand, climate change may also cause previously fragmented forest areas in high latitudes to be covered by forest, thereby reducing the degree of fragmentation [24,25]. To study and compare changes in fragmentation patterns at different latitudes, it is therefore particularly important that future research on forest fragmentation be conducted with a consistent geographical distribution and methodology (Figure 11).
Forest fragmentation and tree mortality pose serious threats to ecosystems and climate stability. Addressing these issues requires a multifaceted approach that integrates ecological restoration, sustainable land use practices, and community engagement. By addressing root causes and implementing targeted strategies, it is possible to mitigate the negative effects of fragmentation, restore degraded forests, and increase ecosystem resilience for future generations.
Although most of the articles analyzed focus on forest fragmentation as the main driver of forest mortality, we are aware that this process is not unidirectional. The most common pattern in the research is fragmentation’s effect on mortality. However, mortality can also affect fragmentation. This leads to a reverse direction of influence. In our study, we did not attempt to assess the effect of mortality on fragmentation. In the context of climate change, it may be of great interest and importance to investigate such direction of these processes. We would also like to emphasize the importance of avoiding a tunnel vision of forest fragmentation as an unambiguously negative process (Figure 12). In addition to the negative effects, we should also be aware of the whole range of processes resulting from forest fragmentation and its positive aspects.

5. Conclusions

We documented that most studies of forest fragmentation and forest mortality have tended to be relatively short-term, focused on a local or regional scale, and based on ground survey data. However, research using remote sensing data is becoming increasingly important, allowing analysis at larger scales. Importantly, our work documents the hotspots of forest fragmentation and mortality research, which are concentrated in North and South America. This suggests a major imbalance and under-representation of research in regions other than the tropical rainforests of the Amazon and the temperate forests of North America. We have shown that the longer the time horizons of the research, the more neutral and positive effects are reported for mortality.
Our findings have led to recommendations and research priorities. Future research on forest fragmentation and mortality should have consistent geographical distribution and methodology. This will ensure a more comparable distribution of research, including in countries with limited research budgets. There is also a great need to efficiently combine existing data types to improve the accuracy of analyses, and to explore methods to harmonize remote sensing and ground data. We would also like to stress the importance of avoiding a tunnel vision of forest fragmentation as a clearly negative process, alongside the negative impacts, it is important to recognize the full range of processes that result from forest fragmentation, as well as its positive features.

Author Contributions

Conceptualization; D.D.F., L.T.-C. and P.N.; methodology; D.D.F., L.T.-C. and P.N.; validation, D.D.F., L.T.-C. and P.N.; formal analysis, D.D.F. and L.T.-C.; investigation, D.D.F. and L.T.-C.; data curtain, D.D.F.; writing—original draft preparation, D.D.F. and L.T.-C.; writing—review and editing, D.D.F. and L.T.-C.; visualization, D.D.F., L.T.-C. and P.N.; supervision, L.T.-C. and P.N.; project administration, L.T.-C. and P.N. All authors have read and agreed to the published version of the manuscript.

Funding

This research was carried out under the project “Assessment of the impact of weather conditions on forest health status and forest disturbances at regional and national scale based on the integration of ground and space-based remote sensing datasets” No 2021/41/B/ST10/04113 funded by the National Science Centre, Poland.

Data Availability Statement

Data will be made available on request. For further inquiries, please contact the corresponding author directly.

Acknowledgments

We thank the editor and the three anonymous reviewers for their constructive comments, which helped us to improve the manuscript.

Conflicts of Interest

The authors assert that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this review paper.

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Figure 1. Flow chart of the PRISMA article search process.
Figure 1. Flow chart of the PRISMA article search process.
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Figure 2. Variation in the number of studies on forest fragmentation and forest mortality over time. The green color indicates the beginning of the period when remote sensing technologies and the use of artificial intelligence (AI) in research became popular (a) and types of data used to study forest fragmentation and forest mortality at different scales. The colors represent the types of data source (b).
Figure 2. Variation in the number of studies on forest fragmentation and forest mortality over time. The green color indicates the beginning of the period when remote sensing technologies and the use of artificial intelligence (AI) in research became popular (a) and types of data used to study forest fragmentation and forest mortality at different scales. The colors represent the types of data source (b).
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Figure 3. Journals in which research has been published and publication trends over time based on bibliographic data. The colors indicate the average year of publication, and the size of the circles indicates the number of articles. Lines between journals indicate co-citations.
Figure 3. Journals in which research has been published and publication trends over time based on bibliographic data. The colors indicate the average year of publication, and the size of the circles indicates the number of articles. Lines between journals indicate co-citations.
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Figure 4. Geographical distribution of studies on forest fragmentation and forest mortality. Colors indicate the number of studies per country.
Figure 4. Geographical distribution of studies on forest fragmentation and forest mortality. Colors indicate the number of studies per country.
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Figure 5. Co-operation between countries, based on co-authorship data. The color represents different clusters of co-authoring countries; the thickness of the lines indicates the strength of cooperation.
Figure 5. Co-operation between countries, based on co-authorship data. The color represents different clusters of co-authoring countries; the thickness of the lines indicates the strength of cooperation.
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Figure 6. Density of studies on forest fragmentation and forest mortality worldwide. Hotspots, representing the highest concentrations of studies, are highlighted in red.
Figure 6. Density of studies on forest fragmentation and forest mortality worldwide. Hotspots, representing the highest concentrations of studies, are highlighted in red.
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Figure 7. Distribution of forest fragmentation and forest mortality studies along the different biomes of the world. The colors represent different biomes.
Figure 7. Distribution of forest fragmentation and forest mortality studies along the different biomes of the world. The colors represent different biomes.
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Figure 8. Number of publications presented by research time scale (a) and by forest fragmentation impact assessment (b).
Figure 8. Number of publications presented by research time scale (a) and by forest fragmentation impact assessment (b).
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Figure 9. The flow of data between the five categories of data. Each sub-category is color-coded.
Figure 9. The flow of data between the five categories of data. Each sub-category is color-coded.
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Figure 10. Temporal trends in the main topics of the research, based on bibliographic data. Colors indicate clusters distinguished by average citation year and keyword frequency. Lines represent co-occurrences between terms.
Figure 10. Temporal trends in the main topics of the research, based on bibliographic data. Colors indicate clusters distinguished by average citation year and keyword frequency. Lines represent co-occurrences between terms.
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Figure 11. Recommendations for research priorities for the development of a more comprehensive and balanced understanding of the process of forest fragmentation and mortality.
Figure 11. Recommendations for research priorities for the development of a more comprehensive and balanced understanding of the process of forest fragmentation and mortality.
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Figure 12. A tunnel view of the process of forest fragmentation and forest mortality.
Figure 12. A tunnel view of the process of forest fragmentation and forest mortality.
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Table 1. Proposed classification of approaches to determining forest fragmentation based on metrics and data used in analyses. Note: NDVI (normalized difference vegetation index), NDWI (normalized difference water index), LAI (leaf area index), EVI (enhanced vegetation index), LPI (land property identifier), LCI (life cycle inventory).
Table 1. Proposed classification of approaches to determining forest fragmentation based on metrics and data used in analyses. Note: NDVI (normalized difference vegetation index), NDWI (normalized difference water index), LAI (leaf area index), EVI (enhanced vegetation index), LPI (land property identifier), LCI (life cycle inventory).
Features Types of Approach
Geometry
Based		Remote Sensing
Based		Ecology
Based		Mixed
Metrics Used
  • Patch density
  • NDVI
  • LPI
  • The fusion of metrics used in the three previous approaches
  • Largest patch index
  • NDWI
  • LCI
  • Edge density
  • LAI
  • LC contrast index
  • Landscape shape index
  • EVI
  • Radius of gyration
  • Land cover classification
  • Cohesion
Data Used
  • Ground survey and forest inventories
  • Satellite imaging
  • Ground survey & forest inventories
  • Ground survey and forest inventories
  • Remote sensing
  • data
  • Aerial photography
  • Remote sensing data
  • Remote sensing data
Table 2. Classification of the approaches used to define forest mortality based on metrics and data used in analyses.
Table 2. Classification of the approaches used to define forest mortality based on metrics and data used in analyses.
FeaturesTypes of Approach
QuantitativeQualitative
Metrics Used
  • Number of dead trees/stands
  • Forest cover density
  • The volume of deadwood/biomass
  • Forest cover change
  • Vegetation indices
  • Degree of crown browning
  • Percentage of crown defoliation
  • Stage of crown defoliation
Data Used
  • Ground survey and inventories
  • Satellite imaging
  • Remote sensing data
  • Aerial photography
Table 3. Summary of the variables extracted from the analyzed papers.
Table 3. Summary of the variables extracted from the analyzed papers.
DataOutcome
Metrics and data used in forest fragmentation studiesClassification of approaches to measuring forest fragmentation
Metrics and data used in forest mortality studiesClassification of approaches to measuring forest mortality
Location of study area
  • Scale of study (local, regional, country, continental)
  • Specific location
  • Country of study
  • Type of biome according to [31]
Information on data types usedGroups of data used (field data, combined data, and remote sensing data)
Information on effect of forest fragmentationPositive, negative, and neutral
Publication Years, duration of study
  • Temporal variation in number of studies
  • Time scale of the study (short-term, medium-term, long-term)
Bibliometric data
  • Keyword analysis
  • International collaboration analysis
  • Publishing trends analysis
Table 4. Types of approaches used in the analyzed studies for the determination of forest fragmentation and forest mortality.
Table 4. Types of approaches used in the analyzed studies for the determination of forest fragmentation and forest mortality.
Type of Approaches Used to Determine Forest FragmentationTotal(%)Type of Approach Used to Determine Forest MortalityTotal(%)
Geometry based (GBA)4033.6Quantitative10789.9
Ecology based (EBA)3226.9Qualitative1210.1
Remote sensing based (RBA)2218.5
Mixed (MIX)2521
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Feleha, D.D.; Tymińska-Czabańska, L.; Netzel, P. Forest Fragmentation and Forest Mortality—An In-Depth Systematic Review. Forests 2025, 16, 565. https://doi.org/10.3390/f16040565

AMA Style

Feleha DD, Tymińska-Czabańska L, Netzel P. Forest Fragmentation and Forest Mortality—An In-Depth Systematic Review. Forests. 2025; 16(4):565. https://doi.org/10.3390/f16040565

Chicago/Turabian Style

Feleha, Debebe Dana, Luiza Tymińska-Czabańska, and Paweł Netzel. 2025. "Forest Fragmentation and Forest Mortality—An In-Depth Systematic Review" Forests 16, no. 4: 565. https://doi.org/10.3390/f16040565

APA Style

Feleha, D. D., Tymińska-Czabańska, L., & Netzel, P. (2025). Forest Fragmentation and Forest Mortality—An In-Depth Systematic Review. Forests, 16(4), 565. https://doi.org/10.3390/f16040565

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