Synthesis and Perspectives on Disturbance Interactions, and Forest Fire Risk and Fire Severity in Central Europe

Abstract

Wildfire risk increases following non-fire disturbance events, but this relationship is not always linear or cumulative, and previous studies are not consistent in differentiating between disturbance loops versus cascades. Previous research on disturbance interactions and their influence on forest fires has primarily focused on fire-prone regions, such as North America, Australia, and Southern Europe. In contrast, less is known about these dynamics in Central Europe, where wildfire risk and hazard are increasing. In recent years, forest disturbances, particularly windthrow, insect outbreaks, and drought, have become more frequent in Central Europe. At the same time, climate change is influencing fire weather conditions that further intensify forest fire dynamics. Here, we synthesize findings from the recent literature on disturbance interactions in Central Europe with the aim to identify disturbance-driven processes that influence the regional fire regime. We propose a conceptual framework of interacting disturbances that can be used in wildfire risk assessments and beyond. In addition, we identify knowledge gaps and make suggestions for future research regarding disturbance interactions and their implications for wildfire activity. Our findings indicate that fire risk in the temperate forests of Central Europe is increasing and that non-fire disturbances and their interactions modify fuel properties that subsequently influence wildfire dynamics in multiple ways.

1. Introduction

1.1. Global Perspective and Rationale

Wildfire is a common disturbance in many biomes, playing a crucial role in various ecosystem processes, such as vegetation dynamics, forest composition, and forest extent [1,2,3]. The frequency and intensity of wildfires are expected to increase with climate change [4], and there is current evidence for this trend across continents [5]. The years 2020, 2021, and 2023 were identified as the fourth, third, and first worst years in global forest fire history [6], and 2022 was the second-worst year in the European Union in terms of the area burned by wildfires [7].

Wildfires are posing threats to human life, settlements, and infrastructure; yet, many natural ecosystems and biomes are adapted to them or can even benefit from infrequent fire events [8]. Over the last years, forest resilience to wildfires declined due to extreme hot droughts [9], while post-fire regeneration is of growing concern [10]. Changes in fire regimes, such as a longer fire season [11], as well as greater in extent, more frequent, or more severe fire events [12], are becoming more prominent [13]. Modified fire regimes are likely to threaten biodiversity and ecosystem functioning [14] and pose a challenge in forest management, as they require advanced adaptive strategies for increased forest resilience [15]. Climate change directly influences forest fire properties by fuelling larger and more severe fires, and it is expected that this will lead to an increase in fire likelihood and severity at a global scale [16,17].

While the general trend of an increase in wildfires is global, Bobek et al. [18] found that, in our study region of Central Europe, there are region-specific patterns linked to the distribution of broadleaved forests. A finding confirming earlier work conducted by Adámek et al. [19] attributed the spread of forest fires to Pinus sylvestris forests. Similarly to the changing weather patterns in many parts of the world [13,20], the projected anomalies in precipitation and temperature patterns in Central Europe are expected to contribute to an increase in fire frequency [21]. However, there are gaps in our knowledge of how pre-fire biotic and abiotic disturbances affect regional forest fire dynamics in Central Europe, as there is missing evidence and only limited understanding on the synergistic effects of individual disturbance agents as drivers of increased fire risk in European temperate forests. Knowledge of one disturbance agent in isolation is proven to be insufficient for managing synergistic or compound disturbance events in the future [22,23], as disturbance interactions are more likely to produce abrupt changes or lead to unforeseen responses of ecosystems [24,25]. Thus, deepening our understanding of disturbance interactions and their cascading effects on forest ecosystems in Central Europe is becoming increasingly relevant for preparing management responses to more complex disturbance events [26].

Here, we discuss how abiotic and biotic forest disturbances interact with wildfire in the temperate ecoregion of Central Europe. We hypothesize that preceding biotic and abiotic disturbances create favourable conditions for succeeding wildfire events in this ecoregion and that the risk from increased wildfire hazard and severity is the result of disturbance interactions and the ensuing cascading effects. Our approach aims to synthesize the most up-to-date literature on this topic, identify research gaps, and bring emerging questions into perspective. We thus aim to reach beyond a review of the current State-of-the-Art and to identify novel trends and emerging risks related to wildfires in times of climate change in Central Europe.

1.2. Changing Fire Regimes in the Temperate Forests of Central Europe

The natural fire regime of a given region is characterized by the prevailing climate and weather conditions (temperature and precipitation patterns, humidity), fuel characteristics (load, moisture, arrangement), topography, as well as vegetation composition and structure [26,27]. In other words, multiple climatic and ecological properties are relevant to fire occurrence [28]. This also includes the interactions, both positive and negative, between the various disturbance agents [29]. Although forest damage in Central Europe is primarily the result of wind and storm disturbances [30], wildfires have become an increasing concern [31,32]. Fire occurrence has increased substantially in recent years in most Central European countries, with global warming being considered the main driver of the shift in fire–vegetation dynamics [31]. The modelling of future scenarios based on data by the European Forest Fire Information System (EFFIS) suggests a significant increase in burned areas in Central Europe in the next few decades [33].

According to the Köppen–Geiger climate classification [34], temperate European forests are associated with the humid zone that is characterized by warm summers with rain [35], currently having the climate reference ‘Western Central Europe (WCE)’ by the Intergovernmental Panel on Climate Change (IPCC) [36]. Divided into two main ecoregions, ‘Central European Mixed Forests’ and ‘Western European Broadleaf Forests’, temperate European forests in their natural states would consist of a mixture of deciduous and coniferous species [37,38]. The regional pyrogeography is classified as “non-fire-prone” [39], and the actual fire regime is characterized by infrequent or low-severity fire events [2]. However, a changing climate leads to rising temperatures and prolonged droughts, creating the conditions for wildfires to ignite and spread [40]. Additionally, human activities such as deforestation, urban expansion, and agricultural practices encroached upon forested areas, lead to an increase in the likelihood of fire ignition [41].

The naturally predominant deciduous and non-fire-prone tree species have been widely replaced by conifers in temperate Europe, particularly by plantations with Scots pine (Pinus sylvestris L.) and Norway spruce (Picea abies (L.) Karst.). Nowadays, Scots pine is among the most widely distributed tree species [42], and due to its high commercial value [43], it is extensively cultivated in monospecific plantations in Central Europe [44,45,46], replacing a large proportion of native broadleaf forests [47]. Native forests primarily composed of deciduous trees were gradually replaced by Scots pine monocultures over the course of industrialization in the 18th–19th centuries and later, after the second World War [48]. Scots pine is also the most common tree species in Northeastern Germany and in the German state of Brandenburg [49,50], an area where many forest fires within the past two decades occurred [51]. Owing to its root system, thick bark, and high regeneration rate [52], as well as a high canopy base height to reduce the likelihood of a surface fire reaching the canopy [53], Scots pine is generally adapted to low-intensity wildfires [54,55]. In contrast, Norway spruce is not well adapted to fires [56], and its branch system can be severely damaged even by low-intensity fires [57]. Overall, fire risk and tolerance to fire is higher in Scots pine than in Norway spruce stands [58]. Moisture content of the humus layer seems to be a determining factor of fire severity in Norway spruce stands [59], which, when burned, are frequently replaced by Scots pine [57]. In a post-fire landscape however, the shade-tolerant Norway spruce may eventually dominate at a later successional stage [60].

1.3. Disturbance Interactions as Drivers of Forest Fire Risk and Severity

Compound disturbances can affect ecosystems in a variety of ways, whereby initial disturbances alter the probability, severity, and intensity of subsequent disturbances [61]. This link between discrete disturbance events can lead to disturbance-driven cascading effects [22,62], which may result in shifts in disturbance regimes [63,64], leading to ecosystem transformations [65,66] or uncertainty about the likelihood of ecosystem shifts and future vegetation states [67]. Disturbance interactions are more likely to take place when individual disturbances increase in frequency, intensity, and duration, which is often a consequence of climate change [27]. Successive [68] or compound [22,69] disturbances may interact in a synergistic way, whereby one disturbance amplifies the effect of a subsequent disturbance; however, they can also act in an antagonistic way, whereby one disturbance buffers the effect of a subsequent disturbance, or in a neutral way, where one disturbance has no effect on another disturbance. As such, implications on the impact of successive disturbances for forest fire risk can vary.

1.4. Wildfire, Drought, and Storms

Wind disturbances, such as storms and windthrow, are widely considered to be the most prominent disturbance agents in Western and Central Europe [70,71]. Wind disturbances interact with subsequent forest fires by altering fuel composition and increasing fuel load [72]. Wind–fire interactions are complex as they can vary in space and time and can be both synergistic and antagonistic [73]. Windthrow and the subsequent accumulation of fine fuel may lead to wildfires, even in less fire-prone forest types [74], while fire severity may increase in blown down stands [75], making pre-fire wind disturbances an important aspect in fire risk assessments. Changes in the forest structure due to wind disturbances can increase the likelihood of wildfire [73] but can also have buffering effects on subsequent wildfires [76]. Drought is another commonly occurring abiotic disturbance in European forests [71]. Drought events are expected to become more severe as well as increase in frequency and duration due to increased global temperatures [77]. This is also predicted for Central Europe [78], which has important implications for forest fire risk, as drought is one of the most common disturbances that precede wildfires [79]. Drought changes both fuel availability and flammability and is positively correlated with wildfire risk and fire extent [80]. Fuel moisture content, a critical component of wildfire dynamics [81], is altered by drought conditions, a process that plays an important role in both fire potential and severity [82,83].

1.5. Wildfire, Insect Outbreaks, and Microbial Pathogens

The interaction between wildfires and insect outbreaks has been extensively studied in the boreal zones of North America (e.g., see review [84]) and Northern Europe [85], as it is an important component of the disturbance regime in those regions. Many findings introduced here are primarily from studies conducted outside Central Europe. This is in order to later discuss relevant findings from our region of interest from a global perspective (see also [86]). Whether insect outbreaks create favourable conditions for wildfire is debated [63], and a global review [87] showed that there can be positive, negative, and neutral legacies of insect pests with regard to wildfire risk and severity. Drought events may intensify the interaction between insect disturbances and wildfire, as drought stress can directly affect the physiology and metabolism of potential tree hosts and render them more susceptible to insect attacks [88]. Drought severity and duration both play an important role in insect activity after a drought event [89], while a warming climate facilitates insect reproduction, survival, and range expansion [90,91]. In Central Europe, the defoliation of deciduous trees, such as oaks (Quercus sp.) and beech (Fagus sylvatica L.), by the gypsy moth (Lymantria dispar L.) and the oak processionary moth (Thaumetopoea processionea L.), as well as attacks by bark beetles, especially the European spruce bark beetle (Ips typographus) [92], can be severe [86,93]. These forest pests may become more relevant in the understanding of the regional wildfire dynamics in the future.

Interactions between bark beetle outbreaks and wildfire depend on the stage and magnitude of the outbreak [85,94], as well as the time that has passed between the initial disturbance and the fire event [84]. That is because changes in fuel characteristics, exerted by damage from insects on trees, can modify fire behaviour in multiple ways, and the effects on fire properties are often stage-dependent of the outbreak [95]. Tree damage by bark beetle infestations are commonly classified in three stages (green, red, grey) that reflect the crown colour of the infested tree, as this changes depending on the stage of the feeding process that the bark beetle is in [86,96]. The build-up of surface fuels, due to the loss of needles and bark during the grey stage, may increase the intensity of surface fires irrespective of infestation phase [84]. However, the ways by which each infestation stage influences fire properties can be complex, and there is uncertainty about this, as many other factors (e.g., fallen snags, the post-infestation growth of vegetation) play a role in fuel arrangement and availability [97].

The effects of bark beetle outbreaks on subsequent wildfire activity can be positive, negative, or neutral. Kulakowski and Veblen (2007) [75] found that pre-fire bark beetle (Dendroctonus spp.) outbreaks had no effect on the fire extent, whilst Agne et al. (2016) [98] reported an inverse relationship between fire severity and mountain pine beetle, whereby higher disturbance severity decreased the severity of a subsequent fire event. Other studies (e.g., [99]) found no influence of mountain pine beetle (MPB: Dendroctonus ponderosae) outbreaks on the occurrence of high-severity wildfires. Furthermore, research generally suggests that the risk of active crown fires in lodgepole pine or spruce–fir forests in North America is not increased by preceding bark beetle (Dendroctonus sp.) outbreaks [100]. Likewise, Meigs et al. (2016) [101] found that outbreaks of MPB and western spruce budworm (Choristoneura freemani) decreased the severity of wildfires by reducing the available biomass that could serve as fuel.

Similarly to insect outbreaks, tree mortality caused by microbial pathogens may have a profound effect on fuel arrangement and promote synergistic interactions between fire and tree pathogens. Although there is currently little research to confirm the effects of pathogens on wildfire dynamics for Central Europe, it was found that tree mortality by Sudden Oak Death (SOD), a widespread disease of oak (Quercus sp.), and other tree species that is caused by Phytophthora ramorum, may change the fuel load on the forest ground and is strongly associated with changes in fire behaviour in parts of North America [102,103]. Further, interactions between tree diseases and fire may negatively impact ecosystem services of forests, particularly carbon sequestration and nutrient retention. The leaching of carbon and other soil nutrients through the forest floor in burned forest patches that have also been attacked by Phytophthora ramorum is higher [104]. Despite the uncertainty around the impacts of tree disease by pathogens in the temperate biome of Europe, the spread of bacterial and fungal diseases is being increasingly recognized as a threat to common deciduous trees (e.g., Quercus sp.) in this region [105,106,107], which might have implications for forest fire potential in the future.

2. Materials and Methods

As disturbance interactions can be reflected on individual fire features [87], we considered studies that explicitly looked at the interaction(s) between naturally occurring non-fire forest disturbances and their influence on wildfire risk, likelihood, extent, spread, and severity, in the temperate ecoregion of Central Europe. We reviewed the relevant literature using Google Scholar©, the Semantic Scholar© research tool provided by the Allen Institute for AI, and Web of Science™ provided by Clarivate™, between December 2023 and July 2024. We set the timeframe of our search (‘date range = 2000–2024’) and performed multiple literature screenings using various combinations of search terms. Accounting for limitations in the search algorithms and the variability of the used keywords, we conducted searches by using both general (e.g., abiotic disturbances) and specific (e.g., wind disturbances) terms. We also accounted for the fact that different terminology might be used in other disciplines and research fields. For example, natural disturbances, including wildfire, may also be referred to as ‘natural hazards’ (e.g., [108]). Additionally, considering the variability in the terminology of terms such as fire risk, hazard, and potential [109], we used the term ‘fire risk’ in a broad sense, treating it as nearly synonymous with fire hazard and fire potential. That said, in the context of this manuscript, ‘fire risk’ refers to the likelihood of wildfire occurrence as driven by pre-fire disturbance interactions. Beyond that, we considered similar words that could be used instead of interaction, such as synergism and synergistic.

In detail, we performed keyword-based searches consisting of combinations of the following search terms: (“(disturbance OR cascading effects OR interactions OR precondition OR predisposing OR natural hazard OR perturbation) AND (fire OR forest fire OR wildfire) AND (Europe OR European) AND (oak-pine OR pine OR Pinus sylvestris OR spruce OR Picea abies OR forest)”). Additional searches using related terms showed that the above combinations sufficiently covered the relevant literature (e.g., fire also yielded results for fire regime, fire risk, fire severity, fire hazard, fire potential etc.; disturbance for disturbances etc.). Filtering of the search results and subsequent article selection were based on how well each study or publication met our criteria; textbooks or other scientific books were excluded in this step. We evaluated each publication, including review or synthesis papers, on an individual basis and recorded meta-data on the location of the study, disturbance types and interactions, methodological approaches, and major findings. In this process, we checked whether the study area of the respective publication was within our geographical region of interest (Figure 1) or within neighbouring countries where climate is comparable and conifer plantations may be found (e.g., Hungary, Slovakia). We also checked whether the studied disturbances took place prior to fire events. Finally, we additionally considered a few synthesis articles that provide an essential background on the topic of disturbance interactions, often from a global perspective.

3. Results

Our literature review revealed that most studies looking at forest disturbance interactions, or cascading effects with particular reference to forest fire risk and/or severity, were conducted in North America and other fire-prone regions of the world, such as in the Mediterranean countries of Southern Europe, countries of the boreal ecozone, South Africa, and Australia. There are only a few studies on this topic that were conducted in the temperate biome of Central Europe (see also [26]), and those primarily focused on the most prominent disturbance agents that occur in this region, namely wind- and insect- related disturbances. In total, our literature review yielded 35 scientific articles (Table S1) that fulfilled our outlined criteria for inclusion in our discussion. Of those, one publication was a European Commission’s Joint Research Centre (JRC) report, 18 were articles of individual studies, and 16 were synthesis papers, including review, conceptual, or opinion articles. To propose a conceptual framework of interacting disturbances (Figure 2a,b), we firstly considered the studies conducted in Central Europe (Figure 2a), and, then, we considered the synthesis articles (Figure 2b), whose scope often goes beyond Central Europe yet whose results and perspective we see as particularly relevant to our findings and synthesis.

4. Discussion

4.1. Disturbance Interactions and Implications for Forest Fire Dynamics in Central Europe

Kane et al. (2017) [112] proposed that differences in the fire history between the Western United States and Europe may explain the high number of studies on disturbance interactions in North America in comparison to Europe. More efficient techniques for fire detection contributed to the increase in the absolute number of forest fire occurrences in Europe [71,113]. However, the growing urban population and density have increased the complexity of the interface between urban and forested areas [114], exposing human settlements to higher fire risk [51] and bringing forward policies for fire suppression. As such, better techniques for the detection of forest fires and a big focus on fire suppression mean that most fires are extinguished on their early stages, including fire events that could have otherwise provided opportunities to study forest fire dynamics. A recently published literature review that looked at the relationships between common biotic and abiotic natural hazards revealed that the pair of hazards between “Wind—Insects” was the one most studied in Europe for the period between 1916 and 2020 [115]. Worth noting is, however, that the publications considered by the authors were reviewed from a forest economics perspective, a field that commonly considers disturbances as independent from one another, even if one precedes or succeeds the other or even if they occur simultaneously [115]. Considering the above, we posit that the relatively small number of studies on fire and non-fire disturbance interactions in Central Europe may be the combined result of firefighting practices to protect human settlements, a focus on the more common disturbance types in Central Europe, especially wind and insect disturbances, and the regional fire history.

An overall intensification of non-fire disturbances could alter fuel characteristics, such as increasing fuel load and connectivity and increasing wildfire risk in disturbed forest patches [116]. The modification of fuel characteristics [84] and changes in fuel availability following an initial disturbance [112] can thus be seen as key points in our understanding of the process through which non-fire disturbances may promote fire events. Empirical analyses also suggest that fuel properties, particularly fuel availability, moisture, and continuity, are the most important drivers of wildfire extent and severity [117]. In this regard, there is growing evidence that changes in fuel properties, induced by a disturbance event other than fire, is the linking point between this preceding event and subsequent wildfire occurrence and behaviour. For instance, branch breakage following storm events can directly increase fuel load and ultimately affect fire characteristics, such as its extent and severity [118]. Likewise, beetle outbreaks may affect fuel properties by altering fuel load, composition or arrangement, or even by modifying fuel moisture and other chemical characteristics influencing forest fire risk [119]. Indeed, Beetz et al. (2024) [120] found that the accumulated fuel on the forest floor, after infestation by bark beetles, promoted fire intensity in Bohemian-Saxon Switzerland, a protected area that is located on the border between Germany and the Czech Republic and is covered primarily by spruce forests, where bark beetle outbreaks have been reported since 2017 and a major fire broke out in 2022. Bark beetle damage on trees can also have indirect effects on fire severity, for example, by altering the structural fuel connectivity in attacked tree stands. Standing deadwood of killed trees can serve as a fuel ladder and allow less severe surface fires reach the tree canopy and thus develop to more severe crown or canopy fires [121]. We would expect a similar mechanism with other disturbance agents, such as windthrow, whereby felled trees would eventually become ladder fuels and promote fire severity.

A large-scale remote sensing analysis of wind and fire disturbances in forests across Europe in the period between 1986 and 2016 suggested that there is little, or even no, interaction between wind and fire disturbances [70]. This finding could be due to the spatial variability of wind and fire disturbances at the continental scale, as storms are more prevalent in Central and Western Europe, whereas fires are more common in Southern and Northern Europe. In addition, storm damage could be exerted on single trees or a small tree stand, which cannot necessarily be mapped using remote sensing methods, such as satellites, whose spatial resolution is insufficient to detect small-scale gap dynamics. Additionally, post-disturbance interventions that took place before the fire season started in Central European countries, such as salvage logging and the removal of felled trees [122,123], may have also influenced the findings of wind—fire interactions in the reference period of the aforementioned study [70]. In addition, since the end of the reference period in 2016, Central Europe has experienced seasons with unprecedentedly intense wildfire activity [7], and, therefore, interactions between wind and fire disturbances after 2016 might have remained undetected or otherwise not yet studied. Notwithstanding, a case study that was conducted in the Italian Alps showed that windthrow increased the rate of spread, flame length, and midflame windspeed of a subsequent wildfire [124]. It is generally known that surface fuels increase after storm events, as branches or other plant material that serve as fine fuel, are deposited on the forest floor [74]. However, deadwood (woody material with a diameter > 10 cm) is likely not a driver of fire risk in European forests, as such pieces of deadwood tend to burn slowly and contribute little to fire intensity [125].

Wind disturbances may influence wildfire occurrence by promoting insect outbreaks or by increasing plant litter on the forest floor, a process that can consequently alter the arrangement of fuel both on the ground and in the canopy. A fossil-based study showed that bark beetles, especially the common species Pityogenes chalcographus and Pityophthorus pityographus, have been interacting with forest fires in Norway’s spruce mountain forests of Central Europe for centuries [126]. It is also documented that the spruce bark beetle (Ips typographus L.), a widespread beetle species across most of Eurasia that often causes epidemics in Norway spruce (Picea abies (L.) Karst.) forests in Central European countries [127], preferentially attacks wind-damaged spruce individuals [128,129] and increases the rates of tree mortality if no intervention (e.g., salvage logging) takes place [130]. Furthermore, insect outbreaks can increase the probability of more severe wildfires [131], which can have important implications when fuel availability is high, for example, after a drought period. Damage by bark beetles in European forests in the past two decades has become more widespread [71]. At the same time, outbreaks of defoliating insects in several Central European countries are promoted by increasing temperatures [132], and an overall increase in insect outbreaks is expected in this region in the future [133]. We expect that these trends will contribute to a higher fire risk or severity, either via direct legacies from plant damage and fuel accumulation by insects or via interactions of multiple disturbances, particularly those between ‘windthrow—insects—fire’ [22,134].

As with insect and wind disturbances, we expect that drought will interact more frequently with wildfires in Central European forests, and this may be manifested in various ways. Tree vulnerability to disturbances, as well as tree mortality, can be amplified after periods with warmer conditions [9]. Drought is associated with a higher tree mortality of Scots pine [135,136], which is independent from the latitudinal location of pine stands [137]. Consequently, tree death can have a direct effect on fuel load and arrangement, which can in turn increase stand vulnerability to fire disturbances, especially in temperate forests [138]. Pre-fire drought conditions increase fire risk by reducing fuel moisture and thus increasing the flammability of live and dead vegetation [80]. In addition, a warmer climate further promotes fuel aridity [139]. This ecological process is expected to create more favourable conditions for large fire events in Europe by the end of the century [140]. The consequences of such drought—fire interactions that we see as becoming potentially more common are a direct increase in fire risk, hazard, and extent following a drought, possibly due to fuel continuity in the landscape. This was the case of the 2018 drought period, after which wildfire activity and burned areas increased in many Central European countries [141,142], or, in other words, larger spatio-temporal disturbance events emerged [143].

We would also expect an indirect influence of drought on fire preconditions, with insect disturbances and plant diseases occurring after drought and subsequently affecting forest vegetation. Owing to this is a warmer climate that promotes more favourable conditions for insect outbreaks, as it facilitates their reproductive success, amplifies their life cycle within a given season, and accelerates their growth [144,145,146]. At the same time, drought and water stress may render forests more vulnerable to insect outbreaks, which can then lead to higher tree mortality and thus the increased likelihood of fire [147]. Furthermore, drought stress renders trees more susceptible to diseases [148], with both Norway spruce and Scots pine suffering increased dieback due to the combined effects of drought and pathogens [149,150,151]. Water stress induced by drought events also increases the susceptibility of Scots pine to infection with the fungus Sphaeropsis sapinea [152,153]. Therefore, drought conditions may facilitate more complex interactions between multiple pre-fire disturbances by first promoting the occurrence of biotic disturbances, all of which successively precede and eventually interact with wildfire activity by potentially altering fuel availability, load, and arrangement.

4.2. Key Factors Influencing Wildfire Dynamics in Central Europe

We considered additional important factors that influence the fire regime in Central Europe. From reviewing the relevant literature, we identified both direct and indirect links between climate change, forest disturbances, and regional wildfire activity. Climate-driven compound disturbances exacerbate the effects of the natural disturbance regime of temperate forests [147,154], and climatic changes will have a significant influence on both biotic and abiotic disturbances by altering their frequency and intensity [155]. Disentangling cascading effects of interacting disturbances from the influence of climate change on fire regimes is certainly not a straightforward task, and research on disturbance interactions in Central Europe needs to consider the impacts of climate change on forest—disturbance dynamics [156]. Research suggests that there is an intensification of forest disturbances by wind and insects in European forests, a process that is largely driven by climate change [157]. The observed increase in fire-affected areas in Europe over the course of the 20th century is thought to be the combined result of higher temperatures and lower relative humidity [113], highlighting that these two environmental conditions are of particular importance when discussing future scenarios and changes in wildfire risk. Disturbance interactions are also purported to become more common due to climate change [158], and an extensive review of the current knowledge on the effects of climate change on several disturbance agents revealed that drought and wind disturbances interact additively to fire disturbances [159].

Meteorological conditions define essential components of fire weather, which is often quantified in Europe using the Fire Weather Index (FWI). The FWI is based on midday readings of temperature, relative air humidity, wind speed, and precipitation of the last 24 h [160], as well as the determination of drought, fuel moisture, and fire behaviour indices [161]. Fire weather and the FWI are important predictors for fire hazard, and thus risk [162], and are projected to become more severe as a result of a warmer climate in Central Europe [11]. A recent study that analyzed the trends of the FWI between 1981 and 2010 showed that Central European countries are expected to face increased fire risk due to changes in temperature and precipitation patterns that would result in warmer and drier summers [21]. In addition, a longer, climate-driven, forest fire season will lead to increased fire potential and eventually to more frequent fire events in periods of the year that would have otherwise faced low danger of fire, such as the spring or autumn [11]. Germany [163] and potentially other Central European countries are expected to experience more severe fire weather in the future [164]. As such, the study of disturbance interactions and their legacies can be tied to fire weather conditions and patterns in the FWI.

Beyond changes in the regional fire weather, projections of future wind activity suggest an increase in storminess [165] and storm intensity [166] in Western Europe. This could promote wind and storm-related disturbances, as well as lightning activity. Lightning may be classified as a disturbance agent from a disturbance ecology perspective yet is often overlooked as such [167]. Increased lightning activity in the alpine areas of Central Europe is thought to be largely driven by global warming, and it was shown that more and more fires are ignited by lightning than in the past [168,169]. Whether it is considered as a disturbance agent or as a source of ignition, lightning activity has implications for the fire regime in the Alps, particularly in Austria [170,171], and submontane coniferous forests [172,173]. We expect to see a similar pattern in other parts of Central Europe, which would be, however, rather challenging to investigate due to the uncertainty in projected thunderstorm and lightning activity [174].

Apart from climatic and fire weather patterns, forest management, particularly tree species selection, plays a crucial role in changing preconditions towards increased wildfire risks [113], as conifer plantations are considered to promote the overall susceptibility of forests to disturbances [157]. We posit that this increased forest susceptibility has contributed to the greater frequency of forest fires in pine forests of Central Europe in recent years. A potential mechanism behind this forest type-driven increase in fire frequency and severity is through modifications in the amount of combustible material by preceding disturbance events that created favourable conditions for fire events. Zald and Dunn (2018) [175], who looked at different management practices of forests in Oregon, USA, showed that conifer plantations indeed play a significant role in modifying wildfire severity, a role that the authors attributed to the spatial homogeneity of fuels that results from such silvicultural practices. There is strong evidence from studies around the world that tree plantations with pine species (e.g., Pinus radiata, Pinus pinaster) are highly prone to fire events (see [176]). Although similar evidence for Scots pine (Pinus sylvestris) plantations is scarce, such managed forests are characterized as vulnerable to wildfire in the state of Brandenburg in Germany [177,178]. We believe that this vulnerability to wildfire could be attributed to the fact that heat release from the burning of litter made of Pinus sylvestris needles is the highest in comparison with other litter types [179]. Additionally, Scots pine litter tends to dry out more rapidly than the litter of other common tree species, such as beech, which is thought to promote fire in pine stands [47]. Nevertheless, worth noting is that, in Germany, anthropogenic ignition sources, primarily by negligence, cause most fires [180], while research conducted in the Czech Republic suggests that it is the combined influence of human activities and climatic factors that drive wildfire occurrence in Central Europe [181]. In addition to Scots pine, another commonly used conifer in plantations is Norway spruce (Picea abies), a species that is notably susceptible to various disturbances, including windthrow [56,182] and drought [183]. Climate change further exacerbates these vulnerabilities, heightening the risk of climate-driven wildfires in Norway spruce plantations [184]. Similarly, evidence suggests high fire risk in plantations of Austrian pine (Pinus nigra) in Hungary [185,186].

4.3. Perspectives and Research Priorities

Emerging evidence indicates that future wildfire risk and severity will increase in Central Europe, possibly due to the overall increased occurrence of disturbance events [69,157]. These changes in wildfire activity are not always evident, although we expect that increased fire risk would not be the result of a single disturbance, but the legacy of multiple disturbances that have occurred in succession and have, each in its own way, altered fuel characteristics. We expect a higher probability of successive disturbances to occur, and, therefore, we posit that the explicit study of disturbance interactions or compound disturbances should be integrated in future research, even if disentangling them from the more direct effects of individual disturbances on fuel characteristics is challenging [62]. Increasing our understanding of disturbance interactions is already seen as a research priority [25,187,188], especially outside North America [189], such as the comparatively understudied region of Central Europe [190]. We generally expect that one-way interactions between ‘drought—fire’, ‘wind—fire’, ‘insects—fire’, as well as multiple interactions between ‘wind—insects—fire’, ‘drought—insects—fire’, and potentially ‘drought—pathogens—fire’, will become increasingly common in Central Europe, and more research on those disturbance interactions is needed. Additionally, the role of tree mortality and deadwood accumulation, following disturbance events, in driving regional wildfire dynamics—particularly fire intensity and burn severity—represents a promising area for future research.

Whether the ecological processes driven by disturbance interactions will lead to a shift in the natural fire regime in Central Europe is debatable; yet, there is growing evidence [21,163,191,192,193] that European temperate forests are currently undergoing a change in the regional fire regime, which is still not fully understood. This trend is also seen in the greater Alpine region of Central Europe [170,194], where forest composition at lower altitudes is comparable to that of the temperate biome. Studies based on modelling approaches (e.g., [158,163,195] showed that severe fire events in less fire-prone areas of Central Europe are more likely to occur in the future and that droughts will render Central and Northern Europe more susceptible to wildfires [21]. Although these modelled future scenarios focused more on the effects of climate change on forest fire occurrence, there is a potential that the interactions between disturbances and ensuing cascading effects will also play a role in this pattern. Following a recent period of considerable increase in fire events, such as the period between 2017 and 2022 [7], it can be argued that natural fire regimes in temperate Europe are currently shifting to novel states characterized by either more frequent or more severe fire events.

4.4. Managing the Emerging Fire Regime in Central Europe

The anthropogenic influence on fire regimes increased since the 18th century with increasing population density and settlement within fire-prone vegetation [20]. The Wildland Urban Interface (WUI), which describes the interlinkage of urban land use and forest fuel mass [196], accounts for about 15% of the land area in Europe [197]. It is at this interface that a high risk for fire ignition meets the threat for the population, calling for an increase in fire prevention strategies [198]. Beyond that, urban planning and encroachment of the WUI may also lead to higher fire risk even in areas where there are fewer effects from climate change [199,200]. In view of this, it is crucial to discuss ways to manage large fire events in Central Europe that may occur within the WUI in the future. We believe that an Integrated Fire Management (IFM) approach is needed [201], in which the acquired ecological knowledge as well as novel technologies are integrated, and wildfire prevention for the protection of the urban–wildland interface is in focus. An IFM should be based on fuel reduction practices, particularly prescribed or controlled burning [202,203] and forest thinning [204], as these practices successfully minimized wildfire potential and mitigated fire impact on ecosystems in other fire-prone regions. Apart from changes in the arrangement of fuel, thinning can facilitate the conversion of Pinus sylvestris plantations to a mixed conifer–broadleaved woodlands [205] that have naturally fire-suppressing elements [206], or less heat would be otherwise released from the burning of mixed plant litter [179], a physical process that could reduce fire intensity. Landscape modifications and fire-preventive silvicultural practices, such as the formation of fuel or fire breaks [207,208,209], should also become an integral part of IFM and adaptation policies to fire events in Central Europe [33]. A case-specific approach is recommended in the use of salvage logging and timber removal after a disturbance event [210], as such practices may also negatively affect ecosystem services and forest regeneration [211,212].

Further, an early warning system that considers the legacies of disturbance interactions in conjunction with fire weather, forest species composition, and climate change is needed to safeguard the temperate forests in Central Europe. Improvements to such a system should include continued research to improve the Global Early Warning System for Wildland Fire (EWS-Fire) [213], enacting regional wildfire monitoring initiatives, and integrating the potential implications of cascading effects for fire properties. We argue that forest fire risk assessments should integrate interactions of other forest disturbances [68] by incorporating the history of non-fire disturbance events in wildfire monitoring. Likewise, forest management should consider the processes by which disturbance interactions could increase fire risk, severity, or magnitude. We assert that the mapping of fuel loads in the temperate forests in Central Europe [214] and developing fuel models that are consistent among countries would be a critical first step towards an effective management of the increasing fire risk in this region. Fuel mapping should be prioritized in disturbed forest patches and conifer plantations, even if they are within areas that have been previously characterized as having a low fire risk. In this respect, the monitoring of tree mortality that results from preceding non-fire disturbances, the transformations of forest structures, and changes in fuel arrangement are crucial [181]. Given the effects of increased lightning activity in the Alps and projected storm patterns in Central Europe, we suggest including lightning and its influence on wildfire ignition in future research [174].

Remote sensing methodological approaches have shown great potential in increasing our understanding of disturbance dynamics in forest ecosystems in Europe, yet complementary field or in situ data on individual disturbance agents are also highly needed [71]. As such, we argue that more effort and resources should be directed towards the collection of data on fuel loads and structures [214], which could serve as reference data to complement remote sensing products and validate statistical, mechanistic, and machine learning models. Many studies demonstrated the potential of modelling approaches for carrying out risk assessments of individual disturbance agents (e.g., [215,216]) and for studying disturbance interactions [63], such as those between windthrow and fire [124]. Machine learning approaches coupled with earth observation data showed promising results in disentangling the effects of interacting disturbances [133] and in predicting the probability of forest fire occurrence [217,218]. These methodologies have also been applied to assess the ecological vulnerability of forests to wildfires at a European scale [219] and to predict wildfire fuels and hazards in Central Europe [32]. Furthermore, Explainable Artificial Intelligence [220] and Graph Neural Networks [221] can be used to study spatio-temporal dynamics and the drivers of wildfire occurrence in Europe. Consequently, modelling and simulation approaches are important tools for understanding how disturbance interactions influence the fire regime in Central Europe and how this knowledge could be incorporated in IFM and decision support systems.

5. Conclusions

Forest disturbances in Central Europe are becoming more frequent, and interactions between disturbances that precede fire events will play an increasingly important role in wildfire risk and severity. Climate change is altering fire weather conditions in ways that will further intensify forest fire dynamics in Central Europe. As Europe is one of the continents most affected by climate change [222], managing wildfire hazards and reducing risks require in-depth knowledge of the effects of disturbance interactions and their impact on wildfire likelihood.

Regional wildfire risk and severity may increase due to interacting disturbances, and we expect that successive disturbance events, especially ‘drought—fire’, ‘drought—insects—fire’, ‘insects—fire’, ‘windthrow—fire’, ‘windthrow—insects—fire’, and potentially ‘drought—pathogens—fire’, will become more common. Drought may influence the magnitude (extent or size) of subsequent wildfires, while such effects of other disturbance agents remain unclear, making this an important research priority. More studies on how these disturbance agents interact with wildfire dynamics in temperate European forests are needed, especially in plantations of Pinus sylvestris, Picea abies and Pinus nigra.

Changes in fuel properties (e.g., load, arrangement) serve as pathways for disturbance interactions. Future research and wildfire prevention efforts in Central Europe should investigate how these changes are driven by interacting disturbances. An Integrated Fire Management approach is needed, where disturbance interactions are a central component, alongside the use of novel technologies for forest monitoring, machine learning, and earth observation methodologies.