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Synchronised disturbances in spruce- and beech-dominated forests across the largest primary mountain forest landscape in temperate Europe
Abstract
Understanding temporal and spatial variations in historical disturbance regimes across intact, continuous, and altitudinally diverse primary forest landscapes is imperative to help forecast forest development and adapt forest management in an era of rapid environmental change. Because few complex primary forest landscapes remain in Europe, previous research has largely described disturbance regimes for individual forest types and smaller isolated stands. We studied the largest but still largely unprotected mountain primary forest landscape in temperate Europe, the Fagaraș Mountains of Romania. To describe historical disturbance regimes and synchronicity in disturbance activity and trends between two widespread forest community types, dominated by Norway spruce (Picea abies (L.) Karst.) and European beech (Fagus sylvatica L.), we established 191 permanent study plots (70 beech; 121 spruce) across 11 valleys, thereby providing information at both stand and landscape
levels. We used a dendrochronological approach to reconstruct and describe the spatiotemporal patterns of
historical disturbances. We observed a diverse spectrum of disturbance severities and timing across the forest landscape. High-severity disturbances created periods of synchrony in disturbance activity at the landscape scale, while moderate- and low-severity disturbances were asynchronous and random in both spruce- and beech-dominated primary forests. We detected a peak of canopy disturbance across the region at the end of the
nineteenth century, with the most important periods of disturbance between the 1890s and 1910s. At the stand scale, we observed periods of synchronised disturbances with varying severities across both forest types. The level of disturbance synchrony varied widely among the stands. The beta regression showed that spruce forests had significantly higher average synchrony and higher between-stand variability of synchrony than the beech-dominated forests. Synchronised disturbances with higher severity were infrequent, but they were critical as drivers of subsequent forest development pathways and dynamics across both forest types.
Our results provide valuable insight into future resilience to climate-driven alterations of disturbance regimes in spruce- and beech-dominated mountain temperate forests in the Carpathians. We suggest that conservation efforts should recognize strictly protecting large continuous and altitudinally diversified forest landscapes such as Fagaraș Mts. as a necessary measure to tackle climate change and ensure temporal and spatial structural heterogeneity driven by a wide range of disturbances. The diverse and synchronous disturbance activity among two interconnected forest vegetation types highlights the need for complex spatiotemporal forest management approaches that emulate disturbance synchronicity to foster biodiversity across multiple forest vegetation types within forest landscapes.
... Drossler, von Lüpke (2005) zdokumentovali, že 10 % porastových medzier tvorili tie s rozlohou 250 až 1000 m 2 a boli s veľkou pravdepodobnosťou spôsobené atmosférickými procesmi. Ďalšie štúdie detekovali stredne silné narušenia so 40% odstránením korunového zápoja v Dinárskych vrchoch alebo v Karpatskom oblúku, kde boli zaznamenané aj disturbancie s vysokou severitou Kameniar et al. 2023). Prírodné disturbancie s rôznymi stupňami narušenia vytvárajú diverzifikovanú štruktúru lesa s kombináciou uzavretého korunového zápoja a s poprepájanými menšími alebo väčšími plochami vyvrátených stromov. ...
... Pre výpočet rozsahu narušenia na úrovni plochy sme použili záznamy o narušení od jednotlivých stromov na výskumnej ploche. Percento narušenej plochy korún stromov na úrovni výskumnej plochy sa následne vypočítalo pre každý rok ako súčet aktuálnych plôch korún stromov reagujúcich na narušenie (vykazujúcich rastové trendy uvoľnenia rastu alebo rýchleho počiatočného rastu v otvorenom poraste), vydelený celkovou plochou korun všetkých vzorkovaných stromov, čím sme dosiahli percento narušenia pre každý rok chronológie (Trotsiuk et al. 2018;Čada et al. 2020;Frankovič et al. 2021;Kameniar et al. 2023). Na vyhladenie všetkých údajov v histograme proxy dát zrekonštruovanej histórie disturbancií sme použili kernelovú funkciu na určenie hustoty pravdepodobnosti (Kernel density estimation function -KDE). ...
Natural disturbances such as windthrows and bark beetle outbreaks are essential in the formation of natural forest ecosystem structures in Central Europe. Therefore, evaluating disturbances’ spatial and temporal extent and synchronicity is critical for understanding of forest dynamics. This study aimed to evaluate the long-term natural disturbance history of primary mixed-beech forests and investigate the species composition-temporal synchronisation relationship. We collected a unique dataset of > 6,000 tree cores across 14 forest stands on 174 study plots, and using dendroecological methods we reconstructed 200-year-long plot-level disturbance histories and evaluated synchronicity within the stands. Subsequently, we used GLMs to evaluate the tree species diversity-synchronicity relationship. The results showed substantial temporal variability of natural disturbances and a prevalence of low and moderate severity disturbances. Disturbance synchronisation was higher in the Western than in the Eastern Carpathian forests, which have a lower proportion of admixture tree species. Moreover, the GLMs showed a strong positive dependence of synchronicity on tree species richness. The results contradict the general assumption that mixed-species forests have higher resilience to disturbances than monospecific forests. The reason behind these findings can be attributed to the substantial admixture of Norway spruce, and its vulnerability to disturbances such as windthrow and bark beetle outbreaks.
... Clear forest cutting, as well as the use of heavy machinery on steep terrains, increases the risks of erosion processes and sediment production. Possible implications of using heavy mechanisation and related forest management challenges were addressed in certain research papers [25][26][27][28]. ...
Erosion is one of the main causes of soil degradation and sediment production. The amount of eroded material that reaches rivers and lakes depends on the terrain but also on the climatic and hydrological characteristics of the basin, as well as the applied land management method. The intensity of sediment production is in direct correlation with the land use type. Repurposing forest land as mining, urban and infrastructural development areas, etc., significantly affects sediment production. Shrinking of forests and unplanned agricultural production are just some of the factors that intensify erosion processes and increase the amount of eroded material, also triggering climate change and the onset of prolonged dry and rainy periods, which increase the risk of erosion in sloping terrain and intense sediment production. The paper presents the correlation between the change in the forest land use method, on the one hand, and soil erosion and sediment production, on the other, by analysing segments of river basins in the territory of the Serbian Municipality of Krupanj. The modelling of sediment production was based on data collected from the experimental territories. The method of erosion potential, data analysis, and procession in GIS surrounding were used for calculating sediment production in the experimental areas. The greatest loss of soil was perceived in the terrain with little or no vegetation on steep slopes covered with material prone to water erosion. The smallest production of sediment was noted in the terrain with vital forest vegetation. The results received point to the heterogeneous estimates of land loss, enabling the modelling of sediment production in wider basin areas and the analysis of the impacts of different land management factors on the erosion processes.
Natural disturbances play a crucial role in shaping forest structural dynamics, directly influencing stand structural heterogeneity. In European forests, disturbances occur across varying scales, from small patches to entire landscapes, significantly affecting ecosystem dynamics. However, detailed information on historical disturbances
and their specific effects on forest structure, particularly tree size distributions in primary mountain forests, remains limited. With global change altering the severity and frequency of these disturbances, understanding their long-term impact has become increasingly critical for forest management and conservation. We addressed two main questions: (1) Is there variability in tree size distributions at both the plot and stand level? and (2) which specific aspects of disturbance regimes, such as severity and timing, are most influential in shaping these distributions? To address these questions, we analyzed data from 11,755 trees across 23 primary European beech forest stands in the Carpathian Mountains of Romania (139 plots) and Slovakia (99 plots). Using linear mixed-effects models, we assessed how historical disturbances have influenced current tree size distributions. Our results showed that tree size distributions across the Carpathians show variability, with the most common pattern being close to a reverse-J shape, indicative of uneven-aged forest structures. Modelling analyses revealed that disturbance severity and timing are key factors influencing present tree size distribution patterns at small scales
in the Carpathian Mountains. High-severity disturbances generally result in unimodal or bimodal distributions, while low-severity disturbances are associated with reverse-J shaped patterns. Specifically, at the plot level (small scale), we observed that last disturbance severity, maximum disturbance severity, and time since the last disturbance all significantly impacted tree size distributions driving them away from a reverse-J shape. Finally, linear mixed-effects modeling revealed that disturbance severity, time since the last disturbance and the interaction between the two were the most influential factors shaping present tree size distributions. Our findings highlight the dynamic nature of forest ecosystems, emphasizing the critical role of historical disturbances in shaping present tree structure and the long-term development of forest stands.
Context
Ecosystem services provided by mountain forests are critically linked to forest structure. Social-ecological disturbance regimes (i.e., the rate, frequency, and patch size distribution of disturbances driven by interacting natural and anthropogenic processes) and land use affect forest structure, but their specific impacts are not fully understood.
Objectives
We examine how differences in disturbance regimes affect patterns of forest structure across three European mountain ranges with similar vegetation types but different land-use histories: the European Alps, the Carpathians, and the Caucasus.
Methods
We related data on horizontal and vertical forest structure, measured by spaceborne lidar (GEDI), with Landsat-derived information on forest disturbances (1986–2020) and topographic, climatic, and anthropogenic predictors.
Results
We found similar social-ecological disturbance regimes in the Alps and Carpathians (average annual disturbance rates of 0.34% and 0.39%, respectively, and median patch size < 0.5 ha), yet much lower disturbance rates and patch sizes in the Caucasus (0.08% yr⁻¹ and < 0.2 ha). Despite different disturbance regimes, we found similar patterns of forest structure. Two alternative states emerged consistently across all mountain ranges: a tall and closed-canopy state in 74–80% of forests and a low and open-canopy state (< 50% canopy cover) in the rest. While forest structure responded consistently to abiotic drivers such as topography and climate, its association with anthropogenic pressures differed between mountain ranges. Stand-replacing disturbances played an important role in the Carpathians, while forest structure in the Caucasus was related to proximity to settlements, reflecting local forest use.
Conclusions
Different social-ecological contexts in mountain regions can produce markedly different forest disturbance regimes. Despite these differences, similar states of forest structures emerge, suggesting strong attractors of structure in temperate mountain forests.
Drought affects the complex interactions between Norway spruce, the bark beetle Ips typographus and associated microorganisms. We investigated the interplay of tree water status, defense and carbohydrate reserves with the incidence of bark beetle attack and infection of associated fungi in mature spruce trees.
We installed roofs to induce a 2‐yr moderate drought in a managed spruce stand to examine a maximum of 10 roof and 10 control trees for resin flow (RF), predawn twig water potentials, terpene, phenolic and carbohydrate bark concentrations, and bark beetle borings in field bioassays before and after inoculation with Endoconidiophora polonica and Grosmannia penicillata.
Drought‐stressed trees showed more attacks and significantly longer fungal lesions than controls, but maintained terpene resin defenses at predrought levels. Reduced RF and lower mono‐ and diterpene, but not phenolic concentrations were linked with increased host selection. Bark beetle attack and fungi stimulated chemical defenses, yet G. penicillata reduced phenolic and carbohydrate contents.
Chemical defenses did not decrease under mild, prolonged drought in our simulated small‐scale biotic infestations. However, during natural mass attacks, reductions in carbon fixation under drought, in combination with fungal consumption of carbohydrates, may deplete tree defenses and facilitate colonization by I. typographus.
Canopy accession strategies reveal much about tree life histories and forest stand dynamics. However, the protracted nature of ascending to the canopy makes direct observation challenging. We use a reconstructive approach based on an extensive tree ring database to study the variability of canopy accession patterns of dominant tree species (Abies alba, Acer pseudoplatanus, Fagus sylvatica, Picea abies) in temperate mountain forests of Europe and elucidate how disturbance histories, climate, and topography affect canopy accession. All four species exhibited high variability of radial growth histories leading to canopy accession and indicated varying levels of shade tolerance. Individuals of all four species survived at least 100 years of initial suppression. Fir and particularly beech, however, survived longer periods of initial suppression, exhibited more release events, and reached the canopy later on average, with a larger share of trees accessing the canopy after initially suppressed growth. These results indicate the superior shade tolerance of beech and fir compared to spruce and maple. The two less shade-tolerant species conversely relied on faster growth rates, revealing their competitive advantage in non-suppressed conditions. Additionally, spruce from higher-elevation spruce-dominated forests survived shorter periods of initial shading and exhibited fewer releases, with a larger share of trees reaching the canopy after open canopy recruitment (i.e. in absence of suppression) and no subsequent releases compared to spruce growing in lower-elevation mixed forests. Finally, disturbance factors were identified as the primary driver of canopy accession, whereby disturbances accelerate canopy accession and consequently regulate competitive interactions. Intensifying disturbance regimes could thus promote shifts in species composition, particularly in favour of faster-growing, more light-demanding species.
Recent megafires and gigafires are contributing to the desertification of conifer forest ecosystems due to their size and severity. Megafires have been increasing in their frequency in the past two decades of the 21st century. They are classed as such because of being 40,469 to 404,694 ha in size, having high complexity, resisting suppression, and producing desertification due to erosion and vegetation type conversion. Increasingly, gigafires (>404,694 ha) are impacting coniferous forest ecosystems. These were once thought of as only pre-20th century phenomena when fire suppression was in its infancy. Climate change is an insidious inciting factor in large wildfire occurrences. Fire seasons are longer, drier, hotter, and windier due to changes in basic meteorology. Conifer forests have accumulated high fuel loads in the 20th and 21st centuries. Ignition sources in conifer forests have increased as well due to human activities, economic development, and population demographics. Natural ignitions from lightning are increasing as a result of greater severe thunderstorm activity. Drought has predisposed these forests to easy fire ignition and spread. Wildfires are more likely to produce vegetation shifts from conifers to scrublands or grasslands, especially when wildfires occur with higher frequency and severity. Severe erosion after megafires has the collateral damage of reducing conifer resilience and sustainability.
Natural disturbances are increasing around the globe, also impacting protected areas. Although previous studies have indicated that natural disturbances result in mainly positive effects on biodiversity, these analyses mostly focused on a few well established taxonomic groups, and thus uncertainty remains regarding the comprehensive impact of natural disturbances on biodiversity. Using Malaise traps and meta‐barcoding, we studied a broad range of arthropod taxa, including dark and cryptic taxa, along a gradient of bark beetle disturbance severities in five European national parks. We identified order‐level community thresholds of disturbance severity and classified barcode index numbers (BINs; a cluster system for DNA sequences, where each cluster corresponds to a species) as negative or positive disturbance indicators. Negative indicator BINs decreased above thresholds of low to medium disturbance severity (20%–30% of trees killed), whereas positive indicator BINs benefited from high disturbance severity (76%–98%). BINs allocated to a species name contained nearly as many positive as negative disturbance indicators, but dark and cryptic taxa, particularly Diptera and Hymenoptera in our data, contained higher numbers of negative disturbance indicator BINs. Analyses of changes in the richness of BINs showed variable responses of arthropods to disturbance severity at lower taxonomic levels, whereas no significant signal was detected at the order level due to the compensatory responses of the underlying taxa. We conclude that the analyses of dark taxa can offer new insights into biodiversity responses to disturbances. Our results suggest considerable potential for forest management to foster arthropod diversity, for example by maintaining both closed‐canopy forests (>70% cover) and open forests (<30% cover) on the landscape.
Abstract Disentangling the relative influence of background versus disturbance related mortality on forest demography is crucial for understanding long‐term dynamics and predicting the influence of global change on forests. Quantifying the rates and drivers of tree demography requires direct observations of tree populations over multiple decades, yet such studies are rare in old‐growth forest, particularly in the temperate zone of Europe. We use multi‐decade (1980–2020) monitoring of permanent plots, including observations of mode of mortality and disturbance events, to quantify rates and drivers of tree demography across a network of old‐growth remnants in temperate mountain forests of Slovenia. Annual rates of mortality and recruitment varied markedly among sites and over time; census intervals that captured intermediate severity canopy disturbances caused subtle peaks in annual mortality (e.g., >2%/year), while rates of background mortality in non‐disturbed intervals averaged about 1%/year. Roughly half of the trees died from modes of mortality associated with disturbance (i.e., uprooting or snapped‐alive). Results of a Bayesian multilevel model indicate that beech (Fagus sylvatica) had a higher likelihood of disturbance related mortality compared to fir (Abies alba), which mainly died standing, and there was a notable increase in the odds of disturbance mortality with increasing diameter for all species. Annual recruitment rates were consistently low at sites (
With accelerating environmental change, understanding forest disturbance impacts on trade-offs between biodiversity and carbon dynamics is of high socioeconomic importance. Most studies, however, have assessed immediate or short-term effects of disturbance, while long-term impacts remain poorly understood. Using a tree-ring-based approach, we analysed the effect of 250 years of disturbances on present-day biodiversity indicators and carbon dynamics in primary forests. Disturbance legacies spanning centuries shaped contemporary forest co-benefits and trade-offs, with contrasting, local-scale effects. Disturbances enhanced carbon sequestration, reaching maximum rates within a comparatively narrow post-disturbance window (up to 50 years). Concurrently, disturbance diminished aboveground carbon storage, which gradually returned to peak levels over centuries. Temporal patterns in biodiversity potential were bimodal; the first maximum coincided with the short-term post-disturbance carbon sequestration peak, and the second occurred during periods of maximum carbon storage in complex old-growth forest. Despite fluctuating local-scale trade-offs, forest biodiversity and carbon storage remained stable across the broader study region, and our data support a positive relationship between carbon stocks and biodiversity potential. These findings underscore the interdependencies of forest processes, and highlight the necessity of large-scale conservation programmes to effectively promote both biodiversity and long-term carbon storage, particularly given the accelerating global biodiversity and climate crises.
Aim
Natural disturbances influence forest structure, successional dynamics, and, consequently, the distribution of species through time and space. We quantified the long-term impacts of natural disturbances on lichen species richness and composition in primary mountain forests, with a particular focus on the occurrence of endangered species.
Location
Ten primary mountain spruce forest stands across five mountain chains of the Western Carpathians, a European hotspot of biodiversity.
Methods
Living trees, snags, and downed logs were surveyed for epiphytic and epixylic lichens in 57 plots. Using reconstructed disturbance history, we tested how lichen species richness and composition was affected by the current forest structure and disturbance regimes in the past 250 years. We also examined differences in community composition among discrete microhabitats.
Results
Dead standing trees as biological legacies of natural disturbances promoted lichen species richness and occurrence of threatened species at the plot scale, suggesting improved growing conditions for rare and common lichens during the early stages of recovery post-disturbance. However, high-severity disturbances compromised plot scale species richness. Both species richness and the number of old-growth specialists increased with time since disturbance (i.e. long-term uninterrupted succession). No lichen species was strictly dependent on live trees as a habitat, but numerous species showed specificity to logs, standing objects, or admixture of tree species.
Main conclusions
Lichen species richness was lower in regenerating, young, and uniform plots compared to overmature and recently disturbed areas. Natural forest dynamics and its legacies are critical to the diversity and species composition of lichens. Spatiotemporal consequences of natural dynamics require a sufficient area of protected forests for provisioning continual habitat variability at the landscape scale. Ongoing climatic changes may further accentuate this necessity. Hence, we highlighted the need to protect the last remaining primary forests to ensure the survival of regionally unique species pools of lichens.
Background
California in the year 2020 experienced a record breaking number of large fires. Here, we place this and other recent years in a historical context by examining records of large fire events in the state back to 1860. Since drought is commonly associated with large fire events, we investigated the relationship of large fire events to droughts over this 160 years period.
Results
This study shows that extreme fire events such as seen in 2020 are not unknown historically, and what stands out as distinctly new is the increased number of large fires (defined here as > 10,000 ha) in the last couple years, most prominently in 2020. Nevertheless, there have been other periods with even greater numbers of large fires, e.g., 1929 had the second greatest number of large fires. In fact, the 1920’s decade stands out as one with many large fires.
Conclusions
In the last decade, there have been several years with exceptionally large fires. Earlier records show fires of similar size in the nineteenth and early twentieth century. Lengthy droughts, as measured by the Palmer Drought Severity Index (PDSI), were associated with the peaks in large fires in both the 1920s and the early twenty-first century.
Changing climate and disturbance regimes are increasingly challenging the resilience of forest ecosystems around the globe. A powerful indicator for the loss of resilience is regeneration failure, that is, the inability of the prevailing tree species to regenerate after disturbance. Regeneration failure can result from the interplay among disturbance changes (e.g., larger and more frequent fires), altered climate conditions (e.g., increased drought), and functional traits (e.g., method of seed dispersal). This complexity makes projections of regeneration failure challenging. Here we applied a novel simulation approach assimilating data-driven fire projections with vegetation responses from process modeling by means of deep neural networks. We (i) quantified the future probability of regeneration failure; (ii) identified spatial hotspots of regeneration failure; and (iii) assessed how current forest types differ in their ability to regenerate under future climate and fire. We focused on the Greater Yellowstone Ecosystem (2.9 × 106 ha of forest) in the Rocky Mountains of the USA, which has experienced large wildfires in the past and is expected to undergo drastic changes in climate and fire in the future. We simulated four climate scenarios until 2100 at a fine spatial grain (100 m). Both wildfire activity and unstocked forest area increased substantially throughout the 21st century in all simulated scenarios. By 2100, between 28% and 59% of the forested area failed to regenerate, indicating considerable loss of resilience. Areas disproportionally at risk occurred where fires are not constrained by topography and in valleys aligned with predominant winds. High-elevation forest types not adapted to fire (i.e., Picea engelmannii-Abies lasiocarpa as well as non-serotinous Pinus contorta var. latifolia forests) were especially vulnerable to regeneration failure. We conclude that changing climate and fire could exceed the resilience of forests in a substantial portion of Greater Yellowstone, with profound implications for carbon, biodiversity, and recreation.
The report (virgin forests at the heart of Europe) provides an overview of the distribution and situation of the last remaining large-scale virgin forests in Central Europe, with a particular focus on Romania. Most people usually associate images of destruction of forests with tropical rain forests. But this also takes place right here on our doorsteps. We in Europe share a global responsibility to protect our unique, irreplaceable natural heritage. These Carpathian forests are some of the last remaining wildernesses, and a precious archive of biodiversity, history, of impressive images and beauty. As consumers, processors and sellers of timber and wood-based products we all have responsibility to stop the pressures placed on these forests, and have the duty to protect this natural heritage for future generations.
Wind is the leading disturbance agent in European forests, and the magnitude of wind impacts on forest mortality has increased over recent decades. However, the atmospheric triggers behind severe winds in Western Europe (large‐scale cyclones) differ from those in Southeastern Europe (small‐scale convective instability). This geographic difference in wind drivers alters the spatial scale of resulting disturbances and potentially the sensitivity to climate change. Over the 20th century, the severity and prevalence of cyclone‐induced windstorms have increased while the prevalence of atmospheric instability has decreased and thus, the trajectory of Europe‐wide windthrow remains uncertain. To better predict forest sensitivity and trends of windthrow disturbance we used dendrochronological methods to reconstruct 140 years of disturbance history in beech‐dominated primary forests of Central and Eastern Europe. We compared generalized linear mixed models of these disturbance time series to determine whether large‐scale cyclones or small‐scale convective storms were more responsible for disturbance severity while also accounting for topography and stand character variables likely to influence windthrow susceptibility. More exposed forests, forests with a longer absence of disturbance, and forests lacking recent high severity disturbance showed increased sensitivity to both wind drivers. Large‐scale cyclone‐induced windstorms were the main driver of disturbance severity at both the plot and stand scale (0.1–∼100 ha) whereas convective instability effects were more localized (0.1 ha). Though the prevalence and severity of cyclone‐induced windstorms have increased over the 20 century, primary beech forests did not display an increase in the severity of windthrow observed over the same period.
The relative importance of environmental factors and species pool in natural regeneration processes is still intensively disputed. Is environmental filtering especially important for species with higher requirements for temperature and soil fertility? Is the seed source limitation especially pronounced among tree species with lower dispersal abilities? Trees, seedlings and saplings measurements were conducted in 600 plots of 0.05 ha distributed in a regular grid in the Tatra Mountains (elevation range 817-1797 m a.s.l.). Boosted regression trees were used to analyse the relationships between the numbers of young trees, factors acting as environmental filters and the potential seed source availability. The most important factor affecting the distribution of young trees of most species was elevation; bedrock type was the second most important factor for Acer pseudoplatanus, while potential solar radiation was important for Sorbus aucuparia. The relationship between the presence of mature trees and abundance of young ones was strong in Fagus sylvatica, moderately strong in A. pseudoplatanus, and weak in Picea abies, Abies alba and S. aucuparia. The role of environmental factors in shaping the abundance of young trees varies strongly among species, while the seed source availability is very important for large-seeded trees.
Extreme fire seasons characterised by very large 'mega-fires' have demonstrably increased area burnt across forested regions globally. However, the effect of extreme fire seasons on fire severity, a measure of fire impacts on ecosystems, remains unclear. Very large wildfires burnt an unprecedented area of temperate forest, woodland and shrubland across south-eastern Australia in 2019/2020, providing an opportunity to examine the impact of extreme fires on fire severity patterns. We developed an atlas of wildfire severity across south-eastern Australia between 1988 and 2020 to test (a) whether the 2019/2020 fire season was more severe than previous fire seasons, and (b) if the proportion of high-severity fire within the burn extent (HSp) increases with wildfire size and annual area burnt. We demonstrate that the 2019/2020 wildfires in south-eastern Australia were generally greater in extent but not proportionally more severe than previous fires, owing to constant scaling between HSp and annual fire extent across the dominant dry-forest communities. However, HSp did increase with increasing annual fire extent across wet-forests and the less-common rainforest and woodland communities. The absolute area of high-severity fire in 2019/2020 (∼1.8 M ha) was larger than previously seen, accounting for ∼44% of the area burnt by high-severity fire over the past 33 years. Our results demonstrate that extreme fire seasons are a rare but defining feature of fire regimes across forested regions, owing to the disproportionate influence of mega-fires on area burnt.
The majority of area burned by wildfire are located in Siberia. Mainly low-intensity surface fires occur in larch forests, whereas in evergreen forests both surface and crown fires are observed. Warming has led to an increase in the frequency and area of wildfires that have reached the Arctic Ocean shore. However, wildfires are the most important factor in taiga dynamics; larch and Scots pine have evolved under conditions of periodic forest fires, thereby gaining a competitive advantage over non-fire adapted species; in the permafrost zone, periodic fires are a prerequisite for the dominance of larch. Wildfires support ecosystem health, biodiversity, and conservation; periodic wildfires decrease the danger of catastrophic wildfires. With an amplified rate of increase in fires, it is necessary to focus fire suppression on areas of high social, natural, and economic value, while allowing a greater number of wildfires to burn in the vast Siberian forest landscapes.
Research Highlights: Past disturbances occurred naturally in primary forests in the Southern Carpathians. High-and moderate-severity disturbances shaped the present structure of these ecosystems, which regenerated successfully without forestry interventions. Background and Objectives: Windstorms and bark beetle outbreaks have recently affected large forest areas across the globe, causing concerns that these disturbances lie outside the range of natural variability of forest ecosystems. This often led to salvage logging inside protected areas, one of the main reasons for primary forest loss in Eastern Europe. Although more than two-thirds of temperate primary forests in Europe are located in the Carpathian region of Eastern Europe, knowledge about how natural disturbances shape the forest dynamics in this region is highly essential for future management decisions. Material and Methods: We established our study in a primary forest valley situated in the centre of the largest temperate primary forest landscape in Europe (Făgăras , Mountains). A dendrochronological investigation was carried out to reconstruct the natural disturbance history and relate it to the present forest structure. Results: The dendrochronological analysis revealed high temporal variability in the disturbance patterns both at the patch and stand level. Moderate severity disturbance events were most common (20-40% of canopy disturbed in 60% of the plots) but high severity events did also occur (33% of the plots). Regeneration was spruce-dominated and 71% of the seedlings were found on deadwood microsites. Conclusions: We conclude that the current structure of the studied area is a consequence of the past moderate-severity disturbances and sporadic high-severity events. The peak in disturbances (1880-1910) followed by reduced disturbance rates may contribute to a recent and future increase in disturbances in the Făgăras , Mts. Our findings show that these disturbance types are within the range of natural variability of mountain spruce forests in the Southern Carpathians and should not be a reason for salvage logging in primary forests from this area.
Despite growing evidence that diverse forests play an important role in ecosystem functioning, ensuring the provision of different ecosystem services, whether such diversity improves their response to drought events remains unclear. In this study, we use a large tree-ring database from thirty case studies across nine European countries and eleven species, covering from Mediterranean to hemiboreal forests, to test if the growth response to site specific drought events that occurred between 1975 and 2015 varied between mixed and monospecific stands. In particular, we quantify how stands resist those specific drought events and recover after them, thus analyzing their resilience. For each drought event and forest stand we calculated resistance, recovery, resilience and relative resilience and we related the variation in these indices between monospecific and mixed stands with type of admixture, tree species identity, site aridity gradient, stand basal area and stand age. We found a large variability among case studies, even for those that share similar species composition and have similar climates. On average, mixed stands showed higher resistance, resilience and relative resilience to drought events than monospecific stands. However, the beneficial effect of mixtures could not be generalized, being greatly modulated by the type of admixture and tree species identity, and depending on site water supply and stand characteristics , such as basal area and age. The increase in resilience in mixtures compared with monocultures was greater on the conifer-broadleaved admixtures, and to a lesser extent in the broadleaved-broadleaved combinations. The observed response patterns to drought largely varied among the eleven studied species, thus
Disturbances have shaped most terrestrial ecosystems for millennia and are natural and essential components of ecological systems. However, direct and indirect human activities during the Anthropocene have amplified disturbances globally. This amplification, coupled with increasingly unfavorable post-disturbance climatic conditions or ecosystem management that intensifies the initial disturbance, is compromising the
resilience of some ecosystems, with cascading effects on Earth system function and ecosystem services. Such dynamics are especially prevalent in forests, which are one of the most important ecosystems on Earth and provide countless ecosystem services for people and nonhuman species. Although climate change and its effects are ubiquitous, they do vary spatially in their intensity, and many ecological systems are more affected
by changing land use than by changing climate. Understanding the geographic variation in relationships and feedbacks among climate, vegetation, disturbances, regeneration, and human activity is necessary for developing management strategies that will promote forest resilience (i.e., facilitate ecosystems tolerating and recovering from novel or intensified perturbations without shifting to alternative states controlled by different processes). Successful management strategies will vary geographically depending on the degree of departure from the ecological dynamics that preceded the Anthropocene and the spatial variability in drivers of change. As global environmental change accelerates, conservation areas, and the species and ecosystem services that rely on them, are particularly vulnerable. Where disturbances increase, expanding the size of protected areas and minimizing secondary anthropogenic disturbances are likely to be the only ways to maintain the minimum dynamic area that will cultivate adequate resilience. Key Words: Anthropocene, disturbances, ecology, ecosystem management, forest resilience.
The combination of drought and heat affects forest ecosystems by deteriorating the health of trees, which can lead to large‐scale die‐offs with consequences on biodiversity, the carbon cycle, and wood production. It is thus crucial to understand how drought events affect tree health and which factors determine forest susceptibility and resilience. We analyze the response of Central European forests to the 2018 summer drought with 10 × 10 m satellite observations. By associating time‐series statistics of the Normalized Difference Vegetation Index (NDVI) with visually classified observations of early wilting, we show that the drought led to early leaf‐shedding across 21,500 ± 2,800 km2, in particular in central and eastern Germany and in the Czech Republic. High temperatures and low precipitation, especially in August, mostly explained these large‐scale patterns, with small‐ to medium‐sized trees, steep slopes, and shallow soils being important regional risk factors. Early wilting revealed a lasting impact on forest productivity, with affected trees showing reduced greenness in the following spring. Our approach reliably detects early wilting at the resolution of large individual crowns and links it to key environmental drivers. It provides a sound basis to monitor and forecast early‐wilting responses that may follow the droughts of the coming decades.
Frequent Amazonian fires over the last decade have raised the alarm about the fate of the Earth’s most biodiverse forest. The increased fire frequency has been attributed to altered hydrological cycles. However, observations over the past few decades have demonstrated hydrological changes that may have opposing impacts on fire, including higher basin‐wide precipitation and increased drought frequency and severity. Here, we use multiple satellite observations and climate reanalysis datasets to demonstrate compelling evidence of increased fire susceptibility in response to climate regime shifts across Amazonia. We show that accumulated forest loss since 2000 warmed and dried the lower atmosphere, which reduced moisture recycling and resulted in increased drought extent and severity, and subsequent fire. Extremely dry and wet events accompanied with hot days have been more frequent in Amazonia due to climate shift and forest loss. Simultaneously, intensified water vapor transport from the tropical Pacific and Atlantic increased high‐altitude atmospheric humidity and heavy rainfall events, but those events did not alleviate severe and long‐lasting droughts. Amazonia fire risk is most significant in the southeastern region where tropical savannas undergo long seasonally dry periods. We also find that fires have been expanding through the wet‐dry transition season and northward to savanna‐forest transition and tropical seasonal forest regions in response to increased forest loss at the “Arc of Deforestation”. Tropical forests, which have adapted to historically moist conditions, are less resilient and easily tip into an alternative state. Our results imply forest conservation and fire protection options to reduce the stress from positive feedback between forest loss, climate change, and fire.
Data recovery and climate reconstruction are an important support for climate change research, as they provide information from periods and areas with sparse meteorological networks. Various sources are currently in use for obtaining valuable evidence of past climate, such as ship logs, diaries, books, monastery documents. This study exploits newspaper reports in order to enrich the historical meteorological information over the territory of Romania, from the last two decades of the 19th century. At that time, the area belonged to the Austro‐Hungarian Empire and Kingdom of Romania, and the meteorological services from Budapest (since 1870) and Bucharest (since 1884) were the providers of official meteorological information. The digital archive of three newspapers (România Liberă, Gazeta de Transilvania and Foaia Poporului) was investigated and the meteorological information was extracted and aggregated into a database containing 2,132 unique entries clearly referenced in time and space. Each entry represents a meteorological event and several associated characteristics, such as date, location, impact and source. A verification procedure, consisting of comparisons with available measurements from the nearby weather stations and with a reanalysis dataset, was applied in order to validate the entries. The results show that the meteorological information was often present in the newspapers of the epoch. Some climatic features could be retrieved such as seasonality of extreme events, temperature and precipitation characteristics. This paper demonstrates the potential of the collected information to further enhance the understanding of climate change, impacts and climate perception at the end of the 19th century in Romania.
Estimates of historical disturbance patterns are essential to guide forest management aimed at ensuring the sustainability of ecosystem functions and biodiversity. However, quantitative estimates of various disturbance characteristics required in management applications are rare in longer‐term historical studies. Thus, our objectives were to (1) quantify past disturbance severity, patch size, and stand proportion disturbed and (2) test for temporal and subregional differences in these characteristics. We developed a comprehensive dendrochronological method to evaluate an approximately two‐century‐long disturbance record in the remaining Central and Eastern European primary mountain spruce forests, where wind and bark beetles are the predominant disturbance agents. We used an unprecedented large‐scale nested design data set of 541 plots located within 44 stands and 6 subregions. To quantify individual disturbance events, we used tree‐ring proxies, which were aggregated at plot and stand levels by smoothing and detecting peaks in their distributions. The spatial aggregation of disturbance events was used to estimate patch sizes. Data exhibited continuous gradients from low‐ to high‐severity and small‐ to large‐size disturbance events. In addition to the importance of small disturbance events, moderate‐scale (25–75% of the stand disturbed, >10 ha patch size) and moderate‐severity (25–75% of canopy disturbed) events were also common. Moderate disturbances represented more than 50% of the total disturbed area and their rotation periods ranged from one to several hundred years, which is within the lifespan of local tree species. Disturbance severities differed among subregions, whereas the stand proportion disturbed varied significantly over time. This indicates partially independent variations among disturbance characteristics. Our quantitative estimates of disturbance severity, patch size, stand proportion disturbed, and associated rotation periods provide rigorous baseline data for future ecological research, decisions within biodiversity conservation, and silviculture intended to maintain native biodiversity and ecosystem functions. These results highlight a need for sufficiently large and adequately connected networks of strict reserves, more complex silvicultural treatments that emulate the natural disturbance spectrum in harvest rotation times, sizes, and intensities, and higher levels of tree and structural legacy retention.
The expected future intensification of forest disturbance as a consequence of ongoing anthropogenic climate change highlights the urgent need to more robustly quantify associated biotic responses. Saproxylic beetles are a diverse group of forest invertebrates representing a major component of biodiversity that is associated with the decomposition and cycling of wood nutrients and carbon in forest ecosystems. Disturbance-induced declines or shifts in their diversity indicate the loss of key ecological and/or morphological species traits that could change ecosystem functioning. Functional and phylogenetic diversity of biological communities is commonly used to link species communities to ecosystem functions. However, our knowledge on how disturbance intensity alters functional and phylogenetic diversity of saproxylic beetles is incomplete. Here, we analyzed the main drivers of saproxylic beetle abundance and diversity using a comprehensive dataset from montane primary forests in Europe. We investigated cascading relationships between 250 years of historical disturbance mechanisms, forest structural attributes and the taxonomic, phylogenetic and functional diversity of present-day beetle communities. Our analyses revealed that historical disturbances have significant effects on current beetle communities. Contrary to our expectations, different aspects of beetle communities, that is, abundance, taxonomic, phylogenetic and functional diversity, responded to different disturbance regime components. Past disturbance frequency was the most important component influencing saproxylic beetle communities and habitat via multiple temporal and spatial pathways. The quantity of deadwood and its diameter positively influenced saproxylic beetle abundance and functional diversity, whereas phylogenetic diversity was positively influenced by canopy openness. Analyzing historical disturbances, we observed that current beetle diversity is far from static, such that the importance of various drivers might change during further successional development. Only forest landscapes that are large enough to allow for the full range of temporal and spatial patterns of disturbances and post-disturbance development will enable long-term species coexistence and their associated ecosystem functions.
The emergence and spread of non-native invasive forest insects represent a major potential threat to global biodiversity. The present study examines the current invasion of the far eastern four-eyed fir bark beetle Polygraphus proximus Blandf. in southern Siberian fir (Abies sibirica Ledeb.) forests. We collected data on 38 large sized (2500 m²) sample plots, situated in fir forests of the Tomsk region. As a direct result of the four-eyed fir bark beetle infestation, stand density decreased by 34-37%, and stand volume by 30%. The mean height, individual age and diameter at the stand level consequently increased. Our results indicated that stands with complete left-sided or normal ontogenetic structure (composed primarily of late virginal firs or firs in young reproductive stage) are more resistant to invasion by the four-eyed fir bark beetle. By contrast, fir forests characterized by more right-sided ontogenetic structure (composed primarily of mature and old reproductive firs), exhibited the least resistance and, with rare exception, degraded rapidly in response to the invasion. Our results also pointed to a mechanism that initiates invasions of the four-eyed fir bark beetle in fir stands of all types of ontogenetic structure, which is the attack of virginal trees and trees in early reproductive stages. Trees up to average diameter are the most susceptible to invasions of the bark beetle. We identified thicker bark, larger DBH and low occurrence of heart rot as the most important parameters for indicating resistance at the single tree level. DBH and bark thickness (p<0.05) correlated significantly with tree health status in infested stands. Our overall assessment of the potential natural regeneration of damaged stands is that the Siberian fir forests are resilient to invasive species and that the fir ecosystems can potentially recover from this disturbance.
Europe’s forests have provided human societies with essential ecosystem services and great economic values for centuries. Some of these values are now increasingly threatened by climate change, which greatly intensifies forest disturbances such as
bark beetle outbreaks. However, some past management practices have also increased the vulnerability of Europe’s forests. For example, due to its good growth performance and favourable properties for forest industry, Norway spruce has been planted extensively in Europe over the past century, including in areas outside its native range. This has created large areas of so-called secondary forests, which have increasingly exhibited problems with health and vitality and are prone to various disturbances: the most important being wind, drought and bark beetles.
This report aims to help European and national policy makers understand the complex roles bark beetles play in our forests, and provide the scientific basis for robust forest policies and management options to address these emerging bark beetle problems.
Aim
Primary forests have high conservation value but are rare in Europe due to historic land use. Yet many primary forest patches remain unmapped, and it is unclear to what extent they are effectively protected. Our aim was to (1) compile the most comprehensive European‐scale map of currently known primary forests, (2) analyse the spatial determinants characterizing their location and (3) locate areas where so far unmapped primary forests likely occur.
Location
Europe.
Methods
We aggregated data from a literature review, online questionnaires and 32 datasets of primary forests. We used boosted regression trees to explore which biophysical, socio‐economic and forest‐related variables explain the current distribution of primary forests. Finally, we predicted and mapped the relative likelihood of primary forest occurrence at a 1‐km resolution across Europe.
Results
Data on primary forests were frequently incomplete or inconsistent among countries. Known primary forests covered 1.4 Mha in 32 countries (0.7% of Europe’s forest area). Most of these forests were protected (89%), but only 46% of them strictly. Primary forests mostly occurred in mountain and boreal areas and were unevenly distributed across countries, biogeographical regions and forest types. Unmapped primary forests likely occur in the least accessible and populated areas, where forests cover a greater share of land, but wood demand historically has been low.
Main conclusions
Despite their outstanding conservation value, primary forests are rare and their current distribution is the result of centuries of land use and forest management. The conservation outlook for primary forests is uncertain as many are not strictly protected and most are small and fragmented, making them prone to extinction debt and human disturbance. Predicting where unmapped primary forests likely occur could guide conservation efforts, especially in Eastern Europe where large areas of primary forest still exist but are being lost at an alarming pace.
The spatial structure of trees is a template for forest dynamics and the outcome of a variety of processes in ecosystems. Identifying the contribution and magnitude of the different drivers is an age-old task in plant ecology. Recently, the modelling of a spatial point process was used to identify factors driving the spatial distribution of trees at stand scales. Processes driving the coexistence of trees, however, frequently unfold within gaps and questions on the role of resource heterogeneity within-gaps have become central issues in community ecology. We tested the applicability of a spatial point process modelling approach for quantifying the effects of seed dispersal, within gap light environment, microsite heterogeneity, and competition on the generation of within gap spatial structure of small tree seedlings in a temperate, old growth, mixed-species forest. By fitting a non-homogeneous Neyman-Scott point process model, we could disentangle the role of seed dispersal from niche partitioning for within gap tree establishment and did not detect seed densities as a factor explaining the clustering of small trees. We found only a very weak indication for partitioning of within gap light among the three species and detected a clear niche segregation of Picea abies (L.) Karst. on nurse logs. The other two dominating species, Abies alba Mill. and Fagus sylvatica L., did not show signs of within gap segregation.
As the terrestrial human footprint continues to expand, the amount of native forest that is free from significant damaging human activities is in precipitous decline. There is emerging evidence that the remaining intact forest supports an exceptional confluence of globally significant environmental values relative to degraded forests, including imperilled biodiversity, carbon sequestration and storage, water provision, indigenous culture and the maintenance of human health. Here we argue that maintaining and, where possible, restoring the integrity of dwindling intact forests is an urgent priority for current global efforts to halt the ongoing biodiversity crisis, slow rapid climate change and achieve sustainability goals. Retaining the integrity of intact forest ecosystems should be a central component of proactive global and national environmental strategies, alongside current efforts aimed at halting deforestation and promoting reforestation.
Determining the drivers of shifting forest disturbance rates remains a pressing global change issue. Large-scale forest dynamics are commonly assumed to be climate driven, but appropriately scaled disturbance histories are rarely available to assess how disturbance legacies alter subsequent disturbance rates and the climate sensitivity of disturbance. We compiled multiple tree-ring based disturbance histories from primary Picea abies forest fragments distributed throughout five European landscapes spanning the Bohemian Forest and the Carpathian Mountains. The regional chronology includes 11 595 tree cores, with ring dates spanning the years 1750 to 2000, collected from 560 inventory plots in 37 stands distributed across a 1000 km geographic gradient, amounting to the largest disturbance chronology yet constructed in Europe. Decadal disturbance rates varied significantly through time and declined after 1920, resulting in widespread increases in canopy tree age. Approximately 75% of current canopy area recruited prior to 1900. Long-term disturbance patterns were compared to an historical drought reconstruction, and further linked to spatial variation in stand structure and contemporary disturbance patterns derived from LANDSAT imagery. Historically, decadal Palmer drought severity index minima corresponded with higher rates of canopy removal. The severity of contemporary disturbances increased with each stand's estimated time since last major disturbance, increased with mean diameter and declined with increasing within-stand structural variability. Reconstructed spatial patterns suggest that high small-scale structural variability has historically acted to reduce large-scale susceptibility and climate sensitivity of disturbance. Reduced disturbance rates since 1920, a potential legacy of high 19th century disturbance rates, have contributed to a recent region-wide increase in disturbance susceptibility. Increasingly common high-severity disturbances throughout primary Picea forests of Central Europe should be reinterpreted in light of both legacy effects (resulting in increased susceptibility) and climate change (resulting in increased exposure to extreme events). This article is protected by copyright. All rights reserved.
Forests are frequently exposed to natural disturbances, which are likely to increase with global change, and may jeopardize the delivery of ecosystem services. Mixed-species forests have often been shown to be more productive than monocultures, but it is unclear whether this results from mixed stands being in part more resistant to various biotic and abiotic disturbance factors. This review investigates the relationships between tree diversity and stand resistance to natural disturbances and explores the ecological mechanisms behind the observed relationships.
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Understanding how species-specific disturbances affect the dynamics of mixed forests is becoming increasingly important due to rapidly changing disturbance regimes. This study estimated the effect of Norway spruce (Picea abies (L.) Karst.) mortality on the disturbance processes in two mixed beech stands of the Western Carpathians that were affected by a bark beetle outbreak. We evaluated the size distribution, fraction of canopy and expanded gaps, the characteristics of gapmakers and the contribution of different species to gap size. The measured canopy gap fraction was <5%, and most canopy gaps were small (<100 m2). Spruce was the most abundant gapmaker, and its share among gapmakers was 3–6 times higher than its share in the canopy. We found that the increase in spruce mortality due to the outbreak resulted in a fine-scale mortality pattern. However, spruce gapmakers did not contribute much to gap area size, as shown by a weak correlation between the number of spruce gapmakers and the area of expanded gaps. Diameter distribution of living versus recently dead trees showed that beech mortality occurred disproportionately in large size classes. However, dead spruce trees were equally frequent in all diameter classes, which means beetles did not exclusively attack larger trees in these stands during the outbreak. We conclude that spruce mortality may have influenced successional processes by giving a competitive advantage to two other species that were not affected by the outbreak, provided that a high deer browsing intensity does not hinder the regeneration of new seedlings.
We created a new dataset of spatially interpolated monthly climate data for global land areas at a very high spatial resolution (approximately 1 km 2). We included monthly temperature (minimum, maximum and average), precipitation, solar radiation, vapour pressure and wind speed, aggregated across a target temporal range of 1970–2000, using data from between 9000 and 60 000 weather stations. Weather station data were interpolated using thin-plate splines with covariates including elevation, distance to the coast and three satellite-derived covariates: maximum and minimum land surface temperature as well as cloud cover, obtained with the MODIS satellite platform. Interpolation was done for 23 regions of varying size depending on station density. Satellite data improved prediction accuracy for temperature variables 5–15% (0.07–0.17 ∘ C), particularly for areas with a low station density, although prediction error remained high in such regions for all climate variables. Contributions of satellite covariates were mostly negligible for the other variables, although their importance varied by region. In contrast to the common approach to use a single model formulation for the entire world, we constructed the final product by selecting the best performing model for each region and variable. Global cross-validation correlations were ≥ 0.99 for temperature and humidity, 0.86 for precipitation and 0.76 for wind speed. The fact that most of our climate surface estimates were only marginally improved by use of satellite covariates highlights the importance having a dense, high-quality network of climate station data.
Natural disturbances strongly influence forest structural dynamics, and subsequently stand structural heterogeneity, biomass, and forest functioning. The impact of disturbance legacies on current forest structure can greatly influence how we interpret drivers of forest dynamics. However, without clear insight into forest history, many studies default to coarse assumptions about forest structure, for example, whether forests are even or unevenly aged. The aim of this study was to analyze the effects of past disturbances on the current diameter distributions of Norway spruce (Picea abies (L.) Karst.)-dominated landscapes throughout the Carpathian Mountains. Our dendroecological dataset comprises tree cores from 339 plots (7,845 total tree cores), nested within 28 primary forest stands, known to vary greatly in the severity of historical disturbances. Our analyses revealed that historical disturbances had a strong and significant effect on the current diameter distribution shapes at the plot level. We demonstrated that mixed-severity disturbance regimes were more frequent and create a complex pattern of diameter distributions at the plot and stand scale. Here, we show that high severity disturbance was associated with unimodal diameter distributions, while low and moderate severity was associated with the reverse J-shaped distribution. This is a result of complex disturbance patterns, with structural biological legacies. Our results will have important management implication in the context of tree size heterogeneity, biomass storage, and productivity as influenced by natural disturbances. Lastly, these results demonstrate that structural changes may arise as consequences of changing disturbance regime associated with global change.
Wildfire risk related to hazards on people and assets is expected to increase in the face of climate change, especially in fire-prone environments such as the Mediterranean Basin. Distinguishing rationalities, i.e., the complex profile of multi-thematic, wildfire-related perceptions that collectively characterize and quantify all of a society’s responses, its interrelations, and influence on its insights, are of primary importance to understand the degree of preparedness and the direction that wildfire management policies are moving. Greece is a country that suffered mega-wildfire events during the first years of the twenty-first century. This paper presents a scheme of advanced multivariate statistical procedures applied on standard social survey questionnaires to uncover different or similar rationalities between fire management services and the general public. Profession-centered versus message-oriented rationalities is defined. They differ mainly on the priorities attributed to strengthening personnel and equipment capacities versus the need for public education and awareness. Both are evaluated against the needs of long-term risk assessment and forest management policies in Greece. The main conclusion is that Greek society, although traumatized by recent fire disasters, is not yet prepared for long-term strategic forestry adaptation and planning.
Outbreaks of tree-killing insects are intensifying globally, affecting economies, human well-being, and driving ecosystem transitions. The Czech Republic has recently become Europe’s epicenter of the outbreak of spruce bark beetle Ips typographus, the most aggressive species in Eurasia. We investigated a countrywide outbreak dynamic during the period 2003–2019, with a special focus on the period 2017–2019 when the outbreak reached an unprecedented intensity. In order to identify main outbreak drivers, we investigated annual time series of the volume of trees killed by bark beetles in the Czech districts (n = 77), and a suite of climatic and forest structure-related predictors using Generalized Additive Models. Finally, we reviewed a large body of public materials to understand broader social, ecological, and economic implications of the outbreak.
We found that bark beetles were damaging 0.2–1.4% of Norway spruce growing stock annually across the Czech Republic in the period 2003–2016. This level increased to 3.1–5.4% in 2017–2019, causing the total depletion of spruce in some regions. The long-term bark beetle dynamics (2003–2019) was driven by the combination of wind disturbance and climatic drivers, represented in our study by annual temperature anomaly and Standardized Precipitation-Evapotranspiration Index. However, the effect of wind was diminished during the period 2017–2019, whereas the effect of drought dominated. Our findings thus suggest a transition from wind- to drought-driven bark beetle dynamics.
The outbreak and subsequent large-scale salvaging and wood transportation affected quality of life of people in a broad vicinity of outbreak areas. Extensive management actions aggravated some of the notorious conflicts between forest management and nature conservation, and highlighted the poor harmonization of respective policies. A decrease in timber price, an excessive workload, and other cascading effects caused severe revenue loss, requiring state interventions amounting to ca 260 million EUR in 2018–2019.
We suggest that increasing frequency of climate extremes in combination with the unfavorable forest structure pushed Central European spruce forests to the margin of their ecological space and unfolded large-scale forest transformations. Effective responses will require fundamental structural changes in the regional forest-based sector, particularly aiming at increased social and ecological resilience.
European beech (Fagus sylvatica) is the dominant tree species of Central Europe’s natural forests and one of the continent’s most important timber species. This highly competitive species is known to be drought-sensitive and thus may increasingly be threatened by climate change-related heat waves and drought in part of its distribution range. Tree responses and tolerance to drought are complex processes that are best understood by adopting a multi-level analysis spanning from the molecular physiological to the tree and stand levels. Here I review recent progress in our understanding of beech drought responses in all relevant fields of research, notably adjustments in cellular biochemistry, responses of photosynthesis, stomatal regulation and leaf water status, the vulnerability of the hydraulic and phloem systems, adaptive responses of the fine root system, leaf area adjustments, long-term declines in radial growth, and drought-induced tree mortality. A special focus is on population differences in drought tolerance as the basis for the selection of drought-hardier provenances. The evidence from the different research fields is brought together with the aim to develop a multi-disciplinary perception of beech drought response, to identify weak components in the species’ drought response strategy, and to suggest future research efforts to close knowledge gaps.
Understanding the processes shaping the composition of assemblages in response to disturbance events is crucial for preventing ongoing biodiversity loss in forest ecosystems. However, studies of forest biodiversity responses to disturbance typically analyze immediate or short-term impacts only, while studies relating long-term disturbance history to biodiversity assemblage dynamics are rare. To address this important knowledge gap, we used a dendroecological approach to link natural disturbance history of 250 years (1750-2000) to structural habitat elements and, in turn, to breeding bird assemblages. We used data collected in 2017 and 2018 from 58 permanent study plots within 10 primary spruce forest stands distributed across the Western Carpathian Mountains of Europe. This dataset contained breeding bird counts and environmental variables describing forest density, tree diameter distribution, tree height, tree microhabitats, deadwood quantity and quality, and regeneration. Bird assemblages were significantly influenced by forest structure which was in turn shaped by disturbance dynamics (disturbance frequency, time since the last disturbance and its severity). Early successional species associated with more open habitats were positively influenced by disturbance-related structure (i.e. deadwood-related variables, canopy cover), while some species responded negatively. At the same time, overall abundance, species richness and Shannon diversity of the bird assemblage remained unchanged under variable disturbance histories. Our results support a view of primary spruce forests as a highly dynamic ecosystem, harbouring populations of bird species at all stages of succession despite significant structural changes and shifting patch mosaics over time due to natural disturbances.
Natural disturbances are key factors in the formation of forest ecosystem structure. Evaluation of the spatial and temporal extent of disturbance regimes is critical for understanding forest dynamics, forest structural hetero-geneity, and biodiversity habitats. Quantifying disturbance regimes is therefore imperative for appropriate management of forests and protected areas. However, natural disturbance regimes have rarely been assessed using dendrochronological methods on a regional scale across primary mixed beech-fir forest stands-one of the dominant forest vegetation types in Europe. To study the natural disturbance regimes of beech-dominated mixed-forest stands, we established 42 permanent study plots with an area of 0.1 ha across three primary forest stands in the Western Carpathians, a region that still contains large areas of primary forest. We reconstructed each stand-level disturbance history using a tree-ring based approach. The temporal synchronicity of disturbance events was then evaluated by delineating stand-level disturbance events using a kernel density function, and through the detection of plot-level disturbances with severities greater than 10 percent. The results obtained from the chronologies showed substantial variability in time and space, especially in the mid-19th century. Low-and moderate-severity plot-level disturbance events were most common, but high-and extremely high-severity plot-level disturbance events also occurred. The observed spatial and temporal variability suggests that the beech-dominated forests were primarily driven by mixed-severity disturbance regimes, with windstorms as the main disturbance agent. This reconstruction of the disturbance regime provided unique insight into the scale of mortality processes in these beech-dominated mixed forests. This information can help guide ecological forestry in areas where both wood production and biodiversity preservation are simultaneous goals, such as by employing more spatio-temporally-complex silvicultural systems that resemble natural disturbance patterns and facilitate heterogeneous forest structures.
Most information on the ecology of oak-dominated forests in Europe comes from forests altered for centuries because remnants of old-growth forests are rare. Disturbance and recruitment regimes in old-growth forests provide information on forest dynamics and their effects on long-term carbon storage. In an old-growth Quercus petraea forest in northwestern Spain, we inventoried three plots and extracted cores from 166 live and dead trees across canopy classes (DBH ≥ 5 cm). We reconstructed disturbance dynamics for the last 500 years from tree-ring widths. We also reconstructed past dynamics of above ground biomass (AGB) and recent AGB accumulation rates at stand level using allometric equations. From these data, we present a new tree-ring-based approach to estimate the age of carbon stored in AGB. The oldest tree was at least 568 years, making it the oldest known precisely-dated oak to date and one of the oldest broadleaved trees in the Northern Hemisphere. All plots contained trees over 400 years old. The disturbance regime was dominated by small, frequent releases with just a few more intense disturbances that affected ≤20% of trees. Oak recruitment was variable but rather continuous for 500 years. Carbon turnover times ranged between 153 and 229 years and mean carbon ages between 108 and 167 years. Over 50% of AGB (150 Mg·ha⁻¹) persisted ≥100 years and up to 21% of AGB (77 Mg·ha⁻¹) ≥300 years. Low disturbance rates and low productivity maintained current canopy oak dominance. Absence of management or stand-replacing disturbances over the last 500 years resulted in high forest stability, long carbon turnover times and long mean carbon ages. Observed dynamics and the absence of shade-tolerant species suggest that oak dominance could continue in the future. Our estimations of long-term carbon storage at centennial scales in unmanaged old-growth forests highlights the importance of management and natural disturbances for the global carbon cycle.
Mortality, driven by both climate and disturbance legacies, is a key process shaping forest dynamics. Understanding the mortality patterns in primary forests in the absence of severe disturbances provides information on background natural dynamics of a given forest type under ongoing climate change. This can then be compared to mortality rates in severely-disturbed stands. Using a large number of sample plots along a gradient from low to high disturbance, we examined the mortality rates and composition of mortality agents in primary mountain Norway spruce (Picea abies (L.) Karst.) forests on different spatial scales. We evaluated the mortality rates and causes of mortality in 28 stands across a large geographical gradient spanning over 1000 km. We resampled (five-year period) 371 plots (16,287 living trees) in primary Norway spruce forests along the Carpathian mountain chain. The estimated overall annual mortality rate was within the previously reported range of background (ambient) mortality, however, stand-level and plot-level mortality rates varied substantially. Over 18% of plots displayed more than 2% annual mortality and 6% of plots even exceeded 10% per year. Stands in the Western Carpathians showed the highest variability in the mortality rate, with 30% of the stands in this region showing annual mortality rates over 5%. At the plot level, mixed-severity disturbances increased variability of mortality rates within most localities. Overall mortality was evenly distributed among size classes up to 50 cm diameter at breast height (DBH). However, the distributions differ for individual mortality agents. Mortality modes were classified into six categories (broken crown, broken stem, uprooted, competition, bark beetle/fungi, climatic extremes). Bark beetle (Ips typographus L.) infestation was the most frequent mortality agent in all stands, whereas the influence of competition as a mortality agent varied substantially. Mortality from abiotically-caused physical damage was similar to that from competition, yet the distribution among modes of physical damage (uprooted, crown, or stem breakage) varied. The lack of clear evidence of mortality agents in some locations implies that many tree deaths are caused by a combination of contributing factors. The results suggest the role of bark beetle as a mortality agent does not equate to severe mortality at large scales. Prevalence of different size classes affected by individual mortality agents underline the high complexity of the mortality process in primary forests.
Given the global intensification of forest management and climate change, protecting and studying forests that develop free of direct human intervention-also known as primary forests-are becoming increasingly important. Yet, most countries still lack data regarding primary forest distribution. Previous studies have tested remote sensing approaches as a promising tool for identifying primary forests. However, their precision is highly dependent on data quality and resolution, which vary considerably. This has led to underestimation of primary forest abundance and distribution in some regions, such as the temperate zone of Europe. Field-based inventories of primary forests and methodologies to conduct these assessments are inconsistent; incomplete or inaccurate mapping increases the vulnerability of primary forest systems to continued loss from clearing and land-use change. We developed a comprehensive methodological approach for identifying primary forests, and tested it within one of Europe's hotspots of primary forest abundance: the Carpathian Mountains. From 2009 to 2015, we conducted the first national-scale primary forest census covering the entire 49,036 km 2 area of the Slovak Republic. We analyzed primary forest distribution patterns and the representativeness of potential vegetation types within primary forest remnants. We further evaluated the conservation status and extent of primary forest loss. Remaining primary forests are small, fragmented, and often do not represent the potential natural vegetation. We identified 261 primary forest localities. However, they represent only 0.47% of the total forested area, which is 0.21% of the country's land area. The spatial pattern of primary forests was clustered. Primary forests have tended to escape anthropogenic disturbance on sites with higher elevations, steeper slopes, rugged terrain, and greater distances from roads and settlements. Primary forest stands of montane mixed and subalpine spruce forests are more abundant compared to broadleaved forests. Notably, several habitat types are completely missing within primary forests (e.g., floodplain forests). More than 30% of the remaining primary forests are not strictly protected, and harvesting occurred at 32 primary forest localities within the study period. Almost all logging of primary forests was conducted inside of protected areas, underscoring the critical status of primary forest distribution in this part of Europe. Effective conservation strategies are urgently needed to stop the rapid loss and fragmentation of the remaining primary forests. Our approach based on precise, field-based surveys is widely applicable and transferrable to other fragmented forest landscapes.
The retrospective study of abrupt and sustained increases in the radial growth of trees (hereinafter 'releases') by tree-ring analysis is an approach widely used for reconstructing past forest disturbances. Despite the range of dendrochronological methods used for release-detection, a lack of in-depth comparison between them can lead researchers to question which method to use and, potentially, increases the uncertainties of disturbance histories derived with different methods. Here, we investigate the efficacy and sensitivity of four widely used release detection methods using tree-ring width series and complete long-term inventories of forest stands with known disturbances. We used support vector machine (SVM) analysis trained on long-term forest census data to estimate the likelihood that Acer rubrum trees experiencing reductions in competition show releases in their tree-ring widths. We compare methods performance at the tree and stand level, followed by evaluation of method sensitivity to changes in their parameters and settings. Disturbance detection methods agreed with 60-76% of the SVM-identified growth releases under high canopy disturbance and 80-94% in a forest with canopy disturbance of low severity and frequency. The median competition index change (CIC) of trees identified as being released differed more than twofold between methods, from −0.33 (radial-growth averaging) to −0.68 (time-series). False positives (type I error) were more common in forests with low severity disturbance, whereas false negatives (type II error) occurred more often in forests with high severity disturbance. Sensitivity analysis indicated that reductions of the detection threshold and the length of the time window significantly increased detected stand-level disturbance severity across all methods. Radial-growth averaging and absolute-increase methods had lower levels of type I and II error in detecting disturbance events with our datasets. Parameter settings play a key role in the accuracy of reconstructing disturbance history regardless of the method. Time-series and radial-growth averaging methods require the least amount of a priori information, but only the time-series method quantified the subsequent growth increment related to a reduction in competition. Finally, we recommend yearly binning of releases using a kernel density estimation function to identify local maxima indicating disturbance. Kernel density estimation improves reconstructions of forest history and, thus, will further our understanding of past forest dynamics.
Canopy gaps play a crucial role for forest dynamics processes, as they largely determine light transmission to lower canopy strata, thereby controlling the turnover of tree individuals in the stand. Even though their functional importance is undisputed, quantitative data on the rate of gap creation and gap closure, and the temporal change in gap size distribution patterns in temperate virgin forests are scarce. We used a repeated inventory (line-intercept sampling) of gap size frequency and fraction in a virgin beech (Fagus sylvatica) forest in the Slovakian Carpathians over a 10-year interval (2003–2013) to test the hypotheses that (i) disturbance intensity and thus gap creation and gap closure rate change only little over time, (ii) gaps persist or even expand, until they are filled primarily by vertical ingrowth of trees from lower strata, and (iii) gap creation promotes the height growth of released saplings and sub-canopy trees. In the 2003 and 2013 inventories, 37 and 30 gaps >20 m² size were mapped along a total of 3217 m transect line investigated. The large majority of gaps was <100 m² in size; large gaps >500 m² were very rare. Gap fraction decreased significantly from 13.6% in 2003 to 8.2% in 2013 (associated with a reduction in mean gap size from 261 to 96 m²), indicating considerable variation in disturbance intensity in the past decades. Before 2003, both large gaps (probably caused by wind throw) and small gaps (from dying trees) have been formed, while only small gaps developed in the period 2003–2013. Small gaps were closed within a few years through rapid horizontal canopy expansion of neighboring beech trees, while vertical gap filling through ingrowth of lower canopy layers and regeneration was the dominant process in larger gaps. Saplings and trees in lower canopy layers formed a heterogeneous understory in large parts of recently formed gaps and responded to this process with increased height growth. We conclude that, despite considerable variation in disturbance intensity over time, this beech virgin forest responds to gap formation with high resilience through rapid lateral canopy expansion in small gaps and ingrowth of saplings and sub-dominant tree layers in larger gaps.
Primary forests are characterized by high vertical and horizontal stand diversity, which provides habitat for a diverse range of species with complex habitat requirements. Detailed knowledge of related ecological processes and habitat development of primary forest species are essential to inform forest management and biodiversity conservation decisions, but relationships are not well documented. We collected dendrochronological data and inventoried numerous structural elements in permanent plots throughout the primary temperate forests within the Carpathian Mountains. We fit and compared multiple predictive models to quantify the importance of 200 years of natural disturbance dynamics on the occurrence probability of an umbrella species – the capercaillie (Tetrao urogallus). We showed that a mixed-severity disturbance regime ranging from low through moderate to high severity disturbances is required to generate diverse forest habitats suitable for capercaillie. The variation in natural disturbance severity and its timing promoted key structural habitat elements, such as low natural regeneration density, low mature tree density, high ground vegetation cover, availability of forest gaps, and abundance of standing deadwood. This study demonstrates the importance of natural disturbance in maintaining the variety of conditions necessary to support primary forest specialist species. Managers of protected areas should be mindful that natural disturbances generate habitat for the capercaillie in mountain Norway spruce forests. Further intervention is unnecessary. Conservation planning and forest reserve design should shift focus to the large-scale spatial requirements needed to ensure that a wide range of forest developmental phases are represented in protected areas.
Natural disturbance regimes are changing substantially in forests around the globe. However, large-scale disturbance change is modulated by a considerable spatiotemporal variation within biomes. This variation remains incompletely understood particularly in the temperate forests of Europe, for which consistent large-scale disturbance information is lacking. Here our aim was to quantify the spatiotemporal patterns of forest disturbances across temperate forest landscapes in Europe using remote sensing data, and determine their underlying drivers. Specifically, we tested two hypotheses: (1) Topography determines the spatial patterns of disturbance, and (2) climatic extremes synchronize natural disturbances across the biome. We used novel Landsat-based maps of forest disturbances 1986-2016 in combination with landscape analysis to compare spatial disturbance patterns across five unmanaged forest landscapes with varying topographic complexity. Furthermore, we analyzed annual estimates of disturbance change for synchronies and tested the influence of climatic extremes on temporal disturbance patterns. Spatial variation in disturbance patterns was substantial across temperate forest landscapes. With increasing topographic complexity, natural disturbance patches were smaller, more complex in shape, more dispersed, and affected a smaller portion of the landscape. Temporal disturbance patterns, however, were strongly synchronized across all landscapes, with three distinct waves of high disturbance activity between 1986 and 2016. All three waves followed years of pronounced drought and high peak wind speeds. Natural disturbances in temperate forest landscapes of Europe are thus spatially diverse but temporally synchronized. We conclude that the ecological effect of natural disturbances (i.e., whether they are homogenizing a landscape or increasing its heterogeneity) is strongly determined by the topographic template. Furthermore, as the strong biome-wide synchronization of disturbances was closely linked to climatic extremes, large-scale disturbance episodes are likely in Europe's temperate forests under climate changes. This article is protected by copyright. All rights reserved.
Forest disturbances are sensitive to climate. However, our understanding of disturbance dynamics in response to climatic changes remains incomplete, particularly regarding large-scale patterns, interaction effects and dampening feedbacks. Here we provide a global synthesis of climate change effects on important abiotic (fire, drought, wind, snow and ice) and biotic (insects and pathogens) disturbance agents. Warmer and drier conditions particularly facilitate fire, drought and insect disturbances, while warmer and wetter conditions increase disturbances from wind and pathogens. Widespread interactions between agents are likely to amplify disturbances, while indirect climate effects such as vegetation changes can dampen long-term disturbance sensitivities to climate. Future changes in disturbance are likely to be most pronounced in coniferous forests and the boreal biome. We conclude that both ecosystems and society should be prepared for an increasingly disturbed future of forests.
Clearing up after natural disturbances may not always be beneficial for the environment. We argue that a radical change is needed in the way ecosystems are managed; one that acknowledges the important role of disturbance dynamics.
In order to gauge ongoing and future changes to disturbance regimes, it is necessary to establish a solid baseline of historic disturbance patterns against which to evaluate these changes. Further, understanding how forest structure and composition respond to variation in past disturbances may provide insight into future resilience to climate-driven alterations of disturbance regimes.
Mountain forests are among the most important ecosystems in Europe as they support numerous ecological, hydrological, climatic, social, and economic functions. They are unique relatively natural ecosystems consisting of long-lived species in an otherwise densely populated human landscape. Despite this, centuries of intensive forest management in many of these forests have eclipsed evidence of natural processes, especially the role of disturbances in long-term forest dynamics. Recent trends of land abandonment and establishment of protected forests have coincided with a growing interest in managing forests in more natural states. At the same time, the importance of past disturbances highlighted in an emerging body of literature, and recent increasing disturbances due to climate change are challenging long-held views of dynamics in these ecosystems. Here, we synthesize aspects of this Special Issue on the ecology of mountain forest ecosystems in Europe in the context of broader discussions in the field, to present a new perspective on these ecosystems and their natural disturbance regimes. Most mountain forests in Europe, for which long-term data are available, show a strong and long-term effect of not only human land use but also of natural disturbances that vary by orders of magnitude in size and frequency. Although these disturbances may kill many trees, the forests themselves have not been threatened. The relative importance of natural disturbances, land use, and climate change for ecosystem dynamics varies across space and time. Across the continent, changing climate and land use are altering forest cover, forest structure, tree demography, and natural disturbances, including fires, insect outbreaks, avalanches, and wind disturbances. Projected continued increases in forest area and biomass along with continued warming are likely to further promote forest disturbances. Episodic disturbances may foster ecosystem adaptation to the effects of ongoing and future climatic change. Increasing disturbances, along with trends of less intense land use, will promote further increases in coarse woody debris, with cascading positive effects on biodiversity, edaphic conditions, biogeochemical cycles, and increased heterogeneity across a range of spatial scales. Together, this may translate to disturbance-mediated resilience of forest landscapes and increased biodiversity, as long as climate and disturbance regimes remain within the tolerance of relevant species. Understanding ecological variability, even imperfectly, is integral to anticipating vulnerabilities and promoting ecological resilience, especially under growing uncertainty. Allowing some forests to be shaped by natural processes may be congruent with multiple goals of forest management, even in densely settled and developed countries.