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In recent years, long periods of drought and heat waves have become increasingly frequent, causing forest dieback phenomena that make stands more sensitive to biotic stressors. How trees may respond to extreme climatic events and which metabolites are involved under stress conditions is still not clear. In this study, using Solid Phase Micro-Extraction (SPME)-GC/MS, we analysed how dieback (D) and non-dieback (ND) Hungarian oak trees from the San Paolo Albanese site respond to these climatic dynamics, focusing on volatile organic compounds (VOCs). For each group of trees, three wood samples were taken, and each was divided into four sub-samples with five growth rings and subjected to SPME and increase in basal area (BAI) analysis of the last 20 years. Dieback trees had a lower number of leaves, and this condition may translate into less photosynthesis, less organic matter production, and lower reserves of carbohydrates being available for growth. Indeed, D trees showed lower radial increases and a lower content of aldehydes, terpenes, and fatty acids than ND trees, indicating a better health of ND trees compared to D trees. Meanwhile, D trees showed a reduction in terpenes, such as α-pinene, γ-eudesmol, and cyperene (with significant insecticidal activity), a reduction in aromatic aldehydes, such as furfural and 5-methylfurfural, and an increase in silanols (with antimicrobial function). Considering the different compounds' contents between D and ND trees, our study could be useful for detecting bio-indicators to identify an early warning signal of dieback phenomena. [ABSTRACT FROM AUTHOR]

Subalpine forests worldwide face the synergistic threats of global warming and increased biotic attack, and the collapse or transition of subalpine forests is predicted in south‐eastern Australia under future climates. The recent widespread dieback of subalpine snow‐gum forests due to increased activity of a native wood‐boring longicorn beetle, Phoracantha mastersii, suggests this process may already be underway. We investigated how variation in tree tissue traits and environmental conditions correlated with elevation‐dependent spatial patterns of forest mortality. We hypothesized that increased vulnerability of subalpine snow gums to wood‐borer‐mediated dieback at intermediate elevations was associated with poorly resolved differences in traits between montane (Eucalyptus pauciflora subsp. pauciflora) and subalpine (E. pauciflora subsp. niphophila) snow‐gum subspecies. We first sought to characterize variation and elevation‐dependent transitions in 20 structural and drought‐related functional traits among 120 healthy trees distributed along a 1000 m elevation transect that spanned the subspecies transition zone. Secondly, we surveyed 774 trees across 53 sites between 1280 and 1980 m a.s.l. to explore associations between borer‐damage severity, elevation, subspecies and a subset of traits that differed between subspecies. We observed evidence for both continuous trait variation in response to changing elevation (10/20 traits) and discrete shifts in mean trait values across the transition between subspecies distributions (5/20 traits). Increased borer‐damage severity across the montane‐to‐subalpine subspecies transition was correlated with lower bark thickness, whereas reduced borer damage at the highest elevations was associated with greater precipitation and lower temperatures. Our results suggest that due to possessing distinct traits associated with increased borer susceptibility, subalpine snow‐gum forests may be subject to an increased risk of severe borer‐mediated forest dieback under warmer and drier future climates. Identifying traits contributing to species' distribution limits and biotic‐agent vulnerability remains critical for predicting, monitoring and possibly mitigating forest and vegetation declines under future climates. [ABSTRACT FROM AUTHOR]

Forest decline caused by climate change has been a growing challenge for European foresters for decades. The accumulation of tree-related microhabitats (TreMs) and deadwood during decline can enhance stand structural heterogeneity and provide crucial habitat features for many forest ecological guilds. We analysed changes in deadwood and TreM assemblages using a trait-based approach in three case studies: drought-induced decline in highland Pyrenean fir and lowland oak forests, and windstorm/pest-induced dieback in highland Bavarian spruce forests. Decline caused significant changes in deadwood and TreM characteristics and composition in three forest contexts. However, tree density with cavities, exudates, or crown deadwood was not linked to decline intensity. Declining conifer forests had more large deadwood and downed woody debris, and their TreM assemblages were more saproxylic, less epixylic, and included more cracks and exposed sapwood. TreM assemblages in drought-declining forests had higher diversity, functional richness, and more dead tops than healthy stands. In Bavarian spruce forests, there was more decayed downed deadwood, and the TreM assemblages were more associated with the base of the tree, snags, and logs. Overall, forest decline significantly boosts ecological niche resources, typically scarce in managed forests, which could benefit many forest biodiversity groups. Though post-disturbance management should respect tree species-dependent economic balance and avoid phytosanitary risks, it should also consider the ecological benefits of decline-induced heterogeneity. [ABSTRACT FROM AUTHOR]

Aim: Forest dieback is increasing from unfavourable climate conditions. Western redcedar (WRC)—a culturally, ecologically and economically important species—has recently experienced anomalously high mortality rates and partial canopy dieback. We investigated how WRC tree growth and dieback responded to climate variability and drought using tree‐ring methods. Location: Pacific Northwest, USA. Taxon: Western redcedar (Thuja plicata). Methods: We collected tree cores from three tree health status groups (no canopy dieback, partial canopy dieback, and dead trees) at 11 sites in coastal (maritime climate) and interior (continental climate) WRC populations. From growth rates, we computed four growth indices that assessed the resilience to drought and estimated the year of death. Results: Warmer and drier climate conditions in May/June that extended the annual July‐to‐September dry season reduced radial growth in 9 of 11 sites (1975–2020). WRC trees recovered growth to pre‐drought rates within 3 years when post‐drought climate conditions were cooler/wetter than average. However, recovery from drought was slower or absent when warmer/drier conditions occurred during the post‐drought recovery period, possibly leading to the recent and widespread mortality across the coastal population. WRC mortality was portended by 4–5 years of declining growth. Annually‐resolved mortality in coastal populations predominately occurred in 2017–2018 (80% of sampled dead trees), a period that coincided with exceedingly hot temperatures and the longest regionally dry period from May to September (1970–2020). In interior populations, mortality was dispersed among years but associated with warmer and drier conditions from August to September. Main conclusions: Our findings forewarn that a warming climate and more frequent and severe summer droughts, especially in consecutive years, will likely increase the vulnerability of WRC to canopy dieback and mortality and possibly other drought‐sensitive trees in one of the world's largest forest carbon sinks. [ABSTRACT FROM AUTHOR]

Between 1954 and the mid-1980s, about 50,000 ha of native montane rainforest on the island of Hawai'i experienced a decline in canopy trees ("'ōhi'a dieback"), leading to great concern about the future of Hawai'i's rainforests. Dieback symptoms particularly affected the dominant tree species, the endemic Metrosideros polymorpha. Early hypotheses postulated that the forest decline was caused by a virulent pathogen or a combination of biotic disease and pest agents. This was ruled out after a decade of intensive disease research in the 1970s. Instead, it turned out that dieback patterns were significantly related to the physical environment, particularly the slope, topography, relative position on the hill slope, annual rainfall, and the type of substrate. Thus, an alternative hypothesis proposed that dieback is initiated by climate anomalies that manifest through soil moisture regimes under certain conditions of forest stand demography. Ironically, scientific perception of this interdisciplinary groundbreaking research that stimulated a global perspective on forest decline vanished while the awareness of climate change and its potential impact on the world's forests started to grow, rapidly becoming a major focus of research in recent years. In this paper, we reinforce memory of the world's first complex discussion on the natural causes of forest dieback as a showcase for the complexity of modern forest mortality research. This case demonstrates the need to rigorously identify, quantify, and fully understand all drivers of tree mortality to realistically project future climate-driven and other risks to forest ecosystem functions and services. Moreover, we summarize recent findings on forest mortality and climate change in the Pacific islands and beyond. [ABSTRACT FROM AUTHOR]

Drought-induced crown dieback depends on plant nutrient status. This research examined the leaves and roots nutrients of Persian oak trees (Quercus brantii Lindl.) under the effects of drought and tree decline. The leaves and roots of oak trees were sampled randomly on the four main sides of their crown to analyze the N, P, K, Ca, and Mg. In healthy and declining trees, the N, P, and K content in the leaves was higher than in the roots, while the Ca content in the leaves was lower than in roots. The N and P in the roots of healthy trees had decreasing temporal changes, but these elements did not have significant temporal changes in the roots of declining trees. Leaf P showed decreasing temporal changes. The temporal decrease of root N and P and leaf P showed a negative effect of drought on Persian oak trees by reducing their ability to absorb water and nutrients and transfer them to their leaves. The higher concentration of elements in the leaves of oak trees, in addition to showing the decrease in the absorption of elements by the roots from the soil, can indicate a protective mechanism of oak trees against drought stress. [ABSTRACT FROM AUTHOR]

Context: A significant forest decline has been noticed these last years in Europe. Managers need tools to better anticipate these massive events. Objectives: We evaluated the efficiency of easily available data about environmental conditions and stand characteristics to determine different levels of vulnerability. Methods: We combined remote sensing images, photo-interpretation, and digital models describing environmental conditions within a modelling approach to achieve spatial vulnerability assessment of the stands. We focused on silver fir and Norway spruce stands in the Vosges mountains (8900 km2, northeastern France), where severe symptoms of decline are visible. Results: Silver fir were predicted highly vulnerable on 7% of their area versus 33% for Norway spruce. Using an independent dataset, we observed ten-times (silver fir) and two-times (Norway spruce) higher mortality rates in the units with a high level of vulnerability than in the others. About half of the model deviance was directly or indirectly explained by variables related to water stress (soils displaying low water availability, having suffered severe drying events these last years). Furthermore, the stands acclimatised to drought conditions were more resilient. Stand characteristics also influenced dieback spread, suggesting that an evolution of silvicultural practices toward mixed stands with broadleaved species and uneven-aged trees can contribute to better adapt to future climate conditions. Conclusion: Vulnerability maps based on easily available geographic information describing climate, soil, and topography can efficiently discriminate canopy mortality patterns over broad areas, and can be useful tools for managers to mitigate the effects of climate change on forests. [ABSTRACT FROM AUTHOR]

Forest decline is a widespread phenomenon on coasts throughout the Mediterranean basin. Pathogenic fungi are amongst the main causes of forest tree diseases. Diplodia species, having a cosmopolitan distribution, are well-known as pathogens of woody plant hosts including Pinus spp. In recent years, symptoms of shoot dieback, necrosis, twig blight, canopy transparency and trunk cankers have been observed on Pinus pinea trees in Tunisian forests. Nevertheless, this has been less well-studied in North Africa and especially in Tunisia. The purpose of this study is to report the occurrence of Diplodia species causing dieback on P. pinea in two forests in northeastern and the northern Tunisia. A collection of fifty-eight isolates were obtained from symptomatic branches of P. pinea trees. Three Diplodia species were identified: D. africana, D. pseudoseriata and D. scrobiculata by means of morphological characteristics, and phylogenetic analysis. Their incidence on P. pinea trees appeared to be significantly correlated to the dendrometric parameters and ecological factors. A pathogenicity test was conducted on 3-years-old P. pinea seedlings, confirmed the virulence of three Diplodia species. Based on the length of vascular necrosis in the wood of the P. pinea inoculated stem, D. scrobiculata was the most virulent species. Data from the present study provide the first report of the species of Diplodia associated with P. pinea dieback in Tunisia. [ABSTRACT FROM AUTHOR]

Amazon forest dieback is seen as a potential tipping point under climate change. These concerns are partly based on an early coupled climate–carbon cycle simulation that produced unusually strong drying and warming in Amazonia. In contrast, the fifth-generation Earth system models (Phase 5 of the Coupled Model Intercomparison Project, CMIP5) produced few examples of Amazon dieback under climate change. Here we examine results from seven sixth-generation models (Phase 6 of the Coupled Model Intercomparison Project, CMIP6), which include interactive vegetation carbon and in some cases interactive forest fires. Although these models typically project increases in area-mean forest carbon across Amazonia under CO2 -induced climate change, five of the seven models also produce abrupt reductions in vegetation carbon, which indicate localised dieback events. The northern South America (NSA) region, which contains most of the rainforest, is especially vulnerable in the models. These dieback events, some of which are mediated by fire, are preceded by an increase in the amplitude of the seasonal cycle in near-surface temperature, which is consistent with more extreme dry seasons. Based on the ensemble mean of the detected dieback events we estimate that 7±5 % of the NSA region will experience abrupt downward shifts in vegetation carbon for every degree of global warming past 1.5 ∘ C. [ABSTRACT FROM AUTHOR]

Oak dieback disease caused by the fungus Paecilomyces formosus threatens oak trees in the Zagros forests in western Iran. Various insects, such as wood-infesting beetles (Coleoptera), may play a role in dissemination of P. formosus. We collected larvae and adult insects from branch wood of oak trees with dieback symptoms in the Zagros forests. For larval identification, the mitochondrial gene cytochrome c oxidase I (COXI) was amplified by polymerase chain reaction (PCR). Fungal isolates from wood and insects collected from the sampled oaks were identified by morphology, acid production on creatine sucrose agar (CREA) medium, phylogeny of DNA sequence data for the β-tubulin gene and the internal transcribed spacer (ITS) rDNA. To detect P. formosus in larvae and adult insects, we used a nested PCR assay with the species-specific primer pairs PaMF and PaMR. The insects that most often tested positive for P. formosus were larvae of the buprestids Acmaeodera sp. and Chrysobothris affinis, and larvae of the cerambycid Trichoferus campestris. Adults of C. affinis and Calchaenesthes diversicolis (Cerambycidae), which were collected from within their galleries, also tested positive. Beetle larvae of Anthaxia sp. (Buprestidae), Latipalpis plana (Buprestidae), Monochamus sp. (Cerambycidae) and Crypticus gibbulus (Tenebrionidae) also tested positive. Larvae that tested negative for P. formosus were species of Cossidae (Lepidoptera), Elateridae (Coleoptera), Gasteruptiidae (Hymenoptera) and Syrphidae (Diptera). Future research is needed to determine whether any of these insects can serve as vectors of P. formosus. These results can be used to target-specific insects for monitoring. [ABSTRACT FROM AUTHOR]

Forest dieback in the last decade has been reported all over the world and not only causes a significant loss of forest yield but also affects successional trajectories. Here, we investigate the plant communities associated with Pinus mugo subsp. mugo dieback patches in Maiella massif. Dieback patches were examined by using satellite images to describe the occurrences and distribution. According to dieback patch size, we identified three ontogenetic stages: small, medium and large. To assess the effect of P. mugo on coexisting species, patch was divided into four belts according to P. mugo healthy status: OUT, FRONT, DEAD and IN zone. Within the four belt zones for each of the three ontogenetic stages, a vegetation analysis was conducted. Overall, 35 dieback patches were recorded and all are characterized by a circular shape and a size ranging from a few meters to 76.1 m. The linear correlation between the width of the inner regeneration belt and the total diameter of the patch suggests that the dieback area expands centrifugally due to the death of standing P. mugo. The increased resources, in terms of light availability caused by dieback, contribute to higher plants diversity in DEAD belts with the highest cover of hemicryptophytes. Contrariwise, in the OUT belts, not affected by dieback, plant diversity was the lowest with the highest cover of phanerophytes and a good contingent of nemoral species. Dieback affects forest structure and leads to sudden vegetation shifts that play an important role in maintaining biodiversity by allowing alternation between forest and grassland ecosystems. [ABSTRACT FROM AUTHOR]

Rear-edge populations at the xeric distribution limit of tree species are particularly vulnerable to forest dieback triggered by drought. This is the case of silver fir (Abies alba) forests located in Southwestern Europe. While silver fir drought-induced dieback patterns have been previously explored, information on the role played by nutritional impairment is lacking despite its potential interactions with tree carbon-water balances. We performed a comparative analysis of radial growth, intrinsic water-use efficiency (iWUE), oxygen isotopes (d18O) and nutrient concentrations in leaves of declining (DD) and non-declining (ND) trees in silver fir in four forests in the Spanish Pyrenees. We also evaluated the relationships among dieback predisposition, intraspecific trait variation (wood density and leaf traits) and rhizosphere soil physical-chemical properties. The onset of growth decline in DD trees occurred more than two decades ago, and they subsequently showed low growth resilience against droughts. The DD trees presented consistently lower foliar concentrations of nutrients such as P, K, Cu and Ni than ND trees. The strong effects of foliar nutrient status on growth resilience indices support the key role played by mineral nutrition in tree functioning and growth before, during and after drought. In contrast, variability in wood density and leaf morphological traits, as well as soil properties, showed weak relationships with tree nutritional status and drought performance. At the low elevation, warmer sites, DD trees showed stronger climate-growth relationships and lower d18O than ND trees. The uncoupling between iWUE and d18O, together with the positive correlations between P and K leaf concentrations and d18O, point to deeper soil/bedrock water sources and vertical decoupling between nutrient and water uptake in DD trees. This study provides novel insights into the mechanisms driving silver fir dieback and highlights the need to incorporate tree nutrition into forest dieback studies. [ABSTRACT FROM AUTHOR]

A critical review of contemporary publications searching for the causes of the decline/dieback of dark-needled forests in the boreal and temperate zones over Northern Eurasia with an emphasis on the southern Siberian mountains is presented. Three groups of factors were considered: primary abiotic factors that weaken the tree defense mechanism (climate aridization, total ozone column, ultraviolet radiation (UV-B), dimming), anthropogenic factors (air pollution), and secondary biotic factors (invasive insect pests and phytopatogens that inhabit weakened trees, which finally leads to their decline/dieback). As there is no irrefutable evidence of the fatal impact of one particular factor, a synergic effect of some of them is possible against a certain climatic background. Further investigations are necessary to find out causes for the dark-needled forest decline/dieback in Northern Eurasia. [ABSTRACT FROM AUTHOR]

We evaluated the composition, structure, and dieback of a montane forest in relation to soils and physiography of an important biogeographic region that has been sparsely studied. Our objectives were to (1) describe the forest composition and structure; (2) assess the current extent of dieback; and (3) relate tree composition, structure, and dieback proneness to edaphic and physiographic measures. We enumerated all live and dead standing plants ≥3 cm dbh, in thirty 20 × 15 m2 overstory plots. We measured all regeneration ≤1 m height in subplots, sampled soils and measures of physiography, and visually rated the proportion of crown dieback, and recorded standing dead trees. Woody plants belonged to 58 species and 25 families, of which 5% were dead. The plant families Lauraceae, Myrtaceae, and Symplocaceae represented over 50% of the basal area; Rubiaceae alone represented 25% of the stems. Multiple regressions revealed measures of edaphic and physiographic factors are associated with tree species richness and diversity, structure (e.g., basal area and stem density), and tree dieback and standing dead trees. Canopy openness and measures of soil toxicity from metals (Al and Cu) on clay‐based soils showed a predictably negative relationship with tree species richness and diversity. Basal area and stem density are positively associated with aspect and slope, and predictably negatively associated with canopy openness. Lastly, dieback and standing dead trees of the dominant canopy tree species are both positively associated with south and southwest aspects. The forest is now in a period of regeneration recovery with high numbers of regenerating secondary successional species. We propose the primary underlying factors that caused the dieback of this forest was a drought during 1970–1980 compounded by accumulations of salts and metals in the soil conveyed by winds from the sea (sodium) and from an ancient period of metallurgy downwind of the forest. However, our results are only suggestive of these causative factors and further study is necessary to confirm or refute these speculative causes for dieback. [ABSTRACT FROM AUTHOR]

Observations suggest that sudden canopy decline and death of widely dispersed individual trees, associated with wood-borer infestations, has recently expanded and intensified within high-elevation Eucalyptus pauciflora and E. lacrimans stands on the Kosciuszko massif, southeast Australia. Despite reports of insect infestations and associated tree decline over four decades, the phenomenon has been poorly understood and the identity of the associated wood-borer taxon only anecdotally resolved. We conducted a systematic study of sub-alpine forests in Kosciuszko National Park, NSW, with the intent of building a knowledge base of the phenomenon's aetiology and guiding ongoing research priorities. Using 10-metre radius plots positioned on elevational transects in areas under shared management for snow-based recreation and conservation and areas managed for conservation-only, we found a strong association between increasing severity of wood-borer feeding galleries and decline of canopy condition. The presence of even the first indicators of insect infestation was associated with a three-fold increase in declining canopy condition. Two-way contingency analysis revealed that increasing severity of feeding galleries corresponded with a highly significant decline in canopy-condition. Indicators of wood-borer infestation held greater explanatory power for deteriorating crown condition, than the converse, indicating that wood-borer infestation more likely precede, than follow, canopy decline and death. Based on insect detections over three years and consistency between observed and previously described wood-borer damage, it is unambiguously clear that the native cerambycid species Phoracantha mastersi is principally responsible for insect galleries in affected trees. Detections were associated with warm daily temperature maxima. Modelling of the incidence and severity of infestations indicated elevation and land-management status were significant predictors for the probability of wood-borer infestation at stand and tree levels. The same predictors were significant for predicting the proportion of stand basal area affected. Combined with temporal patterns in insect detections, our modelling results suggest that low temperatures play a key role in limiting upward migration of infestations. While we suspect that fragmentation of forest cover within shared-management areas favours short-distance dispersal and local intensification, an increase in forest-edges may also account for both greater frequency and severity of stand-level infestations in shared-management areas. Mindful of an apparent lower limit to P. mastersi infestations at the interface of affected/unaffected eucalypt taxa, we advocate for greater understanding of physiological traits affecting tree-level vulnerability to infestation. Further, reflecting their roles in Phoracantha infestations elsewhere, we regard resolving the combined roles of temperature and drought stress in initiating outbreaks as particularly important. Similarly, consistent with the effect of land-management status on P. mastersi infestations, improved knowledge of the spatial and temporal attributes of Phoracantha -induced dieback in snow-gum forests is needed. • A strong association exists between evidence of wood borers and canopy condition. • The native cerambycid species Phoracantha mastersi is principally responsible. • Elevation and land-management are significant predictors of infestation. • Declining temperature with elevation likely limits upward movement of wood borers. • Forest fragmentation may explain greater infestation severity in modified areas. [ABSTRACT FROM AUTHOR]

In 2010-2018, Northern Patagonia featured the longest severe drought of the last millennium. This extreme dry spell triggered widespread growth decline and forest dieback. Nonetheless, the roles played by the two major mechanisms driving dieback, hydraulic failure and carbon starvation, are still not clear and understudied in this seasonally dry region. Here, for the 1800-2017 period, we apply a retrospective analysis of radial growth, wood anatomical traits (lumen area, cell-wall thickness) and δ13C and δ18O stable isotopes to assess dieback causes of the iconic conifer Araucaria araucana. We selected three stands where declining (defoliated) and nondeclining (not defoliated) trees coexisted along a precipitation gradient from the warm-dry Coastal Range to the cool-wet Andes. At all sites declining trees showed lower radial growth and lower theoretical hydraulic conductivity, suggesting a long-lasting process of hydraulic deterioration in their water transport system compared to nondeclining, coexisting trees. Wood anatomical traits evidenced that this divergence between declining and nondeclining trees started at least seven decades before canopy dieback. In the drier stands, declining trees showed higher water-use efficiency (WUE) throughout the whole period, which we attributed to early stomatal closure, suggesting a greater carbon starvation risk consistent with thinner cell walls. In the wettest stand, we found the opposite pattern. Here, a reduction in WUE coupled with thicker cell walls suggested increased carbon assimilation rates and exposure to drought-induced hydraulic failure. The δ18O values indicated different strategies of gas exchange between sites, which are likely a consequence of microsite conditions and water sources. Multiproxy, retrospective quantifications of xylem anatomical traits and tree-ring isotopes provide a robust tool to identify and forecast, which stands or trees will show dieback or, on the contrary, which will likely withstand and be more resilient to future hotter droughts. [ABSTRACT FROM AUTHOR]

Background: Climate warming is amplifying and exacerbating drought stress worldwide. Long-term trends of increasing evaporative demand and decreasing soil moisture availability occur superimposed on severe spells of drought. These rare, extreme droughts have triggered episodes of forest dieback that have led to reduced productivity and rising mortality rates, usually at small scales (dieback hotspots), but affecting biomes worldwide. Aims: This review summarizes and discusses the drivers, patterns and mechanisms of forest dieback caused by drought. Methods: I review studies on forest dieback and tree death linked to dry spells with a focus on tools to forecast dieback. Results: Several mechanisms have been described as physiological drivers of dieback, including hydraulic failure and carbon starvation, however hydraulics-based models have shown little predictive power of dieback and mortality. Field proxies of tree vigour, including changes in canopy defoliation and water content, combined with surrogates of tree functioning (tree-ring growth, wood anatomy, tree-ring δ13C or δ18O composition) may improve predictions of forest dieback or at least render early-warning signals of impending tree death. Conclusions: Drought-induced dieback and mortality are concerning phenomena which lack forecasting tools with sufficient predictive power. Surrogates of tree vigour, growth and functioning should be used to build more accurate models of tree death in response to extreme climate events linked to drought. Here, I argue for combining and comparing those surrogates to better forecast forest dieback. [ABSTRACT FROM AUTHOR]

During the strong El Niño event of 2015–2016 large‐scale dieback of mangrove forests was observed in the Gulf of Carpentaria region of northern Australia. These and other intertidal communities are also extensive along the 7,400 km coastline of north‐eastern Australia. Determination of their floristic composition, potential carbon (C) store and sequestration capacity, and vulnerability to climate extremes is required for their effective conservation management and was the aim of this study.Standardized, state‐wide quantitative classification methods identified five mangrove forest and three saltmarsh communities covering 2,604 km2 along this coastline.Estuarine and oceanic mangrove forests were mapped separately. Carbon storage and sequestration capacity of the intertidal communities and their vulnerability to strong El Niño events were estimated using published data and GIS analyses.An estimated potential 126.2 (± 27.3 SEM) Tg C and 8.3 (±0.4 SD) Tg C were stored in the mangrove forests and saltmarshes, respectively. Comparatively, the rainforests of the region stored an estimated equivalent amount of C but covered three times the area, and the most widespread woodlands (Eucalyptus tetrodonta) of the region stored an estimated 1.5 times the C but covered 16 times the area. The C stored in the intertidal communities was estimated as equivalent to 493.47 Tg of CO2, valued at AU$6.8 billion on the Australian carbon market in December 2018. Annual C sequestration potential was 0.18–0.34 Tg C/year, valued between AU$8.9 and $17 million.Approximately 360 ha of mangrove forest was estimated as lost because of dieback during the 2015–16 El Niño event. Three types of mangrove forest were identified as potentially vulnerable to dieback from El Niño‐driven climate events.This study highlights the national and global significance of these intertidal systems and our findings need to be incorporated in future conservation and development planning of northern Australia. [ABSTRACT FROM AUTHOR]

A massive mangrove dieback event occurred in 2015–2016 along ∼1000km of pristine coastline in the Gulf of Carpentaria, Australia. Here, we use sediment and wood chronologies to gain insights into geochemical and climatic changes related to this dieback. The unique combination of low rainfall and low sea level observed during the dieback event had been unprecedented in the preceding 3 decades. A combination of iron (Fe) chronologies in wood and sediment, wood density and estimates of mangrove water use efficiency all imply lower water availability within the dead mangrove forest. Wood and sediment chronologies suggest a rapid, large mobilization of sedimentary Fe , which is consistent with redox transitions promoted by changes in soil moisture content. Elemental analysis of wood cross sections revealed a 30- to 90-fold increase in Fe concentrations in dead mangroves just prior to their mortality. Mangrove wood uptake of Fe during the dieback is consistent with large apparent losses of Fe from sediments, which potentially caused an outwelling of Fe to the ocean. Although Fe toxicity may also have played a role in the dieback, this possibility requires further study. We suggest that differences in wood and sedimentary Fe between living and dead forest areas reflect sediment redox transitions that are, in turn, associated with regional variability in groundwater flows. Overall, our observations provide multiple lines of evidence that the forest dieback was driven by low water availability coinciding with a strong El Niño–Southern Oscillation (ENSO) event and was associated with climate change. [ABSTRACT FROM AUTHOR]

Premise: Mortality events involving drought and pathogens in natural plant systems are on the rise due to global climate change. In Santa Barbara, California, United States, big berry manzanita (Arctostaphylos glauca) has experienced canopy dieback related to a multi‐year drought and infection from fungal pathogens in the Botryosphaeriaceae family. A greenhouse experiment was conducted using Neofusicoccum australe to test the specific influences of drought and fungal infection on A. glauca. Methods: A full factorial design was used to compare four treatment groups (drought + inoculation; drought – inoculation; watering + inoculation; and control: watering – inoculation). Data were collected for 10 weeks on stress symptoms, changes in leaf fluorescence and photosynthesis, and mortality. Results: Results indicated significant effects of watering and inoculation treatments on net photosynthesis, dark‐adapted fluorescence, and disease symptom severity (P < 0.05), and a strong correlation was found between physiological decline and visible stress (P < 0.0001). Mortality differed between treatments, with all groups except for the control experiencing mortality (43% mortality in drought – inoculation, 83% in watering – inoculation, and 100% in drought + inoculation). A Kaplan–Meier survival analysis showed drought + inoculation to have the least estimated survivorship compared to all other treatment groups. Conclusions: In addition to a possible synergistic interaction between drought and fungal infection in disease onset and mortality rates in A. glauca, these results indicate that young, non‐drought‐stressed plants are susceptible to mortality from N. australe infection, with important implications for the future of wildland shrub communities. [ABSTRACT FROM AUTHOR]

Zagros forest, the second most important renewable source of cellulose for Iran, is one of the best known oak forest reserves worldwide. These vital ecosystems hold great environmental and ecological importance in protecting water resources in the climatic dry conditions of the country. In recent years, however, oak dieback has decimated these forests. The purpose of our work is to evaluate and analyze the relationship of climate variables and the dieback of the Zagros oak forest in Lorestan Province, Iran. Dieback is defined by dieback indices, such as DVI, GEMI, IPVI, MSAVI2, NDVI, EVI, OSAVI, Sarvi2, SR, and VHI, which were obtained for 110 locations in the study area in the period between 2001 and 2017. On the other hand, daily climate variables influencing oak tree dieback were extracted in the same time period. As a first step, the most important climate variables affecting oak tree dieback were detected using the factor analysis method. Next, the so-called G* spatial correlation index was used to compute the covariance of dieback indices with climate variables as well as the spatial patterns of the changes of these covariances. The results of factor analysis showed that three factors, namely, humidity, temperature, and precipitation, were the most significant variables affecting dieback indices. Spatial autocorrelation patterns indicated that humidity played a major role regarding the widespread dieback in the study area. The mismatch of areas of low humidity and dieback indices, however, prompts that other factors, such as fungal diseases, affected the dieback alongside climatic variables. A similar investigation relating temperature with dieback indices indicated that changes in the match of spatial autocorrelation patterns of temperature and other dieback indices were less than 30% in terms of matching area except for NDVI and EVI. The effect of temperature on dieback indices was more limited to the southeast part of the study area. However, the covariance change pattern relating precipitation with dieback indices was related to northwest areas. Among all studied dieback indices, the NDVI was most correlated with all three factors of humidity, temperature, and precipitation, and temperature along humidity was found to play the most important role concerning the dieback of the Zagros oak forest. [ABSTRACT FROM AUTHOR]

Background: The phenomenon of tree dieback in forest ecosystems around the world, which is known to be associated with high temperatures that occur simultaneously with drought, has received much attention. Korea is experiencing a rapid rise in temperature relative to other regions. Particularly in the growth of evergreen conifers, temperature increases in winter and spring can have great influence. In recent years, there have been reports of group dieback of Pinus densiflora trees in Korea, and many studies are being conducted to identify the causes. However, research on techniques to diagnose and monitor drought stress in forest ecosystems on local and regional scales has been lacking. Results: In this study, we developed and evaluated an index to identify drought and high-temperature vulnerability in Pinus densiflora forests. We found the Drought Stress Index (DSI) that we developed to be effective in generally assessing the drought-reactive physiology of trees. During 2001–2016, in Korea, we refined the index and produced DSI data from a 1 × 1-km unit grid spanning the entire country. We found that the DSI data correlated with the event data of Pinus densiflora mass dieback compiled in this study. The average DSI value at times of occurrence of Pinus densiflora group dieback was 0.6, which was notably higher than during times of nonoccurrence. Conclusions: Our combination of the Standard Precipitation Index and growing degree days evolved and short- and long-term effects into a new index by which we found meaningful results using dieback event data. Topographical and biological factors and climate data should be considered to improve the DSI. This study serves as the first step in developing an even more robust index to monitor the vulnerability of forest ecosystems in Korea. [ABSTRACT FROM AUTHOR]

Lentisk dieback has been commonly observed in Tunisian forests. Nevertheless, the identity of the parasitic factors involved in this phenomenon has not yet been reported. Thus, in order to characterize the dieback-associated agents, symptomatic branches were collected from Pistacia lentiscus in the Rimel forest (Northern Tunisia). Two Botryosphaeriaceae species, Diplodia seriata and D. africana, were isolated and identified firstly based on morphological characteristics. Then, molecular identification was performed by sequencing the internal transcribed spacer region (ITS1-5.8S-ITS4), a part of the translation elongation factor 1-α (EF1-α) gene and a partial sequence of the β-tubulin (TUB) gene. The two species were morphologically and phylogenetically distinct from all Diplodia species and represent newly recorded fungal pathogens of lentisk in Tunisia. Pathogenicity tests carried out on asymptomatic detached branches confirmed the virulence of the two species. These findings may confirm D. seriata and D. africana as fungal pathogens associated with lentisk dieback in Tunisia. [ABSTRACT FROM AUTHOR]

A massive mangrove dieback event occurred in 2015/2016 along 1000 km of pristine coastline in the Gulf of Carpentaria, Australia. To gain insights into dieback drivers, we combine sediment and wood chronologies to analyze geochemical and climatic changes. The unique combination of low rainfall and low sea level observed during the dieback event was unprecedented in the previous three decades. Multiple lines of evidence from iron (Fe) chronologies in wood and sediment, wood densities and mangrove water use efficiency suggest low water availability within the dead mangrove forest. Wood and sediment chronologies suggest a rapid and large mobilization of sedimentary Fe, which was likely associated with pyrite oxidation within mangrove sediments. High resolution elemental analysis of wood cross sections revealed 30-90 fold increase in Fe concentrations in dead mangrove areas just prior to mortality. Fe concentrations in wood samples correlated strongly with the El Niño Southern Oscillation (ENSO) index, suggesting ENSO was a major driver of Fe mobilization. Large Fe losses from sediments during the dieback are consistent with Fe uptake in the trees, further implying sediment pyrite oxidation. If our data are representative of the entire dieback region, we estimate that the dieback drove the mobilization and loss of 50 ± 173 Gg Fe, equivalent to 8-50% of annual global atmospheric Fe deposition into the oceans, which is one of the major drivers of surface ocean productivity. Overall, our observations support the hypothesis that the forest dieback was associated with low water availability and Fe toxicity driven by a strong ENSO event. [ABSTRACT FROM AUTHOR]

• We studied the canopy hydrometerological responses to tree dieback phenomenon. • Data from multiple time scales, crown dieback states, and DBH were collected. • We observed significant stemflow variability among canopy dieback states. • Oak decline results in a substantial alteration of canopy ecohydrological parameters. • Our study provides fundamental information for water and forest managers. The potential for sudden tree dieback exists when there is significant variation in perturbation frequency and intensity, which can alter canopy-atmosphere interactions, like canopy rainfall partitioning. In the context of close-to-nature silviculture practices, dieback trees can endure for several decades in specific environments. Therefore, it is warranted to explore the interaction between tree dieback and rainfall partitioning, including throughfall, stemflow, and rainfall interception. The primary aims of this study were to (a) measure the partitioning of incident rainfall into throughfall, stemflow, and rainfall interception, beneath Quercus brantii (Lindl; Brant's oak) canopies in various dieback states in a Mediterranean region of Iran, and (b) quantify canopy and trunk ecohydrological parameters in the various treatments. To do this, we randomly selected 30 trees in three crown dieback states (i.e., healthy, moderate dieback, and severe dieback classes), with two diameters at breast height (DBH) classes (small and medium DBH). The rainfall partitioning, as well as the underlying canopy ecohydrological parameters, differed meaningfully across the oak dieback classes. This is most likely driven by changes in forest canopy structure as oak dieback progresses. The greatest changes included the increase in throughfall and a decrease in interception with leaf mortality. Regarding stemflow input, we observed significant stemflow variability among canopy dieback states—with stemflow percentage comprising 7.6 % of rainfall for healthy small DBH oaks, decreasing by half in severely declined ones—and it was notable that the precipitation thresholds required for stemflow initiation increased from healthy to severely declined trees. However, it should be noted that while decreased intercepted water by declined oak trees may occur, this does not necessarily translate to a significant increase in the proportion of rainfall transformed into stemflow. After the decline of oak trees, there were significant alterations to the ecohydrological parameters of both the canopy and trunk. Notably, canopy ecohydrological parameters (e.g., crown saturation points and canopy storage capacity), which are integral to the interception and storage of rainwater, experienced a marked decrease. In contrast, trunk ecohydrological parameters had not a constant trend among treatments, and it would require further study. These findings can help clarify rainfall partitioning and subsequent ecohydrological parameters dynamics and driving factors across crown dieback treatments, in addition to that offer parameters for hydrological simulations specifically tailored to Mediterranean forests. Studying how rainfall is distributed and the resulting changes in ecohydrological parameters across various crown dieback states, phenological scales, and DBH classes can enhance our comprehension and predictive abilities concerning the contribution of forests to hydrological recharge processes. [ABSTRACT FROM AUTHOR]

Along the Upper Gulf of Thailand, coastal fences and breakwaters have been constructed using bamboo since 2005. Despite their potential benefits, bamboo structures disintegrate within seven years releasing floating debris which severely damages mangrove tree stems. The aim of the study was to investigate whether such stem damage resulted in the decline of Avicennia spp. stands along the Upper Gulf of Thailand. Tree health assessments were conducted to assess the probability of crown dieback in damaged and undamaged trees. Satellite-derived time-series of vegetation indices were used to detect long-term forest decline. In contrast to the unaffected landward mangroves, seaward mangroves were unable to recover from insect-induced defoliation events after the collapse of a nearby fence. Furthermore, there was a significantly higher probability that damaged trees showed signs of moderate-to-severe crown dieback. It is recommended that bamboo fences be secured by replacing individual stems before they become detached. • Debris from deteriorating coastal bamboo fences damage stems of mangrove trees. • Damaged trees are more likely show signs of crown dieback. • Seaward mangrove forests are declining. [ABSTRACT FROM AUTHOR]

Ceratocystis fimbriata sensu lato species complex is a group of highly diverse cryptic species known to be associated with different diseases in a range of host plants. For the accurate identification of fungal pathogens belonging to this species complex, molecular approaches have been employed in juxtaposition with morphological characterization. Different DNA based methods have become the routine practice of mycologist community throughout the world. Among them, multilocus sequence typing (MLST) approach is gaining popularity. Four fungal taxonomy groups have been established MLST scheme for few fungi, to date. However, in Ceratocystis fimbriata species complex, no report on MLST based genotyping is available yet. Therefore, by considering this perspective, we attempted MLST method for genotyping and diversity analysis of this species complex. We investigated ITS and TEF1-α loci, for two multilocus sequence typing scheme. Allelic profile of each locus gave 23 sequence types (STs) and 22 Allele types (ATs). However, the typing efficiency of ITS region was high as compared to TEF1-α. The phylogenetic network parallelograms based on neighbor net algorithm also showed genetic diversity in studied isolates. This relationship study was further supported by generating phylogram, which resolved these isolates in three different clades showing unequivocal phylogenetic diversity and relatedness. [ABSTRACT FROM AUTHOR]

Soil moisture plays a key role in the ecological capability of arid and semi-arid woodland. Reducing soil moisture due to frequent droughts causes pest prevalence and disease outbreak and the consequence of forest dieback. On the other hand, soil moisture is strongly correlated with the amount of radiation received on the Earth's surface. The sun's radiation is traditionally described often by aspect and sometimes by toposhape. The use of the hillshade map for estimating solar radiation is possible through developing GIS. The present study aimed to compare the relationship and the ability of these indices to describe the phenomenon of arid and semi-arid woodland decline better and more accurately in a case study in the west of Iran. To this aim, the aspect and toposhape layers were generated in 5 and 12 classes, respectively. Then, the hillshade map in range of 0-255 was made during the peak of summer heat. The comparison of the dieback ratio in the three characteristic histograms showed that the shade index, unlike the other two indicators, had a significant effect on forest drought (R2=0.91 for linear equation and R2=0.94 for quadratic equation). The results indicated that the application of hillshade in describing and analysing ecological processes by relying on soil moisture such as woodland dieback is superior to the other two indicators. It is suggested that this index be used to obtain a risk model to predict woodlands dieback which are under the pressure of frequent droughts due to climate change or other mortal factors. [ABSTRACT FROM AUTHOR]

Temperate European forests are currently largely under attack by the infection with Hymenoscyphus fraxineus , a fungal pathogen introduced from Asia since at least the early 1990s and causing a major dieback of common ash (Fraxinus excelsior) throughout Europe. At present, ash dieback evokes major problems for forestry, in particular in sensitive forest remnants in Northern Germany, where the disease causes serious concerns for ecosystem conservation. This makes ash dieback a focal area of ecological research. In the present study, we quantified the extent of ash dieback in adult and in young ash trees in Northern Schleswig-Holstein, Germany, in relation to community composition and associated biotic and abiotic factors. Data collection was carried out in 37 plots in communities of ash-rich forests and included floristic inventory, rating of adult and young ash individuals and recording of light and soil conditions. Data were analyzed using non-metric multidimensional scaling and general linear mixed effects models. Forest type was the strongest significant predictor for variation in crown defoliation of adult ash trees. Damage was highest in communities of wet alder-ash forests and lowest in ash-rich beech forests. A further significant predictor of adult crown defoliation was individual height of the ash tree with larger trees being less affected than smaller ones. For juveniles, total species richness displayed a significant positive relationship with the proportional abundance of fungal infection, while the mean damage proportion per individual significantly increased with increasing relative light intensity in the understorey. The study clearly shows a strong relationship between forest type and ecosystem vulnerability to ash dieback. In particular, communities belonging to the species-rich wet alder-ash forests were most severely affected by ash disease, thereby deserving special attention among the vulnerable fragmented forest remnants in Schleswig-Holstein. Co-varying factors, however, seem to differ between juvenile and adult trees, hinting at the relative importance of tree performance for the adult trees and abiotic conditions for the juveniles. Accounting for such differences along a larger ecological gradient of ash forest communities will be necessary to more comprehensively understand effects of ash dieback on the ecosystem and needs to be addressed in future research. [ABSTRACT FROM AUTHOR]

• Dieback models at individual and stand levels were developed using PLS regressions. • Models included abiotic factors and biotic agents: mistletoe and processionary moth. • Biotic agents were crucial to explain Scots pine foliar deficit. • Biotic agents were strongly correlated with climate and soil water balance factors. Forest dieback is a high risk factor for the sustainability of these ecosystems in the climate change context. Productivity losses and increased defoliation and mortality rates have already been recorded for many tree species worldwide. However, dieback is a process that depends on complex interactions between many biotic and environmental factors acting at different scales, and is thus difficult to address and predict. Our aim was to build tree- and stand-level foliar deficit models integrating biotic and abiotic factors for Scots pine (Pinus sylvestris), a species particularly threatened in Europe, and especially in the southeastern part of France. To this end, we quantified foliar deficit in 1740 trees from 87 plots distributed along an environmental gradient. We also measured tree annual radial growth and the abundance of two parasites: the pine processionary moth (Thaumetopoea pityocampa Den. & Schiff.) and mistletoe (Viscum album L.). Topographic, soil, climate and water balance indices were assessed for each plot, together with the stand dendrometric characteristics. Given the large number of environmental factors and the strong correlations between many of them, models were developed using a partial least squares (PLS) regression approach. All the models pointed to a preponderance of the biotic factors (processionary moth and mistletoe) in explaining the intensity of foliar deficit at both tree- and stand- levels. We also show that strong interactions between climate, soil, water balance and biotic factors help to explain the intensity of dieback. Dieback was thus greater in the driest topoedaphic and climatic conditions where the mistletoe and processionary moth were present. This study highlights the need to account for a wide range of biotic and abiotic factors to explain the complex process of forest dieback, and especially the environmental variables that contribute to the water balance on the local scale. The phenomenological modeling approach presented here can be used in other regions and for other species, after a re-calibration and some adaptations to local constraints considering the limited distribution area of some biotic agents. [ABSTRACT FROM AUTHOR]

The current increase of severe droughts associated with climate change is one of the main causes of the observed dieback in Mediterranean holm-oak (Quercus Ilex subs ballota) forests. Effects of forest dieback in soils are greatly variable and depend on a number of local factors, but generally include increased soil organic C due to increased litter inputs, alterations in soil nutrient contents, changes in the nitrogen cycle, and detrimental effects on soil microbial communities and their functioning. There is little evidence, however, of the effects of forest dieback on soil faunal diversity. In this study, we assessed the effects of holm-oak dieback and induced nutrient shifts on soil microfaunal communities and their relationships with soil properties and functioning. We studied 13 Holm oak forest sites affected by tree dieback in the Iberian Peninsula and assessed the relationship between soil properties, microbial functions, and nematode diversity under healthy, defoliated, and dead trees at each site. We found that nematode abundances were variable and significantly patchy across sites. Bacterivore and herbivore nematodes were the most abundant nematode trophic groups. Nematode abundance increased with increasing levels of soil C only if soil P was available, indicating that nematode abundances might be P-limited in these semi-arid systems. Bacterivore nematodes were especially affected by tree death, since tree dieback switched the relationship between bacterivore abundance and soil nutrient contents. Opportunistic bacterivores, able to exploit ephemeral bacterial resources blooming after soil organic enrichment, seemed more resilient to tree death than generalist ones, while fungivores did not clearly respond to forest dieback. We found complex and unexpected effects of tree dieback on soil microfaunal communities, which should receive further attention. [Display omitted] • Nematode communities were patchily distributed in the studied holm-oak forests. • Total number of nematodes increased with organic C only when soil P was available. • Bacterivore nematodes were positively associated to soil N under healthy trees. • Opportunistic bacterivores were linked with mineralization and to N in C-rich soils. • Dieback switched the relationships between soil microfauna and nutrient content. [ABSTRACT FROM AUTHOR]

An Erratum to this paper has been published: https://doi.org/10.3103/S1068373922060097 [ABSTRACT FROM AUTHOR]

Forest decline played a pivotal role in motivating Europe's political focus on sustainability around 35 years ago. Silver fir (Abies alba) exhibited a particularly severe dieback in the mid-1970s, but disentangling biotic from abiotic drivers remained challenging because both spatial and temporal data were lacking. Here, we analyze 14 136 samples from living trees and historical timbers, together with 356 pollen records, to evaluate recent fir growth from a continent-wide and Holocene-long perspective. Land use and climate change influenced forest growth over the past millennium, whereas anthropogenic emissions of acidic sulfates and nitrates became important after about 1850. Pollution control since the 1980s, together with a warmer but not drier climate, has facilitated an unprecedented surge in productivity across Central European fir stands. Restricted fir distribution prior to the Mesolithic and again in the Modern Era, separated by a peak in abundance during the Bronze Age, is indicative of the long-term interplay of changing temperatures, shifts in the hydrological cycle, and human impacts that have shaped forest structure and productivity. [ABSTRACT FROM AUTHOR]

The article discusses the environmental conditions of the Amazon River Region in the 21st century. According to the author, highways, dams, and agriculture have contributed to the dieback of the Amazon forest, which in turn prompted conservation efforts to preserve the tropical forest and advance indigenous peoples' rights in the Amazon. Topics include the environmental impact of greenhouse gas (GHG) emissions on the loss of biodiversity in the Amazon, the ecological impact of Amazon dieback, such as an impairment of the Amazon's ability to regulate climate, and an overview of the World Bank's Amazon dieback report "Assessment of the Risk of Amazon Dieback."

Episodes of drought-induced tree dieback have been recently observed in many forest areas of the world, particularly at the dry edge of species distributions. Under climate change, those effects could signal potential vegetation shifts occurring over large geographical areas, with major impacts on ecosystem form and function. In this article, we studied the effect of a single drought episode, occurred which in summer 2005, on a Scots pine population in central Pyrenees (NE Spain). Our main objective was to study the environmental correlates of forest decline and vegetation change at the plot level. General and generalized linear models were used to study the relationship between canopy defoliation, mortality and recruitment, and plot characteristics. A drought-driven multifactor dieback was observed in the study forest. Defoliation and mortality were associated with the local level of drought stress estimated at each plot. In addition, stand structure, soil properties, and mistletoe infection were also associated with the observed pattern of defoliation, presumably acting as long-term predisposing factors. Recruitment of Scots pine was low in all plots. In contrast, we observed abundant recruitment of other tree species, mostly Quercus ilex and Q. humilis, particularly in plots where Scots pine showed high defoliation and mortality. These results suggest that an altitudinal upwards migration of Quercus species, mediated by the dieback of the currently dominant species, may take place in the studied slopes. Many rear-edge populations of Scots pine sheltered in the mountain environments of the Iberian Peninsula could be at risk under future climate scenarios. [ABSTRACT FROM AUTHOR]

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Simple Summary: Bark beetles serve as vectors to numerous tree pathogens, the most conspicuous guild of which are ophiostomatoid fungi. Most of these fungi are known to cause blue-stain discoloration of wood, and some of them are pathogenic to trees, in certain cases able to kill them. Over the last years, drought-induced stress and attacks by bark beetle Ips sexdentatus resulted in a massive dieback of Pinus sylvestris in Ukraine. Limited and fragmented knowledge is available as to which ophiostomatoid fungi in this geographic area are vectored by Ips sexdentatus, and their roles in tree dieback. It is known, though, that in different parts of Europe those fungal communities might significantly differ. This study represents the first and so far, the most extensive analysis of fungal associates of I. sexdentatus in eastern Europe accomplished combining different methods, using insect, plant, and fungal material, and reports a number of previously unknown insect-vectored pathogens of P. sylvestris. Increasing climate change-related disturbances to forests put reported findings in a broader geographical context. Drought-induced stress and attacks by bark beetle Ips sexdentatus currently result in a massive dieback of Pinus sylvestris in eastern Ukraine. Limited and fragmented knowledge is available on fungi vectored by the beetle and their roles in tree dieback. The aim was to investigate the fungal community vectored by I. sexdentatus and to test the pathogenicity of potentially aggressive species to P. sylvestris. Analysis of the fungal community was accomplished by combining different methods using insect, plant, and fungal material. The material consisted of 576 beetles and 96 infested wood samples collected from six sample plots within a 300 km radius in eastern Ukraine and subjected to fungal isolations and (beetles only) direct sequencing of ITS rDNA. Pathogenicity tests were undertaken by artificially inoculating three-to-four-year-old pine saplings with fungi. For the vector test, pine logs were exposed to pre-inoculated beetles. In all, 56 fungal taxa were detected, 8 exclusively by isolation, and 13 exclusively by direct sequencing. Those included nine ophiostomatoids, five of which are newly reported as I. sexdentatus associates. Two ophiostomatoid fungi, which exhibited the highest pathogenicity, causing 100% dieback and mortality, represented genera Graphium and Leptographium. Exposure of logs to beetles resulted in ophiostomatoid infections. In conclusion, the study revealed numerous I. sexdentatus-vectored fungi, several of which include aggressive tree pathogens. [ABSTRACT FROM AUTHOR]

• Identify early-warning signals of tree mortality is fundamental to predict tree death. • Dead trees suffered from hydraulic constrain following decades of carbon limitation. • Xylem adjustments to increasing summer drought were more plastic in living than in dead trees. • Decreasing wood density and growth warn of upcoming tree death. Tree death is not always preceded by a visible decline in vigor (canopy dieback) or a progressive loss in crown volume. Identifying early-warning signals of incipient decline can help to implement the necessary measures to prevent tree death. The aim of this work is to understand what functional alterations preceded the massive drought-induced death of adult Pinus canariensis trees in an arid stand, located in the Canary Islands. To this aim, we analyzed interannual variations in earlywood and latewood width, wood density and anatomy, and carbon isotope composition, and the relationships among these variables, in dead and living trees from 1980 to 2013. Dead trees grew less since the 1990′s, produced fewer parenchyma rays and resin canals, and exhibited a trend of decreasing latewood density and a marked shift in carbon isotope discrimination over the last 34 years that were reversed in living trees. Higher wood density in living trees resulted from thicker tracheid cell walls rather than narrower lumens. The intrinsic water use efficiency shifted from higher to lower values in dead trees after the 2000s. These results suggest a carbon limitation to maintain hydraulic safety under xylem tension, as well as to maintain storage and defense capacity, which can render trees more vulnerable to severe drought episodes. A long-term trend of decreasing tree-ring density and a reduced intrinsic water use efficiency in the short-term can be early-warning signals of carbon limitation and tree decline in drought-stressed P. canariensis. The analysis of these variables can be used to assess tree decline risks in similarly vulnerable conifer populations inhabiting drought-prone regions. [ABSTRACT FROM AUTHOR]

Rising temperatures and aridification, combined with the stressing effect of some hemiparasitic plants such as mistletoes, may contribute to reduce vigour and growth of trees and shrubs leading to dieback and increasing mortality. This has been rarely explored in pioneer shrubs such as junipers, which are assumed to be more drought tolerant than coexisting trees. To test these ideas, we reconstructed radial growth patterns of common junipers (Juniperus communis L.) with different crown cover and infestation degree by dwarf mistletoe (Arceuthobium oxycedri (DC.) M. Bieb.) in two sites with contrasting aspect and water availability located in north-eastern Spain. We used dendrochronology to study the response of junipers' radial growth to climatic factors (temperature, precipitation, and soil moisture), an index of drought severity, and mistletoe infestation. Juniper growth was constrained by elevated temperatures and low precipitation leading to drought during the growing season. Infestation by dwarf mistletoe contributed to a short-term growth decline in junipers. The interaction between low summer precipitation and high dwarf mistletoe infestation constrained juniper growth, particularly in the north-oriented wetter site, where hosts presented higher growth rates during wet periods. The negative impact of low summer precipitation on juniper growth overrides the effects due to dwarf mistletoe infestation. Aridification and mistletoe infestation could trigger dieback and mortality of shrubs slowing down successional dynamics and delaying shrub encroachment into former croplands and grasslands. [ABSTRACT FROM AUTHOR]

Forest dieback and mortality episodes triggered by droughts are receiving increasing attention due to the projected increases in these extreme climate events. However, the role played by nutrient impairment in dieback is understudied, despite interactions among carbon-water balances and nutrition. Here, we followed a comparative analysis of long-term growth, intrinsic water-use efficiency (iWUE), oxygen isotopes (δ18O) and wood-nutrient composition patterns between living (L) and dead (D) trees of a Nothofagus dombeyi population, showing dieback in Argentina. The onset of the growth decline of D trees occurred ca. 40 years before death. These trees showed higher iWUE, pointing to higher drought stress. Their lower δ18O values, together with the uncoupling between δ18O and leaf-level processes, suggested a deeper source of water uptake for this vigor class. D trees showed a poorer nutritional status than L trees that likely amplified the dieback. This was supported by numerous positive associations of P- and K-concentrations in wood and related ratios with iWUE, δ18O and tree growth. Therefore, drought-related nutrient deterioration can significantly contribute to dieback and be an early warning signal of impending tree death. [ABSTRACT FROM AUTHOR]

• We examined the effect of forest dieback on arthropod biodiversity in Yunnan, China. • Dieback results in compositional change that mimics natural forest-gap dynamics. • We find no evidence that dieback changes overall species richness or diversity. • Pest attack could thus accelerate transition to resilient, heterogeneous forest. China has recently announced a reform of forestry policy, with a major goal being to transform from plantation to heterogeneous forests, which have higher resistance to pests and disease and house more biodiversity. One driver of reform is increased intensity and frequency of pest-induced tree-dieback events. To inform management, we ask what effects these events have on insect biodiversity in Pinus yunnanensis monocultures in Yunnan province, the province with one of the highest proportions of forest cover in China. We sampled aerial arthropods (mostly insect) biodiversity along gradients of Pinus yunnanensis dieback severity using Malaise traps and used metabarcoding to characterise the insect community. We used MS-GDM ('multi-site generalized dissimilarity modelling of zeta diversity'), zeta-decline analysis, and iNEXT ('Interpolation and extrapolation for species diversity') to assess community change as functions of forest-structure covariates. Metabarcoding of Malaise-trapped insects reveals that bark-beetle induced forest dieback does not result in detectable differences in species diversity but does result in compositional change, with the biggest turnover occurring between 0% and infested-0%-open-canopy forests and 20%-infested-20%-open-canopy forests. Zeta-decline analysis found that the insect community in low-infestation forests is characterized by a stochastic assembly, while in high-infestation forests, the community structure is consistent with niche assembly. Our results thus suggest that bark-beetle dieback mimics natural forest-gap dynamics, consistent with the interpretation of bark beetles as a keystone species in European conifer forests, where it has been proposed that forest heterogeneity can be created efficiently by allowing natural disturbances, including bark-beetle outbreaks, to proceed naturally, without being mitigated by deadwood removal and dense replanting. In Yunnan's situation, and given predicted increases in bark-beetle dieback severity and frequency, this strategy should probably be supplemented with anthropogenic treatments, such as deadwood enhancement and planting of multiple tree species, to accelerate the succession of plantations into heterogeneous forests. [ABSTRACT FROM AUTHOR]

Repeated examinations in Quercus pubescens, Quercus cerris and Fagus sylvatica during the summer of 1984 and the winner of 1984/85 showed the possibility of improving the diagnosis for the disease known as 'forest decline' and of facilitating the detection of damage and the epidemiological surveys for these species, probably for other deciduous broadleaves too. The absence of leaves in the rest period allows better identification of branching damage. These pathological features remain unchanged during the rest period, allowing easier epidemiological surveys and sound comparisons between observations made in the same period in different territories. It also allows accurate photographical records which can be useful for comparison between pictures taken in different years. Summer and winter observations on the studied species were consistent and allowed reciprocal integration, which make it easier to differentiate the three damage classes (slight, medium, severe) which are usually employed in epidemiological surveys of forest decline. [ABSTRACT FROM AUTHOR]

Discusses the widespread forest decline and dieback of forests in large areas of Europe and North America. Consideration of problems such as airborne pollutants and natural stresses such as climactic factors and pathogens; Analyzation of the impact of power plants, industries, and vehicles on forest death; Ambient ozone levels in Central Europe and West Germany; How agricultural fertilizers are the main anthropogenic source of airborne nitrogen; Generation of acid deposition from sulfur dioxide, nitrogen oxides, and hydrogen chloride.