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1. Previous investigations of the role of pathogens as drivers of community dynamics in forests have mostly focused on interspecific interactions between canopy trees and seedlings via soil feedbacks. However, feedbacks can also occur directly between canopy trees and seedlings when spores of foliar and stem pathogens fall onto seedlings regenerating underneath the canopy. 2. We studied pathogen spillover between canopy trees and conspecific and heterospecific regeneration in the Pinus nigra-Diplodia sapinea pathosystem. We sampled 70 pine stands distributed across a temperature gradient of 9.5-13.7°C (mean annual temperature). In each stand, we linked spore load and spore survival in the canopy with pathogen biomass and disease severity in regenerating seedlings. The density of regenerating seedlings and the health status of conspecific and heterospecific seedlings were also measured. The strength of canopy-understorey interactions was correlated with both climatic and stand variables. 3. The most severe symptoms of disease in regenerating seedlings were found in the warmest stands. Structural equation models suggested that disease severity in seedlings was mediated by an increased spore spillover from the canopy. Temperature also increased the pathogen survival rate within seedlings, further contributing to disease severity. Interestingly, disease severity in the canopy did not correlate with spore load in the canopy, suggesting that adult trees were spilling pathogens onto the understorey regardless of their health status. Pathogen spillover increased in more open stands. Greater disease severity in seedlings correlated with stands with a higher density of oak seedlings. Synthesis. 4. These results suggest an understudied mechanism of the Janzen-Connell theory, according to which, canopy-understorey negative feedbacks could be driven by canopy pathogens. Our data also suggest that temperature shapes canopy-understorey interactions, which implies that under a warming climate scenario, an acceleration of conspecific negative feedbacks could be expected, with implications for forest regeneration dynamics. M.C. was supported by the AGAUR FI fellowship 2021FI_B00223 from the Secretariat for Universities and Research of the Ministry of Business and Knowledge of the Government of Catalonia and the European Social Fund. M.D.C. was supported by the Ministry of Science and Innovation of Spain via competitive grant CGL2017- 89149-C2-2-R. JO was supported by the ‘Ramón y Cajal’ fellowship RYC-2015-17459 from the Ministry of Science and Innovation of Spain. This project was partly funded by the grant PID2021- 127328OB-I00 from Ministry of Science and Innovation of Spain. We acknowledge the contribution of the anonymous reviewers for their helpful comments.

Purpose of Review Human-caused global change is fundamentally altering natural forest ecosystems. More trees are exhibiting a wide range of symptoms indicative of poor vigour, particularly stressed species at the edge of their native ranges and stands growing on marginal sites. This review will focus on complex tree diseases (declines) caused by native pathogens and the key environmental drivers that contribute to this phenomenon. These systems are frequently complex, with multiple drivers at work. Recent Findings Using four cases studies on different continents, we explored the direct and indirect environmental drivers underlying these decline syndromes. Although climate and weather events seem to be usually associated with forest decline, we found that environmental disturbance by either forest management or land-use changes is also a global predisposing factor of decline which deserves more attention. Changes in land use have directly benefited pathogens such as root rots in the Pyrenees (Spain) or indirectly by making the environment more conducive for canker and foliar diseases in Australia and the USA. Focus on land-use changes could improve understanding of current decline problems such as those affecting Araucaria in Chile. The next century will almost certainly see an unprecedented rise in forest pathogen epidemics, requiring a proactive rather than reactive response. Diseases caused by native pathogens with complex aetiologies will become more common, and recognising, characterising and managing these epidemics are difficult because native pathogens are frequently already widespread, and eradication is not feasible. We need to start approaching these issues from a 'whole ecosystem' perspective, highlighting the many aspects and entanglements of forest declines and allowing us to respond with management options tailored to each scenario. The approach proposed here provides logical steps based on six questions to untangle the direct and indirect environmental drivers of tree declines. Open Access funding enabled and organized by CAUL and its Member Institutions JO was supported by the `Ramon y Cajal' fellowship RYC-2015-17459 from the Ministry of Science and Education of Spain. USDA Forest Service Forest Health Protection partially supported MLS.

The factors shaping the composition of the tree mycobiome are still under investigation. We tested the effects of host genotype, site, host phenotypic traits, and air fungal spore communities on the assembly of the fungi inhabiting Norway spruce needles. We used Norway spruce clones and spore traps within the collection sites and characterized both needle and air mycobiome communities by high-throughput sequencing of the ITS2 region. The composition of the needle mycobiome differed between Norway spruce clones, and clones with high genetic similarity had a more similar mycobiome. The needle mycobiome also varied across sites and was associated with the composition of the local air mycobiome and climate. Phenotypic traits such as diameter at breast height or crown health influenced the needle mycobiome to a lesser extent than host genotype and air mycobiome. Altogether, our results suggest that the needle mycobiome is mainly driven by the host genotype in combination with the composition of the local air spore communities. Our work highlights the role of host intraspecific variation in shaping the mycobiome of trees and provides new insights on the ecological processes structuring fungal communities inhabiting woody plants. This research was supported by the Swedish research council for Environment, Agricultural Sciences and Spatial Planning, FORMAS, project 2016-00798. M.E. and H.D.C were also supported by Formas project 2017-00402. J.O. was partially supported by the 'Ramon y Cajal' fellowship RYC-2015-17459. The authors would like to thank the owners of the seed orchards, Svenska skogsplantor AB and Sodra skogsagarna AB, for allowing us to sample the trees and assisting with the air mycobiome sampling. The authors also thank Antonio Rizzi, Rena Gadjieva, Maria Jonsson, and Katarina Ihrmark for their assistance with the laboratory and field work. The authors would like to acknowledge the support of the National Genomics Infrastructure (NGI)/Uppsala, Genome Center and UPPMAX for assisting us in massive parallel sequencing and computational infrastructure. Work performed at NGI/Uppsala Genome Center was funded by RFI/VR and Science for Life Laboratory, Sweden.

The role of competitive interactions among endophytes and latent pathogens that determine disease on stressed plants is largely unknown. Shoot blight caused by Diplodia sapinea is an emerging disease affecting pine forests in Europe. D. sapinea is an endophyte that becomes pathogenic following drought or hail. Disease expression depends on the availability of nutrient‐rich metabolites produced by trees to alleviate stress. We studied whether competition for these compounds with other endophytes could modulate symptom expression. We compared the shoot‐inhabiting fungal community of trees that developed blight symptoms or remained asymptomatic following a hailstorm that affected four pine species. Communities were characterised by isolation and metabarcoding of the ITS2 region. Endophytes were functionally annotated in their response to C, N and proline. Hail promoted D. sapinea and other endophytes with a rapid colonisation strategy of N‐rich substrates. Asymptomatic trees had a community of endophytes showing a strong negative association with D. sapinea. These antagonistic endophytes were functionally similar to D. sapinea in their response traits to hail, but they were more specialised to proline. The association was conserved across pine species. Our data suggest that after a stress‐inducing event, rapid niche occupation is critical for D. sapinea to cause disease. However, competition with other endophytes for key metabolites can supress the pathogen and prevent trees from developing symptoms. Our findings can pave the way to a better understanding of the contribution of latent pathogens on widespread tree mortality events involving abiotic stressors, such as drought, heat or hail. A free Plain Language Summary can be found within the Supporting Information of this article.

Global change is pressing forest pathologists to solve increasingly complex problems. We argue that understanding interactive effects between forest pathogens and global warming, globalization, and land-use changes may benefit from a functional ecology mindset. Traits can be more informative about ecological functions than species inventories and may deliver a more mechanistic description of forest disease. Myriad microbes with pathogenic potential interact with forest ecosystems at different organizational levels. Elucidation of functional traits may enable the microbial complexity to be reduced into manageable categories with predictive power. In this review, we propose guidelines that allow the research community to develop a functional forest pathology approach. We suggest new angles by which functional questions can be used to resolve burning issues on tree disease. Building up functional databases for pathogenicity is key to implementing these approaches.

Predicting whether naïve tree populations have the potential to adapt to exotic pathogens is necessary owing to the increasing rate of invasions. Adaptation may occur as a result of natural selection when heritable variation in terms of susceptibility exists in the naïve population. We searched for signs of selection on black alder (Alnus glutinosa) stands growing on riverbanks invaded by two pathogens differing in aggressiveness, namely, Phytophthora uniformis (PU) and Phytophthora × alni (PA). We compared the survival and heritability measures from 72 families originating from six invaded and uninvaded (naïve) sites by performing in vitro inoculations. The results from the inoculations were used to assess the relative contribution of host genetic variation on natural selection. We found putative signs of natural selection on alder exerted by PU but not by PA. For PU, we found a higher survival in families originating from invaded sites compared with uninvaded sites. The narrow sense heritability of susceptibility to PU of uninvaded populations was significantly higher than to PA. Simulated data supported the role of heritable genetic variation on natural selection and discarded a high aggressiveness of PA decreasing the transmission rate as an alternative hypothesis for a slow natural selection. Our findings expand on previous attempts of using heritability as a predictor for the likelihood of natural adaptation of naïve tree populations to invasive pathogens. Measures of genetic variation can be useful for risk assessment purposes or when managing Phytophthora invasions.

The effective management of grapevine trunk diseases (GTDs) is an ongoing challenge. Hot water treatment (HWT) is an environmentally friendly and economically viable option; however, the short-term effects of HWT on grapevine (

Mycorrhizal fungi can help plants to cope with drought, but research on the fungal communities that are more resistant to drought or alleviate drought stress of trees is still scarce. In this study, we investigated effects of drought on soil fungal communities and explored potential fungal traits related to drought resistance under greenhouse conditions. We manipulated water availability in pine seedlings belonging to three Spanish Pinus pinaster populations from geographical areas subjected to contrasting summer drought. A set of plant ecophysiological traits were quantified and soil fungi was quantified and profiled using ergosterol and Pacific Biosciences sequencing. Abundance of ectomycorrhizal (ECM) fungi in plants subjected to drought was lower than in well-watered plants. Most ECM taxa in plants surviving drought had long exploration types and were taxa typically forming rhizomorphs and hydrophobic mycelia. By contrast, ECM taxa in well-watered plants had wider range of distinct exploration types. No differences in fungal communities were found among P. pinaster populations. No associations between ECM fungi and plant ecophysiological traits were found, but significant interactions between drought treatments and belowground plant biomass were found for the relative abundances of ECM fungi, particularly ECM with long exploration types. Plants subjected to drought may benefit by associating to ECM taxa previously shown to transport water efficiently., L.S. and R.Z. acknowledge support from Ministerio de Economía y Competitividad/FEDER Grants FUTURPIN AGL2015-68274-C3-2-R, RESILPIN RTI2018-094691-B-C33 and Xunta de Galicia-GAIN grant IN607/2021/03. J.O. was supported by a Ramon ´ y Cajal fellowship (RYC-2015-17459) and J.A.B. was supported by the Serra-Hunter Pro gram-Generalitat de Catalunya

Tree decline caused by exotic Phytophthora pathogens is a serious issue affecting forests around the world. The presence of Phytophthora has been reported in heavily visited natural forests; however, whether recreational forests are at higher risk of being attacked by Phytophthora than non-recreational forests has yet to be explored. In this study, we sampled soil from 44 forests in Catalonia (NE Spain) with high or low recreational use that were dominated by chestnut (Castanea sativa), beech (Fagus sylvatica) or cork oak (Quercus suber). A systematic soil sampling of 277 plots in stands of different tree species, predominantly Pinus and Quercus spp., was also conducted across Catalonia to compare soils of recreational forests with high and low recreational use with the baseline incidence of Phytophthora in the region. The putative impacts of the presence of Phytophthora in recreational areas were assessed by measuring tree defoliation and regeneration, and by performing pathogenicity trials using Phytophthora isolates from soil and root samples to inoculate seedlings of tree species growing in the region. The incidence of native and invasive Phytophthora species was higher in chestnut, beech and cork oak forests than in forests of other tree species. For chestnut and beech forests, we found that the likelihood of finding an invasive Phytophthora in forests with high recreational use was more than three times higher than in forests with low recreational use (76% vs 21%, respectively). No differences were observed in cork oak forests. In plots with invasive Phytophthora species, beech exhibited higher defoliation levels than in plots without Phytophthora. The presence of both native and invasive Phytophthora species was associated with lower chestnut and cork oak regeneration. Our data indicate that Phytophthora is widespread in some forests used for recreation and seems to be associated with defoliation and low regeneration. However, the primary mechanism explaining the high prevalence of Phytophthora in recreational areas remains unclear. Disturbance, soil movement due to construction, or visitors moving inoculum within and across sites could be studied as possible contributing factors to establish effective management actions. This research was partially funded by the grant PID2021-127328OB-I00 from the Spanish National Plan for Scientific and Technical Research and Innovation. D.Š. was supported by the European Union’s H2020 research and innovation program under Marie Sklodowska-Curie grant agreement No 801586. M.C. was supported by the AGAUR FI fellowship 2021FI_B00223 from the Secretariat for Universities and Research of the Ministry of Business and Knowledge of the Government of Catalonia and the European Social Fund. J.O. was partially supported by the “Ramón y Cajal” fellowship RYC-2015–17459. We are grateful to Pere Joan Gelabert for assistance with creating the land-use map in ArcMap and to Jorge Heras (Government of Catalonia) for providing soil samples from the systematic sampling across Catalonia.

The effective management of grapevine trunk diseases (GTDs) is an ongoing challenge. Hot water treatment (HWT) is an environmentally friendly and economically viable option; however, the short-term effects of HWT on grapevine (Vitis vinifera L.) health and production are not fully understood. The aim of this study was to compare the effects of HWT on plant growth and fungal community structure in nursery stock until plants were completely established in the field. We assessed eleven graft and three rootstock varieties from four local nurseries in a region of Catalonia (NE Spain) where GTDs are a serious threat. After treatment, the plants were left to grow under field conditions for two growing seasons. Metabarcoding of the ITS region was used to study the mycobiomes of plant graft unions and root collars. We also assessed the influence of plant physiological indicators in community composition. Hot water treatment caused lasting changes in GTD communities in both the root collar and graft union that were not always characterized as a reduction of GTD-related fungi. However, HWT reduced the relative abundance of some serious GTD-associated pathogens such as Cadophora luteo-olivacea in graft tissues, and Phaeomoniella chlamydospora and Neofusicoccum parvum in the root collar. Treatment had the greatest influence on the total and GTD-related fungal communities of Chardonnay and Xarel center dot lo, respectively. Total community variation was driven by treatment and nursery in rootstocks, whereas HWT most significantly affected the GTD community composition in R-110 rootstock. In conclusion, changes in fungal abundance were species-specific and mostly dependent on the plant tissue type; however, HWT did reduce plant biomass accumulation in the short-term. This research was partially funded by the Generalitat of Catalonia Department of Agriculture (S.B.L.). J.O. was supported by the "Ramon y Cajal" fellowship RYC-2015-17459 through the Spanish Ministry of Science and Education. D.S. was supported by the European Union's H2020 research and innovation program, under Marie Sklodowska-Curie grant agreement No 801586.

The genus Phytophthora comprises many economically and ecologically important plant pathogens. Hybrid species have previously been identified in at least six of the 12 phylogenetic clades. These hybrids can potentially infect a wider host range and display enhanced vigour compared to their progenitors. Phytophthora hybrids therefore pose a serious threat to agriculture as well as to natural ecosystems. Early and correct identification of hybrids is therefore essential for adequate plant protection but this is hampered by the limitations of morphological and traditional molecular methods. Identification of hybrids is also important in evolutionary studies as the positioning of hybrids in a phylogenetic tree can lead to suboptimal topologies. To improve the identification of hybrids we have combined genotyping-by-sequencing (GBS) and genome size estimation on a genus-wide collection of 614 Phytophthora isolates. Analyses based on locus- and allele counts and especially on the combination of species- specific loci and genome size estimations allowed us to confirm and characterize 27 previously described hybrid species and discover 16 new hybrid species. Our method was also valuable for species identification at an unprecedented resolution and further allowed correct naming of misidentified isolates. We used both a concatenation- and a coalescent-based phylogenomic method to construct a reliable phylogeny using the GBS data of 140 non-hybrid Phytophthora isolates. Hybrid species were subsequently connected to their progenitors in this phylogenetic tree. In this study we demonstrate the application of two validated techniques (GBS and flow cytometry) for relatively low cost but high resolution identification of hybrids and their phylogenetic relations This work was supported by the ERA-NET BiodivERsA project “Resipath” (Re sponse of European Forests and Society to Invasive Pathogens) co-financed by national funds: in Belgium through the Belgian Science Policy Office (grant number BR/132/A1/RESIPATH-BE) and in Portugal by the Portuguese Science and Technology Foundation (grant number BIODIVERSA/0002/2012).

Halophytophthora species have been traditionally regarded as brackish water oomycetes, however, recent reports in inland freshwater call for a better understanding of their ecology and possible pathogenicity. We studied the distribution of Halophytophthora fluviatilis in 117 forest streams by metabarcoding river filtrates taken in spring and autumn and by direct isolation from floating leaves. Pathogenicity on six Fagaceae species and Alnus glutinosa was assessed by stem inoculations. The distribution of H. fluviatilis was correlated with high mean annual temperatures (&gt, 93.5% of reports in Ta &gt, 12.2 °C) and low precipitation records. H. fluviatilis was therefore widely distributed in forest streams in a warm–dry climate, but it was mostly absent in subalpine streams. H. fluviatilis was primarily detected in autumn with few findings in spring (28.4% vs. 2.7% of streams). H. fluviatilis was able to cause small lesions on some tree species such as Quercus pubescens, Q. suber and A. glutinosa. Our findings suggest that H. fluviatilis may be adapted to warm and dry conditions, and that it does not pose a significant threat to the most common Mediterranean broadleaved trees.

Patterns of forest disease are shifting worldwide. Pine woodlands in the Mediterranean Basin deliver important functions that may be disturbed by pathogen damage. In this chapter, I briefly review general concepts with examples on how and why forest pathogens are affecting pines. The main threat is the one posed by invasive pathogens that continually enter Europe via trade of living plants. While this aspect is capturing most of the attention, more efforts should be placed on understanding the role of native pathogens that may become more aggressive under global warming. For instance, the role of pathogens driving plant-soil feedbacks in pine woodlands, as well as their role in benefitting from abiotic perturbations such as drought, fire, hail or wind, remains largely unexplored. Another risk may come from future land-use changes. Nowadays, pines cover extensive areas with minimal management. However, increasing demand for timber products may put pressure on these stands and increase the prevalence of roots rots, particularly those caused by Heterobasidion annosum. Simulation data show that stump infection by Heterobasidion can be prevented by spraying thinning stumps with spores of the biocontrol fungus Phlebiopsis gigantea. Promoting mixed stands can be beneficial too. Diplodia sapinea is another pathogen to consider given its association with drought and hail. Concerning exotics, the pine wood nematode still poses the highest risk, however new invaders such as Fusarium circinatum, Dothistroma spp. or H. irregulare require attention. Greater involvement of society and stakeholders in preventing disease is needed. Several general directions for research and some general management recommendations are provided.

Predicting fungal community assembly is partly limited by our understanding of the factors driving the composition of deposited spores. We studied the relative contribution of vegetation, geographical distance, seasonality and weather to fungal spore deposition across three vegetation types. Active and passive spore traps were established in agricultural fields, deciduous forests and coniferous forests across a geographic gradient of ∼600 km. Active traps captured the spore community suspended in air, reflecting the potential deposition, whereas passive traps reflected realized deposition. Fungal species were identified by metabarcoding of the ITS2 region. The composition of spore communities captured by passive traps differed more between vegetation types than across regions separated by >100 km, indicating that vegetation type was the strongest driver of composition of deposited spores. By contrast, vegetation contributed less to potential deposition, which followed a seasonal pattern. Within the same site, the spore communities captured by active traps differed from those captured by passive traps. Realized deposition tended to be dominated by spores of species related to vegetation. Temperature was negatively correlated with the fungal species richness of both potential and realized deposition. Our results indicate that vegetation may be able to maintain similar fungal communities across distances, and likely be the driving factor of fungal spore deposition at landscape level., Vegetation type reinforces non-random patterns of fungal spore deposition.

Black truffle plantations are established on the basis that Tuber melanosporum Vittad. spreads from artificially inoculated trees. Although truffle cultivation has progressed tremendously over the past 30 years, the ecological processes underlying T. melanosporum mycelium expansion and its interactions with the rest of the fungal community over time are not completely understood. Controversy also exists on how mating type distribution evolves with time and its incidence on fruiting. We studied the soil fungal community and mating type distribution in plantations before truffle production (3, 5 and 7 years and after establishment) and during the production period (10, 14 and 20 years) at three distances from the tree stem: 40, 100 and 200 cm. We found that T. melanosporum developed steadily over the years as extraradical mycelium, first at the nearest part of the tree and later up to 200 cm. T. melanosporum development was not correlated with changes in other ectomycorrhizal fungi and was negatively correlated with lower relative abundance of arbuscular mycorrhizal fungi and non-root associated guilds such as moulds, yeasts and plant pathogens. Mating type frequency did not change across years. Twenty years after establishment no signs of replacement of T. melanosporum by other fungi nor biases in mating type abundance were observed in soil mycelia, indicating that T. melanosporum can colonize and dominate the surrounding soil in mature Quercus ilex L. trees for a long time.

Protection against the root rot fungus Heterobasidion annosum s. l. is achieved by mechanically spraying stumps with spores of the antagonistic fungus Phlebiopsis gigantea , commercialized as Rotstop® . An extensive colonisation of the stump by P. gigantea is required for protection. Colonisation may depend on how the biocontrol agent interacts with the fungal community that is endophytically inhabiting the stump, as well as with other fungi establishing after the treatment is applied. In this research, we studied how the distribution and abundance of Heterobasidion and P. gigantea is modulated by the fungal community inhabiting Norway spruce ( Picea abies ) stumps. First thinning stumps were treated with Rotstop and/or artificially inoculated with H. parviporum under field conditions. After two months, P. gigantea or Heterobasidion distribution was measured by qPCR, and both pathogen and biocontrol abundance gradients were correlated with members of the fungal community determined by high throughput sequencing. Rotstop application decreased Heterobasidion biomass by almost 200 times, though the strongest biomass reductions were only achieved when stumps were well covered by P. gigantea . Poor Rotstop applications resulting in, for instance, a drop of 20% in terms of stump covered area by the biocontrol, were associated with a 2-fold biomass increase of the surviving Heterobasidion colonies. Unlike Heterobasidion , some fungi such as Peniophora sp. and F. pinicola did not change their relative abundance following the Rotstop treatment and were associated with areas of low P. gigantea presence. Yeast-like fungi of the genus Debaryomyces increased its relative abundance following Rotstop treatment, and were the only taxa associated with both a higher abundance of the biocontrol, and a lower abundance of the pathogen. Ours is the first study quantifying at microscale the interaction between H. parviporum and P. gigantea in stumps subjected to Rotstop treatment, showing how incomplete stump cover may result in large shifts in terms of biomass for either the pathogen or the biocontrol. Our results stress the importance of achieving a complete cover of the stump surface in practice. Debaryomyces sp. might facilitate P. gigantea colonisation of the stump and could be added as enhancers of Rotstop treatment, though the mechanism of interaction with the pathogen and the biocontrol should be further studied.

Black truffles are a highly valued non-wood forest product. The success of truffle plantations is arousing the interest to establish orchards within forest settings. One main concern is that the forest may act as a source of ectomycorrhizal fungi that could displace Tuber melanosporum in plantations and impair truffle production. We studied the effects of host tree distance to the surrounding forest on T. melanosporum development and on the root-associated fungal community. Our research was carried out in a 5-year old holm oak (Quercus ilex) plantation established in an abandoned pasture surrounded by a Q. ilex forest in the Pyrenees. The spatial distribution of different fungal guilds as well as of T. melanosporum mycelium quantity and mating types frequency was correlated with the distance to the forest and the diameter of the trees. We found a higher relative abundance of non-T. melanosporum EcM fungi associated with the trees closer to the forest. Larger root collar diameter trees had greater biomass of T. melanosporum mycelium and displayed fungal community compositions less affected by the distance to the forest. No associations between the biomass of T. melanosporum mycelium in the soil and the distance to the forest or the abundance of non-T. melanosporum EcM fungi were observed. Our results indicate that T. melanosporum inoculated oaks planted in areas surrounded by forests may be colonised by other ectomycorrhizal species, and develop a distinct microbial community from those usually established in agricultural lands. Further investigations should be carried out to determine whether a different fungal community may affect truffle production in the future, but to date, truffle mycelium does not seem to be impaired. This work was supported by ‘Direcció General d’Ecosistemes Forestals i Gestió del Medi Departament d'Agricultura, Ramaderia, Pesca i Alimentació’ of ‘Generalitat de Catalunya’ and the Project INNOVATRUF (PECT El bosc, el primer recurs de l’economia verda – Fons Europeu de Desenvolupament Regional de la Unió Europea-Programa operatiu FEDER de Catalunya 2014-2020). D. Oliach received support from the ‘Secretaria d’Universitats i Recerca del Departament d’Economia i Coneixement de la Generalitat de Catalunya’ through the program of ‘Doctorats Industrials’. J.A. Bonet benefit from Serra Húnter fellow provided by the Generalitat de Catalunya. J. Oliva was partly supported by a “Ramón y Cajal” fellowship (RYC-2015-17459).

Determining the impacts of invasive pathogens on tree mortality and growth is a difficult task, in particular in the case of species occurring naturally at low frequencies in mixed stands. In this study, we quantify such effects by comparing national forest inventory data collected before and after pathogen invasion. In Norway, Fraxinus excelsior is a minor species representing less than 1% of the trees in the forests and being attacked by the invasive pathogen Hymenoscyphus fraxineus since 2006. By studying deviations between inventories, we estimated a 74% higher-than-expected average ash mortality and a 13% slower-than-expected growth of the surviving ash trees, indicating a lack of compensation by the remaining ash. We could confidently assign mortality and growth losses to ash dieback as no mortality or growth shifts were observed for co-occurring tree species in the same plots. The mortality comparisons also show regional patterns with higher mortality in areas with the longest disease history in Norway. Considering that ash is currently mostly growing in mixed forests and that no signs of compensation were observed by the surviving ash trees, a significant habitat loss and niche replacement could be anticipated in the mid-term. Monitoring of ash dieback is part of the Norwegian forest damage monitoring programme. The study was within the project BiodivERsA RESIPATH: Responses of European Forest and Society to Invasive Pathogens, financed by Research Council of Norway (grant 235947/E40). This research also received funding from the European Union’s H2020 research and innovation programme, under the Marie Sklodowska-Curie grant agreement no. 691149 (SuFoRun). Additional support was received from the Norwegian Institute of Bioeconomy Research and the Norwegian National Forest Inventory (NFI). We would like to thank Rune Eriksen (NIBIO) for kind assistance in retrieving data from NFI database and Halvor Solheim (NIBIO) for sharing his data about the annual advancement of ash dieback in Norway. Jonàs Oliva was supported by the Ramón y Cajal fellowship (RYC-2015-17459).

Predicting whether naïve tree populations have the potential to adapt to exotic pathogens is necessary owing to the increasing rate of invasions. Adaptation may occur as a result of natural selection when heritable variation in terms of susceptibility exists in the naïve population. We searched for signs of selection on black alder (

Whenever a new invasive forest pathogen appears, resources are put into developing countermeasures such as breeding for resistance in the affected host species and/or by developing fungicide/biocontrol treatments. In most cases, little effort is placed into analysing the likelihood of those measures being implemented. Studying the reasoning behind management reactions of forest managers whose forests had been attacked by invasive pathogens can improve implementation of measures in case of future invasions. We used grounded theory to model forest owners’ reactions against chestnut blight caused by the invasive pathogen Cryphonectria parasitica . We focussed on understanding what drove some managers to keep managing chestnut forests and applying biological control, and what made others abandon or even substitute chestnut by exotics. We found that the availability of a market for chestnut products, and not the disease was the main reason for discontinuing chestnut management. The will to continue managing chestnuts was four times higher amongst managers of chestnut stands for nut production (valuable crop), than amongst managers of coppice stands producing small diameter stems, currently without a market. Requesting application of biological control against chestnut blight was four times more likely amongst forest managers obtaining monetary benefits from their land (benefits used as profit or re-investment) than those not obtaining monetary benefits or currently losing money with their land. Substitution with exotic conifers was associated with forest managers that had learned management from family and leaders of opinion, and that have either themselves or previous owners of the land planted exotics beforehand. Abandonment was associated with self-taught managers valuing non-monetary aspects of forests and having moral reasons against exotics. Our research highlights the importance of considering the socioeconomic context around the affected tree species when developing measures against invasive forest pathogens, and raises concerns about the continuity of tree species affected by invasive pathogens with little or no commercial value.

The contribution of non-lethal pathogenic attacks to tree death is still unclear. Manion’s theory of the spiral of decline predicts that tree decline and death occurs because of a sequence of predisposing, inciting and contributing events. To understand whether pathogens can act as predisposing or inciting factors, we tested whether a sequence of non-lethal pathogen attacks causing crown defoliation could lead to a chronic decline in tree health and predispose trees to die. Scots pine (Pinus sylvestris) trees predisposed or escaping (non-predisposed) a first outbreak by the pathogen Gremmeniella abietina (predisposing event) were compared in terms of survival and susceptibility to secondary pests (contributing event) after a second G. abietina outbreak (inciting event). Four years after the inciting event, mortality among predisposed trees was up to five times higher than among trees escaping the first epidemic. Predisposed trees were twice as susceptible to secondary attacks by the common pine shoot beetle (Tomicus piniperda). Ten years after the inciting event, severely predisposed trees had not been able to restore their crowns and still showed stagnated growth. This study showed that pathogen-induced defoliation can act as predisposing and inciting factors for tree death, reducing the capacity of trees to survive short- or long-term stressing events, such as bark beetle attacks. We also showed that tree decline can result from a combination of predisposing and inciting events caused by pathogens.

Alder trees (Alnus spp.) are key nitrogen-fixing riparian species in the northern hemisphere. Inputs of nitrogen-rich leaf litter from alder into stream food webs can contribute significantly to nitrogen dynamics at local and landscape scales. Alder trees also provide habitats for terrestrial and aquatic organisms, and help stabilize river banks. Recently, substantial declines in alder stands have occurred along streams in Europe, with damages observed in some parts of North America also. A major driver has been the invasive oomycete pathogen Phytophthora alni species complex, which can spread rapidly along stream networks. This review synthesises information on the pathogen, processes of spread and infection, and its impacts on alder. We further address the potential ecosystem-level and management consequences of the decline of alder, and highlight research needs. The alder dieback caused by P. alni is associated with reductions in shade and quality and quantity of leaf litter. A decline in the structural integrity of branches and roots further threatens bank stability. Stream banks dominated by other tree species or no trees at all will result in ecosystem-level changes both above and below the waterline. The P. alni taxonomic complex includes different species with varying phenotypes. An improved understanding of their environmental tolerances, virulence and evolution, along with the processes regulating the spread and impacts of the pathogen, would assist in identification of the riparian and stream systems most vulnerable not only to invasion but also to the heaviest disease outbreaks and ecosystem-level impacts. Within the P. alni complex, the highly pathogenic hybrid species P. x alni is favoured by mild winters and warm, but not excessively hot summer temperatures suggesting possible changes in distribution and level of impact under future global climate change.

Disease emergence in northern and boreal forests has been mostly due to tree-pathogen encounters lacking a co-evolutionary past. However, outbreaks involving novel interactions of the host or the pathogen with the environment have been less well documented. Following an increase of records in Northern Europe, the first large outbreak of Diplodia sapinea on Pinus sylvestris was discovered in Sweden in 2016. By reconstructing the development of the epidemic, we found that the attacks started approx. 10 years back from several isolated trees in the stand and ended up affecting almost 90% of the trees in 2016. Limited damage was observed in other plantations in the surroundings of the affected stand, pointing to a new introduced pathogen as the cause of the outbreak. Nevertheless, no genetic differences based on SSR markers were found between isolates of the outbreak area and other Swedish isolates predating the outbreak or from other populations in Europe and Asia Minor. On a temporal scale, we saw that warm May and June temperatures were associated with higher damage and low tree growth, while cold and rainy conditions seemed to favor growth and deter disease. At a spatial scale, we saw that spread occurred predominantly in the SW aspect-area of the stand. Within that area and based on tree-ring and isotope (δ13C) analyses, we saw that disease occurred on trees that over the years had shown a lower water-use efficiency (WUE). Spore traps showed that highly infected trees were those producing the largest amount of inoculum. D. sapinea impaired latewood growth and reduced C reserves in needles and branches. D. sapinea attacks can cause serious economic damage by killing new shoots, disrupting the crown, and affecting the quality of stems. Our results show that D. sapinea has no limitations in becoming a serious pathogen in Northern Europe. Management should focus on reducing inoculum, especially since climate change may bring more favorable conditions for this pathogen. Seedlings for planting should be carefully inspected as D. sapinea may be present in a latent stage in asymptomatic tissues.

Forest decline induced by climate change is a global phenomenon that affects many tree species, mainly in drought-prone areas as the Mediterranean region. In southern Italy, several oak species have shown decline symptoms and elevated mortality since the 2000s due to drought stress. However, it remains to be answered whether decline occurred alone or whether a pathogen was also involved. To this aim, we compared two coexisting oak species in a forest located in southern Italy which are assumed to be less (Quercus cerris) and more tolerant to drought (Quercus pubescens). We sampled fifteen couples of neighboring declining (D) and non-declining (ND) trees of both species. Wood cores were taken from all trees to perform dendrochronological analyses to detect the decline onset and link it to potential climatic drivers. Carbon isotope ratios (d13C) were analyzed in wood of the two vigor classes to compare their water-use efficiency. Phytophthora presence was also assessed in soil samples from ten D-ND couples of trees per species. The oak species most affected by drought-induced decline in terms of leaf shedding and mortality was Q. cerris, i.e., the least tolerant to drought. In both species, the D trees showed a reduced growth rate compared with ND trees from 2000 onward when drought and warming intensified. Q. pubescens showed higher growth sensitivity to precipitation, temperature and drought than Q. cerris. This sensitivity to climate was magnified in D trees whose growth decreased in response to warm and dry conditions during the prior winter and the late summer. The Q. pubescens D trees were more efficient in their water use than ND trees before the growth divergence between D and ND trees amplified. In the studied area, Phytophthora quercina was isolated from 40% of the sampled trees, and tended to be more frequent amongst ND than amongst D trees. Our data suggests that droughts and warm summer conditions triggered oak decline. The high prevalence of P. quercina in the studied area warrants further study as a potential predisposing factor.

Ash dieback is a fungal disease (causal agent Hymenoscyphus fraxineus) infecting Common ash (Fraxinus excelsior) throughout temperate Europe. The disease was first discovered in the UK in 2012 in a nursery in Southern England, in plants which had been imported from the Netherlands. After sampling other recently planted sites across England, more infected trees were found. Tree trade from outside and across the UK may have facilitated the spread of invasive diseases which threaten the sustainability of forestry business, ecological niches and amenity landscapes. Detecting a disease in a nursery at an early stage and knowing how likely it is for the disease to have spread further in the plant trade network, can help control an epidemic. Here, we test two simple sampling rules that 1) inform monitoring strategies to detect a disease at an early stage, and 2) inform the decision of tracking forward the disease after its detection. We apply these expressions to the case of ash dieback in the UK and test them in different scenarios after disease introduction. Our results are useful to inform policy makers’ decisions on monitoring for the control and spread of tree diseases through the nursery trade.

El aumento de la mortalidad y la pérdida de vitalidad de los bosques son procesos que afectan a diversas especies de árboles y biomas. Estos fenómenos de decaimiento forestal se han asociado a la interacción de múltiples factores relacionados con el cambio global, como el aumento de las sequías o la mayor incidencia de factores bióticos que generan estrés. Sin embargo, el papel que juegan estos organismos en el decaimiento del bosque en relación a la sequía no está del todo comprendido. En este trabajo caracterizamos retrospectivamente los efectos que ejercen dichos organismos sobre el crecimiento radial de árboles estresados por sequía utilizando dendrocronología. El efecto combinado del muérdago y las sequías reducen el crecimiento del pino silvestre y aumentan la defoliación, siendo estos efectos más evidentes en el ápice del árbol. La procesionaria induce pérdidas de crecimiento en pino laricio, pero en general se observa una recuperación dos años después. No encontramos un papel determinante de los hongos patógenos de raíz en el decaimiento del abeto. Finalmente, pinos carrasco muertos que presentaban ataques de insectos perforadores mostraron patrones de crecimiento similares a pinos vivos. Este estudio nos permite caracterizar algunos de los complejos y múltiples efectos de diferentes factores bióticos de estrés sobre el crecimiento y la vulnerabilidad de los bosques susceptibles a los efectos negativos de las sequías. Increasing mortality and decline of forests are affecting several tree species and biomes. These events of forest decline have been related to the interaction of several global change elements, like the increase in drought stress or a higher incidence of biotic stress factors. However, the roles played by biotic stressors have been scarcely explored as related to drought stress. In this paper we retrospectively characterize the effects of these organisms on radial growth of trees already stressed by drought using dendrochronology. The combined effect of mistletoe and droughts reduce tree growth in Scots pine and increase defoliation, being these effects more noticeable in the apex. The pine processionary moth induces growth losses in the affected stands of black pine, but they usually recover two years after the outbreak. We did not find a determinate role of root pathogens on silver fir dieback. Finally, dead Aleppo pine trees infested by bark beetles showed similar growth patterns than living pine trees. This study allows characterizing some of the complex and multiple effects of different biotic stressors on growth and vulnerability of forests subjected to the negative effects exerted by droughts. Este estudio ha sido financiado por los proyectos CGL-2007-6066-C04-02/BOS, CGL2008-04847-C02-01, CGL2011-26654 (MINECO), 387/2011 (Organismo Autónomo de Parques Naciona-les) y PROCLIM (La Caixa-Gobierno de Aragón).

Forest dieback is one of the most widespread responses to global-change drivers, such as climate warming-related drought stress and the spread of pathogens. Although both climatic and biotic stressors have been studied separately, much less is known on how drought and pathogens interact and induce dieback, particularly in formerly used forests. We determine the roles played by each of those drivers as factors causing recent dieback in three Pyrenean silver fir stands: a managed site subjected to past logging and two unmanaged sites not logged for the past 50 years. The age, size, recent competition, and basal-area increment (BAI) trends of non-declining and declining trees, and the presence of fungal pathogens were investigated. Growth patterns at yearly to decadal time scales were compared to distinguish the roles and interactions played by the different stressors. In the managed site, declining trees displayed low growth already before logging (1950-1970s). In both unmanaged sites, declining and non-declining trees displayed divergent growth patterns after extreme droughts, indicating that dieback was triggered by severe water deficit. We did not find indications that fungal pathogens are the primary drivers of dieback, since a low proportion of declining trees were infested by primary pathogens (10%). However, trees with the primary fungal pathogen Heterobasidion showed lower BAI than non-declining trees. On the other hand, the secondary fungal pathogen Amylostereum was isolated from a higher number of trees than expected by chance. These findings highlight the importance of legacies, such as the past use in driving recent forest dieback. Past forest use could predispose to dieback by selecting slow-growing trees and thus, making some them more vulnerable to drought and fungal pathogens. © 2015 Elsevier B.V., This study was supported by projects 387/2011 (OAPN, Spanish Ministry of Environment) and CGL2011-26654 (Spanish Ministry of Economy), and by EU COST Strees (FP1106) and Sensfor (ES1203) projects. We sincerely thank C. Rodriguez-García and the personnel of “Ordesa and Monte Perdido” National Park for their help in the field. We thank CRU and AEMET for providing climatic data.

The white-rot fungus Heterobasidion parviporum Niemela & Korhonen establishes a necrotrophic interaction with Norway spruce (Picea abies (L.) H.Karst.) causing root and butt rot and growth losses in living trees. The interaction occurs first with the bark and the outer sapwood, as the pathogen enters the tree via wounds or root-to-root contacts. Later, when the fungus reaches the heartwood, it spreads therein creating a decay column, and the interaction mainly occurs in the inner sapwood where the tree creates a reaction zone. While bark and outer sapwood interactions are well studied, little is known about the nature of the transcriptional responses leading to the creation of a reaction zone. In this study, we sampled bark and sapwood both proximal and distal to the reaction zone in artificially inoculated and naturally infected trees. We quantified gene expression levels of candidate genes in secondary metabolite, hormone biosynthesis and signalling pathways using quantitative polymerase chain reaction. An up-regulation of mainly the phenylpropanoid pathway and jasmonic acid biosynthesis was found at the inoculation site, when inoculations were compared with wounding. We found that transcriptional responses in inner sapwood were similar to those reported upon infection through the bark. Our data suggest that the defence mechanism is induced due to direct fungal contact irrespective of the tissue type. Understanding the nature of these interactions is important when considering tree breeding-based resistance strategies to reduce the spread of the pathogen between and within trees.

The biocontrol agent Phlebiopsis gigantea is widely applied to stump surfaces during commercial thinning operations in Northern Europe to prevent airborne infection of Norway spruce (Picea abies) stumps by the root and butt-rot pathogen Heterobasidion spp. It has been suggested that warmer climatic conditions in the future may affect the interaction between the pathogen and the biocontrol agent, and may enable Heterobasidion annosum sensu stricto, which is currently restricted to southern areas of Sweden, to move further northward. To determine whether warm temperatures do effect the interaction between P. gigantea and Heterobasidion spp., we conducted a series of experiments to investigate the effect of temperature on the growth of P. gigantea and several Heterobasidion spp. and on the ability of P. gigantea to overgrow Heterobasidion spp. in paired interaction experiments in vitro. To validate our in vitro findings, we set up field experiments to examine the interaction between P. gigantea and the Heterobasidion spp. that infect spruce in Sweden, Heterobasidion parviporum and H. annosum s.s., under different temperature regimes. Our study showed that temperature did not diminish the effectiveness of P. gigantea to protect spruce stumps against infection by H. parviporum. The growth rate of P. gigantea in stumps was higher than that of H. parviporum, particularly in sapwood. We found little support for a hypothetical increase in H. annosum s.s. damage under warmer climatic conditions. In vitro studies showed that regardless of the temperature regime, H. annosum s.s. was less resistant to overgrowth by P. gigantea than H. parviporum. Under field conditions, the relatively low infection capacity of H. annosum s.s. to P. abies stumps seems to be the crucial factor determining the outcome of the interaction. The results of paired in vitro experiments between P. gigantea and non-European Heterobasidion species, for example, Heterobasidion araucariae, Heterobasidion irregulare and others, suggest that P. gigantea may be less effective as a biocontrol agent against species or isolates adapted to high temperatures (20–25 °C), particularly those species that are able to grow rapidly at temperatures as high as 25 °C.

Summary Two studies were undertaken in a young New Zealand Pinus radiata stand to find a biological control agent for armillaria root disease caused by Armillaria novae-zelandiae. Fresh thinning stumps were inoculated with saprophytic basidiomycete species as wooden dowel cultures or as homogenized aqueous mycelial suspensions, while all stumps were also inoculated with dowel cultures of A. novae-zelandiae. After a period of between one and two years, no differences were found among test fungi in the percentage of Armillaria species isolated from stumps. However, inoculation of Stereum sanguinolentum as homogenate, and of Phlebiopsis gigantea, Sistotrema brinkmannii, Hypholoma acutum and Rigidoporus concrescens as dowel cultures, significantly increased their isolation incidence in comparison with stumps not treated by these species. Phlebiopsis gigantea, Resinicium bicolor and R. concrescens were also isolated with greater frequency nearer to their dowel inoculation points. Results suggest that P. gigantea and S. sanguinolentum should be included in further testing. However, a future trial is likely to be more effective if evaluation is directed towards the portion of the stump below the soil surface, particularly the root system.

International trade and climate change have increased the movement potential for invasive alien species (IAS), including invasive pests and pathogens (IPPs), to the point where biological invasions are considered one of the major threats to biodiversity. However, practical implementation of plant health with regard to IAS and IPPs is difficult: regulative responsibilities are commonly spread across different authorities, and resources on the ground are often limited. Based on a legislative and literature review and semi-structured qualitative interviews (N = 7), the present study examines the possibilities and potential risks of monitoring and detection of forest invasive species in Sweden, with a particular focus on forest plant nurseries. The study thus adds practical implementation aspects concerning possibilities to limit the spread of invasive species in the plant trade.

Some disturbances can drive ecological systems to abrupt shifts between alternative stages (tipping points) when critical transitions occur. Drought-induced tree death can be considered as a nonlinear shift in tree vigour and growth. However, at what point do trees become predisposed to drought-related dieback and which factors determine this (tipping) point? We investigated these questions by characterizing the responses of three tree species, silver fir (Abies alba), Scots pine (Pinus sylvestris) and Aleppo pine (Pinus halepensis), to a severe drought event. We compared basal area increment (BAI) trends and responses to climate and drought in declining (very defoliated and dying) vs. non-declining (slightly or not defoliated) trees by using generalized additive mixed models. Defoliation, BAI and sapwood production were related to functional proxies of tree vigour measured at the onset and end of the drought (non-structural carbohydrate concentrations, needle N content and C isotopic discrimination, presence of wood-inhabiting fungi). We evaluated whether early warning signals (increases in synchronicity among trees or in autocorrelation and standard deviation) could be extracted from the BAI series prior to tree death. Declining silver fir and Scots pine trees showed less growth than non-declining trees one to three decades, respectively, before the drought event, whereas Aleppo pines showed growth decline irrespective of tree defoliation. At the end of the drought period, all species showed increased defoliation and a related reduction in the concentration of sapwood soluble sugars. Defoliation was constrained by the BAI of the previous 5 years and sapwood production. No specific wood-inhabiting fungi were found in post-drought declining trees apart from blue-stain fungi, which extensively affected damaged Scots pines. Declining silver firs showed increases in BAI autocorrelation and variability prior to tree death. Synthesis. Early warning signals of drought-triggered mortality seem to be species specific and reflect how different tree species cope with drought stress. Highly correlated declining growth patterns during drought can serve as a signal in silver fir, whereas changes in the content of sapwood soluble sugars are suitable vigour proxies for Scots and Aleppo pines. Longer growth and defoliation series, additional vigour parameters and multi-species comparisons are required to understand and predict drought-induced tree death. Early warning signals of drought-triggered mortality seem to be species specific and reflect how different tree species cope with drought stress. Highly correlated declining growth patterns during drought can serve as a signal in silver fir, whereas changes in the content of sapwood soluble sugars are suitable vigour proxies for Scots and Aleppo pines. Longer growth and defoliation series, additional vigour parameters and multi-species comparisons are required to understand and predict drought-induced tree death., This study was supported by projects CGL2011-26654 (Spanish Ministry of Economy and Competitiveness), 1032S/2013 and 387/2011 (Organismo Autónomo Parques Nacionales, Spanish Ministry of Agriculture and Environment, Spain). We thank ARAID for supporting J.J.C. and the AEET for providing climatic data. We also thank M. Maestro and E. Lahoz for performing the chemical analyses and R. Hernández, A.Q. Alla and E. González de Andrés for their help in the field. The authors declare no conflict of interest.

Drought-induced forest mortality is emerging as a widespread phenomenon with potentially large 12 implications for forest function and dynamics (Allen et al., 2010; Anderegg et al., 2012; Martinez13 Vilalta et al., 2012). Although the physiological mechanisms underlying tree mortality are still not 14 completely understood, there is agreement that they involve the storage and transport systems of water 15 and carbohydrates (McDowell et al., 2008; Sala et al., 2010; McDowell, 2011). The xylem of plants is 16 susceptible to drought-induced embolism and severe water deficits may result in the complete loss of 17 xylem hydraulic conductivity and cause tree mortality (hydraulic failure; cf. Tyree & Sperry, 1988; 18 McDowell et al., 2008; Choat et al., 2012). Drought also has detrimental effects on the carbon 19 economy of plants, and it has been hypothesized that reduced assimilation due to stomatal closure 20 may lead to a depletion of stored carbon reserves and, eventually, to tree death due to carbon 21 starvation (Waring, 1987; Martinez-Vilalta et al., 2002; Breda et al., 2006; McDowell et al., 2008). 22 However, only in recent studies has a direct link between reduced carbon reserves and tree mortality 23 been established (Adams et al., 2009; Galiano et al., 2011; Adams et al., 2013; Hartmann et al., 2013; 24 Mitchell et al., 2013; Quirk et al., 2013; Sevanto et al., 2014). Finally, phloem transport could also 25 become impaired due to the inability of plants to maintain phloem turgor under extremely low xylem 26 water potentials, limiting the local availability of carbohydrates for metabolic functions (Sala et al., 27 2010; Sevanto et al., 2014). 28

Stump-to-tree spread is a key step in the root and butt-rot disease caused by Heterobasidion annosum sensu lato . In Norway spruce stands, Heterobasidion parviporum is more likely to spread to neighbouring trees from stumps with a large diameter than from small stumps. However, H. annosum sensu stricto does not appear to share this pattern of infection. Given that the ratio of heartwood to sapwood varies depending on the size of the stump, we investigated the role of heartwood and sapwood in stump infection by H. parviporum and H. annosum s.s. To determine whether heartwood–sapwood susceptibility was different in different host species, both Norway spruce and Scots pine stumps were included in the study. Sapwood, heartwood and the border zone between sapwood and heartwood of spruce stumps and the heartwood and sapwood of pine stumps were artificially inoculated with different genotypes of H. annosum s.s. and H. parviporum. Only one zone was inoculated per stump. Infection was assessed 2 and 10 months after inoculation at 5 and 30 cm below the point of inoculation. On spruce stumps, H. parviporum and H. annosum s.s. were mostly confined to the heartwood. The area infected by H. annosum s.s. was smaller than that infected by H. parviporum. Both sapwood and border infections tended to grow towards the heartwood, whereas heartwood inoculations tended to remain in the heartwood, indicating that heartwood was more favourable for H. parviporum and H. annosum s.s. colonisation in spruce stumps. On pine, heartwood colonisation failed and only H. annosum s.s. was found in the sapwood. Our results suggest a possible association between the size of the stump and the spread of H. annosum s.s. to neighbouring trees mediated by the heartwood content, suggesting that screening for heartwood resistance could be a potentially interesting direction for future research. Treating stumps at final felling would be advisable owing to the large area of heartwood exposed by this operation.

Obtaining reliable and representative mushroom production data requires time-consuming sampling schemes. In this paper, we assessed a simple methodology to detect mushroom emergence by trapping the fungal spores of the fruiting body community in plots where mushroom production was determined weekly. We compared the performance of filter paper traps with that of funnel traps and combined these spore trapping methods with species-specific quantitative real-time PCR and Illumina MiSeq to determine the spore abundance. Significantly more MiSeq proportional reads were generated for both ectomycorrhizal and saprotrophic fungal species using filter traps than were obtained using funnel traps. The spores of 37 fungal species that produced fruiting bodies in the study plots were identified. Spore community composition changed considerably over time due to the emergence of ephemeral fruiting bodies and rapid spore deposition (lasting from 1 to 2 weeks), which occurred in the absence of rainfall events. For many species, the emergence of epigeous fruiting bodies was followed by a peak in the relative abundance of their airborne spores. There were significant positive relationships between fruiting body yields and spore abundance in time for five of seven fungal species. There was no relationship between fruiting body yields and their spore abundance at plot level, indicating that some of the spores captured in each plot were arriving from the surrounding areas. Differences in fungal detection capacity by spore trapping may indicate different dispersal ability between fungal species. Further research can help to identify the spore rain patterns for most common fungal species. IMPORTANCE Mushroom monitoring represents a serious challenge in economic and logistical terms because sampling approaches demand extensive field work at both the spatial and temporal scales. In addition, the identification of fungal taxa depends on the expertise of experienced fungal taxonomists. Similarly, the study of fungal dispersal has been constrained by technological limitations, especially because the morphological identification of spores is a challenging and time-consuming task. Here, we demonstrate that spores from ectomycorrhizal and saprotrophic fungal species can be identified using simple spore traps together with either MiSeq fungus-specific amplicon sequencing or species-specific quantitative real-time PCR. In addition, the proposed methodology can be used to characterize the airborne fungal community and to detect mushroom emergence in forest ecosystems. We are very grateful to the PNIN of Poblet for supporting the process of installing and maintaining the experimental plots. We also thank Katarina Ihrmark and Johanna Boberg for providing the protocol and for advice on how to process and analyze the spore trap samples. We thank Josep Miró and Francesc Bolaño for their assistance with collecting the spore trap and fruiting body samples. We thank Caroline Woods for linguistic revision and for helping to clarify the manuscript. This work was supported by an STSM grant from COST Action FP1203 and by the Spanish Ministry of Economy and Competitiveness (MINECO) through the projects MICOGEST AGL2012-40035-C03 and MYCOSYSTEMS AGL2015-66001-C3. Carles Castaño received the support of the Secretaria d’Universitats i Recerca del Departament d’Economia i Coneixement de la Generalitat de Catalunya through the program of Doctorats Industrials. José Antonio Bonet is supported by the Serra-Hunter fellowship, and Josu G. Alday is supported by the Juan de la Cierva fellowship (IJCI-2014-21393). The funders had no role in the study design, data collection, and interpretation, or in the decision to submit the work for publication.

Rotfinder is a non-destructive decay-sensing apparatus based on resistance measurements in standing trees. The accuracy of Rotfinder in detecting decay was evaluated in 500 standing trees in three Norway spruce ( Picea abies ) plots. Trees were measured at three heights, 0.30, 0.66 and 1.30 m. Sections were later inspected for the presence of decay and reaction zones. Inspected trees were mostly infected by Heterobasidion annosum and showed a large variation in the amount of decay present, ranging from 0.1% to 88.0% of the section. Correctly and incorrectly classified trees were compared in terms of ion and element concentration, density and moisture. Measurements at stump level (0.30 m) were more accurate than measurements at breast height (1.30 m) where the reaction zone and decay columns showed lower moisture content. The accuracy of Rotfinder increased when trees with small decay columns were regarded as ‘non-decayed’. When only trees with more than 15% of the section decayed were regarded as ‘decayed’, Rotfinder had an accuracy of 0.86 when performing assessments at stump level. False negatives, as opposed to true positives, corresponded to trees with smaller and drier decay columns, drier reaction zones and lower K + (potassium) concentration in the decay column. False positives corresponded to trees with large sapwood and high sodium content in the sapwood. Rotfinder represents an alternative to the standard method of using increment core observations to assess decay in living trees.

Rotstand is a computer model that can simulate the development of a forest stand together with the root rot disease caused by Heterobasidion annosum s.l. on several tree species in Europe. We evaluated the accuracy of Rostand model for its use in Sweden by using data from two field experiments of Norway spruce (Picea abies) in which the long term outcome of stump protection methods had been evaluated (14 and 20 plots, respectively). One of the experiments included artificial infection of the stumps that enabled an almost complete parameterization of the model. Artificially infected plots were used for assessing the loss of precision of using average parameter values vs. plot-specific values. Average values obtained from artificially infected plots were used for validating the model on plots subjected to natural infection. Rotstand proved to be able to predict plots with a large variation of decay development (20–90% of stems with decay) as early as 15 years after infection. The parameter controlling the inoculum expansion within the tree root system appeared as a major factor affecting the accuracy of the predictions. Expansion of decay centres of artificially and naturally infected plots in Southern Sweden was not significantly different from natural infected plots in the rest of the country, opening the possibility of using a single average value all over Sweden. By using an expansion rate of 0.20 m year−1, Rotstand gave unbiased predictions of decay development 15 years after infection with a relative error of 38.4%. When using average parameter values, Rotstand tended to underestimate plots showing more than 50% of decay 15 years after infection; however, when simulating those plots beyond the last decay assessment, these errors were predicted to disappear.

The tree pathogenic fungi Heterobasidion annosum s.s. and Heterobasidion parviporum cause root and butt rot in Norway spruce (Picea abies) and produce serious economic losses to the forest sector in Europe. We experimentally studied inter- and intraspecific differences between H. parviporum and H. annosum s.s. in the way they infect stumps and spread into neighbouring trees. Eleven H. parviporum and nine H. annosum s.s. isolates were artificially inoculated on stumps of two spruce stands after first thinning. After 15 years, the same isolates were reisolated from neighbouring trees. Heterobasidion parviporum spread more frequently from the inoculated stumps to the neighbouring trees than H. annosum s.s. The surroundings of H. annosum s.s. stumps that did not spread were often colonized by H. parviporum. Heterobasidion annosum s.s. spread was restricted mainly to the areas of the plot where no other Heterobasidion genotypes had been inoculated. In such cases, H. annosum s.s. tended to develop into bigger genets than H. parviporum. The probability of stump-to-tree spread of H. parviporum depended on the diameter of the stumps, suggesting that H. parviporum spread may relate to the presence of heartwood. Both H. parviporum and H. annosum s.s. proved to be strong pathogens on Norway spruce; however, when competing for the same trees, H. parviporum seemed capable of excluding H. annosum s.s. from the stand.

Summary The tree pathogenic fungi Heterobasidion parviporum and H. abietinum are recently described sibling species that show marked host preference for Picea abies and Abies alba, respectively. H. parviporum shows a higher reproductive character displacement (i.e. higher inter-sterility) towards H. abietinum in sympatry than in allopatry. We inspected whether this pattern was also present in H. abietinum by studying the inter-sterility and gene flow amongst sympatric central European populations of H. abietinum and H. parviporum, and two isolated allopatric populations, Pyrenean H. abietinum and Scandinavian H. parviporum. Inter- and intra-specific fixation index values were calculated from DNA sequence data of the GST-1 locus and four microsatellite loci. Present allopatric and sympatric populations of H. abietinum were equally inter-sterile towards H. parviporum, not suggestive of relaxed reinforcement. Several hypotheses explaining the observed pattern are presented. Genetic differentiation was observed between H. abietinum in the Pyrenees and in the Alps, suggesting geographical structure of H. abietinum in Europe.

Airborne Heterobasidion annosum (Fr.) Bref. sensu lato infections can be controlled by winter thinning or by mechanically spreading urea or Phlebiopsis gigantea (Fr.) Jülich spores on stump surfaces during summer thinning operations. The long-term outcomes of these control methods when applied as part of the conventional forest operations are unclear. We studied the rot incidence and population structure of H. annosum in plots of Picea abies (L.) Karst. thinned in winter or thinned in summer with and without treatment of the stumps. Plots were distributed among 11 stands in Sweden representing two different land use histories: forest and agricultural. After 13 years, the effect of stump treatment on rot incidence was only evident in stands on former agricultural land. In stands planted on former forest land with higher levels of preexisting rot than on former agricultural land, the expansion of preexisting genets of H. annosum might have masked the effects of stump protection. In former forest land, unprotected summer plots showed a greater diversity of H. annosum genotypes and a smaller number of trees infected by each genet than in protected plots, suggesting that protection treatments prevented the establishment of new genets, which may result in a reduced rot incidence in the future.

Forest dieback and mortality events induced by drought stress are widely reported. However, few studies have jointly examined the role played by drought on growth and mortality in tree species inhabiting floodplain forests. Here, we focused on mortality events occurring since the early 2000s on large areas in a floodplain forest located within the Ticino regional park in Northwest Italy, where affected native (pedunculate oak, Quercus robur L.) and introduced tree species (black locust, Robinia pseudoacacia L.) coexist. We related growth with climate data and drought severity to discern if these species were similarly affected by drought. Then, we: (i) evaluated the presence of pathogens of the genus Phytophthora in recently dead oak trees since this was the most affected species and pathogens are often associated with oak decline cases; and (ii) compared xylem vessel diameter and tree-ring C isotope discrimination ( 13C) to highlight differences in water-use strategies between living and dead trees in both species. The radial growth of living and dead trees started diverging in the 1970s, although only after warm-drought periods occurred during 1990s did this divergence become significant. Growth of trees that died responded more negatively to drought than in the case of living trees. Moreover, trees that died formed smaller xylem vessels in the past than living trees and also showed more negative 13C values in both tree species, indicating a higher intrinsic water-use efficiency in living than in dead trees. The pathogen Phytophthora cinnamomi Rands was only detected in one recently dead tree, suggesting that it is unlikely that dead oaks were predisposed to drought damage by the pathogen. We conclude that a climate shift from wet to warm-dry summer conditions in the early 1990s triggered forest dieback and induced mortality in both tree species. Temperate floodplain forests are susceptible to drought-induced dieback. The drought-sensitivity of both species could lead to successional shifts driven by a reduction of N inputs through N-fixing by black locust and the replacement of oak by drought-tolerant species. This research was funded by the projects “Convenzione Rotondella” (Rotondella Administration, Basilicata Region, Italy) and CGL2015-69186-C2-1-R project (Spanish Ministry of Economy). M.C. was supported by the PhD program from the course of Agricultural, Forest and Food Science at the University of Basilicata (Italy)

In the last two decades, stand decline and increased mortality has affected silver fir (Abies alba) forests in the Spanish Pyrenees. Simultaneously severe occurrences of the root rot fungus Heterobasidion annosum s.l. and of the mistletoe Viscum album have been reported. We aimed to improve the understanding of the epidemiology of both pathogens in our region. All H. annosum isolates found on silver fir were typed as H. abietinum. H. abietinum was more frequently observed where cuttings had targeted fir trees rather than other species. H. abietinum fruiting bodies were observed in the most recently cut stumps. V. album was more abundant on more dominant fir trees, and in southern aspect stands. The number of V. album colonies in the stand correlated (R2 = 0.40) with silver fir mortality. Stands with a high level of V. album infection tended to have a smaller percentage of basal area in species other than silver fir, and they tended to be located on more south-facing slopes. H. abietinum was widespread in silver fir forests of the Pyrenees. Our data suggest that, in the Pyrenees, the observed H. abietinum incidence may represent a combination of both primary and secondary spread of the pathogen. Favouring mixed forests should be tested as a potential control method for V. album. The correlation between silver fir mortality and V. album infection warrants further study, as the observed tree mortality could have occurred due to other factors than V. album, such as drought damage.

Stump protection using chemical or biological agents is the main control measure against root and butt rot caused by Heterobasidion annosum s.l. in northern and temperate conifer forests. Long-term effects of urea treatment of stumps are poorly known and here we describe a 15-year study of urea treatment on the rot incidence on Norway spruce (Picea abies). We also tested the effect of urea protection on tree growth and on the resistance of stands against strong winds. Four treatments were made in two replications in two first-rotation P. abies stands in southern Sweden; after first-thinning stumps were (i) treated with urea 35% (w/v), (ii) artificially infected with H. annosum conidia, (iii) half urea treated and half artificially conidia infected, (iv) untreated, therefore naturally infected. After 15 years, the trees were sampled at 20 cm above ground using an increment borer and observed for presence of rot and, following incubation, presence of H. annosum conidia. Tree growth was calculated by measuring the diameter before and after the treatment. Urea treated plots showed the lowest incidence of rot (3%) as well as of H. annosum incidence (0%). Conidia treatment showed the highest incidence of rot (68%), its incidence was higher than that observed in natural infection treatment (43%), but did not differ from that of the 50% conidia treatment (47%). On about 30% of the rotted trees we observed conspicuous H. annosum fruiting. We did not observe growth reduction associated with tree rot. H. annosum was the only fungus observed associated with rotted trees which suggest that it was responsible for most of the rot observed in the investigated plots. Urea treated plots showed the lowest incidence of windthrown trees, and 59% of the windthrown tree incidence among the plots was explained by the incidence of H. annosum. Urea can be regarded as a reliable long-term protection method against root and butt rot of Norway spruce.

The present study examines the influence of ecological conditions and silvicultural practices on the health of silver fir (Abies alba Mill.) stands, with special attention to the implications of Viscum album, Armillaria and Heterobasidion annosum. Health of individual trees was estimated by measuring defoliation, chlorosis, needle retention, cone production, stork nest, epicormic sprouting and lichen cover. For stand health we measured regeneration. We used a systematic sampling throughout the Spanish Pyrenees, and analyzed the data by means of mixed models. We observed damage on 15% of the trees based on defoliation levels and on 10% of the trees based on chlorosis. Defoliation, chlorosis and cone production were more frequent in dominant trees while epicormic sprouting was more concentrated in the dominated layer. Stork nest and lichen cover may be linked to low growth in low quality sites. The incidence of V. album on trees was 13.9%, and positively related to defoliation and chlorosis. We did not find a clear relationship between H. annosum or Armillaria and decline symptoms. Both pathogens appeared to be widespread in silver fir forests. No regeneration difficulties were observed anywhere. At stand level silver fir does not seem to be a species in decline. Silver fir was favoured during the last period of intensive cuttings, whilst Pinus sylvestris and Pinus uncinata nearly disappeared. At tree level, fir decline and health problems are more localized and advanced on seed trees left after the last cuttings.

Melampsorella caryophyllacearum is a heteroecious fungus that produces disease on silver fir resulting in galls that depreciate the value of trees. Currently, little is known about its epidemiology. The purpose of this study was to identify those site or management factors associated with fir broom rust infection. A 29-plot systematic sampling was carried out throughout all silver fir forests in the Spanish Pyrenees. In every sampling plot, we examined each tree for the presence of M. caryophyllacearum galls or witch’s brooms. We differentiated their position as either on the main stem or on the branches of trees. We identified the most susceptible stands by relating a compendium of tree and stand variables with the presence and severity of symptoms. Disease symptoms were observed in 11.8% (95% CI: 5.8–17.9%) of trees. Fir broom rust-infected trees tended to be those of larger diameters. The rust infection was observed in 55.2% (95% CI: 35.7–73.6%) of the stands. We identified a wide range of severities with 5 of 29 stands having more than 30% of trees infected. Fir broom rust appeared to be a widespread disease, although we observed important differences in terms of severity. The presence and severity of the rust in stands was associated with understories composed of more shade tolerant plant species and with longer periods without thinnings. Canopy opening should be experimentally tested as a possible control method.

Redondo, M. A., Boberg, J., Olsson, C. H. B., and Oliva, J. 2015. Winter conditions correlate with Phytophthora alni subspecies distribution in southern Sweden. Phytopathology 105:1191-1197. During the last century, the number of forest pathogen invasions has increased substantially. Environmental variables can play a crucial role in determining the establishment of invasive species. The objective of the present work was to determine the correlation between winter climatic conditions and distribution of two subspecies of the invasive forest pathogen Phytophthora alni: P. alni subspp. alni and uniformis killing black alder (Alnus glutinosa) in southern Sweden. It is known from laboratory experiments that P. alni subsp. alni is more pathogenic than P. alni subsp. uniformis ,a nd that P. alni subsp. alni is sensitive to low temperatures and long frost periods. By studying the distribution of these two subspecies at the northern limit of the host species, we could investigate whether winter conditions can affect the geographical distribution of P. alni subsp. alni spreading northward. Sixteen major river systems of southern Sweden were systematically surveyed and isolations were performed from active cankers. The distribution of the two studied subspecies was highly correlated with winter temperature and duration of periods with heavy frost. While P. alni subsp. uniformis covered the whole range of temperatures of the host, P. alni subsp. alni was recovered in areas subjected to milder winter temperatures and shorter frost periods. Our observations suggest that winter conditions can play an important role in limiting P. alni subsp. alni establishment in cold locations, thus affecting the distribution of the different subspecies of P. alni in boreal regions. Additional keywords: Alnus spp., invasiveness, Phytophthora plurivora.