Forest self-regeneration following clear-felling of dieback-affected Fraxinus excelsior: focus on ash

Due to the dieback caused by invasive fungus Hymenoscyphus pseudoalbidus, ash is threatened in many regions of Europe. In order to predict the situation with ash ecosystems, it is important to examine forest regeneration in areas formerly dominated by ash with long disease history and known management. The main aim of the present study was, therefore, to evaluate forest self-regeneration following clear-felling of ash dieback-devastated stands, focusing on ash. In Lithuania, during 2011, a forest regeneration inventory was performed on twenty clear-cuts, initially dominated by stands of ash, but severely damaged by the dieback caused by H. pseudoalbidus and clear-felled 1–10 years previously. In subsequent forest regeneration, grey alder and birch dominated. Ash regeneration was scarce, and its proportion decreased from 40–100 % in pre-dieback stands to 0–21 % in current stands. Of all observed ash trees (n = 775), 53.9 % were diseased, 16.8 % were dead, and only 29.3 % were visually healthy. Moreover, ash was among the slowest growing species, while grey alder, aspen and birch were the fastest. The regeneration and health condition of ash in forests, where previous to the dieback ash was dominant, demonstrated a sharp decrease in occurrence of this species and a clear shift in species composition towards grey alder and birch.     

First report of Hymenoscyphus fraxineus on Fraxinus excelsior in Montenegro

During the monitoring of the mycological complex on different forest tree species in the Biogradska Gora National Park in north‐east Montenegro, symptoms indicative of ash dieback caused by Hymenoscyphus fraxineus were observed on young Fraxinus excelsior trees in the protected virgin forest, including dieback of plants and branches, wilting of leaves and shoots leading to a “flag‐like” habitus, premature shedding of leaves and longitudinal bark necroses. Using standard isolation methods, slow‐growing cultures with numerous phialides, typical of the asexual phase of the ash dieback fungus, were obtained. In addition, petioles with numerous characteristic apothecia were also recorded. This is the first report of H. fraxineus on common ash in Montenegro. Possible pathways of introduction and implications of the findings are discussed.     

Xylem sap flow and drought stress of Fraxinus excelsior saplings

We report on diurnal and seasonal variations in sap flow rate and stem water potential of Fraxinus excelsior L. saplings growing at the edge of a Fraxino-Aceretum forest in western Germany. Because of shallow soil, the trees were subjected to drought in summer. When soil water availability was not limiting, sap flow rate was related to changes in solar radiation and vapor pressure deficit. Maximum transpiration rates per leaf area were 3.5-7.4 mmol m-2 s-1, and maximum daily totals were 1.7-3.3 kg m-2 day-1. Under drought conditions, stem water potential dropped to midday minima of -2.6 to -3.5 MPa and sap flow rate was strongly related to this parameter. After the drought period, reduced apparent (whole-plant) hydraulic conductance was observed, which was attributed to a continued reduction in stomatal conductance after the drought stress had ceased. A model was developed that linked sap flow rate directly to climatic variables and stem water potential. Good correlation between measured and simulated sap flow rates allowed the model to be used for data interpretation.     

Fungi associated with necrotic apical parts of Fraxinus excelsior shoots

Stem necrosis was investigated in 2–3‐year‐old seedlings of European ash Fraxinus excelsior, both self‐sown and in plantations within four Forest Divisions in eastern, northern and central Poland. Fungi isolated from the apical parts of shoots showing fresh necroses were identified and the pathogenicity of some of them was determined. Some species were isolated at all four sites; the most consistent members of this group were Cladosporium cladosporioides, Cytospora ambiens, Discula sp., Fusarium lateritium, Phoma sp., Phomopsis scobina and P. controversa. Diplodia mutila was isolated at all the sites, but with variable frequency. It induced necrotic lesions following wound inoculation of stems of 2‐month‐old and 2‐year‐old seedlings of F. excelsior in growth chamber and greenhouse conditions.     

On the longevity of Hymenoscyphus pseudoalbidus in petioles of Fraxinus excelsior

Air‐dried pseudosclerotia produced by the ash dieback pathogen Hymenoscyphus pseudoalbidus (anamorph: Chalara fraxinea) can survive up to 3 months. Under field conditions, they are capable of postponing fructification for 1 year. The implications of this observation for epidemiology and control of ash dieback are discussed.     

A review of black heart of ash (Fraxinus excelsior L.)

Black heart is a non-fungal stain of ash (Fraxinus excelsiorL.) which results in a price penalty compared with white wood.The formation of black heart varies within individual trees,between trees and on different sites; the evidence availablesuggests that it can be present in up to 45 per cent of treesat age 60. Once formed the proportion of cross-sectional areaaffected increases with age. Previous work has suggested thatblack heart has no effect on timber properties or strength butits occurrence has been linked with high soil moisture. Howeverthese studies were conducted in the 1950s and need to be confirmedusing more rigorous research methods. Scope exists for innovativemarketing of black heartwood to emphasize that it is a naturalfeature and can be visually pleasing; this may lead to betterreturns for the grower.     

Differences in susceptibility to ash dieback in Czech provenances of Fraxinus excelsior

The impact of ash dieback caused by Hymenoscyphus fraxineus on 17 provenances of Fraxinus excelsior and one provenance of Fraxinus angustifolia was studied in an extensive field trial established in the Czech Republic prior to the H. fraxineus invasion in 1999. A difference in the level of resistance to ash dieback between the species was found: F. angustifolia was significantly less affected by the disease than F. excelsior. Moreover, particular provenances of F. excelsior showed important differences in the level of resistance to H. fraxineus. A relationship between the impact of ash dieback and altitude was also discovered – the provenances from altitudes above 600 m a.s.l. were less affected by the pathogen than were the provenances from lower areas. No difference in the impact of the disease among provenances of F. excelsior from different ecotopes (ravine, calcareous ravine and alluvial) was found. Substantial among‐tree variability in resistance to H. fraxineus was observed throughout the trial – promising genotypes (with crown defoliation up to 5%) were identified in all 18 tested provenances. In regard to this finding, it appears that the main source of resistance to the pathogen is probably at the individual genotype level in the trial. A secondary but massive attack by Hylesinus fraxini was identified in the trees that had been greatly damaged by ash dieback, and the beetle caused their health to deteriorate significantly. A significant negative effect of the presence of collar necroses caused by H. fraxineus and browse damage was also identified.     

Fraxinus excelsior seed is not a probable introduction pathway for Hymenoscyphus fraxineus

We investigated the transmission of Hymenoscyphus fraxineus from infested seed to germinating seedlings of common ash (Fraxinus excelsior) in order to determine the potential risk associated with intra‐ and intercontinental movement of seed. Neither fungal isolations from necrotic or healthy embryos nor PCR testing with H. fraxineus‐specific primers detected the pathogen. Similarly, H. fraxineus was not detected in axenically grown seedlings generated from infested seed lots. The results help clear up prior confusion of the pathogen being seed‐borne. Any remaining surface contamination by pathogen spores could be washed off seeds as a quarantine measure.     

Non-destructive monitoring of early stages of white rot by Trametes versicolor in Fraxinus excelsior

? Non-destructive detection of fungal decay in living trees is relevant for forest management of valuable species, hazard tree assessment, and research in forest pathology. A variety of tomographic methods, based on stress wave timing, radioactive radiation, or electrical resistivity have been used to detect decay in standing trees non-destructively. But apart from mobile gamma ray computed tomography (Habermehl and Ridder, 1993) which is virtually unavailable, the detection of incipient stages of decay is still not possible.

? Wood moisture and electrolyte content influence the electric resistivity of wood. Both are changed by fungal decay. Therefore electric resistivity tomography (ERT) should detect decay in its early stages. Then it could be used to monitor the spatial and temporal progress of degradation.

? We infected four Fraxinus excelsior trees with Trametes versicolor using wooden dowels and measured two-dimensional electric resistivity tomograms 3, 10, 13 and 21 months after infection. Immediately after the last electric resistivity measurement trees were felled for further analyses of stem cross-sections. Wood moisture content and raw density had significantly increased in infected areas, but dry density had not significantly changed after 21 months. Areas of very low electric resistivity around the infected wounds correlated very well with infected wood in the stem cross-sections. Increasing areas of low electric resistivity around the infected wounds during consecutive measurements indicate increasing areas of infected wood.

? We conclude that the growth of white rot by Trametes versicolor can be monitored with electric resistivity tomography (ERT) beginning from incipient stages, even before wood density decreases. ERT could therefore be a powerful research tool for decay dynamics as well as a method for diagnosing wood decay in forestry and arboriculture.

     

Escuside,a new coumarin-secoiridoid from Fraxinus ornus bark

A new coumarin-secoiridoid, named escuside, was isolated from the bark of Fraxinus ornus and its structure determined by spectroscopic methods.     

Natural infection of Fraxinus excelsior seeds by Chalara fraxinea

The occurrence of Chalara fraxinea, the fungus responsible for dieback of European ash (Fraxinus excelsior), was investigated in the current and previous seed years collected from symptomatic trees in Latvia and Sweden using molecular techniques (DNA extraction, ITS‐PCR, Sanger sequencing). Molecular analysis of seeds revealed the presence of 30 different fungal taxa. Chalara fraxinea was detected in 8.3% of seeds tested from the current year originating from Latvia. The presence of C. fraxinea in seeds of F. excelsior is of great concern to phytosanitary protection authorities in countries outside the current zone of infestation.     

Genetic variation in dieback resistance in Fraxinus excelsior confirmed by progeny inoculation assay

Ash dieback caused by the pathogenic fungus Hymenoscyphus fraxineus [previously known as H. pseudoalbidus (sexual stage) and Chalara fraxinea (asexual stage)] is a widespread problem in Europe. Here, we assess crown damage from natural infection and necrosis development following artificial controlled inoculations on full‐sib and half‐sib progeny from Danish Fraxinus excelsior clones with contrasting and well‐characterized levels of susceptibility to the disease. The inoculation assay was performed on a total of 123 offspring, and necrosis development monitored over two years. The offspring from low susceptible mother clones developed smaller necroses when compared to offspring from susceptible clones. Their crown damage due to natural infections was also significantly less. The correlation coefficient between average crown damages of mother clones and the average of their progeny was 0.85 (natural infections), while the correlation between crown damage of mother clones and the average necrosis development in their progeny after controlled inoculation was 0.73. The correlation between resistance of parent trees and crown damage/necrosis development on their offspring confirms the presence of heritable resistance and indicates that a bioassay based on controlled inoculations has the potential of becoming a fast and cost‐effective tool for estimation of dieback susceptibility in breeding programmes for resistance in ash trees.     

Genetic resistance to Hymenoscyphus pseudoalbidus limits fungal growth and symptom occurrence in Fraxinus excelsior

The European common ash (Fraxinus excelsior) is currently threatened by a pathogenic fungus, Hymenoscyphus pseudoalbidus, which seems to enter the trees through the leaves. Continuous assessments of 39 clones in Danish field trials have shown that there are significant differences in the susceptibility of clones to the new disease. Interestingly, clones that showed early leaf senescence in the autumn were in general less susceptible to the disease than late‐senescing clones. Thus, variation in susceptibility could be owing to phenological differences associated with the infection biology. To test whether differences in susceptibility are driven by genetically based factors other than phenology, we compared inoculations with H. pseudoalbidus on four highly susceptible clones with those of four less susceptible clones. Development of necrosis was hereafter followed regularly. The growth of the fungus in the inner bark was further detected with species‐specific PCR primers. The severity of the response to infection shows significant differentiation among clones and significant correlation with clone susceptibility, as assessed from natural infections in field trials. The fungus was detected in tissues immediately surrounding the necrosis but showed some signs of endophytic growth. The results suggest that healthier clones are able to limit the growth and spread of the fungus and thereby minimize the occurrence of symptoms. This gives hope for the future preservation of F. excelsior in Europe through selection and breeding.     

Die Esche(Fraxinus excelsior L.) — ein Baum des Leipziger Auwaldes?

Zusammenfassung Die historische Analyse der Waldentwicklung des Leipziger Auwaldes ergab, da?Fraxinus excelsior bis Mitte des 19. Jahrhunderts nicht in der Baumschicht vorkam. Ihre Einbringung erfolgte durch forstliche Ma?nahmen beginnend am Ende des 18. Jahrhunderts und wurde ab 1870 verst?rkt. Heute ist die Esche die h?ufigste Baumart im Leipziger Auwald. Weitere Beispiele zeigen, da? der Eschenanbau keine lokale Erscheinung darstellt, sondern auch in anderen europ?ischen Gebieten zur gleichen Zeit erfolgte.      

Mycological and histological investigations of Fraxinus excelsior nursery saplings naturally infected by Chalara fraxinea

Ash dieback is an emerging disease of Fraxinus excelsior in Germany. To date, economical damage is significant in nurseries, which also contribute towards spread of the disease, but damage to forests is increasing. The study presents the results of mycological and histological investigations on three hundred 3‐year‐old nursery ash saplings. The infection rate by the causative pathogen was determined for bark, outer and inner xylem, the pith and also separately for the above‐ground portion and root system of the plants. The invasion and colonization strategy of the fungus in the woody stem was examined. In addition, the presence of soil‐borne Oomycetes as possible primary or accompanying causal organisms was investigated. The results verify the dominant role of Chalara fraxinea as a causal agent of ash dieback and rule out the role of Oomycetes in the disease process. We conclude that C. fraxinea is not primarily endophytic in nature and spreads very effectively in the central stem tissues, which enables colonization of the woody stem in all three dimensions. Infections arising in the upper part of plants can thus spread extensively to lower parts.     

Differences in chemical composition relative to functional differentiation between petioles and laminas of Fraxinus excelsior

Differences in structural and nonstructural carbohydrates, lignin and chlorophyll, and Rubisco (ribulose-1,5-bisphosphate carboxylase/oxygenase) activity between petioles and leaflets were studied along a canopy light gradient in Fraxinus excelsior L., which has pinnate compound leaves and up to 20% of foliar biomass invested in petioles. Long-term light conditions at the sampling locations were characterized by values of seasonal mean integrated quantum flux density (Q(int), mol m(-2) day(-1)) estimated by combining data from hemispherical photographs at the sampling locations with measurements of global solar radiation above the canopy during the growing season. The contribution of petioles to leaf carbon assimilation was disproportionally lower than that of leaf laminas. Though the light relationships of assimilative compounds-foliar chlorophyll concentration increasing with decreasing Q(int) to improve leaf absorptance, foliar N concentration and Rubisco activity being relatively constant along the light gradient-were similar for both petioles and leaflets, petiole nitrogen and chlorophyll concentrations were only 30% and 10%, respectively, of those of leaflets. Nonstructural carbohydrate concentration was about 20% higher in petioles than in leaf laminas, indicating that petioles also serve as storage tissues for photosynthates. Relationships between foliar structural carbon components and irradiance-increasing lignin (L) and decreasing structural polysaccharide (SP) concentrations with increasing Q(int)-were qualitatively similar for petioles and leaflets. However, petioles had lower L, but higher SP and total investment in structural compounds (L + SP) than leaflets. Greater lignification at high irradiances in leaflets than in petioles was attributed to greater water stresses at high light, and to more variable water contents of actively transpiring leaflets. Low lignin concentration in combination with high osmotically active carbohydrate concentrations in petioles suggest that turgor plays an important role in the mechanical properties of petioles. As a result of lower lignin and protein concentrations, the glucose cost of petiole construction (g glucose per g dry mass) was about 5% lower than that of leaf laminas.     

Genetic variability and divergence among Italian populations of common ash (Fraxinus excelsior L.)

The level of genetic variation throughout the Italian range of common ash (Fraxinus excelsior L.) was estimated using six microsatellite markers. High levels of allelic diversity was detected. The levels of expected heterozygosity for each of the populations ranged from 0.726 to 0.871, with an average of 0.798, and indicated that populations have a high level of genetic variation. A general and significant homozygote excess was found at most loci in all populations, with an overall mean F _IS of 0.284. Possible explanations for such situations are discussed. Only 4.9% of the total diversity was attributable to differentiation among populations. Although divergence among pedo-climatic regions explained only a small part of the variance it was possible to observe some partial clustering of populations belonging to the same regions. The contribution of the results in relation to the definition of the most appropriate strategies to collect forest reproductive material is discussed.