Common Ash Fraxinus Excelsior Research Articles

Long-term woodland restoration on lowland farmland through passive rewilding.

Natural succession of vegetation on abandoned farmland provides opportunities for passive rewilding to re-establish native woodlands, but in Western Europe the patterns and outcomes of vegetation colonisation are poorly known. We combine time series of field surveys and remote sensing (lidar and photogrammetry) to study woodland development on two farmland fields in England over 24 and 59 years respectively: the New Wilderness (2.1 ha) abandoned in 1996, and the Old Wilderness (3.9 ha) abandoned in 1961, both adjacent to ancient woodland. Woody vegetation colonisation of the New Wilderness was rapid, with 86% vegetation cover averaging 2.9 m tall after 23 years post-abandonment. The Old Wilderness had 100% woody cover averaging 13.1 m tall after 53 years, with an overstorey tree-canopy (≥ 8 m tall) covering 91%. By this stage, the structural characteristics of the Old Wilderness were approaching those of neighbouring ancient woodlands. The woody species composition of both Wildernesses differed from ancient woodland, being dominated by animal-dispersed pedunculate oak Quercus robur and berry-bearing shrubs. Tree colonisation was spatially clustered, with wind-dispersed common ash Fraxinus excelsior mostly occurring near seed sources in adjacent woodland and hedgerows, and clusters of oaks probably resulting from acorn hoarding by birds and rodents. After 24 years the density of live trees in the New Wilderness was 132/ha (57% oak), with 390/ha (52% oak) in the Old Wilderness after 59 years; deadwood accounted for 8% of tree stems in the former and 14% in the latter. Passive rewilding of these ‘Wilderness’ sites shows that closed-canopy woodland readily re-established on abandoned farmland close to existing woodland, it was resilient to the presence of herbivores and variable weather, and approached the height structure of older woods within approximately 50 years. This study provides valuable long-term reference data in temperate Europe, helping to inform predictions of the potential outcomes of widespread abandonment of agricultural land in this region.

Disturbance by invasive pathogenic fungus alters arthropod predator-prey food-webs in ash plantations.

According to the disturbance-succession theory, natural disturbances support biodiversity and are expected to increase the complexity of food-webs in forest ecosystems by opening canopies and creating a heterogeneous environment. However, a limited number of studies have investigated the impact of disturbance by invasive pathogenic species and succession on arthropod predator-prey food-webs in forest ecosystems. Hymenoscyphus fraxineus is a pathogenic fungus of ash trees that is invasive in Europe and causes massive dieback, mainly of the common ash Fraxinus excelsior across its native range. Here we investigated how this pathogenic fungus affects food-webs of web-building spiders and their prey in understorey vegetation of ash plantations. In 23 young and middle-aged ash plantations that were distributed along a gradient of infestation by H. fraxineus (29%-86% infestation), we measured the vegetation structure (canopy openness, shrub coverage, herb/grass coverage), the trait composition of local spider communities (web type, body size), the prey availability and the prey intercepted by spider webs. We then evaluated the multivariate prey composition (prey type, body size) and network properties. Hymenoscyphus fraxineus opened the ash tree canopy, which resulted in denser shrub coverage. The dense shrub vegetation changed the composition of web types in local spider communities and increasing fungus infestation resulted in reduced mean body size of spiders. Infestation by H. fraxineus reduced the availability of predaceous Coleoptera and increased the availability of herbivorous Coleoptera as potential prey. The mean body size of captured prey and the per capita capture rates of most prey groups decreased with increasing fungus infestation. Hymenoscyphus fraxineus infestation indirectly reduced the complexity in bipartite networks and the trophic functional complementarity in local web-building spider communities. The plantation age affected the vegetation structure but did not affect the studied food-webs. Forest disturbance by the invasive pathogen affected four trophic levels (plant-herbivore-coleopteran intermediate predator-top predator web-building spiders) and, contrary to the disturbance-succession theory, disturbance by the fungus simplified the web-building spider-prey food-webs. The results support the view that H. fraxineus represents a threat to the biodiversity and ecosystem functioning in the simplified ecosystems of ash plantations.

Interspecific Variability of Water Storage Capacity and Absorbability of Deadwood

The aim of the study was to determine the water storage capacity and absorbability of deadwood of different tree species with varying degrees of decomposition. Coniferous (Silver fir—Abies alba Mill.) and deciduous (Common hornbeam—Carpinus betulus L., Common ash—Fraxinus excelsior L., Common alder—Alnus glutinosa Gaertn., and Common aspen—Populus tremula L.) species were selected for the research. The study focuses on the wood of dead trees at an advanced stage of decomposition. Deadwood samples were collected at the Czarna Rózga Nature Reserve in central Poland. Changes over time of the water absorbability and water storage capacity of deadwood were determined under laboratory conditions. The research confirmed the significance of the wood species and the degree of wood decomposition in shaping the water storage capacity and absorbability of deadwood in forest ecosystems. Fir wood was characterized by having the highest water storage capacity and water absorbability. Among deciduous species under analysis, aspen wood was characterized by having the highest water storage capacity and absorbability. Our research has confirmed that deadwood may be a significant reservoir of water in forests.

Initial regeneration success of tree species after different forestry treatments in a sessile oak-hornbeam forest

Ecological, economic, and social demands triggered a shift in the management of temperate deciduous forests from rotation forestry system towards more nature-based forest management techniques such as continuous cover forestry. However, there is insufficient knowledge on the regeneration success of different tree species—especially oaks—within this management system. Through a systematic experiment, we compared the regeneration processes of a sessile oak-hornbeam forest after gap-cutting (as an element of continuous cover forestry system) to regeneration after clear-cutting, preparation cutting, and in retention tree groups (treatments of rotation forestry system). A managed, closed, mature forest was used as control. Several different aspects of the regeneration were studied: (1) seed supply of sessile oak—Quercus petraea (Matt.) Liebl., (2) species number and abundance of the natural regeneration, (3) survival and growth of individual saplings of five tree species (sessile and Turkey oak—Quercus cerris L., hornbeam—Carpinus betulus L., beech—Fagus sylvatica L., and common ash—Fraxinus excelsior L.). The number of acorns was high in closed forest, intermediate in preparation cutting and retention tree group, low in gaps, and zero in clear-cutting. Four years after the interventions, there was no detectable treatment effect on the species number of regeneration. Survival increased in every treatment compared to control, but there was no significant difference in this measure between the differently treated sites. Height growth was highest in the gaps and clear-cuts, intermediate in preparation cuts, and lowest in retention tree groups and controls. Species with different seed dispersal mechanisms responded differently to treatments: oaks were dispersal-limited in the gaps and clear-cuts, while anemochorous species (e.g., hornbeam and manna ash) were present in every treatment. The survival and growth pattern of the particular species proved to be similar, but the intensity of the response differed: shade-tolerants (hornbeam, beech, and ash) showed better survival than oaks in most treatments, and their height growth was larger. According to our results, oak regeneration establishes successfully in oak-hornbeam forests not only in the case of rotation forestry, but also during continuous cover forestry (gap-cutting). The survival and growth of the saplings are similar in cutting areas and gaps, but keeping in mind other considerations (such as preserving forest continuity, balanced site conditions, and forest biodiversity), continuous cover forestry should be preferred.

Spread of Hymenoscyphus fraxineus in Latvia: Analysis based on Dynamics of Young Ash Stands

Abstract In Latvia, during the last 15 years (2000–2015), the area of common ash Fraxinus excelsior forest stands has decreased by 40.6%. The dieback was predominantly caused by the fungal pathogen Hymenoscyphus fraxineus. Mostly young stands (up to 40 years old) were affected, accounting for 77.3% of the area of dieback. In this paper, we analysed the dynamics of young ash stand area within nature regions in Latvia to attempt to determine patterns of spread and the major migration routes of H. fraxineus. As suggested by the available data, the invasion of the fungal pathogen began in the southern part of Latvia, and then gradually dispersed across the country. The largest decline of young ash stands, during the period 2000–2010, occurred in lowlands. According to our estimate, the average rate of dispersal in Latvia was 40 km per year. At the scale of Latvia, the dispersal routes of fungal pathogen H. fraxineus largely coincide with the major migration corridors of biota and are related to macro-relief forms and their configuration.

Population structure of the invasive forest pathogen Hymenoscyphus pseudoalbidus.

Understanding the genetic diversity and structure of invasive pathogens in source and in introduced areas is crucial to the revelation of hidden biological features of an organism, to the reconstruction of the course of invasions and to the establishment of effective control measures. Hymenoscyphus pseudoalbidus (anamorph: Chalara fraxinea) is an invasive and highly destructive fungal pathogen found on common ash Fraxinus excelsior in Europe and is native to East Asia. To gain insights into its dispersal mechanisms and history of invasion, we used microsatellite markers and characterized the genetic structure and diversity of H. pseudoalbidus populations at three spatial levels: (i) between Europe and Japan, (ii) in Europe and (iii) at the epidemic's front in Switzerland. Phylogenetic and network analysis demonstrated that individuals from both regions are conspecific. However, populations from Japan harboured a higher genetic diversity and were genetically differentiated from European ones. No evident population structure was found among the 1208 European strains using Bayesian and multivariate clustering analysis. Only the distribution of genetic diversity in space, pairwise population differentiation (GST) and the spatial analysis of principal components revealed a faint geographical pattern around Europe. A significant allele deficiency in most European populations pointed to a recent genetic bottleneck, whereas no pattern of isolation by distance was found. Our data suggest that H. pseudoalbidus was introduced just once by at least two individuals. The potential source region of H. pseudoalbidus is vast, and further investigations are required for a more accurate localization of the source population.

Gene exchange across a postglacial contact zone in Fraxinus excelsior L.

AbstractHybridization between divergent lineages of common ash Fraxinus excelsior L. was studied in Slovakia and adjacent regions of Hungary, the Czech Republic and Poland in the contact zone between two postglacial colonization routes originating from different glacial refugia. Thirty-eight common ash populations were studied using a combination of 6 maternally inherited chloroplast microsatellite markers and 7 biparentally inherited nuclear microsatellites (nSSR). Two predominant chloroplast lineages were identified, between which the boundary was very sharp. Populations containing a mixture of different haplotypes were found only in the immediate proximity of the boundary. Bayesian analysis of population structure based on nSSR loci revealed the existence of two clusters, which largely coincided with chloroplast lineages. Both haplotype frequencies and proportions of clusters identified by the Bayesian analysis exhibited a clinal transition over the hybrid zone, with cline widths of 36 km for chloroplast haplotype frequencies (reflecting gene flow by seeds) and 275 km for Bayesian clusters based on nSSR (reflecting gene flow by pollen and seeds). Chloroplast haplotype diversity increased along the boundary between lineages not only because of admixture, but also due to the presence of rare haplotypes. In contrast, diversity at nuclear loci did not exhibit any geographical trend.

Chilled but not frosty: understanding the role of climate in the hybridization between the Mediterranean Fraxinus angustifolia Vahl and the temperate Fraxinus excelsior L. (Oleaceae) ash trees

AbstractAimTo examine mechanisms related to the formation of hybrid zones between the Mediterranean narrow‐leaved ash tree Fraxinus angustifolia Vahl and the common ash Fraxinus excelsior L., a mostly temperate tree species, at the continental scale.LocationTemperate and Mediterranean Europe and the western part of the Black Sea basin.MethodsWe used species distribution models to determine the potential zones of sympatry between the two species, which remain largely unknown. In addition, we analysed 58 populations and 456 samples of ash tree that spanned most of the distribution of the two species across Europe, and included both parental species and selected hybrid populations. Levels of hybridization in the 58 populations were estimated using 19 nuclear microsatellite loci, including six anonymous nuclear single sequence repeat (SSR) markers and 13 recently developed single sequence repeats from expressed RNA sequence tags (EST‐SSRs).ResultsBayesian assignment supported the notion of two separate gene pools regardless of the type of marker used, which suggest an ancient population structure. Populations located within the predicted overlap zones had intermediate levels of admixture with a tendency for hybrid populations to occur towards temperate areas. Selection analyses indicated that six of the EST‐SSRs had been subjected to stabilizing selection whereas two others had been subjected to directional selection. Results of spatial filtering on the allele frequencies of the loci under directional selection suggest that the number of days of frost and summer temperatures are both ecological factors that can limit the extent of the hybrid zone. Moreover, areas associated with known or predicted hybrid zones showed abrupt changes in allele frequencies compared with the periphery of the distributions.Main conclusionsOur analyses suggest that the hybrid structure in these closely related ash species is ancient and asymmetric and that climate‐driven selection, in particular cold weather, can potentially limit the extent of hybrid populations.

Many ways to die – partitioning tree mortality dynamics in a near‐natural mixed deciduous forest

SummaryPartitioning of tree mortality into different modes of death allows the tracing and mechanistic modelling of individual key processes of forest dynamics each varying depending on site, species and individual risk factors. This, in turn, may improve long‐term predictions of the development of old‐growth forests.Six different individual tree mortality modes (uprooted and snapped (both with or without rot as a predisposing factor), standing dead and crushed by other trees) were analysed, and statistical models were derived for three tree species (European beechFagus sylvatica, hornbeamCarpinus betulusand common ashFraxinus excelsior) based on a repeated inventory of more than 13 000 trees in a 28 ha near‐natural deciduous forest in CentralGermany.The frequently describedU‐shaped curve of size‐dependent mortality was observed in beech and hornbeam (but not ash) and could be explained by the joint operation of processes related to the six distinct mortality modes. The results for beech, the most abundant species, suggest that each mortality mode is prevalent in different life‐history stages: small trees died mostly standing or being crushed, medium‐sized trees had the highest chance of survival, and very large trees experienced increased rates of mortality, mainly by uprooting or snapping. Reduced growth as a predictor also played a role but only for standing dead, all other mortality modes showed no relationship to tree growth.Synthesis. Tree mortality can be partitioned into distinct processes, and species tend to differ in their susceptibility to one or more of them. This forms a fundamental basis for the understanding of forest dynamics in natural forests, and any mechanistic modelling of mortality in vegetation models could be improved by correctly addressing and formulating the various mortality processes.

Estimating Coextinction Risks from Epidemic Tree Death: Affiliate Lichen Communities among Diseased Host Tree Populations of Fraxinus excelsior

At least 10% of the world’s tree species are threatened with extinction and pathogens are increasingly implicated in tree threats. Coextinction and threats to affiliates as a consequence of the loss or decline of their host trees is a poorly understood phenomenon. Ash dieback is an emerging infectious disease causing severe dieback of common ash Fraxinus excelsior throughout Europe. We utilized available empirical data on affiliate epiphytic lichen diversity (174 species and 17,800 observations) among 20 ash dieback infected host tree populations of F. excelsior on the island Gotland in the Baltic Sea, Sweden. From this, we used structured scenario projections scaled with empirical data of ash dieback disease to generate probabilistic models for estimating local and regional lichen coextinction risks. Average coextinction probabilities (Ā) were 0.38 (95% CI ±0.09) for lichens occurring on F. excelsior and 0.14 (95% CI ±0.03) when considering lichen persistence on all tree species. Ā was strongly linked to local disease incidence levels and generally increasing with lichen host specificity to F. excelsior and decreasing population size. Coextinctions reduced affiliate community viability, with significant local reductions in species richness and shifts in lichen species composition. Affiliates were projected to become locally extirpated before their hosts, illuminating the need to also consider host tree declines. Traditionally managed open wooded meadows had the highest incidence of ash dieback disease and significantly higher proportions of affiliate species projected to go extinct, compared with unmanaged closed forests and semi-open grazed sites. Most cothreatened species were not previously red-listed, which suggest that tree epidemics cause many unforeseen threats to species. Our analysis shows that epidemic tree deaths represent an insidious, mostly overlooked, threat to sessile affiliate communities in forested environments. Current conservation and management strategies must account for secondary extinctions associated with epidemic tree death.

Root damage and water stress: treatments affecting the exploitation of the buds of common ash Fraxinus excelsior L., by larvae of the ash bud moth Prays fraxinella Bjerk. (Lep., Yponomeutidae).

Damage to the buds of the common ash (Fraxinus excelsior L.) by larvae of the ash bud moth Prays fraxinella Bjerk. was investigated in trees growing in the presence, or absence, of an adjacent ditch. The presence of a ditch was correlated with increased damage levels due to bud moth. Saplings were used in an experiment to compare the effects of artificial drought and root damage with those of ditches. Both root damage and drought elicited analogous responses to those observed in trees growing near a ditch, being associated with elevated levels of bud moth exploitation and reduced growth of shoots. There exists a documented link between ditches, root disturbance, and the dieback of hedgerow ash trees in Great Britain; the response of P. fraxinella to environmental disturbances associated with dieback is hypothesised to be evidence of the possible role of moth damage as a causal factor in dieback.