The Effect of Felling on the Occurrence of Microfungi Stimulatory to Armillaria Rhizomorph Formation in Thin Roots of Quercus robur

Eight fungal assemblages were isolated from thin branch roots (0.5–1 mm diameter) of living oak trees (Quercus robur) and their stumps 2 years after felling. The roots were serially washed or surface‐sterilized with ethanol and hypochlorite. The thin roots were inhabited by fungi that are stimulants of Armillaria rhizomorphs as found previously in thick roots (0.5–1 cm diameter). However, unlike thick roots, the densities of the fungi in serially washed roots were significantly less in stump roots than in living roots. Among the most common ‘stimulants’, the frequencies of Cylindrocarpon didymum, Mortierella gracilis, Mor. microspora var. macrocystis, Mycelium radicis atrovirens, Nectria grammicospora, Pseudogymnoascus roseus and Sporothrix schenckii were usually significantly greater in stump roots than in living roots, while the frequencies of Chrysosporium merdarium, Cy. destructans, Mor. hygrophila, Penicillium adametzii, Pe. citrinum, Pe. daleae, Pe. janczewskii and Pe. spinulosum were usually significantly less in stump roots. The frequency of Trichoderma viride was less in the stump roots. Serial washing of roots was more effective than chemical surface‐sterilization in the detection of Armillaria rhizomorph ‘stimulants’. The significance of these findings in the ecology and risk assessment of Armillaria in plantations is discussed.     

Genetic Diversity within an Italian Population of Forest Armillaria gallica Isolates as Assessed by RAPD-PCR Analysis

The genus Armillaria includes harmful fungal pathogens that cause root rot and wood decay in a broad range of host plants throughout the world. The aim of this study was to detect, by means of Random amplified polymorphic DNA-polymerase chain reaction (RAPD-PCR) markers, the level of intraspecific variability within isolates of an Armillaria gallica population sampled from a Quercus spp. stand located in Gravina in Puglia, southern Italy. UPGMA cluster analysis of RAPD profiles generated by decamer primers grouped the isolates in subclusters demonstrating relatively low intraspecific genetic variability. Moreover, RAPD pattern analysis yielded clusters which did not correspond to the groups discriminated by vegetative compatibility tests performed by a previous investigation on the same population. The findings of this research pose the question of whether somatic incompatibility, which involves an undefined number of genes and alleles per gene, might still be considered an effective tool for the epidemiological studies of A. gallica , whereas molecular analyses are more useful for assessing genomic variation within the species.     

A conceptual model for the development of Phytophthora disease in Quercus robur

Here, a conceptual model is presented for the development of Phytophthora disease in pedunculate oak. The model is presented using the causal loop diagram tool and gives an overview of how various abiotic and biotic factors, such as soil moisture, nutrient availability and mycorrhizal colonization, may affect the reproduction and the infective capacity of soil-borne Phytophthora species, the susceptibility of the host and subsequent disease development. It is suggested that the link between the root damage caused by Phytophthora species and overall tree vitality is in the assimilation and allocation of carbon within the plants. The potential impact of environmental factors on these processes is discussed. The model is presented with reference to scenarios related to variation in soil moisture and nutrient availability. The need for species-specific validation of the model and the implications of the model are discussed.     

Tree rooting patterns and soil water relations of healthy and damaged stands of mature oak (Quercus robur L. and Quercus petraea [Matt.] Liebl.)

At three sites in northwestern Germany, which represent the centres of the present oak damage, root distribution and biomass beneath healthy and damaged trees of mature pedunculate oak (Quercus robur L.; Neuenburg site) and sessile oak (Q. petraea [Matt.] Liebl.; Lappwald and Sprakensehl sites) were investigated, and soil texture, bulk density, duration of waterlogging periods and the water available in the mineral soil were determined. For Neuenburg and Sprakensehl, the available soil water was related to leaf water parameters determined in a separate investigation. At the clayey and hydromorphic sites of Neuenburg and Lappwald, the measurements were performed in each one healthy and one damaged part of the site, which differed in the number of oaks with crown damage. In the damaged stand of Neuenburg, the clay content of the subsoil was higher than in the healthy stand, and the soil water availability was reduced especially in dry periods. Compared to healthy oaks of the healthy stand, the density of finest plus fine roots as well as the biomasses of finest roots were lower beneath damaged oaks of the damaged stand. With decreasing relative available soil water (actually available water in relation to water available at the saturation state), the relative leaf water content decreased in damaged, but not in healthy oaks. At Lappwald, similar differences in soil water availability between the healthy and the damaged stand were found, but had no effect on the distribution or biomass of the roots. At the sandy site (Sprakensehl), the available soil water decreased drastically during a dry period, and predawn leaf water potentials of both healthy and damaged oaks declined with decreasing relative available soil water. However, the damaged oaks were not inferior to the healthy ones with respect to root density and biomass. It is concluded that, in the damaged stand of Neuenburg, the high clay content of the subsoil, which results in prolonged periods of waterlogging, in sharp changes from waterlogging to drought and decreased water availability in dry periods, is the reason for the reduced biomass and density of roots of the pedunculate oak. Thus, in northwestern Germany, unfavourable soil water relations are considered as a factor contributing to crown damage of pedunculate oak at hydromorphic sites, but not to damage of sessile oak.     

Isolation and characterization of polymorphic simple sequence repeat loci in Armillaria ostoyae

A modified hybridization strategy was used to construct a microsatellite enriched library from DNA of Armillaria ostoyae, a serious root pathogen on pine. Sequence characterization of 19 random clones revealed 12 distinct loci harbouring a repetitive motif. Primer design from the flanking regions allowed for their development as polymerase chain reaction based markers. Polymorphic assessment at both the population and global levels revealed levels of variation useful for genetic studies. The level of cross‐species amplification observed with closely related Armillaria species was high, raising the possible exploitation of these primers across the genus.     

Fungal Endophytic Communities in Healthy and Declining Quercus robur L. and Q. cerris L. Trees in Northern Italy

Healthy and declining English oak (Quercus robur) and Turkey oak (Q. cerris) in north‐western Italy, in a plain oak forest showing decline for oak puzzle disease, were tested to assess possible variations in the composition of their fungal endophytic communities and their relation to the health status of trees. Samples collected in spring (buds) and in autumn (leaves, annual shoots and twigs) were surface‐sterilized, cut into fragments and placed on potato dextrose agar for a month; 26 fungal species were isolated, the most frequent being Tubakia dryina, Dendrodochium sp., Eutypella sp. and a sterile mycelium. Correspondence analysis showed significant qualitative differences between assemblages inhabiting twigs and herbaceous tissues that were due to the low frequency of Tubakia dryina in twigs and its higher frequency in buds, leaves and shoots. Tubakia dryina was isolated more frequently from leaves of declining oaks and from buds of healthy oaks; Monochaetia monochaeta showed a preference for healthy trees, especially leaves and buds. According to the Shannon–Wiener index, endophytic fungal communities of leaves, twigs and buds of declining English oak were poorer than those of declining Turkey oak, but there were no significant differences between healthy hosts.     

Carbon isotope discrimination and wood anatomy variations in mixed stands of Quercus robur and Quercus petraea

The two most common oak species in western Europe, Quercus robur and Quercus petraea, display different ecological behaviours, particularly with respect to their responses to drought. The ecophysiological basis of this niche difference is not understood well. Here we test the hypothesis that these two species present distinct water use efficiencies (WUEs), using the carbon isotope discrimination approach. Leaves and 13 dated ring sequences were sampled in 10 pairs of adult trees growing side by side. Carbon isotope composition was measured on cellulose extracts. In addition, relationships between carbon isotope discrimination and wood anatomy were assessed at the tree level. Quercus robur displayed a 1·0‰ larger isotopic discrimination than Q. petraea, and therefore a lower intrinsic WUE (?13%). This interspecific difference of isotopic discrimination was quite stable with time and independent of tree radial growth and climate fluctuations. A strong positive correlation was observed between average tree values of earlywood vessel surface area and ¹³C isotopic discrimination. This correlation was even higher with ¹³C of the 1976 dry year (r = 0·86). These observations led to the hypothesis that hydraulic properties of xylem could exert a constraint on leaf gas exchange, resulting in a larger WUE for individuals with smaller vessel cross‐section area.     

Species distinction in Irish populations of Quercus petraea and Q. robur: morphological versus molecular analyses

BACKGROUND AND AIMS: Populations of oak (Quercus petraea and Q. robur) were investigated using morphological and molecular (AFLP) analyses to assess species distinction. The study aimed to describe species distinction in Irish oak populations and to situate this in a European context. METHODS: Populations were sampled from across the range of the island of Ireland. Leaf morphological characters were analysed through clustering and ordination methods. Putative neutral molecular markers (AFLPs) were used to analyse the molecular variation. Cluster and ordination analyses were also performed on the AFLP markers in addition to calculations of genetic diversity and F-statisitcs. KEY RESULTS: A notable divergence was uncovered between the morphological and molecular analyses. The morphological analysis clearly differentiated individuals into their respective species, whereas the molecular analysis did not. Twenty species-specific AFLP markers were observed from 123 plants in 24 populations but none of these was species-diagnostic. Principal Coordinate Analysis of the AFLP data revealed a clustering, across the first two axes, of individuals according to population rather than according to species. High F(ST) values calculated from AFLP markers also indicated population differentiation (F(ST) = 0.271). Species differentiation accounted for only 13 % of the variation in diversity compared with population differentiation, which accounted for 27 %. CONCLUSIONS: The results show that neutral molecular variation is partitioned more strongly between populations than between species. Although this could indicate that the populations of Q. petraea and Q. robur studied may not be distinct species at a molecular level, it is proposed that the difficulty in distinguishing the species in Irish oak populations using AFLP markers is due to population differentiation masking species differences. This could result from non-random mating in small, fragmented woodland populations. Hybridization and introgression between the species could also have a significant role.     

Comparison of earlywood vessel variables in the wood of Quercus robur L. and Quercus petraea (Mattuschka) Liebl. growing at the same site

The differences in the microscopic structure of wood based on the variables of earlywood vessel area and tree ring width were analysed in 6 trees of Pedunculate oak (Quercus robur L.) and 6 trees of Sessile oak (Quercus petraea (Mattuschka) Liebl.) in the same forest stand at a site in the Vizovice Highland (Czech Republic). The aim of this paper was to assess any differences between the two oak species when grown in the same location. Also the effect of tree-ring width and the effect of age on earlywood vessel variables were analysed. Mean values of tree-ring widths were not different between species. Earlywood vessel area chronologies were synchronized well between species. The mean values of earlywood vessel area (average vessel area, average vessel area of the first row of vessels, area of the largest vessel) showed a significant difference. Finally, we can conclude that there were differences in vessel features between these oak species.     

Survivorship of the sexual and agamic generations of Andricus quercuscalicis on Quercus cerris and Q.robur

ABSTRACT.

• 1 Survivorship within the galls of the sexual and agamic generations of Andricus quercuscalicis (Burgsdorf) is estimated from field populations.
• 2 The sexual generation on Turkey oak suffers a 20% death rate through parasitism by Mesopolobus spp. The emerging parasitoids are virtually all males.
• 3 The agamic generation on the acorn cups of English oak is not attacked by any parasites or inquilines at our Berkshire site despite the fact that most of the species which parasitize the gall in continental Europe are on the British list.
• 4 Agamic galls cause average losses of between 30% and 50% of the total acorn crop in different years. Individual trees differ greatly in their rates of loss, many losing over 90%, others losing virtually none.

     

Network formation by rhizomorphs of Armillaria lutea in natural soil: their description and ecological significance

Armillaria lutea rhizomorphs in soil were mapped over areas of 25 m2 at a Pinus nigra (site I) and a Picea abies (site II) plantation. Rhizomorph density was 4.3 and 6.1 m m(-2) soil surface with 84% and 48% of the total rhizomorph length in the mapped area interconnected in a network at site I and site II, respectively. At site I there were only two network attachments to Pinus stumps, but at site II many more to Picea roots and stumps. Anastomoses of rhizomorphs resulted in cyclic paths, parts of the network that start and end at the same point. Connections between different rhizomorph segments were shown to allow gaseous exchange. The network at site I consisted of 169 rhizomorphs ('edges'), and 107 rhizomorph nodes ('vertices'). Disruption of two critical edges ('bridges') would lead to large parts (13% and 11%) being disconnected from the remainder of the mapped network. There was a low probability that amputation of a randomly chosen edge would separate the network into two disconnected components. The high level of connectedness may enhance redistribution of nutrients and provide a robust rhizomorph structure, allowing Armillaria to respond opportunistically to spatially and temporally changing environments.     

Porosity and available water of temporarily waterlogged soils in a Quercus robur (L.) declining stand

Pedunculate oak (Quercus robur L.) is particularly sensitive to decline in clayey soils presenting a high-perched temporary water table. These soils induce two successive constraints in one-year cycle: water excess (and hypoxy) in winter and early spring, and water shortage in summer (water stress being more restrictive to oak). We determined the porosity and water properties of temporarily waterlogged clayey soils supporting forest stands of decliningQuercus robur trees in a 101yr-old oak stand in Belgium (50°06N, 4°16E). Roots unevenly colonized the soil down to 1.6m: fine roots (diameter<5mm) were mostly distributed on the surface horizons (0–0.3 m) and around 1.3m deep, despite dense clayey horizons appearing at 0.35m depth. Clay content below this depth reached 46–51. Illite and vermiculite were the dominant clay minerals. The clayey horizons exhibited marked shrink–swell properties: bulk density at 30kPa increased from 1.41 to 1.88gcm^–3 from the surface to 2m depth. There was also evidence of hypoxic conditions caused by water saturation of pore space in the rooting zone from October to mid-April. Extractable water (EW), calculated between moisture tensions of 5 and 1600kPa was 152.8mm. The level of perched temporary water table strongly depended on the seasonal rhythm of water uptake by trees and on the shrink–swell behaviour of soil.     

Comparison of Isoprene Emission, Intercellular Isoprene Concentration and Photosynthetic Performance in Water-Limited Oak (Quercus pubescens Willd. and Quercus robur L.) Saplings

Abstract: The influence of prolonged water limitation on leaf gas exchange, isoprene emission, isoprene synthase activities and intercellular isoprene concentrations was investigated under standard conditions (30 °C leaf temperature and 1000 μmol photons m^-2 s^-1 PPFD) in greenhouse experiments with five-year-old pubescent oak ( Quercus pubescens Willd.) and four-year-old pedunculate oak ( Quercus robur L.) saplings. Net assimilation rates proved to be highly sensitive to moderate drought in both oak species, and were virtually zero at water potentials (Ψ_pd) below - 1.3 MPa in Q. robur and below - 2.5 MPa in Q. pubescens . The response of stomatal conductance to water stress was slightly less distinct. Isoprene emission was much more resistant to drought and declined significantly only at Ψ_pd below - 2 MPa in Q. robur and below - 3.5 MPa in Q. pubescens . Even during the most severe water stress, isoprene emission of drought-stressed saplings was still approximately one-third of the control in Q. robur and one-fifth in Q. pubescens . Isoprene synthase activities were virtually unaffected by drought stress. Re-watering led to partial recovery of leaf gas exchange and isoprene emission. Intercellular isoprene concentrations were remarkably enhanced in water-limited saplings of both oak species during the first half of the respective drought periods with maximum mean values up to ca. 16 μl l^-1 isoprene for Q. pubescens and ca. 11 μl l^-1 isoprene for pedunculate oak, supporting the hypothesis that isoprene serves as a short-term thermoprotective agent in isoprene-emitting plant species.     

Colonization of Fungi and Bacteria in Stumps and Roots of Scots Pine after Thinning and Treatment with Rotstop

The research areas were located in the Pisz Forest District, northeast Poland, in 10‐year‐old Scots pine (Pinus sylvestris L.) plantations, established in 2004 on a clear‐cut area. Reforestation was performed without a biological treatment against root pathogens, despite the presence of Heterobasidion annosum and Armillaria ostoyae in roots and stumps of trees growing previously. The aim of this research was to evaluate how thinning and treatment with the biological control agent Rotstop influences bacterial and fungal communities within roots and stumps. Twelve months after thinning, samples were collected from five stumps in each of two seasons, autumn and spring, from stands on two types of site, one previously forested and one agricultural (20 stumps in total). Wood samples were cultured on agar media, and (i) fungi in the upper part of the stump and (ii) in roots and (iii) bacteria in roots were genetically identified. Sequences were genetically identified by comparing sequences with records held in the GenBank database. We found great differences in the frequency of both fungi and bacteria in roots: they were more frequent (i) in healthy stumps compared to stumps infected with pathogens (H. annosum and A. ostoyae), (ii) in postagricultural soil than in forest soil and (iii) after spring rather than autumn biological treatment. The introduced species Phlebiopsis gigantea was only identified in the parts of the stumps which were above ground level. The bacterium Paenibacillus pini was associated with the presence of H. annosum infecting the stumps from the roots side. In areas seriously threatened by root pathogens, biological treatment can play only a limited role. It can spread to the upper part and impede the production of fruitbodies; however, it has no impact on the development of pathogens in deeper root areas.     

Effect of indole-3-acetic acid (IAA) and sucrose on vessel size and density in isolated stem segments of oak (Quercus robur)

The effects of several concentrations of indole-3-acetic acid (IAA) and sucrose on xylogenic cambial activity and secondary xylem differentiation were investigated in isolated stem segments of Quercus robur L. supplied with liquid medium in aseptic conditions. After 5 weeks of culture auxin controlled cambial cell division and the number and size of vessel elements even without sugar in the medium. Sucrose modified these IAA effects, although little cambial activity occurred without auxin. The xylem increment correlated with changes of auxin concentration with the optimum at 28.5 μ M IAA. The formation of wide vessels was correlated with the optimal concentration of auxin. The frequency of vessel differentiation increased with auxin concentration. High concentrations of sucrose (0.24 M and 0.96 M ) reduced both the number of vessels and their diameter. The frequency of vessel formation was inhibited more than the vessel size by changes of sugar concentration. The vessels formed under low concentrations of IAA were circular in transverse section. With increase in IAA concentration the shape of the vessel cross-section changed to oval with the largest dimension in the radial direction.     

蜜环菌糖苷水解酶家族基因在菌索与菌丝间的差异表达分析

蜜环菌是一种兼性腐生和寄生的真菌,通过降解伴栽基质并为药用植物(天麻)或菌物(猪苓)提供营养物质,而糖苷水解酶是这一过程的主要酶类.本研究从蜜环菌Armillaria mellea 541菌株转录数据库中共获得糖苷水解酶家族基因170个,分布于39个亚家族.进一步分析发现,这些家族基因编码的糖苷水解酶家族蛋白(glyc...