The Effect of Bud Position on Branch Growth and Bud Abscission in Quercus petraea (Matt.) Liebl.

The time at which a bud began to expand was related to its positionnot only on an individual shoot but also within the crown. Thedistribution of buds and branches on the shoot was uneven; theshoot tip, where they were densely clustered, was termed the'whorl; and the remainder of the shoot, where they were widelyspaced, the ‘interwhorl’ stem. In spring, the terminalbud started expanding before the ’whorl’ buds whichpreceded the ‘interwhorl’ stem buds; completionof the flush of growth, determined by the end of leaf expansion,occurred in the reverse order, ‘interwhorl’     

Characterisation and natural variation of a dehydrin gene in Quercus petraea (Matt.) Liebl

For the first time in sessile oak [Quercus petraea (Matt.) Liebl.], the isolation and characterisation of a full-length dehydrin gene and its promoter region, as well as its allelic variation in natural populations, is reported. Dehydrins (Dhn) are stress-related genes important for the survival of perennial plants in a seasonal climate. A full-length dehydrin gene (Dhn3) was characterised at the nucleotide level and the protein structure was modelled. Additionally, the allelic variation was analysed in five natural populations of Quercus petraea (Matt.) Liebl. sampled along an altitudinal gradient in the French Pyrenees. The analysed sequences contain typical domains of the K(n) class of dehydrins in the coding region. Also, the 5'untranslated region (promoter) of the gene was amplified, which shows typical motifs essential for drought- and cold-responsive gene expression. Single nucleotide substitutions and indels (insertions/deletions) within the coding region determine large biochemical differences at the protein level. However, only low levels of genetic differentiation between populations from different altitudes were detectable.     

Three divergent rDNA clusters predate the species divergence in Quercus petraea (Matt.) Liebl. and Quercus robur L

Quercus petraea and Quercus robur are two closely related oak species that frequently hybridize. We sequenced 70 clones containing the 5.8S and ITS2 regions of ribosomal DNA (rDNA) from these two species and did not detect a species-specific difference. Surprisingly, three divergent (up to 12.6%) rDNA families were identified in both species, indicating that they predate the speciation event. Despite a large between-rDNA-families divergence, rDNA sequences were very similar within families, suggesting ongoing concerted evolution. Expression analysis, relative-rate tests, and mutation spectrum analyses indicated that only a single rDNA family is functional. We propose that past hybridization events, combined with nucleolar dominance, were the evolutionary processes underlying the contemporary rDNA variability in Q. petraea and Q. robur.     

Regeneration Patterns of European Oak Species (Quercus petraea (Matt.) Liebl., Quercus robur L.) in Dependence of Environment and Neighborhood

Quercus robur L. (pedunculate oak) and Quercus petraea (Matt.) Liebl. (sessile oak) are two European oak species of great economic and ecological importance. Even though both oaks have wide ecological amplitudes of suitable growing conditions, forests dominated by oaks often fail to regenerate naturally. The regeneration performance of both oak species is assumed to be subject to a variety of variables that interact with one another in complex ways. The novel approach of this research was to study the effect of many ecological variables on the regeneration performance of both oak species together and identify key variables and interactions for different development stages of the oak regeneration on a large scale in the field. For this purpose, overstory and regeneration inventories were conducted in oak dominated forests throughout southern Germany and paired with data on browsing, soil, and light availability. The study was able to verify the assumption that the occurrence of oak regeneration depends on a set of variables and their interactions. Specifically, combinations of site and stand specific variables such as light availability, soil pH and iron content on the one hand, and basal area and species composition of the overstory on the other hand. Also browsing pressure was related to oak abundance. The results also show that the importance of variables and their combinations differs among the development stages of the regeneration. Light availability becomes more important during later development stages, whereas the number of oaks in the overstory is important during early development stages. We conclude that successful natural oak regeneration is more likely to be achieved on sites with lower fertility and requires constantly controlling overstory density. Initially sufficient mature oaks in the overstory should be ensured. In later stages, overstory density should be reduced continuously to meet the increasing light demand of oak seedlings and saplings.     

Natural hybridisation between Quercus petraea (Matt.) Liebl. and Quercus pubescens Willd. within an Italian stand as revealed by microsatellite fingerprinting

Interspecific gene flow is frequently reported in the genus Quercus . However, interfertile oak species often seem to remain distinct, even within areas of sympatry. This study employed molecular markers to verify, at a fine scale, the presence of interspecific gene flow in a natural population of Quercus petraea and Quercus pubescens . Within a delimited area of 6 ha, all adult trees belonging to the studied oak complex and seeds from a subsample of such trees were collected and analysed using molecular microsatellite markers. A low interspecific genetic differentiation and a high level of interspecific genetic admixture suggested past hybridisation. Paternity inference of seeds allowed the estimation of pollination frequencies from the three groups of pollen donors ( Q. petraea, Q. pubescens , intermediate). We also assayed pollen viability and germinability of each species group. We observed natural hybridisation between Q. petraea and Q. pubescens, with a predominant component in the direction Q. petraea → Q. pubescens : Q. pubescens displayed a higher level of heterospecific pollination by Q. petraea (25.8%) and intermediate morphotypes (14.7%), compared to Q. petraea acting as pollen receptor (with less than 5% heterospecific pollinations). Intermediate 'mother trees' were pollinated in similar proportions by Q. petraea (23.1%), Q. pubescens (37.8%) and intermediate morphotypes (39.1%). The asymmetrical introgression observed for the studied generation may be caused, among other factors, by the relative abundance of trees from each species group in the studied area.     

Genetic variation at allozyme and RAPD loci in sessile oak Quercus petraea (Matt.) Liebl.: the role of history and geography

The nuclear genetic variation within and among 21 populations of sessile oak was estimated at 31 RAPD loci in conjunction with previous estimates of variation at eight allozyme loci. The aim of the study was to assess the relative role of isolation-by-distance and postglacial history on patterns of nuclear variation. Because of its small effective population size and maternal transmission, the chloroplast genome is a good marker of population history. Both kinds of nuclear variation (RAPD and allozyme) were therefore compared, first, to the geographical distances among populations and, secondly, to chloroplast DNA restriction polymorphism in the same populations. Multiple Mantel tests were used for this purpose. Although RAPDs revealed less genetic diversity than allozymes, levels of genetic differentiation ( G _ST) were identical. The standard genetic distance calculated at all RAPD loci was correlated with geographical distances but not with the genetic distance calculated from chloroplast DNA data. Conversely, allozyme variation was correlated with chloroplast DNA variation, but not with geography. Possibly, divergent selection at two allozyme loci during the glacial period could explain this pattern. Because of its greater number of loci assayed, RAPDs probably provided a less biased picture of the relative role of geography and history.     

Genome scanning for interspecific differentiation between two closely related oak species [Quercus robur L. and Q. petraea (Matt.) Liebl.

Interspecific differentiation values (G(ST)) between two closely related oak species (Quercus petraea and Q. robur) were compiled across different studies with the aim to explore the distribution of differentiation at the genome level. The study was based on a total set of 389 markers (isozymes, AFLPs, SCARs, microsatellites, and SNPs) for which allelic frequencies were estimated in pairs of populations sampled throughout the sympatric distribution of the two species. The overall distribution of G(ST) values followed an L-shaped curve with most markers exhibiting low species differentiation (G(ST) < 0.01) and only a few loci reaching >10% levels. Twelve percent of the loci exhibited significant G(ST) deviations to neutral expectations, suggesting that selection contributed to species divergence. Coding regions expressed higher differentiation than noncoding regions. Among the 389 markers, 158 could be mapped on the 12 linkage groups of the existing Q. robur genetic map. Outlier loci with large G(ST) values were distributed over 9 linkage groups. One cluster of three outlier loci was found within 0.51 cM; but significant autocorrelation of G(ST) was observed at distances <2 cM. The size and distribution of genomic regions involved in species divergence are discussed in reference to hitchhiking effects and disruptive selection.     

AFLP markers demonstrate local genetic differentiation between two indigenous oak species [ Quercus robur L. and Quercus petraea (Matt.) Liebl.] in Flemish populations

The nuclear genetic variation within and between four sessile ( Q. petraea) and six pedunculate ( Q. robur) autochthonous Flemish oak populations was investigated with AFLP markers. One sessile and one pedunculate oak population were additionally screened for detailed leaf characteristics using an image analysis system. Principal coordinate analysis on the AFLP data classified the oaks in two main groups, according to their taxonomic status. No species-specific AFLP markers were found using four primer combinations, but marker frequency differences up to 71% were recorded between both species. Analysis of the genetic structure showed that the divergence between species, as observed by ordination, was significant. Both species revealed similar diversity levels. A smaller though significant differentiation was also revealed for both species among populations within species. Molecular and morphology based approaches showed a high degree of consistency. Screening of 60 AFLP primer combinations using a bulking strategy did not allow identifying species-specific markers, which supports the conclusions reached in previous studies. The distribution of genetic variability at the species and at the population level is discussed.     

Pollen dispersal inferred from paternity analysis in a mixed oak stand of Quercus robur L. and Q. petraea (Matt.) Liebl.

Paternity analysis was used to determine the spatial distribution of male parents of 984 offspring collected from 13 identified mother trees in a natural stand of 5.76 ha and comprising 296 adult trees of Quercus petraea and Q . robur . For seven of the 13 maternal progeny arrays sampled, we found an excess of nearby matings and a preferential direction of pollination. For the remaining progeny arrays, no departure from random distribution of male parents was detected. A common trend among all families was a high percentage (averaging 65% for Q. robur and 69% for Q. petraea ) of offspring that were pollinated by male parents from outside the study site. By pooling the data over all families, the average pollen dispersal curve within the stand was inferred and fitted to a negative exponential distribution. This model extrapolated for distances over the spatial scale of the study stand was insufficient to explain the high level of gene flow detected by the paternity analysis, suggesting a substantial level of long-distance pollination events. The genetic composition of the pollen pools received by each maternal tree was compared and showed significant differentiation that could be attributed to differences in male reproductive success. By contrast, no significant differentiation between the pollen clouds from outside and inside the study stand was detected, suggesting genetic homogeneity between the surrounding forest and the study stand.     

Soil N and C trace gas fluxes and microbial soil N turnover in a sessile oak (Quercus petraea (Matt.) Liebl.) forest in Hungary

During two intensive field campaigns in summer and autumn 2004 nitrogen (N₂O, NO/NO₂) and carbon (CO₂, CH₄) trace gas exchange between soil and the atmosphere was measured in a sessile oak (Quercus petraea (Matt.) Liebl.) forest in Hungary. The climate can be described as continental temperate. Fluxes were measured with a fully automatic measuring system allowing for high temporal resolution. Mean N₂O emission rates were 1.5 μg N m⁻² h⁻¹ in summer and 3.4 μg N m⁻² h⁻¹ in autumn, respectively. Also mean NO emission rates were higher in autumn (8.4 μg N m⁻² h⁻¹) as compared to summer (6.0 μg N m⁻² h⁻¹). However, as NO₂ deposition rates continuously exceeded NO emission rates (−9.7 μg N m⁻² h⁻¹ in summer and −18.3 μg N m⁻² h⁻¹ in autumn), the forest soil always acted as a net NO _x sink. The mean value of CO₂ fluxes showed only little seasonal differences between summer (81.1 mg C m⁻² h⁻¹) and autumn (74.2 mg C m⁻² h⁻¹) measurements, likewise CH₄uptake (summer: −52.6 μg C m⁻² h⁻¹; autumn: −56.5 μg C m⁻² h⁻¹). In addition, the microbial soil processes net/gross N mineralization, net/gross nitrification and heterotrophic soil respiration as well as inorganic soil nitrogen concentrations and N₂O/CH₄ soil air concentrations in different soil depths were determined. The respiratory quotient (ΔCO_{2 resp} ΔO_{2 resp}⁻¹) for the uppermost mineral soil, which is needed for the calculation of gross nitrification via the Barometric Process Separation (BaPS) technique, was 0.8978 ± 0.008. The mean value of gross nitrification rates showed only little seasonal differences between summer (0.99 μg N kg⁻¹ SDW d⁻¹) and autumn measurements (0.89 μg N kg⁻¹ SDW d⁻¹). Gross rates of N mineralization were highest in the organic layer (20.1–137.9 μg N kg⁻¹ SDW d⁻¹) and significantly lower in the uppermost mineral layer (1.3–2.9 μg N kg⁻¹ SDW d⁻¹). Only for the organic layer seasonality in gross N mineralization rates could be demonstrated, with highest mean values in autumn, most likely caused by fresh litter decomposition. Gross mineralization rates of the organic layer were positively correlated with N₂O emissions and negatively correlated with CH₄ uptake, whereas soil CO₂ emissions were positively correlated with heterotrophic respiration in the uppermost mineral soil layer. The most important abiotic factor influencing C and N trace gas fluxes was soil moisture, while the influence of soil temperature on trace gas exchange rates was high only in autumn.     

Osservazioni Preliminari Sull'ecologia di Semenzali di Rovere (Quercus Petraea (Matt.) Liebl.)

Abstract

Preliminary observations on the ecology of sessile oak (Quercus petraea (Matt.) Liebl.) seedlings.—The responses of sessile oak seedlings to four different light intensities and to increasing drought (three soil moisture contents) have been investigated in a nursery experiment in order to improve the knowledge of the ecology of this species in the Mediterranean area. Data have been compared with observations on natural regeneration established in different shade and soil moisture conditions in a sessile oak stand. The results of the two experiments showed that low light intensities (transmittance <5%) did not limit the seedlings establishment but induced lowest growth rates, while in full daylight seedlings mortality was very high (61%). In accordance with the observations in natural conditions, seedlings showed the best performances at intermediate light levels (50–80% of full daylight) and with increasing soil moisture: mortality was very low and average height increment, dry weight, relative growth rate (RGR) and net assimilation rate (NAR) showed the highest values. The differences in specific leaf area (SLA), leaf area ratio (LAR), root/stem ratio enabled to discuss the sessile oak seedlings ecology and adaptations to different light intensities and soil moisture content.     

Gene flow and hybridisation in a mixed oak forest (Quercus pyrenaica Willd. and Quercus petraea (Matts.) Liebl.) in central Spain

Oaks are long-standing models for the study of gene flow and hybridisation. Temperate (Quercus petraea) and sub-Mediterranean (Quercus pyrenaica) oaks coexist in central Spain, showing remarkable differences in population size and structure. Q. petraea has a scattered distribution in central Spain, where it is at one of the southernmost limits of its range, and forms low-density stands; in contrast, Q. pyrenaica is widespread in the region. We selected a mixed population of the two species ( approximately 13 ha, 176 adults and 96 saplings) to compare the patterns of gene flow within each species and the extent of introgression between them. Using five nuclear microsatellite markers, we performed a parentage analysis and found considerable immigration from outside the stand ( approximately 38% for Q. petraea and approximately 34% for Q. pyrenaica), and estimated average seed-dispersal distances of 42 and 14 m for Q. petraea and Q. pyrenaica, respectively. Introgression between species was also estimated using our microsatellite battery. First, we developed a multivariate discriminant approach and, second, we compared our results with a widely used clustering method (STRUCTURE). Both analyses were consistent with a low level of introgression between Q. petraea and Q. pyrenaica. Indeed, only 15 adult trees, approximately 8.5%, were identified as putative hybrids when both methods of analysis were combined. Hybrids may be most common in contact zones due merely to physical proximity.     

Morphological and physiological reactions of young deciduous trees (Quercus robur L., Q. petraea [Matt.] Liebl., Fagus sylvatica L.) to waterlogging

One-year-old seedlings of Quercus robur L., Q. petraea (Matt.) Liebl. and Fagus sylvatica L. were cultivated in lysimeters and subjected to waterlogging for 17 weeks, interrupted by a five-week drainage period during summer. The growth of Q. robur was less affected by waterlogging than that of Q. petraea and Fagus. Waterlogging resulted in the formation of adventitious roots in Q. robur and Q. petraea, but not in Fagus. In contrast to Fagus, Q. robur and, to a lesser extent, Q. petraea were able to generate roots even below the water table. The hydraulic conductance of the excised root systems, the stomatal conductance and, in Fagus, the leaf water potential and the leaf-mass related hydraulic conductance were decreased by waterlogging. The decrease in the hydraulic conductance was largest in Fagus, and smallest in Q. robur. The roots of Fagus responded to anaerobic conditions with an increase in ethanol concentration. The measurements of nitrate reductase activities in roots and leaves provided no indications of a persistent contribution of NO₃ ⁻ metabolism to the alleviation of waterlogging-induced stress. It is concluded that Q. robur and, to a lesser extent, Q. petraea can tolerate waterlogging periods better than Fagus due to a different pattern of root formation, and to a better adjustment of leaf biomass production to the hydraulic conductivity of the root system. This revised version was published online in June 2006 with corrections to the Cover Date.     

The effect of lime and gypsum applications on a sessile oak ( Quercus petraea (M.) Liebl.) stand at La Croix-Scaille (French Ardennes) II. Fine root dynamics

Fine root distribution, quantities, dynamics and composition were studied in a sessile oak coppice stand in the French Ardennes on an acidic soil (< pH-H2O 4.5), one to five years after lime or gypsum applications. Fine root biomass and length increased and specific root length decreased after lime or gypsum treatments. The treatment responses were strongest four to five years after the applications, but the tendencies after one year were similar. The effects were pronounced in the top 15 cm but also at 30–45 cm four to five years after liming. The latter effect suggests an indirect positive feedback from the aerial parts of the trees into the deeper soil layers. Sequential sampling for two years revealed large differences in total fine root length between the years, and also indicated that fine root turnover was lower after liming or gypsum applications than in the control. This seemed to be related to a lower fine root mortality and higher longevity rather than to increased fine root production. The improved nutrient status of the fine roots corroborates this and coincides with improved foliar nutrition and tree growth. Moderate doses of lime and gypsum appeared effective in enhancing root system uptake function, resulting in increased above ground growth.     

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.     

Identification of triterpene saponins in Quercus robur L. and Q. petraea Liebl. Heartwood by LC-ESI/MS and NMR

A detailed structural study of the major acidic triterpene saponins from European oak heartwood, Quercus robur L. and Q. petraea Liebl., revealed the presence of 2 alpha,3 beta,19 alpha-trihydroxyolean-12-ene-24,28-dioic acid (1), 2 alpha,3 beta,19 alpha-23-tetrahydroxyolean-12-ene-24,28-dioic acid (2), and their corresponding glycosides, 28-beta-D-glucopyranosyl-2 alpha,3 beta,19 alpha-trihydroxyolean-12-ene-24,28-dioic acid (3), and 28-beta-D-glucopyranosyl-2 alpha,3 beta,19 alpha,23-tetrahydroxyolean-12-ene-24,28-dioic acid (4). Compounds were isolated and purified by successive flash chromatography and semi-preparative HPLC, and structurally determined by NMR and LC-ESI/MS in the negative ion mode. Compounds 2 and 4 have been identified for the first time in Quercus species and are new compounds.     

Photosynthesis of oak trees [Quercus petraea (Matt.) Liebl.] during drought under field conditions: diurnal course of net CO2 assimilation and photochemical efficiency of photosystem II

Adult trees of Quercus petraea were submitted to controlled water shortage in a natural stand near Nancy, France. Diurnal course of net CO₂ assimilation rate (A) was measured in situ together with chlorophyll a fluorescence determined on dark adapted leaves. In 1990, trees experienced a strong water stress, with predawn and midday leaf water potentials below –2·0 and –3·0 MPa, respectively. Diurnal course of A of well-watered trees exhibited sometimes important midday decreases in A related to high temperature and vapour pressure deficit. Decreases in initial (F_o) and maximal (F_m) fluorescence and sometimes in photochemical efficiency of photosystem II (F_v/F_m) were observed and probably revealed the onset of mechanisms for thermal de-excitation. These mechanisms were shown to be sensitive to dithiothreitol. All these effects were reversible and vanished almost completely overnight. Therefore, they may be considered as protective mechanisms adjusting activity of photosystem II to the electron requirement for photosynthesis. Water stress amplified these reactions: A was strongly decreased, showing important midday depression; diurnal reductions in F_m and F_v/F_m were enhanced. The same trends were observed during summer 1991, despite a less marked drought. These protective mechanisms seemed very effective, as no photoinhibitory damage to PS II could be detected in either water stressed or control trees.     

The effects of liming and gypsum applications on a sessile oak (Quercus petraea (M.) Liebl.) stand at La Croix-Scaille (French Ardennes) I. Site characteristics, soil chemistry and aerial biomass

In a former 45 to 50 year old sessile oak ( Quercus petraea (M.) Liebl.) coppice mixed with birch (Betula pubescens Ehrh.) and rowan (Sorbus aucuparia L.) on a poor acidic forest soil at la Croix-Scaille in the French Ardennes, several liming amendments were applied in 1990 and 1994. Data on soil and soil solution composition, as well as stand growth and foliar composition were collected between 1994 and 1997. All treatments, containing 1.4 t ha-1 equivalent of CaO supplied as lime, gypsum or a mixture of the two, resulted in an increase of cation exchange capacity and base saturation down to 15 cm and for CaSO4 treatments down to 30 to 45 cm, increases of soil pH and Ca concentration at the surface and a decrease of Al concentration in the soil and soil solution in the surface layers. No negative effects like increased nitrate or cation leaching were observed. Although Mg nutrition was not improved by the treatments (not containing Mg), a relative and maintained gain of radial increment of sessile oak in the order of 40% for both lime and gypsum applied, was observed immediately from the first year on, after the application (1991).