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.     

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.     

Comparison of radial increment and volume growth in stems and roots of Quercus petraea

Dendrochronological studies dealing with roots, stems and branches are very rare or often take the form of short notes. The difficulties of detecting rings and of quantifying the radial growth in roots have already been described for various species. In oak the anatomical root structure differs from stemwood. The roots are radial-porous or diffuse-porous, and there is often no clear distinction between individual rings. In our study visual and radiographic techniques were used to examine radial increment in roots of sessile oak (Quercus petraea L.) which was compared with radial growth in branches and along the stem. Coarse roots were cut from four 30- to 34-year-old trees that had been uprooted mechanically and disks were taken at different distances from the stem-root base. Ring widths were measured in the stem at height of 0.3 m, at breast height (1.3 m), beneath the crown, in branches of the crown, and in roots every 20 cm. The ring widths were cross-dated, and the heterogeneity of growth within a root and within the root system were analysed. Asymmetric growth frequently occurred in roots so that ovals, I-beam and T-beam shapes were developed. With the method used in our study the annual growth layers close to the central cylinder could be distinguished as well as beneath the bark. Pointer years were detected in all sections of the tree and permitted correction of ring widths in roots. Root system, stem and branch showed a basic similarity in their radial sequence of ring width. The annual biomass increment was weaker and more variable with several consecutive changes in the roots than in the stems. The root/shoot ratio reached a minimum rather early, beginning at the cambial age of 20 years. This revised version was published online in June 2006 with corrections to the Cover Date.     

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.     

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’     

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.     

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.     

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 werewidely spaced, the ‘interwhorl’ stem. In spring,the terminal bud started expanding before the ‘whorl’buds which preceded the ‘interwhorl’ stem buds;completion of the flush of growth, determined by the end ofleaf expansion, occurred in the reverse order, ‘interwhorl’> ‘whorl’ > terminal. Similarly bud expansionstarted at the top of the crown and progressed downwards, andthe first shoots to complete their flush were at the bottomof the crown. Approximately 60% of the buds on each shoot beganexpanding in spring but only about half of these formed branches.Bud abscission began in May and by Sep. 45% of buds originallypresent had abscised. Most of-the buds that did not abscisewere the small buds at the base of the shoot that were not originallyassociated with a leaf. Approximately 42% of ‘whorl’buds and 28% of MnterwhorP stem buds formed branches. ‘Whorl’branches were approx. 60% longer that ‘interwhorl’stem branches; buds on the lower surface of the shoot producedlonger branches than those on the upper surface. The implicationsof the results for the development of crown form and selectionof superior oak are discussed. Quercus petraea, oak, buds, branches, crown form     

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.     

Factors affecting radial growth of Quercus ilex L. in a coppice stand in southern France

Quercus ilex woodlands in the south of France are characterized by low productivity. Several hypotheses have been put forward to provide an explanation for this phenomenon: (i) senescence of coppice stumps; (ii) limited availability of water and nutrient resources; (iii) intershoot and inter-plant competition. To test some of these hypotheses, growth response to rainfall distribution and nutrient supply (NPK), and the influence of edge effects after clear-cutting were studied in a 40-yr old stand.Curves of radial increment were compared with precipitation and umbrothermic patterns for the 6 years studied. Water availability apparently accounted for some inter-annual differences in growth. However, it was not mean annual rainfall, but rather rainfall occurring in late spring-summer (June, July) which played a large role in determining radial growth. Radial growth was also increased by nutrient supply. Its positive effect was strengthened by favourable rainfall in 1987. Edge-effects following a clear-cut were also found, both in the first year and in four succeeding years following the cut. This comprised in greater radial growth rate for all shoots, with reduced repression of smaller shoots by the larger ones. The effect decreased over a distance of 10 m with increasing distance from the cut field. This could be explained by increased availability of water, nutrient resources, and light.     

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.     

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.     

Ontogeny of the proventitious epicormic buds in Quercus petraea.

In the present work, we described the fate of proventitious epicormic buds on the trunks of 40-year-old Quercus petraea trees and in parallel the vascular trace they produced in the wood. Our results show that small and large individual epicormic buds can survive as buds for 40 years and that both are composed of a terminal meristem and scales. Meristematic areas are detected in the scale axils of small buds; in addition to these meristems the large buds also have secondary bud primordia. The small buds are connected to the pith of the main stem by a unique trace, whereas the large buds are connected by one or multiple traces. A single trace might imply that the whole bud is still alive and multiple traces might indicate that the terminal meristem has died. In the latter case, each trace is connected to a secondary bud of the large bud. The buds found in a cluster are composed of a terminal meristem and scales with axillary meristems in the scale axils. A cluster is connected to the pith of a stem either by a unique trace when it seems to be the result of partial abscission of an epicormic shoot or multiple traces when it might have originated from an epicormic bud in which the terminal meristem has died. Whatever the type of the bud, the vascular trace in the bark is composed of a cambium, secondary xylem and parenchyma cells and the trace present in the wood had parenchyma cells with vestiges of secondary xylem. Each year, the vascular trace should be produced in the bark by the cambium of the tree but not by the bud itself. On 40-year-old Q. petraea, we observed a proliferation of epicormic buds and in parallel a multiplication of the number of vascular traces in the trunk, but the knots caused by the traces of epicormic buds in the wood, either as individuals or in clusters, are minor since their colours are only slightly darker than those of woody rays and they are less than 2 mm in diameter. The knots will appear when epicormic buds develop into shoots. Received: 30 March 1999 / Accepted: 09 June 1999     

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.     

Water relations and gas exchange in Fagus sylvatica L. and Quercus petraea (Mattuschka) Liebl. in a mixed stand at their southern limit of distribution in Europe

Water status and gas exchange of beech (Fagus sylvatica L.) and sessile oak [Quercus petraea (Mattuschka) Liebl.] were studied in a mixed stand in the Montejo de la Sierra forest (central Iberian Peninsula), one of the southernmost locations of both species in Europe. Gas exchange and water potential were measured in leaves at different canopy levels over several days in two growing seasons. The daily variation pattern was established with the measurements of three selected dates per year, representative of the soil moisture content situations in early, mid- and late summer. A similar daily time course of leaf water potential was found for the two species. Nevertheless, beech showed a most noticeable decrease of water potential at midmorning and maintained lower leaf water potential than oak in the early afternoon. In 1994 the sessile oak saplings showed higher values of predawn water potential (Ψ_pd) than beech at the end of summer, when soil moisture content was lowest (20 cm depth). Beech showed a significantly lower net assimilation rate (A) than sessile oak for leaves under the same PPFD. Maximum net photosynthesis values (A _max) for beech and sessile oak on sunny leaves were 10.1±0.4 μmol m^–2 s^–1 and 17.8±1.7 μmol m^–2 s^–1 respectively, and those for water vapour stomatal conductance (g _wv) were 265±31 mmol m^–2 s^–1 and 438±74 mmol m^–2 s^–1. Differences in A and g _wv between the two species were maintained throughout the day on all measurement dates. No clear relationship was found between water status of saplings and stomata performance; there was only a negative correlation between Ψ_pd and g _wvmid in beech. Nevertheless, a significant response to the air vapour pressure gradient between leaf and air was translated into stomata closure on an hourly basis, more intensively in beech. Received: 4 March 1999 / Accepted: 21 December 1999