Sorbus Aria Research Articles

The introduction of Citrus to Italy, with reference to the identification problems of seed remains

While some consensus exists about the roles of southwestern China and northeastern India in the origin and diversification of the genus Citrus, the scarcity of its archaeological remains, as well as some methodological limits in unequivocally identifying taxa, do not facilitate reconstruction of the tempo and mode of spread of the genus towards other areas, notably the Mediterranean. Recent discoveries of archaeobotanical macro-remains (seeds and fruits) and pollen records from some important Italian sites in the Vesuvius area and Rome can be used to shed new light on this history. However, due to their morphological variability and the changes derived from the preservation processes, Citrus seeds appear difficult to recognise. In this paper, we present criteria to facilitate their precise identification, based on the observation of the morphology of modern seeds, and most of all the seed-coat patterns. The reference material consisted of “archaic” varieties of C. medica L. (citron), C. × limon (L.) Burm. f. (lemon) and seeds of C. × aurantium L. (bitter or Seville orange), C. × aurantiifolia (Christm.) Swingle (lime) and C. reticulata Blanco (tangerine, mandarin orange). Considering the fact that the general morphology of seeds, especially when mineralised, can confuse the identification of Citrus with Maloideae types, we also add criteria for the recognition of Cydonia oblonga Mill. (quince), Malus domestica Borkh. (apple), Pyrus communis L. (pear), Sorbus aria (L.) Crantz (whitebeam) and S. domestica L. (service tree). The observation of the keels and cell patterns was mostly useful to identify new material from Pompeii and Rome dating from the 3rd/2nd century b.c. and the Augustan period around the beginning of the Common (Christian) Era as C. medica L. (citron) and C. cf. × limon (L.) Burm. f. (possible lemon). The classical Greek and Latin sources helped us to understand the use and status of citrus fruits in the ancient world and, in combination with all available archaeobotanical remains compiled in this paper, have allowed us to discuss the spread of Citrus from its regions of origin to the eastern Mediterranean and then within the Mediterranean.

Vegetation databases provide a close-up on altitudinal tree species distribution in the Bavarian Alps

Thermal limits of tree species are of paramount interest in monitoring and projecting effects of climate warming. Based on the large phytosociological database BERGWALD, a database of forest inventory plots, and phytosociological tables of vegetation above treeline, tree species occurrence along a regional elevation gradient was assessed separately for tree and regeneration layer individuals. Upper limits in the database were compared to altitudinal limits given in Oberdorfer’s regional flora and to northern latitudinal limits given in worldwide distribution maps. For 26 of 32 tree species, the known distribution limits had to be raised based on plot data, demonstrating the high potential of vegetation databases to deliver ecological information. In most species, regeneration layer occurrences markedly exceeded the altitudinal limit of trees, indicating a potential for rather rapid advances of treelines under climate warming. Regional altitudinal and global latitudinal limits were quite closely related in the majority of tree species. However, several tree species climb to much higher elevation in the Alps than their latitudinal limits suggest. The boreal-subalpine mismatch in cold limits is only partly explained by vicariism and endemism. Sorbus aria, Acer pseudoplatanus, Fagus sylvatica and Taxus baccata are important elements of temperate mountain forests, which do not reach thermally equivalent northern latitudes for unknown reasons.

Leaf traits in parental and hybrid species of Sorbus (Rosaceae)

Knowledge of functional leaf traits can provide important insights into the processes structuring plant communities. In the genus Sorbus, the generation of taxonomic novelty through reticulate evolution that gives rise to new microspecies is believed to be driven primarily by a series of interspecific hybridizations among closely related taxa. We tested hypotheses for dispersion of intermediacy across the leaf traits in Sorbus hybrids and for trait linkages with leaf area and specific leaf area. Here, we measured and compared the whole complex of growth, vascular, and ecophysiological leaf traits among parental (Sorbus aria, Sorbus aucuparia, Sorbus chamaemespilus) and natural hybrid (Sorbus montisalpae, Sorbus zuzanae) species growing under field conditions. A recently developed atomic force microscopy technique, PeakForce quantitative nanomechanical mapping, was used to characterize the topography of cell wall surfaces of tracheary elements and to map the reduced Young's modulus of elasticity. Intermediacy was associated predominantly with leaf growth traits, whereas vascular and ecophysiological traits were mainly parental-like and transgressive phenotypes. Larger-leaf species tended to have lower modulus of elasticity values for midrib tracheary element cell walls. Leaves with a biomass investment related to a higher specific leaf area had a lower density. Leaf area- and length-normalized theoretical hydraulic conductivity was related to leaf thickness. For the whole complex of examined leaf traits, hybrid microspecies were mosaics of parental-like, intermediate, and transgressive phenotypes. The high proportion of transgressive character expressions found in Sorbus hybrids implies that generation of extreme traits through transgressive segregation played a key role in the speciation process.

Two newSorbus(Rosaceae) species from the Bakony Mts, Hungary

Two new species of the genus Sorbus are described from Hungary, illustrations and their distributions are presented. Both belong to nothosubgenus Tormaria (Sorbus latifolia group), putative hybridogenous apomictic species originated from a cross between a member of S. aria group and S. torminalis and occur sympatrically in dolomite slopes of the Bakony Mts (NW Hungary). One of them S. barabitsii is a stenoendemic species to the hill Malom-hegy above villages Márkó and Bánd where it grows in mixed forests of scree rocky slopes (Primulo veris-Tilietum) and in mixed karstic forests (Fago-Ornetum) of the north-facing slopes accompanied by other Sorbus species, such us S. aria, S. danubialis, S. graeca, S. pannonica, S. torminalis, as well as the local endemism S. veszpremensis. Population of the other species S. polgariana occurs in the valley Burok-völgy nearby town Várpalota (Királyszállás), where the majority of the individuals grow in mixed karstic forests (Fago-Ornetum), rocky beech forests (Daphno laureolae-Fagetum) and in the margin of closed thermophilous oak woodlands (Vicio sparsiflorae-Quercetum pubescentis) as well. This species grows sympatrically with Sorbus aria, S. danubialis, S. graeca, S. pannonica, S. redliana and S. torminalis.

Activity-guided isolation and identification of free radical-scavenging components from various leaf extracts of Sorbus aria (L.) Crantz

Nine phenolics were obtained from the leaves of Sorbus aria (L.) Crantz by activity-directed isolation: isorhamnetin 3-O-β-glucopyranoside (1), astragalin (2), isoquercitrin (3), hyperoside (4), kaempferol 3-O-β-glucopyranoside-7-O-α-rhamnopyranoside (5), quercetin 3-O-β-glucopyranoside-7-O-α-rhamnopyranoside (6), rutin (7), chlorogenic acid (8) and neochlorogenic acid (9). The isolates were identified by spectral methods (UV, 1H- and 13C-NMR, COSY, HMQC and HMBC), and their free radical-scavenging activity was tested using the l,l-diphenyl-2-picrylhydrazyl (DPPH) method. The antioxidant potential of the different extracts obtained in the fractionation process was evaluated using the DPPH test in relation to the HPLC contents of the isolates 1–9, total phenolics and total proanthocyanidins. Among the analytes tested, superior activity was expressed by isoquercitrin (3, EC50 = 2.76 mg L−1) and the ethyl acetate extract (EC50 = 2.99 mg L−1). Five strongly active isolates 3, 6, 7, 8 and 9 were found to be major components and to be principally responsible for the radical-scavenging activity of S. aria extracts.

Wood traits in parental and hybrid species ofSorbus

The genus Sorbus is an example of taxonomic complexity arising from the combined effects of hybridization, polyploidy, and apomixis. In this study, a comparison of the diagnostic microscopic characteristics of wood, together with the quantitative traits of vascular anatomy, fiber morphology, and cell wall constituents in parental ( Sorbus aria , Sorbus aucuparia , Sorbus chamaemespilus ) and hybrid ( Sorbus haljamovae , Sorbus montisalpae , Sorbus zuzanae ) taxa, was undertaken to discriminate each one of the examined taxa from the other and also to examine relatedness among parental and hybrid taxa. Chemical profiles of the degree of polymerization of cellulose were found to be the discriminating chemotaxonomic marker for all taxa. Sorbus haljamovae was determined to be the only taxon distinctively identified microscopically and distinguishable from the other examined taxa. The use of cell wall constituent traits provided a better segregation of hybrid taxa from their putative parental species than did the application of vascular anatomy variables. Sorbus zuzanae and Sorbus haljamovae formed isolated clusters that were distinctively segregated from all the other examined taxa. The results of this study indicate that the majority of wood traits in Sorbus hybrids do not exhibit an intermediate position between the parental taxa.

Diversity of leaf unfolding dynamics among tree species: New insights from a study along an altitudinal gradient

Leaf unfolding is a key ecosystem parameter controlling carbon and water fluxes and affecting forest dynamics. This parameter is highly sensitive to temperature and, consequently, often used as an indicator of global change. In this paper, we analyzed weekly leaf unfolding dynamics for seven temperate species (Fagus sylvatica L., Acer opalus Mill., Sorbus aria L., Quercus pubescens Willd. Abies alba Mill., Pinus sylvestris L., Pinus nigra Arnold). The effects of temperature on leaf unfolding were studied in situ using several methods and proxies. First, in a spatial approach, leaf unfolding dates were measured along two altitudinal gradients situated on the north and south face of the Mont Ventoux to test altitudinal and slope effects. Second, in a temporal approach, the year effect was analyzed on the north face by comparing two contrasted years (2006 and 2007). Finally, the role of temperature was investigated directly by linking leaf unfolding patterns and temperatures recorded during the leaf unfolding process itself.Two major conclusions were obtained. First, three distinct leaf unfolding patterns were revealed: a rapid sigmoid pattern for the deciduous species group, a slow kinetic pattern for the pines and an intermediate pattern for A. alba. Second, we found an unexpected pattern of variation in the response to spatial or temporal variation of temperature. The more sensitive a species was to temperature variations between years, the less sensitive it was to temperature variations due to altitude. Finally, we discuss that these results can be correlated to two major life history traits: evergreen vs. deciduous and shade tolerant vs. shade intolerant.

Disentangling factors controlling fruit and seed removal by rodents in temperate forests

AbstractFleshy fruits fall on to the ground together with cleaned seeds previously ingested by primary dispersers, offering a wide range of fruits and seeds to the ground foragers. Although nutritional properties strongly differ between fruits and seeds, this different seed presentation (cleaned seeds versus seeds within the pulp) has not been addressed in seed removal studies. This study reports on the removal of fruits versus their seeds in five fleshy-fruited species in a temperate forest. We found that rodents removed most of the seeds and partially consumed most of the fruits, preferring seeds to fruits. Rodents bit the fruits to extract the seeds, leaving most of the pulp. We found a preference ranking for the seeds (Sorbus aucuparia>Ilex aquifolium>Sorbus aria>Rosa canina>Crataegus monogyna) but no preferences were found for the fruits, probably due to their similarities in pulp constituents. Seed and fruit choice were affected by chemical and physical properties and not by their size. The presence of alternative and preferred seeds (nuts) delayed the encounter of the fruits and seeds and diminished their removal rates. We found that higher rodent abundance is not necessarily associated with higher removal rates of fleshy fruits. Rodent abundance, fruit size and seed size are minor factors in the removal of fleshy fruits and their seeds. This study underlines that scatter-hoarding rodents are important removers of fleshy fruits and their seeds, producing a differential seed removal depending on the seed presentation (with or without pulp), the nutritional properties of the seeds (but not of the fruits) and the presence of alternative food.

Isolation and characterization of microsatellite loci in Sorbus aria (Rosaceae)

Sorbus aria (L) Crantz (Common Whitebeam) is native to Europe, east of the Balkans and in North Africa; it is also present in the Canary Islands. To evaluate the genetic diversity in natural populations of this vulnerable species, nine novel polymorphic microsatellite markers were isolated from enriched libraries. Microsatellite loci were screened in 97 individuals from La Palma (Canary Islands) and Sierra Nevada (Granada, Spain). Examination of the microsatellite profiles shows that S. aria individuals have up to three alleles per locus. The cloned sequences in microsatellite loci confirmed the polyploidy status of the plants. The number of alleles ranged from 5 to 14 per locus. The phenotype diversities across loci (H′ T ) ranging from 0.653 to 0.847.

A seeding experiment for testing tree-community recruitment under variable environments: Implications for forest regeneration and conservation in Mediterranean habitats

As a result of historical and ecological factors, woody-plant communities are composed of species with different biogeographic origins that presumably diverge in their ability to recruit under variable environments. The aim of this study is to evaluate the conservation implications of the differences in recruitment of six tree species of different biogeographical origin coexisting in a Mediterranean mountain forest. Under field conditions, we experimentally sowed a total of 16,650 seeds belonging to two Mediterranean ( Quercus ilex and Q. pyrenaica), two Sub-Mediterranean ( Acer opalus subsp. granatense and Sorbus aria), and two boreo-alpine ( Pinus sylvestris and Taxus baccata) species. We selected the three most common successional scenarios where seeds are dispersed (woodland, shrubland, and open areas), and half of the seeds were irrigated simulating a rainy summer in terms of Mediterranean ecosystems. Seedling survival was strongly affected by the type of habitat as well as irrigation. The recruitment of the six species was possible only in wooded areas under wet conditions (20–60%). In shrubland, only Mediterranean and Sub-Mediterranean species recruited in typical dry years (10–50%), but irrigation increased their survival (25–65%) and allowed P. sylvestris to recruit (32%). Open areas constituted the most adverse scenario for seedling recruitment, especially for non-watered seedlings (0% for boreo-alpine species, 1–24% for the rest). Overall, the benefit of a wet summer was more critical for the recruitment of boreo-alpine species ( P. sylvestris and T. baccata) than for the rest, although a rainy summer is also necessary for the recruitment of A. granatense and S. aria in shrubland habitats. Our experimental results indicate that the maintenance of current species composition in Mediterranean mountain forests is critically dependent on the periodic, scattered occurrence of wet summers. From a conservational standpoint, the predictions of less rainy summers and greater aridity in the future imply a change in the relative abundance of species in the mature forest, with Mediterranean species becoming dominant and boreo-alpine species becoming even rarer and, consequently, a serious threat for the persistence of boreo-alpine species at their southern distribution limit.

Consistent pattern of habitat and species selection by post-dispersal seed predators in a Mediterranean mosaic landscape

There is still little information on effects of habitat degradation on post-dispersal seed predation at the landscape scale. The aim of this study was to determine the influence of habitat degradation and seed species on the variability of post-dispersal seed-predation rate. Experimental seed removal was investigated in six Mediterranean woody plant species, four trees (Pinus sylvestris, Quercus ilex, Acer opalus ssp. granatense, and Sorbus aria) and two shrubs (Berberis vulgaris and Crataegus monogyna), in an extensively used mosaic landscape on the Sierra Nevada massif (SE Spain). Seed depots were distributed over 2 years in five differently degraded landscape units, each one with three plots: shrubland; native forest; and dense, cleared and fenced reforestation stands. Predation was the highest in native forest, shrubland, and fenced reforestation, and the lowest in dense and cleared reforestation stands, being partially due to a positive correlation between shrub cover and post-dispersal seed predation. However, the main factors driving post-dispersal seed predation were intrinsic to seeds, as species preference explained most of the variance in our model for predation. The plant-species ranking was Quercus > Pinus > Sorbus > Berberis > Acer > Crataegus, the dominant tree species being the most depredated. These findings are novel because they suggest for the first time that species-selection patterns by post-dispersal seed predators tended to remain constant through both study years in all habitats comprising a mosaic landscape, whether native forest, reforestation stands or successional shrubland.

Frost sensitivity of Fagus sylvatica and co-occurring deciduous tree species at exposed sites

In Central-European forest stands on southerly to south-westerly exposed slopes of hills with shallow soil, European beech ( Fagus sylvatica L.) loses its predominance over other deciduous tree species such as oak ( Quercus spp.) or whitebeam ( Sorbus aria [L.] Cr.) due to limited availability of water. To test whether winter frost is an additional factor affecting the performance of beech at such exposed sites, we measured the minimum air temperatures between the sampling dates, and determined the frost sensitivity of F. sylvatica and of co-occurring tree species ( Quercus petraea [Matt.] Liebl., Quercus pubescens Willd., Quercus robur L., S. aria [L.] Cr.) eight times during the winter months, from October to March, at two sites in the low mountain ranges of Central Germany, which share the same climatic, topographic and edaphic conditions. At each site, two localities (stand margins and the interior of the stands) were compared. The frost sensitivity was determined in twigs of the trees using the electrolyte leakage method. Throughout the winter, frost sensitivity of F. sylvatica was higher at the stand margin than in the forest interior at one of the sites. At the other one, the frost sensitivity of S. aria was lower at the stand margin than within the stand. No consistent differences between the localities were detected in the Quercus species. Generally, the differences in frost sensitivity between the localities were only small; this was probably due to the slight differences in the minimum air temperature. Thus, it appears unlikely that, in exposed localities, the performance of the investigated tree species is limited by winter frost. More consistent differences were found among species, with F. sylvatica proving to be the most frost-sensitive, and S. aria, the least frost-sensitive species, particularly at the stand margins. The role of frost for the occurrence and performance of F. sylvatica should be further studied at the limit of its distribution along the boundary to the semihumid areas of Central Europe.

Separation of quercetin, sexangularetin, kaempferol and isorhamnetin for simultaneous HPLC determination of flavonoid aglycones in inflorescences, leaves and fruits of three Sorbus species

A reversed-phase high-performance liquid chromatographic (RP-HPLC) method was developed and validated for the simultaneous determination of four flavonol aglycones (quercetin, QU; sexangularetin, SX; kaempferol, KA; isorhamnetin, IS) in hydrolyzed extracts from different plant parts of Sorbus aucuparia L., Sorbus aria (L.) Crantz. and Sorbus intermedia (Ehrh.) Pers. Separation of the four compounds was accomplished on a C18 Lichrosphere 100 column (5 μm, 250 mm × 4.6 mm, i.d.) with a methanol gradient elution and recorded at 370 nm. The high resolution of critical bands – SX, KA and IS – was achieved with retention of the last peak (IS) in 19.5 min. The equilibration of the standard mixture by addition of HCl to an acid concentration equal that of hydrolyzed extracts injected was found to be necessary when minimizing calibration error. The correlation coefficients of all the calibration curves showed good linearity ( r > 0.9991) over the test range. The relative standard deviation of the method was less than 2.8% for intra- and inter-day assays, and the average recoveries were between 95.5 and 102.5%. High sensitivity was demonstrated with detection limits between 0.050 and 0.085 μg/ml. The level of total aglycones was found to be in the range of 687–1515 mg/100 g of dry weight in the inflorescences, 424–1078 mg/100 g in the leaves and 20–60 mg/100 g in the fruits depending on the Sorbus species.

Parentage of endemic Sorbus L. (Rosaceae) species in the British Isles: evidence from plastid DNA

The parentage of polyploid Sorbus species in the British Isles was investigated using plastid DNA microsatellites. Four hundred and fifty-three samples from 30 taxa were screened using six microsatellite fragments, which gave 28 haplotypes. The haplotypes formed groups clearly related to the ancestral diploids Sorbus aria, Sorbus aucuparia, and Sorbus torminalis. Species in the Sorbus aria group all had Aria haplotypes (with the exception of one English S. aria), species in the Sorbus anglica group had an Aucuparia haplotype, and species in the Sorbus latifolia group had a Torminalis haplotype. Sorbus intermedia had an Aucuparia haplotype. This indicated that the hybridization events that led to the formation of species in the S. anglica and S. latifolia groups usually did so with S. aria s.l. as the pollen-donating (paternal) parent. The polyploids S. anglica, Sorbus bristoliensis, Sorbus croceocarpa, Sorbus decipiens, Sorbus devoniensis, Sorbus hibernica, Sorbus lancastriensis, Sorbus leptophylla, Sorbus leyana, Sorbus minima, Sorbus rupicola, Sorbus subcuneata, Sorbus vexans, Sorbus whiteana, Sorbus wilmottiana, and three unnamed taxa may each be derived from a single maternal lineage. The polyploids Sorbus eminens, Sorbus porrigentiformis, and S. latifolia have multiple maternal lineages. The two primary diploid hybrids S. × thuringiaca and S. × vagensis have arisen many times independently. © 2007 The Linnean Society of London, Botanical Journal of the Linnean Society, 2007, 154, 291–304.

Deposition velocities to Sorbus aria, Acer campestre, Populus deltoides × trichocarpa ‘Beaupré’, Pinus nigra and × Cupressocyparis leylandii for coarse, fine and ultra-fine particles in the urban environment

Trees are effective in the capture of particles from urban air to the extent that they can significantly improve urban air quality. As a result of their aerodynamic properties conifers, with their smaller leaves and more complex shoot structures, have been shown to capture larger amounts of particle matter than broadleaved trees. This study focuses on the effects of particle size on the deposition velocity of particles (Vg) to five urban tree species (coniferous and broadleaved) measured at two field sites, one urban and polluted and a second more rural. The larger uptake to conifers is confirmed, and for broadleaves and conifers Vg values are shown to be greater for ultra-fine particles ( Dp<1.0 μm ) than for fine and coarse particles. This is important since finer particles are more likely to be deposited deep in the alveoli of the human lung causing adverse health effects. The finer particle fraction is also shown to be transported further from the emission source; in this study a busy urban road. In further sets of data the aqueous soluble and insoluble fractions of the ultra-fines were separated, indicating that aqueous insoluble particles made up only a small proportion of the ultra-fines. Much of the ultra-fine fraction is present as aerosol. Chemical analysis of the aqueous soluble fractions of coarse, fine and ultra-fine particles showed the importance of nitrates, chloride and phosphates in all three size categories at the polluted and more rural location.

Forest dynamics in unexploited birch ( Betula pendula) stands in the Vosges (France): structure, architecture and light patterns

Two adjacent forest stands composed of birch ( Betula pendula) and beech ( Fagus sylvatica) in the Vosges (France), aged 15 and 50 years, respectively, and representing a possible chronosequence of 35 years of forest evolution, have been studied. The objectives of the research were to compare structural and architectural changes at the level of tree population and forest ecosystem. In each forest stand, a profile of 2500 m 2 was carried out in which trees and juveniles were measured and mapped, as well as dead woody debris. Forest architecture was represented by crown projection and vertical drawings. In addition, we have drawn some typical birch and beech morphologies observed in the two plots and interpreted them on the basis of their social status: tree of the present, potential tree, suppressed tree, tree of the past. These concepts have been developed by the architectural model of [Oldeman, R.A.A., 1990. Forests: Elements of Silvology. Springer, Berlin]. Changes in canopy geometry (canopy openness, LAI), and light pattern (PPFD; R:IR) were examined with the help of hemispherical photographs and analysed by two computer programs (Gap Light Analyser, GLA and Winphot). The study of this temperate succession shows that there is a functional link between structure, tree architecture, forest architecture, light pattern and succession. Analysis of birch architecture during succession reveals a remarkable variety of architectural developments that are related to the strong infraspecific competition during site colonisation. The architecture of beech depends on the forest environment created by the birch. Beech is a very flexible species, which may grow in light as well as in alternating conditions of light and shade. In birch forest, the good light availability explains its architectural traits where growth is close to the open. Because such good light conditions are constant during the whole development phase of the birch eco-unit, beech growth does not pass through alternating periods of suppression and released growth as observed under beech canopy. Forest architecture and structure change significantly during succession, with a decrease in birch density and a global increase in tree dimensions for the birch and the understorey beech. Juveniles are more numerous among the dense herb coverage. Like young beeches, other tree species (mainly Sorbus aria and Quercus petraea) appreciate the light pattern created by birch canopy, with both good light availability and protection against wind and drought. After 50 years of dominance, the birch begins to die, creating big gaps in the canopy which favour beech growth and a rich herbaceous coverage. We predict that after 80 years, the canopy will be dominated by beeches with very broad crowns under which juveniles will grow with periods of suppression.

The origin of intermediate species of the genus Sorbus.

The genus Sorbus in Europe contains five diploid species, Sorbus aria, Sorbus aucuparia, Sorbus torminalis, Sorbus chamaemespilus and Sorbus domestica, classified into five different subgenera. The subgenus Aria (or the S. aria aggregate) contains apomictic triploid and tetraploid species. Within the genus there are, in addition, a number of species, morphologically intermediate between plants of the four main subgenera, which are considered to reproduce partly, or exclusively, by apomixis. These are believed to have originated by hybridisation between species in the S. aria aggregate and a species from another subgenus (either S. aucuparia, S. torminalis or S. chamaemespilus). We have used restriction fragment length polymorphism (RFLP) analysis on a total of 178 Sorbus accessions to test this model. The genome relationships of the different groups have been assessed, and the hybrid nature of the major intermediate groups is unequivocally demonstrated. Polyploid species in the S. aria aggregate show genetic variation, indicating the possibility of multiple origins and/or facultative apomictic breeding behaviour. A major finding, confirmed by microsatellite analysis, is that the 'intermediate' species S. intermedia is shown to have genomes from S aria, S aucuparia and S torminalis. Polymorphic mitochondrial DNA markers were used to determine the direction of the crosses that gave rise to new 'hybrid species'; in the majority of cases the pollen was provided by the parent from the S aria aggregate.

Pollen- versus seed-mediated gene flow in a scattered forest tree species.

We examined the spatial distribution of maternally inherited chloroplast DNA markers over the French part of the range of Sorbus torminalis, a scattered temperate forest tree native to most of Europe. The survey by restriction analysis of polymerase-chain-reaction amplified fragments for 880 individuals distributed among 55 populations allowed the detection of 25 haplotypes. The coefficient of differentiation among populations computed on the basis of haplotype frequency (G(STc) = 0.34) was one of the lowest found in forest trees so far, and the mean within-population diversity was relatively high, indicating multiple-mother foundation events. A significant but slight geographical pattern was observed, up to distances of about 100 km. This pattern of differentiation was compared to the genetic structure of the same populations revealed by biparentally inherited markers (isoenzymes), and a new method to quantify the relative importance of seed and pollen dispersal was derived, based on isolation-by-distance models. Neither pollen- nor seed-mediated gene flow was predominant in S. torminalis, a finding that differs from those for the majority of tree species studied so far. This result was most likely due to an extinction-recolonization dynamics based on efficient seed dispersal strategies. The joint screening of 31 individuals of the related Sorbus aria and of 163 hybrid individuals shows that hybridization occurs predominantly in one direction and is rarely followed by cytoplasmic introgression. As a consequence, interspecific gene flow should not significantly affect the diversity dynamics within S. torminalis.

POLLEN- VERSUS SEED-MEDIATED GENE FLOW IN A SCATTERED FOREST TREE SPECIES

We examined the spatial distribution of maternally inherited chloroplast DNA markers over the French part of the range of Sorbus torminalis, a scattered temperate forest tree native to most of Europe. The survey by restriction analysis of polymerase-chain-reaction amplified fragments for 880 individuals distributed among 55 populations allowed the detection of 25 haplotypes. The coefficient of differentiation among populations computed on the basis of haplotype frequency (GSTc = 0.34) was one of the lowest found in forest trees so far, and the mean within-population diversity was relatively high, indicating multiple-mother foundation events. A significant but slight geographical pattern was observed, up to distances of about 100 km. This pattern of differentiation was compared to the genetic structure of the same populations revealed by biparentally inherited markers (isoenzymes), and a new method to quantify the relative importance of seed and pollen dispersal was derived, based on isolation-by-distance models. Neither pollen- nor seed-mediated gene flow was predominant in S. torminalis, a finding that differs from those for the majority of tree species studied so far. This result was most likely due to an extinction-recolonization dynamics based on efficient seed dispersal strategies. The joint screening of 31 individuals of the related Sorbus aria and of 163 hybrid individuals shows that hybridization occurs predominantly in one direction and is rarely followed by cytoplasmic introgression. As a consequence, interspecific gene flow should not significantly affect the diversity dynamics within S. torminalis.Corresponding Editor: S. Tonsor

Growth and water relations of four deciduous tree species (Fagus sylvatica L., Quercus petraea [Matt.] Liebl., Q. pubescens Willd., Sorbus aria [L.] Cr.) occurring at Central-European tree-line sites on shallow calcareous soils: physiological reactions of seedlings to severe drought

The reactions of growth and water relations to drought were tested in young plants of four deciduous tree species ( Fagus sylvatica L., Quercus petraea [M att .] L iebl ., Q. pubescens W illd ., Sorbus aria [L.] C r .) which occur at drought-caused tree-line sites in Central Europe on shallow calcareous soils. Two-year-old seedlings were planted into lysimeters and subjected to two periods (eleven and ten weeks, respectively) of severe drought, interrupted by thorough rewatering after the first drought period. Control plants were always well watered. Drought stress responses of these plants were investigated by pV analysis, and by measurements of leaf conductance and actual water potentials. Additionally, desiccation experiments were performed with excised shoots of some of the trees. Drought-stressed F. sylvatica exhibited the highest rates of visible drought damage. However, this species was able to regain a high relative leaf water content (RWC) quickly after rewatering. S. aria, which can grow even on extremely exposed sites, was drought-adapted as shown by the following: its high leaf mass per unit area; its high leaf tissue elasticity; its high relative water content of saturated leaves (WC sat ); and the low relative water content of its symplasm, which facilitates large changes in osmotic pressure in spite of low dry-mass related solute concentrations. High δ 13 C ratios (deviation of the leaf's 13 C fraction from a standard) indicated a high long-term water-use efficiency by this species. These features allow an effective gas exchange even at low leaf water potentials. In contrast to F. sylvatica and S. aria , the Quercus species exhibited adaptability to drought, which was evident due to significant morpho-anatomical differences between the drought-stressed and control trees. In Q. petraea , adaptability was achieved by reductions in the leaf/root ratio, and in the number of second flushes and buds. In Q. pubescens , adaptability was found at the leaf level; this is indicated by larger differences between the osmotic pressures at full turgor and at turgor loss, in higher WC sat , and in a decrease in the RWC at turgor loss. These alterations are obviously due to changes in the leaf tissue properties. In none of the species was osmotic adjustment found as an adaptation to drought stress.