Shrub facilitation of Quercus humilis regeneration in succession on calcareous grasslands

Abstract. In southern France, the natural invasion by Quercus humilis of calcareous grassland takes place in a mosaic of herbaceous and scrubby patches. We hypothesized that the presence of the shrubs Buxus sempervirens and Juniperus communis alter the rate and the pathway of the succession by facilitating the regeneration of Q. humilis. To infer the process of facilitation at a large scale, the spatial distribution of Q. humilis was studied in relation to acorn sources and the type of plant cover in grazed and ungrazed sites. Abundant recruitment up to 80 m from the wood margins and from isolated oak trees in grassland shows that acorns are dispersed effectively. At the three study sites, the density of Q. humilis individuals was higher under shrubs than in grassland, suggesting that facilitation may occur. This density difference was much higher in the grazed sites than in the ungrazed site. Moreover, before grazing by livestock, the distribution of first-year seedlings is independent of vegetation cover. Thus, shrubs improve Q. humilis regeneration by protecting individuals from grazing. The high density of individuals at the northern edge of shrubs suggests that a second facilitation mechanism may exist, probably related to improved germination conditions. Facilitation by shrubs appears to be very important for Q. humilis dynamics.     

佛坪国家级自然保护区秦岭箭竹克隆结构的SSR分析

利用SSR分子标记技术分析了佛坪国家级自然保护区秦岭箭竹(Fargesia qinlingensis)的克隆多样性和克隆结构,以探讨小尺度范围内秦岭箭竹自然居群遗传变异的分布特征,对该种开花特性、高山地区生态环境维护和大熊猫的保护提供重要依据。结果表明7对SSR引物共扩增出79个位点,其中多态性位点77个,多态位点百分率(PPB)为97.47%。秦岭箭竹的142个分株共形成107个克隆,最大克隆可达5 m。克隆多样性略高于其他克隆植物的平均值(D=0.62,G/N=0.17,E=0.68),基因型比率(G/N)、Simpson指数(D)、平均克隆大小(N/G)和Fager均匀性指数(E)分别为0.7535、0.9680、1.3271和0.5109。克隆空间结构分析表明秦岭箭竹的克隆构型为密集型,各克隆呈镶嵌性分布,同一克隆的分株排列紧密。克隆聚类分析表明各克隆之间聚类不明显,总体上来自同一样地的克隆被聚为一类。空间自相关分析显示在空间距离为36 m范围内,分株比基株有更显著的空间遗传结构,空间自相关系数r的取值范围分别为0.084—0.626和0.024—0.288,说明克隆繁殖在一定程度上限制了空间遗传结构的范围。样地内秦岭箭竹个体在空间距离小于44 m时存在显著的正相关空间结构,特别是在4 m处表现出最大的空间自相关系数(r=0.626),表明空间距离相距4 m内的个体最有可能属于同一克隆,4 m比5 m更能表现出清晰的克隆结构,X-轴截距为52.280,代表了秦岭箭竹不规则克隆的平均最小长度。秦岭箭竹的克隆多样性和克隆结构与初始苗补充、花粉散播方式和微环境差异有关。     

Floral distribution,clonal structure,and their effects on pollination success in a self-incompatible <Emphasis Type="Italic">Convallaria keiskei</Emphasis> population in northern Japan

In plant species, when clonal growth produces a patchy structure and flowering ramets are clustered, the amount of pollen contributing to reproductive success is often regulated by pollinator efficiency and geitonogamy. The spatial population structure may influence reproductive success. We examined the clonal structure, the spatial ramet distribution, and their combined effects on fruit set in a natural population of the insect-pollinated, self-incompatible clonal herb, Convallaria keiskei, in northern Japan. The number of shoots, flowers, and fruits in 1-m² quadrats were counted at every 5 m grid point in an established 100 × 90-m study plot. From all the quadrats where shoots existed, leaf samples were collected for allozyme analysis. Using the two spatial parameters of flowering ramet densities and genotypes, we then constructed individual-based fruit-set models. A total of 236 quadrats contained shoots, and 135 contained flowering ramets, which indicated expanded distribution of this plant throughout the study plot, while shoots, flowers and fruits all showed clustering distributions. Allozyme analysis of 282 samples revealed 94 multilocus genotypes. The largest clone extended to more than 40 m, whereas 56 genotypes were detected in only one sample. Several large clones and many small clones were distributed close to each other. Fine-scale spatial modelling revealed that the neighbouring flower numbers of different genotypes, compared with local genet or flower diversity, more influenced fruit set, in which the range of the neighbour was 14.5 m. These findings indicate that the compatible pollen dispersed by insect pollinators has a significant effect on sexual reproduction, in this C. keiskei population. Consequently, the spatial structure, which includes both genet distribution and clonal expansion by ramets, had a significant effect on pollination success.     

Spatial pattern of Quercus ilex and Quercus pubescens recruitment in Pinus halepensis dominated woodlands

Abstract. European Mediterranean landscapes have undergone changes in structure in recent years as a result of widespread agricultural land abandonment and cessation of silvicultural regimes. Studies concerning the regeneration dynamics of dominant forest species have become critical to the prediction of future landscape trends in these changing forest stands. Quercus ilex (holm oak) and Q. pubescens (downy oak) are considered to be the terminal point of secondary succession in extensive areas of the Mediterranean region. Recent studies, however, have suggested the existence of recruitment bottlenecks in oak genet populations as a result of current management regimes. In this study, we present evidence of the successful establishment of Q. ilex and Q. pubescens in Pinus halepensis (Aleppo pine) woodlands. We investigate the distribution patterns and spatial relationships among oak recruits and resident pines. Established P. halepensis is randomly distributed throughout the study area. Oak seedlings are positively associated with pine trees, suggesting that P. halepensis individuals provide safe sites for oak genet recruitment. We show that spatial patterns of recruitment are in agreement with the general model of spatial segregation described for other Mediterranean plant communities, with seeder species colonizing large openings after disturbance, followed by a more aggregated recruitment of resprouter species.     

Clonal and genetic structure of two Mexican oaks: Quercus eduardii and Quercus potosina (Fagaceae)

Quercus eduardii and Q. potosina are dominant oak species in Sierra Fría, Aguascalientes, Mexico. These species have been exploited for multiple purposes since the 16th century. Both species produce clonal offspring through root suckering and acorns through sexual reproduction. To understand clonality for the implementation of the most adequate actions for the conservation of these species, we addressed the following questions: (a) what is the spatial clonal structure of both species? (b) How much clonal and genetic diversity is maintained in these species? Random Amplified Polymorphic DNAs (RAPDs) were used as molecular markers for these analyses. Genets of both species have few ramets and these grow close the parent tree. Autocorrelation analyses at the ramet level showed an aggregated distribution at short distances and a random spatial distribution at larger distances. Also, at the genet level the autocorrelation analyses showed a random distribution. Clonal diversity was high in both species (Q. eduardii: D=0.963, G/N=0.60; Q. potosina: D=0.985, G/N=0.65). Genetic diversity was high within populations (Q. eduardii: H _e =0.33±0.11; Q. potosina: H _e =0.35±0.11). Low levels of genetic differentiation among populations were observed (Q. eduardii ? _st =0.19, P < 0.002; Q. potosina ? _st =0.13, P < 0.002). Both species maintain high levels of clonal and genetic diversity, probably due to successful sexual reproduction, which allows gene flow among populations. Conservation and/or reforestation programs must include seed collections and germplasm banks. Due to the small genet size and the high clonal diversity of these species, seeds can be collected in any place in Sierra Fría, Aguascalientes.     

Large-Scale Patterns of Quercus ilex, Quercus suber, and Quercus pyrenaica Regeneration in Central-Western Spain

In Central-Western Spain, forests and woodlands composed of Quercus sp. support outstanding levels of biodiversity, but there is increasing concern about their long-term persistence due to a lack of regeneration. We hypothesize that this regenerative lack is operating on a large geographic scale; that there are differences in the abundance of regeneration between three oak species; that oak regeneration is governed mainly by forest management and structure; and that shrubs act as important physical protectors of seedlings and saplings. We analyzed whether densities of oak seedlings and saplings in several size classes were related to stand-structure, understory, and physiographic variables potentially affecting regeneration. Data collected at a regional level (1 km × 1 km grid) by the Spanish Forest Inventory were evaluated from 2,816 plots. Results revealed that regeneration failure was common for all size categories, from small seedlings to large saplings, and for the three oak species studied, especially the evergreens. Of the Quercus ilex, Q. suber, and Q. pyrenaica plots studied, 49%, 62%, and 20% were lacking any small seedlings, and 82%, 96%, and 56% did not have any large saplings, respectively. Regeneration was positively correlated with tree cover and density, especially of small and medium-sized trees, and negatively correlated with the presence of large trees, indicating that regeneration failure is mostly associated with more open, uniform, and/or aged woodlands. Regeneration densities of Q. ilex and Q. suber were positively correlated with all understory variables, suggesting that the presence of pioneer shrubs represent a major safe site for early tree recruitment, independent from specific shrub species.     

An analysis of the costs and benefits of physiological integration between ramets in the clonal perennial herb Glechoma hederacea

Summary The costs and benefits, measured in terms of dry weight, of physiological integration between clonal ramets, were analysed in two experiments conducted on the clonal herb Glechoma hederacea. Firstly, integration between consecutively-produced ramets was examined in an experiment in which stolons grew from one set of growing conditions (either unshaded or shaded and either nutrient-rich or nutrient-poor) into conditions in which light or nutrient level was altered. Comparisons were made between the dry weight of the parts of the clones produced before and after growing conditions were changed, and the dry weights of the corresponding part of control clones subjected to constant growing conditions. In a second experiment, integration between two distinct parts of G. hederacea clones was investigated. In this experiment clones were grown from two connected parent ramets and the parts of the clone produced by each parent ramet were subjected independently to either nutrient-rich or nutrient-poor conditions. Ramets in resource-rich conditions provided considerable physiological support to those in resource-poor conditions. This was measured as a dry weight gain compared with the weight of the corresponding part of the control clones growing in resource-poor conditions. However, when stolons grew from resource-poor conditions into resource-rich conditions, there was no similar evidence of the resourcepoor ramtes receiving support from resource-rich ramets. Physiological integration did not result in dry weight gains when this would have necessitated basipetal translocation of resources.Because of the predominantly acropedal direction of movement of translocates in G. hederacea, the structure of the clone was important in determining the effectiveness of integration between ramets. Where physiological integration was effective, the cost to the supporting ramets in terms of dry weight was insignificant. Physiological integration allows clones to maintain a presence in less favourable sites with insignificant cost to ramets in favourable sites, thereby reducing the probability of invasion by other plants, and providing the potential for rapid clonal growth if conditions improve. Integrated support of ramets in unfavourable conditions also enables the clone to grow through unfavourable sites, thus increasing the probability of encountering more favourable conditions by wider foraging.     

Trade-offs among growth, clonal, and sexual reproduction in an invasive plant Spartina alterniflora responding to inundation and clonal integration

The purpose of this article was to study the trade-offs among vegetative growth, clonal, and sexual reproduction in an aquatic invasive weed Spartina alterniflora that experienced different inundation depths and clonal integration. Here, the rhizome connections between mother and daughter ramets were either severed or left intact. Subsequently, these clones were flooded with water levels of 0, 9, and 18 cm above the soil surface. Severing rhizomes decreased growth and clonal reproduction of daughter ramets, and increased those of mother ramets grown in shallow and deep water. The daughter ramets disconnected from mother ramets did not flower, while sexual reproduction of mother ramets was not affected by severing. Clonal integration only benefited the total rhizome length, rhizome biomass, and number of rhizomes of the whole clones in non-inundation conditions. Furthermore, growth and clonal reproduction of mother, daughter ramets, and the whole clone decreased with inundation depth, whereas sexual reproduction of mother ramets and the whole clones increased. We concluded that the trade-offs among growth, clonal, and sexual reproduction of S. alterniflora would be affected by inundation depth, but not by clonal integration.     

Position of tillers in a clone determines their ontogeny: example of the clonal grass <Emphasis Type="Italic">Phalaris arundinacea</Emphasis>

For practical reasons, all ramets of a clonal plant are considered to be independent individuals in demographical studies, but there are reports that ramet ontogeny is affected by their position in a clone. This means that ramets growing from buds located in certain parts of the rhizome system differ in size and probability of flowering from ramets emerging from buds at various positions. We studied the growth characteristics and ontogeny of tillers (ramets) of a rhizomatous grass in relation to their position in a clone and examined whether it is affected by the disturbance regime. We assessed the position of tillers in a clone, the developmental stage of their apical meristem and growth characteristics in mown and unmown stands of the rhizomatous grass Phalaris arundinacea at two sites, one representing optimal (lowland) and the other suboptimal (highland) environmental conditions. We found that tiller characteristics and the probability of a tiller to start flowering are related to the tiller position in a clone. Tiller differences were preserved even in suboptimal environmental conditions but disappeared after mowing, which equalized size and ontogeny. Our results imply that ramets of clonal plants are not independent demographical units, thus their position in a clone should be taken into account.     

The clonal structure of Quercus geminata revealed by conserved microsatellite loci

The scrub oak communities of the southeastern USA may have existed at their present locations for thousands of years. These oaks form suckers, and excavations of root systems suggest that clones may occupy very large areas. Resolution of the clonal nature of scrub oaks is important both to manage the tracts of this ecosystem that remain, and in conducting long-term ecological studies, where the study area must substantially exceed the area occupied by any single clone. Microsatellites were used to determine the genetic diversity of a dominant oak species within a 2-ha long-term experimental site on Merritt Island at the Kennedy Space Center. This area contains a long-term study of the effects of elevated CO2 on the ecosystem. Conservation of seven microsatellite loci, previously identified in the sessile oak, Quercus petraea, was tested in two Florida scrub oak species, Q. geminata and Q. myrtifolia. Sequence analysis revealed that all seven microsatellite loci were conserved in Q. geminata and five loci were conserved in Q. myrtifolia. Six microsatellite loci were polymorphic in Q. geminata and these were subsequently used to investigate the clonal structure of the Q. geminata population. Twenty-one unique combinations of microsatellites, or haplotypes, occurred only once, whereas the remaining 26 individuals belonged to a total of seven different haplotypes. Trees with identical haplotypes were in close proximity, supporting the interpretation that they were clones. The results showed that there is significant genetic diversity within the 2-ha experimental site. Microsatellites provided a powerful and noninvasive tool for distinguishing individual genotypes and determining an adequate area for long-term ecosystem studies.     

The Description of Clones of Quercus robur L. and Q. petraea (Matt.) Liebl. with Microsatellites and AFLP in an Ancient Woodland

Abstract: In densely populated areas autochthonous Quercus robur L. (pedunculate oak) and Q. petraea (Matt.) Liebl. (sessile oak) (Fagaceae) populations have been maintained as ancient devastated woodlands. The continuous cutting, grazing and resprouting of such woodlands has enabled the development of clonal structures. For conservation purposes, an analysis of the actual number, size and spatial distribution of clones is necessary, especially when there is an interest in genetic variation of the population. This study describes for the first time - based on microsatellite and AFLP™ analysis - clones in an autochthonous mixed Q. robur and Q. petraea population that has been coppiced and grazed for several centuries. Based on six microsatellite loci and 69 polymorphic AFLP markers, only 14 unique genotypes were detected in a plot that consisted of 80 trees. Clones were observed for both Q. robur and Q. petraea. The largest clone diameters were observed for Q. robur, with distances up to 5.8 m. The observed clone sizes may indicate the age of the trees.     

北京山区栓皮栎林优势种群点格局分析

采用点格局分析方法对北京山区栓皮栎林主要种群和不同发育阶段种群的空间分布格局及相互关系进行分析。结果表明:(1)群落物种组成简单,栓皮栎和油松是群落的优势种群,优势种群以中径木为主,缺乏可供更新的幼苗,种群呈现出衰退趋势;(2)在研究尺度内,栓皮栎、油松单种的空间分布格局以聚集型分布为主,随着尺度的增大呈现出随机分布和均匀分布的趋势;栓皮栎和油松呈现显著负相关;(3)优势种群不同发育阶段的空间分布格局与研究尺度紧密相关,栓皮栎小树和中树以聚集型分布为主、大树呈随机分布,而油松不同发育阶段均以随机分布为主;(4)栓皮栎、油松种群不同发育阶段之间的空间关联主要发生在小尺度范围内,小树和中树以正关联为主,而中树和大树、小树和大树以负关联为主;随着研究尺度的增大趋于无关联。     

Influence of light and nitrate assimilation on the growth strategy in clonal weed <Emphasis Type="Italic">Eichhornia crassipes</Emphasis>

We studied as to how the inter-connected modular architecture of clonal Eichhornia crassipes allows nutrient to transfer from established ramets to developing ramets, and nitrate translocation within clonal system and how such a strategy may play an important role in successful establishment and expansion of this clonal plant. Using this stoloniferous E. crassipes as a model, we studied the effects of light and nitrate availability on growth and nitrate assimilation in inter-connected parent and offspring ramets. Our results showed that increase in light and nitrate availability significantly increased growth rate of the whole clonal fragments and reproduction of offspring ramets in E. crassipes. In addition, increases in nitrate reductase (NR) activity and glutamine synthetase (GS) activity were observed in both parent and offspring ramets with increase in light density and nitrate supply. We also found that nitrate translocation is greater in offspring ramets than in parent ramets under abundant light and nitrate environment in this fast-growing clonal plant. Consequently, majority of nitrate assimilation in offspring ramets is beneficial to the growth of whole clonal system, as indicated by a close correlation between nitrate assimilation in offspring ramets and RGR of whole clonal fragments. We strongly contend that nitrate translocation and assimilation within clone system is important for efficient utilization of nitrogen in alien clonal plant E. crassipes during establishment and expansion, and thus for increase in its invasiveness in natural water columns.     

Natural hybridisation among Quercus glabrescens,Q. rugosa and Q. obtusata (Fagaceae): Microsatellites and secondary metabolites markers

• Natural hybridisation has significant ecological, genetic and evolutionary consequences altering morphological and chemical characters of individuals. Quercus glabrescens, Q. rugosa and Q. obtusata are white oak species well separated by their morphological characters when they occur in allopatry in Mexican temperate forests. However, in sympatry, individuals with atypical morphology have been observed, suggesting hybridisation events.
• In this study, we determined, with microsatellites and secondary metabolites, if interspecific gene flow occurs when these three oak species coexist in sympatry. In total, 180 individuals belonging to seven populations [three allopatric (one for each parental species) and four sympatric sites] were analysed.
• Allopatric populations represent well‐defined genetic groups and the sympatric populations showed genetic evidence of hybridisation between Q. glabrescens × Q. rugosa and Q. glabrescens × Q. obtusata. The hybridisation percentage varied between sites and combination of involved species. We registered the presence of unique flavonoid compounds for Q. glabrescens (caffeic acid and flavonol 2), Q. rugosa (flavonol 5) and Q. obtusata (flavonol 1). Three compounds (quercetin rhamnoside, flavonol 3 and alkyl coumarate) were expressed in all taxa. Finally, the hybrid genotypes identified in this study (Q. glabrescens × Q. rugosa and Q. glabrescens × Q. obtusata) showed specific chemical profiles, resulting from a combination of those of their parental species.
• These results show that hybridisation events between these oak species alter chemical expression of secondary metabolites, creating a mosaic of resources and conditions that provide the substrate for different combinations of foliar‐associated species such as herbivores, endophytic fungi or epiphyte plants.

     

Leaf morphological analyses in four European oak species (Quercus) and their hybrids: A comparison of traditional and geometric morphometric methods

Abstract

In this study, leaf morphology was assessed in a mixed oak stand (western France) using two geometric morphometric (landmark and outline) datasets and one dataset of 19 leaf measures. Adult oaks (817 oaks), comprising four white oak species (Quercus petraea, Q. robur, Q. pubescens and Q. pyrenaica), were sampled for DNA extraction and genetic analysis (nuclear microsatellites). Leaf morphology was assessed on 336 oaks, comprising pure species and hybrids as determined by genetic assignment. This comparative study of oak leaf morphology, based on the use of two free size geometric morphometric methods and a set of leaf measurements, combined with the genetic assignment of individuals to pure species or hybrids, provided information about the differences among species and the intermediate leaf morphology of their hybrids.     

Plant clonality: Biology and diversity

The current approaches to the study of clonal plants are reviewed. Most studies concentrate at the level of the ramet and clonal fragment exploring the “microscopic” view of clonal plants, dealing with the translocation of resources, clonal integration, plasticity of growth etc. The information gained, by this approach can be used in the understanding of higher levels of organization within the clonal system either with the help of spatially explicit modelling techniques, or by using means and distributions of size within a population instead of studying individual ramets separately. Plant scientists use the term clone with two meanings, viz. (a) a set of physiologically connected, but potentially independent ramets, and (b) a set of genetically identical, but potentially physically separated individuals. The overlap of these terms differs between individual plant species, depending on the extent of physical separation of the ramets and the degree of physiological integration between the ramets; the lower the frequency of ramet separation, the closer are the physiological and genetic concepts of the clone. Three critical areas seem to be neglected in clonal plant research: (a) the interrelationship between hierarchical levels in clonal plants, (b) the particular spatial structure of their environment, and (c) the importance of clonal plants in different ecological communities.     

The effect of local resource availability and clonal integration on ramet functional morphology in Hydrocotyle bonariensis

Summary Within a physiologically integrated clone, the structure and functioning of an individual ramet is determined by: 1) the response of that ramet to its local environment and 2) its response to resource integration within the clone. In a multifactorial experiment, Hydrocotyle bonariensis ramets were grown in limiting resource environments with and without the benefit of basipetal resource movement from another branch of the clone. Ramets were analyzed for their morphological responses to variation in local light, water and nitrogen availability and to the superimposed effect of resource integration on these conditions. The expression of ramet morphology, from induction to development, was highly plastic in response to variable local resource availability. Resource integration changed a ramet's local response in a variety of ways depending on the resource(s) being translocated and the character involved. Among leaf characteristics (leaf weight, petiole height, blade area), resource translocation into the shade resulted in an enhancement of the local response. Similarly, the translocation of nitrogen and water generally increased clonal proliferation and sexual reproduction among ramets. In contrast, the translocation of water reversed the effect of local low water conditions on ramets by inhibiting root production. Some characters such as internode distance and leaf allometry were unaffected by integration. The maintenance of connections between ramets as a Hydrocotyle clone expands allows for resource sharing among widely separated ramets and can result in an integrated morpological response to a resource environment that is patchy in time and space.     

Fungal Community Analyses of Endophytic Fungi from Two Oak Species,Quercus mongolica and Quercus serrata,in Korea

Fungal endophytes have been recorded in various plant species with a richness of diversity, and their presence plays an essential role in host plant protection against biotic and abiotic stresses. This study applied the Illumina MiSeq sequencing platform based on the amplification of fungal ribosomal ITS2 region to analyze fungal endophytic communities of two oak species (Quercus mongolica and Q. serrata) with different oak wilt disease susceptibilities in Korea. The results showed a total of 230,768 sequencing reads were obtained and clustered at a 97% similarity threshold into 709 operational taxonomic units (OTUs). The OTUs of Q. serrata were higher than that of Q. mongolica with the number of 617 OTUs and 512 OTUs, respectively. Shannon index also showed that Q. serrata had a significantly higher level of fungal diversity than Q. mongolica. Total of OTUs were assigned into 5 fungal phyla, 17 classes, 60 orders, 133 families, 195 genera, and 280 species. Ascomycota was the dominant phylum with 75.11% relative abundance, followed by Basidiomycota with 5.28%. Leptosillia, Aureobasidium and Acanthostigma were the most abundant genera detected in Q. serrata with the average relative abundance of 2.85, 2.76, and 2.19%, respectively. On the other hand, Peltaster, Cladosporium and Monochaetia were the most common genera detected in Q. mongolica with the average relative abundance of 4.83, 3.03, and 2.87%, respectively. Our results indicated that fungal endophytic communities were significantly different between two oak species and these differences could influence responses of host trees to oak wilt disease caused by Raffaelea quercus-mongolicae.     

Radial growth and climate responses of white oak (Quercus alba) and northern red oak (Quercus rubra) at the northern distribution limit of white oak in Quebec, Canada

Aim The objectives of this study were: (1) to compare radial growth patterns between white oak (Quercus alba L.) and northern red oak (Quercus rubra L.) growing at the northern distribution limit of white oak; and (2) to assess if the radial growth of white oak at its northern distribution limit is controlled by cold temperature. Location The study was conducted in three regions of the Ottawa valley in southern Québec. All stands selected were located at the northern limit of distribution of Q. alba. Methods Twelve mixed red and white oak stands were sampled and increment cores were extracted for radial growth analyses. For each oak species, 12 chronologies were derived from tree‐ring measurement (residual chronologies). Principal components analysis and redundancy analysis were used to highlight the difference between radial growth in both species and to determine their radial growth–climate association. Results There was little difference between the radial growth of each species; Q. alba, however, exhibits more year‐to‐year variation in growth than Q. rubra. More than 65% of the variance in radial growth was shared among sites and species. Both species showed a similar response to climate, which suggested that the limit of distribution of Q. alba might not be determined by effects on growth. Both species had a classic response to climate and drought in the early growing season. Main conclusions The northern distribution limit of Q. alba does not appear to be directly controlled by effects on growth processes as indicated by the similarities in radial growth and response to climate between the two species. The location of the stands on southern aspects suggested that cold temperature could have been a major factor controlling the distribution limit of Q. alba. However, it is speculated that stands growing on southern aspects may be more prone to forest fires or to drought, which would favour the maintenance and establishment of oaks, and of Q. alba in particular. Models relating the northern distribution limits of species to broad climate parameters like annual mean temperature will need to be reviewed to incorporate more biologically relevant information. Such assessments will in turn provide better estimates of the effect of climate changes on species distribution.     

Fine‐scale asexual and sexual reproduction in Quercus crispula var. horikawae,a stunted shrub oak,on a mountain with deep snow in central Japan

Quercus crispula var. horikawae, a stunted shrub oak, occurs on mountains with deep snow on the Sea of Japan side of Japan. This oak generates patches of multiple creeping stems. It is unclear whether these patches are the result of asexual or sexual reproduction, or both. We therefore aimed to describe the clonal structure and gene flow in Q . crispula var. horikawae on Mount Nasu in central Japan by using nuclear microsatellites. Genotypes of 331 stems with no distinct connection with roots and creeping stems above the ground were determined using nine loci in two study plots, and 64 acorns from three mother genets in a plot were determined using eight loci. The results of the clonal identification indicated that the patches consisted of 51 genets; at least 85% of the stems may have been derived from asexual reproduction through sprouting and layering. The prominence of asexual reproduction may be a result of adaptation to the snowy environment. In contrast, 15% of the ramets in the study plots probably originate via sexual reproduction by seedling regeneration. Analyses of the spatial genetic structure and paternity showed that limited ability of the pollen and seeds to disperse might result in the spatial aggregations of closely related offspring at a relatively short distance (<10 m), and inbreeding, a factor that might reduce sexual reproduction, was not observed. Thus, sexual reproduction could be reduced by ecological rather than genetic factors, namely the hindrance of seedling regeneration by the dense coverage of dwarf bamboo (Sasa) on the forest floor.