Search

In our study, we performed a population genetic analysis in all three native oak species of Cyprus and addressed whether two intermediate individuals arose from intersectional hybridization between Quercus coccifera ssp. calliprinos and Q. infectoria ssp. veneris. For this purpose, we successfully tested chloroplast and nuclear microsatellites initially developed for other oak species. We identified a set of markers that can be efficiently used for species identification and study of hybridization. Haplotypes based on the chloroplast microsatellites could resolve different maternal lineages and distinguish between the two sections represented in the study area. Using the nuclear microsatellites, we described patterns of genetic diversity across species with the endemic Q. alnifolia exhibiting reduced genetic diversity. Additionally, we employed a multivariate analysis, which could clearly differentiate among the three species. The two intermediate individuals clustered within the point cloud of Q. infectoria ssp. veneris and also possessed a chloroplast haplotype typical for this species. Therefore, we rejected the hypothesis of intersectional hybridization and interpreted their phenotypic appearance as the result of high phenotypic plasticity within Q. infectoria ssp. veneris.

Genetic monitoring of populations currently attracts interest in the context of the Convention on Biological Diversity but needs long-term planning and investments. Genetic diversity has been largely neglected in biodiversity monitoring, and when addressed is treated separately, detached from other conservation issues, such as habitat alteration due to climate change. Genetic monitoring supports the conservation and management of fisheries, game, and threatened populations. It also can contribute to the assessment of predicted and realized impacts of climate change, and their management. We report the first accounting of genetic monitoring efforts among countries in Europe (their ‘genetic monitoring capacity’, GMC) to determine where GMC suggests the combination of national infrastructure, political support and resources for continued and expanded monitoring. Overlaying GMC with areas where species ranges approach current and future climate niche limits (i.e., niche marginality) helps identify whether GMC coincides with anticipated climate change effects on biodiversity. Our analysis suggests that country area extent, financial resources, and conservation policy influence GMC, high values of which inconsistently match joint species patterns of climate niche marginality. Populations at niche margins likely hold genetic diversity that is important to adaptation to changing climate, and our results illuminate the need in Europe for expanded genetic monitoring across the climate gradients occupied by species, a need arguably greatest in southeastern European countries.

Europe has a history rich in examples of successful and less successful introductions of non-native trees (NNT) - trees with an evolutionary origin outside of Europe. NNT have been introduced for various reasons such as ornamental purposes, food production, land reclamation/amelioration and timber production. The use of NNT also causes controversies due to potential negative impacts on natural ecosystems of NNT that become invasive. Many international legal frameworks responded to the presence and use of NNT, such as international treaties and conventions ; and also the European Union deals with the issue of invasive alien species in Regulation (EU) No. 1143/2014. However, especially the national and regional responses to NNT affect current and future management decisions in the forest sector and furthermore shape forest landscapes of Europe. In this comprehensive study of the legal landscape concerning NNT we identified all relevant legislations, the different legal approaches and the regulatory intensity in every European country (except microstates). Hard and relevant soft law items at the national and sub-national (regional) level for 40 European countries were collected and proofed using a targeted questionnaire sent to country experts across Europe mainly coming from the forestry and nature conservation sector. In addition, international and national legislation information systems and databases were consulted to complete the information. Legislations included updates until June/July 2019. In total, 320 national and regional law items were identified to be relevant for the regulation of NNT in the investigated 116 legislative units (countries, and regions with own legislation) in Europe. Countries and regions were empirically categorised according to ad hoc created legislation indicators dealing with the general ban of introduction of non-native species, the generally allowed and prohibited NNT, approval mechanisms, e.g. mandatory forest management plans, and areas and cases where NNT are restricted or completely prohibited. Our study revealed a very diverse legal landscape in Europe, with a large variety of approaches being pursued to regulate NNT, and the intensity of regulation ranging from very little restrictions on the species choice and plantation surface area to a complete ban of NNT in forests.

Non-native forest tree species have been introduced in Europe since the 16th century, but only in the second half of the 20th century the significance of the seed source origin for their economic use was recognized, resulting in the establishment of numerous provenance trials at a national, regional, European and International level, as those led by IUFRO. Breeding programs have also been launched in the continent for the most economically important species. Aim of this work is the formulation of provenance recommendations for planting of five non-native tree species in Europe (Douglas fir, grand fir, Sitka spruce, lodgepole pine and black locust), based on the information obtained from twenty countries, in the frame of the EU FP-1403 NNEXT Cost Action. The survey revealed that official and non-official national recommendations, based on provenance research results, have been elaborated and followed at a different level and extend for the above five species, but only for Douglas fir recommendations exist in almost all the participating to the survey countries. The compilation of provenance recommendations across Europe for each species is presented in the current work. Besides the recommended introduced seed sources, European seed sources are also preferred for planting, due to ease of access and high availability of forest reproductive material. European breeding programs yielding genetic material of high productivity and quality constitute currently the seed source of choice for several species and countries. Consolidation of trial data obtained across countries will allow the joint analysis that is urgently needed to draw solid conclusions, and will facilitate the development of ‘Universal-Response-Functions’ for the species of interest, rendering possible the identification of the genetic material suitable for global change. New provenance trial series that will test seed sources from the entire climatic range of the species, established in sites falling within and outside the environmental envelopes of their natural ranges, are urgently needed to pinpoint and understand the species-specific climate constraints, as well as to correlate functional traits to the seed origin and the environmental conditions of the test sites, so that the selection of suitable forest reproductive material of non-native tree species in the face of climate change can be feasible, This paper is an output from the European COST Action FP1403 ‘Non-native tree species for European forests—experiences, risks and opportunities’ (NNEXT), and specifically the Working group “Pathways”, in which all authors participated and contributed information in the relevant questionnaire

Rapid human-induced environmental changes like climate warming represent a challenge for forest ecosystems. Due to their biological complexity and the long generation time of their keystone tree species, genetic adaptation in these ecosystems might not be fast enough to keep track with conditions changing at such a fast pace. The study of adaptation to environmental change and its genetic mechanisms is therefore key for ensuring a sustainable support and management of forests. The 4-day conference of the European Research Group EvolTree (https://www.evoltree.eu) on the topic of “Genomics and Adaptation in Forest Ecosystems” brought together over 130 scientists to present and discuss the latest developments and findings in forest evolutionary research. Genomic studies in forest trees have long been hampered by the lack of high-quality genomics resources and affordable genotyping methods. This has dramatically changed in the last few years; the conference impressively showed how such tools are now being applied to study past demography, adaptation and interactions with associated organisms. Moreover, genomic studies are now finally also entering the world of conservation and forest management, for example by measuring the value or cost of interspecific hybridization and introgression, assessing the vulnerability of species and populations to future change, or accurately delineating evolutionary significant units. The newly launched conference series of EvolTree will hopefully play a key role in the exchange and synthesis of such important investigations.

Purpose of Review Non-native tree species (NNT) raise a range of different associations and emotions—to many citizens they are just an exotic curiosity in parks, to many conservationists they are an evil to native ecosystems that should be eradicated, to a rising group of foresters they are part of the solution to climate change and an increasing timber demand, and to others they are already daily forestry business. In this review, where we also summarise the findings of the recent COST Action FP1403 (NNEXT) ‘Non-native tree species for European forests: experiences, risks and opportunities’, we highlight opportunities and challenges in the light of climate change, ecological risks and legislative limits of growing non-native tree species in Europe. Recent Findings Few NNT in Europe show invasive behaviour and are listed as prohibited species or as species to be monitored. A larger number of NNT is utilised in productive forestry and forest restoration due to their superior growth, valuable timber properties and good performance under harsh growing conditions. Current species distribution, experiences with success and failures and environmental concerns differ profoundly across Europe, with Western Europe overall revealing higher shares in NNT and showing a stronger interest of forestry related stakeholder groups to continue planting NNT. Summary Many more NNT are already used in forestry than previously thought, but relatively few species have major importance in terms of area, mainly in western European countries. Diversification, mixing and avoidance of invasion in relation to NNT are necessities that are relatively new on the agenda. In contrast, provenance research of major NNT has been going on for many decades and now provides important information for climate change adaptation. Despite the limitations to the use of NNT either through legal restrictions or forest certification that differ considerably across Europe, the careful integration of a range of tested NNT also into future forest management planning shows a high potential for climate change adaptation and mitigation.

Ailanthus altissima is a common tree species of the European non-native flora. In Eastern Austria, Ailanthus has both a long tradition, dating back to the late 18th century, and a high frequency of occurrence. Here, we apply molecular markers from the nuclear and chloroplast DNA in order to study the origin, as well as the clonal and genetic structure of the species in forest stands of this region. Our results indicate a single area of origin for all our study populations from the north-eastern part of the native range, in agreement with historical reports. Within populations, vegetative reproduction has resulted in extensive clonal structures at some sites. Long presence and repeated vegetative recruitment might have led to expansion of clones in areas of early introduction. In addition, limited mate availability may also have promoted vegetative reproduction. Tests for recent bottlenecks were significant in almost all stands, possibly reflecting founder effects since the introduction of Ailanthus in our study area. Among populations, we found a very pronounced and significant genetic structure, which did not follow a spatial pattern. We argue that founder effects due to population establishment with a low number of propagules – whether naturally or artificially – have caused genetic drift and a structured gene pool. The lack of spatial pattern or isolation-by-distance suggests that seed transfer by humans might have been a major factor shaping the genetic variation of Ailanthus in Eastern Austria.

Douglas fir is expected to play an increasingly important role in Swedish forestry under a changing climate. Thus far, seed orchards with clones of phenotypically selected trees (plus trees) have been established to supply the market with highly qualitative reproductive material. Given the high genetic variation of the species, its growth properties are significantly affected by the provenance. Here, we applied microsatellite markers to identify the origin of clones selected within the Swedish breeding programme. Moreover, we analysed the timing of bud burst in open-pollinated families of these clones. In particular, we aimed to explain the provenance effect on phenology by using molecular identification as a proxy. A Bayesian clustering analysis with microsatellite data enabled the assignment of the clones to one of the two varieties and also resolved within-variety origins. The phenological observations indicated an earlier bud burst of the interior variety. Within the coastal variety, the northern provenances exhibited a later bud burst. We found a significant effect of the identified origin on bud burst timing. The results of this study will be used to support further breeding efforts.

Sycamore maple (Acer pseudoplatanus L.) is a broadleaved forest tree species mainly occurring in mountainous areas of Europe, preferring fresh to moist, deep, calcareous soils. As was the case with other important forest tree species, sycamore maple was absent from the area north of the Alps during the last ice age. Therefore, we investigated the origin of sycamore maple in this area and its genetic variation across the landscape as the result of its postglacial migration into this area. We compared our results with the current delineation of sycamore maple into regions of provenance in Germany. Altogether 1043 trees from Germany, but also, in few cases, from bordering countries, were analysed using 11 nuclear and one chloroplast microsatellite (SSR) markers. Both the analysis of chloroplast (cpDNA) haplotypes and the Bayesian cluster analysis based on nuclear SSR data revealed distinct genetic structures of sycamore maple in Germany, suggesting more than one origin. The spatial distribution of a particular cpDNA haplotype (haplotype ‘105’) might be the result of a migration event with an eastern origin, probably along the Eastern Alps, which could have occurred during the Holocene. The other haplotype (haplotype ‘102’) might be of (south-) western origin, representing a migration pathway along the Western Alps. This hypothesis is further supported by a Bayesian cluster analysis based on nuclear SSRs. We identified two clusters, one distributed in the western part of the study area and one in the eastern. Between them, a zone of genetic introgression occurs, where populations exhibit a mixed membership to both clusters. This zone of genetic admixture is narrower in the south. An analysis of the landscape shape confirms that average genetic differentiation per distance unit is higher in the south. This is also in agreement with higher genetic distances between regions of provenance in the south. The mountainous landscape of this region could have contributed to this higher genetic differentiation, by posing geographic barriers to gene flow. On the contrary, wide-scale seed movement, also beyond the limit of native range, as well as a flat relief could have resulted in a wider zone of admixture in the north. Based on our results, we argue that the current delineation of provenance regions for sycamore maple in Germany fits to the inferred patterns of genetic differentiation.

Douglas-fir (Pseudotsuga menziesii) is a conifer tree native to western North America. In central Europe, it shows superior growth performance and is considered a suitable substitute for tree species impaired in vitality due to climate change. Maintenance and improvement of growth performance in a changing environment is a main challenge for forest tree breeders. In this context, genetic variation as a factor underlying phenotypic variation, but also as the basis for future adaptation, is of particular interest. The aims of this study were to analyse (1) genetic diversity of selected Douglas-fir provenances, (2) variation in height growth among provenances, and (3) to assess the link between genetic and phenotypic variation in height growth. Genotyping was done on microsatellite loci. Effects of ‘provenance’, ‘genotype’, and ‘site’ on height growth were assessed by fitting mixed linear models. The most significant genetic differentiation was observed between provenances of the coastal variety, versus a provenance of the interior variety originating from British Columbia. Although genetic differentiation among provenances of the coastal variety was lower, genetic structures within this variety were identified. Moreover, genetic diversity showed a latitudinal gradient with the southernmost provenances being more diverse, probably reflecting the species’ evolutionary history. The modelling approach revealed that height growth differed significantly by ‘provenance’, ‘site’, and the interaction between ‘site’ and ‘provenance’. Additionally, this analysis showed that genetic variation captured by the genotyped microsatellite loci was significantly related to variation in height growth, providing statistical evidence for a genetic component in the observed phenotypic variation.

Since the nineteenth century, Douglas-fir seed sources have been widely used for establishment of forest stands outside its natural distribution range. In Europe, some of these old Douglas-fir stands are registered as seed stands and provide seed sources for nurseries, although it is unclear from which region in North America they originate. In recent years, the interest in planting Douglas-fir has increased substantially because the species is seen as a potential adaptation option to climate change. This makes the assignment of European Douglas-fir stands of unknown seed origin to their geographic origin in North America increasingly important, because the genetic quality of these plantations must be guaranteed. In this study, we use 13 nuSSR loci to investigate the origin of 67 Austrian and German Douglas-fir stands of unknown origin. We performed a hierarchical Bayesian cluster analysis using 38 native Douglas-fir populations. The resulting clusters are used as reference populations to assign the 67 Central European Douglas-fir stands from Austria and Germany planted more than 80 years ago. Our results suggest that the majority of our investigated Douglas-fir stands come from central Washington (USA), the recommended seed zones for Central Europe. Some stands were located outside the suggested area, e.g. central Oregon and Santa Fe (New Mexico). The accuracy assessment of our approach revealed the best performance for the highest hierarchical level, e.g. assigning populations either to the coastal or the Rocky Mountain variety. As expected, the uncertainty increases with decreasing hierarchical level. The final assessment, if an admixture of seed sources within the European Douglas-fir stands is evident suggests that 23 of the Douglas-fir stands show an admixture which was not detected in our Douglas-fir reference populations growing in the natural distribution range.

Bayesian clustering methods have been widely used for studying species delimitation and genetic introgression. In order to test the effect of phylogenetic relationships and sampling scheme on the inferred clustering solution and on the performance of Bayesian clustering analysis, I simulated genotypes of the interfertile oak species Quercus robur, Quercus petraea, and Quercus pubescens and I run analyses using two popular software programs, STRUCTURE and BAPS. First, based on purebred simulations, I compared clustering solutions resulting from different sample size configurations. While clustering solution generally reflected the taxonomic relationships when equal samples of each species were included, spurious partition was inferred by STRUCTURE when some species were represented by larger and others by smaller samples. In very unbalanced configurations, STRUCTURE failed to identify the three species, even if three subpopulations were assumed. By contrast, BAPS could properly identify the three species under any sampling scheme. Second, based on simulations of purebreds and hybrids, I tested the performance of individual assignments with variable number of loci. This analysis showed that STRUCTURE can detect introgressed individuals more efficiently than BAPS. However, BAPS could assign purebreds more efficiently with a lower number of loci. Method performance also depended on phylogenetic relationships. In the case of Q. petraea, Q. pubescens, and their hybrids, method performance was lower due to their phylogenetic affinity. Inclusion of three instead of two species into the analysis led to reduction of performance, and to misclassification of hybrids, which often reflected the phylogenetic affinity between Q. petraea and Q. pubescens.

Douglas-fir (Pseudotsuga menziesii) is a conifer tree native to western North America. In central Europe, it shows superior growth performance and is considered a suitable substitute for tree species impaired in vitality due to climate change. Maintenance and improvement of growth performance in a changing environment is a main challenge for forest tree breeders. In this context, genetic variation as a factor underlying phenotypic variation, but also as the basis for future adaptation, is of particular interest. The aims of this study were to analyse (i) genetic diversity of selected Douglas-fir provenances, (ii) variation in height growth among provenances, and (iii) to assess the link between genetic and phenotypic variation height growth. Genotyping was done on microsatellite loci. Effects of provenance, genotype, and site on height growth were assessed by fitting mixed linear models. The most significant genetic differentiation was observed between provenances of the coastal variety, versus a provenance of the interior variety originating from British Columbia. Although genetic differentiation among provenances of the coastal variety was lower, genetic structures within this variety were identified. Moreover, genetic diversity showed a latitudinal gradient with the southernmost provenances being more diverse, probably reflecting the species' evolutionary history. The modelling approach revealed that height growth differed significantly by provenance, site, and the interaction between site and provenance, demonstrating that height growth is under strong genetic control. Additionally, this analysis showed that genetic variation captured by the genotyped microsatellite loci was significantly related to variation in height growth, providing statistical evidence for a genetic component in the observed phenotypic variation.

The evergreen Quercus alnifolia and Q. coccifera form the only interfertile pair of oak species growing in Cyprus. Hybridization between the two species has already been observed and studied morphologically. However, little evidence exists about the extent of genetic introgression. In the present study, we aimed to study the effects of introgressive hybridization mutually on both chloroplast and nuclear genomes. We sampled both pure and mixed populations of Q. alnifolia and Q. coccifera from several locations across their distribution area in Cyprus. We analyzed the genetic variation within and between species by conducting Analysis of Molecular Variance (AMOVA) based on nuclear microsatellites. Population genetic structure and levels of admixture were studied by means of a Bayesian analysis (STRUCTURE simulation analysis). Chloroplast DNA microsatellites were used for a spatial analysis of genetic barriers. The main part of the nuclear genetic variation was explained by partition into species groups. High interspecific differentiation and low admixture of nuclear genomes, both in pure and mixed populations, support limited genetic introgression between Q. alnifolia and Q. coccifera in Cyprus. On the contrary, chloroplast DNA haplotypes were shared between the species and were locally structured suggesting cytoplasmic introgression. Occasional hybridization events followed by backcrossings with both parental species might lead to this pattern of genetic differentiation., Comment: 28 pages, 6 figures, author's accepted manuscript

Oak species (Quercus spp.) in Central Europe grow on a relatively wide range of sites. Due to the economic importance of oak for its wood and other products, oak forests have long been managed by humans. This raises the question whether adaptation and/or human activities—especially the moving of propagules—have left their footprints on the genetic variation of oak populations. To address this question, we focused on the Upper Rhine Valley, a densely populated area today that was settled by humans early on. Here, the three most common native Central European oak species can be found. We studied their genetic variation across a large number of oak stands, growing on different sites and having different silvicultural histories, using neutral and EST-derived microsatellite markers. At the interspecific level, we showed that Quercus robur is relatively well delimited, while Quercus petraea and Quercus pubescens are more closely related. Natural hybridization might explain the increased genetic introgression between these two species. Within species, we found a low differentiation among populations of Q. robur and Q. petraea. In spite of forest fragmentation, we detected no spatial genetic barriers. However, we found that populations of Q. pubescens, a species with a marginal distribution in the study area were spatially structured. Genetic drift but also unidirectional introgressive hybridization with Q. petraea may account for this. Regarding the question of adaptation, we considered soil flooding, texture, drainage, and calcium carbonate in the upper horizons as physiologically important site condition variables. But with multivariate statistics, we could not find any significant effects of these parameters on genetic differentiation. Although there was no evidence for natural selection due to adaptation in stands of Q. robur, we demonstrated that age had a significant effect on their genetic variation and that stands established after the end of the Second World War had higher genetic diversity. We interpret these findings as being the result of an increase in large-scale transfers of reproductive materials during this time period and discuss arguments supporting this hypothesis. Finally, we consider the implications of these results for forest management.

Genetic analysis was carried out in order to provide insights into differentiation among populations of two interfertile oak species, Quercus petraea and Quercus robur. Gene flow between the two species, local adaptations and speciation processes in general, may leave differential molecular signatures across the genome. Three interspecific pairs of natural populations from three ecologically different regions, one in central Europe (SW Germany) and two in the Balkan Peninsula (Greece and Bulgaria) were sampled. Grouping of highly informative SSR loci was made according to the component of variation they express - interspecific or provenance specific. Species and provenance discriminant loci were characterized based on FSTs. Locus specific FSTs were tested for deviation from the neutral expectation both within and between species. Data were then treated separately in a Bayesian analysis of genetic structure. By using three species discriminant loci, high membership probability to inferred species groups was achieved. On the other hand, analysis of genetic structure based on five provenance discriminant loci was correlated with geographic region and revealed shared genetic variation between neighbouring Q. petraea and Q. robur. Small sets of highly variable nuclear SSRs were sufficient to discriminate, either between species or between provenances. Thus, an effective tool is provided for molecular identification of both species and provenances. Furthermore, data suggest that a combination of gene flow and natural selection forms these diversity patterns. Species discriminant loci might represent genome regions affected by directional selection, which maintains species identity. Provenance specific loci might represent genome regions with high interspecific gene flow and common adaptive patterns to local environmental factors., Comment: 26 pages, 5 figures, accepted author's manuscript

The indigenous oak species (Quercus spp.) of the Upper Rhine Valley have migrated to their current distribution range in the area after the transition to the Holocene interglacial. Since post-glacial recolonization, they have been subjected to ecological changes and human impact. By using chloroplast microsatellite markers (cpSSRs), we provide detailed phylogeographic information and we address the contribution of natural and human-related factors to the current pattern of chloroplast DNA (cpDNA) variation. 626 individual trees from 86 oak stands including all three indigenous oak species of the region were sampled. In order to verify the refugial origin, reference samples from refugial areas and DNA samples from previous studies with known cpDNA haplotypes (chlorotypes) were used. Chlorotypes belonging to three different maternal lineages, corresponding to the three main glacial refugia, were found in the area. These were spatially structured and highly introgressed among species, reflecting past hybridization which involved all three indigenous oak species. Site condition heterogeneity was found among groups of populations which differed in terms of cpDNA variation. This suggests that different biogeographic subregions within the Upper Rhine Valley were colonized during separate post-glacial migration waves. Genetic variation was higher in Quercus robur than in Quercus petraea, which is probably due to more efficient seed dispersal and the more pronounced pioneer character of the former species. Finally, stands of Q. robur established in the last 70 years were significantly more diverse, which can be explained by the improved transportation ability of seeds and seedlings for artificial regeneration of stands during this period., Comment: 33 pages, accepted author's manuscript

Conservation of 16 nuclear microsatellite loci, originally developed for Quercus macrocarpa (section Albae), Q. petraea, Q. robur (section Robur) and Q. myrsinifolia, (subgenus Cyclobalanopsis) was tested in a Q. infectoria ssp. veneris population from Cyprus. All loci could be amplified successfully and displayed allele size and diversity patterns that match those of oak species belonging to the section Robur. At least in one case, limited amplification and high levels of homozygosity support the occurrence of 'null alleles', caused by a possible mutation in the highly conserved primer areas, thus hindering PCR. The sampled population exhibited high levels of diversity despite the very limited distribution of this species in Cyprus and extended population fragmentation. Allele sizes of Q. infectoria at locus QpZAG9 partially match those of Q. alnifolia and Q. coccifera from neighboring populations. However, sequencing showed homoplasy, excluding a case of interspecific introgression with the latter, phylogenetically remote species. Q. infectoria ssp. veneris sequences at this locus were concordant to those of other species of section Robur, while sequences of Quercus alnifolia and Quercus coccifera were almost identical to Q. cerris., Comment: 18 pages, 2 figures, author's accepted version

Hybridization and introgression between Quercus alnifolia Poech and Q. coccifera L. is studied by analyzing morphological traits, nuclear and chloroplast DNA markers. The study site is a mixed stand on Troodos Mountains (Cyprus) and the analyzed material includes both adult trees and progenies of specific mother trees. Multivariate analysis of morphological traits shows that the two species can be well distinguished using simple leaf morphometric parameters. A lower genetic diversity in Q. alnifolia than in Q. coccifera and a high interspecific differentiation between the two species are supported by an analysis of nuclear and chloroplast microsatellites. The intermediacy of the four designated hybrids is verified by both leaf morphometric and genetic data. Analysis of progeny arrays provides evidence that interspecific crossings are rare. This finding is further supported by limited introgression of chloroplast genomes. Reproductive barriers (e.g. asynchronous phenology, post-zygotic incompatibilities) might account for this result. A directionality of interspecific gene flow is indicated by a genetic assignment analysis of effective pollen clouds with Q. alnifolia acting as pollen donor. Differences in flowering phenology and species distribution in the stand may have influenced the direction of gene flow and the genetic differentiation among effective pollen clouds of different mother trees within species., Comment: 30 pages, 7 figures, author's accepted manuscript