Tree species with wide distributions often exhibit different levels of genetic structuring correlated to their environment. However, understanding how environmental heterogeneity influences genetic variation is difficult because the effects of gene flow, drift and selection are confounded. We investigated the genetic variation and its ecological correlates in a wind-pollinated Mediterranean tree species, Fraxinus angustifolia Vahl, within a recognised glacial refugium in Croatia. We sampled 11 populations from environmentally divergent habitats within the Continental and Mediterranean biogeographical regions. We combined genetic data analyses based on nuclear microsatellite loci, multivariate statistics on environmental data and ecological niche modelling (ENM). We identified a geographic structure with a high genetic diversity and low differentiation in the Continental region, which contrasted with the significantly lower genetic diversity and higher population divergence in the Mediterranean region. The positive and significant correlation between environmental and genetic distances after controlling for geographic distance suggests an important influence of ecological divergence of the sites in shaping genetic variation. The ENM provided support for niche differentiation between the populations from the Continental and Mediterranean regions, suggesting that contemporary populations may represent two divergent ecotypes. Ecotype differentiation was also supported by multivariate environmental and genetic distance analyses. Our results suggest that despite extensive gene flow in continental areas, long-term stability of heterogeneous environments have likely promoted genetic divergence of ashes in this region and can explain the present-day genetic variation patterns of these ancient populations.

Populations occurring in areas of overlap between the current and future distribution of a species are particularly important because they can represent "refugia from climate change". We coupled ecological and range-wide genetic variation data to detect such areas and to evaluate the impacts of habitat suitability changes on the genetic diversity of the transitional Mediterranean-temperate tree Fraxinus angustifolia. We sampled and genotyped 38 natural populations comprising 1006 individuals from across Europe. We found the highest genetic diversity in western and northern Mediterranean populations, as well as a significant west to east decline in genetic diversity. Areas of potential refugia that correspond to approximately 70% of the suitable habitat may support the persistence of more than 90% of the total number of alleles in the future. Moreover, based on correlations between Bayesian genetic assignment and climate, climate change may favour the westward spread of the Black Sea gene pool in the long term. Overall, our results suggest that the northerly core areas of the current distribution contain the most important part of the genetic variation for this species and may serve as in situ macrorefugia from ongoing climate change. However, rear-edge populations of the southern Mediterranean may be exposed to a potential loss of unique genetic diversity owing to habitat suitability changes unless populations can persist in microrefugia that have facilitated such persistence in the past.

Aim To examine mechanisms related to the formation of hybrid zones between the Mediterranean narrow‐leaved ash tree Fraxinus angustifolia Vahl and the common ash Fraxinus excelsior L., a mostly temperate tree species, at the continental scale. Location Temperate and Mediterranean Europe and the western part of the Black Sea basin. Methods We used species distribution models to determine the potential zones of sympatry between the two species, which remain largely unknown. In addition, we analysed 58 populations and 456 samples of ash tree that spanned most of the distribution of the two species across Europe, and included both parental species and selected hybrid populations. Levels of hybridization in the 58 populations were estimated using 19 nuclear microsatellite loci, including six anonymous nuclear single sequence repeat (SSR) markers and 13 recently developed single sequence repeats from expressed RNA sequence tags (EST‐SSRs). Results Bayesian assignment supported the notion of two separate gene pools regardless of the type of marker used, which suggest an ancient population structure. Populations located within the predicted overlap zones had intermediate levels of admixture with a tendency for hybrid populations to occur towards temperate areas. Selection analyses indicated that six of the EST‐SSRs had been subjected to stabilizing selection whereas two others had been subjected to directional selection. Results of spatial filtering on the allele frequencies of the loci under directional selection suggest that the number of days of frost and summer temperatures are both ecological factors that can limit the extent of the hybrid zone. Moreover, areas associated with known or predicted hybrid zones showed abrupt changes in allele frequencies compared with the periphery of the distributions. Main conclusions Our analyses suggest that the hybrid structure in these closely related ash species is ancient and asymmetric and that climate‐driven selection, in particular cold weather, can potentially limit the extent of hybrid populations.

Populations residing near species' low‐latitude range margins (LLMs) often occur in warmer and drier environments than those in the core range. Thus, their genetic composition could be shaped by climatic drivers that differ from those occurring at higher latitudes, resulting in potentially adaptive variants of conservation value. Such variants could facilitate the adaptation of populations from other portions of the geographical range to similar future conditions anticipated under ongoing climate change. However, very few studies have assessed standing genetic variation at potentially adaptive loci in natural LLM populations. We investigated standing genetic variation at single nucleotide polymorphisms (SNPs) located within 117 candidate genes and its links to putative climatic selection pressures across 19 pedunculate oak (Quercus robur L.) populations distributed along a regional climatic gradient near the species' southern range margin in southeastern Europe. These populations are restricted to floodplain forests along large lowland rivers, whose hydric regime is undergoing significant shifts under modern rapid climate change. The populations showed very weak geographical structure, suggesting extensive genetic connectivity and gene flow or shared ancestry. We identified eight (6.2%) positive FST‐outlier loci, and genotype–environment association analyses revealed consistent associations between SNP allele frequencies and several climatic variables linked to water availability. A total of 61 associations involving 37 SNPs (28.5%) from 35 annotated genes provided important insights into putative functional mechanisms in our system. Our findings provide empirical support for the role of LLM populations as sources of potentially adaptive variation that could enhance species’ resilience to climate change‐related pressures.

Summary Lowland riverine floodplains are among the most dynamic and heterogeneous of ecosystems, but many have been extensively modified. Their provision of important habitats provides a key motivation for restoring natural flow regimes (in terms of magnitude, frequency, duration, timing and rate of change of hydrological conditions). Despite general recognition that aquatic insects respond to changes in water availability, there is little agreement about the influence of flow regime components on community structure in floodplains. We analysed a 6‐year (2005 and 2007–2011) data set of aquatic insects (Heteroptera and Coleoptera) in a large Danubian floodplain characterised by pronounced hydrological variability. We hypothesised that abundance and assemblage structure would depend on flow regime components. In particular, we predicted that high‐flow events in the mainstream and concurrent high‐magnitude floods of prolonged duration would have a positive effect on diversity and abundance and that insect assemblages would differ between phases of hydrological connectivity. In addition, we assessed whether the survival of two threatened aquatic insects, Graphoderus bilineatus and Berosus geminus, depends on hydrological connectivity. Abundance and species richness of Heteroptera and Coleoptera increased during years with frequent extremely high flood pulses and prolonged flooding. Multivariate ordination showed subtle relationships between hydrological conditions, hydrological connectivity and insect assemblages with number of days in flood providing the best fit. Spring floods had a more pronounced effect on the assemblages than autumn floods. The time lag between flood occurrence and its apparent effect on aquatic insects was 17–31 days, reflecting insect life cycles. The abundance of G. bilineatus (Coleoptera: Dytiscidae) was affected by mean water level during the month preceding sampling, whereas the abundance of B. geminus (Coleoptera: Hydrophilidae) was related to the occurrence of prolonged high‐magnitude floods. The current water regime of the Danube River favours generalist species with high dispersal capacities and broad niches and food resources. Changes in flow management practices could remove the positive influence of floods on aquatic insect communities and lead to losses of threatened species that depend on these river–floodplain habitats. Our results contribute to the understanding necessary for appropriate evaluation and prioritisation of international floodplain conservation efforts.