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Mediterranean refugial areas are generally underrepresented in large-scale genetic surveys of forest trees. In the case of silver fir (Abies alba Mill.), this has led to divergent hypotheses about the exact location of glacial refugia and the trajectory of recolonization routes. Based on the comprehensive sampling of Apennine populations, we aimed to reconcile discrepancies about the number and location of refugia for silver fir in the Apennines and test alternative demographic scenarios developed from palaeobotanical and genetic data. Location Mediterranean Basin; the Apennines and surrounding areas.Methods 1167 individuals from 16 Apennine populations, extensively covering the species’ distribution along the Italian Peninsula, and eight populations from the Alps and Eastern Europe were genotyped at 16 nuclear and three chloroplast microsatellite markers. The geographical distribution of genetic variation was explored using Bayesian clustering and multivariate methods. Based on the inferred genetic structure, the demographic history of A. alba was assessed by the approximate Bayesian computation (ABC) analysis.Results Two unexpected characteristics of genetic structure emerged: a sharp genetic boundary in the central Apennines and a tight genetic connection between southern Apennine and Eastern European gene pools. Two Apennine areas, corresponding precisely with refugial areas hypothesized in most recent palaeobotanical syntheses, have high genetic diversity on a par with Eastern European populations. ABC analysis showed an ancient separation between Apennine and Eastern European gene pools followed by an admixture event that, mainly through directional gene flow via pollen, might have established the genetic similarity between southern Apennine and Eastern European populations. In addition, there was evidence that the central Apennines acted as a small-scale, isolated refugium during the Last Glacial Maximum. Main conclusionsSilver fir rear edge populations have experienced a complex demographic history across several glacial-interglacial cycles, leading to unexpected genetic structure. Our study provides new insights into forest tree dynamics in the Mediterranean, showing the putative presence of multiple refugia for silver fir in the Apennines and a trans-Adriatic connection between silver fir populations in the southern Italy and the Balkans

BackgroundLocal adaptation is a key driver of phenotypic and genetic divergence at loci responsible for adaptive traits variations in forest tree populations. Its experimental assessment requires rigorous sampling strategies such as those involving population pairs replicated across broad spatial scales.MethodsA hierarchical Bayesian model of selection (HBM) that explicitly considers both the replication of the environmental contrast and the hierarchical genetic structure among replicated study sites is introduced. Its power was assessed through simulations and compared to classical ‘within-site’ approaches (FDIST, BAYESCAN) and a simplified, within-site, version of the model introduced here (SBM).ResultsHBM demonstrates that hierarchical approaches are very powerful to detect replicated patterns of adaptive divergence with low false-discovery (FDR) and false-non-discovery (FNR) rates compared to the analysis of different sites separately through within-site approaches. The hypothesis of local adaptation to altitude was further addressed by analyzing replicated Abies alba population pairs (low and high elevations) across the species’ southern distribution range, where the effects of climatic selection are expected to be the strongest. For comparison, a single population pair from the closely related species A. cephalonica was also analyzed. The hierarchical model did not detect any pattern of adaptive divergence to altitude replicated in the different study sites. Instead, idiosyncratic patterns of local adaptation among sites were detected by within-site approaches.ConclusionHierarchical approaches may miss idiosyncratic patterns of adaptation among sites, and we strongly recommend the use of both hierarchical (multi-site) and classical (within-site) approaches when addressing the question of adaptation across broad spatial scales.

Pollen gene flow is not impeded in the fragmented agricultural landscape investigated. High pollen immigration and extensive pollen dispersal distances are probably counteracting the potential loss of genetic variation caused by isolation. Some evidence was also found that U. minor and U. pumila can hybridize when in sympatry. Although hybridization might have beneficial effects on both species, remnant U. minor populations represent a valuable source of genetic diversity that needs to be preserved.

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The ever-increasing use of paternity analysis to estimate the dispersal capability of forest trees calls for a quantitative evaluation of potential errors due to sampling design. Previous studies on optimal sampling strategies for seed trapping experiments suggested a link between sampling effort and error rate in the reconstruction of the seed dispersal kernel. We considered 92 papers on paternity analysis to quantitatively assess the sampling strategy used to study the characteristics of pollen dispersal patterns (pollen immigration rate, distribution of male reproductive success and estimates of pollen dispersal kernel parameters). For each studied stand we report data on the sampling effort (the total number of sampled seeds, the number of mother trees and the number of seeds per mother tree) and additional information on the studied species and characteristics of the sampling areas. The reviewed papers used a median of 8 mother trees (acting as pollen traps in paternity analysis studies), a median of 29 seeds per mother tree and a median of 240 total sampled seeds. These are values (especially the number of mother trees) lower than usually found in classical seed trapping studies, for which accuracy and precision of seed dispersal estimates had already been assessed. These findings underline the need of evaluating the consequences of realistic sampling efforts on the estimation of parameters describing the pollen dispersal pattern to provide the basis for meaningful guidelines to paternity analysis