Cyclic environmental changes have occurred during the last two million years. Living organisms react to changing phases through adaptation strategies, otherwise they get extinct. Forest trees are among the most ancient living organisms and in many cases they symbolise the last representatives of taxonomic units that underwent severe selection and/or extinction. The present distribution of trees in the forest landscape is the result of population dynamics experienced during the last millennia. In order to understand the mechanisms determining the evolution and the diversity of forest tree populations, it is important to consider historical factors, such as climatic and geological events, and evidences provided by fossil pollen records. The availability of fossil pollen records is restricted to few taxa, i.e., those producing large amount of pollen and characterised by wind pollination. Recently, the application of molecular markers to population genetics has provided additional research tools.
Important phylogeographic studies have been made on plant species with the analysis of organellar (chloroplast and mitochondria) DNA. Chloroplast and mitochondria represent haploid genomes, uniparentally transmitted through the generation: mitochondria are passed though the female parent in both animal and plant organisms, chloroplast are generally maternally inherited in angiosperms (through seed), and parentally inherited in gymnosperms (though pollen). For maternally inherited genomes, seed dispersal mechanisms determine the geographic structure of genetic diversity. Furthermore, the influence of human activities plays an important role in shaping the genetic diversity of forest tree populations.
Within the frame of the European project CYTOFOR (Measuring molecular differentiation of European deciduous forests for conservation and management, FAIR5 - CT97 - 3795), we analysed chloroplast diversity in twenty-two angiosperm shrub and tree species. These were sampled in the same 25 European forests selected on the basis of their high species richness and limited human influence. The innovative aspect of this study is the multispecies approach: genetic diversity was described not only for single species, but also at the forest level.
To compare forests with each other, we calculated the mean number of haplotypes and within-population gene diversity by averaging across species in each forest. Furthermore, a measure that expresses the average genetic divergence of the forest from all remaining populations was also calculated.
Seed dispersal mechanisms influence the distribution of the haplotipic diversity. Low values of genetic differentiation (expressed by the coefficient GST), indicating high levels of gene flow through seeds, were found in Salix and in Populus, genera characterized by light, wind-dispersed seeds The species characterised by animal-ingested seeds also tended to have below-average values. In contrast, species with animal-cached seeds (i.e., nuts) exhibited higher than average values.
The highest values of genetic divergence were observed in Corsica, Italy and the Balkans, including Croatia and Romania. This finding is in agreement with the location of the most important glacial refugia. On the other hand, lower values of genetic divergence were estimated in the other European regions (Fig. 1). Patterns of diversity across forests were very different; both mean number of haplotypes (Fig. 2) and gene diversity were higher in Central F
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