Riparian trees, especially relict trees, are attractive and important for research to understand both past and recent biogeographical and evolutionary processes. Our work is the first study to elucidate the genetic diversity and spatial genetic structure of the canopy-dominating riparian (Juglandaceae) along two altitudinal gradients in different river systems of the Hyrcanian forest, which is one of the most important refugium of relict trees in Western Eurasia. Altitudinal gradients were chosen along two river systems at 100, 400 and 900 m a.s.l. Leaf samples were collected from 116 trees, and the genetic diversity was evaluated with eight SSR markers. Overall, 39 alleles were identified for all of the populations studied. The observed heterozygosity (Ho) varied from 0.79 to 0.87 (with a mean of 0.83). The results of the AMOVA analysis indicated that the variation within populations was 88%, whereas the variation among populations was 12% for all of the gradients. A structure analysis indicated that 93% of the trees were grouped in the same gradient. The genetic distance based on Fst confirmed the structure result and indicated a high rate of gene flow among the investigated populations. Based on high gene flow (low differentiation of the population along the same river) and the clearly distinct genetic structure of the investigated gradients, it can be concluded that rivers are the main seed dispersal vector among populations. The genetic diversity of did not show any trend from upstream to downstream. The high level of gene flow and uniform genetic diversity along each river suggest the “classical” metapopulation structure of the species. Pterocarya fraxinifoliaP. fraxinifoliaP. fraxinifolia

Aim of study: The Caucasian apple (Malus orientalis Uglitzk.) is distributed throughout the Hyrcanian forest. The species keeps a gametophytic self-incompatibility reproduction system based on a single S-allele. In order to ensure a high mating probability and thereby long-term survival of the populations, a high S-allele diversity is needed.Area of study: Three populations of Caucasian apple along an altitudinal gradient in northern Iran were studied.Material and methods: Here, we evaluated the S allele frequency and genetic diversity based on a small set of individuals.Main results: Moderate genetic diversity within population and high genetic differentiation among populations are suggested. In total, 18 S-alleles were identified among the three populations with between 10 and 12 S-alleles at each population site.Research highlight: The diversity of S-alleles showed a significant positive correlation with altitude, which is consistent with the positive correlation of genetic diversity.