Morphological and molecular genetic methods were used to study specimens of Populus suaveolens s.l. (including P. suaveolens s. str., P. maximoviczii,
Populus koreana, etc.) from the natural Russian range of this species; compari-son with specimens of P. nigra and P. laurifolia was made. P. nigra, P. laurifolia and P. suaveolens s.l. differ from each other in 22–25 morphological characters out of 30 examined, and these differences in many characters are significant in quantitative terms and appear in the vast majority of cases. They are also sharply different in molecular genetic terms, although P. nigra and P. laurifolia are much closer to each other than to P. suaveolens s.l. due to overlapping ranges and hybridisation. P. suaveolens s.str. differs from P. maximoviczii, P. ko-reana, etc. of more southern and coastal forms in 14 morphological characters out of 30 studied, but all these differences are quantitative, revealed only statistically. Molecularly and genetically, these taxa are identical and possess a common molecular polymorphism. Within the range of P. suaveolens s.l., at least 13 morphological characters change in a regular way from regions with harsh climate (in the north and in the interior of the continent) to regions with mild climate (in the south and closer to the sea): the size of the leaf and its parts increases, the ratio of leaf length to leaf width decreases, the pubescence of some leaf parts increases, leaves with basal glands become more numerous, and leaves with a narrow short “spout” at the apex become fewer. However, these changes are gradual, small, detectable only statistically, and overlap many times with the variability of morphological characters in each particular region. P. koreana and P. maximowiczii growing in Primorye are practically identical both morphologically and molecularly; they do not deserve a distinctive systematic status, but, taken together, on the basis of morphological characters, they can be contrasted with P. suaveolens s. str in Khabarovsk Krai, as well as to the north and west. It is shown that P. suaveolens s.l. carries genes of P. nigra, P. talassica, P. afghanica, P. simonii in small quantities, and they can be found in any part of its range (the idea of syngameon of Euroasian black and balsam poplars). It is shown that P. suaveolens s.l. has a well-defined molecular-genetic polymorphism; there are three distinct groups (three “branches” on the dendrogram), with group I being close to P. talassica and P. afghanica in terms of genes studied; group II – to P. simonii and P. longifolia; group III is isolated from all taxa. Representatives of all three molecular-genetic groups were found in all regions from which at least 10 specimens for molecular-genetic study were taken, i.e. in Primorye, Khabarovsk Krai and Transbaikalia. Nevertheless, the regions are likely to differ in the ratio of these molecular genetic groups: in regions with a relatively mild climate (Primorye, Khabarovsk Krai), with a probability of 84%, the proportion of P. suaveolens group II is slightly higher, and with a probability of 60%, the proportion of P. suaveolens group III is slightly lower. In regions with more severe and sharply continental climate (Transbaikalia, Yakutia), the opposite picture is observed. The corresponding difference between the regions can be explained logically: in regions with mild climate on the southern periphery of the Populus suaveolens range, gene flows from southern poplars penetrate more easily, while regions with harsh climate (especially Yakutia) are maximally isolated and therefore have a higher proportion of P. suaveolens group III, which is maximally isolated on the dendrogram. In addition, at the western and southern limits of its range, P. suaveolens is heavily influenced by gene flow from P. laurifolia, but this fact was established earlier.
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