Populus suaveolens Fisch. morphology in the populations of the North-East of Russia

The fragrant poplar (Populus suaveolens Fisch, section Tacamahaca Mill.) is a mountain-valley species inhabiting riverine alluvial deposits and confined to floodplain and first floodplain terraces of rivers from the upper Yenisei to northern Japan, Korea, Mongolia and China in the south to the Chukchi Peninsula in the north. Despite its wide distribution, it remains the least studied and almost unused to “green building” even within its range. Data on intraspecific variability of P. suaveolens are fragmentary, and studies of the variability of species-specific traits at the population level are insufficient. Such trait as basal glands (glands from above at the junction of petiole and blade) has been recorded recently and only in one part of the range. As a consequence, there is no clear understanding of the scope of the taxon, the boundaries of its range and natural hybridization with neighboring species of the Tacamahaca section. Since hybridization introduces some difficulties in studying intraspecific variability in species of the genus Populus, it is of theoretical and practical interest to study variability in populations remote from its current zones. One of such regions of P suaveolens range is Chukotka Autonomous Okrug, where it grows in the floodplains of rivers in periodically flooded areas, forming with Chosenia arbutifolia (Pall.) A.K. Skvortsov poplar-chosenia forests. The aim of this study is to characterize the variability of morphological traits of vegetative organs of P suaveolens in populations of Chukotka Autonomous Okrug. We studied P. suaveolens populations in 2020 in Bilibinsk district of Chukotka Autonomous Okrug. The study area is characterized by a low-mountain relief: with increasing altitude of the terrain a change in vegetation belts is traced. Forest communities occupy river valleys and the lower parts of the slopes of hills. Poplar-chosenia forests grow on sandy-pebble substrates along the river floodplains in small strips. As our observations have shown, P suaveolens is widely used in landscaping of settlements of Chukotka Autonomous Okrug and Magadan Region. Three of the studied populations: “Baimka”, “Tallag” and “Peschanka” are located at the latitude of the Arctic Circle in the floodplains of the eponymous rivers in the river basin Bolshoi Anyui. All of them are isolated from each other by mountains. The northernmost population “Bilibino” is confined to the right bank of the Bolshoi Keperveem River (Maly Anyuy River basin) on the border of Bilibino, it is 200 km distant from the others. All populations were sampled in poplar-chosenia floodplain forests with varying degrees of anthropogenic transformation of the territory (Table 1). To exclude age variability, we collected herbarium material only from shortened shoots of the southern side of the middle part of the crown of reproductively mature trees. The tendency of balsamic poplars to form clones was taken into account, and repeated collections within a clone were carefully avoided. From each individual, fifteen fully developed, undamaged leaves were taken. The main morphometric traits, as well as the derivatives (indices) were measured on each leaf. A total of 1800 leaves were used for morphological study. We studied a complex of traits: blade shape, its apex and base, presence of basal glands, and petiole pubescence. Poplar-chosenia forests in the studied area grow in a narrow strip on alluvial sandy-pebble deposits in the floodplains of rivers (See Fig. 1). The stand formed by P suaveolens is 12-14 m high, 16-20 cm in diameter; trunks are straight and with low taper. Bark in a lower part is oblong-cracked at the height of 1-1.5 m, above it is smooth and whitish-gray. The crown is usually compact, spreading in older trees; branches are of medium thickness. Clearance from dry branches is good. Leaf blades on shortened shoots are bent upwards at the edges and folded in the form of a “boat” in all the studied plantings. This is probably due to the absence of stomata on the upper side of the blades and this “folding” facilitates transpiration. Perhaps it is an important adaptation in the subarctic conditions of the region. P suaveolens in all four populations is characterized by two blade forms: obovate and elliptic (See Table 2). As in other Populus species investigated, several forms can be observed at the endogenous level (within the individual), but one - obovate, with the maximum width above the middle of its length (57-70%) prevails. On shortened shoots in the middle part of the crown, all leaves of one tree always have only one shape of the apex and base of the leaf blade. In all studied populations of P. suaveolens, individuals with a shortly pointed (sharply pointed) apex dominated, but in 4-6 trees in the plantation the leaves had a gradually pointed shape. Only in Baimka population, plants with a wedge-shaped base of the leaf blade prevailed, in the rest these were roundly wedge-shaped ones (See Table 2). As noted above, one of the morphological traits of leaves characteristic of P. suaveolens is a differently expressed cordate base of the blade and the presence of a small notch. The notch at the base of leaves of shortened shoots is a quite frequent trait, especially in leaves with a rounded wedge-shaped base; it often overlaps with rounded lobes. In our opinion, it is caused by the character of petiole-to-leafplate transition. The fact is that the petiole of P. suaveolens is located in different planes with the blade and enters it at an angle from below, at some distance from the base edge (See Fig. 2). The groove, expressed on the upper side of the petiole, continues to be seen at some distance on the main vein, gradually tapering conically. Here, the first lateral veins branch off at the place where the groove enters. The levels of variability of the considered morphological traits in the populations studied differ in the development of basal glands (See Tables 3 and 4). At the endogenous level, this trait varies greatly from complete absence to 1-4 glands at the base of the blade. However, only in populations “Bilibino” and “Baimka”, 3 individuals (10%) with leaves without glands were noted, and on the contrary, in “Peschanka” leaves with developed glands prevailed on all plants (See Table 2). In the first two plantations, the greatest number of trees with developed glands on less than 50% of leaves was noted. Leaf blades with two glands (32.2% to 67.7% in the population) and one (8.8% to 17.3%) were typical (See Fig. 3). The proportion of leaves without them in the populations ranged from 18.6% to 52.2%. Leaves with three and four glands were noted singularly in all populations except “Bilibino”. Consequently, the sign of the presence of basal glands varies both at the endogenous and individual levels. In our opinion, its occurrence in the populations of North-East Russia is caused by ancient introgressive hybridization (possibly repeated) of this taxon with P. balsamifera in the Pliocene-Pleistocene on the territory of Beringia. Obtained as a result of introgression by the fragrant poplar, this trait gradually spread as a result of intraspecific interbreeding from the east deep into the continent. Consequently, the presence of basal glands as a trait in modern populations of P. suaveolens of North-East Russia can be considered as evidence of the existence of Eurasian-American syngameon at certain stages of evolution of the genus. Weak differentiation of the studied populations, despite their considerable remoteness, indicates that modern poplars in the territory of North-East Russia are the result of postglacial recolonization. The paper contains 3 Figures, 4 Tables and 47 References. The Authors declare no conflict of interest.

Keywords

Authors

References