EVALUATION OF ECOLOGICAL PLASTICITY AND STABILITY OF SOME PROMISING VARIETIES OF POPLARS

A.P. Tsarev, Doctor of Agriculture, Prof.; ResearcherID: S-6639-2019, ORCID: https://orcid.org/0000-0001-8019-0016 V.A. Tsarev, Candidate of Agriculture, Assoc. Prof.; ORCID: https://orcid.org/0000-0002-3921-9339 R.P. Tsareva, Candidate of Agriculture, Senior Research Scientist; ORCID: https://orcid.org/0000-0002-6949-4665 N.V. Laur, Doctor of Agriculture, Assoc. Prof.; ORCID: https://orcid.org/0000-0003-1989-0384 All-Russian Research Institute of Forest Genetics, Breeding and Biotechnology, ul. Lomonosova, 105, Voronezh, 394087, Russian Federation; e-mail: antsa-55@yandex.ru, vad.tsareff@yandex.ru, tsarais42@mail.ru Petrozavodsk State University, prosp. Lenina, 33, Petrozavodsk, 185910, Russian Federation; e-mail: laur@petrsu.ru


Introduction
Most species of the genus Populus L. are fast-growing woody plants. In lowforest regions of the world, poplars are the most promising species of woody plants that can quickly fill the existing shortage of wood. That was the reason for their wide reproduction and breeding in many countries of the world. pests to which local varieties are not adapted. This happened with the most heavyproductive variety of poplar in Italy I-214. Due to its high quality it became widely used in various countries of Western Europe (its share in the total poplar cultivation reached 30 % and more). But suddenly there was an outbreak of a disease caused by the pathogen Marssonina brunnea [38], which became a disaster for this poplar. It was necessary to urgently develop work on breeding new varieties resistant to this disease.
The varieties bred, tested and shown excellent results in one region do not always fulfill the expectations in other regions. Thus, in the Central Chernozem Region the magnificent Italian varieties of black poplars could not withstand winter frosts, and winter-hardy poplar hybrids bred by P.L. Bogdanov in Leningrad were not drought-resistant [19,24].
In accordance with the world practice, especially after the disaster of the 1960's in Western Europe, associated with the disease Marssonina brunnea, technologies for creating poplar plantations from a set of many different varieties with similar practical properties, but with different genotypic structure were adopted. In particular, O. Lange [37] recommended creating plantings not from a single variety, but from 30-50 clonal varieties that are homogeneous in growth strength, ecological stability and morphological similarity, but distinguished by genotypic composition.
Considering the above mentioned, when cultivating poplars, like other woody and non-woody plants, it is necessary to take into account not only their productivity, but also ecological plasticity and stability. This is precisely what is missed in many cases due to the need for long-term, and often, multi-regional research. Research in this direction has relevance and novelty.
There are a number of methods for evaluation of ecological stability and plasticity of plant varieties which were developed by G. Wricke [44], S. Eberhart and W. Russell [29], G. Tai and D. Young [43], M.A. Fedin and D.Ya. Silis [18], V.G. Potanin et al. [12]. The analysis of some of these methods conducted by Novosibirsk researchers V.G. Potanin, A.F. Aleynikov and P.I. Styopochkin showed that each of them has advantages and disadvantages [12].
In our country, the method of Eberhart and Russell was often used. It was considered in detail by V.Z. Pakudin [11] and used by different researchers for evaluating the ecological stability and plasticity of different plants in different regions [8,9,16]. However, for poplars it was only used in the Astrakhan semi-desert [19]. Therefore, in this paper, we have tried for the first time to extend the application of this method.
The purpose of our work was to evaluate the ecological plasticity and stability of some promising poplar varieties for the South-Eastern part of European Russia and Ukraine using the Eberhart and Russell method and to determine their suitability for practical use in various cultivation conditions.

Research objects and methods
To evaluate the ecological plasticity and stability, three variety testing sites were selected. One of them located in the upland conditions of the Voronezh region and the others in the floodplain conditions of the Volgograd and Donetsk regions. On these sites, for evaluation of the level of ecological stability, five of the same promising varieties were selected. Among them there are three euramerican hybrids  [4,26].
In the Voronezh region research was carried out on the Semiluky populetum laid out in 1974 by A.P. Tsarev in accordance with the methods of field testing [5,28]. Preplanting preparation of soil was in the fall plowing to a depth of 30 cm. Planting was carried out with stem cuttings of different varieties and clones of poplars; 84 representatives of different sections of poplars were introduced to the site [19].
The site area was 4.6 ha, the size was 296×155 m. The soil is leached chernozem (black soil). The terrain is a weak water-dividing slope to the Veduga river. The groundwater level was 4-5 m. The forest site type is D 2 , according to the integrated scheme of P.S. Pogrebnyak and P.P. Kozhevnikov, which means that the soil is fertile (grey forest and chernozem) and slightly moist. Plant spacing was 5×4 m.
The experiment was carried out in 4 replications. Plot allocation in replications was randomized. There were 6 ramets (one clone plants) on the plot. So, each clone or variety was represented by 24 plants on an area of 480 m 2 . Materials of field observations of 25-year old trees were used for this site.
In the Volgograd region, the study object was pilot-production crop testing site laid out in 1988 by R.P. Tsareva together with employees of the Podtelkovsky forestry in the floodplain of the Kumylga River. The soil is intrazonal floodplain chernozem-meadow layered-grained heavy loam. The area of the plot is 2.0 ha. Planting is carried out by 1-year old rooted seedlings with plant spacing of 4×4 m. Each variety is represented by at least two replications. Eighteen different clones and varieties of poplar were planted on the site, 30-70 copies of each. The studied varieties were represented as follows: Vernirubens, 49 copies; Marilandica, 31 copies; Pioneer, 44 copies; Regenerata, 38 copies; and E.s.-38, 70 copies [25]. Materials of field observations of 29-year old trees were used for this site.
In the Donetsk region research was carried out on the variety testing site laid out in 1977 by A.P. Tsarev together with employees of the Fedorovsky forest district of the Zhdanov (now Mariupol) forestry pertaining to the Donetsk Forestry Management Department in Urochishche Ksenevka (no. 50), located in the floodplain of the Karatysh River. The forest site type is E 2-3 , according to the integrated scheme of P.S. Pogrebnyak and P.P. Kozhevnikov, which means that the soil is low-fertile (degraded chernozem and transitional to them slightly alkalinized loams), slightly moist and moist. The occurrence of ground water varied from optimal (1.0-1.5 m) to complete swamping. Comparative studies were carried out on the part of the site with optimal groundwater occurrence. The total area of the site was 1.2 ha, including the accounting area of 0.61 ha (135×45 m). Plant spacing is 3×3 m. The total number of plants was 1,215 pcs, including 675 accounting plants. The planting pattern was the method of complete random blocks. Plot allocation on the site and the mixing of plants on the plot were randomized. There were 3 replications. Planting was carried out with stem cuttings 20 cm long. The number of plants in the plot was 25 pcs. Into the test 9 clones and varieties of poplars were introduced [23]. Materials of field observations of 21-year old trees were used for this site.
Plant survival, growth dynamics in height (at an early age using a measuring pole, and then -using the German Blume Leiss altimeter) and diameter at breast height (1.3 m) were established by complete enumeration of trees. Trunk volumes were determined by volume tables for poplar [32].
Wood stock was determined by summing up the actual volumes of survived trees of each variety or clone on each plot for each replication and converting the resulting stock value to the area of 1 ha using the equation (1): where -calculated wood stock, m 3 /ha; W´-actual wood stock on the plots, obtained as the sum of stocks on each plot, m 3 ; S p -total area of all plots of each variety, m²; 10,000 -area of a hectare, m².
Standard valuation and mathematical methods as well as Microsoft Excel program were used for data processing [5,14,17].
The average increments in stocks of stem wood were determined based on these indicators. Since the stands were of different ages, the average increments in wood stocks were used to evaluate the ecological stability. According to our data [24], the average increments of poplars in the period of ontogenesis from 20 to 30 years vary slightly, so their use in this study is quite acceptable.
The method of Eberhart and Russell, according to which the ecological plasticity and stability of the studied varieties were evaluated, uses two main parameters: -Coefficient of productivity regression for changes in environmental conditions, which allows characterizing of varieties by their plasticity; -The standard deviation of actual productivity indices from the regression line, which characterizes the stability of productivity in different environmental conditions.
In general, this method is expressed by the following dependency (equation 2): where -productivity of the i-th variety in the j-th testing site; i = 1, 2, 3, …, ; = 1, 2, 3, …, ; X iaverage productivity of the i-th variety for all testing sites;the regression coefficient of the i-th variety on changing of wood stock productivity (see equation 4); I jindex of environmental conditions for the j-th testing site (see equation 3); d ijdeviation from the regression line of the i-th variety in the j-th testing site.
Additional details of the method are presented in the discussion of research results.

Research results and discussion
The average indices of survival, height and diameter of the studied varieties in different environmental conditions are shown in table 1.

T a b l e 1 Survival and growth of poplars in different environmental conditions
where V -number of testing varieties, V = 5; n -number of testing sites, n = 3.
Calculations have shown that for the Semiluky populetum testing site I 1 = = -0.9; for the Kumylga testing site I 2 = -6.4 and for the Ksenevka testing site I 3 = = +7.3.
Based on the fact that the higher index of environmental conditions, the more favorable environmental conditions for the testing varieties, the most favorable for poplars were the Ksenevka environmental conditions in the Donetsk region. The average environmental conditions can be attributed to Semiluky populetum and the least favorable for growth of poplars were the environmental conditions of the Kumylga testing site, located in the dry-steppe zone of the European Russia.
The regression coefficient of the i-th variety for the change in the average increment r i was calculated using equation 4: The calculations have shown that for the Vernirubens variety r 1 = 1.466, for Marilandica r 2 = 1.147, for Regenerata r 3 = 1.148, for Pioneer r 4 = 0.106 and for E.s.-38 r 5 = 1.126.
According to the hypothesis of S. Eberhart and W. Russell, the higher regression coefficient, the greater response of the variety to change in environmental conditions. In other words, it has higher plasticity and responds positively on improving of environmental conditions. And vice versa, the lower regression coefficient, the lower plasticity of the variety and it less changes its productivity depending on change in environmental conditions. In our case, the ranking has shown that the highest rank (1) was in the Vernirubens variety, it had the highest plasticity. The average ranks were for the varieties Regenerata, Marilandica and E.s.-38 (2-4, respectively), and the lowest (5) -for the Pioneer variety. That is, in this case, the Pioneer variety, whose growth was less than other poplars (most likely this is due to its lower rooting and survival), has shown the lowest plasticity. Thus, its productivity, depending on improvement or degradation of environmental conditions in comparison with other poplars, changed less.
Hereafter, the theoretical average increments (X ij ) were calculated using equation 5: 2 X ij = X i + r i I j .
They are presented in the table 3.
T a b l e 3 In the general case, to get the value of ecological stability variance for each variety, the sum of squared deviations of the theoretical from the actual values is divided by the number of degrees of freedom (equation 6):

Theoretical average increments of poplars in different environmental conditions (m 3 /ha/year)
where S i 2 -variance of ecological stability for each variety. In our case, since the number of testing sites is 3, the stability variance is equal to the sum of squared deviations given in table. 4.
As can be seen from the table 4 data, the Marilandica variety is distinguished by the highest stability of the average trunk wood increments in all of the studied testing sites. The average stability of the average trunk wood increments can be attributed to the following poplar varieties: E.s.-38, Vernirubens and Pioneer. The most productive poplar variety Regenerata in all environmental conditions of the studied testing sites showed the lowest variance of ecological stability.

Conclusion
The conducted research of a group of promising poplars on three testing sites of the forest-steppe and steppe zone of our country allowed to establish not only their phenotypic characteristics on survival, growth and productivity, but also their ecological plasticity and stability.
It was found that the most productive of the studied varieties was Regenerata. It was characterized by the average plasticity, but the lowest stability. So, it can perform high productivity only in the favorable environment for growth. Poplar Pioneer, despite the fact that it was inferior in productivity to other varieties, was the most plastic with average ecological stability. Its productivity did not change too much with change in environmental conditions.
The Vernirubens variety has shown the minimum ecological plasticity with medium ecological stability.
Average indices of plasticity and stability are noted in the poplar variety E.s.-38, which allows it to be used in various environmental conditions. Poplar Marilandica has demonstrated average plasticity and high stability.
In general, the studies have shown that poplar variety testing in different environment conditions allows obtaining a more complete characteristic of them, taking into account ecological plasticity and stability. This makes it possible to determine more appropriate ways for their using in afforestation and other applications in forest management.