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plastic traits depended on the frequency and duration of flooding (Huber et al., 2008). On contrary, in more dry and warm conditions leaves had smaller blades and smaller petiole size and hairiness. The significant differences in leaf blade and petiole size were observed between the typical and hairy type of European black poplar which were in accordance with climate difference in respective habitats of continental riparian forests and Submediterranean type of climate (Kajba et al., 2004, 2015; Ballian, 2017). We examine the variability of a specific leaf area, petiole length, and developmental stability (composite leaf index of fluctuating asymmetry FALEAF) with the aim of assessing which of them could be considered the key element of suboptimal environmental conditions for riparian tree Populus nigra L.. The statistically significant impact of flooding was obtained only for leaf geometric size and shape, so these traits could be recognized as indicators of flooding suboptimal environment. For all analysed leaf traits genetic variability were confirmed with statistical significant effect of tree variation (all P > 0.05), except for the developmantal instability (FALEAF) (Table 1, Table 2). According to the study by Herrera et al. (2009) trees photosynthetic acclimation to flooding in the Mapire River could not be explained by leaf anatomy traits. Plants have adaptive survival strategies in the period of flooding which depend of the seasonal or perennial flooding in certain areas. For example, plants reduce leaf expansion and new leaf production because the root systems are losing the capacity to absorb water and nutrients in an oxygen-poor environment (de Oliveira and Joly, 2010). The leaf shape and structure are defined mainly during a brief period of primary morphogenesis based on the possible role of the reaction-diffusion system and can be altered by the allometric expansion (Xu et al., 2009). A large variation in the values of the leaf traits exists within one individual species (Xu et al., 2009). Morphological adaptation can improve tissue aeration during flooding. However, flooding negatively affects the ultrastructure of leaves, especially photosynthetic organs, decreasing photosynthetic capacity (Du et al., 2010) and stomatal conductance (Rood et al., 2010). Negative effects on plant growth and total biomass accumulation in many species with different tolerance to flooding were also reported (Du et al., 2008; Rood et al., 2010). The human activity-induced global changes in environment, espacially changes in the flooding regime, represent the most important stress factor potentially affecting wetland ecosystems. The flooding regulation sistems significantly altered the natural regeneration capacity of black poplar, and favoured the succession of poplar stands by hardwood forests (Ballian, 2017). The knowledge of plant strategies of wetland vegetation across a range of flooding and not flooding gradients is therefore very important (Lou et al., 2016), and it is the base for future large-scale studies.The study showed the results of development instability and genotypic variability of the P. nigra leaf morphological traits in natural populations which depend on the changes in the habitat conditions, primarily from the water regime, which have been altered significantly in the last century. The results have confirmed that P. nigra is highly tolerant of long floodings as well of the changing water regime, so it could be used in a restoration programme aimed at the recovery of areas that are naturally subject to longer periods of flooding. In this way, we obtained the base for the conservation and the use of the available gene pool of these species, as well as the guidelines for breeding programs in riparian ecosystems. Preservation of genetic resources, or genetic variability of natural populations, is the basis for improvement of the existing state of black poplar forests, as well as precondition for further improvement of vitality and conservation of biodiversity. The assessment of endangered species may also give guidelines for a certain degree of protection of the investigated threatened areas. Acknowledgement This study was supported by the Ministry of Education, Science and Technological Development of the Republic of Serbia Grants No. 173025 title “Evolution in heterogeneous environments: mechanisms of adaptation, biomonitoring and conservation of biodiversity”. Many thanks to Jelena Mladjenović (English Professor) for manuscript language revision and Dr Bojan Tubić for producing map. Authors’ Contributions DM conceived the idea and designed the study, performed morphometric and statistical analyses. DČ conducted fieldwork and performed laboratory measures. DM and DČ wrote and prepare the final version of the manuscript. Conflict of interest The authors confirm that there is no conflict of interest in relation to this article. References Al Afas, N., Pellis, A., Niinemets, Ü., Ceulemans, R., 2005. Growth and production of a short rotation coppice culture of poplar. II. Clonal and year-to-year differences in leaf and petiole characteristics and stand leaf area index. Biomass and Bioenerg. 28(6), 536–547. Al Afas, N.A., Marron, N., Ceulemans, R., 2007. Variability in Populus leaf anatomy and morphology in relation to canopy |