Evaluation of the stability of the balsam poplar (Populus balsamifera L.) under conditions of mass leaf rust infestation

Methods of fluctuating asymmetry are quite often used to assess the developmental stability of tree stands under conditions of industrial pollution in urban areas. The aim of the study was to evaluate the developmental stability of balsam poplar (Populus balsamifera L.) stands using fluctuating asymmetry of leaves from unpruned, pruned trees and root suckers, which were weakened by leaf rust and brown leaf spot, in different districts of Bratsk, Irkutsk Oblast. The research employed the fluctuating asymmetry method according to V.M. Zakharov’s technique, as well as the method by E.V. Avdeeva. Bratsk is a complex urban agglomeration where residential areas are separated by tens of kilometres, which allows the assessment of the relationship between the degree of developmental stability of balsam poplar trees and pollution levels with a high degree of reliability. The study was conducted during 2023–2024 on different tree categories: unpruned and pruned trees, as well as root suckers, which poplar produces abundantly. It was found that infection of balsam poplar by leaf rust and brown leaf spot is the decisive factor affecting the developmental stability of the stands, whereas the effect of pruning and the pollution level of diseased stands did not, as usually observed, exert a significant influence on the fluctuating asymmetry indices. Moreover, in the first year of mass disease outbreak, leaf rust did not affect developmental stability, but in subsequent years after the epiphytotic, severe weakening of the trees occurred, manifested in early defoliation, which led to a critical state of developmental stability. In this case, the assessment of developmental stability appears to be of little effectiveness. The main factor affecting the developmental stability of urban balsam poplar (Populus balsamifera L.) stands infected by poplar leaf rust and brown leaf spot is the weakening of the trees’ vital status caused by pathogens, leading to a loss of ornamental value, premature defoliation, and loss of symmetry in leaf blades.