Resilience of Fraxinus excelsior (Oleaceae) and Quercus robur (Fagaceae) Stands in Fragmented Forests of Western Tatarstan

The article discusses the resilience of Quercus robur L. and Fraxinus excelsior L. stands located in the northeastern part of their geographic range. The as a model, a forest outlier of the Central Russian – Volga linden-oak forests with admixture of European ash growing in Tatarstan to the west of the Volga River is used. The total forest cover of the western Tatarstan is 12 %, although in the 18-th century it was almost three times as much as today. The forests surrounded by arable lands have functioned as forest outliers for almost two centuries. The average size of most forest areas does not exceed 500–1000 ha. The model forest is one of two relatively large ones measuring 11,000 ha in size. The dynamics of the taxation characteristics of forest stands for six dates of the state forest inventory (1926, 1947, 1958, 1980, 2011 and 2020) was analyzed. The resilience of forest stands is considered as the ability to restore its presence and numbers after disturbances (clearcutting) and under stress factors (climatic and anthropogenic). The zonally determined forest composition and the main phytocenotic strategies of species are taken as standards. It is shown that at the beginning of the studied period, species were characterized by sustainable phytocenotic strategies: Quercus robur as an upper-storey primary foundation species (edificator), Fraxinus excelsior as a forest stand secondary foundation species (assectator), but these strategies have changed over time. By constructing generalized linear models (GLM), factors significantly associated with these changes are determined. The effects of forest fragmentation in a sparsely forested region aggravate the decrease in resistance to unfavorable climatic conditions, what is common to all species growing at the distribution range margins. In the forest outliers, Quercus robur loses its ability to compete and act as a stable edificator, when the internal integrity of habitats is disturbed due to the economic activities. This provides assectators, including Fraxinus excelsior, with ability to pass from assectators to edificators. Fraxinus excelsior exhibit selectivity in relation to the relief and position relative to the forest boundaries, which allows it to offset the limiting effect of unfavorable climatic conditions. However, only relatively large forests occupying watershed spaces can provide such an opportunity, which are almost nonexistent in a heavily developed region. Therefore, even with the ability to show a progressive strategy, the species cannot find new suitable habitats.