Population of common oak (Quercus robur) in the widening of the Luga River floodplain (Luzhsky district of the Leningrad region): dendrological, phytopathological, soil-ecological characteristics

This study aims to provide a comprehensive dendrological, phytopathological, and soil-ecological analysis of the English oak (Quercus robur) population within a natural habitat at the northern extreme of its distribution. The habitat in question lies in the southern taiga subzone, specifically an extension of the Luga River floodplain near its confluence with the Kemka River. The focus is on old-growth trees, particularly late-generational and subsenile individuals. A preliminary survey identified 31 English oak trees affected by seasonal spring flooding in the area. A detailed investigation was conducted on the four subsenile-aged trees, ranging from 84 to 134 years. These trees occupy distinct microhabitats: Tree N 1 grows on an alluvial riverbank uplift; Tree N 2 – on a flat isthmus within the central floodplain zone; Trees N 3 and N 4 are situated in a terrace depression sloping toward a bedrock bank. The trees represent three ecotypes – forest (Trees N 1 and N 4), floodplain (Tree N 2) and intermediate (Tree N 3) – and include different biotypes. Three are classified as summer forms (N 1, 3, and 4), while one is a winter form (N 2). All trees are located primarily in well-drained zones despite the periodic flood conditions. Under the tree canopies, oak regeneration appears robust and plentiful. However, young saplings face competition from shade-tolerant broadleaved species – such as linden, maple, elm, and aspen – as well as boreal and nemoral shrubs. Phytopathological assessments concluded that the trees are generally in good health, with an average condition score of 2. Notably, these natural forest oaks undergo self-regulated crown cleansing rather than human intervention, such as pruning. Central rot caused by the polypore Laetiporus sulphureus was identified in most trees except for Tree N 2, which represents the winter form. The soils in this area are classified as alluvial soddy acidic soils, shaped by short-term seasonal flooding. They possess a well-developed humus horizon and exhibit high fertility levels that surpass those of typical podzolic soils found in southern taiga floodplains. This fertile soil underpins a diverse ecosystem typical of northern oak forests. The site encompasses geobotanical features representative of lily-of-the-valley oak forests, sedge oak forests, and slope nemoral grass forests dominated by spruce and linden. While the central floodplain oaks are likely of anthropogenic origin, the other mature specimens reflect habitats where natural regeneration of English oak occurs consistently without human interference. This location serves as an illustrative fragment of northern oak forest ecology, encapsulating key environmental dynamics of this habitat type.

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