2025, Robakowski, Piotr, Jagiełło, Radosław, Baranowska, Marlena, Bułaj, Bartosz, Dering, Monika, Hauke-Kowalska, Maria, Korzeniewicz, Robert, Łukowski, Adrian, Szmyt, Janusz Stanisław, Zadworny, Marcin, Wierzbicka, Anna, Popek, Robert, Przybysz, Arkadiusz, Kowalkowski, Wojciech, Uniwersytet Przyrodniczy w Poznaniu

In response to ongoing climate warming, tree species adapted to colder climates are expected to shift their geographic ranges northward. Within the framework of long-term ecological monitoring in Wigry National Park (northeastern Poland), observed changes in forest biocenoses reflect the combined influence of climate change and natural ecological dynamics. This study compares dendroflora composition and diversity between two monitoring periods, 2011 and 2024, as part of an ongoing effort to track climate-related ecological shifts. Tree observations and meas­urements were carried out using concentric circular plots. In the largest plots, all trees with a diameter at breast height (d.b.h.) ≥ 12 cm were recorded by species, and their d.b.h. was measured. In the smaller plots, all trees with a d.b.h. ≥ 2 cm and < 2 cm but taller than 30 cm were similarly identified and measured. Data were recorded with Field-Map software integrated with an electronic calliper. The species-level taxonomic data, individual counts and basal area per species and plot were used to calculate biodiversity indices. Over the 13-year interval, a marked increase in overall dendroflora diversity was observed. Notably, the dominance of canopy-forming conifers – Pinus sylvestris and, to a lesser extent, Picea abies – measured as the proportion of individuals or stem density, has declined. This decline of coniferous species has been accompanied by an increase in the abundance and diversity of broadleaved deciduous species, including Tilia cordata, Quercus robur, Betula pendula, and Acer platanoides. Other thermophilous deciduous taxa also ex­hibited upward trends in both presence and abundance. Furthermore, the exponential of Shannon entropy, reached the highest value when evergreen conifers comprised 35% of the stand composition in 2011 and 18% in 2024. This finding suggests that maximum dendroflora diversity reaches its highest level at an intermediate proportion of conifers presence, rather than under conifers dominance or absence. Collectively, the pro­cesses occurring in Wigierski National Park illustrate the gradual shift in ecotonal forest ecosystems from cold-adapted coniferous species to broadleaved deciduous taxa due to ongoing climate change.