Changes in biodiversity and species composition of meso- and eutrophic broadleaved forests in southern Poland over 30 years: a resurvey study based on semi-permanent plots

Since forest vegetation dynamics is a process lasting decades to centuries, we aimed to check if 30 years is enough to expect any significant qualitative and quantitative changes in the herb layer composition in mature broadleaved forests. We identified the trends in the selected global environmental drivers that may lie behind the observed changes in plant communities. We focused on understorey species since they are good indicators of altered habitat conditions and because 80% of plant species richness in forests is located in the herb layer. We conducted the study in two forests located in S Poland. Both sites are situated on the edge of these extensive forests. One of the regions, Wapienica Valley, situated in the Silesian Beskids, is protected as Natura 2000 Beskid Śląski region and as Wapienica Valley nature-landscape complex. The second study region, Rudno Stream Valley Nature Reserve, located on Kraków Upland, was designated a forest nature reserve in 2001. Both sites are characterized by a significant proportion of mature forest communities of natural character, with a very high proportion of herb layer species typical for forest interiors. The baseline vegetation surveys and resurveys were conducted in similar years, in 1991-2020 and 1992-2023, for Wapienica Valley and Rudno Stream Valley, respectively. Thus, the interval between the studies was ca. 30 years. During this period, forests were free from natural disturbances or direct human-induced pressure due to forest management, grazing, or severe trampling. No alterations in forest structure due to dynamic successional changes were detected in the 30-year interval since the initial survey was conducted in mature stands with diverse vertical profiles and species composition typical for natural forests. The Braun-Blanquet method was implemented in the initial survey and resurvey studies documenting forest floor vegetation composition. Meso- and eutrophic communities from the Querco-Fagetea and Alnetea glutinosae classes were surveyed. To assess temporal shifts in climate, we used data from the Kraków-Obserwatorium (1960-2024; ~25 km away from Rudno) and Brenna (1980-2024; ~20 km away from Wapienica) meteorological stations, provided by the Institute of Meteorology and Water Management. Based on modelled gridded data provided by the EMEP program, we compiled information on temporal changes (1990-2022) in the total deposition of NOx and SOx. We conducted all statistical analyses using R. Apart from species richness and Shannon index; we calculated three metrics of phylogenetic diversity for each plot: Faith's PD, mean nearest taxon distance index, and mean pairwise phylogenetic distance. Low PD values hint at a strong phylogenetic clustering of species, i.e., the phylogenetic distances between species are shorter, making the abundance of plant taxa representing particular clades less random than in the phylogenetic tree. Data on four functional traits of plant species recorded in plots were compiled from the LEDA, BIEN, BiolFlor, and Pladias databases: canopy height, specific leaf area, leaf dry matter content, and seed mass. Using the ecological indicator values (EIV), we characterized the ecological requirements of plant species. For each plot, we calculated the community weighted mean values of plant functional traits and EIVs. To characterize vegetation's functional diversity, we computed functional richness, divergence, dispersion, and evenness for each plot. To identify the patterns of species composition shifts between the two sampling periods for each vegetation type, we conducted the NMDS ordination. We used mixed-effects models to evaluate temporal changes in plant species diversity metrics within each vegetation type. While mean annual temperature increased prominently in both study sites, the number of days with snow cover decreased significantly from 1960 to 2024. No significant shifts over time were observed regarding mean annual precipitation. In both study sites, we observed significant decreases in the atmospheric deposition of sulphur and nitrogen oxides. Regarding the plots representing Fraxino- and Ribeso nigri-Alnetum , as well as Lunario-Aceretum and Dentario glandulosae Fagetum , we observed consistent directions of compositional changes over time, while temporal shifts in plant species composition in Luzulo-Fagetum forest were plot-specific. In the resurveyed Fraxino- and Ribeso nigri-Alnetum plots, the species richness, functional richness, functional evenness, and the contribution of species with lower seed mass and higher temperature demands increased significantly compared to the initial survey period. In resurveyed Lunario-Aceretum plots, we observed a significant decrease in functional richness, dispersion, and divergence, as well as leaf dry matter content CWM, along with an increase in canopy height and seed mass CWMs values and the contribution of species with higher light, moisture, nitrogen, and reaction demands compared to historical data. Considering the Luzulo-Fagetum plots, only values of the mean nearest taxon distance index were higher in resurveyed plots than in historical vegetation data. In the Dentario glandulosae-Fagetum plots, we identified a significant increase in the mean nearest taxon distance and species richness over time. Over the last decades, significant changes in the climatic parameters and a severe decrease in NOx and SOx atmospheric deposition were observed. The reaction of vegetation to these shifts in the global environmental drivers is inconsistent and varies in forest communities of different fertility and moisture regimes.