Long-term changes of forest vegetation caused by global and local environmental change drivers

Multiple environmental changes pose major challenges for nature conservation efforts and seriously threaten the sustainability and biodiversity of temperate forest plant communities in the 21st century. Four main processes of the vegetation alteration will be in our focus in the proposed project. As a result of climate warming, vegetation is shifting towards higher frequency of plant species more tolerant to warmer and drier climate (thermophilization). The increasing frequency, intensity and spatial extent of natural and management disturbances cause forest stands to be more open, which facilitates the establishment of non-forest ruderal species (ruderalization). Atmospheric nitrogen deposition is increasing nitrogen availability in the forest soil, which can lead to higher share of nitrophilous species in communities (eutrophication). The gradually increasing compositional similarity among different communities, leading to a decrease in beta diversity, is a consequence of several factors (biotic homogenization). Monitoring the status and temporal alterations in the species richness and composition of the understory forest vegetation is of paramount importance, as the knowledge gained from resurvey studies provides valuable insight into the driving factors. However, it can be difficult to disentangle the effects because vegetation shifts are usually caused by the combined effects of simultaneously operating environmental changes. We will test whether environmental changes are reflected in the understory vegetation of temperate forest in Slovenia. By resurveying research plots first sampled several decades ago, we will assess changes in forest plant communities associated with global warming, increasing intensity and frequency of forest disturbances, effects of atmospheric nitrogen deposition, invasive spread of non-native species and ungulate browsing. The objectives are to quantify the: 1) Contribution of climate warming and forest disturbances to the vegetation thermophilization signal. 2) Contribution of forest disturbances to the signal of ruderalization and thermophilization of vegetation. 3) Contribution of nitrogen deposition to the vegetation eutrophication signal. 4) Contribution of climate warming, forest disturbances, nitrogen deposition, invasive alien species and deer browsing to the potential process of biotic homogenization in vegetation composition. Detection and appropriate assessment of effects of environmental changes on forest vegetation require inclusion of multiple sites with different ecological backgrounds and baseline conditions. The following research objects will be studied: i) permanent research plots in managed forests of Quercus robur and Quercus petraea, established in the early 1990s; ii) semi-permanent research plots in unmanaged old-growth forest reserves of Fagus sylvatica and Quercus robur, established in the late 1970s and early 1980s; iii) permanent plots for intensive monitoring in representative Slovenian forests, established in 2004; iv) research plots in canopy gaps in Dinaric fir-beech forests, created in 2012. Forest vegetation will be resurveyed in all research plots using the same procedure as in the original studies. In addition, soil sampling will be performed in oak permanent research plots and canopy gaps will be equipped with sensors for microclimatic measurements. The observed vegetation changes will be compared with those from other European countries by implementing relevant data from the ICP-Forests database. Changes in vegetation will be assessed at several levels (species, species groups and communities). We will use various community-level metrics that capture different facets of diversity (taxonomic, functional, phylogenetic indices). This project will develop models to predict future trends in plant community composition. The results will help develop mitigation strategies to maintain the diversity and functioning of temperate forests in Slovenia and beyond.