Mixed signals of environmental change and a trend towards ecological contraction in ground vegetation across different forest types

Abstract

Abstract Forest ground vegetation (GV) may serve as an early warning system for anthropogenic global-change impacts on temperate forests. Climate warming induce decline of cool-adapted species to the benefit of more thermophilous plants. Nitrogen deposition has been documented to potentially result in eutrophication, which can increase the proportion of species with higher nutrient requirements. Abiotic forest disturbances are changing the light and soil conditions and temporal vegetation dynamics may be altered by deer browsing. In this resurvey study, we tested the magnitude and direction of change in GV at different forest types (11 monitoring sites and 174 permanent vegetation plots within the ICP-Forests network) in Slovenia over 15 years (2004/05–2019/2020). Using the plant-derived characteristics (Ellenberg-type indicator values) and by testing a priori predictions concerning expected effects of environmental drivers, we show that the magnitude and direction of floristic changes varied greatly between forest sites. Divergent response of GV at different sites resulted in low net change and overall weak environmental signal. Sites positioned at the ends of the gradients are losing their original ecological character and are becoming more similar to mid-gradient sites which exhibited smaller changes. Shifts towards the middle of the gradients (process of ecological contraction) was driven by the decline of ecological specialists and their replacement by species with greater ecological amplitudes. Our results point to the importance of local stand dynamics in explaining the temporal trends in GV. Ground vegetation in Slovenian forests is changing in directions dictated by multiple local, regional and global change drivers.