Importance of litter and fire effects during woody species invasion in Transylvanian dry basiphilous grasslands

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Grasslands are among the most species-rich ecosystems worldwide; among them are the dry basiphilousgrasslands occurring in the Transylvanian plain in Romania. These grasslands evolved under continuoushuman utilisation, as meadows and pastures. In the absence of management, these grasslands experienced accumulation of litter and invasion by shrub and trees, which in general both decrease plant species diversityin grasslands. Transylvanian grasslands are not only invaded by bushes and trees native to the region but also by non-native woody species, which further increase the concerns of nature conservancies. Effects of invadingbushes and trees on the invaded plant community might be mediated through direct and/or indirect effects, with the latter being in part translated through plant litter. Effects of litter on seed/seedling performance,biomass and species richness of different ecosystems are predominantly negative. Only in grasslands litter effects were found to be also positive. But the effect size and sign of this pattern is further modulated by differences among litter quality and quantity, and knowledge about the interplay of litter of native and non-native species on invasibility of species-rich dry basiphilous grassland is scarce. Distinct responses might be linked to physical, mechanical, structural and chemical differences. Where ecosystems are invaded by non-native species, their effects are consequently in part also mediated by the outcome of the interplay of non-native and resident species. In the study region this pattern gets more complex by the widespread use of fire to remove litter mats and to stop invasion by bushes and trees after the cessation of regular management.Those studies addressing the effects of fires on species invasion highlight trait-based differences in species response. Furthermore, there seems to be no linear correlation between fire frequency and woody species.Apart from disturbance related effects of fire events, fire acts as a direct germination cue through heat and smoke. These effects are well established for naturally fire prone ecosystems of e.g. the Americas, but lesswell understood in systems like the dry basiphilous grasslands of Transsylvania. The following working hypotheses will be adressed:- Invasion of native and non-native woody species differs in its impact on species diversity of the grasslands - Litter mediates this process and therefore litter type, i.e. grassland vs. woodland, and non-native vs. nativewill show differential effects on seed germination and seedling establishment of herbs as well as woody species. Litter effects will be further accentuated by fire through heat and changes of the chemical environment.- Successful seedling establishment of bushes, will depend on competition by herbaceous resident grassland species. In the presence of fire there will be a trade-off between negative (competitive) and positive (protective) effects of resident grassland species.