Comparative plant diversity and seed germination patterns of alpine species in the context of climate change

Climate is a major determinant of the distribution of ecosystems on earth and influences the latitudinal and altitudinal distribution of both plant species and vegetation. Over the least decades, global warming has been affecting ecosystems worldwide, especially cold regions such as the arctic and alpine biomes. European mountains are considered hotspots of biodiversity, hosting approximately 20% of the continent's native vascular flora, with a high number of endemic species. Yet, these rich and diverse environments are projected to experience one of the highest rates of climate warming, compared to other regions of the world. Climate change has direct impact on plant species, causing species migration and extinction. Consequence, vegetation dynamics in these cold adapted ecosystems are difficult to predict. In this Thesis, I firstly investigated the variation of species richness, α-diversity, β-diversity and total cover of plant functional types on an alpine long-term monitoring site (belonging to the GLORIA network, https://www.gloria.ac.at), with the aim to promote a better understanding of climate-driven changes of alpine vegetation. I identified significant increment in plant species richness, along with trend of biodiversity loss and signals of biotic homogenization: Cold-adapted and rare species declined while dominant species like nitrophilous graminoids and shrubs increased. The results obtained highlight that long-term vegetation monitoring activities paired with multiple measures of diversity are required to properly assess biodiversity and to obtain useful indications for future conservation activities in alpine environments. Secondly, I investigated the germination ecology of 75 alpine plants of interest, to evaluate the role of seed germination as a driver for plant species population dynamics in response to climate warming. One of the key processes in determining the species capacity to migrate, establish and persist is, in fact, seed dispersal, followed by seed germination and seedling development. In alpine plants, seed germination is under strong environmental control, suggesting that climate change will inevitably affect recruitment success. In the present study, I selected 75 alpine plant species which populations either increased, decreased or remained stable in five long-term monitoring alpine study sites (belonging to the GLORIA network), along a time period of 15 years. Increasing species germinated at a broader spectrum of environmental conditions than those of the other groups, and were characterized by higher germination, especially at low temperature after cold stratification treatment). These results suggest that germination during early spring, right after snowmelt, may be an advantage in a warmer climate, promoting seedling emergence when water is more available and drought/heat hazards are low. Finally, I investigated the relationship between seed germination and environmental cues of 28 species of the genus Saxifraga, with the aim to disentangle their germination response from ecological cues and phylogenetic relatedness. Saxifraga seed germination resulted overall promoted by cold stratification and cool temperatures, while being strongly inhibited by darkness. Germination traits in the genus Saxifraga were overall unconstrained by phylogeny, while they were driven by the species ecological niche, as it was possible to observe different germination strategies depending on species requirements for water availability, soil acidity and texture. These results highlight that microenvironmental variables play a major role than phylogeny in determining the germination strategies across species of this genus.