Supervisors: prof. Meelis Pärtel (TÜ) ja prof. Scott D. Wilson (Regina Ülikool, Kanada)
Opponent prof. James F. Cahill Jr. (Alberta Ülikool, Kanada)
Our current understanding about plant species diversity and coexistence is almost entirely based on studies of aboveground vegetation. In some species-rich grassland ecosystems, nearly 75% of plant biomass occurs belowground, however, little is known about below-ground processes due to difficulties in distinguishing between morphologically similar roots of different species. Belowground root systems are more extensive in space and more persistent in time than short-lived shoots, causing belowground species richness to be higher than aboveground per volume. In this thesis I explored the differences between the patterns of below-and aboveground plant diversity with adding new insights to the knowledge of mutualistic relationships between plants and root dwelling fungi and the functioning of plant communities as a whole. Application of next-generation sequencing revealed that belowground plant species richness can be up to twice as high as aboveground. By using the same root samples I identified arbuscular mycorrhizal fungi forming tight mutualistic relationships with plants and found that fungal species diversity and phylogenetic diversity increases with increasing plant diversity and phylogenetic diversity. Above- and belowground plant diversity responded differently to soil fertility gradient as well. Similar to previous studies, aboveground richness declined with increasing soil fertility; in contrast, the number of species found only belowground increased with fertility. Investigating the plant community assembly patterns above- and belowground revealed substantial differences - there is more evidence for biotic interactions, such as light competition aboveground, whereas abiotic and stochastic processes, such as soil nutrient gradients, soil pH, etc. dominate belowground. Results of this thesis suggest that by considering only aboveground vegetation we merely see the "tip of an iceberg", whereas real species coexistence occurs belowground. This thesis for the first time highlights the importance of belowground processes in the research of plant species coexistence and ecosystem processes.