Thursday, 15. November 2012 Kadri Koorem: "The influence of abiotic and biotic factors on small-scale plant community patterns and regeneration in boreonemoral forest".
Supervisor: vanemteadur Mari Moora (TÜ)
Opponent: prof. Guillaume Decocq (Jules Verne Picardie Ülikool, Prantsusmaa)
Plants can perceive their close surrounding and adapt to those conditions. For example, it is known that plants can locate their roots into nutrient rich areas; and increase growth in the presence of symbiotic micro-organisms like arbuscular mycorrhizal (AM) fungi that help plants to acquire nutrients from soil. In addition, plants in forest understorey are highly influenced by overstorey and this can occur through the effects of tree and shrub litter. Despite this knowledge, we have still little information about the relative importance of abiotic and biotic factors (e.g. the ones listed above) in determining understorey productivity, regeneration success and species richness in natural ecosystem.
Boreonemoral spruce forest with hazel sub-canopy and species-rich understorey was selected as a model system to fill abovementioned gaps in our knowledge. I found that high availability of soil nutrients and abundance of AM fungi enhances plant aboveground biomass, which implies to the importance of light competition among understorey plants. At the same time, soil nutrient content and the abundance of AM fungi did not influence plant belowground biomass. Plants seem to be able to acquire more nutrients directly from the soil when nutrient level is high or alternatively via symbiotic relationship with AM fungi when nutrient content is low. This mechanism seems to operate also at regeneration phase as I found that seedling growth of a characteristic forest plant species benefited from the presence of AM fungi when soil nutrient content was low. At the same time, seedling growth of a plant species occurring in a wide range of habitats was not influenced by AM fungi despite of soil nutrient content. In addition, the regeneration of the plant species in the understorey was highly influenced by presence and type of overstorey litter as I found that hazel litter is more beneficial to seedling growth and emergence than spruce litter. The effect of different factors, especially the effect of litter, leads to spatial differentiation in species composition in the forest understorey as I recorded twice as high herbaceous species richness under hazel compared to spruce.
Results of this thesis lead to better understanding of forest understorey dynamics. Improved knowledge enables to preserve the species richness characteristic to forests as most of the of plant species growing in forests can be found in the understorey.