1.3. Aims of the study

In this thesis I examine the role of nutrients at several organisational levels from nutrient acquisition strategies of individual plants to nutrient cycling at the ecosystem level. I show the existence of a trade-off in resource competition under stable and variable conditions (I, II) which may provide new insight into the costs and benefits of mycorrhizal symbiosis (I) and the phenotypic plasticity of above and below ground growth allocation (II). The strong interaction between plants and soil microbes motivates the unification of the theories concerning the function of organisms in and above ground, which will promote the development of both (III). This functional approach guided the study of primary succession at the island of Hailuoto, the Gulf of Bothnia, where ecosystem development and the associated changes in plant and soil microbial communities are a consequence of post-glacial land uplift (IV). The largest scale in nutrient-mediated interactions is the nutrient cycle of a whole ecosystem. Many studies have discussed direct nutrient flows while ignoring indirect trophic interactions. I will propose a measure for the strength of indirect interactions and their contribution to the dynamics of nutrient cycling (V).