Ecosystem Functioning and Plant-Soil Interactions in Forests Influences of quality and diversity of resources

The aim of this thesis was to investigate the role of resources in driving ecosystem processes and in influencing soil-and plant communities in boreal and temperate forests, through four complementary experimental studies. In the first study, plant and soil microbial responses to the quality and diversity of added organic substrates from boreal forests were investigated. The substrate-diversity effects were greatest for plants and depended on the presence of high-quality substrates. In the second study, the impact of fire disturbance in boreal forests on litterfall composition and decomposition rates was assessed along a fire chronosequence, by collecting litterfall data and performing a litterbag experiment. Time since last fire reduced tree litterfall quality by increasing the proportion of twigs (low-quality litter) in the litterfall and this resulted in a decline in overall litter decomposition rates. In the third study, effects of resource availability on the performance of coexisting boreal tree seedlings were examined by growing seedlings in pots with low-and high fertility soil under different light filters. The growth and mycorrhizal colonisation of seedlings in response to light and soil conditions differed between species, which affected the relative competitive abilities of the seedling species. In the fourth study, impacts of different canopy tree species on tree seedling recruitment and soil communities were assessed by planting seedlings in stands of native and invasive tree species in temperate forests. Treatments were applied to test for effects of mycorrhizal connections and root competition from canopy trees on seedling performance. Tree seedlings were unaffected by root competition and mycorrhizal treatments, but both seedlings and soil organisms were strongly affected by canopy species identity through influences on the microenvironment. In combination, these studies demonstrate different mechanisms for how resources may influence nutrient dynamics, soil communities and tree seedling recruitment in forests, i.e. through regulating decomposer activities and decomposition rates, nutrient cycling, mycorrhizal colonisation, plant growth, and through influencing feedbacks between soil-and plant communities. These studies are also relevant for a better understanding of global change issues such as impacts of invasive species, biodiversity loss, and increased carbon sequestration.