Effects of Tree Species Diversity on Forest Structure and Ecosystem Services in Boreal Forests

Boreal forests provide numerous ecosystem services, including carbon storage, maintenance of biodiversity, and recreation and economic resources for people. Trees are functionally important in forests, building the three-dimensional structure that controls light and water availability, biochemical cycles, and creating habitat for other forest dwelling organisms. Boreal forests typically consist of few tree species and small increases in tree richness will significantly alter forest structure and functions that directly impact on ecosystem services. This thesis aims to understand how variation in tree species richness affects forest canopy structure and ecosystems services. Utilizing the Satakunta Forest Diversity Experiment in southwest Finland, variation in boreal forest canopy structure were assessed between monocultures and mixed forests of two-, three- and five tree species using UAV surveys. The effect of tree species richness, composition, and structural variation on the ecosystem services of aboveground biomass storage, understorey plant diversity, and edible berry production were analysed. This study found that canopy structure, measured through vertical complexity, was positively affected by tree species richness, and measures of canopy rugosity and canopy gap fraction were not affected. Additionally, tree species richness weakly affected plot-level above-ground biomass, but no other ecosystem services. Tree identity affected above-ground biomass, understorey species diversity and edible berry production. Percentage of silver birch positively influenced aboveground biomass and understorey diversity and the percentage of silver birch and Scots pine promoted edible berry cover. Conversely, Norway spruce negatively affected understorey species richness and edible berry cover. Measures of canopy structure had mixed effects on ecosystem services, with canopy rugosity and gap fraction promoting understorey diversity and vertical complexity index promoting bilberry cover. The results of this thesis demonstrate that tree species richness alone did not promote all ecosystem services and suggest that light availability was an important mechanism promoting ecosystem services.