Mesotopografian ja geomorfologian vaikutukset kasvillisuuden beta-diversiteettiin arktis- alpiinisessa ympäristössä

Species composition of a local habitat is a sample of regional species pool. Stress and disturbance define the environmental conditions and control the vegetation on the local scale. The variation of stress and disturbance inside a region control fluctuations in species richness and species composition. This thesis examines the effect of local stress and disturbance on the relationship between local and regional vascular plant species pools in arctic-alpine environment. The relationship is studied through beta-diversity, which measures the ratio of local and regional species richness. It tells how many times larger the regional species pool is compared to the local species pool. If stress, disturbance and biotic competition didn't limit the species dispersal at the local scale, there would be the same species composition in every habitat and no variance in beta-diversity. Stress level is studied through soil temperature, moisture and pH. Stress variables determine the resource shortage, which limits the primary production and species richness. Earth surface processes (ESP) are a major disturbance in a low-productive environment. ESPs modify physical conditions of a habitat, destroy vegetation and change the availability of resources. Stress and disturbance are distributed by local topography, which is measured with a mesotopographical index. The research was carried out by analyzing an environmental data, which was sampled in Finnish Lapland in 2011–2013. The data consists of 21 research grids that hold together 3360 research plots. A research grid is considered a region for which regional species richness was computed based on its plots' species pools. Beta-diversity was computed to every research plot as a ratio of regional and local species richness. The effect of environment on beta-diversity was studied through Spearman correlation coefficient and statistical modeling. Generalized linear models (GLM) were used to find out the best combination of stress variables in explaining the variation of beta-diversity and the change in explained variation after bringing ESPs in the model. The relational shares of stress and disturbance variables in explaining beta-diversity were studied with generalized boosted regression models (GBM). Beta-diversity is strongly distributed spatially by mesotopography. Local species richness increases and beta-diversity decreases when moving from a ridge through a slope to a valley. Mesotopography controls habitats' stress level and ESPs distribution, which is why it has such a strong impact on vegetation. Other noteworthy controllers of beta-diversity are soil moisture, wind driven erosion, known as deflation and fluvial disturbance. Deflation and fluvial disturbance are more effective in controlling beta-diversity than stress variables. It was found that stress and disturbance affect beta-diversity both in positive and negative ways. Mesotopograhy divides the environment in different parts, where there are different controllers of local habitat and vegetation. In the valley there is fluvial disturbance which is related to productive habitat and low values in beta-diversity. Fluvial disturbance lowers stress level and competition which removes the species dispersal obstacles. A large share of regional species pool has dispersed in valleys. Deflation operates on the ridge where it erodes soil and raises stress level. This prevents many species from dispersing to ridges and in doing so raises the beta-diversity. There are no strong ESPs in the slope, for which beta-diversity is controlled by variable snow cover driven soil moisture. Moist habitats have higher species richness and lower beta-diversity than dry habitats. The absence of ESPs may increase competition between plant species. This research shows that the variance of beta-diversity is driven by the descending local species richness along mesotopographical index. Beta-diversity is controlled by ESPs which also are distributed by mesotopography. Stress is a minor driver of beta-diversity.