Understanding plant community assembly and succession has long preoccupied ecologists, with a plethora of different theories (stochastic, deterministic and an intermediate situation of both) put forward over time. Currently, the role of historical contingency in forming alternative vegetation states is attracting increasing attention, with priority effects caused by order of arrival of different species producing long-term and significant effects on ecosystem functioning and diversity. The role of nutrient availability in modulating the strength of priority effects is an important consideration, since significant effects of order of arrival on communities may depend strongly on how many nutrients are available in the soil. The range and the effect size of these key drivers of assembly (historical contingency, nutrients) along temporal, spatial and resource related gradients are rarely addressed. The main underlying goal is to understand community assembly better to gain knowledge that can be applied in the restoration of species-rich grasslands, so that specific desired goals of an ecosystem can be met. The topic of this dissertation is the investigation of priority effects in relation to soil nutrient availability over time during assembly of semi-natural European grasslands. The focus is on both community traits and species-specific traits (including intraspecific responses of traits). The main grassland field experiment (Manuscript 2) showed that priority effects do play a role in community assembly of dry acidic grasslands, but the stability over time depended on the variables measured, with stable priority effects being found in relation to community aboveground biomass and plant functional group composition but not for community composition or species richness. The low nutrient availability negatively influenced the establishment of target species and the breadth of the priority effect was not as widely found in mesic grasslands, which suggested the important role of abiotic factors on community assembly. These results supported the findings of Sutherland (1974), that the vegetation in some ways reached a stable state in terms of the plant functional group relative composition but not in terms of species richness of species composition. Furthermore it could be shown that not the species richness of the initial seed mixture was the determining factor, but functional composition especially the plant functional group of legumes have an abiding positive effect on community traits (aboveground productivity and cover) in these grasslands, which could be also demonstrated using a new automated field measurement system (Manuscript 1). In Manuscript 3 the positive priority effect of the plant functional group of legumes on productivity found in the field, could be confirmed in a pot experiment using more nutrient-rich potting soil. In terms of productivity and functional group composition the simultaneously sown controls and the legume first treatment developed similarly. Which also underlines the importance of…

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