Fitosociology is a field in vegetation science studying plant communities, trends, changes, and drivers of these changes in their species richness and composition. Its basic data consist of small-scale records (plot-level) and give exhaustive information on species composition and abundance. Annual weed and ruderal vegetation represents vegetation types found on anthropogenically disturbed sites with varying frequency, intensity, and predictability of the disturbances which (together with other abiotic and biotic factors) strongly affect its species composition and abundance. We used the so far largest dataset of phytosociological data to explore and describe patterns in annual weed and ruderal vegetation on a European scale. In the first study, we tackled the long-lasting issue of classification of European weed and ruderal vegetation for the first time on a joined dataset. Using several different classification methods, we had found that four groups best describe the variability of this vegetation and consequently proposed changes to the current classification scheme. In the second study, using two types of weed vegetation in cereal fields, we asked if the general ecological theory on the relationship between species abundance regarding the position in the distribution area holds also for weed vegetation. We found contrasting results: strong support for the theory in weed vegetation on base-rich soils and no support in weed vegetation on acidic soils. We argued that the proposed methodology we had adopted has certain flaws and that the scope of the study should be extended. In the third study, we explored one of the longest known gradients in ecology – decrease in species richness with the increasing distance from the equator. We found that alpha diversity show a much less clear pattern than gamma diversity studies. However, disturbance regimes influence the patterns and can also increase species richness at high latitudes. In the fourth chapter, we addressed the issue of neophyte species in weed and ruderal vegetation, which has been found as the most invaded in several regional studies. We found strong differences in the level of invasion between seven biogeographical regions (Atlantic region having the largest neophyte species pool) and three disturbance regimes (ruderal vegetation having the largest neophyte species pool). From all the studies we can conclude that the inherent feature of the selected vegetation type (dominant annual species and the close relation to human activities) makes it a very valuable study object since it enables us to identify exceptions in some generally accepted theories on the one hand and to solidify other theories with this extreme vegetation type on the other.
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