Colonization processes and seed bank dynamics in flood meadows and their implications for ecological restoration
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Flood meadows, typical plant communities of large lowland river valleys, have declined drastically across Europe during the last decades and now belong to the most threatened plant communities. This thesis deals with the ecology and restoration of flood meadows in the Northern Upper Rhine Valley in south-western Germany. Its main objectives were (1) to evaluate the success of plant material transfer as a restoration tool and (2) to explore the factors governing soil seed bank dynamics in flood meadows. The thesis comprises a total of four studies that deal with different aspects of the topic. The first was a case study with the flood-meadow plant species Arabis nemorensis. Its objective was to evaluate the success of grassland restoration via plant material transfer by comparing restored and remnant sites with regard to population dynamics and spatial patterns. The results show that plant material transfer rapidly triggered the formation of spatially-structured populations that closely resembled those of remnant sites. The overall habitat structure of the restoration sites - which may determine whether they will also provide suitable habitat conditions for other taxa - rapidly approximated that of remnant sites in the course of early post-restoration succession. The second study dealt with vegetation development on restoration sites 7-8 years after plant material application. Its aim was to test the assumption that plant material strips act as colonization initials for transferred species and to evaluate whether it is feasible to restore entire sites by spatially-restricted plant material application. It turned out that transferred species, which were absent from the sites until restoration began, now occurred in the above-ground vegetation, the seed rain and the soil seed bank on and adjacent to the plant material strips. Almost 90 % of all species that had established on the strips had also spread into their surroundings. Apparently, the strips really acted as colonization initials for flood-meadow restoration, i.e. the implicit assumption underlying the layout of plant material transfer seems to hold. The first two studies also gave evidence for rapid seed bank formation during early post-restoration succession, and it seems likely that the restoration measures will eventually result in the establishment of self-sustaining populations of the target species. Vertical seed translocation occurred particularly rapidly, which could at least partially be explained by seed entrapment due to desiccation cracks. Such cracks regularly form on the study sites during periods of extended summer drought, when desiccation causes the fine-grained alluvial soils to shrink and thus leads to the formation of distinctive cracks. The third study presented in this thesis comprises empirical and experimental investigations on the impact of desiccation cracks on soil seed bank formation and dynamics. Its results show that the cracks act as natural seed traps and contribute to a rapid incorporation of shed seeds into the soil. Mapping of crack patterns in consecutive dry periods revealed that the cracks were at least short-term spatially constant, which has important consequences for their trapping potential: If cracks frequently open in the same positions they can accumulate a large number of seeds, which may result in a clumped distribution of the soil seed bank, as indeed found in the case study with A. nemorensis. The implications of seed entrapment by desiccation cracks were investigated by means of two burial experiments, which are presented in the third and fourth study compiled here. Seeds of five herbaceous flood-meadow species were buried in different depths and partly also different substrates and tested at regular intervals. It turned out that the likelihood of survival during burial generally increased with increasing depth. This indicates that the seeds possess depth-sensing mechanisms that lead to germination inhibition when they are buried so deep that successful seedling emergence would be unlikely. Furthermore, seed fate differed pronouncedly between species and seemed to be connected with seed size: Smaller-seeded species had a higher likelihood of survival during burial than larger-seeded species, whereas the latter were more likely to emerge successfully after germination in greater depths. Substrate type did not have a pronounced influence on the fate of seeds and seedlings. These results indicate that desiccation cracks may favour the development of long-term persistent soil seed banks and could thus indirectly influence the composition of the plant community. All in all, the results presented in this thesis show (1) that plant material transfer is a suitable method for flood-meadow restoration and (2) that seed bank dynamics in this disturbance-prone ecosystem are strongly influenced by abiotic conditions such as desiccation cracks.Verknüpfung zu Publikationen oder weiteren Datensätzen
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Restoration Ecology, DOI: 10.1111/j.1526-100X.2010.00668.x; Biological Conservation, DOI: 10.1016/j.biocon.2010.08.018; Plant and Soil 333, 2010: 351-364; Seed Science Research 20, 2010: 189-200