Plant-plant interference is inherently local and seed dispersal generally limited. Bothudprocesses generate spatial and genetic structure within plant populations and communities thatudneed to be better understood in order to predict dynamic community changes due for exampleudto biodiversity loss or global change. There is increasingly strong theoretical evidence thatudspatial pattern is an essential factor controlling the species dynamics of many communities. Inudparticular, one conclusion from spatial models is that intraspecific aggregation promotesudcoexistence by slowing down competitive exclusion. Whereas local interactions contribute toudinterspecific segregation, limited seed dispersal leads to aggregation at two hierarchicaludlevels: i) species within communities and ii) genetically related individuals (e.g. siblings)udwithin populations. However, especially for plant communities there is a need forudexperimental tests of the predictions generated from spatial models.udThe principal goal of this thesis was to narrow the gap between theoretical andudempirical investigations on the role of spatial pattern in plant communities and populationuddynamics. I focused on the effects of spatial pattern on the dynamics of experimental plantudcommunities at the level of species as well as at the level of genotypes within species. Inudparticular, I (i) manipulated the spatial pattern, i.e. the relative frequency of intra- vs.udinterspecific contacts and (ii) contrasted the performance of genetically related (half-sibs) vs.udnon-related individuals. The basic goal of the experiments was to investigate whetheruddifferent spatial patterns (random vs. aggregated) and relatedness of neighbors had any effectsudon population dynamics within experimental plant communities.udThe experiments provided interesting results and showed essential aspects of the roleudof intraspecific aggregation and sibling interference in regulating the dynamics of populationsudwithin experimental plant communities. I showed that weak competitors increased theirudfitness (e.g. biomass and seed production) when grown in neighborhoods of conspecificsudcompared to neighborhoods of heterospecifics, at least in the short run. The data furtherudsuggested that the advantages of intraspecific aggregation for weaker competitors might beudindependent of the species identity and that all other species are best avoided.udAn additional aggregation at the level of genotypes (e.g. seed families) suggested speciesspecificudeffects linked with seed size. For instance, I found negative sibling competitionudeffects for the small-seeded species (Capsella), while rather positive effects for the largeseededudspecies (Stachys). Negative effects of sibling competition were also observed amongudrelatives of sunflower seed families. By contrast, genetically similar individuals of the dimorphic species Senecio jacobaea increased their fitness (e.g. biomass) compared toudgenetically dissimilar individuals. However, also this species suggested seed traits specificudrelatedness effects (e.g. dispersal ability). Positive relatedness effects were more evident byudseeds expected to aggregate more locally (without pappus) than by seeds expected to disperseudwider (with pappus). Generally, I observed lower size variation (measured as coefficients ofudvariation) among related compared to non-related individuals. This might be a consequence ofudmore genetic uniformity and / or kin selection among relatives compared to non-relatives.udAlthough, I could not provide strong evidence for sibling competition or kin selection, Iudbelieve that relatedness among plants, especially for species with highly localized dispersal,udshould play a considerable role in the regulation of local population dynamics. Similar to theudspecies level, there must be subtle trade-offs (e.g. between neighbour relatedness and density)udthat determine the complicated local dynamics of plant communities. However, the questionudunder which circumstances and to which extent relatedness effects are species-specificudremains open and deserves further investigation.udAt the level of species, effects of intraspecific aggregation on the dynamics ofudexperimental plant communities were clear and consistent throughout my experiments. Byudcontrast, at the level of genotypes, they were less clear and to some extent contrasting. Thisudemphasized the importance for further investigations on population dynamics at levels belowudthat of species.udFrom an applied point of view, findings of this thesis might help to give better information forudmanagement practices (e.g. restoring species rich communities). For example, by varyingudspatial pattern (random vs. intraspecifically aggregated) of selected species in wildflowersudstrips or fallows, the dominance of undesired species (e.g. Dipsacus sp.) and the exclusion ofudweaker species can be delayed.
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