Thesis #1017

One of the great challenges in ecology is to explain how large numbers of plant species are able to coexist in natural communities. The role of spatial structure for maintaining plant coexistence has so far mainly been explored by theory. Spatial structure involves two main processes: dispersal and competition. Competitive interactions between plants occur over relatively small spatial scales. Spatially limited dispersal together with local interactions can result in individual neighbourhoods much different from mean population densities. Theory suggests that seed dispersal may contribute substantially to population dynamics and plant coexistence. However, additional processes affect the survival and fitness of established individuals, and the consequences of seed dispersal for local community dynamics are still under-explored. Individual-based models examine population dynamics by modelling survival and growth for each individual separately. As a consequence, assumptions have to be made about the distances over which neighbourhood interactions occur and how these attenuate with distance. Theory has shown that a competitively weaker species can invade a population of a superior species if the average distance at which conspecifics compete is longer than the average distance at which heterospecifics compete (heteromyopia). However, empirical knowledge on the spatial scales of competition lags behind, and heteromyopia has not been reported so far. Arbuscular mycorrhizal fungi (AMF) are symbiotic fungi that colonise the roots of most plant species and enhance their hosts' nutrient supply. Increasing evidence for host-specificity and the fact that AMF can connect the roots of many plant species suggest that they might be involved in the spatial scales of competiton. In my thesis I experimentally tested basic model assumptions and theoretical predictions on how dispersal and competition may contribute to maintain plant species coexistence. In a field experiment, I examined the consequences of seed dispersal distance for spatial pattern and local population dynamics of the perennial forb Prunella grandiflora. I found that only individuals in the vegetative but not in the reproductive stage responded to dispersal manipulation. Increasing dispersal distance lead to more vegetative individuals, and decreasing dispersal distance resulted in a more aggregated spatial distribution. In two target-neighbour competition experiments I tested for heteromyopia in co-occurring forbs from calcareous grasslands. I explored the spatial scales of intra- and interspecific competitive interactions, how these attenuate with distance and the role of AMF therein. Although the distances over which intra- and interspecific competition could be detected varied substantially, I found no evidence for hetereomyopia. AMF neither influenced the distances over which competition occurred nor how the strength of competition declined over distance. However, AMF reduced the effects of relative size differences between neighbouring plants. The intensity of competitive interactions was primarily determined by relative size differences between target and neighbour plants, irrespective of their con- or heterospecific status. However, a conspecific neighbour may be more important than a heterospecific neighbour but only as the neighbour becomes very large compared to the target individual (size-identity interaction). Finally, I also tested for within-population host-specificity of genetically different isolates of Glomus intraradices. The different AMF isolates altered plant biomass and differed in their efficiency to colonise plant roots. Interestingly, plant species differed substantially in their susceptibility to different functional differences between these isolates, and this seemed to be positively linked to the percentage root colonisation. The results of my thesis emphasise the importance of both dispersal and competition as spatial mechanisms promoting plant coexistence and point towards novel aspects of AMF in spatial plant ecology. I could confirm theory in that dispersal affects local population dynamics of natural plant communities - at least in the short run. From my target-neighbour experiments, I conclude that resource competition and AMF can be ruled out as potential mechanisms for heteromyopia. My results suggest