Interactions between plants generally are of great importance for successional processes and plant community structure and thus, research on the underlying mechanisms is essential for a attaining a comprehensive understanding in the field of vegetation ecology. In this regard, the relevance of belowground interactions has long been neglected, although they represent the dominant interaction type in several biomes of global importance, such as grassland ecosystems. In particular, the impact of the mutualistic relationship between plants and arbuscular mycorrhizal fungi (AMF) has been ignored for a long time. Although it has been revealed that AMF are involved in the nutrition of the majority of grassland plants and may have considerable influence on belowground plant-plant interactions between them, there is still only poor knowledge on the underlying mechanisms. Intensive research is particularly required on the relevance of mycotrophy (i.e., the species-specific dependency of a plant on AMF for nutrient acquisition) for the competitive ability of a plant, as well as on the mediation of plant-plant interactions via common mycelial networks (CMNs; mycorrhizal mycelia that interlink different plant individuals). The present study addresses these questions and presents the results of four controlled pot experiments in this connection. The experiments were performed on five grassland plant species of Central Europe.

In a first experiment (Ch. 2), competition between the grassland forbs Hieracium pilosella and Plantago lanceolata was investigated to test the suitability of foraging via AMF compared to foraging via roots with respect to the competitive ability. The results revealed that, although a highly mycotrophic life-style (with predominantly AMF-mediated foraging) may be a very successful trait on the individual scale, it may be a disadvantageous trait for the competitive ability as compared to a more root-mediated nutrition. Further, it is concluded that for making predictions on the outcome of a competitive interaction, both, the mycotrophy level as well as root properties of the involved plants need to be considered.

The second experiment (Ch. 3) compared differences between growth dynamics and nutrient depletion capacities of mycorrhizal hyphae and roots between the coarse-rooted forbs P. lanceolata, H. pilosella and Hypochaeris radicata, and the fine-rooted grasses Corynephorus canescens and Festuca psammophila. The results demonstrated completely contrasting foraging strategies (i.e., AMF-mediated vs. root-mediated nutrient acquisition) in potentially competing plant species. It further revealed that in highly mycotrophic plants, initiation of an adequate phosphorus (P) uptake is strictly dependent on presence of AMF. Nevertheless, the results led to the conclusion that AMF-mediated foraging may provide some (competitive) advantages over root-mediated foraging in terms of rapid exploitation of P from of bare soil patches, due to comparatively high growth rates of hyphal absorptive surface area.

CMN-effects on plant-plant interactions and the underlying mechanisms were investigated in two seedling facilitation experiments (Ch. 4 and 5), using pots with AMF-accessible, root-excluding compartments as main experimental tool to achieve a separation of CMN- from root-mediated interactions. The results showed that the main CMN-mediated facilitative adult plant effect on seedlings was an accelerated mycorrhizal colonization of seedling roots, which occurred to be particularly critical for P-uptake and seedling establishment of highly mycotrophic species. Promotion of CMN-growth should be highest in adult plants belonging to productive, highly mycotrophic species, such as P. lanceolata, which revealed as a potential 'key species' for CMN-growth. Nevertheless, high adult plant carbon-investment into a CMN did not reduce the CMN-costs to seedlings, but, in contrast, rather increased them. Further, the two facilitation experiments revealed that (root- and CMN-mediated) competitive pressure by adult plants may overlay any facilitative effects, resulting in net neutral or negative effects on seedling growth. Net CMN-mediated seedling facilitation might be highest when pronounced mycelium growth is combined with low competitive pressure by the adult plant, as e.g., exhibited by the highly mycotrophic forb H. pilosella.

Summarizing, this study demonstrates the generally high relevance of mycorrhizal parameters for plant-plant interactions and emphasizes the pronounced species-specificity of mycotrophy levels and CMN-effects. Regarding these factors, this study gives some new insights into the mechanisms underlying AMF-effects on plant-plant interactions.