Occurrence of non-mycorrhizal plant species in south Swedish rocky habitats is related to exchangeable soil phosphate

1 Plant species that do not typically form mycorrhiza are most likely to be successful under conditions where mycorrhizal fungi are not important for plant coexistence or where the costs of symbiosis outweigh the benefits. The relative occurrence of non-mycorrhizal species was investigated in relationship to exchangeable soil P in herbaceous vegetation, where arbuscular mycorrhizal (AM) associations are generally common. We investigated a total of 439 sites in rocky habitats with sparse vegetation and 110 meadows with dense vegetation. 2 In both rocky habitats and meadows, soil pH was inversely related to exchangeable P. In rocky habitats plant species richness increased greatly between pH 3 and 5 and was also inversely related to P. 3 Plant communities in rocky habitats contained a relatively larger proportion of non-mycorrhizal species than those in meadows. More non-mycorrhizal species occurred at high soil P in the rocky habitats, but no such relation was found in meadows. 4 Non-mycorrhizal species in rocky habitats were most common at low soil pH (high P availability). Plant species that thrived at extreme soil pH were often non-mycorrhizal. 5 The occurrence of fewer non-mycorrhizal plants in meadows than in rocky habitats supports the hypothesis that AM associations are more important in ecosystems with intense competition among plants. In rocky habitats, where abiotic stress may restrict photosynthesis more than nutrient limitation does, it is adaptive for plant species to utilize strategies other than mycorrhiza, particularly at low pH sites where P availability is likely to be adequate.