Mycorrhizal colonizations of ferns and lycophytes on the island of La Reunion in relation to nutrient availability

Very little is known about the degree and ecological correlates of mycorrhization of lycophytes and ferns, although mycorrhizae are believed to be crucial for the majority of land plants. We screened the degree of mycorrhizal colonization for all 75 fern species recorded in nine survey plots across a wide range of soil conditions on the island of La Reunion, including five plots on lava flows of different age. Overall, 66% of all samples had mycorrhizae, but ground-dwelling species had significantly higher colonization levels (78%) than epiphytes (58%). Among ground-dwelling species, 98% of the individuals belonged to species with mycorrhizae, which was significantly higher than at the species level. Most samples had glomeromycetes as fungal partners, but septate endophytes were found in 41% of the colonized samples. While we found no significant relationship between the percentage of fern species with mycorrhizae per plot and seven ecological parameters, the percentage of fern individuals with mycorrhizal colonizations per plot significantly increased on shallow soils with high pH values and high base-saturation. This supports the idea that mycorrhizal colonizations confer an ecological advantage to colonized individuals, and that this advantage is more pronounced on nutrient-deficient sites. Our study thus provides evidence for an ecological advantage of mycorrhizae for ferns, but raises the question why, despite this advantage, species level mycorrhization among ferns (68%) is so much lower than the average for land plants (85%).