Low and facultative mycorrhization of ferns in a low-montane tropical rainforest in Ecuador

Arbuscular mycorrhizal fungi (AMF) are amongst the most studied plant symbionts and regularly found in terrestrial plants. However, global estimates of AMF abundance amongst all land plants are difficult because i) the mycorrhizal status of many non-commercial, wild plant species is still unknown ii) numerous plant species engage in facultative symbiosis, meaning that they can, but do not always do, associate with mycorrhiza and iii) mycorrhizal status can vary amongst individuals of one same plant species at one location, as well as for different plant species within a given genus or family. To gain new insights to the pristine distribution of the plant-AMF symbiosis, we investigated the mycorrhizal status of one of the oldest lineages of extant vascular plants, Polypodiophyta (aka ferns), in one of the hotspots of natural plant diversification, the tropical rainforest. Providing a new data set of AMF abundance for 79 fern species, we hypothesized that (1) AMF would be found in 60-80% of the studied plants and (2) plant species with AMF symbionts would be more abundant than non-mycorrhizal species. Both hypotheses were rejected while unexpected observations were made: (1) AMF occurred in 33% of studied species, representing 56% of the studied fern families, (2) AMF colonisation was not correlated with species abundance, (3) a small but significant proportion of AMF-positive ferns was epiphytic (7%) and (4) as expected, mycorrhization was inconsistent among different populations of the same species. Together this empirical data supports recent reservations regarding global abundance of AMF, and further demonstrates that mycorrhization is not a taxonomic trait. In addition, the occurrence of AMF in epiphytic plants and no net benefits of AMF for plant abundance indicate that the mycorrhization observed here is on the commensalism, possibly parasitism, side of the symbiosis spectrum. HighlightsO_LISmall fraction of fern species mycorrhized (33% species level, 56% of families) C_LIO_LIMycorrhization of species can vary with location (facultative symbiosis) C_LIO_LIAMF colonisation does not increase plant species abundance C_LIO_LIAMF presence in epiphytic ferns suggests commensalism / parasitism C_LI