[en] Arbuscular mycorrhizal fungi (AMF) are amongst the most studied obligate 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, asso-
ciate with mycorrhiza, and iii) mycorrhizal status can vary within families, genera, and
species. To gain deeper insights to the distribution of the plant-AMF symbiosis we
investigated the mycorrhizal status in some of the oldest lineages of extant vascular
plants, Polypodiophytina (ferns) and lycophytes, in one of the hotspots of natural
plant diversification, the tropical rainforest. Providing a new data set of AMF abun-
dance for 82 fern species representing 19 families, we hypothesized that (1) AMF
would be found in 60–80% of the studied plants and (2) plant species with AMF sym-
bionts would be more abundant than non-mycorrhizal species. Both hypotheses were
rejected while the following observations were made: (1) AMF occurred in 30.5% of
studied species, representing 63% of the studied fern families, (2) AMF colonisation
was not correlated with species abundance, (3) a small proportion of AMF-hosting
ferns was epiphytic (6%) and (4) mycorrhization was inconsistent among different
populations of the same species (facultative mycorrhization). While these observa-
tions align with previous studies on ferns, they emphasise that mycorrhization is not
a taxonomic trait and underscore the challenges in estimating the global abundance
of AMF. In addition, the occurrence of AMF in epiphytic plants and no net benefits of
AMF for plant abundance indicate that the mycorrhization observed in this study likely
comprises the commensalism to parasitism range of the symbiosis spectrum.
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