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

Abstract

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, associate 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 abundance 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 symbionts 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 observations 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.

Fig 1. Location of the study area in the lowlands of the Eastern Andes in Ecuador in the province Morona Santiago (via [41]).

Fig 2. Abundance of AMF amongst 19 fern families of the Ecuadorian low-montane tropical rain forest.

Number in brackets behind family name indicates number of samples investigated within each family, total number of samples n = 97, corresponding to n = 82 species. Dotted lines indicate average root colonisation with AMF across all investigated fern species (30.5%) and across all investigated fern families (63%) respectively.

Fig 3. Relationships between (A) AMF colonisation and plant habitat (terrestrial, hemi-epiphytic or epiphytic), (B) Relative abundance of fern species and their AMF colonisation (mycorrhizal, non-mycorrhizal, sometimes mycorrhizal (facultative)), (C) Fern α-diversity at plot level, soil exchangeable cations (CEC) and degree of AMF-colonisation (indicated by the size of the circle), (D) Fern α-diversity at plot level, soil pH and degree of AMF-colonisation (indicated by the size of the circle), (E) Fern α-diversity at plot level and degree of AMF-colonisation and (F) Soil pH (triangles), cation exchange capacity (circles) and degree of AMF-colonisation.