Ferns on ferns: gametophyte assemblages on tree ferns reveal associations with accidentally epiphytic ferns

Tree ferns are substantial hosts for epiphytes, often hosting distinct communities that are considerably different from other hosts. While previous research has highlighted the prevalence of epiphytic ferns growing on tree ferns, studies have yet to explore this host relationship for the gametophyte generation. This study aims to investigate the epiphytic fern gametophyte communities growing on low trunks of Alsophila spinulosa (Cyatheaceae).

A high-throughput DNA barcoding approach was used to identify fern gametophytes collected from tree fern trunks and neighboring angiosperm hosts across six established plots. Co-occurring sporophytes were surveyed along the same trunks. Gametophyte identification was achieved by sequencing multiplexed trnL-F amplicons derived from tissue-direct PCR on an Illumina MiSeq platform.

Tree fern trunks harbored a higher abundance and species richness of fern gametophytes than angiosperm trunks, and hosted more cordiform gametophytes, most of which were otherwise terrestrial ferns considered to be accidentally epiphytic. In comparison, angiosperm hosts were associated with non-cordiform, obligately epiphytic ferns. The humid microclimate of tree fern trunks simulate a terrestrial-like environment, offering an indistinct boundary from the forest floor which provides habitat for fern species that typically establish on the ground. Our survey also recovered three species we interpret as independent fern gametophytes Antrophyum henryi., Callistopteris apiifolia, and Haplopteris yakushimensis, given that their sporophyte phases were not present and are not known from nearby localities.

DNA barcoding techniques provide an opportunity to study the distribution and habit of fern gametophytes, the life history stage of ferns which is often overlooked. Our results demonstrate that gametophyte growth form and habit are correlated with habitat preference and community structure. Our findings support the importance of incorporating gametophytes into broader studies of fern ecology and distribution.