A contribution to Gymnosphaera (Cyatheaceae) in mainland Asia: two new species, reinstatement of Cyathea bonii, and their phylogenetic positions

<p><em>Gymnosphaera</em> represents a minor lineage within the scaly tree-fern family Cyatheaceae. Tropical and subtropical mainland Asia is a main distribution area of <em>Gymnosphaera</em>. However, the species diversity of <em>Gymnosphaera</em> is currently incompletely known in mainland Asia due to lacking critical revision. Here we present new findings of species diversity and their relationships of mainland Asian <em>Gymnosphaera</em> based on field surveys, the examination of herbarium collections, and phylogenetic analyses of sequences of multiple chloroplast and nuclear regions. Two new species, <em>G. saxicola</em> from southwestern Yunnan and <em>G. bachmaensis</em> from central Vietnam, are established. Traditionally recognized <em>G. podophylla </em>is revealed to be a complex, from which <em>G. bonii</em> is reinstated as a distinct species. Our phylogenetic analyses identified four clades within <em>Gymnosphaera</em> in mainland Asia: <em>G. denticulata</em> clade, <em>G. gigantea</em> clade, <em>G. podophylla </em>clade, and <em>G. salletii </em>clade. The new species <em>G. saxicola,</em> which is special for its saxicolous habitat, was resolved as sister to <em>G. austroyunnanensis</em> in the <em>G. salletii</em> clade. The newly discovered <em>G. bachmaensis</em>, which is characterized specially by the spathulate frond, was positioned in the <em>G. podophylla</em> clade, being sister to <em>G. bonii</em>. The mountainous region from south-central Vietnam northwards to western Yunnan is a diverse center of <em>Gymnosphaera</em> and more species of this group are probably to be discovered there.</p>