Actinostachysminuta, a new species of grass fern from Mindanao, Philippines

Abstract Actinostachysminuta Amoroso & Coritico (Schizaeaceae), from Mindanao, Philippines, is described herein as a new species. This species is distinguished from all other species of Actinostachys (grass ferns) by its notably short and narrow fronds, distinct triangular stipe, and bifid apex of the sorophore lamina with profuse white long hairs. This species is distinct from the other known Philippine species of Actinostachys by its diminutive epiphytic habit and a habitat restricted to the trunks of the tree fern Sphaeropterispolypoda (Baker) R.M.Tryon. A taxonomic key to the species of Philippine Schizaeaceae that incorporates the new species is provided.


Introduction
The fern family Schizaeaceae comprises two genera, Actinostachys and Schizaea, and ca. 30-35 species widely distributed in tropical and south-temperate regions (PPG I 2016;Smith et al. 2006;Chen et al. 2017). In the Philippines, the family is represented by two species of Schizaea: S. dichotoma (L.) Sm. and S. malaccana Bak., and two species of Actinostachys: Actinostachys digitata (L.) Wall. and A. inopinata (Selling) C.F.Reed (Barcelona et al. 1996). Of these, S. dichotoma and S. malaccana are distinctive by having the sorophores attached pinnately to an elongate axis. Schizaea dichotoma is easily recognized by the dichotomously branching sterile portions of the frond, whereas S. malaccana has an unbranched sterile portion. The two species of Actinostachys, A. digitata and A. inopinata, both have digitately arranged sorophores and are differentiated by the relative width of their lamina and by the number of sporangial rows (4-seriate or biseriate, respectively) (Holttum 1955; Barcelona et al. 1996). Moreover, the different species of Schizaea and Actinostachys are classified into sub-genera and sections based on the exospore (smooth or striated to pitted) and spore size (Reed 1947).
All four Philippine species of the family Schizaeaceae are reported in Mount Hamiguitan Range Wildlife Sanctuary (MHRWS), a protected UNESCO World Heritage Site (Amoroso et al. 2016;Amoroso et al. 2018). During fieldwork in MHRWS in 2016 and 2018, we encountered unusual individuals of Actinostachys growing on the trunks of tree ferns. On careful examination of these plants, available type images from JSTOR Global Plants, and in consultation of the literature, we conclude that they represent a species new to science. Here we describe this new species and provide detailed photographs of it along with a key to the five Philippine species of Schizaeaceae, all of which are found in MHRWS.

Taxonomy
Actinostachys minuta Amoroso & Coritico, sp. nov. urn:lsid:ipni.org:names:77209910-1 Figures 1, 2 Diagnosis. This new species Actinostachys minuta is most similar to Actinostachys plana (Fourn.) Reed but differs by its shorter and narrow fronds with a distinct triangular stipe, sorophore lamina longer and narrower with white long hairs and sorophores 1-4 but usually 1. It differs from the other four Philippine species by its restricted epiphytic habit on the trunk of the tree fern Description. Epiphytic on trunks of the tree fern Sphaeropteris polypoda with rhizome embedded between adventitious roots. Rhizome: short-creeping to erect, black, becoming elongate, attached to the persistent tuberous gametophyte with profuse, long, uniseriate, pale brown hairs. Fronds: crowded, pendulous, grass-like, unbranched, up to 3.0-4.5 cm long; stipe distinct, black, oblong to triangular in transection, 5-8 mm long, with short scattered glandular hairs, with a few large cortical sclerenchymatous cells, vascular tissues reduced with single flattened xylem strand; lamina (sterile portion) simple, unbranched, flattened, up to 2.5-3.7 cm × 0.8-1.0 mm wide, margin entire, adaxial surface with distinct costa and scattered uniseriate hairs, the basal cells of the hairs persistent and forming scattered warts and disappearing distally; stomata arranged in one row (uniseriate) on each side of costa; sorophores 1 to 4 per frond but mostly 1, sessile or attached by a short stalk at apex of lamina, digitately arranged, 4-6 mm long; laminae of sorophores covered with profuse white long hairs adaxially, margin entire, apex bifid; sporangia in 2 rows, nearly symmetrically arranged, completely covering abaxial surface and protected by reflexed edge of sorophore lamina, sessile, ellipsoidal, with distal annulus, surface striated, glabrous; spores monolete, smooth.
Distribution and habitat. This species is currently known only within the buffer zone located outside the boundaries of MHRWS in San Isidro Municipality, in shaded habitat in lowland tropical rainforest at 622 m a.s.l. It grows strictly as an epiphyte on trunks of the tree fern Sphaeropteris polypoda with rhizomes embedded between adventitious roots in association with the moss genus Leucobryum and has not been observed terrestrially. The vegetation surrounding Sphaeropteris polypoda with Actinostachys minuta consists of trees 20-30 meters tall, including Canarium asperum Etymology. The specific epithet refers to the diminutive size of the fronds relative to the other species in the genus.
Suggested common name. Diminutive grass fern. Notes. The traditional treatment of Schizaeaceae includes all species with digitately (connate) and pinnately arranged sorophores in Schizaea as in Barcelona et al. (1996), Barcelona (2011), Holttum (1955 and Kramer (1990). Here we follow the classification of PPG 1 (2016) segregating them into two genera Schizaea (species with pinnately arranged sorophores) and Actinostachys (species with digitately arranged sorophores) and by less easily observed features of their gametophytes (tuberous in Actinostachys versus filamentous in Schizaea) (Bierhorst 1968(Bierhorst , 1971) and based on phylogenetic evidence (Labiak and Karol 2017;Wikstrom et al. 2002). The size of the fronds and sorophores is used in identifying the species of the family Schizaeaceae (Barcelona et al. 1996). However, the length of the sorophore is a much more reliable character than the length of the whole frond because the short sorophore is much less influenced by environmental conditions than the whole frond (Brownsey and Smith-Dodsworth 2000). In this respect, the new species and the closest related species (A. plana) differ from all other species of Actinostachys in its very short sorophores, about 2.5-6 mm long.
We compare the new species to three other digitate species of Actinostachys that are most similar to it in morphology based on the published descriptions of Bierhorst (1968), Holttum (1955), Barcelona et al. (1996), Reed (1947) and Sofiyanti et al. (2019), as well as examinations of JSTOR type images viz., A. plana, A. spirophylla, and A. wagneri. Our new species is closest morphologically to A. plana, followed by A. spirophylla Troll, and A. wagneri Sell. (Table 1). Actinostachys minuta shares an epiphytic habitat with A. plana, A. spirophylla and A. wagneri, growing in moss cushions on tree fern trunks, although Holttum (1955) did not explicitly mention tree fern trunks but simply trees.
It is interesting to mention that we found a persistent gametophyte in our new species as also reported by Bierhorst (1968) in Actinostachys oligostachys Bierh., A. melanesica C.F. Reed, A. intermedia (Mett.) C.F. Reed, and A. laevigata (Mett.) C.F.Reed. The tuberous gametophyte is attached to the well-developed sporophyte ( Fig. 2A).
All five species of Philippine Schizaeaceae are found in MHRWS in shaded forest. Except for A. minuta which is epiphytic on the trunks of tree ferns, the other four species are terrestrial, inhabiting an ultramafic soil with fallen leaves of Gymnostoma rumphianum.
Conservation status. Although MHRWS is a protected area, we only have observed this species from the type locality. The species occurs within the buffer zone of San Isidro, MHRWS with an estimated number of 30 individuals growing strictly on trunks of tree ferns. Its location in the buffer zone and the over-collection of tree fern trunks as a medium to grow other plants and for the making of handicrafts will likely reduce the populations of the species if this threat continues. Thus, we recommend listing the species as Critically Endangered (CR) based on its very small and restricted population with ≤ 50 mature individuals and the extent of occurrence estimated to be < 10 km 2 (IUCN Standards and Petition Committee 2019).
Key to the genera and species of grass ferns (Schizaeaceae) from the Philippines