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 Sphaeropteris polypoda (Baker) R.M. Tryon.

Philippines • Mindanao Island. Davao Oriental: San Isidro Municipality, Mount Hamiguitan Range Wildlife Sanctuary, 622 m a.s.l., 10 October 2016, V.B. Amoroso 11213 with F.P. Coritico (holotype: PNH; isotypes: BRIT, CMUH).

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.

Figure 1. 

A the tree fern Sphaeropteris polypoda with associated vegetation. The trunks of this tree fern species serve as the substrate of Actinostachys minuta B close-up view of in-situ epiphytic habitat of A. minuta embedded in the trunk of S. polypoda with the moss Leucobryum sp.

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 Benth., Dillenia philippinensis Rolfe, Gymnostoma rumphianum (Miq.) L.A.S.Johnson, Lithocarpus spp., Pittosporum euphlebium Merr., and Shorea polysperma Merr., and tree ferns such as Alsophila lurida (Blume) Hook. and Sphaeropteris elmeri R.M.Tryon. The ground cover is dominated by other fern and lycophyte species such as Dicranopteris linearis (Burm. f.) Underw., Lindsaea gueriniana (Gaudich.) Desv., Nephrolepis biserrata (Sw.) Schott, Schizaea dichotoma, Selaginella jagori Warb., Selliguea taeniata Parris, and Taenitis blechnoides (Willd.) Sw., as well as several species of Calamus.

Philippines, Mindanao, Davao Oriental Province, Municipality of San Isidro, Mt. Hamiguitan Range Wildlife Sanctuary, 06°44'15.24"N, 126°08'59.36"E, 622 m a.s.l., 16 June 2018, V.B. Amoroso 13515 with F.P. Coritico (CMUH).

The specific epithet refers to the diminutive size of the fronds relative to the other species in the genus.

Diminutive grass fern.

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, 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.

The sorophores of A. plana and A. spirophylla come closest in length to those of A. minuta (2.8–8 mm long versus 4–6 mm). In addition to the shorter sorophores, however, our new species is distinguished from A. plana by several other features, viz., shorter (2.5–3.7 cm long) and narrower (0.8–1.0 mm) lamina (sterile portion) with distinct triangular stipe up to 8 mm long (versus longer and wider (5.5–10.2 cm × 1.4–1.74 mm) and with flattened stipe in A. plana) and longer and narrower sorophores (4–6 × 0.5–0.6 mm vs. 2.5–4.8 × 0.5–0.8 mm).

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).

Figure 2. 

Actinostachys minuta A habit with distinct stipe (st), unbranched fronds with 3 sorophores (so) attached at the tip of lamina (la), (inset) elongate rhizome (rh), with persistent gametophyte (ga) and uniseriate hairs B transection of the stipe showing flattened xylem (x) C lamina with distinct costa (co) D single row of stomata (sm) on each side of costa E elongated rhizome with long uniseriate hairs (uh) F digitate sorophores attached at the tip of the sterile lamina G bifid apical lamina of sorophore H portion of sorophore with reflexed lamina margins and biseriate sporangia, I. Monolete spore.

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.

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 km² (IUCN Standards and Petition Committee 2019).