The genus Amblovenatum was first recognized by J. P. Roux in 2009 (Roux, 2009). It consists of 12–15 species distributed in India, Australia, Malesia, and the Pacific islands, especially in New Guinea (Holttum, 1971). So far, the genus was known by a single species A. terminans in Vietnam. However, comprehensive research including nomenclatural review and critical study of the type specimens was awaited to confirm the actual number of species for the genus.

During our routine survey work in Gia Lai province, north highland of Vietnam, we collected several specimens of Amblovenatum. A comparison was made between those with morphologically similar A. terminans (Table 1). On the basis of available literature (Holttum, 1977; Loc, 2001; Roux, 2009), and careful investigation of specimens of Amblovenatum in the herbaria of Vietnam Academy of Science and Technology (HN) and Taiwan Forestry Research Institute (TAIF), our specimen was identified as A. immersum (Blume) Mazumdar, the first record for Vietnam (Fig. 1). While checking TAIF specimens, we found some specimens of A. immersum collected from Lam Dong province (Vietnam), however, they were never been critically examined and documented for its occurrence in Vietnam. Here, we provide a detailed description, habitat, distribution, color photographs, illustration and comparison table for easy identification of Amblovenatum immersum.

The newly recorded species was collected at Kon Chu Rang Nature Reserve in Vietnam. The specimens collected from the natural habitats were thoroughly processed using standard herbarium techniques (Jain and Rao, 1977) and deposited in HN. Specimens housed in TAIF, K, L and P (acronym follows Thiers, 2020) were also scrutinized. Morphological characters were studied by consulting the relevant literature (Holttum, 1971, 1977; Tagawa and Iwatsuki, 1988; Roux, 2009; Mazumdar, 2017).

Amblovenatum immersum (Blume) Mazumdar, Int. J. Adv. Res. Innov. Ideas Educ. 3: 5960, 2017; Aspidium immersum Blume, Enum. Pl. Javae 2: 156, 1828; Thelypteris immersa (Blume) Ching, Bull. Fan Mem. Inst. Biol. Bot. 6: 306, 1936; Amphineuron immersum (Blume) Holttum. in B. K. Nayar & S. Kaur, Companion Handb. Ferns Br. India 203, 1974. (Figs. 2–4).—TYPE: INDONESIA: Java, C. L. Blume s.n. (holotype: L, isotype: K, K000548430, photo!).

Plants 2.6–3 m tall. Rhizomes strong, woody, and ascending. Fronds approximately 2 m long; stipes nearly 1 m long, ca. 1 cm in diam., bases with thick lanceolate scales, distally with sparse short setae, dark stramineous; laminae oblong-lanceolate, 150–190 × 55–60 cm, bases nearly tapering or sometimes proximal pair of pinnae slightly shortened, pinnate-pinnatifid, acuminate and pinnatifid at apices, papery, when dry grass-green, pinnae abaxially ± with yellow spherical small glands along veins, elsewhere glabrous, adaxially along grooves with dense grayish white fine and long acicular hairs and along veins sparsely shortly hairy; pinnae mostly subopposite, spreading, sessile; middle pinnae linear-lanceolate, 25–35 × 2–3 cm, bases symmetrical, truncate, pinnatifid and reaching both narrow wings of costae, lobed to 1 mm from costa or deeper; lobes separated by wide sinuses, almost at right angles to costa, not falcate, distal lobes more oblique; apices caudate-acuminate; segments 60–70 pairs, flatly spreading, pectinately arranged, linear, 8–14 × 2–3 mm, entire, rounded or acute at apices. Veins evident, lateral veins simple, 10–14 pairs per segment, proximal pair arising from base of costules, all reaching margins above sinuses or basal acroscopic veinlet close to sinus. Sori orbicular, medium-sized, dorsifixed at middle of lateral veins, 8–10 pairs per segment; indusia orbicular-reniform, somewhat leathery, glabrous, sulfur-colored glands along margins, persistent. Sporangia with slender stalks which bear short hairs with glandular tips like the glands on indusia. Spores orbicular-reniform, perispore echinate.

Distribution: Vietnam (Gia Lai and Lam Dong Provinces), S. China (W. Hainan), Malesia, N. Queensland, New Hebrides, New Caledonia, Loyalty Islands (Fig. 1).

Ecology, habitat, and conservation status: Amblovenatum immersum grows on slopes, secondary forests, roadside, and exposed disturbed areas at an elevation range of about 800 m a.s.l., in association with Glaphyropteridopsis erubescens (Wall. ex Hook.) Ching, Pronephrium megacuspe (Baker) Holttum, Pneumatopteris truncata (Poir.) Holttum, Christella dentata (Forssk.) Brownsey & Jermy, Ficus sp., Lithocarpus sp., Madhuca sp., Musa sp., Dioscorea sp., Lygodium sp., Polystichum sp., and Chromolaena sp. We found its distribution in Gia Lai province consisting of at least 10 to 15 mature individuals in >10 populations. Its distribution in Bao Loc Pass in Lam Dong province is documented here based on the specimen record at the Herbarium of Taiwan Forestry Research Institute (TAIF). Further field investigations are needed to confirm its occurrence in other areas of Vietnam. In accordance with the IUCN Red List Categories (2012, ver. 3.1), we propose a temporary listing of this taxon under the Data Deficient (DD) category.

Specimens examined: VIETNAM. Gia Lai Province: Kbang District, Kon Chu Rang Nature Reserve, 14º31′11″N, 108º36′31″E, elev. 870 m, 6 Sep 2018, Son H. D. Son 75 (HN). Lam Dong Province: Da Huoai District, Bao Loc pass, 28 Jun 2018, elev. 407 m a.s.l., Cheng-Wei Chen et al. 5182 (TAIF).