Nephrolepis hirsutula
(sword fern)

Nephrolepis hirsutula is a fern native to Asia and the Pacific, and is not recorded as having spread outside of this region. N. hirsutula is reported as an invasive species is in Cuba, however this is due to a misidentification of Nephrolepis brownii. N. brownii, which is considered an aggressive invasive species, is an Asian fern species that has been introduced in the New World. Within its range in Southeast Asia, Australia and the Pacific, N. hirsutula is not considered invasive.

According to Hovenkamp and Miyamoto (2005) N. hirsutula occurs in the southeast of Asia, throughout Malesia, to Australia and the Pacific, and distribution records elsewhere are due to a misidentification of N. brownii. The species has been wrongly reported as occurring in North America, Central America, South America and the Caribbean (Oviedo Prieto et al., 2012; García-Lahera, 2016; Encyclopedia of Life, 2018; Missouri Botanical Garden, 2018; USDA-ARS, 2018).There is some debate about the extent of the native region of the species at some of the Pacific region, being regarded as both native and introduced in Hawaii (Moomaw and Takahashi, 1960; Vernon and Ranker, 2013)

The wrongful attribution of synonyms is also adding to the confusion in the distribution reported for N. hirsutula. For example, in Missouri Botanical Garden (2018) some of the synonyms listed for N. hirsutula are synonyms of N. exaltata or N. brownii. In USDA-ARS (2018) GRIN online database N. multiflora is used as a synonym for N. hirsutula.

Nephrolepis hirsutula is recorded as as an invasive species in Cuba by Oviedo Prieto et al. (2012), but this is due to a misidentification of N. brownii, which is considered an aggressive invasive species and has been introduced in the Caribbean (Kairo et al., 2003). N. brownii is not listed as invasive by Oviedo Prieto et al. (2012). The authors cite Florida Exotic Pest Plant Council (2009) for N. hirsutula as being invasive in Florida, USA. Neither the FLEPPC invasive plant species list cited or the most recent one for 2019 (Florida Exotic Pest Plant Council, 2019) list N. hirsutula as an invasive species for Florida. The Nephrolepis species appearing in the list are N.brownii (= N. multiflora) and N. cordifolia. In an invasive report for N. hirsutula in Cuba, Caluff and Regalado (2011) report the synonyms of the species as being N. multiflora and Davallia multiflora, which are synonyms of N. brownii (Hovenkamp and Miyamoto, 2005; The Plant List, 2013). 

Without further details about the biology, the environmental requirements and the possible effects over non-native habitats, it is difficult to assess the risk of introduction. However, with the use of other Nephrolepis species as ornamentals, their spread facilitated by its stoloniferous vegetative reproduction and the easiness of spore dissemination it is highly possible that N. hirsutula could spread from its native area and slowly progress its way into suitable areas.

N. hirsutula is reported as growing in grasslands, forest edges, open forests, ridges, scrublands, rainforests, and low elevations (Ridley, 1926; Kingston and Waldren, 2003; Banaticla and Buot, 2005; Bell, 2017). It is also reported at margins of freshwater lakes in Papua New Guinea (Chambers, 1987).

Genetics

Molecular work shows that N. hirsutula is more closely related to N. biserrata than to N. brownii (Yahaya et al., 2016). The authors also report the species as being a tetraploid (2n= 82, 164), although Manton and Sledge (1954) report the species as being diploid (n=41).

Nephrolepis hirsutula has been suggested as one of the parent species of the hybrid Nephrolepis exaltata var. bostoniensis (Encyclopedia of Life, 2018). However molecular studies by Yahaya et al. (2016) found that N. hirsutula did not contribute to this hybrid species.

Reproductive Biology

Although no details are given for the reproduction of N. hirsutula, it is highly likely this species reproduces vegetatively through stolons or by spores, as most of the other Nephrolepis species (Yahaya et al. 2016).

Environmental Requirements

There is little detailed information available for the environmental requirements of N. hirsutula. The species grows on clay banks near water or in crevices of rock outcrops in Australia (Bell, 2017; 2018). In Papua New Guinea it is reported as growing in lowlands, in infertile, serpentine soils and in organic alluvium; with a mean annual rainfall of 3000 – 4000 mm (Takeuchi, 2003).

In Guangdong, China, it is reported in red soils in subtropical areas with an annual average temperature of 22°C and an annual rainfall of 2380 mm (Wang et al., 2006). In Hawaii, the species is reported as growing in nutrient poor soils, gravelly silty clay and volcanic soils with high infiltration rate; in an average temperature of 23°C, with a range between 13°-90°C; and in an annual precipitation of 1250 to 3810 mm with a relative high humidity (Moomaw and Takahashi, 1960).

Environmental

• Amenity
• Wildlife habitat

General

• Sociocultural value

Human food and beverage

• Vegetable

Materials

• Mulches

Medicinal, pharmaceutical

• Traditional/folklore

Hovenkamp and Miyamoto (2005) provide keys for the identification of the Nephrolepis species. N. hirsutula is mostly confused with N. brownii. N. brownii has the sori close to the margin while in N. hirsutula the sori are less close to the margin to nearly medial. N. hirsutula has dense, rufous rachis scales with a very strongly dentate acumen. N. brownii has usually sparser, paler rachis scales, with a longer, nearly always entire narrow apex. The upper costae in N. hirsutula is glabrous.

N. hirsutula can be distinguished from N. biserrata as the latter is not densely rusty wooly, but scaly. N. cordifolia and N. exaltata pinnules are oblong to oblong-lanceolate and acute to round at the apex, while N. hirsutula pinnules are linear-lanceolate, and attenuate toward the apex (Fosberg, 1943).

Molecular studies are needed to assess generic and family delimitations in ferns (Smith et al., 2006; Christenhusz et al., 2011). More information on the biology and the environmental requirements for N. hirsutula is needed. Herbaria collections should be verified, and field work is needed to better assess the distribution of the species.

16/07/2018 Original text by:

Jeanine Vélez-Gavilán, UPR-RUM, Mayagüez, Puerto Rico