Christella dentata
(soft fern)

Pictures

Picture	Title	Caption	Copyright
Title Habit and habitat Caption Christella dentata (soft fern, downy wood fern); habit and habitat. Honokowai Ditch Trail, Maui. June 21, 2010. Copyright ©Forest Starr & Kim Starr - CC BY 4.0	Habit and habitat	Christella dentata (soft fern, downy wood fern); habit and habitat. Honokowai Ditch Trail, Maui. June 21, 2010.	©Forest Starr & Kim Starr - CC BY 4.0
Title Habit on bridge Caption Christella dentata (soft fern, downy wood fern); habit on bridge - many plants growing, in a line, below bridge parapet. Puaa Kaa Park, Hana Hwy, Maui. July 02, 2009 Copyright ©Forest Starr & Kim Starr - CC BY 4.0	Habit on bridge	Christella dentata (soft fern, downy wood fern); habit on bridge - many plants growing, in a line, below bridge parapet. Puaa Kaa Park, Hana Hwy, Maui. July 02, 2009	©Forest Starr & Kim Starr - CC BY 4.0
Title Habit Caption Christella dentata (soft fern, downy wood fern); habit. Waihee Ridge Trail, Maui. September 25, 2002 Copyright ©Forest Starr & Kim Starr - CC BY 4.0	Habit	Christella dentata (soft fern, downy wood fern); habit. Waihee Ridge Trail, Maui. September 25, 2002	©Forest Starr & Kim Starr - CC BY 4.0
Title Fronds and habit Caption Christella dentata (dsoft fern, owny wood fern); fronds and habit. Garden of Eden Keanae, Maui. March 30, 2011 Copyright ©Forest Starr & Kim Starr - CC BY 4.0	Fronds and habit	Christella dentata (dsoft fern, owny wood fern); fronds and habit. Garden of Eden Keanae, Maui. March 30, 2011	©Forest Starr & Kim Starr - CC BY 4.0
Title Sori Caption Christella dentata (soft fern, downy wood fern); sori. Sacred Garden of Maliko, Maui. January 24, 2011 Copyright ©Forest Starr & Kim Starr - CC BY 4.0	Sori	Christella dentata (soft fern, downy wood fern); sori. Sacred Garden of Maliko, Maui. January 24, 2011	©Forest Starr & Kim Starr - CC BY 4.0
Title Close-up of sori Caption Christella dentata (soft fern, downy wood fern); sori. Sacred Garden of Maliko, Maui. January 24, 2011 Copyright ©Forest Starr & Kim Starr - CC BY 4.0	Close-up of sori	Christella dentata (soft fern, downy wood fern); sori. Sacred Garden of Maliko, Maui. January 24, 2011	©Forest Starr & Kim Starr - CC BY 4.0
Title Hybrid habit Caption Cyclosorus x intermedia (hybrid christella); habit. Makawao Forest Reserve, Maui. April 05, 2003 Copyright ©Forest Starr & Kim Starr - CC BY 4.0	Hybrid habit	Cyclosorus x intermedia (hybrid christella); habit. Makawao Forest Reserve, Maui. April 05, 2003	©Forest Starr & Kim Starr - CC BY 4.0
Title Pinnules and sori Caption Cyclosorus x intermedia (hybrid christella); pinnules and sori. Makawao Forest Reserve, Maui. April 05, 2003 Copyright ©Forest Starr & Kim Starr - CC BY 4.0	Pinnules and sori	Cyclosorus x intermedia (hybrid christella); pinnules and sori. Makawao Forest Reserve, Maui. April 05, 2003	©Forest Starr & Kim Starr - CC BY 4.0

Identity

Preferred Scientific Name

• Christella dentata (Forssk.) Holttum

Preferred Common Name

• soft fern

Other Scientific Names

• Aspidium molle Sw., J. Bot. (Schrader) 1800: 34 (1801)
• Aspidium nymphale (G. Forst) Schkuhr (1804)
• Christella dentata var. caespitosa Holttum (1986)
• Cyclosorus dentatus (Forssk.) Ching
• Cyclosorus nymphale (G. Forst) Ching (1941)
• Cyclosourus nymphalis (G.Forst.) Ching
• Dryopteris dentata (Forssk.) C. Chr. (1920)
• Dryopteris molle (Sw.) Hieron (1907)
• Dryopteris nymphalis (G. Forst.) Copel., 59:46 (1929 [1929])
• Nephrodium dentatum (Forssk.) Kümmerle
• Nephrodium molle (Sw.) R.Br. (1810)
• Nephrodium nymphale (G. Forst) Desv. (1827)
• Nephrodium remotum Heward (1842)
• Polypodium dentatum Forssk (1775)
• Polypodium molle Jacq. (1791)
• Polypodium nymphale G. Forst. (1786)
• Thelypteris dentata (Forssk.) E.P. St. John
• Thylepteris nymphalis (G. Forst) C. F. Reed (1968)

International Common Names

• English: downy maiden fern; downy shield fern; downy wood fern; tapering tri-vein fern
• Spanish: helecha

Local Common Names

• Cuba: helecho

Summary of Invasiveness

C. dentata is a small fern widespread throughout the tropics and appears to be introduced in Hawaii and the Americas, where it was collected for the first time only relatively recently, having probably established prior to 1900 (Strother and Smith, 1970; Wagner, 1972). Elsewhere in the Pacific it is well established but its status as an introduced species is debated. With long distance dispersal (100s of kilometers) being possible via dispersal of its spores, it is believed to have arrived in New Zealand under its own volition. 

Throughout its range it has a tendency to hybridize with other congeners, including other weeds like C. parasitica (=Cyclosorus parasiticus) in Guam, Hawaii and Taiwan, and the endemic C. cyatheoides in Hawaii, and even species from other genera such as Pneumatopteris afra (Fosberg and Sachet 1981; Quansah and Edwards 1986; Palmer 2003). In addition it can be difficult to tell C. dentata apart from some other congeners (Wagner 1950; Brownlie 1959; Lange et al. 1999).

In Hawaii it can form dense stands in disturbed sites, where it can outcompete some native species (J Beachy, US Fish and Wildlife Service, personal communication, 2010a, 2010b, 2010c). It can be an early colonizer after invasive pigs disturb a site (C Buddenhagen, Florida State University, personal observation).

C. dentata is not invasive in New Zealand. Of the two forms present (see Biology and Ecology), the 'true' form occurs only in a few very restricted places in northern New Zealand and is very rare. The thermal form, although more abundant, shows no sign of spreading out from its specialized habitat to other parts of the country. In New Zealand the 'true' C. dentata is regarded as 'Nationally Critical', whilst the other form is deemed 'At Risk'.

Taxonomic Tree

• Domain: Eukaryota
•     Kingdom: Plantae
•         Phylum: Pteridophyta
•             Class: Filicopsida
•                 Family: Thelypteridaceae
•                     Genus: Christella
•                         Species: Christella dentata

Notes on Taxonomy and Nomenclature

Christella dentata (Forssk.) Brownsey & Jermy has been in use for this species since 1973, and much of the existing literature refers to this name (Brownsey and Jermy, 1973). However, in North America and elsewhere Thelypteris dentata  (Forssk.) E.P. St. John is still applied to this taxon (USDA, 2012).

It appears though that the genus Christella Léveillé is not monophyletic, and is best subsumed within Cyclosorus, making Cyclosorus dentatus (Forssk.) Ching the preferred name under modern systematic standards (Smith et al., 2006; Evenhuis and Eldredge, 2011; He and Zhang, 2012). All these names are derived from the basionym: Polypodium dentatum Forsskål 1775.

Several common characters exhibit continuous variation across generic boundaries in the clade containing Christella, and some taxa currently within Christella are polyphyletic (He and Zhang, 2012). Christella could be made monophyletic if the few taxa that are polyphyletic were dropped and placed elsewhere; such a decision may be justifiable on the basis of morphological and or molecular (DNA) differences (He and Zhang, 2012). It is worth noting that the type specimen (from Yemen) is unhelpful because it consists of a single portion of the middle of a frond, and so does not show the critical features people traditionally use to define the species (P de Lange, personal communication). Unless this is resolved, understanding when a Christella specimen is naturalized or indigenous anywhere in the Pacific or New World where that name is used is likely to be difficult (Brownlie, 1959).

Given the possibility of Christella becoming monophyletic again, and in view of the fact that The Plant List (2013) lists Christella dentata as a preferred name and Cyclosorus dentatus as a synonym, this datasheet uses the name Christella dentata as the preferred scientific name.

Description

Stems short-creeping, 4-6 mm in diameter. Leaves often somewhat dimorphic, evergreen, often closely placed, 50-150 cm, fertile, with longer petioles and more contracted pinnae. Petiole often purplish brown, 15-50 cm by 3-6 mm at the base with brown, linear-lanceolate, hairy scales. Blade (25-) 40-100 cm, 1-4(-6) proximal pairs of pinnae reduced, blade gradually tapered to pinnatifid apex. Pinnae 7-17 by 1-3 cm, incised 1/2-3/4 of width; segments rounded at apex, basal acroscopic segment of proximal pinnae often auriculate; proximal pair of veins from adjacent segments united at obtuse angle below sinus with excurrent vein 2-4 mm. Indument abaxially of uniformly short hairs 0.1-0.2 mm on costae, veins, and blade tissue; veins adaxially with stouter hairs, also with hairs 0.1-0.2 mm on blade tissue. Sori round, medial to supramedial; indusia tan, pubescent, hairs 0.1-0.3 mm; sporangial stalks with orangish, stalked glands (Flora of North America, 1993).

Plant Type

Distribution

C. dentata is native to the Old World tropics (widespread in Asia and Africa) and temperate parts of Australia and New Zealand (McCarthy, 1998), but is adventive in North and Central America. It has been described as having a tropical cosmopolitan distribution (Iwatsuki, 1963), but it also occurs in subtropical sites that are frost-free or nearly so, perhaps surviving a few frosts a year in northern Florida (GBIF, 2012).

C. dentata was first recorded in Japan in the 1950s and has since spread northward (possibly as a greenhouse contaminant) at a rate of 3 km per year, persisting through the winters probably as a result of urban heat islands (Murakami et al., 2007). In Hawaii the species was collected much earlier and was probably introduced in and established by the late 1800s (MacCaughey, 1918; Wagner, 1950).

It is not clear whether the two forms of C. dentata present in New Zealand (see Biology and Ecology) arrived within historic time or whether they have been established there rather longer, although the species was recorded early in the country's history, so it seems unlikely to have been introduced (P Brownsey, Museum of New Zealand, personal communication). There is similar uncertainty over C. dentata in Fiji.

C. dentata is regarded as native in Lord Howe Island, Norfolk Island, Tonga and Tahiti, although one publication suggests it is introduced in Tahiti (Iwatsuki 1963; Rodd and Pickard 1983; Murdock and Smith 2003; Orchard, 1994; Meyer et al., 2009). It has also been recorded from New Caledonia, Vanuatu, Samoa and the Cook Islands.

Distribution Table

The distribution in this summary table is based on all the information available. When several references are cited, they may give conflicting information on the status. Further details may be available for individual references in the Distribution Table Details section which can be selected by going to Generate Report.

History of Introduction and Spread

C. dentata is native to the Old World tropics (widespread in Asia and Africa) and temperate parts of Australia and New Zealand (McCarthy, 1998), but is adventive in the Americas, where it was most likely introduced around 1900 as a greenhouse contaminant (Strother and Smith, 1970; Holttum, 1976; Smith, 1993). It has been described as having a tropical cosmopolitan distribution (Iwatsuki, 1963), but it also occurs in subtropical sites that are frost-free or nearly so, perhaps surviving a few frosts a year in northern Florida (GBIF, 2012). Specimens of this plant were not collected in the New World (e.g. Brazil, Florida) prior to the 1930s, and in some cases only after the 1940-1950s (e.g. Costa Rica and Hispaniola) (Strother and Smith, 1970).

C. dentata was first recorded in Japan in the 1950s and has since spread northward (possibly as a greenhouse contaminant) at a rate of 3 km per year, persisting through the winters probably as a result of urban heat islands (Murakami et al., 2007). In Hawaii the species was collected much earlier and was probably introduced in and established by the late 1800s (MacCaughey, 1918; Wagner, 1950).

It is not clear whether the two forms of C. dentata present in New Zealand (see Biology and Ecology) arrived within historic time or whether they have been established there rather longer, although the species was recorded early in the country's history, so it seems unlikely to have been introduced (P Brownsey, Museum of New Zealand, personal communication). There is similar uncertainty over C. dentata in Fiji.

C. dentata is regarded as native in Lord Howe Island, Norfolk Island, Tonga and Tahiti, although one publication suggests it is introduced in Tahiti (Iwatsuki 1963; Rodd and Pickard 1983; Murdock and Smith 2003; Orchard, 1994; Meyer et al., 2009).

Risk of Introduction

C. dentata is an effective disperser that has reached some Pacific islands (e.g. New Zealand, Fiji, Raoul and Norfolk Islands) probably via natural means (Brownsey and Smith-Dodsworth, 1989; Wolf et al., 2001).

Its presence in Hawaii and the Americas is likely to be the result of accidental introduction (Strother and Smith 1970). The most probable human-mediated pathway for this plant’s introduction is accidentally via the horticultural trade. However, C. dentata is rarely traded as an ornamental plant now (Ko et al., 1997).

Habitat

C. dentata is regarded as a facultative wetland plant. Both in its native and introduced ranges forested wetlands, damp woods and disturbed or open wet areas are typical habitats (Viane 1985; Flora of North America Editorial Committee 1993; USDA 2012). In Hawaiian forests it has colonized trails and road sides, weedy gulches and areas disturbed by pigs (C Buddenhagen, Florida State University, personal communication). In general it grows well in disturbed, wet sites. It often co-occurs with another widespread adventive fern, C. parasitica (=Cyclosorus parasiticus).

It is tolerant of a range of soil conditions, although it has been suggested as an indicator of calcareous conditions in South Carolina (Hill 1992) and has been found on limestone substrates in the Bahamas (Correll 1976). C. dentata can tolerate some shade.

Fronds of C. dentata are killed by exposure to -3 degrees Celsius for 2 hours or more (Given 1980). Frosts seem to limit the range of this species, although occasional frosts may be tolerated considering that it has been found growing in Kyoto (Japan) and South Carolina and Georgia (USA) (Norsworthy, 1966; Faircloth, 1975; Hill, 1992; Murakami et al., 2007; GBIF, 2012; USDA, 2012). In New Zealand C. dentata occurs in geothermal sites (Lange et al. 1999).

Habitat List

Host Plants and Other Plants Affected

Biology and Ecology

Genetics

2 n = 144 (Flora of North America, 1993).

For other chromosome counts see Löve et al. (1977). There is also evidence of polyploidy in C. dentata.

Diversity

There are believed to be two forms in New Zealand: one is the true C. dentata, with a long-creeping rhizome and which occurs only in a couple of places in the northern North Island (and is also reasonably common on Raoul Island and Norfolk islands) (P Brownsey, Museum of New Zealand, personal communication). The other has an erect or short-creeping rhizome and occurs around thermal areas and also on Raoul Island (although occurs less commonly there than true C. dentata) (Brownsey and Smith-Dodsworth, 1989). The relationship between these two forms and whether they are both attributable to C. dentata remains to be determined.

Climate

Latitude/Altitude Ranges

Air Temperature

Soil Tolerances

Soil drainage

• free
• impeded
• seasonally waterlogged

Soil reaction

• alkaline
• neutral

Soil texture

• light
• medium

Means of Movement and Dispersal

Occasional long-distance dispersal (100s of kilometers) from Australia or elsewhere has been suggested as an explanation for C. dentata occurrences in New Zealand and South Pacific islands (Given, 1980; McGlone et al., 2001).

Its presence in Hawaii and the Americas is probably the result of accidental introduction (Strother and Smith, 1970). The most likely human-mediated pathway for this plant’s introduction is accidentally via the horticultural trade.

Spores and gametophytes could easily be transported and are likely contaminants of nurseries, greenhouses, house plants and the potted plant trade, provided the climate is suitable.

Pathway Causes

Pathway Vectors

Impact Summary

Threatened Species

Risk and Impact Factors

• Invasive in its native range
• Proved invasive outside its native range
• Has a broad native range
• Abundant in its native range
• Highly adaptable to different environments
• Is a habitat generalist
• Pioneering in disturbed areas
• Tolerant of shade
• Highly mobile locally
• Long lived
• Has high reproductive potential
• Reproduces asexually

• Damaged ecosystem services
• Ecosystem change/ habitat alteration
• Modification of successional patterns
• Monoculture formation
• Reduced native biodiversity
• Threat to/ loss of endangered species
• Threat to/ loss of native species

• Competition - monopolizing resources
• Competition - shading
• Competition - smothering
• Hybridization
• Interaction with other invasive species

• Highly likely to be transported internationally accidentally
• Difficult to identify/detect as a commodity contaminant
• Difficult to identify/detect in the field

Uses List

General

• Botanical garden/zoo

Detection and Inspection

This species is easily confused with C. parasitica (=Cyclosorus parasiticus)(Wagner 1950). Living plants of C. parasitica are paler, with a duller green, and its proximal pinnae are narrower. Most importantly, its fronds are not dimorphic and they do not have the sharp break between sterile and fertile fronds that C. dentata has. C. dentata’s sterile fronds are broader, shorter, more spreading than the others and generally entirely without sori.

Similarities to Other Species/Conditions

This species is easily confused with C. parasitica (=Cyclosorus parasiticus) (Wagner, 1950). Living plants of C. parasitica are paler and a duller green, and have narrower proximal pinnae. Most importantly, its fronds are not dimorphic and do not have the sharp break between sterile and fertile fronds that C. dentata has. C. dentata’s sterile fronds are broader, shorter, and more spreading than the others, and generally entirely without sori.

A key to Chinese species of the subgenus Cyclosoriopsis (including both Cyclosorus parasiticus and Christella dentata) is provided by Li et al. (2013).

Prevention and Control

With long-distance dispersal (100s of kilometers) being possible via the dispersal of spores, as evidenced from its likely unassisted arrival to New Zealand and other South Pacific Islands, this is a species that may be difficult or impossible to contain once established in a region.

Hand weeding or treatment with herbicide have been used to control C. dentata. The application of triclopyr at standard foliar rates or at high concentrations with biodiesel to the rhizome has been shown to work (J Beachy, US Fish and Wildlife Service, personal communication).

Gaps in Knowledge/Research Needs

More could be done to understand the history of the spread of this species around the world, in both its introduced and its native range. The genetic connectivity between populations could tell us more about this species’ spread in short- and long-term evolutionary timeframes.

References

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Organizations

USA: University of Hawaii UH, Manoa, Honolulu, http://www.hawaii.edu

USA: US Fish and Wildlife Service, http://www.fws.gov/

Contributors

14/11/12 Original text by:

Buddenhagen CE, Florida State University, USA

Distribution Maps