Adiantum raddianum (delta maidenhair fern)

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Title Leaves Caption Adiantum raddianum (maidenhair fern); leaves at Waikapu Valley, Maui. February 29, 2012 Copyright ©Forest & Kim Starr Images-2012. CC-BY-3.0	Leaves	Adiantum raddianum (maidenhair fern); leaves at Waikapu Valley, Maui. February 29, 2012	©Forest & Kim Starr Images-2012. CC-BY-3.0

Identity

Preferred Scientific Name

• Adiantum raddianum C Presl., 1836

Preferred Common Name

• delta maidenhair fern

Other Scientific Names

• Adiantum amabile Liebm.
• Adiantum boliviense Chr. & Rosenst.
• Adiantum colpodes Moore
• Adiantum cuneatum Langsd. & Fisch.
• Adiantum mexicanum Presl.
• Adiantum werckleanum Christ.

International Common Names

• English: delta maidenhair fern
• French: capillaire

Local Common Names

• Germany: Frauenhaarfarn
• Sweden: snittadiantum

EPPO code

• ADIRA (Adiantum raddianum)

Summary of Invasiveness

A. raddianum is a delicate fern native to tropical and subtropical South America. The fern grows terrestrially or on rocks and erects arching fronds, up to 50 cm high, growing out of a short rhizome. The plant has become naturalized in various tropical and subtropical islands and is considered to be invasive in Hawaii and French Polynesia. The fern readily spreads and becomes locally abundant. In Hawaii it was first observed around 1910 and is now the most common Adiantum species. It grows best in moist and shady places and appears to replace the closely related native Adiantum capillus-veneris. It also threatens an endemic species of silversword, Dubautia plantaginea subsp. humilis, and another native fern, Pteris lidgatei.

Taxonomic Tree

• Domain: Eukaryota
•     Kingdom: Plantae
•         Phylum: Pteridophyta
•             Class: Filicopsida
•                 Family: Pteridaceae
•                     Genus: Adiantum
•                         Species: Adiantum raddianum

Notes on Taxonomy and Nomenclature

The taxonomic treatment of Adiantum raddianum has frequently changed. This has resulted in a number of synonyms, of which Adiantum cuneatum is still widely used. The name A. raddianum honours the Italian botanist Giuseppe Raddi (1770-1829). This fern species is an important ornamental and more than seventy cultivars have been developed. The cultivars vary in their cultural requirements, their frond shape and growth forms. Closely related species of A. raddianum include the brittle maidenhair fern (A. tenerum) and the common maidenhair fern or Venus' hair fern (A. capillus-veneris). In Hawaii, the naturalized Adiantum 'Edwinii' is probably a cultivar of A. raddianum, or a cultivar or hybrid of A. concinnum.

Description

A. raddianum is a fern with a short-creeping and irregularly-branched rhizome up to 50 mm long and ca 2 mm wide. The rhizome and the bases of the frond stalks are covered with dark-brown scales of less than 1 mm length. Fronds are arched to erect, 10-50 cm long and 6-20 cm wide, and triangular in shape. Frond stalks and axes are dark reddish-brown to blackish and shining. The frond stalk is usually longer than the lamina. Laminas are 3-4-pinnately divided, with the ultimate segments delicate, herbaceous and up to 1 cm wide. Ultimate segments are wedge-shaped and have slender red-black stalks. Segments of sterile fronds, if present, are larger than fertile fronds. Spore cases (sori) are 'U'-shaped and arranged either at the edge of veins or at their tips and less than 4 mm wide. Each sorus is covered with a pale or whitish membrane (indusium).

Plant Type

Distribution

In its native range, in tropical and sub-tropical South America, from Mexico down to Argentina, the fern is found from 0 to 4000 m (Luteyn, 1999; Zuloaga et al., 2008). It has been quite widely introduced elsewhere as an ornamental and now occurs in Asia, Africa and the Pacific. In Hawaii, the fern occurs on all major islands and ascends from sea level to 4,400 m. On the Azores it is found on all islands except Terceira and Corvo and ascends to 400 m (Schäfer, 2002; Palmer, 2003).

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

On Hawaii, A. raddianum was first collected in the wild on the island Kaua'i in 1910, but was reportedly already seen outside of cultivation in 1907 (Palmer, 2003). On Madeira, the plant has been naturalized since 1911 (Press and Short, 1994).

Risk of Introduction

The fern is available through the horticultural trade and sold in many nurseries. New specimens, possibly including different cultivars, are likely to be imported into many regions. Since the spores are easily dispersed and germinate readily, any specimen with fertile fronds will act as a potential source for new ferns. Dispersal of spores is an important vector in all regions with a subtropical to tropical climate where the fern can be grown outdoors.

Habitat

The fern grows best in damp and shady places but does not tolerate very heavy shade. It is found in a wide range of habitats including forest floors, rock crevices, walls, roadside banks, river banks, coastal cliffs and basalt banks along trails and streams (Palmer, 2003).

Habitat List

Host Plants/Plants Affected

Biology and Ecology

Genetics

Reported chromosome numbers are 2n=60, 2n=114, 2n=58 and 2n=228 (Missouri Botanical Garden, 2012).

Reproductive Biology

As a fern, A. raddianum possesses the typical life cycle with a sporophyte generation and a gametophyte generation. The sporophyte (spore-bearing plant) is the conspicuous fern plant with fronds, roots and stems. It produces spores and its tissues are diploid. A spore germinates and forms a tiny green prothallus, which is the gametophyte (gamete-bearing plant). The prothallus is haploid and has structures on its surface producing the gametes or sex cells, e.g. sperm and egg cells. After fertilization a new sporophyte grows out of the prothallus. Fertilization is in most cases cross-fertilization. This is ensured by the large number of prothalli lying next to each other at a site where spores germinated.

The fern spreads mainly by its spores, which are produced abundantly and dispersed by wind and water. The rhizome is short but fragments may resprout if large enough. Rhizome fragments may be carried to new places by heavy rains or soil disturbances. 

Physiology and Phenology

The broad elevational range of this fern species suggests a broad environmental tolerance, including high and low temperatures, shade and bright light.

Environmental Requirements

In Hawaii, the fern has a remarkable altitudinal distribution, from 0 to 4400 m (Palmer, 2003). It grows on young volcanic soils such as moist cinder and basalt banks. In French Polynesia, it occupies a similar range between 0 and 4000 m.

Climate

Soil Tolerances

Soil drainage

• free

Soil reaction

• acid
• neutral

Soil texture

• heavy
• medium

Special soil tolerances

• infertile
• shallow

Notes on Natural Enemies

Healthy plants do not suffer greatly from pests. Some generalist herbivores such as slugs, snails, earwigs and caterpillars can damage rhizomes and young fronds. Aphids are common pests on uncurling fronds in potted plants. Grey mould caused by a fungus of the genus Botrytis may occur on plants in stagnant and wet conditions (Jones, 1987).

Means of Movement and Dispersal

Long-distance dispersal is by spores carried by wind and water. Local spread may be achieved by rhizome fragmentation and pieces growing to new plants, and from spores carried by animals. Accidental introduction by man to new sites is likely by clothing and shoes carrying spores. Delta maidenhair fern is the most commonly grown member of the genus in the world. Naturalizations are the result of imported plants used as ornamentals. Distributing the fern by nurseries and private persons is an important pathway for intentional introduction and dispersal.

Pathway Causes

Pathway Vectors

Impact Summary

Economic Impact

A. raddianum has been reported to be weedy in rice fields (Moody, 1989) and under tea bushes (Schaffer-Fehre, 2006).

Environmental Impact

A. raddianum has become the most common fern of Adiantum in the Hawaiian Islands (Palmer, 2003). In Hawaii, the native Adiantum capillus-veneris has become uncommon and seems to have been replaced by A. raddianum in places where both species grow (Wilson, 1996). Two endangered plant species native to Hawaii (Pteris lidgatei and Dubautia plantaginea sub. humilis) are threatened by the spread of A. raddianum, besides habitat alterations and spread of other invasive species (Torres-Santana et al., 2007; Freifeld et al., 2009).

Threatened Species

Risk and Impact Factors

Impact mechanisms

• Competition - monopolizing resources
• Competition - shading
• Competition - smothering
• Rapid growth

Impact outcomes

• 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

Invasiveness

• Abundant in its native range
• Benefits from human association (i.e. it is a human commensal)
• Fast growing
• Gregarious
• Has a broad native range
• Has high reproductive potential
• Highly adaptable to different environments
• Is a habitat generalist
• Long lived
• Proved invasive outside its native range
• Reproduces asexually
• Tolerant of shade

Likelihood of entry/control

• Highly likely to be transported internationally deliberately

Uses

A. raddianum is an important horticultural plant and sold in many nurseries. A high number of cultivars are available on the market.

Uses List

General

• Botanical garden/zoo

Ornamental

• Potted plant

Similarities to Other Species/Conditions

Common maidenhair fern (Adiantum capillus-veneris) is of similar size, but the frond stalk is usually shorter than the lamina. The ultimate segments are up to 3 cm wide, deeply lobed and irregularly shaped. Spore cases are not roundish and not 'U'-shaped, but longer than they are wide and arranged at the edge of ultimate segments. Brittle maidenhair fern (A. tenerum) has larger fronds (30-100 cm long) and the ultimate segments of blades are variable in shape, mostly triangular to diamond shaped. Stalks of ultimate segments are enlarged and form saucer-like discs at attachments.

Gaps in Knowledge/Research Needs

Studies on the ecological impacts of A. raddianum are badly needed, as well as studies on the establishment of this fern in natural habitats. Threats to endangered plant species and its overall invasive character are largely based on anecdotal and correlative observations. Particularly noteworthy is the broad elevational range of A. raddianum on the Hawaiian Islands, i.e. 0 to 4,400 m. Here, studies on physiological tolerances or adaptations would be fruitful, especially on the effects of light and low temperatures on growth performance. Another question relates to longevity of spores, as it is not known for how long spores remain viable in the soil.

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Links to Websites

Contributors

20/09/12 Original text by:

E Weber, Consultant, Switzerland

Distribution Maps

• = Present, no further details
• = Evidence of pathogen
• = Widespread
• = Last reported
• = Localised
• = Presence unconfirmed
• = Confined and subject to quarantine
• = See regional map for distribution within the country
• = Occasional or few reports

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