Species Description: Old World climbing fern, Lygodium microphyllum, is a large and distinctive ground-to-crown climbing fern species. It has long climbing, stem-like fronds (= leaves) and wiry brown rhizomes. Fronds spread along the ground, overgrow shrubs, and also climb up and overgrow trees and other structures by twining around them. The rhizomes can accumulate to form dense (1 m or more) mats on the ground. The leaflets (pinnules) are of two types: non-reproductive leaflets are unlobed along their margins, and oblong to lanceolate (pointed); reproductive leaflets (see below) have tiny lobes along their margins covering the sporangia (Langeland and Craddock Burks 1998).
Potentially Misidentified Species: In Florida, L. microphyllum may potentially be confused with its congener L. japonicum, the Japanese climbing fern which is also non-native to the state and highly invasive in habit. The non-reproductive leaflets of L. japonicum are lobate rather than oblong-lanceolate in form.
Regional Occurrence: L. microphyllum is believed native from Africa to Southeast Asia, Australia, and the islands of the South Pacific (Langeland and Craddock Burks 1998).
In Florida, Pemberton et al. (2002a) report that L. microphyllum is a common invader in wet habitats such as bald cypress (Taxodium distichum) stands, wet prairies, saw grass (Cladium jamaicense) marshes, Everglades tree islands and mangrove marshes. Upland pine flatwoods are also vulnerable to invasion as are disturbed habitats.
As of 2002, L. microphyllum was thought to be limited to approximately the southern one-third of Florida from Highlands County south on the Gulf coast and from Brevard County south on the east coast (Pemberton et al. 2002a). The species is particularly threatening to tree islands within the Everglades region where both the density and spread of L. microphyllum are steadily increasing (Ferriter et al. 2005).
IRL Distribution: Within the IRL watershed, L. microphyllum occurs in all counties except Volusia County. It is substantially more abundant and problematic in the southern half of the watershed, primarily within Martin and Palm Beach counties (Langeland and Craddock Burks 1998). This part of the southern IRL watershed (essentially the Loxahatchee River basin) has been identified as the center of dispersal of Old World climbing fern in Florida (Beckner 1968, Nauman and Austin 1978).
Age, Size, Lifespan: L. microphyllum is a long-lived perennial vine that can overgrow trees to reach a height of more than 30 m (Langeland and Craddock Burks 1998, Langeland and Hutchinson 2005).
Vegetative growth and spore/gamete production reportedly occur year-round (Langeland and Craddock Burks 1998, Pemberton et al. 2002a).
Abundance: Old World climbing fern is more abundant in the southern half of the state and the acreage occupied by this pernicious invader is rapidly expanding. A 1993 South Florida Water Management District regional survey estimated the species occupied approximately 11,000 acres in South Florida. By 1997, this number had climbed to more than 15,000 acreas, and by 1999 the coverage was estimated at an alarming 43,000+ acres (Pemberton and Ferriter 1998, SFWM 2004).
Reproduction: Old World climbing ferns employ a reproductive strategy that is typical of ferns, alternating between vegetative and sexual reproductive forms in successive generations. As with other ferns, spores require a moist environment to germinate and grow (Lott et al. 2003).
Sporangia along the margins of the reproductive leaflets produce vegetative spores which disperse and give rise to the haploid (one set of chromosomes) gametophyte generation of the fern. The sexual reproductive gametophytes are small, cryptic plants similar to liverworts. These plants are monoecious, having the reproductive organs of both sexes present on the same plant. Male and female swimming gametes are produced and these unite (often through self-fertilization) to form an embryo that gives rise to the familiar diploid sporophyte form of the fern. Production of reproductive pinnules and sporangia occurs year-round (Langeland and Craddock Burks 1998).
Lott et al. (2003) demonstrated that intragametophytic selfing (the union of egg and sperm originating from the same gametophyte) is a common reproductive strategy of both Old World climbing fern and Japanese climbing fern in Florida. These authors and others speculate that intragametophytic selfing has facilitated the rapid spread of both of these invasive ferns across the state and that it is likely the primary sexual reproductive mode at the beginning of a new infestation. Lott et al. (2003) support this contention by noting that the strategy is employed by other pioneering (colonizer) ferns (e.g., Asplenium platyneuron, Onoclea sensibilis).
Embryology: Spores typically germinate in six to seven days (Brown, 1984). They represent the hardy dispersal stage of L. microphyllum and remain viable for an extended period (at least two years). An 80% germination rate has been reported for 5-month old spores, and spores can remain viable for at least two years (Brown 1984). Spores can be transported through dispersal facilitated by wind or water with wind being the typical mechanism of long-range dispersal.
Invasive stands of L. microphyllum are capable of producing large numbers of spores. Pemberton and Ferriter (1998) reported more than 800 spores/cubic meter/hour were captured from a sporulating Florida L. microphyllum infestation.
Temperature: L. microphyllum is a subtropical to tropical species thus far limited in Florida to the southern 1/3 of the peninsula although climatic analysis indicates the northern thermal limits of the species on the east coast of Florida may be up near the Georgia border and north of Tampa on the Gulf coast of the state (Pemberton et al. 2002b).
Hydrology: Old World climbing fern is capable of growing in wet soils and even in standing water, under light conditions ranging from shade to full sun (Pemberton et al. 2002a). The spores require moist conditions to germinate (Brown 1984).
Fire Tolerance: L. microphyllum can be killed back by wildfire or controlled burn, but the species is difficult to completely eliminate (Maithana et al. 1986).
Trophic Mode: Autotrophic (photosynthetic).
Associated Species: L. microphyllum tends to overgrow and dominate co-occurring vegetation (see below), reducing habitat value for associated community fauna.
Invasion History: A broad native range is reported for L. microphyllum, essentially extending throughout the moist habitats of the Old World tropics (Lott et al. 2003). Much of tropical Asia, Africa, India, Australia, New Guinea, and several Indo-Pacific nations fall within the reported native range. Although the species is highly invasive where it has been introduced, within its native range, it rarely if ever exhibits the traits of an invasive species.
L. microphyllum was first found to be naturalized in Florida in 1965 (Pemberton et al. 2002a). The putative center of dispersal in Florida is within the Loxahatchee River basin straddling Martin and Palm Beach counties.
The spread of the species across the state has been rapid, and it can now be found on the Gulf coast north into Hillsborough County (Pemberton and Ferriter 1998).
Old World climbing fern is listed as a Category I invasive species (most invasive) by the Florida Exotic Plant Pest Council (Langeland and Craddock Burks 1998).
Potential to Compete With Natives: Authors have noted that the species often becomes initially established at the ecotone (boundary) between wetlands and drier upland pine habitats (Langeland and Craddock Burks 1998). It can quickly overgrow, smother, and completely dominate the native vegetation in these habitats, resulting in reduced native biodiversity. Moreover, it is capable of spreading into new areas in the absence of prior habitat disturbance (Pemberton et al. 2002a).
In addition to conveying a competitive advantage, the overgrowing and tree-climbing habit of Old World climbing ferns likely enhances long-range, wind-facilitated spore dispersal ability of the species (Lott et al. 2003).
Once L. microphyllum has invaded and overgrown an area, it is unlikely that native species will regain space because growing up through the thick rhizome mats is difficult. Because it is a ground-to-crown climbing species, Old World climbing fern can have a significant effect as a fire carrier, allowing fire to travel upward into the canopies of trees that would be otherwise protected (e.g., by standing water or lack of fuel at trunk height) from most wildfires or prescribed burns (Roberts 1998, Pemberton et al. 2002a).
Craddock Burks (1996) indicate that some rare Florida plant species are particularly vulnerable to L. microphyllum overgrowth, including arboreal bromeliads such as Tillandsia utriculata which have suffered population decline in the Loxahatchee Slough (Palm Beach County) due to climbing fern infestation.
Possible Economic Consequences of Invasion: L. microphyllum is a non-native species that is considered an invasive weed in Florida's natural areas. It is an aggressive invader currently threatening many moist habitats of south and central Florida (Pemberton and Ferriter 1998). Pemberton et al. (2002a) refer to the species as "one of the most dangerous weeds in southern Florida."
Limited uses have been found for L. microphyllum, including use as a handicraft weaving material and assorted uses as an herbal remedy, e.g., as an anti-diuretic and an anti-inflammatory agent (FLEPPC 2001).
Chemical herbicide and mechanical control of L. microphyllum are employed as partial remedies, but they are expensive and not entirely effective. Such strategies can also cause unintentional harm to native vegetation that is usually intertwined with the fern.
Where infestations are fairly accessible (e.g., roadsides and residential areas), herbicidal control can cost less than $1,000/ha. Treatment of infestations in more remote settings may be more costly, however. Bailey and Thomas (reported in Pemberton et al. 2002a) noted that chemical treatment of L. microphyllum infestations within Florida's Loxahatchee National Wildlife Refuge in 2000 cost approximately $3,750/ha. These authors noted that L. microphyllum grew back afterward and re-treatment in 2001 was required.
Scientists from the USDA Agricultural Research Service have investigated a couple of insects as potential biological controls of L. microphyllum in Florida. One candidate that appears promising at this point is a mite, Floracarus perrepae, that has been documented to preferentially consume L. microphyllum in Australia and Thailand (Wood 2004).
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