Are mangroves in the tropical Atlantic ripe for invasion? Exotic mangrove trees in the forests of south Florida

Authors: JW Fourqurean, TJ Smith III, J Possley, TM Collins, D Lee and S Namoff

 

DOI: 10.1007/s10530-009-9660-8

 

See the paper at the journal Biologial Invasions's website, or email JW Fourqurean for a reprint

 

Abstract of the paper:

Two species of mangrove trees of Indo-Pacific origin have naturalized in tropical Atlantic mangrove forests in south Florida after they were planted and nurtured in botanic gardens. Two Bruguiera gymnorrhiza trees that were planted in the intertidal zone in 1940 have given rise to a population of at least 86 trees growing interspersed with native mangrove species Rhizophora mangle, Avicennia germinans and Laguncularia racemosa along 100m of shoreline; the population is expanding at a rate of 5.6% y-1. Molecular genetic analyses confirm very low genetic diversity, as expected from a population founded by two individuals. The maximum number of alleles at any locus was three, and we measured reduced heterozygosity compared to native-range populations. Lumnitzera racemosa was introduced multiple times during the 1960ís and 1970ís, it has spread rapidly into a forest composed of native R. mangle, A. germinans, Laguncluaria racemosa and Conocarpus erectus and now occupies 60,500 m2 of mangrove forest with stem densities of 24,735 Ha-1. We estimate the population growth rate of Lumnitzera racemosa to be between 17 and 23% y-1. Populations of both species of naturalized mangroves are dominated by young individuals. Given the long life and water-dispersed nature of propagules of the two exotic species, it is likely that they have spread beyond our survey area. We argue that the species-depauperate nature of tropical Atlantic mangrove forests and close taxonomic relatives in the more species-rich Indo-Pacific region result in the susceptibility of tropical Atlantic mangrove forests to invasion by Indo-Pacific mangrove species.


 

 

Bruguiera gymnorrhiza at The Kampong of the National Tropical Botanic Garden

 

A small specimen of Bruguiera gymnorrhiza with unopened red flower buds

 

Photo credit: David Lee

A comparison of the leaves of the native Rhizophora mangle (top) and the exotic Bruguiera gymnorrhiza (bottom).  Note the more elongated shape of the exotic, the more pointed leaf tips, and the longitudinal folds running parallel to the midrib of the exotic leaf.

 

Photo credit: Jim Fourqurean

A Bruguiera gymnorrhiza sapling growing in the understorey of the mangrove forest.  Note the substantial herbivory on the leaves.

 

Photo credit: Jim Fourqurean

A small branch from a small Bruguiera gymnorrhiza tree in the understorey.  Note the pronounced points on the leaves compared to the native Rhizophora mangle

 

Photo credit: Jenn Possley

Note the density of saplings and small trees of Bruguiera gymnorrhiza (marked with orange flagging tape) in the understorey of native Avicennia germinans and Rhizophora mangle.

 

Photo credit: Jim Fourqurean

Base of the trunk of the largest specimen (ca. 15 cm DBH) of Bruguiera gymnorrhiza at the Kampong.  Note the beginning of buttress roots and lack of prop roots compared to Rhizophora mangle.

 

Photo credit: Jim Fourqurean

A closer view of the base of the trunk of a medium-sized Bruguiera gymnorrhiza.  Note color and texture of bark.

 

Photo credit: Jenn Possley

Unopened flower buds on Bruguiera gymnorrhiza.

 

Photo credit: David Lee

Partially opened flower on Bruguiera gymnorrhiza.

 

Photo credit: David Lee

Fully opened flower on Bruguiera gymnorrhiza.

 

Photo credit: David Lee

Recently uprooted propagule of Bruguiera gymnorrhiza.  Note the similar appearance to the propagules of the native Rhizophora mangle. The exotic propagules have a slightly hexagonal cross section compared to the natives- note longitudinal ridge along the hypocotyl.

 

Photo credit: Jim Fourqurean

Recently uprooted propagule of Bruguiera gymnorrhiza, for scale.

 

Photo credit: Jim Fourqurean


 

 

Lumnitzera racemosa growing at Fairchild Tropical Botanic Garden and Matheson Hammock county park.

A small Lumnitzera racemosa tree growing amongst dwarf native Rhizophora mangle in Matheson Hammock park.

 

Photo credit: Jenn Possley

Many small Lumnitzera racemosa trees growing amongst dwarf native Rhizophora mangle in Matheson Hammock park.

 

Photo credit: Jenn Possley

Close up of a stem of Lumnitzera racemosa, showing an open flower and young fruit. Note leaf shape and indentation on distal end of leaf.

Photo credit: Jenn Possley

Close up of a stem of Lumnitzera racemosa, showing fruits.  Note venation pattern in leaves.

 

Photo credit: Jim Fourqurean

Newly germinated Lumnitzera racemosa seedling.

 

Photo credit: Jim Fourqurean

Many Lumnitzera racemosa seedlings on the shore of a freshwater pond at Fairchild Tropical Botanic Garden.

 

Photo credit: Jim Fourqurean

Many Lumnitzera racemosa seedlings among the dwarf native Rhizophora mangle at Matheson Hammock.

 

 

 

Photo credit: Jenn Possley

Saplings of Lumnitzera racemosa.

 

Photo credit: Jim Fourqurean

A large specimen of Lumnitzera racemosa (between Jenn Possley on left, David Lee on right) in Fairchild Tropical Botanic Garden

 

Photo credit: Jim Fourqurean