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Species Description:
Dicerandra immaculata is a rare and
endangered species of mint that grows to a height of approximately
50 cm (1.6 feet). Young plants and those growing
in open sunlight have an erect growth habit and may form dense mats,
while older plants and ones growing in shaded areas are arching
or sprawling. Leaves are opposite (but may appear to
be whorled) and linear to narrowly oblong in shape, with inrolled
margins and rounded tips (FNAI 2006; Nelson 1996).
Leaves measure 1.5 - 3 cm (0.6 - 1.2 inches) in length and 2 - 4
mm (0.07 - 0.16 inches) in width. Both the upper and lower
surfaces of the leaves are covered with fine dots. Flowers
are two-lipped and tubular in shape, measuring 1.5
- 2 cm (0.6 - 0.8 inches) in length. Color ranges from pink
to rose or lavender, and there
are no spots on the petals, a trait that differs from most other
Dicerandra species. Anthers are white, and extend
beyond the flower. Stamens are spurred.
The inflorescence is 15-25 cm (5.9 - 9.8 inches) long and has overlapping
cymes, each bearing 1,3 or 5 flowers (Kral
1982) in leaf axils. Fruit is a pale-colored, rounded
nutlet approximately 1mm (0.04 inches) in length.
Potentially
Misidentified Species:
Dicerandra immaculata is distinguished from the other
4 Dicerandra species that occur in Florida by their flowers,
which, as suggested by the scientific name, are immaculate, or lacking
in spots.
II. HABITAT AND DISTRIBUTION
Regional Occurrence:
Dicerandra immaculata is extremely rare and endemic to
the Atlantic Coastal Ridge of Florida. It was first described
in 1963 (Lakela 1963), and known to occur naturally only in coastal
scrub and pine scrub areas in Indian River and St. Lucie Counties,
with most known plants in an area of old dune with an elevation
of 45 feet. This area is approximately 1/2 mile wide and 3
miles long, lying between Vero Beach and Ft. Pierce (USFWS 1999).
An experimental population was introduced into Hobe Sound National
Wildlife Refuge in Martin County in 1991 and 1992 (Race 1994).
IRL Distribution:
The bulk of the Dicerandra immaculata population lies on
remnant Pleistocene dunes between Vero Beach and Ft. Pierce, FL.
Until recently, no populations occurred in protected areas (USFWS
1999).
III. LIFE HISTORY AND
POPULATION BIOLOGY
Age, Size, Lifespan:
Lakela's mint grows to approximately
50 cm (1.6 feet) in height. Leaves measure 1.5 - 3 cm (0.6
- 1.2 inches) in length and 2 - 4 mm (0.07 - 0.16 inches) in width. Flowers
are two-lipped and tubular in shape, measuring 1.5
- 2 cm (0.6 - 0.8 inches) in length.
Abundance:
Dicerandra immaculata is extremely rare and endemic to
a tiny range that extends over 2 Florida Counties (USFWS 1999; Nelson
1996).
Reproduction:
New
leaves emerge from February through August. Lakela's
mint is documented to bloom sporadically throughout the year, with
peaks in September - November (FNAI 2006). It only reproduces
through seedlings and requires insect pollinators, though which
specific species of insects actually pollinate this plant is unknown.
However, it produces spurred anthers, which require an insect
to trigger them before pollen is dispersed (USFWS 1999; Nelson 1996).
Fruiting occurs from October
to December, and sporadically through the year (Austin et al 1980).
Dicerandra immaculata apparently
has limited seed dispersal. Race (1994) reported that the
experimental population, introduced to two sites in Hobe Sound National
Wildlife Refuge in 1991 and 1992, had spread no more than 2m (6.6
feet) from the original parent population. As
of 1994, 1/3 of the original plants survived at first site, and
1/2 survived at the second. Both sites contained new seedlings.
As of 1997, the first site continued to produce new seedlings;
plants at the second site were flowering; and most newer plants
had become well established (USFWS 1999). Based on monitoring
of this population, Race (1994) suggested that plants grew best
when planted in the cooler spring months, and when not irrigated,
as irrigation promoted the growth of competing species.
IV. PHYSICAL TOLERANCES
V. COMMUNITY ECOLOGY
Competitors:
Dicerandra immaculata competes for space with other scrub
species, most likely saw palmetto (Serenoa repens) and
other understory plants that colonize open gaps in scrub areas (USFWS
1999).
Habitats:
Dicerandra
immaculata is found in open
scrub, sand pine scrub, and sandhills on remnants of old coastal
dunes. Optimum
habitats are open "gap" areas in scrub having varying
degrees of cover, from bare sands to areas littered with other species
growing nearby. Growth is most vigorous in sun or lightly
shaded areas, but
plants become weak and sprawling as woody species and shrubs such
as saw palmetto (Serenoa repens) become established in
open areas (Nelson 1996; USFWS 1999).
Specific substrate
types are white or yellow sands (St. Lucie,
Paola, and Astatula) that are deep and acidic, occurring
on high dune-like ridges (USFWS 1999).
Associated Species:
The essential oils of
Lakela's mint protect it from most herbivory by insects (USFWS 1999).
It is subject to mildew that grows on nectary glands. Mildew
infection destroy fruits and the viability of seeds before seeds
can be dispersed (Austin et al. 1980).
VI. SPECIAL STATUS
Special
Status:
Dicerandra immaculata has been Federally
listed as an Endangered Species since
July 23, 1984. It was originally listed due to its extremely
limited range and because of the rate in which its primary habitat
along the Atlantic Coastal Ridge was being rapidly developed for
commercial and residential uses.
The State of Florida lists
Dicerandra immaculata as Endangered.
However, under Florida law, endangered plants on privately held
land are considered the property of the owner, and are thus subject
to removal when land is sold for development. Until recently, the
only protected population of this species was experimentally introduced
in 1991 and 1992 to two sites in Hobe Sound National Wildlife Refuge
in Martin County, an area outside the species' original, limited,
range.
The Florida Natural Areas
Inventory (FNAI) considers Dicerandra immaculata to be
critically imperiled (5 or fewer occurrences, or a total of less
than 1000 individuals), both because it is a rare endemic with a
limited range, and because coastal development is encroaching on
the few, highly fragmented populations in existence.
Management issues:
Management efforts are ongoing to monitor and protect existing populations,
to purchase environmentally sensitive lands, to enforce current
protections, and to learn more about the ecology of Dicerandra
immaculata.
The management plan for
D. immaculata recommends prescribed burning as an important
conservation measure that both rejuvenates existing plants, and
also maintains the quality of the limited habitat by removing understory
plants in open scrub, thus reducing competition with other species
(USFWS 1999). To date, however, the optimum level of prescribed
burning has not been established (USFWS 1999).
Continuation of
ex situ conservation conservation efforts like those ongoing
at Hobe Sound are also recommended in the USFWS (1999) management
plan. The plan further recommends conservation of D.
immaculata germ plasm by long-term seed stage. Additionally,
more rigorous enforcement of protective measures already
being undertaken, such as prohibition of offroad vehicle use in
Dicerandra areas, and prohibiting cutting or transplanting
of existing plants, is also recommended.
St. Lucie County
has purchased environmentally sensitive lands where Lakela's mint
grows in order to protect existing plants and to reintroduce Lakela's
mint to suitable areas. Two such purchases were a
23-acre site of scrubland, and a 296-acre remnant
of the Atlantic Coastal Ridge. Both sites were within the
3-mile historical range of Lakela's mint, and border U.S. Highway
1. These sites were surveyed and found to harbor small populations
of Dicerandra immaculata. The County has been working
with area botanical gardens to monitor existing plants and introduce
new plants to these areas.
VII. REFERENCES
Austin D.E., C.E. Nauman, and B.E. Tatje.
1980. Final report:
endangered and threatened plant species
survey in southern Florida
and the National Key Deer and Great White
Heron National Wildlife
Refuges, Monroe County, Florida. U.S. Fish
and Wildlife Service; Atlanta,
Georgia.
FNAI. 2006. Florida Natural Areas
Inventory website. Statewide Tracking
List and on-line field guide. Accessed
online at www.fnai.org. June 28,
2006.
Kral, R. 1982. Some notes on Dicerandra
(Lamiaceae). Sida
9(3):238-262.
Kral, R. 1983. A report on some
rare, threatened, or endangered
forest-related vascular plants of the South.
Athens, GA, U.S. Dept. of
Agriculture Forest Service. U.S.
Dept. of Agriculture Forest Service
Technical Publication R8-TP2, Vol. 1.
Lakela, O. 1963. Dicerandra immaculata
Lakela, sp. nov. (Labiatiae).
Sida 1(3):184-185.
McCormick, K.D., M. Deyrup, E.S. Menges, S.R.
Wallace, J. Meinwald, and
T. Eisner. 1993. Relevance of chemistry
to conservation of isolated
populations: The case of volatile leaf
components of Dicerandra mints.
Pages 7701-7705 in Proceedings of the National
Academy of Science,
Volume 90.
Menges, E. S. 1992. Habitat preferences and
response to disturbance for
Dicerandra frutescens, a Lake
Wales Ridge (Florida) endemic plant.
Bulletin of the Torrey Botanical Club 119(3):
308-313.
Nelson, G. 1996. The Shrubs and
Woody vines of Florida. Pineapple
Press, Sarasota, FL.
Robinson, A.F. 1981. Status review of Dicerandra
immaculata (Lakela's
mint). Unpublished report prepared for
U.S. Fish and Wildlife Service;
Jacksonville, Florida.
Race, T. 1994. Establishment of a new population
of Dicerandra
immaculata at the Hobe Sound National
Wildlife Refuge in Florida. Unpubl.
report prepared for the U.S. Fish and Wildlife
Service; Jacksonville,
Florida. On file at U.S. Fish and Wildlife
Service, South Florida Ecosystem
Office, Vero Beach, Florida.
St. Lucie County Environmental Resources Department.
2006.
Environmentally Significant Lands Program.
Accessed online at
www. stlucieco.gov/esl/. June 28, 2006.
U.S. Fish and Wildlife Service [FWS]. 1987.
Recovery plan for three Florida
mints. U.S. Fish and Wildlife Service;
Atlanta, Georgia.
U.S. Fish and Wildlife Service [FWS].
1999. South Florida multi-species
recovery plan. Atlanta, GA.
2172 pp.
Wunderlin, R.P. 1982. Guide to
the Vascular Plants of Central Florida.
University Presses of Florida, University
of Florida, Gainesville. 175 pp.
Report by:
K. Hill, Smithsonian Marine Station
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Page last updated: June 27, 2006 |
