Species Account: Dicerandra frutescens (Lamiaceae) Shinners
(revised 14 October 2003)
Common Name: Scrub balm, Lake Placid scrub mint
Distribution: Scrub balm is federally and state listed as endangered and is endemic to the Lake Wales Ridge in south-central Florida. Most populations are found on the southern Lake Wales Ridge from about Lake Placid southward (this group has been described as D. frutescens Shinners subspecies frutescens). A few populations are found near Davenport in Polk County on the Lake Wales Ridge (D.frutescens subspecies modesta Huck).
Habitats: Scrub balm is found almost exclusively on well-drained yellow sands, in Florida scrub. Populations occur in scrub dominated by oaks, especially myrtle oak. Some areas have abundant scrub hickory or sand pine. This species is not generally found in sandhill vegetation. Scrub balm also grows well in disturbed areas on appropriate soils, including roadsides, firelane edges, and power-line right-of-ways. Studies of its microhabitat preferences confirm that it is a specialist for gaps in Florida scrub dominated by evergreen, xeromorphic oaks. Microsite occupancy decreases with litter depth, litter cover, and shade.
Life History: Scrub balm is a short-lived perennial shrub, to about 0.5 m tall. Plants older than 8 years are very uncommon based on our long-term demography data. The plants are branched at the base from a single taproot. Plants are generally killed by fire, although patchy fires allow survival of individual plants. Population recovery from complete fires is exclusively via dormant seeds in a persistent soil seed bank. Seed dispersal is very limited, and patches of postfire plants are generally in the same locations as the prefire population.
Phenology: Seedlings typically germinate in the winter, flower in September and October, and form fruits later in the fall. Seeds and fruit disperse (very locally) in the fall and winter. The basal parts of the plants are perennial and maintain leaves year-round. During the summer and fall, annual shoots with leaves are produced. Flowers and fruits appear on these shoots. The annual shoots die back each winter.
Breeding System and Pollination: Although described in one publication as an obligate outcrosser, self-pollinations do produce viable, germinating seeds. Inbreeding and outbreeding depression are possibilities but require more research. Plants are pollinated primarily by bee-flies in southern Highlands county. Exprosopa fasciata was the bee-fly responsible for 95% of flower visits, and the average flower received about 46 visits by this insect. Plants in open habitats and with more flowers received more bee-fly visits. The bee-flies tend to contact anthers in the morning and stigmas in the afternoon, increasing the chances of cross-pollination. Pollen is concealed in the anthers. When bee-flies contact spur-like appendages on the anther, this triggers pollen release. The pollen is deposited on the bee-fly and carried to another flower. Bee-flies were common in most habitats and concentrated on scrub balm, so pollinator limitation of fecundity appears unlikely.
Genetics: Genetic variation (studied using isozymes) is particularly low in D. frutescens, compared with other Lake Wales Ridge endemics. There seems little differentiation in isozyme variation among populations, suggesting that they once may have been linked on long ridges of yellow sand, before most of these areas were converted to citrus. On the other hand, there is abundant morphological variation among wild populations within D. frutescens. Additional genetic research on the genus is being conducted by Doug and Pam Soltis and collaborators (University of Florida). D. frutescens subsp. frutescens is a hexaploid (n=24), while D. frutescens var. modesta is a tetraploid (n=16).
Population Dynamics: Scrub balm has variable population sizes. Seedling recruitment varies widely (ca. 50-fold) from year to year and survival of seedlings is often low during dry periods in late spring. Seedling microsites may require a combination of mineral soil substrates and partial shade in many years, but seedlings do survive and grow well in open postfire sites. Population size peaks about 6-8 years postfire, and most demographic parameters also peak during the first decade. After this, populations decline due to low recruitment, slow growth, and fairly high mortality (> 20% annually) of these short-lived plants. Mortality can be considerably higher during droughts. A population viability analysis suggests that optimal fire return intervals are 6-10 years. Spatial dynamics in scrub balm are very subtle due to limited seed dispersal. There are likely gap-level dynamics. Limited dispersal and gap closure between fires combine to make habitat patches smaller and more isolated between fires, and larger and more connected shortly after fire. Fire kills all plants directly affected by high temperatures, but patchy fires allow survival of individual plants. Population recovery from complete fires is exclusively via dormant seeds in a soil seed bank. Clipping of plants has the same effect as fire on individuals, but mowing above these low growing mints can create favorable competitive conditions.
Interesting Facts: The various Dicerandra species are aromatic. The chemical compounds in their tissues serve to deter herbivores. Chemical studies of these scrub mints has resulted in the discovery of one compound previously unknown in nature. These compounds give scrub balm a characteristic minty odor. The subspecies of D. frutescens are distinguished by flower characters and chromosome number. The five species of narrowly distributed, allopatric, perennial Dicerandra species in peninsular Florida are of biogeographic interest. They may be recently evolved (neoendemics). D. christmanii populations are about 10.5 km north of the range of D. frutescens.
Data Collected by Archbold Biological Station’s Plant Ecology Lab: Since 1988, we have collected data on individually marked plants at a number of sites. Data collection was initially monthly but now is quarterly or annually depending on population. Our dataset now consists of 11 populations and over 4000 plants. We also have conducted seed germination experiments in the field and laboratory, sub-sampled for fecundity in many years, and conducted research on seed dispersal. Genetic work has also been completed.
Data Availability: We provide a summary of part of the data on our Web site.
Deyrup, M.A. and E.S. Menges. 1997. Pollination ecology of the rare scrub mint Dicerandra frutescens (Lamiaceae). Florida Scientist 60:143-157.
Evans, M.E.K., E.S. Menges, and D.R. Gordan. In revision. Mating systems and limits to seed production in two Dicerandra mints endemic to Florida scrub. Biodiversity and Conservation
Finer, M. and E.S. Menges. In preparation. Seed dynamics of the endemic Florida scrub mint, Dicerandra frutescens: Implications for Fire Management.
Huck, R.B. 1987. Systematics and evolution of Dicerandra (Labiatae). Phanerogamarum monographiae tomus XIX.
Huck, R.B. 2001. Two new intraspecific taxa in Florida Dicerandra (Labiatae). Novon 11:417-420.
Huck, R.B., W.S. Judd, W.M. Whitten, J.D. Skean, Jr., R.P. Wunderlin, and K.R. Delaney. 1989. A new Dicerandra species from the Lake Wales Ridge of Florida, with a cladistic analysis and discussion of endemism. Systematic Botany 14:197-213.
McCormick, K.D., M.A. Deyrup, E.S. Menges, S.R. Wallace, and J. Meinwald. 1993. Relevance of chemistry to conservation of isolated populations - B the case of volatile leaf components of Dicerandra mints. Proceedings of the National Academy of Sciences 90:7701-7705.
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 119:308-313.
Menges, E.S., P.J. McIntyre, M.S. Finer, E. Goss, and R. Yahr. 1999. Microhabitat of the narrow Florida scrub endemic Dicerandra christmanii, with comparisons to its congener D. frutescens. Journal of the Torrey Botanical Society 126:24-31.
Menges, E.S., R.W. Dolan, R. Yahr, and D.R. Gordon. 2001. Comparative genetics of seven plants endemic to Florida’s Lake Wales Ridge. Castanea 66:98-114.
Menges, E.S., P.F. Quintana-Ascencio, C.W. Weekley, and O.G. Gaoue. In preparation. Population viability analysis of an endemic Florida scrub mint.
Archbold Biological Station
123 Main Dr
Venus, FL 33960