(revised May 2017)
Common Name: Scrub balm, Lake Placid scrub mint, scrub mint, Lloyd’s mint
Distribution: Scrub balm is federally and state listed as endangered and is endemic to the Lake Wales Ridge in south-central Florida. All populations are found on the southern Lake Wales Ridge from about Lake Placid southward (this taxon has also been described as D. frutescens Shinners subspecies frutescens). The closely related D. modesta (Huck) Huck; blushing scrub balm) includes a few populations are found near Davenport in Polk County on the Lake Wales Ridge (formerly known as 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: Seeds typically germinate in the winter. Plants 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 . The genetics of all Dicerandra species groups D. frutescens with other perennial congeners, and apart from annual congeners. D. frutescens is a hexaploid (n=24), while D. 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 (greater than 20% annually) of these short-lived plants. Mortality can be considerably higher during droughts. Vital rates show 3-5 times greater mortality, 7 times less progression to reproduction, and 6 times greater regression from flowering to vegetative states for plants greater than 6 years postfire vs. 3-6 years postfire. Population viability analyses suggest that optimal fire return intervals are 6-12 years or 24-30 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. However, germination percentages are not affected by smoke. Clipping of plants has the same effect as fire on individuals, but mowing above these low growing mints can create favorable competitive conditions. Hurricanes have little effect on this low-growing plant.
Interesting Facts: The various Dicerandra species are aromatic. The chemical compounds in their tissues serve to deter herbivores. However, one moth larvae herbivore is tolerant to the plant’s terpene-based chemical defenses, regurgitating a fluid that deters its own predators. Chemical studies of these scrub mints have resulted in the discovery of one compound previously unknown in nature. These compounds give scrub balm a characteristic minty odor. 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 7500 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.
Data Availability: We provide a summary of part of the data on our website. Raw data are available in Menges (2008).
Contact Person: Eric S. Menges