Macbridea is a small genus of two perennial herbs in the mint family (Lamiaceae), first described in 1818. Both species are native to the southeastern United States, where they inhabit the wetlands and fire-maintained flatwoods of the Atlantic and Gulf coastal plains, and both are considered rare.
Plants in the genus are herbaceous perennials, typically producing slender, sometimes branching stems that reach 30 to 40 centimetres in height. The stems and paired leaves are often glandular, giving the foliage a sticky texture. Flowers are arranged in a thyrse — a raceme subdivided into compact cymes — and are nestled among tightly packed, pointed bracts. The bilabiate (two-lipped) corolla is distinctive: in Macbridea alba, it is white with occasional pale purple markings in the throat; the open flowers and rounded buds sitting among the green bracts give both species their shared common name "birds-in-a-nest." Blooming occurs from May through July, and pollination is carried out primarily by bumblebees (Bombus spp.).
The genus contains exactly two species. Macbridea alba (white birds-in-a-nest or white macbridea) is endemic to four counties in the Florida Panhandle — Gulf, Liberty, Franklin, and Bay — where it grows in pine flatwoods, seeps, wet savannas, and the margins of swamps and sandhills on wet, infertile, peaty sandy soils. Roughly 40 percent of all known individuals occur within the Apalachicola National Forest. It is federally listed as a threatened species in the United States. Macbridea caroliniana (Carolina birds-in-a-nest or Carolina bogmint) has a somewhat wider distribution across the Atlantic coastal plain of North Carolina, South Carolina, and Georgia, occurring in swamp forests, though it too is considered rare throughout its range.
Distribution
Macbridea is native to the southeastern United States. M. alba is endemic to the Florida Panhandle (Gulf, Liberty, Franklin, and Bay counties), with the majority of its roughly 10,000 individuals concentrated in the Apalachicola National Forest. M. caroliniana ranges across the Atlantic coastal plain from North Carolina and South Carolina to Georgia, with unconfirmed reports from Florida, Alabama, and Mississippi.
Ecology
Both species occupy wetland-associated habitats maintained by periodic disturbance. Macbridea alba is found in pine flatwoods, seeps, wet savannas, and ecotones bordering swamps and sandhills, favouring wet, infertile, peaty or sandy soils. M. caroliniana occurs in swamp forests of the coastal plain. The flatwoods habitat of M. alba depends critically on periodic wildfire: fire suppression leads to woody vegetation build-up, shading out the herb layer; populations tend to increase markedly in the years following fire. Flowers are pollinated primarily by bumblebees (Bombus spp.).
Conservation
Macbridea alba is federally listed as a threatened species under the US Endangered Species Act. Approximately 10,000 individuals remain across several scattered populations in four Florida Panhandle counties, and several formerly known occurrences have been extirpated. Key threats include urban development and road construction, drainage and habitat alteration, and the suppression of the natural fire regime on which pine flatwoods depend. About 40 percent of occurrences fall within the Apalachicola National Forest. Macbridea caroliniana is not federally listed but is considered rare throughout its Atlantic coastal plain range.
