Habitat
T. racemosa grows on the Coastal Plain from New Jersey to north Florida and west to southeast Texas. It is common in North Carolina, South Carolina, Georgia, Mississippi, and Florida. A disjunct population exists in Tennessee which is isolated from those in the Coastal Plain. T. racemosa occurs in wet, sandy-peaty soils, pine savannas and flatwoods, shrub bogs and seepage slopes.
At the northern edge of the species' range in New Jersey there are five confirmed extant occurrences (New Jersey Natural Heritage Program, 1994). All are on state owned land in open bogs or savannas edged by white cedar swamps and pitch pine lowlands, with a total number of plants estimated at less than one thousand. There are five additional historical sites, three of which may be extant. The remaining two historical sites were destroyed by agricultural activity (New Jersey Natural Heritage Program, 1994). Within New Jersey it is often found growing with a closely related and also endangered lily species which is now restricted to the New Jersey Pine Barrens, Narthecium americanum (Stone, 1911; Schuyler, 1990). Associated herbaceous species at Narthecium locations include Aster nemoralis, Calamagrostis pickeringii, Calamovilfa brevipilis, Juncus caesariensis, Lophiola americana, Muhlenbergia torreyana, Platanthera integra, Pogonia ophioglossoides, Rhynchospora oligantha, Schizaea pusilla, and Sphagnum spp. (Schuyler, 1990).
In Delaware the species is considered historical at five locations and possibly extirpated from the state. The species was once present at sites described as sea-level fens, herbaceous wetlands just above high-tide and adjacent salt marshes. These wetlands contain deep peaty muck and are formed from low-nutrient, acidic fresh ground water seepage from adjacent uplands (McAvoy, pers. comm.). The historical sites are now either Spartina alterniflora or Phragmites australis marshes.
In Virginia the species was always rare (Harvill, 1973). There is now only one extant site, estimated at 28 individuals in 1992 (Virginia Natural Heritage Program, 1992). The plants persist in a railroad right of way near the edge of a boggy swale in a habitat unusually dry for this species. Other species at the site include Pteridum aquilinum, Sericocarpus linifolia and Gaylussacia spp.
In Tennessee there is one extant site of several hundred individuals which is disjunct from the rest of the Coastal Plain populations. The site is an open, wet savanna within the oak barrens of the southeast highland rim. This area supports several other species disjunct from the Coastal Plain including Platanthera nivia and Carex barattii (Pyne, pers. comm.).
T. racemosa becomes increasingly common along the Gulf Coast. It is common throughout west Florida panhandle bogs, seepage slopes and wet prairies to just east of the Appalachicola river, at probably 50 to 100 sites (Hilsenbeck, pers. comm.). Habitat types in the Florida panhandle were described by Schwartz (1994). T. racemosa is listed as one of the species co-occurring at several sites in the Florida panhandle with a newly described species, Eriocaulon nigrobracteatum (Orzell and Bridges, 1993). The habitat that these plants grow in was characterized as deep unstable sapric muck soils of lower slope seepage fed herbaceous communities. Orzell and Bridges (1993) refer to the habitat as poor fens, which are oligotrophic to weakly minerotrophic, somewhat acidic, nutrient deficient mires poor in species of Sphagnum. The occurrences of these sites is correlated with the escarpment of sandy upland terraces reflecting various relict shorelines. Poor fens apparently form along the middle and upper reaches of small stream valleys in upland terraces. In these areas topographical relief attracts groundwater from the unconsolidated surficial aquifers (Orzell and Bridges, 1993). Other west Florida wetland habitats were described in a study of pine savannas containing Cladium mariscoides, a species recently reported for the Florida flora (Bridges and Orzell, 1993).
The range of T. racemosa continues along the coast, with probably several hundred occurrences within the appropriate habitat type in southern Mississippi. In southeast Louisiana to the east of the Mississippi there are 26 confirmed occurrences in St. Tammany and Washington Parishes. These eastern Louisiana savannas support more state-rare plant species than any other habitat type in the state (Louisiana Natural Heritage Program, 1990). The westernmost edge of the species' range is in southeast Texas.
Ecology
In New Jersey, T. racemosa flowers from late-June to mid-July. Little is known about the pollination biology of the species. Pollination studies of other bog species revealed that Diptera and Lepidoptera were the common pollinators of T. glutinosa and other white flowered species (Douglas, 1983). Population growth in T. racemosa is predominantly due to rhizomatous spread rather than from sexual reproduction (Summerfield, 1974). No reproduction from seed was observed in transplanted populations of the European T. pusilla, which was instead successfully established through vegetative growth in a manner similar to T. racemosa (Cranston and Valentine, 1983).
Flowering is probably inhibited by shading, as for many other bog species including Narthecium americanum (Schuyler, 1990). The lack of competition with woody vegetation in open bog and savanna habitats promotes growth and flowering of bog species.
Maintenance of open habitat may be due to hydrology, soil conditions, disturbance such as fire, or a combination of factors. Bog areas have very hydric, acidic and nutrient poor soils, providing poor conditions for vigorous growth of tree species. Frequent fire contributes to these edaphic factors by providing disturbance that eliminates young invading tree seedlings and keeps these areas open (MacRoberts and MacRoberts, 1993).
Fire is important for the stimulation of flowering of herbs and shrubs, eliminates invading hardwoods, and exposes mineral soil necessary for seedling microsites (Louisiana Natural Heritage Program, 1990). Fire also alters the time and duration of flowering in pine flatwoods (Platt et al., 1988). In the absence of fire an open longleaf pine flatwood savanna can become overgrown with shrubs in as little as five years (Louisiana Natural Heritage Program, 1990). The pre-historic natural frequency of fires in southeastern Louisiana pine savannas has been estimated to be once every 1-4 years, with lightning fires occurring in the early growing season from late March to early June along the Gulf Coastal Plain (Louisiana Natural Heritage Program, 1990). Northern savannas in the New Jersey Pine Barrens, however, are believed to be maintained primarily by hydrology, with the effects of fire less clear.
T. racemosa is apparently dependent on water moving through the substrate, as may be other species occurring in this habitat such as N. americanum (Schuyler, 1990). Permanent flooding will eliminate these species, such as that caused by beaver (Hammerson, 1994), as will alterations in hydrology by cranberry culture common in the New Jersey Pine Barrens (Stone, 1911). Natural flooding patterns will likely maintain open habitat for the species.