Rank Reasons104 extant occurrences known, but most populations (72%) are very small, C- or D-ranked occurrences of less than 100 rooted crowns. Nine occurrences (8%) are A-ranked quality with 1000 or more rooted crowns per site (one site in West Virginia with over 100,000 rooted crowns), and 20 occurrencess (20%) are B-ranked quality with between 101-999 rooted crowns. Total estimated number of rooted crowns globally : 106,955. Most historically known sites are extirpated, reducing its known range to small portions of West Virginia, Ohio, Kentucky and Indiana, with one C-RANKed site in Missouri. Ohio, and West Virginia rank this element "S2", Kentucky ranks it "S2S3", and Indiana and Missouri rank it S1. It's habitat is primarily limited to moderately disturbed areas such as old roads, old home sites, cemeteries, jeep trails, etc., which pass through mesic woodlands in regions underlain with limestone (with notable D-RANKed exceptions, e. g. - sandy stream terrace at Cotton Hill, WV), and soils with low Phosphorus content.
Threat Impact CommentsCampbell et al (1988) listed several hypotheses for the decline of running buffalo clover: 1) initial habitat destruction by the wave of new settlers, 2) poor dispersal to new sites following the elimination of bison and other large herbivores, 3) loss of the natural grazing regime, 4) increased consumer pressure from increased cattle herds and rabbit populations, 5) increased competition from exotic plants, 6) reduced fire frequency resulting in the loss of open woodlands, and 7) lack of rhizobial infection. At the Running Buffalo Clover Information Sharing Meeting in December of 1995, these hypotheses were reviewed and some participants felt that there was no credible evidence to support numbers 6 & 7 (reduction of fire frequency and lack of rhizobial infection) as a valid cause of the decline of the species. Dr. Michael Vincent expressed concern that there was no solid evidence that these species ever historically had rhizobial associates, and further stated that openings in the canopies to produce woodland habitat for running buffalo clover likely happened with or without fire involvement.
The following suggestions were made at the 1995 Cincinnati meeting regarding current major threats to this species, namely: 1) any irreversible, catastrophic disturbance, such as road construction that completely destroys the habitat and/or kills all plants and seeds within the path of the disturbance, 2) the closing of forest canopies through succession to the point of severe shading, leading to reduced flower and fruit production, 3) loss of habitat through natural or human causes, 4) a reduction of hooved mammals for dispersal of the species' seeds and vegetative fragments, 5) low genetic diversity among populations of T. stoloniferum as reported by Hickey et. al. (1991) and Hickey & Vincent (1992) and Crawford 1995; 6) low population size (less than 30 rooted crowns, or "D-ranked quality" of 41% of the extant occurrences and associated fragility and susceptibility to destruction by vehicle and foot traffic, use of heavy equipment, etc.; 7) a range of viruses that have been observed attacking the species at Missouri Botanical Garden and in introduced populations in Missouri, 8) herbivory by mammals, especially rabbits, groundhogs, etc., 9) fungal diseases, including "tar spot", 10) canopy closure cause too much shade, or canopy removal causing too much sunlight, 11) reduction in a plant pollinator, 12) competition from non-native invasive plant species, especially from Microstegium vimineum, Trifolium repens, and Alliaria petiolata, 13) over grazing, and 14) disruption of moderate prolonged disturbance.
Researchers at that meeting from West Virginia (Harmon, pers. comm.), and Ohio (Windus, pers. comm.) reported the permanent loss of small populations of C- & D-ranked, and one B-ranked quality sites
Until recently, most populations were reported to be small, with plants scattered over an area of several meters (Cusick, pers. comm., Leblanc and Aldrich 1988). Since 1990, 9 large, A-ranked quality sites each with over 1000 rooted crowns present, and 16 B-ranked quality occurrences with 101-1000 rooted crowns present, have been found in West Virginia, Kentucky and Ohio (Harmon 1995, White 1995, Windus 1995). Very small populations are reported to be at high risk of extirpation through physical disruption, disease and inbreeding depression (USFWS 1989).
It has been suggested that T. stoloniferum has a poor dispersal mechanism (Cusick 1989b). According to Cusick, deforestation created many new habitats for the species, but with the loss of bison, elk and eventually deer, there were no effective means of dispersal remaining. White-tailed deer and bison were effectively eliminated from the landscape due to over-hunting. Only recently have deer returned to pre-settlement numbers. According to this theory, habitat in which T. stoloniferum lived, gradually grew closed due to the absence of disturbance by ungulates and fire. Although a presumed primary disperser (deer) is again present, relatively few populations have survived (compared to presettlement numbers of populations.
Jacobs and Bartgis (1987) suggested that along with the destruction of habitat through the clearing of agricultural lands, the introduction of the non-native species, European white clover (Trifolium repens), and bluegrass (Poa pratensis) may have contributed to the decline of this native clover.
D. White (1995) of Kentucky Heritage Program reported that the exotic grass microstegium is a serious threat to many native populations in Kentucky, in that it is strongly competing for resources with running buffalo clover. The white clover is more aggressive and may have invaded the habitat of running buffalo clover, out-competing it for available resources, as well (Jacobs and Bartgis 1987).
Homoya et al. (1989) stated that the removal or suppression of native vegetation by bison [or other large mammals, PJH] may have created the open understory and light gaps necessary for this species. Various researchers at the 1995 meeting in Cincinnati supported the hypothesis that appropriate habitat was more likely produced in presettlement time through canopy gaps created by the falling of large, old-growth trees. Jacobs and Bartgis (1987) suggested that the bison may have provided the right balance of periodic disturbance, soil enrichment, seed dispersal and seed scarification necessary to maintain the clover. It was reasoned that without some level of disturbance, a site would become too shaded to provide enough sunlight for the species (Homoya et al. 1989, Cusick 1989b, Cusick 1988a).
According to Homoya et al. (1989),however, the removal of bison does not completely explain the range-wide depletion of this species. They suggested that there was not a sufficient time interval between bison loss and cattle introduction to account for the rarity now present in the species, since cattle should have satisfied the same biological necessities as bison. However, many researchers currently feel that the ecological equivalency of bison and cattle is uncertain (Vincent, pers. comm., Windus, pers. comm., Harmon, pers. comm.). Davis (1987) likewise disputed the attribution to bison as the cause of the species' decline. In any case, overgrazing by the modern counter-part of the bison, cattle, appears to pose a substantial risk to this species at some sites (Leblanc and Aldrich 1988).
Jacobs and Bartgis (1987) suggested that the decline of this species may have partially centered around a pathogen introduced from the exotic white clover, however no specific disease had been identified at that time. A number of viral and fungal diseases are reported to have attacked the species in greenhouses at Missouri Botanical Garden, including cucumber mosaic virus and the como virus.
The species seems to be a favorite food of some herbivores. At the Missouri Botanical Garden, running buffalo clover plants are habitually gnawed to the ground by rabbits, slugs and other rodents (Pickering 1989). Only protected plants did well. Similar observations have been made in Kentucky (Baskin pers. comm., Davis 1987) and in West Virginia (Foster pers. comm.). It should be noted, however, that herbivores are also the likely dispersers of seeds for this species, so palatable greens are an evolutionary advantage for the species as a whole (M. Vincent, pers. comm.).
Unlike all other species within the genus Trifolium, running buffalo clover lacks a rhizobial associate. Small nodules, uninfected by Rhizobium, have been observed in cultivation, suggesting past associations (USFWS 1989). An experiment has been conducted cooperatively at the Fernow Experimental Forest by the Northeast Forest Experiment Laboratory, the United States Department of Agriculture Agricultural Research Service, Beckeley, West Virginia, and the West Virginia Natural Heritage Program, DNR. Unpublished results suggest that 1) this species does not fix organic nitrogen, 2) that no root nodules of the nature typical of rhizobium infections in other clovers were observed, and 3) that this species seems to respond well to 3 inch clipping. It was stated by Morris, et. al. (1995) that these rusults suggest that T. stoloniferum can tolerate moderate grazing, and is vigorous despite the lack of organic nitrogen fixation.
Land development and the consequential loss of habitat is also a serious concern. In Ohio, urban sprawl around Cincinnati is believed to have destroyed much of the available habitat for this species (Cusick 1987).
Taylor, et. al. (1994) and Hickey, et. al. (1991) cite evidence that inbreding depression may have contributed to the decline of T. stoloniferum. Trifolium spp. native to eastern North America have strong barrier to cross pollination. To a limited degree, T. stoloniferum can set self seed, but inbreeding and loss of vigor were demonstrated by Taylor, et. al. (1994).