The following Participating Institutions are custodians for this species in the CPC National Collection:
Rae Selling Berry Seed Bank & Plant Conservation Programs
University Of Washington Botanic Gardens
The conservation of Trifolium thompsonii is fully sponsored.
Edward Guerrant, Ph.D. contributed to this Plant Profile.
Trifolium thompsonii probably doesn't look like any other clover you've ever seen. It grows up to two feet tall and its large, spherical, reddish-lavender flower heads rise above the swaying grass like a sea of cheerleader pom-poms. Unlike most clovers, the leaves of this plant are comprised of 3 to 8 narrow, pointed leaflets (most clovers have three leaflets, hence the name "trifolium" or "three leaves"). It grows only within a fairly limited geographical distribution (only 20 miles north to south), but Trifolium thompsonii can be found in a wide variety of habitats. It grows in open ponderosa pine woods, grass and herb dominated areas, and sagebrush steppe. Steep slopes, along ridgelines, in alluvial fans and along canyon bottoms anywhere from 140 feet to 3760 feet (40m to 1130 m) in elevation are likely places to find this species.
Distribution & Occurrence
Trifolium thompsonii grows in a wide variety to habitats. The majority of populations grow on the fringe of the ponderosa pine (Pinus ponderosa) zone in open ponderosa pine woods or grass and herb dominated areas. It grows along ridgelines, on steep slopes, in alluvial fans, and in canyon bottoms anywhere from 140 feet to 2760 feet (40 m to 1130 m).
|Approximately 15 sites have been observed since 1986. Numbers range from as low as 30-40 to as high as several thousand per site. 11 of the known sites are on Forest Service Land (WNHP 2000).|
Conservation, Ecology & Research
Trifolium thompsonii is found primarily in microsites that are intermediate between well exposed, dry, south-facing slopes and more shaded, mesic, and highly vegetated sites (WNHP 1999). Occasional fires may have played an important role in maintaining suitable habitat for Thompson's clover. It is not known why this species has such a varied habitat but such a limited distribution.
Like many other members of the pea family, Trifolium thompsonii is able to form a symbiotic relationship with nitrogen-fixing bacteria. Thus, this plant has an advantage over competing vegetation in soils that are low in available nitrogen (WNHP 1999).
The primary pollinators appear to be a butterfly (Plebejus saepiolus) and several species of bumblebee (Bombus sp.). Long distance pollen transfer may be fairly common between nearby populations. Seed predation can often be very high. In one study, over 14% of observed seeds had some kind of insect boring (Canfield 1977).
The invasion of exotic species such as knapweed (Centaurea diffusa) and cheatgrass (Bromus tectorum).
Illegal ORV use in population areas.
Germination trials at The Berry Botanic Garden were inconclusive. Seeds were subjected to either 8 weeks of cold stratification or no cold stratification followed by either constant 68F (20C) or alternating 50 /68F (10/20C) temperatures. Without cold stratification, both temperature treatments showed 40% germination. With cold stratification, the constant 68F treatments showed 40% germination while the alternating 50/68F treatment showed 80% germination (BBG File).
Study potential factors influencing the limited distribution of Trifolium thompsonii.
Determine optimum germination conditions. Test conditions corresponding to natural elevation and habitat gradients for this species.
Determine propagation and reintroduction protocols.
WNHP. (2000). Washington Natural Heritage Program Database. Olympia, Washington.