|Listed||August 26, 1988|
|Description||Shrubby evergreen with gray-green leaves.|
|Habitat||Chihuahuan Desert; dry limestone slopes.|
|Threats||Limited distribution, hybridization.|
Hinckley oak, Quercus hinckleyi, is a shrubby evergreen that grows as a single stem up to 4 ft (1.2 m) in height. It is usually seen in clonal groups that form dense thickets. From a distance, shrubs can be identified by the smoky appearance of the gray-green foliage. The leaves are about 0.6 in (15 mm) long, glabrous, and have spine-tipped margins. Acorns are produced annually and mature during the fall.
Hinckley oak is a component of the middle elevation Chihuahuan Desert scrub vegetation, and grows on dry limestone slopes at roughly 4,500 ft (1,370 m) in elevation. The habitat receives an average of 10 in (25 cm) of rainfall per year and has a frost-free season of 260 days. The species shares this habitat with other desert scrub plants, including lechuguilla (Agave lechuguilla ), whitethorn acacia (Acacia constricta ), and mariola (Perthenium incanum ).
Before the area's desertification about 5,000 years ago, Hinckley oak was probably more widely distributed across the southwestern United States and northern Mexico. A general warming and drying trend caused the decline of the shrub and may explain its current limited distribution in Texas.
Botanists surveyed similar habitats in the Mexican state of Coahuila but discovered no additional colonies. Hinckley oak may occur in the Dead Horse Mountains where the habitat is similar. There is evidence of acorns in fossil samples dating back several thousand years, but so far none of the shrubs have been found there.
Presently, four populations of Hinckley oak have been documented. Because of the clonal nature of the shrub, population estimates are highly subjective. Three colonies are present in the Solitario, a circular laccolith (an underground lava formation) approximately 8 mi (13 km) in diameter in Presidio and Brewster Counties, Texas. The largest known population occurs there, estimated between 300 and 500 plants in 1988. The two other Solitario sites contain fewer than 50 plants each. A fourth population, surveyed in south-central Presidio County, was estimated at 30-40 plants in 1982. Three of the four documented populations occur on private land and one population occurs on state of Texas land.
Several previously documented sites in Presidio County have not been relocated, despite extensive searches made in 1982.
Three of the known populations of the Hinckley oak occur on private land and are unprotected. In 1988, it was determined that a fourth colony stood on state land, which was leased to a private ranch for grazing. Levels of cattle grazing are currently low enough so that plants are not harmed. Should exotic goats or sheep be introduced to the habitat— as has occurred on nearby ranches—the Hinckley oak would probably be jeopardized. As food sources are scarce in this desert environment, most of the annual acorn crop is consumed by animals.
Because other species of oak grow in the same habitat, Hinckley oak is susceptible to "genetic swamping," the loss of a pure genetic strain through the hybridization of two similar species. The plant hybridizes so readily that botanists at Texas A & M University were forced to discontinue propagation efforts when seedlings were determined to be hybrids.
Overcollecting of acorns has also contributed to the species decline and current status. Loss of acorns in turn reduces recruitment. The oak has a potential as a cultivary as it is very attractive. Propagation research was conducted at Texas A & M Research and Extension Center, Dallas, but the work ceased because researchers believed they were dealing with hybrids. Although the plant grows easily from acorns, some people wanting to cultivate or continue propagation of the species may collect whole plants, further damaging the population.
Conservation and Recovery
Recovery strategies for the species include fencing the current state-owned site, and working with local landowners to ensure the integrity of the three privately owned sites. Further attempts to propagate the shrub for reintroduction to suitable tracts of habitat are anticipated.
Miller, D. J., and A. M. Powell. 1982. "Status Report on Quercus hinckleyi." U.S. Fish and Wildlife Service, Endangered Species Office, Albuquerque.
Muller, C. H. 1951. "The Oaks of Texas." Contributions from the Texas Research Foundation 1:40.
U.S. Fish and Wildlife Service. 1992. "Threatened and Endangered Species of Texas. Hinckley's Oak (Quercus hinckleyi )." U.S. Fish and Wildlife Service, Washington, D.C.
Van Devender, T. R., et al. 1978. "Full-Glacial and Recent Vegetation of Livingston Hills, Presidio County, Texas." Southwestern Naturalist 23:289-302.