San Diego Mesa Mint
San Diego Mesa Mint
|Listed||September 28, 1978|
|Description||Annual herb with aromatic leaves and lavender flowers.|
|Habitat||Seasonal pools atop mesas.|
|Threats||Road construction, off-road vehicles.|
As an herbaceous annual, the San Diego mesa mint, Pogogyne abramsii, occasionally reaches 1 ft (30 cm) in height and typically blooms from May to early July. The plant can be very branched, and the vegetative and floral portions give off a strong, sweet mint odor. The vegetation develops a reddish tinge as the plant matures and flowers. The flowers are purple with white or yellow throats. The San Diego mesa mint typically has only two flowers per node, a hairy calyx, and thin bracts subtending each flower
The reproductive patterns are highly dependent on the amount of seasonal rainfall. Submerged plants grow very slowly and develop different leaves. If the pools are continuously filled with water, reproduction may be delayed several months. Plants around the perimeter of the pool, where there is less water, flower much earlier.
Seeds usually remain inside the calyx, which is attached to the parent plant until disintegration occurs with the next seasonal rains. The seeds are then carried from one pool to another by water overflows during heavy rains. It is thought that wind and birds also disperse some seed.
This mint prefers the dried beds of vernal pools atop stony mesas. These pools, ranging in size from a few square meters to several acres, have standing water about four months a year (December through April). Seedling mortality is greatest in the regions of the pool that are inundated with deep water for long periods of time; mortality ceases when the pool dries, and almost all plants that have survived to this point will reach reproductive age. Surrounding vegetation is chaparral and coastal sage scrub.
The San Diego mesa mint is endemic to San Diego County, California. The northern limit of distribution for this species is Del Mar Mesa, and it occurs south on Mira Mesa, Marine Corps Air Station (MCAS) Miramar, and Kearny Mesa, with a few scattered populations in western Tierrasanta. Populations have been extirpated from the Linda Vista area, the vicinity of Balboa Park, Normal Heights, and the area surrounding San Diego State University. Although most of these extirpated populations from the San Diego Mesa are labeled as P. nudiuscula on herbarium collections, these specimens have not been annotated and should be probably be considered P. abramsii.
It survives at disjunct locations in declining numbers. The populations fluctuate greatly with the amount and timing of rainfall during any given season. Seed set is minimal in a dry year, but seed storage is sufficient to allow populations to survive several successive dry seasons.
Much of the land within the suspected historical range of this plant, particularly in the vicinity of Otay Mesa, was long ago converted to agricultural uses, initiating the mesa mint's decline. Road improvement work in the 1970s, particularly along the Miramar Road corridor, destroyed dozens of vernal ponds and eliminated several known populations. Various planned road improvement projects near Miramar Naval Air Station and Miramar Mounds Natural Landmark (north of San Diego), or proposed construction on Highway 52, would further shrink the amount of available habitat.
The popularity of recreational off-road vehicles has also become a threat to the San Diego mesa mint. In some areas, these vehicles have severely damaged the fragile, semi-arid habitat by stripping plant cover from the soil.
A pollinator is required for seed to be set; populations that are restricted to small pools that are isolated from adjoining natural habitat may have reduced numbers of pollinators, which would reduce reproduction.
Conservation and Recovery
The primary objective of the Recovery Plan is to secure the species by preserving all essential vernal pools on private lands (114 acres; 46.1 hectares) and the habitat on the Miramar Naval Air Station. Additionally, the Recovery Plan recommends developing artificial habitats; developing plans for wet-land habitat management and modification; salvaging plants and seed stocks from wild populations; and preventing disturbance to vernal pools by enforcing laws; restricting vehicular travel near the pools, and appealing to public support.
The San Diego mesa mint was listed by the State of California as endangered in January 1979, under the California Endangered Species Act, and is currently recognized as such.
U. S. Fish and Wildlife Service
Regional Office, Division of Endangered Species
Eastside Federal Complex
911 N.E. 11th Ave.
Portland, Oregon 97232-4181
Armstrong, W. P. 1978. "Four Wildflowers Vanishing from Northern San Diego County." Environment Southwest 480 (Winter):3-6.
Oberbauer, T. A. 1978. "San Diego County and its Rare Plants." Fremontia 5(4):12-15.
U. S. Fish and Wildlife Service. 1998. "Recovery Plan for Vernal Pools of Southern California." U. S. Fish and Wildlife Service. Portland. 160 pp.