It has been documented that San Diego thorn-mint can re-establish after fire. An
herbarium specimen from the Rancho Santa Ana Botanic Gardens documents that San
Diego thorn-mint is present after fire. The plant specimen was collected in 1995
and reported near the Cuyamaca/Laguna Mountains region in a "burned
area" [7].
During the October 2003 Cedar and Paradise wildfires on the Cleveland National
Forest, chamise chaparral populations of San Diego thorn-mint on Viejas Mountain experienced high-severity fire.
Soon after the fires (November 2003), San Diego thorn-mint plants were not located,
but San Diego thorn-mint was expected to regenerate from the soil seed bank
[32]. No further monitoring results were available as of this write-up
(2005).
This description provides characteristics that may be relevant to fire ecology, and is not meant for identification. Keys for identification are available [15,22,37].
San Diego thorn-mint is a low-growing, native annual herb. Stem height ranges from 2 to 8 inches (5-20 cm), with few to many branches [15,22,31,32,37]. The head-like inflorescence is terminal; the flowers are in whorls each having distinct spiny bracts below. The 0.47-inch (12 mm), funnel-shaped flowers have 2 lips, the upper lip smaller than the lower lip. The fruit is a schizocarp containing 4 smooth, ovoid nutlets. The nutlets remain in the flower calyx until the rainy season [1,15]. San Diego thorn-mint is distinguished from other species of thorn-mints by having sterile upper stamens and flowers with hairless anthers and styles [15,35].
Fire adaptations: San Diego thorn-mint establishes from seed. Seeds of San Diego thorn-mint are stored in the soil seed bank [1,32].
FIRE REGIMES: San Diego thorn-mint occurs in a mediterranean climate, with mild winters and hot, dry summers [18]. The summer drought generates low fuel moistures in habitats that typically experience stand-replacing fire.
Chaparral: The chaparral ecosystem with natural fire return intervals of 50 to 80 years usually experiences high-severity fires [25]. In fall, southern California typically experiences hot, dry "Santa Ana" winds that drive chaparral fires. Such fires can easily burn thousands of hectares, killing most aboveground vegetation [4]. Coastal sage scrub experiences fires of similar severity, but with shorter frequency intervals of 20 to 25 years [36]. Abundant postfire growth of herbs and sprouting shrub species combined with a dry fire season can result in chaparral and coastal sage scrub reburns in only 1 to 2 postfire years. Short fire return intervals may convert shrublands to annual grasslands [29].
Annual grassland: The presettlement fire frequency for California grasslands is assumed to be similar to present fire frequency, although fire size may have been larger. Historically, California grasslands consisted of perennial grasses and forbs, but most have been converted to annual nonnative grass species [14]. Nonnative grass dominants may influence the fire regime [13]. California annual grasslands characteristically support fast-moving wildfires. Annual grasses germinate, set seed, and die in a single season, quickly curing into flashy fuels. Grassland fire return intervals range from 4 to 20 years, depending on local climate and ignition sources [25].
The following table provides fire return intervals for plant communities and ecosystems where San Diego thorn-mint is found. Find further fire regime information for the plant communities in which this species may occur by entering the species name in the FEIS home page under "Find FIRE REGIMES".
Community or Ecosystem Dominant Species Fire Return Interval Range (years) California chaparral Adenostoma and/or Arctostaphylos spp. 25] coastal sagebrush Artemisia californica 20-25 [22,36] California annual grasslands Avena, Bromus. and/or Hordeum spp. 4-20 [25]San Diego thorn-mint can be found in chaparral, coastal sage scrub, and California
annual grassland habitats.
Chaparral shrub species associated with San Diego thorn-mint include chamise (Adenostoma fasciculatum),
red shank (Adenostoma sparsifolium), manzanita (Arctostaphylos spp.),
ceanothus (Ceanothus spp.), and sumac (Rhus spp. ) [25]. California sagebrush (Artemisia
californica), California buckwheat (Eriogonum fasciculatum), and woolly
sunflower (Eriophyllum lanatum var. obovatum) dominate southern coastal
sage scrub communities where San Diego thorn-mint occurs. Black sage (Salvia
mellifera), purple sage (S. apiana), chaparral yucca (Yucca
whipplei), and golden yarrow (Eriophylllum confertiflorum var.
confertiflorum) are also common shrub species associates [8,25].
Nonnative grasses such as oat (Avena spp.), brome (Bromus spp.)
barley (Hordeum spp.), and rattail fescue (Vulpia myuros) dominate
the annual grasslands of California where San Diego thorn-mint is located
[19]. Common native grass associates are purple needlegrass (Nassella pulchra)
and other needlegrass (Nassella spp.) species [2].
Herbaceous species often found with San Diego thorn-mint are wild celery (Apiastrum
agustifolium), golden stars (Bloomeria crocea), lilies (Calochortus
spp.), small flowered soap plant (Chlorogalum parviflora), fringed
spine flower (Choriznthe fimbriata var fimbriata), and slender
tarweed (Hemizonia fasciculata) [2].
San Diego thorn-mint establishes from seed after fire [32]. Postfire establishment most likely occurs shortly after fire. Research is needed on the postfire response of the San Diego thorn-mint.
Darkness has been shown to inhibit germination [1], and San Diego thorn-mint is apparently an early seral species [15,22,23,32]. Therefore, fire may benefit San Diego thorn-mint by creating open spaces in shrub canopies, which increases light availability for germination and early successional establishment.
San Diego thorn-mint reproduces sexually [32].
Breeding system: San Diego thorn-mint has bisexual flowers [15,22,37].
Pollination: San Diego thorn-mint appears to be insect-pollinated. Bauder and Sakrison [1,2] observed several insect species visiting San Diego thorn-mint. T>he most frequent visitors were bees and checkered beetles [1,2]. Two similar species of thorn-mint, San Mateo thorn-mint (Acanthomintha duttonii) and heartleaf thorn-mint (A. obovata ssp. cordata), are self-pollinating and insect-pollinated by medium to large-sized bees [28]. Because San Diego thorn-mint has sterile upper stamens, it probably does not rely on self-pollination as a breeding mechanism [15]. Additional studies are needed on the pollination biology of San Diego thorn-mint.
Seed production: San Diego thorn-mint has decreased seed production when invasive species are present [1,2].
Seed dispersal: No information is available on this topic.
Seed banking: Seed banking is documented for San Diego thorn-mint [1]. Population numbers fluctuate annually depending on environmental conditions (i.e. precipitation, insolation), indicating that seeds may remain dormant and viable for several years [32].
Germination: Optimal germinating conditions for San Diego thorn-mint require a long daily cool period of 50 °F (10 °C). Bauder and Sakrison [1] found seed age was associated with germination. Older seeds showed an increase in mean percent germination and also germinated at warmer temperatures. Darkness inhibited germination, especially in younger seeds [1,2].
Seedling establishment/growth: Seedling survivorship was at least 80 % in a weeding study of San Diego thorn-mint. Weeding out nonnative species has a positive effect on fecundity. Plant size and fecundity are depressed by environmental stress and growth interference from invasive species [1,2].
Acanthomintha ilicifolia, known by the common name San Diego thornmint, is a rare species of flowering plant in the mint family. It is native to Baja California and San Diego County, California, where it is a resident of the chaparral and coastal sage scrub plant communities and vernal pools.[1]
Acanthomintha ilicifolia has been extirpated from many of the sites where it was previously noted in San Diego County. It is a federally listed threatened species in the United States under the Endangered Species Act of 1973 (ESA). This species is also listed as endangered under the California Endangered Species Act which means that killing or possessing the plants is prohibited within California unless authorized by the California Department of Fish and Game.[2]
Acanthomintha ilicifolia is a petite annual herb growing up to about 15 centimeters (5.9 in.) in maximum height.[2] It has rounded to oval serrated leaves up to 1.5 (0.59 in.) centimeters long. The inflorescence is a cluster of flowers with oval-shaped bracts nearly a centimeter long which are edged with long spines. Each flower is about a centimeter wide and white, often tinted purple to pink, with a hooded upper lip and a slightly longer lower lip. The flowers of the San Diego thornmint distinguish it from the other members of its genus by the hairless anthers and style.[3]
The San Diego thornmint is an annual plant that germinates in late winter, flowers in April–May, and sets its seeds in June–July.[4] Each flower can produce a maximum of 4 seeds, with each plant producing between 70-200 seeds.[5] The species has been shown to have a seed bank.[3] Seed age is associated with germination, with higher rates of germination recorded among older seeds.[3]
While relatively little is known about the pollination of the San Diego thornmint, it is believed to be pollinated by bees and checkered beetles. Two other members of the Acanthomintha genus, the San Mateo thornmint (Acanthomintha duttonii) and heartleaf thornmint (A. obovata ssp. cordata), are known to self-pollinate, and there is evidence of this for the San Diego thornmint as well.[3][6]
The San Diego thornmint is found in chaparral, coastal sage scrub, and grassland habitats. It grows in gabbro and calcareous clay soils, usually on gentle slopes between 15-20 degrees.[2][4]
This species is generally limited to San Diego County, California, and select areas of Baja California, Mexico. The distribution is higher in the United States, with over 80 historical occurrences listed in 2009 versus 13 occurrences in Mexico.[1][4]
As of 1998, the population of the San Diego thornmint is estimated to be between 150,000 and 170,000. Most occurrences have between 100-10,000 plants, with the largest occurrence boasting 60,000 in 1993. When the species was listed under the ESA in 1998, there were an estimated 52 known occurrences of the plant in the United States and 9 in Mexico, and 32 known extant occurrences. At the time of the 2009 5-year review, estimates identified around 80 known occurrences with approximately 55 extant occurrences.[1]
Seed dispersal for the San Diego thornmint is limited, so the range of the species has remained largely the same. Although more occurrences of the plant have been found since it was listed under the ESA, it is likely that they existed before.[1]
Major threats toward the San Diego thornmint fall into five categories:[1]
This is the most significant factor affecting the species. Older threats that have since lessened in significance include mining operations that alter soil state. Newer threats include destruction by fire and invasive species. Some of the most common invasive species in the habitat are Avena spp., Brachypodium distachyon, Brassica nigra, Centaurea melitensis, Cynara cardunculus, and Foeniculum vulgare. Other examples of habitat modification for the species include general urbanization, grazing, dumping, and off-road vehicle usage.
At the time that the species was listed as threatened, there were concerns that over-collection was a key detrimental factor. This includes legal and illegal collection for scientific or vandalistic reasons. However, this factor has become less critical in the species’ survival.
Two largest predators are rabbits and snails that have possibly eliminated a few occurrences of thornmint. Generally, predation is not the top concern among conservationists.
The Endangered Species Act of 1973 is the primary legislation that protects the thornmint species. Regional efforts from the city of San Diego and the surrounding county provide species regulation as well. However, many of these regional efforts are based around the official ESA listing. These local regulations are ineffective without federal guidelines. Along with this, occurrences of the species are not protected under Mexican legislation, exposing many occurrences to danger.
Many occurrences of the thornmint are found in small population sizes, which may make them vulnerable to changes. Other factors include animal grazing and climate change.
The San Diego thornmint was listed as endangered under the California Endangered Species Act in January 1982. It was federally listed as threatened under the ESA on October 13, 1998.[1]
5-year reviews have been initiated in 2008, 2010, 2013, and 2021. Only one 5-year review has been published in 2009. The review compiled scientific data concerning the thornmint’s conservation status and recommended cooperation between state and landowners to continue effective conservation. The review also recommends a status change to level “8”, having moderate threats with a high likelihood of recovery. The overall degree of severity was reduced and may be the reason why further 5-year plans have not been published.[1]
The 5-year review found that only 2 occurrences of the San Diego thornmint have gone extinct since its listing under the ESA in 1998. In addition, 39 of the estimated 55 extant occurrences are protected in conserved areas, compared to 7 at the time of listing. The review points to working with private landowners to conserve the remaining 16 occurrences as an important step in conservation of the species.[1]
There is not currently a formal species status assessment or recovery plan for the San Diego thornmint. In 2008, the U.S. Fish and Wildlife Service designated 641 acres of both private and state/federal land in San Diego County as “Critical Habitat” for the San Diego thornmint.[7]
Recent efforts to conserve the San Diego thornmint, such as the San Diego Thornmint Expansion Project,[8] focus on reducing the impact of invasive species, habitat loss, and climate change. Studies on the pollination[6] and gene flow[5] of this species are also important for conservation and expansion of the plant into new areas.
Acanthomintha ilicifolia, known by the common name San Diego thornmint, is a rare species of flowering plant in the mint family. It is native to Baja California and San Diego County, California, where it is a resident of the chaparral and coastal sage scrub plant communities and vernal pools.
Acanthomintha ilicifolia has been extirpated from many of the sites where it was previously noted in San Diego County. It is a federally listed threatened species in the United States under the Endangered Species Act of 1973 (ESA). This species is also listed as endangered under the California Endangered Species Act which means that killing or possessing the plants is prohibited within California unless authorized by the California Department of Fish and Game.
