|© Roxanne Bittman and CNPS|
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. ) . 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 . Common native grass associates are purple needlegrass (Nassella pulchra) and other needlegrass (Nassella spp.) species .
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) .Two uncommon native forbs, chocolate lily (Fritillaria biflora) and Palmer's grappling-hook (Harpagonella palmeri), co-occur with San Diego thorn-mint in chamise chaparral and at vernal pool edges .
© 1996 Christopher L. Christie
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].
RAUNKIAER  LIFE FORM:
San Diego thorn-mint reproduces sexually .
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 . Because San Diego thorn-mint has sterile upper stamens, it probably does not rely on self-pollination as a breeding mechanism . 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 . Population numbers fluctuate annually depending on environmental conditions (i.e. precipitation, insolation), indicating that seeds may remain dormant and viable for several years .
Germination: Optimal germinating conditions for San Diego thorn-mint require a long daily cool period of 50 °F (10 °C). Bauder and Sakrison  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].SITE CHARACTERISTICS:
Fire regimes: San Diego thorn-mint occurs in a mediterranean climate, with mild winters and hot, dry summers . 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 . 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 . Coastal sage scrub experiences fires of similar severity, but with shorter frequency intervals of 20 to 25 years . 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 .
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 . Nonnative grass dominants may influence the fire regime . 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 .
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.||< 35 to < 100 |
|coastal sagebrush||Artemisia californica||20-25 [22,36]|
|California annual grasslands||Avena, Bromus. and/or Hordeum spp.||4-20 |
Darkness has been shown to inhibit germination ,
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.
DISCUSSION AND QUALIFICATION OF PLANT RESPONSE:
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" .
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 . No further monitoring results were available as of this write-up (2005).FIRE MANAGEMENT CONSIDERATIONS:
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