Slender-horned spineflower grows at elevations from 656 to 2,296 feet (200-700 m) [4,7,19]. Surveys of 8 slender-horned spineflower populations found the species was most common on slightly acid (pH 6.4) silts with low levels of nitrogen, phosphorus, and organic matter and low electrical conductivity and “fairly low” cation exchange .PLANT COMMUNITIES:
Slender-horned spineflower is noted in grasslands dominated by nonnative annuals; in coastal sage scrub [1,7], chamise (Adenostoma fasciculatum), and other chaparral [1,7], and in woodlands dominated by coast live oak (Quercus agrifolia) , western sycamore (Platanus racemosa), California juniper (Juniperus californica), and Fremont cottonwood (Populus fremontii) [1,19].
Slender-horned spineflower is sometimes found on soil crusts with lichens and mosses [1,19]; however, some slender-horned spineflower habitats have little to no soil crust cover . In western Riverside and San Bernardino counties, percent soil crust cover was negatively correlated with the number of involucres/slender-horned spineflower plant (r = -0.49, P=0.03), while percent annual grass or forb cover was not significantly correlated with slender-horned spineflower presence .
See the Fire Regime Table for a list of plant communities in which slender-horned spineflower may occur and information on the fire regimes associated with those communities.
Field studies show evidence of insect pollination. Both ants and flying insects were observed visiting slender-horned spineflower flowers, although a wasp (Plenoculus davisii) was the only species observed actually pollinating flowers .
Establishment: As an annual, slender-horned spineflower establishes solely from seed. It produces 3 to 507 seeds/plant . Small spines and hooks on the involucres likely aid animal dispersal of the seeds. The achenes have no mechanisms for dispersal, but since slender-horned spineflower occurs on alluvial soils, sheet flows during heavy rains may disperse the achenes .
Seeds are stored in the soil seed bank. Seed viability was unknown as of 2012, although seeds stored in the soil seed bank are likely long-lived . Ferguson and others  speculated that in a given year, recruitment may result from seeds stored across several years and recommended prioritizing research on seed bank dynamics of slender-horned spineflower.
Current-year precipitation is critical in determining slender-horned spineflower’s rate of establishment and seed production. In a year with steady summer rains in western Riverside and San Bernardino counties, many hand-marked slender-horned spineflowers favored vegetative over reproductive growth. The next year, which was drier, marked plants dried out with summer heat. However, at the next rainstorm, plants appeared in the same locations as the marked, dried-out plants measured earlier. The authors interpreted this as evidence that the dried-out plants had sprouted from their root crowns in response to rain .
Sheltered microsites appear to aid slender-horned spineflower seedling establishment. In western Riverside and San Bernardino counties, crevices by rocks imbedded in the soil surface were favored establishment sites. At a former dumpsite, partially buried trash served the same function; in both cases, crevices likely trapped seeds and retained moisture . Similarly, slender-horned spineflower establishment might have been facilitated by the presence of woody plants after a fire on the Cleveland National Forest.
Because slender-horned spineflower is an annual, population sizes fluctuate widely across and within years [6,19]. Over 3 years in western Riverside and San Bernardino countries, slender-horned spineflower seedlings established throughout the growing season after intermittent rainstorms. Due to dry conditions between rains, many plants died before setting seed; plants establishing early in the growing season apparently had highest survival. Monthly population size was positively correlated with precipitation from the previous month (P=0.003) . Establishment may be poor during drought years . However, density of a population in Los Angeles County was not directly correlated with rainfall (Sapphos Environmental, Inc. 2008 cited in ).
Disturbance may promote establishment ; slender-horned spineflower establishment was noted in tire tracks on a site near San Jacinto (Meyer personal communication cited in ). Near the Santa Ana River, slender-horned spineflower establishment was noted in slight depressions that may have originated from rodent burrowing . In western Riverside and San Bernardino counties, herbivory reduced slender-horned spineflower growth and reproduction in some years. Floral stems were favored, while leaves were rarely consumed. The authors surmised that cyclically high numbers of grasshoppers were responsible for the herbivory .
Field studies in western Riverside and San Bernardino counties suggest that within a growing season, slender-horned spineflower sprouts from the root crown after die-back from drought .
Postfire regeneration strategy :
Herbaceous root crown, growing points in soil
Ground residual colonizer (on site, initial community)
Crown residual colonizer (on site, initial community)
Initial off-site colonizer (off site, initial community)
Secondary colonizer (on- or off-site seed sources)
Fire adaptations and plant response to fire:
Fire adaptations: Slender-horned spineflower establishes solely from soil-stored seed. Its seed bank is likely long-lived , so it may establish from seed after fire or other disturbances . Animals and/or sheet erosion may aid in seed dispersal from off-site parent plants (see Establishment).
Postfire growth and seed production may depend partly on season of fire. Slender-horned spineflower is unlikely to have time to reproduce after late-summer fire. In field studies, plants that died back completely in response to summer drought apparently sprouted from their root crowns after later summer rains, but they produced relatively few seeds . This suggests that within a growing season, slender-horned spineflower plants might resume growth after top-kill from fire, but seed production would be low. Next-year recruitment from the soil seed bank is probably more important for slender-horned spineflower’s postfire regeneration than current-year sprouting and subsequent seed set.
Plant response to fire: Limited information suggests that fire may benefit slender-horned spineflower populations. The US Fish and Wildlife Service  reports that wildfires in the Agua Tibia Wilderness-Vail Lake area of the Cleveland National Forest “have burned over occupied Dodecahema leptoceras habitat in the past with no apparent adverse effect; in fact, the observed number of plants increased dramatically after fire”. Postfire monitoring of slender-horned spineflower was conducted following the August 1989 Vail Wildfire in the Aqua Tibia Wilderness Area. Plants established in burned areas during postfire year 1. Populations on both sides of a creek were expanding, and many large plants (10-12 inches (25-30 cm) diameter) were apparent. In September of 1994 (postfire year 5), both populations had >300 plants. The populations were concentrated near woody plants: a large oak tree on one bank and a clump on chamise on the other , suggesting possible facilitation by the woody plants. Most slender-horned spineflowers were of “fairly good size”—about 2 to 3 inches (5-8 cm) in diameter—with around 20 achenes/plant .
The long-term fire response of slender-horned spineflower was unknown as of 2013.
The Fire Regime Table summarizes characteristics of fire regimes for vegetation communities in which slender-horned spineflower may occur. 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".LEGAL STATUS AND MANAGEMENT:
Information on state-level protection status of plants in the United States and Canada is available at NatureServe.Other management:
Other threats to slender-horned spineflower include soil disturbance from off-highway vehicle (OHV) usage, trash dumping, and trails created by unauthorized human activities near alluvial habitats .
Conservation efforts focusing on protecting existing slender-horned spineflower populations and their habitats are recommended. Limited seed bank studies suggest artificial establishment of new populations may require a large investment of seeds over many generations before slender-horned spineflower establishes naturally . Slender-horned spineflower plants have shown low seed production in the greenhouse [6,21]. Ferguson and others  were unable to raise slender-horned spineflower plants to maturity despite easy germination.
For restoration purposes, soil and nonnative grass species in the proposed habitat should be considered before planting slender-horned spineflower seed. Ideal sites would have silt (not loam) soil with pH around 6.4 and low electrical conductivity, with no annual grasses present . In light of their difficulties in growing slender-horned spineflower in the greenhouse, Ferguson and others  call for further studies on artificial regeneration of slender-horned spineflower. They note that “natural recruitment from seed is no problem within populations, but the exact conditions for growing plants ex situ remain elusive” .Further research is needed on all aspects of slender-horned spineflower ecology.
|Fire regime information on vegetation communities in which slender-horned spineflower may occur. This information is taken from the LANDFIRE Rapid Assessment Vegetation Models , which were developed by local experts using available literature and/or expert opinion. This table summarizes fire regime characteristics for each plant community listed. The PDF file linked from each plant community name describes the model and synthesizes the knowledge available on vegetation composition, structure, and dynamics in that community. Cells are blank where information is not available in the Rapid Assessment Vegetation Model.|
|Vegetation Community (Potential Natural Vegetation Group)||Fire severity*||Fire regime characteristics|
|Percent of fires||Mean interval
|Coastal sage scrub||Replacement||100%||50||20||150|
|Coastal sage scrub-coastal prairie||Replacement||8%||40||8||900|
|Surface or low||62%||5||1||6|
|California oak woodlands||Replacement||8%||120|
|Surface or low||91%||10|
Replacement: Any fire that causes greater than 75% top removal of a vegetation-fuel type, resulting in general replacement of existing vegetation; may or may not cause a lethal effect on the plants.
Mixed: Any fire burning more than 5% of an area that does not qualify as a replacement, surface, or low-severity fire; includes mosaic and other fires that are intermediate in effects.
Surface or low: Any fire that causes less than 25% upper layer replacement and/or removal in a vegetation-fuel class but burns 5% or more of the area [3,10].
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