By Christal Niederer
Cirsium fontinale var. fontinale
Not all thistles are bad! Fountain thistle is a rare, native thistle currently found only in serpentine seeps or perennially moist areas in San Mateo County. A member of the Sunflower family (Asteraceae), it is distinguished from other Cirsium species by its nodding flower heads and glandular-hairy upper leaf surfaces. This herbaceous perennial is federally and state-listed endangered, and a CNPS 1B.1.
Fountain thistle is no longer found at Edgewood, but there is a chance it could be reintroduced. It was last seen in the late 1990s at a seep along the Clarkia trail. There are plans to reintroduce it to the Triangle, the San Francisco Water District property on the west side of Highway 280, adjacent to Edgewood.
Dispersal
Like its close relative the Mount Hamilton thistle, found at Coyote Ridge and other places in Santa Clara County, fountain thistle appears to be designed for limited dispersal. Unlike other thistles that are wind-dispersed, Mount Hamilton thistle has nodding heads and a deciduous pappus, which allows seeds to fall near the parent plant, thus increasing the likelihood of landing in appropriately wet habitat. Wind-dispersed seeds would be more likely to end up in inappropriately dry upland areas.
Fountain thistles also appear to have lost their original dispersal agents. Biologist Jessica Shors Appel has found that fountain thistles are myrmecochorous (adapted for dispersal by native ants). At the distal end of each fountain thistle seed is an elaiosome, which is a fat-and protein-rich protuberance. Many native ant species are known to carry elaiosome-bearing seeds to their nests, eat the elaiosomes, and leave the seed intact and buried in the ant nests. In addition, several of the native ant species that carry elaiosome-bearing seeds are known to drop seeds along the way to their nests. The potential benefits of the interaction between elaiosome-bearing seeds and native ants are long-distance and medium-distance dispersal, dispersal to superior germination and establishment sites, reduced interspecific and intraspecific competition, reduced predation, and protection from disease and fire (Pemberton and Irving 1999; Christian 2001). Appel has also found that most of the range of fountain thistle has been invaded by Argentine ants (Linepithema humile). Argentine ants are known to displace most native ant species (reviewed in Holway et al. 2002). They are also known to eat elaiosomes without dispersing elaiosome-bearing seeds (Christian 2001), leaving the seeds under the parent plants and vulnerable to predation, competition, disease, and fire. Native ants are believed to be absent from most, if not all, of the extant fountain thistle colonies (Jessica Shors Appel, pers. comm. 2010). The invasion of the Argentine ants is likely to be a significant cause of fountain thistle decline.
Pollination and Reproductive Biology
Fountain thistle is pollinated mainly by the common native yellow-faced bumblebee Bombus vosnesenskii, common honeybee Apis mellifera, bees in the Halictidae family, and unidentified flies (Diptera). To test autogamy (the ability to self-pollinate), flower heads were experimentally bagged with mesh to exclude pollinators. This led to a 90% decrease in seed set, indicating that pollinator presence is important for reproductive success (Powell et al. 2009; Knight et al.). The plants are monocarpic, meaning they flower and bear fruit once, then die (Powell et al. 2009).
Competitive Abilities
Powell and Knight (2009) hypothesized that as a serpentine endemic, fountain thistle would be a weak competitor in high nutrient conditions. Surprisingly, their research did not demonstrate this. Fountain thistle increased its aboveground biomass as nutrient conditions increased, and competed well even against the known invasive bull thistle (Cirsium vulgare). Of the four native Cirsium species tested, fountain thistle biomass was least affected by interspecies competition. They concluded that traits that allow fountain thistle to survive in its low-nutrient environment do not appear to trade off with traits that allow rapid growth in nutrient-rich environments. The question as to why fountain thistle is limited to serpentine sites remains unanswered.
Seed Predation
Seed predation has been noted by several sources (CDFG 2010, Knight pers. comm. 2010, Herr, 2000, USFWS 1998). In particular, studies have noted nontarget effects of the USDA-approved biocontrol Rhinocyllus conicus, a seedhead weevil introduced to control invasive thistles such as bull thistle and Italian thistle (Carduus pycnocephalus) (Bossard 2000).
Herr (2000) reports that although R. conicus does utilize fountain thistle and other native thistles, its observed level of seed predation was insufficient to limit seedling recruitment in Mount Tamalpais thistle (Cirsium hydrophilum var. vaseyi). He did not test seed predation intensity on fountain thistle. While studying Mt. Hamilton thistle, Hillman (2007) noted R. conicus at 4 of 5 study sites, and 18% infestation of all inspected flower heads. She also found crab spiders, stink bugs, ants, a fly, and unknown larvae. Seed loss from pre-dispersal predation varied among sites, from 13 to 42%. Despite finding several sources of pre- and post-dispersal seed loss, however, this demographic study found recruitment was sufficient to maintain population viability.
The SFPUC (2010) fountain thistle summary mentions a 1995 study by Herr that found evidence of R. conicus oviposition on 39.3% of sampled fountain thistle flower heads, along with an accompanying reduction in seed production. The same summary states that Mike Pitcairn of the California Department of Food and Agriculture found fountain thistle seed heads to be heavily predated by native fruit flies, resulting in up to 90% seed loss. When R. conicus is present, the native predators are absent. It is currently unclear whether R. conicus or other predators are creating seed limitation in fountain thistle.
Some people have noted what appeared to be excessive aphid infestations at the CalTrans site (L9) (Paul Heiple, CNPS Santa Clara Valley board member, pers. comm. 2010). These appear to be associated with Argentine ants, which may be repelling predators and parasites of the aphids (Don Thomas, SFPUC IPM Manager, 2010).
References
Bossard, C. C., J. J. Randall, and M. C. Hoshovsky. 2000. Invasive Plants of California’s Wildlands. Berkeley, CA: University of California Press.
California Department of Fish and Game (CDFG). 2010. Seed Collection and Banking of 50 Plant Species of Critical Conservation Concern. Final report, phase 1. U.S. Fish and Wildlife Service: Endangered Species Act (Section 6) Grant-in-Aid Program, E-2-P-31.
California Native Plant Society (CNPS). 2010. Inventory of Rare and Endangered Plants (online edition, v7-10d). California Native Plant Society. Sacramento, CA. Accessed on Wed, Dec. 1, 2010 from http://www.cnps.org/inventory
Corelli, T. and Z. Chandik. 1995. The Rare and Endangered Plants of San Mateo and Santa Clara County. Half Moon Bay, CA: Monocot Press.
Christian, C. E. 2001. Consequences of a biological invasion reveal the importance of mutualism for plant communities. Nature 413: 635.
Herr, J. C. 2000. Evaluating Non-target effects: The Thistle Story. California Conference on Biological Control II, The Historic Mission Inn Riverside, California, USA, 11-12 July, 2000.
Hickman, J. C., Ed. 1993. The Jepson Manual: Higher Plants of California. Berkeley, CA: University of California Press.
Hillman, J. 2007. Constraints on population recruitment for a rare serpentine seep thistle. Master’s thesis, San Francisco State University.
Holway, D. A., L. Lach, A. V. Suarez, N. D. Tsutsui, and T. J. Case. 2002. The causes and consequences of ant invasions. Annual Review of Ecological Systems 23: 181-233.
Pemberton, R. W. and D. W. Irving. 1990. Elaiosomes on weed seeds and the potential for myrmecochory in naturalized plants. Weed Science 38: 615-619.
Powell, K. 2007. Pollination biology and competitive abilities of six rare, common, and invasive species of Californian thistle (genus Cirsium). Honors thesis. Washington University.
Powell, K. and T. Knight. 2009. Effects of nutrient addition and competition on biomass of five Cirsium species (Asteraceae), including a serpentine endemic. International Journal Plant Science 170: 918-925.
Powell, K., K. Krakos, and T. Knight. 2009. Comparing the reproductive success and pollination biology of an invasive plant to its rare and common native congeners: A Case Study in the genus Cirsium (Asteraceae). Biological Invasions. Published Online.
Thomas, D. and G. Ciardi. 2008. Jubatagrass control and natural regeneration of Cirsium fontinale var. fontinale (fountain thistle). Accessed online at http://www.cal-ipc.org/symposia/archive/pdf/2008/Proceedings_2008.pdf
Thomas, J. H. 1961. Flora of the Santa Cruz Mountains of California. Stanford, CA: Stanford University Press.
U.S. Fish and Wildlife Service (USFWS). 1998. Recovery Plan for Serpentine Soil Species of the San Francisco Bay Area. Portland, Oregon. 330+ pp.
Photo by Toni Corelli