Research from the Pine Hill gabbro formation

Research from the Pine Hill gabbro formation

The Pine Hill gabbro formation began 175 million years ago when a volcano on the Pacific plate was pushed into the continental plate in a process known as plate tectonics. The soil that arose from this intrusion is termed gabbro soil, and more specifically the USGS has described the soils as the “Rescue series”. A description of this soil and how it relates to plants is in Medieros et al 2015.

The plant communities associated with this area are grassland, oak woodland, and chaparral and were described in a paper by Wilson et al 2009 and in a webinar here: pdf of talk, Ayres Pine Hill Jan 2015 CDFW). Over 740 plant species are found in this 30,000 acre site, making Pine Hill one of the most floristically diverse areas of the world. Five plant species were listed as endangered or threatened in 1996 by the federal government; these are the only federally listed species in El Dorado County. Stebbins’ morning-glory (Calystegia stebbinsii), Pine Hill ceanothus (Ceanothus roderickii), Pine Hill flannelbush (Fremontodendron decumbens), and El Dorado bedstraw (Galium californicum ssp. sierrae) were listed as endangered; one plant was listed as threatened, Layne’s butterweed (Packera (formerly Senecio) layneae); and one plant was listed as a species of concern, El Dorado mule-ears (Wyethia reticulata). Preserves for the rare species were established in 2001; a description of the species, preserve sites, management goals and a brief history are found here:

Within the Pine Hill formation where do the rare species occur? Knowing which habitats are suitable for the rare species allows us to target specific areas for preservation. Melanie Gogol-Prokurat developed a predictive model to address this question (Gogol-Prokurat 2011) and characterized rare plant habitat in a later paper (Gogol-Prokurat 2014).

In addition to the community studies, much of the research that has been done in the Pine Hill area examines the population biology and genetics of the rare species.

Stebbins’ morning-glory is a vining, trailing perennial with large trumpet shaped white flowers and highly dissected leaves—the only morning glory to exhibit this character. R. K. Brummitt, an authority on morning glories, first described Calystegia stebbinsii in 1974 calling it “A Remarkable New Species of Calystegia Convolvulaceae from California USA” (Brummitt 1974); remarkable due to the leaves. It was a focal species in Gogol-Prokurat’s studies and also in the Wilson et al 2009 paper; these studies found it grew best on drier chaparral sites. Ayres (2011) conducted studies of seed survival, seedling growth, and population persistence before and after experimental fires. She found that seeds of this species could survive temperatures in the soil in excess of 200°C, germinated profusely the first spring after fire, and plants began flowering heavily three years after the fire. It was absent before fire, and plant density declined over 98% within 10 years. These observations suggested that adult plants do not survive a long (> 10 year) inter-fire period. However, despite the shortness of its life, there was enough time to replenish the soil seed bank; seeds that would await the next fire. This obligately seeding pattern is one mechanism by which chaparral plants survive fire.

In 2010, another remarkable Calystegia was found by Kathy Van Zuuk of the USDA Forest Service near Foresthill in Placer County. Specimens were sent to Brummitt and Namoff (2013) for evaluation. It was determined that this was a new species that likely arose though hybridization between C. stebbinsii and C. occidentalis and was named in honor of Van Zuuk, C. vanzuukiae. Herbarium records show it was first collected at Otter Creek in El Dorado County in 1939 and has subsequently been found at Traverse Creek in the County by members of the El Dorado Chapter of the CNPS.

Pine Hill Ceanothus is a low growing evergreen shrub with clusters of white to blue (to pink) flowers in the early spring. Mostly absent in mature chaparral, the plants are killed by fire requiring that populations reestablish themselves by seed in the soil after fire (Boyd 2007, Ayres 2011)—the pattern of an obligate seeder like Stebbins’ morning-glory. Ceanothus roderickii is restricted to three major occurrences, totaling 828 acres of the Pine Hill formation, with a current estimated population of about 33,000 individuals (Burge et al 2013). It grows in the drier chaparral on south facing slopes (Wilson et al 2009; Gogol-Procurat 2011, 2014). Dylan Burge has conducted a number of studies on the genetics and evolution of this species and its place in the larger evolutionary history of the genus Ceanothus in California, the center of Ceanothus diversity in the world.

Burge and his colleagues in 2011 developed DNA base pair sequences from a part of a gene in the cell nucleus for an enzyme, nitrate reductase (NIA). They used divergence in the sequence of DNA base pairs to develop the evolutionary history (phylogeny) for the species in this genus, including C. roderickii. Ceanothus plants are important to the ecosystem as they have a symbiotic relationship with a nitrogen-fixing bacterium known as Frankia that adds nitrogen to forests and shrublands. The genus split into two subgenera, Cerastes and Ceanothus, 13 million years ago perhaps in response to continental uplift that created arid conditions in what was to become California. Divergence of species within the two subgenera began 6 million years ago. This corresponds to the uplift of the Coast Ranges and faulting and tectonic movement that exposed diverse geology, creating new, discontinuously arrayed habitats that favored geographic and edaphic (soil) speciation. Pine Hill ceanothus is a member of the Cerastes subgenus that specialized on gabbro soil in the geographically restricted area of Pine Hill.

Burge and Manos in 2011 studied the edaphic (soil) ecology and genetics of Ceanothus roderickii using the nuclear gene nitrate reductase (NIA) sequence data they developed in order to elucidate the evolutionary history of C. roderickii. Their results indicated a very close relationship between the gabbro-endemic C. roderickii and the less soil-specialized C. cuneatus var. cuneatus. They posit that C. roderickii arose from C. cuneatus var. cuneatus as an edaphic specialist on gabbro soil.

Burge and his colleagues in 2013 investigated hybridization between Ceanothus roderickii and C. cuneatus using Amplified Fragment Length Polymorphic (AFLP) genetic markers they developed for C. roderickii and C. cuneatus. Apparent hybrids between C. roderickii and C. cuneatus are sometimes encountered in the field, recognized by their intermediate morphology and tendency to form what appear to be small hybrid swarms. However, these plants seem to be confined to areas that have been subjected to recent soil disturbance, for example by bulldozers. They found that controlled cross pollination by hand between C. roderickii and nearby C. cuneatus resulted in fruits and seeds. However, under natural conditions there was little genetic exchange between the species, despite close proximity and shared pollinators. This lack of genetic exchange may be maintained by the edaphic disjunction between the species—C. roderickii grows well on nutrient poor gabbro, while C. cuneatus occurs on more fertile soil. Results from experimental growth trials and AFLP markers provide evidence for natural selection against hybrids, indicating a possible mechanism—soil adaptation—for reproductive isolation and speciation in the C. cuneatusC. roderickii system.

Burge and his colleagues in 2012 developed 10 microsatellite markers from Ceanothus roderickii to be used in future population genetic studies of the species and to test the robustness of their 2013 conclusions that soil conditions prevent establishment of hybrids between C. roderickii and C. cuneatus.

Pine Hill flannelbush is an evergreen shrub that is almost entirely restricted to the immediate vicinity of Pine Hill itself. Unlike its tall yellow-flowered cousin California flannelbush, Pine Hill flannelbush grows only to about 3′ and has bronze colored flowers. It was first collected by Beecher Crampton in 1956 and described by Lloyd in 1965 from plants growing at Pine Hill.

During the 1990s Robert Boyd conducted much work on this species. In 1994 he conducted careful pollination observations and determined that insect pollinators were required for pollination and 98% of this work was performed by solitary native bees, highlighting the importance of incorporating native bee habitat in recovery planning for this endangered shrub. The post-pollination fate of seeds and seedlings were studied by Boyd and Serrafini (1992). Due to insect herbivory less than 2% of flowers set fruit, and rodent herbivory after seed fall eliminated 90% of ripe seed. Rodents, insects, and summer drought eliminated all seedlings they planted. When they excavated small plants naturally occurring in the field they found all were connected by roots to larger shrubs indicating that asexual reproduction played a large role in the persistence of the species.

Boyd (1996, 2001) also examined the effect of elaiosomes on seed dispersal and reproductive success. Elaiosomes are nutrient packets attached to seeds that attract ants. The ants take the seeds to their nests to consume the elaisosomes and then discard the seeds on their midden piles. In this way seeds are dispersed away from the shrub canopy where rodents are active in consuming them and shrubs pose competition for light and water. The effect of seed dispersal on seedling survival was pronounced following fire where dispersal by ants of as much as 12 meters reduced predation and competition.

Since 1966 decumbent Fremontodendron of uncertain taxonomy have been collected from Yuba, Butte, and Nevada Counties. In 2006 Kelman et al. conducted genetic analyses to determine the taxonomic affiliations of F. decumbens from Pine Hill, widespread F. californicum, and decumbent plants from Yuba County. They found that F. decumbens is genetically distinct from F. californicum, and that the plants from Yuba County contain a mixture of alleles from both these species suggesting that they are hybrids.



Layne’s butterweed, formerly Senecio layneae, was first collected in May 1883 by Mary Katharine Layne-Curran from “Sweetwater Creek, near Folsom”, and described by Greene (1883) who named it after her. In 2002 Marsh and Ayres conducted a genetic analysis of the species from the three population centers that were known at that time in El Dorado, Tuolumne, and Yuba Counties. Each population was genetically distinct. Work on the community associates of Packera layneae was reported in Wilson et al (2009) and Gogol-Prokurat (2011, 2014).

The first collection of El Dorado mule ears was also made by Layne-Curran in July 1883 and reported by Greene as occurring “on Sweetwater Creek, in El Dorado County” (Greene 1884 Bull. Cal.Acad. Sciences). Weber (1946) in his monograph of Wyethia reported W. reticulata as occurring “from a few individuals to dense stands of 100 or more” and noted it to be an “aggressive colonizer of disturbed areas” (Weber, 1946, page 420). These impressions lasted until the genetic work of Ayres and Ryan (1997, 1999) when it was found that the plants were extensively clonal with single individuals spreading 10s of meters via underground rhizomes. After disturbance by fire, or grading, the plants responded by prolific flowering giving the impression of colonization but, in fact, the plant resprouted flowering stems from these underground rhizomes. Its responses to fire were detailed in Ayres (2011) and its community associations included in Wilson et al (2009) and Gogol-Prokurat (2011, 2014).

Our newest new species, chaparral sedge, is not completely new—it has been thought to be Brainerd’s sedge, Carex brainerdi, for many years. However, a recent examination of this species (Zika et al 2014) has determined it is not C. brainerdi but a completely new species given the name C. xerophila. Chaparral sedge grows on gabbro and serpentine soils in the central California foothills, but its exact distribution is a focus the CNPS Rare Plant Treasure Hunt.