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Assam Horse-chestnut
Aesculus assamica
California Buckeye
Aesculus californica
  (Spach) Nutt.
Chinese Horse-chestnut
Aesculus chinensis
  Owner:   C. Michael Hogan    

Leafless California Buckeye on slope, Sonoma County

California Buckeye
Aesculus californica

C. Michael Hogan PhD
September 13, 2008



DISTRIBUTION AND ASSOCIATIONS
A. californica occurs below 1200 m in elvation within the the Klamath and Coast Ranges from Siskiyou County south to Los Angeles County, and in the Cascade Range and the foothills of the Sierra Nevada, it is found from from Shasta County south to Kern County. Less commonly it occurs in the Central Valley in Colusa, Yolo and Stanislaus Counties. California Buckeye is found in the following diverse vegetative associations: chaparral, montane chaparral, California mixed oak forest, California mixed evergreen forest, Ponderosa shrub forest, cypress forest, redwood forest, mixed conifer forest and silver fir/douglas fir forest. A. californica prefers sandy, sandy-loam, or gravelly-loam soils.

California Buckeye woodland is recognized as a distinct plant community (Halvorson);. furthermore, it can also co-dominate mixed oak woodland, where Q. wislizenii, Q. agrifolia and Q. douglasii are common.(Ratliff) In chaparral, it is sometimes a dominant species. A. californica is considered a climax indicator in mixed oak forest, chaparral and California Buckeye woodlands. Typically rainfall within the range of California Buckeye is 25 to 55 cm per annum, and summer temperatures usually reach above 30 degrees C; it grows on dry slopes and canyons, but also along streambanks, particularly in its Northern California and California Central Valley occurrences; in Northern California, a typical tree associate in riparian areas is Fraxinus latifolia. Common understory associates are Toxicodendron diversilobum, Arctostaphylos manzanita and Heteromeles arbutifolia.



MORPHOLOGY

Close-up of autumnal foliage and seed formation
A.Californica is a low growing tree attaining a height of up to eight meters, but often grows as a mulitfurcate large shrublike form Bark is a mottled light gray in color. Deciduous palmately compound leaves range from from six to 14 cm in length. Flowers bloom in late spring and occur on a terminal panicle nine to 19 cm long. Fruits when developing are fuzzy and light olive-green in coating, thence transforming to a glossy chestnut brown inner nut, shedding its husk by late fall; the smooth nut is fig shaped and approximately two to 3.5 cm in diameter. A prominent lighter colored hilum is exhibited. The California Buckeye is one of the first trees in its range to shed leaves in the summer (early summer in the Central Valley and late summer in Klamath and Coast Ranges), and is one of the first to leaf out in early spring.



REPRODUCTION AND ECOLOGY
Insects are the primary pollinators of California Buckeye, even though there is some evidence that visits to this tree may be toxic to species which have not co-evolved with A. californica. California buckeye reproduces from seed, a mature tree producing 70 to 100 seeds per annum; however, it can also resprout from its root crown after cutting or fire. Dispersal of seeds is hindered by their considerable mass and size, as well as unpalatability to most wildlife. Seed viability persists only within a one year cycle, germination usually taking place between late November to late February. Soil temperature must exceed four degrees C to effect germination.

A. californica is an important tree species for regeneration after forest fires, chiefly due to the resilience of the root crown subsequent to disturbance by cutting or fire. This is particularly useful on slopes that benefit from root cohesion subsequent to vegetation loss from fires or other deforestation. Regrowth from the root crown is supported by tapping nutrient and water reserves of the mature root system This remarkable tree is considered a preferred browse species, (Hedrick) even though there is some evidence of toxicity to deer; moreover, the leaves and shoots are clearly regarded as palatable to browsers.



CONSERVATION
Although the conservation status of A.californica is secure, the issue is more appropriately viewed as the integrity of the relevant plant communities. Particularly in the counties where population and agricultural pressure is most intense, attention must be given to the issues that threaten habitat quality within Sonoma, Marin, Solano, Sacramento, Contra Costa, Alameda, Santa Clara, Monterey, Santa Cruz and San Mateo Counties plant communities: chaparral, montane chaparral, California mixed oak forest. In these areas urban development, grazing and deforestation pose the most significant threats to the woodland and chaparral systems. The integrity of these plant communities must be measured in terms of habitat quality and absence of habitat fragmentation, since the habitats enhanced by California Buckeye support a complex system involving mammals, reptiles, avafauna, reptiles and arthropods. (Solano) Thus the main issues are not only preservation of these habitats, but also careful attention to migration corridors, in order to maximize the chance of survival for the numerous threatened species that depend on these habitats. A. californica is a key plant utilized in habitat restoration and enhancement within the above counties where woodland and chaparral habitats are threatened. (Solano)(California)



HUMAN INTERACTION
Some prehistoric Native Americans in California used California Buckeye nuts as a food source, after boiling and leaching toxins from nut meats for at least 48 hours. This process was necessary since fruit, bark and leaves of A. californica embed the neurotoxic glucoside aesculin, which induces hemolysis of red blood cells. Some early tribes such as the Yokut, Pomo and Luiseño, employed the toxins deriving from California Buckeye to stupefy entire schools of fish in mountain streams in order to render the fish easier to capture. Bee-keepers must be aware of the proximity of A. californica, when bees are kept that have not co-evolved with the California Buckeye, since the flower nectar may be toxic to honeybees and other insecta, which species lack such co-evolution.



REFERENCES
* William L. Halvorson, and Ronilee A.Clark (1989). Vegetation and floristics of Pinnacles National Monument, Tech. Rep. No. 34. Davis, CA: Univ.of California, Davis, Institute of Ecology, Cooperative National Park Resources Study Unit, 113 p.
* Raymond D.Ratliff; Don A. Duncan and Stanley E.Westfall (1991) California oak-woodland overstory species affect herbage understory management implications, Journal of Range Management. 44(4): 306-310.
* Donald W. Hedrick, (1951) Studies on the succession and manipulation of chamise brushlands in California Texas, Agricultural and Mechanical College. College Station, Texas, 113 p. Dissertation
* Solano County Habitat Conservation Plan: Inner Coast Ranges (2007) Solano County Water Agency and LSA Associates
* California Dept of Fish and Game (2001) Native Species Planting Guide for the Elkhorn Slough National Estuarine Reserve


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