Geography 316:  Biogeography 

The Biogeography of Raven's Manzanita (Arctostaphylos hookeri ssp. ravenii)

by Brianna Schaefer, student in Geography 316
 

Taxonomic Levels

Kingdom:  Plantae Phylum:  Magnoliophyta Class: Magnoliopsida Order:  Ericales Genus:  Ericaceae Species:  Arctostaphylos hookeri Subspecies: A. hookeri ssp.ravenii *no longer accepted as A. hookeri ssp.ravenii, now referred to as A. pungens var. ravenii (ITIS 1999).

Introduction

    The genus Arctostaphylos consists of over 100 taxa and is considered to be one of the larger and more complex genera of the Pacific Slope of North America (Markos 1995) [See Figure 1].  There are over 80 taxa in the state of California alone, many of which are also endemic (Parker 1997).  Species diversity remains highest in the coastal regions where over 30 taxa can be found from Mendocino County to San Luis Obispo (Markos 1995).  There is always an exception to the rule however, the uva-ursi, the most wide ranging species of Arctostaphylos is not only circumpolar, existing in boreal latitudes, but also occupies mountainous habitats to the south as well as coastal areas of California (US Fish and Game 1984) [See Figure 2].
   Arctostaphylos, known in California as manzanita (Spanish for little apple spawning from the shape of the fruit), is most commonly found in chaparral, coastal prairie, and northern coastal scrub communities (CalFlora Database 1999). Many manzanita, 47 % of the taxa are restricted to certain soil types (Markos 1995). For example, A. silvicola can only be found in Miocene marine sandstone in the Santa Cruz Mountains, or A. elegans restricted to volcanic substrate (Markos 1995).  Most Arctostaphylos can be found on serpentine and shale substrates in chaparral communities (CalFlora 1999).  Arctostaphylos range from small prostrate woody plants, to tree size forms and some can also be creepers (Parker 1999).  The leaves are scherloferous meaning leathery, and all are evergreen (1999).

Natural History

Although every manzanita has similar characteristics, even the smallest detail can spawn a new species.  Due to widespread hybridization and minute differences between each species it has made this genus very difficult to measure (Wells 1988).  This can in fact be seen in the species hookeri which has been divided into five subspecies: 1) A. hookeri  G. Don ssp. franciscana (Eastw.) Munz, 2) A. hookeri G. Don ssp. hearstiorum (Hoover & Roof) P.Wells, 3) A. hookeri G. Don ssp. hookeri, 4) A. hookeri G. Don ssp. montana (Eastw.) P. Wells, and 5) A. hookeri G. Don ssp. ravenii P.Wells  (Markos 1995).
 
 

When looked at closely, the two subspecies, franciscana and ravenii seem very similar, and have been documented as occurring sympatrically at three locations in San Francisco.  Although the size of the flowers are different, franciscanaís being larger, both plants have the same creeping low to the ground appearance (Wells 1968).  Known to occur on serpentine outcrops in Laurel Hill Cemetery, Masonic Cemetery and Mount Davidson franciscana was thought to be itís own species by Eastwood in 1905 until it was distinguished as a subspecies by Munz in 1958 (Markos 1995).  When franciscana went extinct due to urbanization in the late 1930ís to early 1940ís, it was thought that ravenii, which was evidently found in the same areas had gone extinct as well  until it was rediscovered by Wells in 1968 (US Fishing and Wildlife Services 1984).

    Although every manzanita has similar characteristics, even the smallest detail can spawn a new species.  Due to widespread hybridization and minute differences between each species it has made this genus very difficult to measure (Wells 1988).  This can in fact be seen in the species hookeri which has been divided into five subspecies: 1) A. hookeri  G. Don ssp. franciscana (Eastw.) Munz, 2) A. hookeri G. Don ssp. hearstiorum (Hoover & Roof) P.Wells, 3) A. hookeri G. Don ssp. hookeri, 4) A. hookeri G. Don ssp. montana (Eastw.) P. Wells, and 5) A. hookeri G. Don ssp. ravenii P.Wells  (Markos 1995).
   When looked at closely, the two subspecies, franciscana and ravenii seem very similar, and have been documented as occurring sympatrically at three locations in San Francisco.  Although the size of the flowers are different, franciscanaís being larger, both plants have the same creeping low to the ground appearance (Wells 1968).  Known to occur on serpentine outcrops in Laurel Hill Cemetery, Masonic Cemetery and Mount Davidson franciscana was thought to be itís own species by Eastwood in 1905 until it was distinguished as a subspecies by Munz in 1958 (Markos 1995).  When franciscana went extinct due to urbanization in the late 1930ís to early 1940ís, it was thought that ravenii, which was evidently found in the same areas had gone extinct as well  until it was rediscovered by Wells in 1968 (US Fishing and Wildlife Services 1984).

Distribution
 

The single plant of A. hookeri ssp. ravenii can be found growing near the World War II Pacific Theater Monument on the grounds of the San Francisco Presidio, it is the last of itís variety in the wild (US Fishing and Wildlife Services 1984) [See Figure 3].  Ravenís Manzanita, as described by Wells (1968), is a low shrublet creeping with branchlets rooting right along top of the serpentine soil.  The flowers about 6-10 with inflorescence and fruits are small as in the ssp. hookeri but smaller than those of franciscana and the leaves are round or broadly elliptical (Wells 1968). Elevation:  The site of the wild plant is on an exposed, west-facing slope of about 20 degrees situated about 90 meters above the Pacific ocean near the World War II monument (US Fishing and Wildlife Services 1984).  The elevations at the historic sites ranged from 60 meters to a possible high of 300meters near Mount Davidson (1984). Temperature:  Moderated by a maritime influence, the climate is Mediterranean with temperatures ranging from an average minimum of 47 F (8 C) to a maximum of 61 F (16 C).  About 58% of possible daylight hours are sunny.  Although frosts and snowfall are almost unknown, the actual site of the manzanita considering itís location near the Golden Gate is probably influenced by fog and wind (US Fishing and Wildlife Services 1984).

Evolution

   Arctostaphylos, having a rich fossil record is considered to be of Miocene origin, approximately 15 million years ago with radiation of the genus occurring 1.5 million years ago during the Pleistocene ( Markos 1995).  In terms of a geologic record, the remains of two species from the Pleistocene were found in the Tomales Region while the oldest remains of Arctostaphylos come from the lower Pliocene Claredon beds of Texas (Adams 1935).  The great abundance of manzanita in southern California and itís relation to the Pleistocene flora of Tomales  suggests that the Pleistocene center of distribution and variation was located in southern California and that the center has moved northward and coastal since then (Adams 1935).

Regeneration
   As Jepson states,(1993) the radiation in the genus which has occurred predominantly in the chaparral of the Coast Ranges can be attested to not only by the large number of species there, but also by the very large number of minor forms.  The prevalence of the species in the Coast Ranges can be attributed to two things: the frequency of fire in the chaparral and the influence of the regenerative responses  of the genus (US Fishing and Wildlife Services 1984).
   In the case of maintaining coastal chaparral and coastal-scrub plant communities, fire has often been implicated as a major influence.  For the manzanita fire is one of the most important factors in regeneration by helping to break the dormancy of seed or preparing and appropriate seed bed (US Fish and Wildlife Service 1984).  Following a fire, species of Arctostaphylos typically have two modes of regeneration: obligate seed germination and resprouting from the root crown.  The sprouting is considered to be the primitive condition occurring when a below ground portion of the plant survives the fire (Wells 1968).  The obligate seed germination  occurs when adult plants are completely killed by the fire, which stimulates the seed bank and then dormant seeds germinate (Markos 1995).  It is proposed that this may be the reason why the single plant of  Ravenís manzanita is not regenerating, due to a lack of fire the dormant seeds with increasing competition from introduced species of Monterey cypress and Monterey pine is suffering from limited resources and unable to produce ( US Fish and Wildlife Service 1984).
   The tendency for this genus to hybridize has been noted and looked at by many authors and field workers (US Fish and Wildlife Service 1984).   Markos states that there are no evident reproductive barriers among the species of Arctostaphylos and suspected hybrids often occur  in sympatric populations (1984).  Wells suggests that the variety within the genus has become a superfluous category, with botanists collecting hybrid individuals that later become the new types of taxa at the species level (1968).  With a bewildering array of minor variations, even on the same plant, the extreme foliar variability for example the existence of tall and short forms in the same taxon, has given way to the idea of hybridization within the genus US Fishing and Wildlife Services (1984).  Raven feels that the overall pattern of variation tends to become more and more obscure as the new taxa are proliferated (1969).  The only hope is that some of the studies of this genus which are under progress will eventually provide a new synthesis (1969).
 

  Interesting Extras
  
    Within the state of California alone as of October 1999, there are 128 state-listed endangered species six of which fall into the genus Arctostaphylos, two more listed as rare (Dept. of Fish and Game 1999).  Of these eight species six are also federally listed as threatened or endangered (1999).  The A. hookeri ssp. ravenii was state listed in November 1978 and moved to the federal list in October 1979.  Attempts for recovery were started in 1984 after the proposition by the US Fish and Wildlife Service, however, the species remains extremely rare and isolated (CalFlora Database 1999).
 
 
 

Bibliography
 

 Adams, Joseph Edison.  1935.  A Systematic Study of the Genus Arctostaphylos.   Berkeley: Berkeley Press.

CalFlora Database. Arctostaphylos hookeri ssp. ravenii. Available: http://www.calflora.org [1 November 1999].

Department of Fish and Game.  1999.  State and Federally Listed  Endangered, Threatened and Rare Plants of California.
Available:  http://www.dfg.ca.gov/whdab/index.htm  [1 November 1999].

ITIS Classification Report.  Arctostaphylos hookeri ssp. ravenii.  Available:                          http://www.itis.usda.gov/pl.../class_report.cgi?kingdom=Plantae& input=183563&input_type= [1 November 1999].

Jepson, Willis Linn.  1993.  The Jepson Manual: higher plants of California.  Berkeley:University Press.

Markos, Satci E.  Phylogeny of Arctostaphylos hookeri (Ericaceae) based on  Ribosomal DNA Sequence From the Its Region.  San Francisco: 1995.

Parker, Tom and Michael Vasey. 1997.  Class IV -- Arctostaphylos  (Manzanitas) Ericaceae.  Available:  http://ucjeps.berkeley.edu/jep_classes/Arctostaphylos.html  [29 November  1999].

Raven, Peter H.  Review of Phillip A. Munz. 1969.  A Supplement to A California  Flora.  Madrono 20(4): 239-240.

US Fish and Wildlife Service.  Ravenís Manzanita Recovery Plan.  Portland:  US Fish and Wildlife Service, 1984.

Wells, Phillip V.  1988.  New Combinations in Arctostaphylos: annotated list of  changes in status.  Madrono 35(4): 330-341.

Wells, Phillip V.  1968.  New Subspecies of Arctostaphylos.  Madrono 19(6): 197-208.
 
 
 
 
 
 
 

send comments to Bsch8fer@aol.com
 

Geog 316 homepage  Back to Geography home page     Back to SFSU homepage