San Francisco State University
Department of Geography

Geography 316:  Biogeography 

The Biogeography of  Blue Oak (Quercus  douglasii)

by James Vest, student in Geography 316, Fall 1999

Kingdom: Plantae
Division: Tracheophyta
Sub Division: Pteropsida
Class: Angiospermae
Order: Fagales
Family: Fagaceae
Genus: Quercus
Species: Quercus douglasii
 

Description of Species:

    Blue oaks were named in 1831 by a Scottish botanist, Daivd Douglas, for a bluish color caused by a waxy coating on its leaves (Pavlik et al. 1991).  Its leaves, which appear blue/green in late summer, are usually 1-3 inches long with wavy margins, occasionally shallowly and irregularly lobed (Pavlik et al. 1991).  The Blue oak has a dark brown heartwood and a light brown sapwood(Pavlik et al. 1991).
 

 Regeneration

    Blue oaks seedlings can grow very slowly and even decrease in height during severe conditions and still survive (Phillips 1996).  Blue oaks can also reproduce vegetatively, from either root crown sprouts or stool sprouts from burnt or cut stumps, although they are considered weak sprouters (McDonald 1999).
    Sprout growth varies greatly due to conditions, topography and stand density.  Blue oaks tend to be taller on deep soils, near the base of hillsides or near ephemeral streams in canyons or draws (Callaway et al.). 90% of blue oaks grow from a single stem that may for, the remaining 10% may sprout from multiple acorns (McDonald 1999).  The blue oak grows so slowly, small plants may be over 25 years old (Phillips 1996).  Under normal conditions growth slows after reaching 65cm(26”) d.b.h.(diameter breast height), seldom exceeding 125cm(49”)d.b.h. or  25m (82’)height, although one champion blue oak in Alameda County is 196cm(77”)d.b.h. and 28.7m(94’) tall . 
                                          Young seedling
    Blue oak reproductive height is 5-25m and it flowers from late March to mid-May  depending on elevation, aspect and climate.  Warmer areas tended to bloom first (McDonald 1999).  Blue oak flowers are unisexual and wind pollinated, with little energy put into reproduction until after pollination (Koenig et al. 1994).  Its acorns mature in one season, weighing 2-12 grams and can germinate in with in one month of falling as opposed to some other oaks that can only germinate in the spring (Pavik et al.1991).  Germination requires thick leaf cover or loose soil and rarely occurs on the surface when there is adequate moisture and low temperatures. October rain can stimulate root growth and the root will continue downward growth in spite of cool soil temperatures in the winter.  Most of the growth is put into root instead of shoots, as much as 73% of the dry weight in the first year of growth is from below ground (McDonald 1999).
 

Habitat

     Blue Oaks are adapted to dry climates and drought and can survive in regions where the temperature exceeds 100 F for weeks (Pavik et al.1991).    The blue oak’s leaves are tough and thick to conserve water.  Draught deciduous leaves are rare in oaks and trees in general while more common in chaparral and desert scrubs. In extremely hot and dry years, the blue oak can perform estivation, dropping its leaves and going dormant in the summer (Pavik et al.1991).  Even after dormancy, the drought-stricken blue oaks will continue to produce acorns from food stores.  After estivation, growth resumes in the spring and not in the fall (Pavik et al.1991).    The mean maximum July temperatures ranges between 21o and 380C(700-1000 F) and the mean January low temperature of –120 to 20 C (100-350 F) depending on location and elevation (Pavik et al.1991).  The frost-free growing season varies from 150-300 days.  Within the main distribution of blue oaks, the average annual precipitation, usually in the form of rain, varies from 510 to 1020 mm (20-40”) occurring November through April (McDonald 1999).  The precipitation varies greatly from 1520mm(60”) in Shasta County to 250mm(10”) in Kern County.
 

Evolution
  
Blue oaks developed from angiosperms that evolved in the Early Cretaceous period.  The blue oak is in the Fagales order and the Fagaceae family, which is spread throughout the world.  The blue oak’s genus, Quercus, is found throughout the Northern hemisphere.  Blue oaks have most likely evolved recently to drier and warmer climates during the last 10 million years (Pavlik et al. 1991).  Paleobotanic record suggest that the blue oak has a Miocene progenitor, the Quercus douglasoides which had a much wider distribution (McDonald 1998).  It is possible that following the development of the Sierra Nevada mountains, that the population became isolated and thus evolved into a new species.
The blue oak is capable of hybridizing with a number of white oak trees, including valley oak, Oregon white oak and California scrub oak (Pavlik et al. 1991).  The resulting offspring of a blue oak Oregon oak cross is called a Epling oak and is found in Marin, Sonoma and Napa counties(Pavlik et al. 1991).  The blue oak cross with a desert scrub oak is called a alvord oak and can be found from Monterey County to the Tehachapi mountains(Pavlik et al. 1991).

Distribution

   The blue oak, Quercus douglasii, is endemic to California (McDonald 1999).  It has a disjunctive distribution and is widely spread in 39 of 58 counties in California extending as far north as Montgomery Creek in Shasta county, as far south as Santa Ynez valley in Santa Barbara County, and the Liebre Mountains of Los Angeles County (Pavlik et al.1991).  Blue oaks are found mostly in the foothills of coastal ranges and Sierra Nevada, roughly forming a ring around the central valley; ranging in elevations from 100-1200m (Allen-Diaz et al. 1991).  The range of the blue oak is estimated to cover between 1 and 3 million hectares with an average density of 10 to 50 trees per hectare (Barbour 1977).  The density of blue oak stands tends to be greater on slopes with moderate, gentle grades, with quickly drained soils.  Areas in the blue oak’s natural range with heavy clay soils or near surface hardpan do not support the oaks (Whitney 1994). The blue oak also tends to be limited in deep fertile soils where it is out competed by taller interior live and black oak trees. (McDonald 1999).   Because blue oaks are shade intolerant, it is rare to find them in understories, and when located with mixed woods, they are usually place in an area receiving direct overhead sun for long duration (MCDonald 1999). Blue oaks occur often with gray pines (Pinus sabiniana) which also tend to be taller but do not create a dense canopy that would limit blue oak sapling growth (Whitney 1994).  Blue oaks are also found with ponderosa pine (Pinus ponderosa), knobcome pine (P.Attenuata), toyon (Heterondes arbutifolia) and California buckeye (Aesculus californica).  Blue oaks are often found with tall grasses, originally the perennial bunch grasses (Stipa), but since replaced by introduced species like wild oats (Svera fatua, A. barbatta) (McDonald 1999).  Blue oaks can facilitate or interfere with understory productivity (Callaway et al. 1991).  Litterfall adds nutrients beneath their canopies, while the negative effect shade on undergrowth is minimal (Callaway et al 1991).   According to the study (Callaway et al. 1991) blue oak root structure morphology played a significant role in the various effects of the oaks on their understory.  Large blue oaks with deeper tap roots facilitated surrounding grasses while smaller trees had finer roots, closer to the surface that competed with grasses for scarce water.
 

Map of Distribution:

 
Other interesting issues

   Blue Oak wood is prone to dry rot and it does not tent to grow straight, thus the commercial value of the tree has remained low.  Its principal uses are for firewood and fence posts.  Although the number of blue oak trees has decreased there has been no natural regeneration, the tree is not considered endangered due to its wide distribution.
   

Bibliography

Allen-Diaz, Barbara and Holzman, Barbara. 1991.“Blue Oak Communities in California” Madrono 38 (2): 80-95.

Barbour, M. 1987. Community ecology and distribution of California hardwood forests and woodlands.  USDA, Forest Service, Washington D.C. p.18-23.

Callaway, Ragan, Nadkarni, Nalini and Mahall, Bruce. 1991.“Facilitation and Interference of Quercus Douglasii on Understory Prodctivity in Central California.” Ecology 72(2): 1484-1499.

Koenig, W. Mumme, R. Carmen, W. and Stanback, M. 1994. “Acorn production by  oaks in Central Costal California: variation within and among years.” Ecology. 75() 99-109.

Mensin, Scott. 1992. “The impact of European Settlement on Blue Oak (Quercus Douglasii) Regeneration and Recruitment in the Tehachapi Mountains, California.” Madrono 39(1): 36-46.

Pavlik, Bruce M. Muick, Pamela C. Johnson, Sharon G. and Popper Marjorie. 1991. Oaks of California.
Cachuma Press, Los Olivos, CA.

Phillips R.L. 1996. “Blue Oak Seedlings may be older than they look.” California Agriculture 50(3) 14-19.

McDonald, Philip M. (June 2, 1998). Quercus douglassi Hook.&Arn. [Online]. Available: http://willow.ncfes.umn.edu/silvics_manual/volume_2/quercus/douglasii.htm [ 17 September, 1999].

Whitney, Stephens. 1994. Western Forests. National Audubon Society Nature Guides, New York.
 
 
 
 

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