Geography 316: Biogeography
Geography 316: Biogeography
The Biogeography of valley oak (Quercus
lobata Nee)
by Patrick Tice, student in
Geography 316
Thank you for visiting our site. This
web pages was written by a student in Geography 316: Biogeography and edited by
the instructor, Barbara Holzman, PhD. All photos and maps are posted with
specific copyright permission for the express use of education on these web
pages. The students have tried to be as accurate as possible with the
information provided and sources and references are cited at the end of each
page.
Species Name: Quercus lobata Nee
Kingdom: Plantae
Division:
Tracheophyta
Class: Angiosperm
Order: Fagales
Family:
Fagaceae
Genus: Quercus
Species: Quercus lobata
Description of Species:
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At first, valley oak trees maintain a straight, pole-like form during the first 10 to 30 years of growth (Pavlic et al. 1991). Older specimens of valley oak are extremely noble in stature, reaching heights of 100 feet or more at a mature age. The bark is very thick (5 to 6inches) and checked, resembling the knobby hide of an alligator's back. The leaves vary in size from 2 1/2 to 3, and sometimes even 4 inches long, 1 to 2 inches wide (Peattie, Donald, 1991). The upper surface of the leaves are a dark green with a felt-like texture. The undersides are pale green with yellowish veins. Acorns of this species come solitary or in pairs and grow to lengths of 3 inches. At first they are glossy green, then turn a chestnut brown before dropping; the acorn cups are thick, and warty. The branches usually bear woody, spherical galls that harbor the larvae of small native wasps (Pavlic et al. 1991). This gull is formed when wasps lay eggs inside the soft tissue of the tree. |
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Natural History: Prior to European contact The Great Valley of California was full of life. It may have been one of the wildest places on earth. Rivers ran wild therefore heavy winter precipitation produced great floods in the valley supporting the life of many riparian areas. Preceding the mid 1800’s, riparian forests covered some 800,000 acres in the Great Central Valley of California. Of this great coverage, Valley oaks were a primary component of these forests. Valley Oak (Quercus lobata) is the monarch of California oaks by virtue of its size, age and beauty (Pavlic et al. 1991). This magnificent tree is endemic to California and quite possibly is the largest North American oak, declaring it very unique. Descriptions of its remarkable stature appear in the diaries of many early visitors to California (Pavlic et al. 1991). |
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As the name clearly conveys, Valley oak are dwellers of fertile soils found in
bottomlands and valley floors. During the months of low solar declination,
this winter deciduous tree drops its leaves to conserve necessary food
during this period of minimal photosynthesis. As
spring commences, and the sun makes its way into the higher latitudes, Valley
oak put out lobed, felt-covered leaves to provide a dense canopy of shade during
the intense sun drenching of summer. Valley oaks flower with the arrival
of spring. Flowering is finely tuned to and triggered by temperature
change. When cold nights give way to warm days of spring, hormones known
as auxins promote bud burst by increasing the tree's intake of water (Keator,
Glenn 1998). The combination of fertile soils,
constant water and intense sunlight produce perfect conditions to achieve high
rates of photosynthesis and rapid growth. As a result, many rather
impressive trees, with trunks three or four feet in diameter, are relatively
young – only 150 to 250 years old (Pavlic et al. 1991).
Valley oaks, as well as other oak species, exhibit characteristics of opportunism, which means that they have evolved to subsist in environments, which typically are not favorable to many plant species. Although Valley oaks thrive in fertile, riparian areas, they can subsist on hillsides as well. They do not require excessive water, yet being plugged into some source is favorable. A young (10 to 40 years old) Valley oak’s taproot can reach 60 feet deep, to search for ground water (Kaweah Oaks Preserve, 2002). This opportunistic approach allows the Valley oak, as well as other California oak species, to thrive in a landscape blighted by drought. A valley oak savanna is typified by grassland expanses dotted with mingling Valley oaks and other species- usually Blue, Coast live, and Interior live oaks. With the ability to tap groundwater sources they have evaded competition with other plant species for resources necessary for viability.
Valley oak riparian forests have the most complex structure of any vegetation type within California, as a result, they are among the most diverse in terms of the animal life, which they support (Griggs et al. 1989). Within a Valley oak forest many species of songbirds, migratory waterfowl, raptors, herons, and egrets as well as small mammals, such as squirrels are housed within this diverse ecosystem. Of these life forms, many have made their name on the threatened or endangered species list of state and federal agencies
In the late 1800’s the rich soils of the Sacramento River floodplain were cleared of riparian vegetation to provide fencing, lumber, fuel for steam ships and open areas for agriculture (Griggs et al. 1989). Since this time, an astonishing number of dams have been erected on major California waterways to contain the water for agricultural and municipal uses as well as the threat of catastrophic floods. This ongoing conversion of riparian forests to agricultural and urban uses had plagued the viability of Valley oaks as well as countless other flora and fauna. Grazing cattle in the oak savannas relentlessly trample and browse the grasses and leaf litter under the canopy, which is the area where ultimately all acorns must germinate to escape the harsh elements of this environment; young saplings are also browsed by these grazing animals making viable regeneration extremely difficult if not impossible. Today, a large majority of Valley oak habitat has been converted to other uses. Remnants of these huge forests and savannas exist throughout the Central Valley but most Valley oak forests and savannas now exist in smaller valleys of the inner Coast Ranges and Sierra foothills.
Evolution:
Tracing the steps of a species’ evolutionary path can be a difficult and rather controversial pursuit. Tracing evolutionary steps relies heavily upon the examination of fossils, yet the complete story is often obscured due to breaches in the fossil record. Fossils posses a bias toward those organisms, which are easily preserved making evolutionary inferences tentative and debatable.
Angiosperms (flowering plants) arrived in the fossil record as far back as 135 million years ago, yet the first remains of an undeniably Quercus plant didn’t arrive until approximately 40 to 56 million years ago (Keator, Glenn). If we were to travel back in time approximately 200 million years ago (let us not forget that geologic time is extremely approximated) the continents were melded together into one super-continent—Pangea. As millennia passed, there was a gradual separation into two continental landmasses, a northern landmass—Laurasia—and a southern—Gondwanaland. Whether or not angiosperms appeared before the separation is still unknown, but more importantly, ancestral species of Quercus plants (beeches, chestnuts), occurred along the margins of the now separated landmasses (Keator, Glenn).
According to Glenn Keator, around 60 million years ago another burst of flowering species occurred. The ancient lineages that had survived took on new, more specialized forms. As these new forms encountered further changes from climate and geologic activity, subfamilies of the fagaceous lineage sprung up; this is how the true oak subfamily came about.
As I mentioned before, to grasp the entirety of the story is impossible. Only reasonable inferences can be made. Whether or not North America was the origin of the Quercus plants, the fact is, they are present here. In the Southwestern portion of North America, which would eventually be delineated by the political boundaries California, major historical changes took place. The mountain building episode responsible for the Sierra Nevada range occurred bringing with it climatic changes as well. These changes created a new environment for which the existent Quercus plants would need to adapt. From all of this change would evolve Quercus lobata.
Distribution
Quercus lobata is an endemic species to California, meaning that it only exists there naturally. It is widely distributed in the Great Central Valley (Shasta Lake to southern San Joaquin Valley), the inner Coast Ranges from the Eel River (Mendocino County) south, and the Transverse Ranges (Tehachapi Mountains to San Fernando Valley)( Pavlic et al. 1991 ). Although remnant groves are still found in the Central Valley, the most impressive valley oak woodlands and savannas are seen in the valleys of the Coast Ranges (Pavli et al. 1991 ).
Valley oaks reside mainly in five general land use/land cover settings: 1) valley oak or mixed oak riparian forests; 2) valley oak and mixed oak woodland and savanna; 3) upland mixed oak or evergreen forest; 4) as relict trees in pastures, croplands, vineyards; and, 5) as relict trees in residential and developed recreational areas (e.g. golf courses, county parks, etc.)(UCSB, 2002).
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| Valley oak distribution (Pavlic, 1991) |
The savanna community type is found on alluvial soils of valley floors and on some broad ridge tops of the Coast Ranges, which provide an adequate water source. Valley oaks are not found in valleys directly exposed to coastal winds, as the species is sensitive to salt aerosols. On the margins of meandering river systems throughout the Central Valley the more, dense, riparian stands are found (UCSB, 2002).
In the northern portion of California, valley oaks reside in areas where elevations range from approximately 500 to 800 feet above sea level. In the southern region, where the climate is more benign, the valley oak has a greater range of tolerance, thus it can reside in areas with elevations up to 5,600 feet above sea level (IHRMP).
Links
http://www.rain.org/~audubon/sbaskisner.html
http://oaksymposium.uaex.edu/abstracts.asp
http://danr.ucop.edu/ihrmp/val1a.html
Bibliography
Pavlic, Bruce M.; Pamila C. Muick; Sharon G. Johnson; Marjorie Popper. 1991. Oaks of California. Cachuma Press. P 10,12.
Santa
Barbara Audubon Society. 1998. “The Importance of Oak Woodlands.” [On-line]
http://www.rain.org/~audubon/sbaskisner.html [Oct. 12, 2002].
Kaweah Oaks Preserve. [On-line] http://kaweahoaks.com/html/valley_oaks_new.html
[Oct. 12, 2002]
Peattie, Donald. 1981. A Natural History of Western Trees.
Houghton Mifflin, Boston. P 419.
Keator, Glenn. 1998. The Life of an Oak: An Intimate Portrait.
Heyday Books. P 182-196.
Photographs
All photographs taken by Patrick Tice 2002.
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