Oak: Evolutionary History and Ecological Interactions
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Oak: Evolutionary History and Ecological Interactions

In northern California oak woodlands cover nearly 4.5 million acres of typically rolling terrain that is bounded by valley grassland, chaparall, and montane forest (Holmes, 1990). Oaks are mostly found at high elevations under cool or temperate climate. The oak communities in California include blue oak (Quercus douglasii), blue oak-foothill pine (Pinus sabiniana), coast live oak (Quercus agrifolia), and valley oak (Quercus lobata) (Purcell, 2006). Throughout their evolution oak trees acquired adaptations that allowed them to grow, reproduce, and survive in a dry environment. Oak species vary in age and distribution and occur in hybrid, spindly, robust, and scrubby forms (Holmes, 1990). Most of them form trunks from which branches stem out, though others produce branches that shoot out from the ground. Both deciduous and evergreen oaks exist. Their leaves for the most part are pinnate though some elliptical are found as well. The inflorescence is catkins and the fruit are acorns. Their distribution and abundance depends on various factors. Here I address the history and ecological interactions of oaks.

In northern California oak woodlands cover nearly 4.5 million acres of typically rolling terrain that is bounded by valley grassland, chaparall, and montane forest (Holmes, 1990).  Oaks are mostly found at high elevations under cool or temperate climate.  The oak communities in California include blue oak (Quercus douglasii), blue oak-foothill pine (Pinus sabiniana), coast live oak (Quercus agrifolia), and valley oak (Quercus lobata) (Purcell, 2006).   Throughout their evolution oak trees acquired adaptations that allowed them to grow, reproduce, and survive in a dry environment.  Oak species vary in age and distribution and occur in hybrid, spindly, robust, and scrubby forms (Holmes, 1990).  Most of them form trunks from which branches stem out, though others produce branches that shoot out from the ground.  Both deciduous and evergreen oaks exist.  Their leaves for the most part are pinnate though some elliptical are found as well.  The inflorescence is catkins and the fruit are acorns.  Their distribution and abundance depends on various factors. Here I address the history and ecological interactions of oaks.    

            Cattle and sheep introduced during the European explorations to North America had a great impact on oak community.  The Spanish coastal missions, including Santa Clara and San Jose acquired approximately four million sheep by 1880 and nearly 1 million cattle by 1890 (Holmes, 1990).  This created a demand for oak browsing, which led to changes in ecological biomes such as the conversion of oak woodlands to grasslands. Over time range modification has greatly reduced the distribution of oaks in northern California, especially in the Sierra foothills (Holmes, 1990). Animal species that impact the oak community are rodents. Squirrels and pocket gophers use both acorns and seedlings heavily as food sources (Holmes, 1990).  The influence of rodents on oaks is more visible in recent times due to an increase in their population.  Their populations have increased due to the removal of rodent predators (Holmes, 1990).  Rodent and oak interactions could be one of the reasons of why the blue oak regeneration has slowed down.

            The Blue oak (Quercus douglasii) is the most common species of oak tree in the oak woodland.  Its common name comes from the fact that it appears blue from a distance.  They cover an estimated 3 million hectare of woodlands and savanna in the interior valleys and foothills of Central California (Dahlgren, 1997). Blue oak woodlands occur in dry, hilly terrain in the western foothills of the Sierra Nevada, Cascade Ranges, Tehachapi Mountains, and the eastern foothills of the Coastal Ranges (Purcell, 2006).  This species happens to be one of the dominant oaks though this may potentially change in upcoming decades since low reproduction rates have been recorded.  Very few juvenile blue oaks have been observed suggesting that its life cycle might be obstructed by factors related to reproduction.  This could be caused by variations in environment conditions such as soil degradation and climate shifting.

            Blue oak regeneration is a topic of concern since little to none oak sprouting is observed in the field.  These populations do not appear to be regenerating at rates capable of maintaining current distributions and densities, causing great concern over sustainability of these ecosystems (Dahlgren, 1997).  Oaks regenerate from acorns that fall not far away from the canopy.  Animals also plant acorns, in many cases far away from the canopy.  The oaks that sprout under the canopy produce large root structures but are held back due to overshadowing from older trees.  These understory seedling may survive for years, producing a strong root system but little shoot growth (Swiecki, 1998).  At the moment there is no concrete reason as to why its regenerations has slowed down but it has been suggested that grazing, clear cutting, and man manipulation of fire have provided the most damage. California Indians burned to modify plant and animal communities for their benefit (Purcell, 2006).  Though extensive fire decimates the oak population it is sometimes needed to plant regeneration.

            An important factor in oak regeneration is fire.  Fire is an essential component of the disturbance regime in oak woodlands of California (Purcell, 2006). Oak life cycle is highly dependent on fires since its seeds depend on the space created by fires. Weeds may reduce seedling success in unburned areas, as they compete with oak seedlings for light and moisture (Holmes, 1990). Acorn and leaf production of blue oaks increase as a result of reduced competition with understory vegetation after fire (Purcell, 2006). Burning also releases nutrient into the ground that could be used by sprouting oak seedlings.

            Studies on oak woodland have suggested that oak trees promote high nutrient concentrations and cycling in the soil.  The characteristics measured in soil are pH, concentrations of organic carbon, nitrogen, phosphorus, and other minerals like alkaline elements. A study revealed that concentrations of carbon, nitrogen, and phosphorus were higher in all soil beneath the oak canopy compared to the open grassland (Dahlgren, 1997).  Leaf litter density is responsible for the soil’s larger nutrient concentrations.  Inputs of grass litter over a three-year period averaged 1758 and 2911 kg/ha/yr for the oak understory and open grasslands, respectively (Dahlgren, 1997).The persistent shedding and recycling of plant litter causes accumulation of nutrients on the soil’s surface. Oak trees create islands of enhanced fertility through organic matter incorporation and nutrient cycling (Dahlgren, 1997).  Total and available soil nutrients are generally shown to be higher beneath the oak canopy than in surrounding grasslands (Dahlgren, 1997).

            History and ecological interactions provide a better understanding of oak growth survival, growth and reproduction.  The early disturbances created by Indians and early settlers affected the distribution of oak in California.  The blue oak though the one of the most dominant species of oaks is currently under stress to reproduce.  We know that fire is a helpful factor but it doesn’t provide all the elements for successful oak seedling sprouting.  The soils seems fertile for oaks though this may not induce its regeneration and probably only promotes its survival.  From my perspective what we should focus on to understand they low regeneration rate in climatic variation since it seems to be in constant fluctuation.   

Works Cited

Dahlgren, R. A.; Singer, M. T.; Huang, X.. "Oak Tree and Grazing Impacts on Soil

Properties and Nutrients in a California Oak Woodland." Springer 39.1 (1997): 45-64. Print.

Holmes, Tyson H. "Botanical Trends in Northern California Oak Woodland." Society

for Range Management 12.1 (1990): 3-7. Print.

Purcell, Kathryn L.; Stephens, Scott L.. "Changing Fire Regimes And The Avifauna Of

California Oak Woodlands." Studies In Avian Biology 30 (2006): 33-45. Print.

Swiecki, Tedmund J.; Bernhardt, Elizabeth. "Understanding Blue Oak Regeneration."

Fremontia 26.1 (1998): 19-26. Print.

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Comments (1)

An excellent piece about oaks.

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