The common Valley Oak (Quercus Lobata) is found endemically in California and is well adapted to the Mediterranean climate of wet winters and hot dry summers. It is easily characterized by its large velvety round-lobed leaves and arc-like branches. The purpose of this paper is to describe the anatomy, development, growing habits, reproduction and ecology of this species of oak and relate how evolution has caused its structure to follow functions necessary for survival and propagation.
Q. lobata is a deciduous tree with lobe-shaped leaves (about 5-10 cm in length)(Munz, 1973). Each leaf is covered with tiny hairs giving it a velvety texture. The tree itself can exceed 2-3 meters in diameter and be over 30 ft. tall (Munz 1973; Bolsinger 1988). Acorns from this species are 3-5 cm. long and are slenderer than other species (Munz 1973; Bolsinger 1988). Q. lobata is a part of the Lepidobalanus lineage of oaks, often termed white oaks (Tilson & Muller 1942). These oaks are characterized as having short styles, less tannin content in acorns, basal abortive ovules, equal cotyledons and leaves with either broadly rounded lobes or pointed with tooth-like mucronate tips (Tilson & Muller 1942). The bark of this lineage tends to be soft and grey as opposed to the dark-colored bark of Erythrobalanus the red oaks (Tilson & Muller 1942). One difference between these two lineages is that Erythrobalanus tends to have a thick-walled and rounded summerwood vessels (vessels found in the darker denser growth of the tree) whereas the summerwood vessels in Lepidobalanus can be characterized either as either thin-walled and angular or thick-walled and circular (Tilson & Muller 1942). Furthermore, Lepidobalanus tends to have more numerous vessels than does Erythrobalanus (Tilson & Muller 1942). Tyloses are much more common in the wood of white oaks than in red oaks (Tilson & Muller 1942). Tyloses, which are outgrowths of parenchyma cells, push through the pits of vessel members and block water movement. This makes the wood of Lepidobalanus better for watertight crafts such as wine casks or ship hulls. Q. lobata is a species that has thin walled numerous summerwood vessels unlike those of Erythrobalanus oaks (Tilson & Muller 1942). Q. lobata can be distinguished from other species of oak from the appearance of its leaves, bark, acorns, and summerwood vessels.
In Lepidobalanus, acorns are produced yearly. Fertilization occurs 1-2 months after the deposition of pollen. Once acorns are fertilized maturation takes another 3 months and then the acorns fall (Ducousso, Michaud & Lumaret, 1993). Seeds do not need a period of dormancy (due to the mild California winters) but usually require a depth of 5-10 cm for successful seed germination in soil (Griffin 1971; Tietje, Nives, Honig &Weitkamp, 1991). Q. lobata exhibits hypogeal germination-meaning that seeds germinate underground and do not need light for germination (Tietje et al. 1991). Shallowly buried or surface acorns are prone to desiccation or predation (Tietje et al. 1991). Seedlings grow best in partial shade where moisture is adequate (Griffin 1980). Seedlings are susceptible to herbivory and to competition from fast-growing annuals and generally have slow growth and long life-spans (Danielsen and Havorson 1991; Griffin, 1976).
Q. lobata is a phanerophyte--which means that it bears perennial buds at least 25 cm above the soil (Munz, 1973). Generally, Quercus sp. has very broad crowns and a highly branching growth habit (Griffin, 1976). Broader trees tend to gather more sunlight and outcompete their neighbors and also tend to have the requisite energy to produce more acorns. Q. lobata typically has many taproots going as deep as 30 meters thus requiring a water table within 30 m of the surface (Griffin 1977;1973; Barbour 1987). Historically the water table would be replenished from periodic inundations of rivers in the Central Valley (Granholm, Henry, Kanemoto & Palmer 1988). These oaks also have extensive networks of horizontal roots that access water from winter rains (Griffin 1977;1973). Q. lobata is intolerant of heavy salts; trees within 4 miles of the coast tend to have smaller leaves, are weakened and prone to disease (Ogden, 1980).
Q. lobata is a monoecious angiosperm that relies on the wind for pollination (Ducousso, Michaud & Lumaret, 1993). Each tree has unisexual catkins that require the pollination of a different tree for the formation of acorns (Ducousso, Michaud & Lumaret, 1993). Catkins of different sexes have different inflorescences. Staminate flowers are characterized by their formation on the inner bud scales on the lower portions of branches. They start forming in late spring; the following spring meiosis occurs immediately before the emergence of newly formed catkins. Pistillate flowers are characterized by their formation on the axils of new leaves; they often emerge a few days after male catkins. They develop during late winter and upon emerging 3-5 styles appear and become red and sticky (Ducousso, Michaud & Lumaret, 1993). Fertilization occurs when the pollen tube enters the ovule through the micropyle; only one ovule in the ovary is allowed to develop into a seed (Ducousso, Michaud & Lumaret, 1993). Oak trees have their first crop of acorns at and yield generally correlates with the size of the tree (Ducousso, Michaud & Lumaret, 1993). The process of fertilization allows the oak to establish more genetic diversity in its offspring than it would be able to do via self-fertilizing.
Q. lobata is distributed from Shasta County to Los Angeles County predominating in lower elevations, in the foothills and valleys beside the Coast Ranges and Sierras (Bartoleme, 1987). It is tolerant of both drought and floods (both conditions are characteristic of the Mediterranean climate of California) (Bartolome, 1987). These oaks dominate two distinctive plant communities in California: the valley oak woodland and valley oak riparian forest (Holstein, 1984). The valley oak Woodland is characterized by grassy savannahs with widely dispersed oaks (Holstein, 1984). These grasslands historically were populated by perennial bunchgrasses (Heady 1977). The now prevalent annuals compete with juvenile oaks for moisture often suffocating them (Danielson and Havorson 1991). Near waterways, on levees, and floodplains the valley oak riparian forest predominates (Hehnke and Stone 1979; Holland and Roye 1989). This plant community is characterized by a heavier understory of bushes grasses and vines including sumac, blackberries and grasses (Bolsinger, 1988). Historically it extended .6-5 miles on either side of major waterways (Hehnke and Stone, 1979; Barbour 1987). Q. lobata favors the rich deep silty loam and alluvial sand provided by regularly flooding rivers (Holland, 1986). It is also well adapted to periodic fires. While mature trees are fire-resistant, seedlings can sprout from the root crown after burning (Griffin, 1980 ). Fire does not burn buried acorns and it clears competing plants that might take moisture and suffocate new oak seedlings (Griffin, 1980). Q. lobata has a symbiotic relationship with many bird and mammal species providing nesting space and food (Carmen, Koenig and Mumme, 1987). It is dependent on cavity caching animals such as the scrub jay and the ground squirrel which often bury acorns at the requisite depth for germination and forget their caches (Carmen, Koenig and Mumme, 1987). Scrub Jays in particular tend to bury caches in recently burned areas which coincidentally allows the seedling to establish themselves with lower levels of competition (Odion, Bornstein & Carroll, 1988 ).
Recent changes brought by Euro-American settlement have created challenges to the propagation of Q. lobata forests. The damning of major rivers prevents the replenishment of nutrients in alluvial soils while also lowering the water table (Rossi, 1980). Fire control prevents the cyclical clearing of forests needed for the success of seedlings (Griffin, 1980). Feral pigs and cattle eat new seedlings before they have a chance of establishing (Havorson and Clark, 1989). Q. lobata is a prime example of how changes in environment and ecology can either force a species to evolve or go extinct. Since many of these changes are anthropogenic, changes in forest management can help preserve Californian oak forests.
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