Oak trees of the Sonoran Desert region differ from oak trees in northern climates in two respects. First, most desert oak trees are not seasonally deciduous, i.e, they do not drop their leaves in the fall. Desert oak trees stay green through the winter and drop just a few leaves in spring and early summer as new leaves appear. The replacement takes two to three weeks. More leaves will drop during drought conditions.
Secondly, desert oak trees, and other leafy plants of the desert, have relatively small leaves. Small leaf size is a trait called microphylly. (The foothills paloverde is the extreme example).
The leaves are small because of the geometry and physics of heat dissipation. Desert plants must maintain an internal temperature of less than 115 degrees Fahrenheit to survive. The size of their leaves helps them do that.
First the geometry. Consider a sphere; this is the shape which contains a maximum volume in the minimum surface area. If you cut up the sphere, it still has the same volume with an increased surface area. Tiny-leafed plants take this to the extreme. Now the physics. The best radiators are those with the most surface area per unit of volume. Another consideration is how air flows around the leaves. A large leaf actually develops a large boundary layer of stagnant air around its surface. For the plant to cool itself, it must open its pores and transpire water to get some evaporative cooling – not a good idea if you are a desert plant and water is scarce. With small leaves, the boundary layer air does not develop, so the leaves can radiate heat without transpiration, even in relatively calm air. Convective heat exchange is more efficient with tiny leaves.
Worldwide, there are about 600 species of oaks (genus Quercus). At least 21 species occur in the Sonoran Desert region (source). Eight to 12 species occur in Arizona (the number seems to depend on who is counting), and four are very common. Oaks occur in grasslands and mountains in three biomes from about 4,500 feet to 7,000 feet.
According to the Arizona-Sonora Desert Museum:
White, emory, and shrub oak acorns were gathered by Yavapai, Pima, and Tohono O’odham Indians…. Some Indian groups ate acorns raw or ground into meal; others roasted the acorns first. Some acorns have a very astringent taste. These were shelled, ground into meal, and leached by placing a bag of the acorn meal in a stream for several hours. The resulting sweet-tasting meal was used to thicken stews, and for mush and bread.
Ground acorns were roasted and brewed into a coffee-like beverage. Tea made from green oak bark was used as an astringent, an anti-diarrhetic, and a cure for bad-smelling feet.
Dye made from the bark was used by Indians and pioneers. Oak tannin was used for curing buckskin.
Acorns are eaten by many birds, bears, bighorn sheep, ring-tails, foxes, racoons, squirrels, chipmunks, javelina, and deer.
The four most common oak species in Arizona are the Emory, Silverleaf, Shrub Live, and the Arizona White oak. Less common are the Canyon Live oak , Net-leaf oak , Wavy-leaf oak, Gambel oak (the only winter deciduous oak), and Dunn oak (also called the Palmer oak). Click links to see photos.
The Arizona White oak is the largest in the southwest. It can reach 60 feet tall with a trunk diameter of three feet. Its leaves are one to four inches long and one-half to 2 inches wide. The leaves can have spiny teeth or can be smooth. They are blue-green above and pale and densely hairy below. Bark is ash-gray and vertically fissured into narrow plates.
The Emory oak ranges from shrub size to 50 feet tall with a trunk diameter of 2.5 feet. Leaves are one to two inches long and up to one inch wide and are either smooth edged or with a few spiny teeth. The leaves are leathery and yellowish-green on both sides. Bark is black and deeply furrowed into squarish plates.
The Silverleaf oak grows as a shrub or tree up to 30 feet tall with a trunk 2.5 feet in diameter. Leaves are two to four inches long, 0.5 to 1 inch long and sharply pointed. They are shiny green above and white-woolly below.
The Shrub Live oak is usually a shrub up to eight feet tall. Its leaves are holly-like up to 1.25 inches long with spiny teeth. Leaf color is blue-green above and yellow-green and hairy below.
For more information on oaks and the general plant biology of “Sky Islands,” see a paper by Brusca and Moore here. Brusca states: “The greatest diversity of oak species in the world is found in Mexico, where at least 125 species have been identified,…” He does not say if all these species are within the Sonoran Desert region.
For information on other desert plants, see: