Wildflower is published quarterly by the Wildflower Center. Its content is national in scope with articles about the conservation and use of native plants as well as news from the Wildflower Center. A subscription is provided to Wildflower Center members as a benefit of membership.
"Monarch of the woodland is the oak, of all trees most dear to us who live in northern lands..."
–"Aristocrats of the Trees" (1930)
Ernest H. Wilson, Horticulturist, Arnold Arboretum, Boston
By Bibi Wein
The oak (Quercus spp.) is the most widespread hardwood in the Northern Hemisphere, prized for its shade, the beauty of its architecture and foliage, and for its strength, longevity and durability in both landscape and lumber. Worldwide, some 600 species make their home in forests on every wooded continent but Australia. According to Cornell plant scientist Kevin C. Nixon, Ph.D., different oak species worldwide are “adapted to a broad array of habitats.”
This is true in the United States as well, where more than 90 native species grow, with some in every state but Alaska, Idaho and Hawaii, in a striking variety of conditions. Although most oaks prefer drier soils, according to Nixon some – like the swamp white oak (Q. bicolor), pin oak (Q. palustris) and bur oak (Q. macrocarpa) – tolerate standing water. And although most require acidic soil, several like the chinkapin oak (Q. muhlenbergii) and Shumard oak (Q. shumardii) are well-adapted to alkaline conditions. Texas has the most species – 74 percent of North American natives – largely due to its size and the diversity of habitat. Various parts of Texas can accommodate oak species of the Southeastern forests, Southwestern woodlands and mountains, and South-central plains.
Oaks have been classified in several different ways. One of the simplest divides them into three primary groups – red, white and live – according to the shape of their leaves. Red oaks (Q. rubra and others) have lobed leaves with sharply pointed tips; the leaves of white oaks (Q. alba and others) are similar in shape but have rounded or blunt tips; and live oaks are characterized by their round-tipped oval leaves.
In the United States, Quercus rubra [hardiness zones 3-8] and Q. alba [zones 3-9] are the most widespread of all individual species in the wild, found almost throughout the Midwest and East. (However, Q. rugosa extends from Arizona to Guatemala.) Both Q. rubra and Q. alba are prized for their fall color. White oak species live longer and are preferred for their tougher lumber; red species, noble and stately, are especially valued for their adaptability to urban settings. Live oaks (Q. virginiana and others) are Southern trees, revered for magisterial spread and shade. They are sometimes called evergreens, but they do shed their leaves in the spring, just as new ones are forming. There are red, white and live oak species native to the West, and the West has more-specialized species like canyon oak (Q. chrysolepis), which is limited to mountain ridges, moist slopes and canyons in California, Oregon, Nevada, New Mexico and Arizona.
The uneducated eye doesn’t immediately see that the spreading live oaks are related to the towering deciduous reds and whites, to say nothing of some of the small Western oaks, better known as shrubs. No other broadleaf genus has such a wide size range. In fact, Quercus is the most diverse genus of trees in North America. The common link – what makes an oak an oak – is the flower structure, to which most of us give little notice.
We Americans love our oak trees. The names of at least 150 California communities include reference to oaks. Six states and the District of Columbia have chosen various Quercus species as their official tree. The oak (species unspecified) is our national tree as well, so designated by Congress in 2004, after 100,000 people voted for it in a National Arbor Day Foundation poll. We’ve even perpetrated falsehoods about our favorite oaks to encourage their protection, as a journalist reputedly did with the landmark Jacksonville, Florida, “Treaty” Live Oak, under which no treaty has actually proved to have been signed. We’ve coined a word – “re-oaking”– for planting new oak forests where old have died off, and when Austin’s beloved Treaty Oak was the victim of attempted arboricide, children sent it get well cards.
In the few hundred years of American history, individual oak trees have achieved local and sometimes national status as kissing trees, hanging trees, treaty trees, legacy trees and champion trees. (At least one historic tree in each of these categories can be found in Texas). The oak’s venerable natural longevity is the source of many of these designations.
South Carolina’s Angel Oak, on John’s Island near Charleston, is purported o be about 1,400 years old, perhaps the oldest living thing east of the Rockies. The 65-foot-tall live oak (Q. virginiana) was severely damaged by Hurricane Hugo but has since recovered and remains the focal point of many cultural events.
The Angel Oak, in all its gnarled complexity, suggests a living being of awe-inspiring spiritual power and makes it easy to understand why the shade of a single oak, or an oak grove, was a sacred place for many ancient peoples. In these shaded spaces, ancient peoples prayed, worshipped, conducted sacrifices and sought oracular advice. Later, Christians in the British Isles spread the word under certain “Gospel Oaks,” trees that also served to mark parish boundaries.
The history of Quercus is intricately linked with the history of humankind in North America, throughout Europe and in much of Asia – wherever the genus grows.
Trees in general figure prominently in Western mythology and folklore, but the oak stands out as “The King of Trees” and in some cultures was treated as the personification of the human king – who took on the oak’s strength by association. The word “robust” comes from the Latin robur (specific epithet of the English Oak), which means both “strength” and “oak.”
Contrasting somewhat with its formidable role as king, the oak was the Tree of Life for many northern European cultures, including the Teutons, the Celts and the Druids, as well as in tropical Central America, where it was associated with fertility, birth and the voluptuous abundance of rain forest flora and fauna. Both Greeks and Romans regarded the oak as a progenitor of mankind, and the ancient Greeks called it “the Mother Tree” for the sustenance gleaned from its acorns, which, if not “man’s first food,” as Ovid claimed, certainly provided a vital source of nutrition for primitive peoples – among them Greeks, Romans, Nordic tribes and Native Americans – who subsisted primarily on acorns and nuts.
Oaks play a vital ecological role wherever they grow. When our ancestors relied directly on trees for food, warmth and shelter, oaks were among the most reliable. Today, acorns feed more than 100 species of wildlife, including game birds, as well as pigs and poultry in some regions, and they shelter countless species of birds with their massive frame, huge limbs and lush canopy.
But oaks are more than mere trees. Older individuals in all three groups may be massive, towering ecosystems that have evolved over centuries. Known in Celtic times as “The Garden in the Forest,” older oaks may support harmless fungi and lichen on their trunks alongside tendrils of ivy. Live oaks are home to romantic epiphytes such as Spanish moss and parasites such as mistletoe. Such profound mystical powers have been invested in the oaks of lore and legend that plants said to have magical properties – such as mistletoe – are invested with still more potent magic if found growing on an oak.
The line between legend and history is far from clear in Britain’s Sherwood Forest, home of the fabled Robin Hood. Whether or not Robin and his reputedly merry band of outlaws actually roamed these woods, Sherwood Forest is the very real home of nearly 1,000 beloved trees that made international news recently. Its core collection of ancient oaks (Q. robur and Q. petrea), said to be one of the greatest in Europe, is in mortal danger due to age, industry, development and possibly climate change.
Some of the oaks here likely date back to the 13th century, at least as saplings. But only 450 acres, plus some outlying patches, remain of what was once a 100,000-acre forest. Rangers cite this fragmentation as one of the factors increasing the vulnerability of the ancient trees. At least 15 organizations are developing jointly a rescue plan that features planting 250,000 trees to knit the forest back together.
Americans are equally devoted to our favorite oaks: case in point, Austin’s famous Treaty Oak. It’s improbable that any treaty was signed in the shade of the 500-year-old live oak on Baylor Avenue, but it is definitely the last of 14 historic trees known as the Council Oaks, where Comanches and Tonkawas celebrated feasts, held religious ceremonies and war dances, and smoked the peace pipe. In 1927 Treaty
Oak was proclaimed “The most perfect specimen of a North American tree” and was inducted into the American Forestry Association’s Hall of Fame. In 1989 it boasted a spread of 127 feet when an unrequited lover set out to cast a spell, or so he claimed, attempting to murder the tree with a powerful hardwood herbicide. Arborists expected the tree to die, even after industrialist Ross Perot wrote a blank check to support intensive efforts to save it. Almost two-thirds of the tree did succumb, and more than half of its crown had to be pruned away, but the tree survived. In 1997 it produced its first new crop of acorns since the poisoning, which were gathered, germinated and planted to ensure its legacy. The poisoner served nine years in prison.
Paradoxically, our toughest and most durable tree is also one of our most vulnerable. Its timber, a favorite for fine woodcrafts, furniture, bridges and ships, is so strong that the USS Constitution got the nickname “Old Ironsides” because British cannonballs merely bounced off of its white oak hull. Yet ancient peoples around the world linked oaks to their storm gods because of their propensity to be struck by lightning, and numerous insects and diseases have always threatened oaks in rural, urban and forest settings. There are other more subtle threats today. The live oak, which needs lots of elbow room to thrive, is threatened by both suburban sprawl and reforestation, according to University of Arkansas researchers. A 2003 study found that 90 percent of the live oaks in the suburbs of Gainesville, Florida, were dangerously crowded by other trees. Even ongoing changes in squirrel distribution may impact oaks: Acorns gathered by the gray squirrel appear more likely than those hoarded by the red squirrel to result in tree seedlings, as a recent Indiana study suggests.
Millions of government dollars have been allocated to battle two major threats facing oaks today, sudden oak death and oak wilt. There’s a sense of urgency in the intensive efforts to unlock the secrets of the pathogens responsible for these threats, and plant scientists are learning more about them each year. But there is much more to be done to ensure that America’s favorite tree endures to feed small mammals, shelter birds and provide pleasure, wonder and inspiration for human generations to come.