Oaks: An Enduring Legacy

Story by Colleen Pelles Madrid

I’m an acorn. You know! An oak nut. How could I not be, growing up in California where there are 20 species of oaks and 20 more oak hybrids? They are my favorite tree and when I became an ecologist I understood why. Oaks are, by far, the most ecologically important tree species in the forests of the North American continent and have played a large role in human history.

To understand their importance, you must understand their evolution. During their roughly 56 million-year evolution oaks have evolved into 435 species found on five continents. They range from Canada to Colombia and from Norway to Borneo. Oaks are especially prominent in the Americas, where about 60 percent of all oak species live. They account for more woody plant biomass than any other genus in North America and Mexico.

How did a tree with heavy seeds repopulate postglacial North American faster than species with lighter, wind-dispersed seeds? As oaks were evolving various species of birds in the Corvid family coevolved with them, jays. Jay species evolved a hook on the tip of their beaks ideal for opening the husk of acorns. Jays harvest acorns in the fall to bury for later. They can carry five acorns at a time in their gullet, carrying them up to a mile away. On a typical oak, which is wind pollinated, only about 10 percent of acorns are fertile. Jays have been shown to select these viable acorns 90 percent of the time, increasing the odds for germination. A jay can bury up to 5,000 acorns but only “remembers” where one out of four acorns is buried. Jays have potentially grown thousands of oaks.

The National Park Service refers to oak as a keystone genus because it supports a greater diversity of organisms “than any other North American tree genus including fungi, insects, birds, and mammals.” Seventy-five percent of insect food required by birds is produced by only a few plant genera: oaks, cherries, willows, hickories, birches, pines and maples. Of these, oaks host by far the greatest number of insect species. Almost 900 species of caterpillars are found in the oak forests of the United States, more than any other genus of tree. Most songbirds in North America are insectivorous, especially during the nesting season when seeds and berries only supplement their diet. Nestlings are unable to digest seeds at all. Most bird species cannot reproduce without a ready supply of insects. Ninety-six percent of bird species feed their young on insects. At breeding time 92 percent of hummingbird diets are insects. It takes between 6,000 and 9,000 caterpillars to rear one clutch of chickadees.

There are more than a dozen species of oak found in New Mexico including bur oak (Quercus macrocarpa), chinkapin oak (Quercus muehlenbergii), Gambel’s oak (Quercus gambelii), gray oak (Quercus grisea) and shrub live oak (Quercus turbinella). Bur oak was once common in the environs around Santa Fe until development altered the terrain. It has the largest acorns of all oaks and is an important food for wildlife. It is an exceptionally large tree growing up to 105 feet. Chinkapin oak has a limited range in New Mexico and occurs only in the high desert of the southeast. It also can grow to 100 feet in height and is valued for its sweet tasting acorns. Gambel’s oak occurs in the foothills and mountain slopes throughout New Mexico. It grows 1060 feet depending upon the precipitation. Shrub live oak is common in the Rio Grande valley. It is a small shrub-like tree that seldom grows higher than 15 feet and commonly forms thickets.

In addition to being ecological wonders, oaks are long lived. The Angel Oak in South Carolina is estimated to be 400-500 years old. What better legacy for our world than to nurture oaks. Plant your legacy, plant an oak.

References:

Tallamy, D.W. The Nature of Oaks. Timber Press (2021)

Tallamy, D.W. Nature’s Best Hope. Timber Press (2019)

Hipp, A., et al. How Oak Trees Evolved to Rule the Forests of the Northern Hemisphere. Scientific American (2020).

National Park Service, Region 1. Spotlight on Oaks.