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Editor's Picks

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An attempt to settle the matter of this controversial name
Allen Coombes and Roderick Cameron | Aug 16, 2021
The IOS logo in black and white
The story of how the IOS logo came into being
Allen Coombes | Jul 31, 2021
Phylogenetic tree
IOS members Paul Manos (Duke University) and Andrew Hipp (...
Andrew Hipp | Jun 16, 2021

Plant Focus

The Compton oak at Colonial Williamsburg
A natural hybrid between Quercus lyrata (overcup oak) and Q. virginiana (Southern live oak)

Oaks: The Key to Sustainable Landscaping

Many cultures have historically viewed humans as beings separate from, and superior to, the natural world. At best, nature has been viewed as entertaining but not essential. And, above all else, nature has been confined to somewhere else—someplace to visit, but certainly not to live in. Consequently, we have designed landscapes for aesthetics and convenience, with no thought to ecological function. We have an adversarial relationship with nature, which is a false human construct that does not recognize our total dependence on the natural world. We are products of nature and cannot exist without the essential life-support that healthy ecosystems deliver daily. Unfortunately, building landscapes that degrade local ecosystems instead of enhance them has led to a dismal record of biodiversity loss. We now face global insect decline, the documented loss of one-third of North America’s breeding birds and two-thirds of earth’s wildlife, not to mention the imminent extinction of 1 million species.

Lucky for us that there is nothing inevitable about the demise of nature. We can forge a collaborative relationship with the natural world, but an essential component of that relation will be giving up the notion that humans are here and nature is somewhere else. We must learn to coexist with nature where we live, work, play, and farm. There are four ecological functions every landscape must perform if we hope to persist on this planet: support a diverse and viable food web, capture and store atmospheric carbon, contribute to watershed management, and sustain a diverse community of generalist and specialist pollinators. Oaks perform three out of four of these ecological tasks better than any other temperate plant genus.

Food web support

From an animal’s perspective, along with oxygen production, the most important thing plants do is convert sunlight to simple sugars and carbohydrates, the basis of the foodstuffs that feed nearly all animal life on planet Earth. But the food that plants produce is of no use to animals if it remains within plant tissues. Most vertebrates do not eat plants directly; they eat invertebrates that eat plants (think insects), and caterpillars transfer more energy from plants to more kinds of animals than any other type of insect. For example, 96% of North American terrestrial birds rear their young on insects, and in 16 of the 20 most common bird families, caterpillars dominate nestling diets. In short, a landscape that does not support lots of caterpillars is a landscape with very few of the animals that help run local ecosystems.

Laughter fig caterpillar
Caterpillars such as the laugher (Charadra deridens), shown here, fuel the food web, and more species depend on oaks than on any other plant

But here’s the key: plants differ widely in their ability to sustain caterpillars. Native plants sustain far more types and numbers of caterpillars than do non-native plants, but even among native plants there are vast differences in how well caterpillar development and reproduction is supported. In fact, just 14% of the native plant genera in North America support 90% of its caterpillar species. I call these genera “keystone plants” because, like the essential keystone that supports a Roman arch, if keystone plants are removed from a food web, that food web collapses. In North America, and undoubtedly elsewhere as well, oaks are the best keystone plants because they support more caterpillar species than other plants: over 950 caterpillar species recorded on oaks to date. No other plant genus comes close to this level of productivity.

White-eyed Vireo Vireo griseus
White-eyed Vireo (Vireo griseus) feeding a caterpillar to its young

Carbon capture

All plants store atmospheric carbon dioxide in their tissues and in the soil around them, but not all plants do this equally well. Large, long-lived plants with dense cell structure store more carbon in their tissues than other plants, and plants with large root systems deposit the most carbon in soil where it can remain out of harm’s way for thousands of years. Oaks are superior to other plants in both of these regards. Although not all oaks are large, most are; and even small species are very long lived. The average oak species has a 900-year life cycle; 300 years of growth, 300 years of stasis, and 300 years of decline. Moreover, the large species have enormous root systems that can extend 300 feet or more in all directions from the trunk. These traits make oaks the obvious choice when planting for carbon storage.

Watershed management

Everyone lives within a watershed, and no one has the ethical right to degrade that watershed. Unfortunately, most people don’t realize that the degree to which their landscape purifies rainwater water, manages stormwater runoff, and facilitates infiltration for water table recharge depends on the type and number of plants they choose for their landscape. Plants with large root systems and broad canopy spread manage watersheds far better than small plants with shallow root systems, such as turf grass. Again, oaks are superior to other plants in this regard. Not only do their canopies and roots manage heavy rain, but the leaf litter they create provides a better buffer against soil erosion than other types of leaf litter because shed oak leaves take up to three years to break down—far longer than leaves produced by maples, birches, hickories, willows, poplars, and other deciduous trees.

Oak leaf litter
Oak leaf litter takes up to three years to break down, making it an excellent buffer against erosion

Pollinator support

This is the only important landscape function in which oaks do not lead way; they are wind pollinated! But even here, there is growing evidence that oaks contribute. Apparently, a number of early spring bees do gather pollen from oak catkins; they just don’t deliver that pollen to female oak flowers.

Concluding remarks

Despite their many landscape attributes, oaks are in trouble. Nearly one third of the 430 species of oaks worldwide are threatened with extinction. In most oak habitats, the old giants are gone, long ago harvested for their wood products. We humans live our lives out in a brief instant of ecological time. We cannot return ancient oaks to our landscapes during that instant, but we can—indeed, we must—start the process. I have planted a number of massive old oaks on our property, except they are only 19 years old and are not so massive yet. They are growing, though, and several have topped 40’ at this writing. In a blink of ecological time they will be large enough and old enough to fully assume their keystone positions in our yard. And just in time. So please, plant an oak: plant a living, sustainable community, plant the future!

White oak
A mature white oak (Quercus alba) supports 537 species of butterfly and moth caterpillars

 

Editor's Note

Douglas Tallamy is well known to gardeners in the U.S., where for years he has promoted the use of native plants in landscaping. His 2007 book, Bringing Nature Home, was an introduction to the subject of the strong relationships linking native plants and native wildlife. In Nature’s Best Hope, a New York Times bestseller published in 2020, he urged homeowners to take conservation into their own hands. Now in his latest book, The Nature of Oaks, he has turned his advocacy to our favorite genus. The book reveals what is going on in oak trees month by month, highlighting the seasonal cycles of life, death, and renewal. From woodpeckers, who collect and store hundreds of acorns for sustenance, to the beauty of jewel caterpillars, Tallamy illuminates and celebrates the wonders that occur in backyards in the United States. He also shares practical advice about how to plant and care for an oak, along with information about the best oak species for certain areas. You can find his book on Amazon and other major bookstores. You can also listen to him talk about his recent work in an interview with Margaret Roach here. And check out Homegrown National Park, a non-profit organization he co-founded, which defines itself as "a grassroots call-to-action to regenerate biodiversity and ecosystem function by planting native plants and creating new ecological networks."


Photos © Douglas Tallamy