It’s always a pleasure speaking with Dr. Doug Tallamy, a renowned entomologist and advocate for native gardening. Based on the feedback his past appearances on the podcast have received, I know listeners will be just as excited as I am that he is back this week discussing North America’s most important native tree: the oak.
Doug holds both a master’s and a Ph.D. in entomology — the study of insects — and he’s a University of Delaware professor in the Department of Entomology and Wildlife Ecology. He has long studied the interactions between plants and insects, and the interactions between insects and the birds and other wildlife that eat them. His earlier books on the importance of conservation and native ecology include “Nature’s Best Hope” and “Bringing Nature Home,” and his new book is “The Nature of Oaks: The Rich Ecology of Our Most Essential Native Trees.”
To most people, Doug says, an oak is just a tree that drops leaves they have to rake up. But what he wants people to learn and appreciate is that an oak is an entire community of living things that changes throughout the year. He wrote “The Nature of Oaks” to provide a roadmap of what’s happening on and in an oak tree any time of year and to explain what to look for.
Knowledge generates interest, and interest generates compassion, Doug says, so landowners, armed with knowledge, will realize how critically important oaks are and maybe feel motivated to add another to their property.
Before proceeding with Doug’s explanation of why oaks are so vital, a quick reminder that you can sign up to be notified when enrollment opens for my upcoming Online Gardening Academy™ course, Growing Epic Tomatoes, co-led by Craig LeHoullier. This brand new course will turn you into a tomato-growing expert in no time.
Blue Jays and Oaks
Before Doug and his wife, Cindy, purchased their 10-acre property, it was routinely mowed for hay. Doug recalls that once they built their house and the mowing stopped, the rootstock of Oriental bittersweet and other invasive vines began to grow unabated. They set about removing the vines and multiflora rose bushes, another invasive. After he wrapped a chain around the base of the bushes and pulled them out with a tractor, disturbed areas were left behind. In spring, oaks and the occasional beech would grow out of those areas.
Doug says he was scratching his head because his property had very few oaks, none of which were old enough to bear acorns. But then he happened to pick up a photography magazine at the barber and came across a photo of a blue jay flying with an acorn. “Bingo! That is what’s happening,” he says.
A single blue jay can bury 4,500 acorns during a mast year — a year in which oaks produce more acorns than normal. But, Doug says, blue jays only remember where they buried one in four acorns. That means they plant 3,300 new trees in a mast year.
Jays and oaks co-evolved starting 65 million years ago in Southeast Asia, forming a mutualism, Doug says, and because of jays, oaks move faster around the world than any other tree genus.
Most animals that harvest acorns will eat each acorn they pick up, and even squirrels that bury acorns only bury them feet from the tree. But jays will fly an acorn a mile away before burying it, Doug says.
Golden-Crowned Kinglets and Oaks
The golden-crowned kinglet is a tiny, insectivorous bird that somehow manages to overwinter in cool climates. How could that be? Bernd Heinrich, a University of Vermont professor emeritus of biology, was the one who figured it out, Doug says.
Heinrich dissected kinglets in Maine in January and found their crops were full of caterpillars. It turns out that the kinglets were eating overwintering inchworms. Those inchworms stop eating and growing once oaks drop their leaves in fall, and then they imitate sticks all winter. Kinglets, chickadees and titmice all see past the camouflage and eat those inchworms.
The Problem with Non-native Plants
The difference between native plants and non-native plants is that natives have co-evolved with the plants and animals around them, Doug says.
In order for insects to eat a plant, they need to be able to bypass that plant’s defenses. To eat a plant that would otherwise be toxic, the insects develop adaptations. But when we import plants — most often from Asia — the local insects can’t handle the chemical defenses in those non-native plants. Non-native plants just sit there, not contributing to the local food web.
“Caterpillars transfer more energy from plants to other animals than any other type of creature,” Doug notes. When caterpillars don’t have native plants to eat, they won’t be around to provide that energy to birds and other wildlife. A landscape without a lot of caterpillars will be a dead-end for the food web.
There are 435 species of oaks worldwide, and 91 of them are found in the United States. Oaks support more caterpillar species than any other genus of plants in North America — not to mention all the animals that eat acorns. That’s why Doug calls oaks “keystone plants.” If you take the keystone plant out of the local food web, the food web collapses.
In his research, Doug has found that in 84 percent of the counties in North America, oaks are the most productive trees when it comes to producing food. Additionally, about 5% of North America’s native plants make 75% of the food.
Among the places where oaks are not the most important trees are Northern Minnesota and Northern Washington, where oaks are far less common, and then there are desert counties where oaks can’t grow. But oaks are found in a wide variety of habitats, from wet areas to dry areas and from rocky areas to acidic and piney areas, Doug says.
One hypothesis of why oaks became so valuable to the food web concerns their chemical defenses. While other plants produce toxins, like cyanide, or bitter compounds, which make plants unappetizing, oaks only produce tannins. Tannins slow the absorption of proteins, which is a defense that’s much easier for insects to adapt to, Doug says.
Another hypothesis is that oaks have been around for so long that insects have had more time to adapt to oaks than newer plants. Individual oaks are large and can live for several hundred years, and that also gives them a lot of exposure to insects and a lot of time for those insects to adapt.
Host Plant Specialization
Of the insects that eat plants, 90% can only eat a few plant lineages — the plants that they have specialized on, Doug says.
Every plant lineage is protecting itself with a unique cocktail of chemical defenses. An insect species can’t adapt to all of those various defenses, so it gets really good at circumventing the defenses of a small group of plants, Doug explains. One example is the monarch, which can get around the cardiac glycosides and latex sap in milkweeds. If there are no milkweed plants around for monarch butterflies to lay their eggs on, they can’t lay eggs on oaks instead, because monarch caterpillars haven’t evolved a way around the oaks’ defenses. They can’t lay their eggs on Asian imports either, for the same reason.
“You can plant all the Asian ornamentals you want, and monarchs will go extinct way before they adapt to them because it’s a big deal to change host plant,” Doug says.
The Importance of Insects as a Food Source
A 2019 Cornell Lab or Ornithology study found that there are 3 billion fewer birds in North American than there were 50 years ago. In 2018, The New York Times wrote the headline “The Insect Apocalypse Is Here.” And going all the way back to 1987, naturalist and writer E.O. Wilson penned “The Little Things that Run the World” on the importance of the conservation of invertebrates and what it would mean if the world lost its insects.
Losing insects would mean losing most pollinators of flowering plants. Losing flowering plants would mean losing most of the energy that drives the food webs that support our animals — amphibians, reptiles, mammals, birds and many freshwater fish would disappear. Additionally, the loss of insect decomposers that recycle nutrients quickly would mean organic material would only break down slowly by bacteria and fungi.
These are the kinds of changes that humans wouldn’t survive, Doug says, comparing global insect decline to losing oxygen.
Instead of judging insects by the few species that annoy us, like mosquitos, think of the bulk of them, Doug says. There are about 3.4 million insect species that drive the food web.
“Nature is not optional for humans,” Doug says. “We’re products of nature. We’re totally dependent on it. And insects are such a crucial part of the natural world that they’re essential. We can’t lose them.”
What’s Happened to Our Oaks
Doug points out that there are 50% fewer oaks in eastern forests than there were 100 years ago. There’s been logging and land clearing that has contributed to oak loss, and there are oak diseases as well, such as sudden oak death syndrome, bacterial leaf scorch, and oak wilt.
The Morton Arboretum in Illinois studied native U.S. oaks and found 28 of the 91 species are of conservation concern. Doug adds that a third of the oak species worldwide are endangered, as many species have very limited ranges.
Smaller Oaks for Smaller Landscapes
Doug often hears that oaks are too large and homeowners just don’t have the space for an oak in their yard. He tells them that he has a list that’s two pages long of species of oaks that are small trees, shrubs or ground covers that can be used in small landscapes. For example, in the east, there’s Quercus prinoides, known as the dwarf chestnut oak or dwarf chinquapin oak, which grows acorns when it’s just 5 feet tall and maxes out at 20 feet tall. And there’s Quercus georgiana, the Georgia oak or Stone Mountain oak, which grows to 50 feet, compared to Quercus alba, the white oak, which grows 100 feet tall.
From an Acorn, a Mighty Oak
I had the great fortune to visit Doug’s property for the filming of an episode of my public television series “Growing a Greener World” and to spend some time with him touring the grounds. Right in his front yard, there is a gorgeous white oak that he grew from an acorn, a fulfilling experience that he writes about in “The Nature of Oaks.”
Most of the oaks on his property were planted there as acorns — and they were free. He says it’s possible to pay $3,000 to have an oak dug, root-pruned and transported, but there’s really no reason to, outside of instant gratification. Plus, a root-pruned oak tree will spend at least a decade trying to regrow that root system before it concentrates its efforts on top growth.
Doug also says not to buy an oak grown in a pot because it is sure to be rootbound, and those roots will grow to strangle the oak later.
An acorn planted the same day as a balled-and-burlapped tree will surpass that tree after 10 years growing side by side, Doug says. Not only is the acorn-planted tree taller, but it’s also much healthier. It spent the first year of its life growing 10 times more root biomass than leaf biomass, and that extensive root system will pay off later with rapid growth of the tree after just a few years.
Oak Tree Masting
Every two to five years, oak trees will have a mast year, when they produce many, many more acorns than they produce in the years in between. Doug explains that mast seeding, or masting, occurs for “predator satiation.” Those predators include birds, mice, squirrels, deer, bears, raccoons and other animals that enjoy acorns. In the case of acorn weevils, which specialize on acorns, the weevil population can build up so much that they can hit 90% of the acorns on a tree.
If oaks produced the same number of acorns every year, the populations of those animals that depend on acorns would stabilize around that number and those animals would eat almost every single acorn.
During a mast year, oaks produce more acorns than there are animals to eat them, so the animal populations grow. The following year, when there are far fewer acorns to be had, the animal populations crash. When the next year comes, a normal amount of acorns will be enough to exceed the number of animals eating them. “It’s a reproductive strategy that works,” Doug says.
Another hypothesis for why oaks have mast years is the conservation of energy. It’s hard to expend energy on both growth and reproduction, so every few years oaks prioritize reproduction over growth.
Interestingly, oaks synchronize masting with other species that belong to the same group. So the white oak group will have a mast year all at once, and the red oak group will mast together another year.
The Importance of Oak Leaf Litter
“There are more species that live in the soil than above the soil,” Doug says. “They’re all tiny, but they all need soil moisture. They all need a lot of organic material in the upper layers of the soil.
These unappreciated organisms in the soil are breaking down and returning nutrients to the trees, and they are vital parts of the ecosystem, he says. When that leaf litter is taken away, the soil dries out, it starts to erode, and all of those underground creatures and mycorrhizal networks are clobbered.
The best leaf litter is leaf litter that will not degrade quickly, Doug says, and that’s where oaks come in. Maple, birch and tulip tree leaves don’t make it through the summer because they degrade rapidly, but whole oak leaves are so full of tannins and lignans that a single oak leaf can take up to three years to break down. In an oak forest, there’s never bare soil, and that’s exactly what the doctor ordered to keep organisms in soil happy, Doug says.
Oak leaves also stand up to invasive jumping worms, which eat leaf litter and seeds and change the soil chemistry. Fortunately, jumping worms don’t like oak leaves.
Then there’s Japanese stiltgrass, which seems to grow just about anywhere, sun or shade — except in oak leaf litter. A mat of oak leaf litter is very effective at stopping the germination, growth and spread of an annual like stiltgrass.
Oak leaf litter also prevents compaction of soil by pounding rains, while the tree canopy also takes some of the impact and the bark absorbs some water. The slower that water hits the ground, the more chance of soil infiltration before the water runs off. The carbon in soil from broken-down oak leaves also acts as a water filter, which makes for healthier watersheds.
All the leaves that fall on your property should stay on your property, Doug says. Oak leaves are holds of nutrients, and when they decompose they return those nutrients to the soil to be taken up again by oaks and other plants. Raking and removing leaves will remove those nutrients from the nutrient cycle, and it also kills the many insects that overwinter in leaf litter, including the 70 species of moths that eat dead oak leaves.
Oaks trees sequester carbon not just in their biomass but also in the soil, where they deposit very stable carbon that can remain there for thousands of years.
I hope you enjoyed learning from Doug Tallamy as much as I did. If you haven’t already done so, you can listen to this episode now by scrolling to the top of the page and clicking the Play icon in the green bar under the page title.
Have you planted an oak on your property and watched it grow? Let us know in the comments below.
Links & Resources
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joegardener Online Gardening Academy™: Popular courses on gardening fundamentals; managing pests, diseases & weeds; seed starting and more.
joegardener Online Gardening Academy Beginning Gardener Fundamentals: Essential principles to know to create a thriving garden.
joegardener Online Gardening Academy Growing Epic Tomatoes: Tomato expert Craig LeHoullier joins me in leading this course on how to grow healthier, productive tomato plants and how to overcome tomato-growing challenges. You can sign up to be notified when enrollment opens.
“The Nature of Oaks: The Rich Ecology of Our Most Essential Native Trees” by Douglas W. Tallamy
“Nature’s Best Hope: A New Approach to Conservation That Starts in Your Yard” by Douglas W. Tallamy
“Bringing Nature Home: How You Can Sustain Wildlife with Native Plants” by Douglas W. Tallamy
“The Insect Apocalypse Is Here” from The New York Times
“The Little Things that Run the World” by E.O. Wilson
Disclosure: Some product links in this guide are affiliate links, which means we would get a commission if you purchase. However, none of the prices of these resources have been increased to compensate us. None of the items included in this list have any bearing on any compensation being an influencing factor on their inclusion here. The selection of all items featured in this post and podcast were based solely on merit and in no way influenced by any affiliate or financial incentive, or contractual relationship. At the time of this writing, Joe Lamp’l has professional relationships with the following companies who may have products included in this post and podcast: Rain Bird, Corona Tools, Milorganite, Soil3, Exmark, Greenhouse Megastore, High Mowing Organic Seeds, Territorial Seed Company, Wild Alaskan Seafood Box and TerraThrive. These companies are either Brand Partners of joegardener.com and/or advertise on our website. However, we receive no additional compensation from the sales or promotion of their product through this guide. The inclusion of any products mentioned within this post is entirely independent and exclusive of any relationship.