EVERY TIME over the years that I’ve spoken to today’s guest, one word comes up: oak. If entomologist and University of Delaware Professor Doug Tallamy sounds a little fixated on native oaks, it’s because they are the most powerful plant there is, and he wants us to be very clear on that and plant them.
In his new book, “The Nature of Oaks: The Rich Ecology of Our Most Essential Native Trees” (affiliate link), he makes the case more strongly than ever, with twists and turns and the tales of all the creatures we depend on, who depend on the genus Quercus.
Doug Tallamy is well-known to most every gardener as a longtime leading voice speaking in the name of native plants. His 2007 book, “Bringing Nature Home,” was for many of us, an introduction into the entire subject of the unbreakable link between native plants and native wildlife. He followed up with the 2020 “New York Times” bestseller, “Nature’s Best Hope,” and now, just out, “The Nature of Oaks.”
We talked about the importance of oak leaf litter, about galls on oaks, about mast years when there’s a bumper crop of acorns, and even about how periodical cicadas and oaks have some things in common. And of course we talked about planting oaks.
Read along as you listen to the April 5, 2021 edition of my public-radio show and podcast using the player below. You can subscribe to all future editions on Apple Podcasts (iTunes) or Spotify or Stitcher (and browse my archive of podcasts here).
Plus: Enter to win a copy of the new book by commenting in the box near the bottom of the page.
And: Read the interview with Doug I did in my column in “The New York Times” for more on the story of oaks and their importance.
the power in an oak tree, with doug tallamy
Margaret Roach: Yeah. You should see my copy of your new oak book, there’s Post-its sticking out everywhere and four sheets of folded-up paper inside, covered in notes.
Doug Tallamy: We’ll have to get you a new one then.
Margaret: No, no, no, no, no. I didn’t write on the pages. But so much to learn [laughter]. And so maybe we should sort of start with a little CV, a little resume of the genus Quercus. What you found out about its powers when you started research all those years ago, comparing native and non-native plants and their ecological value and so forth.
Doug: Well, the first thing I found, thinking about their role in ecosystems, I always focus on food webs first. Plants of course, are capturing the energy from the sun and turning it into food, and then how well they pass that energy on to animals depends on how many animals are there. And not all plants are willing to pass it on. A lot of plants are very well-protected and they hang on to that energy.
But oaks pass it on better than any other plant genus in North America, so that’s what I focused on. But in doing that, I learned they’re also great at other ecological roles. They’re great at sequestering carbon, which we certainly need today. They’re great at pumping that carbon into the soil, which we certainly need today. They’re great at managing our watershed, because they have such big canopies and big root systems. The only thing they’re not great at is supporting pollinators because they’re wind-pollinated, but three out of four, that’s pretty good.
Margaret: [Laughter.] And the numbers are staggering. You often speak in caterpillar counts, which I tease you about, but how many caterpillar species, many of them from moths, are making use of a particular genus of native plants and oaks really are powerhouses on that score, yes?
Doug: Yes, the most productive plant in this country. Over 900 species of Lepidoptera—and most of them are moths—use oaks and there’s no other genus that comes close to that. The reason that’s important is that caterpillars are transferring more energy from plants to other animals than any other type of plant eater. Measuring the caterpillars in your ecosystem is a really good measure of how well that energy is being transferred out and how much biodiversity you actually have in that ecosystem. [Above, a one-spotted variant, a kind of inchworm, is among the caterpillar species reliant on spring oak foliage.]
Margaret: Right. Maybe we’d better, right off the bat, recite sort of an homage to leaf litter because it’s spring cleanup time in a lot of places. And I really shudder after reading the new book, to think of all those vacuums and shredders going at it around the nation, in the name of garden cleanup. It’s a good thing that oak leaves are plentiful and that they resist breaking down, right? It’s not some plot to drive gardeners mad.
Doug: [Laughter.] That’s right. Leaf litter of course, it’s the blanket that protects our soil. And if that blanket doesn’t make it through the summer, which many other trees’ leaf litter doesn’t, so things like maple and birches and tulip trees, they disintegrate very quickly, but oak, oak-leaf litter can last up to three years after it falls. That provides a permanent cover and that’s exactly what all of the creatures that live in the soil, and there are more species that live in the soil than live above the soil, they need that protective blanket to maintain the moisture level, to return nutrients to the soil, put that organic material into the soil. And again, oak leaves are better than most other trees in terms of doing that.
One of the things we’re learning and it’s something I hadn’t thought about earlier, is that those caterpillars that are so vital to the food web, most of them develop on trees, but then they fall from the tree. And they’ve got to complete their development—either they tunnel into the ground and pupate underground or they spin a cocoon in the leaf litter under the tree.
And if you look around you and see the way we landscape, there is no leaf litter [below] under the tree, we have grass right up to it or bare soil or cement and we compact our soil so much by mowing and walking on it, that it’s very difficult for those caterpillars to get underground. The way we landscape underneath the tree is now becoming just as important as the type of tree that we choose for our yards, in terms of allowing that vital part of the food web to complete its life cycle.
Margaret: Right. I was interested in the book to note that you note that two possible, very practical advantages that leaf litter, oak leaf litter in particular, offers is that it may suppress the incredible spread of the invasive Japanese stiltgrass, and also Asian jumping worms, which are another threat to forest, especially forest, ecosystems.
Doug: Right. And it’s hard to figure out which one’s worse, but Japanese stiltgrass just, it takes over it. It produces seeds, not only at its terminals, but at its axils right next to the ground, so you can mow it as often as you want and it’ll still make seeds. It’s an annual, but it keeps coming back, blankets the ground and excludes everything else.
And then those jumping worms eat all the leaf litter, and that’s where oak leaves again, stand out because they’re so tough and full of tannins and lignins that it’s the one type of leaf litter that jumping worms really don’t like. I’ve heard of examples, yeah, they’re eating some of it. But typically when you have an oak forest, those worms stay away.
Margaret: Some years with oaks, anyone who lives with oaks knows, there’s wall to wall acorns on the ground in fall [below]. And so can we talk about mast years, the phenomenon of mast years, and how they work and maybe what some of the hypotheses are as to why that happens, why sometimes there’s such a profusion, such a big crop.
Doug: Yeah. That’s called oak masting, and usually it’s within one of the oak groups, the white oak group or the red oak group will mast a particular year. And often they don’t mast the same year. But you’re right, they produce a lot of acorns in a single year and then go one or two or maybe even more years without producing very many at all. Why do they do that? One of the hypotheses and the most popular one is called predator satiation. Things that would eat acorns we call acorn predators. And there’s a lot of things, all the deer and the squirrels and the turkeys and so many birds and the acorn weevils.
And if you look at what the acorn collection looks like under a tree after all these things are eating the acorns, there’s very little left on a typical year. But on a mast year, so many acorns are produced, it swamps the population size of the squirrels and all the things that are eating those acorns. It’s a strategy for overwhelming the predator pressure that is on acorns. If they produce the same amount every year, the population of all those things that eat acorns would be steady as well, but at a very high level and there would be very few acorns left to make new oaks. That’s one of the hypotheses.
Another one is that it takes a lot of energy to make acorns. And it takes a lot of energy to grow new shoots and get bigger as a tree. And typically there’s not enough energy to do both, so oaks—and there are other trees that mast as well—but they allocate that energy. Sometimes some years they put it towards reproduction, sometimes they put it towards growth, but rarely both.
There’s also a hypothesis that because they’re wind-pollinated, if all the oaks produce their catkins and produce an awful lot of pollen in a single year, then pollination is much more effective, it’s much more efficient.
And there’s a fourth hypothesis that in producing a whole lot of acorns one year, it allows things like mice and squirrels to really explode their populations. They get very, very numerous. But then the next year there’s hardly any acorns and those populations crash, which means if they produce acorns the third year after that, there’ll be very few things around to eat it. It’s close to predator satiation, but a little bit different.
And none of those hypotheses are mutually exclusive, they all could be happening at the same time.
Margaret: Right. And that was the case with so many things in the book. In fact, you end a lot of the sections by saying, “And like I’ve said, it could be all of these things, because it’s interconnected.” [Above, a white oak.]
Doug: We humans like to make it black and white, very simple. It’s this or that, but it’s often a lot of things happening together. It can be very complicated.
Margaret: Yeah. A lot of chapters in the book, which you’ve arranged month by month through the year, starting in chapter one is October, they tell these sort of intimate, intricate stories of creatures, like the ones in the leaf litter and so forth. And of course I expected caterpillars to be a prominent character in the book about oaks, but the list was like, oh my goodness: katydids, walking sticks, tree crickets, lace bugs, plant hoppers. Tree hoppers, gall wasps, blah, blah, blah. And I could go on and on and we could fill a page with the animals that are involved with oaks.
But then maybe most surprising one to me and you’re going to have at your house, in your area, a brood emergence of them there this year, in 2021, is the periodical cicadas. I didn’t know they had anything to with oaks. Tell us a little bit about that.
Doug: Well, the periodical cicadas are not specific to oaks, but they love them. Of course cicadas, the periodical cicada, comes out in two broods, either the 17-year brood or the 13-year brood. The one at our house is going to be the 17-year brood. It’s been 17 years since they appeared. And the oaks that I planted at our house have grown a lot in that 17 years, so it’ll be interesting to see the size of the population, because they were just small trees when the cicadas around the last time.
And I’m not expecting that many cicadas because the eggs were laid when they were small trees, there weren’t that many around. But this year there’ll be a lot of eggs laid and then 17 years from now, we’ll probably have a pretty good emergence.
So periodical—they’re periodical for the same reason that you have oak mast. There’s no predator of periodical cicadas that can track that 17 or 13 year old period. They can’t wait that long to come out to eat again.
Margaret: To eat [laughter].
Doug: There’s a lot of things that eat those cicadas, a lot of birds and all the rodents and everything, but they swamp them. They come out by the millions and then they successfully get to reproduce. I do have one recommendation:
The media loves to sensationalize everything and they’re making this brood sound like it’s just going to be the worst scourge in the world. It’s terrible. Everybody should hide.
It’s actually one of the most fantastic biological phenomena that you’re ever going to see. You should go out and appreciate it. But it’s probably a good year not to plant small trees in the spring. Wait till the fall, because the cicadas do lay their eggs in the terminal twigs of branches and it kills the branch from that point on, it causes what we call flagging. And if it’s a very small tree, 3 or 4 feet, it really can knock it back. Wait till the fall this year to plant your trees and you’ll be happy you did.
Margaret: Am I correct in understanding also from the book, that part of the reason—and I’m going to get this imprecise, I’m sure. But part of the reason that the cicadas take either 13 or 17 years to grow and eventually emerge as adults is because living underground all that time, they are sustained by sucking xylem from the roots of trees, including oaks and this is a very watery substance, not a lot of nutrients, and so you grow really slow to adulthood when you live on xylem. Again, I’m paraphrasing, but is that another sort of connection?
Doug: Yes. Xylem is practically pure water. There are very few nutrients in it. But it doesn’t totally explain the very long periods, because of course we have the annual cicada, which comes out every year, but we don’t actually know how long it takes the annual cicada to develop underground. We know they come out every single year, but each individual was underground more than a single year, but it’s not 17 or 13 years, so they can develop faster than those periods.
But xylem is the worst plant resource that’s available [laughter]. It’s water with just a tiny little bit of nutrients. But it’s one of the reasons you can have so many sucking on a tree and unless you have a serious, serious drought and I mean serious drought, they really don’t harm the tree much at all. They’re taking very little from it, and very slowly.
A friend of mine did his PhD on cicadas, trying to measure the impact on trees, and it was immeasurable. He couldn’t measure any difference on trees that had 20,000 cicadas on their roots versus trees that had none. In terms of what the nymphs are doing underground, don’t worry about it.
Margaret: Yeah. Galls: I want to just touch on galls for a minute or two here. And I have to confess that honestly, over the last year, when I saw the initial pictures of the coronavirus spike protein, I kept thinking, oh, it looks like a gall; it looks like a gall [laughter].
Doug: [Laughter.] It does, it does.
Margaret: I shouldn’t laugh about that, but it does to me. Various insects that make sort of bumps and balloons and lumps and other odd formations on leaves—and we’re talking about oak leaves here—gallers, I think you even refer to them. What startled me, especially about that and people may have seen these things, was that there’s also a way to see it from the oak’s side of the equation. Maybe that it might actually help the oak, these gall formations.
Can you explain what a gall is? And people may have seen them, some of them just look like kind of balloons or anyway, yeah, can you explain a little bit about galls?
Doug: Yeah. The ones on oaks are made by the little wasp in the family Cynipidae, so cynipid gall wasp. And the female will lay an egg in the buds of oaks, the meristematic tissue. And along with that egg, she injects plant hormones that manipulate the growth of the cells. Those are undifferentiated cells, and it creates a species-specific growth on the tree that we call a gall. It’s been likened to cancerous growths, and there’s some similarity, but it’s very controlled. Cancer keeps growing; these do not. And it provides a little house that the galler can develop in. Some nutrients from the tree are directed to that gall.
It’s a good deal for the galler, but the tree is not a total loser here, because way back when, those were insects that essentially tunneled into tissue. And if they tunneled up and down a branch the way a number of beetles do, they wreck the vascular system of the entire branch, and the damage to the tree from a single insect borer can be a whole lot more than the damage by a single galler, which is contained in one single place.
People have had traditionally looked at galls as something that happens to a tree, and it’s a disease type of thing, and it’s all terrible for the tree. When in fact it’s really in part, the tree’s response to this insect in a way that it can manage without too much damage. There are a lot of species of galls. There are 5,000 species of cynipid gallers. Most of them are on oaks. There can be 70 species of gallers on a single oak tree.
Doug: And it’s even more complicated than that because they have what they call alternation of generations. There’s two generations a year. The first-generation galler has a particular morphology. It looks like a particular type of wasp. The second generation looks totally different. It’s the same species, but it looks totally different. And both generations make galls that are totally different from each other. I’m still amazed at the oldtime taxonomists were ever able to figure out that we’re really talking about the same species here, but in different times of the year, everything—the gall and the insect itself—look totally different.
Margaret: Right. But the oak gets a benefit that the herbivory is confined to these little spots, not all over the tree. That kind of was a light bulb for me. It was like, oh, O.K., that’s a good deal. That’s a good deal.
Doug: I know what’s what some people are thinking because there have been some introduced species of gallers. There’s one introduced from Europe, and you know what that means, they’re here without their natural enemies. And they can get very numerous on a tree. People say, “Oh no, the galler’s going to kill the tree.” And in some cases that galler can get so bad it actually can.
But those aren’t the native ones that we’re talking about here. Gallers, for some reason, host more species of parasitoids—the other wasps that lay eggs in the gallers—more species of parasitoids than any other type of insects. They’re clobbered by these natural enemies, and that keeps them in check. But these introduced gallers don’t have any, and that’s why they go crazy.
Margaret: In the last four or five minutes, I wanted to talk about maybe the most important mandate of all in the book, which the final chapter is about it, and it’s mentioned throughout: We need to plant them. We need to get past all our gardener/human objections about, “Oh, the leaf litter sticks around. It’s too long. It’s too heavy. It lasts too long. There’s acorns everywhere. Oh the tree is too big for my yard. It lifts up the sidewalk.”
You and your wife, Cindy, 20 years ago basically set about planting acorns when you got to your new home and you’re super-happy that you did obviously. And so how to plant them. If I see them popping up, tell us what Doug Tallamy would like us to do to be sort of parents to some oaks [laughter].
Doug: Well, two of the objections of oaks is that they’re too expensive, and they grow too slowly. You can get past both those objections if you plant them very small. We really did plant a lot of our oaks as acorns, and that makes them free, or 2-foot bare root whips, $1.50 each. They did very well. What you’re doing is you’re allowing a very small tree to build its full-size root system.
And that allows it, it takes a couple years to build that root system. In the first year, gee, what’s the figure?—something like, there’s 10 times more root biomass laid down by the oak in its first year than leaf biomass. That’s what they’re doing. It looks like they’re not growing. They’re just sitting there, but they’re really building that root system that then allows them to take off.
And if you buy a 15-foot oak tree with a giant root ball and you plant it, that tree has been root-pruned so seriously, it will sit there without growing a bit for 10 years, trying to rebuild the roots. Good chance it will die, and it does cost thousands of dollars. Do yourself a favor and plant small trees that will be much healthier and they will pass that big tree once they get going in not too many years.
Margaret: Right. And when these oak seedlings or saplings are babies, we want to protect them from herbivorous animals, from animals and so forth that are going to chew on them, so we put them in what, like a 5-foot cage to protect them in their early years?
Doug: Yeah. We’re talking about deer. It’s the old deer problem. If you’ve got a fenced property, you don’t have to do that. But I certainly had to do that, because the deer—just like the caterpillars—the deer love oaks. They will snip off those babies. It doesn’t kill it right away, but it keeps it in a bush shape. There’s so many deer now that they’ll do it forever, and eventually they will kill it. I’d go to Lowe’s or Home Depot and get those 5-foot galvanized wire rolls and make a nice cage around it. You don’t want it tight against the tree. You want the branches to be able to spread. And keep it there until it grows past the point where the deer can eat it to death.
Then I call that graduation. You take the tree off and I’m still using the cages that I made 20 years ago when we moved in. I just keep moving them around. It’s a downside. The problem is not with the oaks, it’s with the deer [laughter].
Margaret: I want to be sure in our last minute here, just to do a shout out for your recently launched website, homegrownnationalpark.org. And besides information about your work and your books and so forth, there’s a call to action for all of us to sort of join in and put our properties, where we’re growing native plants and doing the kind of work that you inspire, the kind of planting that you inspire—to put it on the map, there’s an interactive map. And more than 5,000 people have already answered that call. I want to ask my readers and listeners to have a look there too and get involved, yes?
Doug: Yes, absolutely. The biodiversity crisis that we’re trying to address here is it’s a global crisis, but it has a grassroots solution. Every one of us can manipulate the part of the planet that we live on in a positive way. And that’s what this get-on-the-map effort is all about, trying to get people interested into joining. We’re tribal, we love to belong to something. We’re going to belong to Homegrown National Park here and put life back where we live. And we can do that by simply shrinking the area that we have in lawn right now.
Margaret: Right. Well, Doug Tallamy, I am loving the new book. I’m on my second pass through, because as I said, I have so many Post-its and notes and whatever. Such an eye-opener about really how complex what’s going on with our oaks out there is, and how important they are. Thank you very much, and thanks for making the time today in your busy schedule.
Doug: Well thank you, Margaret.
(All photos above from “The Nature of Oaks” were shot by Doug Tallamy; author photo by Rob Cardillo.)
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