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DR. JAMES NARDI says you can tell a lot about a tree by the company it keeps. From life in the soil around their roots to the action up in their canopies, trees are swarming with engagement—unseen microbes and fungi, countless insects and other arthropods, and vertebrates like birds, squirrels, and even porcupines.
Jim Nardi spoke about their diverse community of companions. He is research scientist in the School of Integrative Biology at The University of Illinois Urbana-Champaign, and the author and illustrator of several previous books. His latest is “The Hidden Company That Trees Keep: Life from Treetops to Root Tips.”
Plus: Enter to win a copy of the new book by commenting in the box near the bottom of the page.
Read along as you listen to the March 13, 2023 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).
the company trees keep, with dr. james nardi
the company trees keep, with dr. james nardi
Margaret Roach: Welcome, Jim. Now you don’t just know all this and write all this, but you illustrate these two books of yours?
Jim Nardi: Yes, I do.
Margaret: Oh my goodness.
Jim: I find it very soothing and relaxing, and I learn a lot from such close observation that’s required in order to draw these creatures.
Margaret: Oh, the illustrations are just, I’m so charmed, I’m just so drawn in.
Jim: Thank you.
Margaret: And it’s funny, when I was reading the book and looking at the illustrations I was thinking of another favorite author, the biologist and prolific author, Bernd Heinrich, formerly from University of Vermont.
Jim: Oh, yes. Sure.
Margaret: And because he illustrates his, too. And so I thought oh my goodness, what talent [laughter]. So anyway, we’re going to talk about these big organisms, kind of I don’t know, are they the biggest organisms in our average world over here, these trees?
Jim: Well, yes. I think the record is held by a redwood tree. They can certainly be very large, but they can also be very ancient and very small.
Margaret: Yeah. But in our average world, those of us who are gardeners listening in our backyards, I mean, they’re the biggest creatures we see day to day, they’re the giants of the landscape. So you start the book with the sentence: “You can tell a lot about a tree by the company it keeps.” And so just to get us started, the range is from what to what, we’re tiny to what?
Jim: Oh well, we can begin with the viruses and then move on. The microbes, we’re discovering a new world out there that we didn’t know existed just a few years ago, and that’s the microbial world that’s associated with each individual—with not only ourselves, but with the trees and the small companions that they keep. Even small bacteria contain viruses and smaller bacteria within them.
Margaret: That’s very small [laughter].
Jim: Indeed it is.
Margaret: And as I said in the introduction, trees companions go up to mammals and other large animals and so forth. But where I am, black bear tend to like running up a tree. If they get a sight of you they go run up a tree.
Jim: Oh, how fortunate you are to have black bears.
Margaret: Yes. My best thing was the other day I had a mink in the backyard, swimming in the water garden that was semi-frozen, but that’s a digression.
Jim: What a treat.
Margaret: Crazy. Crazy. But a lot of the companions are insect species. And so I was fascinated in the book, you talk about the diversity, the great range, and certain tree species have a lot of diverse insects that engage with them, don’t they?
Jim: Oh, indeed. We usually think of the insects that feed on trees, but there are far more insects that prey on these plant-eating insects and even far more that are parasitic on these insects. And these help maintain a balance in the life of the tree that so many of us are unaware of—the importance of these predators and parasites in maintaining a balance in the life of the tree.
Margaret: And you also write about how lately, especially, we talk about boosting our immune systems and immune diseases and immune this and that, but we animal types aren’t the only ones with immune systems by any means. So the tree has this really incredible, impressive immune system as well. Yes?
Jim: We normally think of trees as passive creatures, however, they’re quite capable of mounting a robust defense, an immune response. And by immune response, we mean the ability to distinguish self from non-self. So non-self would include everything from other insects, microbes, the bite from a porcupine. And so they’re not only able to defend themselves with their own sophisticated immune system, but they’re able to recruit animal and microbial allies to help defend them.
Margaret: And I think if you asked gardeners, well, what allies do trees have? They’d probably say, “Oh, well, I guess some animals disperse their seeds, and some pollinate them.” They’d know that maybe, but they wouldn’t know all these other really complex ally relationships.
Jim: We know that birds help maintain, help control, the populations of the leaf-eating and wood-eating insects. But there’s a whole world out there of parasitic insects. And this is, in terms of diversity, it’s probably one of the most diverse: 15 percent of all animal species are believed to be parasitic insects, and parasitic insects make up about 10 percent of the number of species of insects.
These are incredibly diverse. Some of them are very specific; they have specific hosts on the tree. And some of them are generalists. There are flies called tachinid flies that whose larvae feet on over a hundred different species of tree herbivores.
Margaret: Wow. And it’s incredibly complex. And as I said, when we began speaking, the illustrations in the book really give you a look at some of these creatures, some of which are quite small or you may never have seen. And I loved, it’s not like one organism is getting all the benefit, or the other organism is the harmful one. We can’t put those judgements. It’s a complex sort of interaction, dance, whatever. I love one illustration in the book, there’s a bird in a nest tree—so it’s taking advantage of the protection of the nest to the branch to hang his nest on and the leafy protection. But it also gets advantages from being there and also provides services to the tree. Right?
Jim: Oh, indeed. Each of these birds will feed on, in its lifetime, will feed on thousands of caterpillars plant eating insects. And so the tree never has to worry about being, usually never has to worry about being defoliated with the help of its avian as well as insect allies.
Margaret: Right. And as sort of cryptic, as hidden as many of those insects, those caterpillars and so forth try to be in the book. You say “the birds leave no leaf unturned.” They look and look, and look. Right, they hunt out their dinner.
Jim: Oh indeed. And all the crevices in the bark and yeah, they’re very thorough and on the ground beneath the tree, among the leaf litter.
Margaret: The bark is something; the bark is really special. One of the ways that sometimes in winter, for our tree ID, we can start to tell what species of tree it is. But I live in a spot where outside my window, where I sit most days and work, my little home office is this very, very old triple-trunk Thuja, very giant tree with shaggy, kind of cinnamon-colored bark.
And so all of the decades that I’ve been here, every year regularly I see a brown creeper, the bird. And you actually have a beautiful illustration of the brown creeper on some bark on the back of the book. And if not for that bark, I wouldn’t see that bird, who loves to investigate [laughter]. And they have a relationship, right? I mean…
Jim: Oh, yes. So the brown creepers climb up the trunk of trees and the nuthatches climb down the trunks of trees. You probably have nuthatches on the same tree.
Margaret: I do. And I call it the two-lane highway. Yes. Yeah. One going up, one going.
Jim: [Laughter.] Right.
Margaret: Yeah. And what are they doing? I mean, there’s birds that look, there’s also birds that stash things, cache things, in bark. But what are they doing? I mean, what’s going on there and does it serve the tree as well, or what’s going on?
Jim: Oh, they’re little insects that hide out in those crevices. And so they’re constantly finding these insects, and enriching their diets with them. The bark is alive with not only insects, but small creatures that are known as tardigrades. These can only been seen with a microscope. So it’s an alien world there, but it’s a world that can readily be seen if one… just in a simple way. You can simply take some of this bark and extract some of these living creatures and view them with a digital microscope. It’s something that could be done in classrooms, it can be done at home. So it’s a wonderful education about an alien world that most people don’t know about. [Above, microscopic creatures of the bark, lichens and mosses including nematodes (top), protozoa (bottom), rotifers (left) and tardigrades (right).]
Margaret: Right. Because there’s a lot of action going on in the bark. It’s not just this impenetrable… The lichens and the mosses, and as you say, all these small, like the tardigrades and all these tiny creatures.
Jim: Yes. Some bark is covered extensively with lichens [illustration below]. There’s certain trees that appear to be very popular with lichens, and most of their branches are almost completely covered. So that’s a world unto itself there. There’s so many different creatures that associate with the lichens.
Margaret: I think there’s three species of lichen moths here, I can’t remember, but I think there are three. And they’re the painted, and I forget what the other ones are. I never understood that, and then suddenly it was like, “Oh wait, lichen is in their name because they have this relationship with the lichen and that has a relationship with the tree,” etc., etc., etc. Or the rocks in some cases.
Now, you were talking about this a little bit before, but you also in the book liken, not L-I-C-H-E-N, but L-I-K-E-N [laughter], sorry, I stepped right into that. You liken a tree to having its leaf nibbled to a human getting, a mosquito bite that it sets off this awareness, this and then this chemical response.
Jim: Yes.
Margaret: I know it’s very complex, but-
Jim: It is complex. But this was first reported in the journal “Science” about two years ago, and the authors compared their finding to what we observe when our skin is irritated by something like, let’s say a mosquito bite. What happens at that time is the adjacent cells release a simple chemical. It’s a derivative of the amino acid, glutamic acid. It’s glutamate. And when this is released, it excites the adjacent neurons and these release calcium. And this wave of calcium is propagated all the way to the brain to let us know that we’ve been bitten by a mosquito.
And something similar happens when an insect bites a leaf or a fungal hyphae penetrates the surface of a leaf. One of the first responses is the release of glutamate, and this triggers the release of calcium. And this is propagated throughout the cells of the plant in the way similar to the propagation of a nerve response.
And this in turn triggers a cascade of chemical reactions within the plant, ultimately resulting in the production of the master regulators of the trees’ immune response, two hormones known as salicylic acid and jasmonic acid. And these in turn, activate certain genes, defense genes.
And just one example would be a protease that damages the proteins of the intruder, whether it’s an insect or a fungus. So it’s a complex series of events in the plant as well as in our own bodies that we’re still unraveling. And we have much to learn about this. But we have bits and pieces that we understand, and we just know it’s a complex cascade.
Margaret: Yeah. You say more than 1,000 volatile compounds may be released in defense against herbivory, pathogens, even a wasp sticking its ovipositor into a tree—that may even prompt a response.
Jim: It triggers a mild immune response. And as I point out, every tree is associated with microbes. They’re referred to as endophytes because they live within the tree tissues, fungi, and bacteria. And they are comparable to the bacteria that live within our own guts, our microbiome. So we can refer to this as the tree’s microbiome.
And when these enter the tree, they trigger a slight immune response. But it’s a give and take. So the tree and the microbes come to a compromise. And what it does, it primes the immune system of the tree so that when the tree is exposed to real pathogens and noxious intruders, it can mount a strong immune response. So the endophytes, the bacteria and the fungi that live in harmony with the tree, do not help the tree when it really needs to defend itself, by priming the immune system.
Margaret: There’s so many… If you had asked me before I read your new book, “The Hidden Company That Trees Keep,” if you had said, “Well, what’s a sapsucker?” I would have said, “Oh, yellow-bellied sapsucker. The bird that drills the perfect lines or grid of holes.” But there’s tiny adelgid insects, and scale insects. And so you write about all of these, and you illustrate all these incredible interactions.
And my favorite example of leaf nibbling, if you could call it that, I always love to see the activity of a leaf-cutter bee [above, in redbud leaves]. Isn’t that just astonishing?
Jim: Oh, yes. It’s amazing. It’s a perfect little circle. It’s able to cut these perfect little circles from the edges of… The leaf-cutter bees seem to have preferences in which leaves they choose. They seem to have a particular fondness for redbud trees, and they also seem to be fond of rose leaves. They tend to avoid trees like oaks that have sturdier and thicker leaves. So I think they choose some of the tender leaves like red buds.
Margaret: So I wanted to go down the tree to the base, and trees drop their leaves, whether annually or on a less frequent cycle. And then there’s all this material beneath, and there’s this intricate network of helpers who facilitate the process of returning all that to the soil and to the tree, I suppose. And you say in the book that “recycling is a community effort.” So a couple of creatures there. Yeah, I love millipedes. I love the burying beetles. I love lots of those creatures in the soil. Tell us a little bit; let’s take a couple minutes to talk about some of those guys, the recyclers.
Jim: Sure. Well, the microbes are important here, too. Down in the leaf litter, the bacteria and fungi are chewing, are digesting leaf tissue. But some of these larger little creatures, like mites and springtails and millipedes [above] and wood lice are chewing away at the fallen leaves and fragmenting them, so that the bacteria in the fungi have a larger surface area on which their enzymes can work. And so it is a community affair. There are thousands, millions of these little critters down in the soil that are chomping away. And ultimately, they liberate the mineral nutrients like calcium, magnesium, manganese, iron from the leaves.
And they also leave behind this extremely important part of the soil known as humus. And this is the really hard to digest remains of things like the bacterial cell walls, a hard-to-digest material in plant cell walls known as lignin. And this humus has a wonderful attribute of latching onto mineral nutrients, because it has a negative charge, and it holds on to positive-charged nutrients like calcium and magnesium, and iron, and holds them into place so that they’re not leached down into the deep soil. And we often forget about the importance of these recyclers, but one of my favorite quotes comes from a book entitled, “Teaming With Microbes.”
Margaret: Oh, yes.
Jim: You probably know it.
Margaret: Jeff Lowenfels.
Jim: By Jeff Lowenfels. And yeah, it’s that “no one has ever had to fertilize an old-growth forest.” You don’t have to add fertilizer because these little decomposers and recyclers are liberating the nutrients. And then they’re also producing the humus that holds these nutrients in place, and gives the soil this wonderful spongy texture, a structure that it would not have if it weren’t for the humus. And not only are they liberating these minerals, but they’re actually mixing the mineral particles of the soil with the organic parts of the soil.
Margaret: So I have a very different relationship in recent decades than I did when I was a beginning gardener with dead and dying trees. We used to sort of erase them from the landscape: “Oh, I’ve got to get that out of here,” right down to the stump grinder or whatever. And now I really revere them, and I first make them safe and make sure that nobody’s going to get harmed. But I leave them standing as snags or wildlife trees as long as possible and so forth. And then hopefully their carcass ends up beside where they grew. And all this biomass… So let’s just talk. I suspect you revere them, too [laugh].
Jim: Oh, yes.
Margaret: Tell us a little bit about a dead tree and why that’s an important creature, too, in the whole world.
Jim: Well, it provides a welcoming habitat for so many of these important in insects. And it provides food and habitat for brown creepers that love to go navigate its bark.
And it’s adding nutrients. As it decays, it’s returning minerals that were borrowed during the tree’s lifetime to the soil so they can be used by other trees, other plants. Very important.
A person that I met a couple years ago, Nancy Lawson, who writes a column for “All Animals” magazine [published by the Humane Society of the United States] called The Humane Gardener, recently wrote a book entitled, “The Humane Gardener,” in which she points out the importance of leaving these welcoming habitats for all these creatures that we should welcome. The insects, many of them are predators and parasites which help control pests in our garden. So if you have a vegetable garden or flower garden, you want to have habitat for these other creatures as well, so that they can help control whatever plant-eating insects come to your vegetable or flower garden.
Margaret: Right. Nothing is separate. Nothing is separate from anything else.
Jim: We’re all intertwined.
Margaret: Well, Jim Nardi, the book is “The Hidden Company That Trees Keep.” And the illustrations are beyond charming and it’s just so packed with information. So thank you very much, and thanks for making time today.
(All illustrations by James Nardi from his book; used with permission.)
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