Brian Wahl teaching from the branches of a tree (SoWBA photo).
This episode is so fun and full of information that it’s in two parts! In part one, we answer questions like “How many trees are there in the world?” and why that’s not such an easy question to answer. In part two, we learn about “mast” and what animals benefit from a “mast year” with our tree expert Brian Wahl, the Urban Forestry Coordinator at the Wisconsin Department of Natural Resources!
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Transcription
Part One
Hey, and welcome to Questions Asked by Curious Kids or Quack, a podcast made by Southern Wisconsin Bird Alliance. This is a podcast where we gather questions about nature from kids to be answered with a local expert. My name is Mickenzee. I'm an educator, and I'll be the host for this series. This is a two part episode where I'll be interviewing Brian Wahl, an urban forestry specialist at the Department of Natural Resources, and today we'll be answering questions all about trees. Oh, and just a note for all of you listening. The audio ended up a little bit funky on this one, so if you notice the voices get kind of echoey, or maybe a little quiet. Don't worry, it's not your device just some wonky stuff happened in editing. All righty, let's get started with Brian.
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Mickenzee: Hey, Brian, welcome to the show. Before we get started with the questions from the kids, can you tell us a little bit about what you do for the Department of Natural Resources?
Brian: Yeah, sure, Mickenzee. So, yeah. So I am an urban forestry coordinator with the Wisconsin Department of Natural Resources. So really what that means is, well, first off, urban forestry. What is that? So those are and I kind of explain it like any time we have infrastructure. So buildings, roads, side walks, they interact with trees and people. So no matter where you are, you don't have to be in Milwaukee or Madison to consider yourself urban. You could be in Philips, Wisconsin or Medford, you know, if you have buildings, trees and people interacting. To me, that's urban forestry. And so what do I do specifically? So I work with communities and nonprofits, really to understand that they have a resource to, to manage, like they have an urban forest that needs care. And then I work with them on some strategies, and what they can do, what steps they can take to make sure that those trees get managed. And we also have a nice grant program that can help them do some of those activities.
Mickenzee: Awesome. So conveniently, this episode is all about trees. Could you tell us about how you came to know so much about trees, and why you like them?
Brian: Right, so most people come to urban forestry, actually come to it not directly. And so I'm going to throw that out to any listeners who are out there who may be third graders thinking of a career. That again, as I as I just said, most people don't come to urban forestry or culture for that matter, as a first career, it's something they sort of discover along the way. And for me, yeah. So wildlife was my undergrad major. And then I actually went back to tech school, in an arboriculture program because I'm like, wow, these trees are fascinating. And I wanted to get my hands on something. And to be able to, you know, prune a tree and actually see the results of that, like, right away. And then, so I learned a lot, actually, from my associate's degree. And then I did end up going, after working in the private sector for just a bit, I went back to school and I got my masters, natural resource emphasis on urban forestry. So that added to that knowledge pool, but quite, quite frankly, you learn mostly by being out there interacting with nature. Touching those trees every day, seeing how they respond, see how they change over the years. Not understanding what you're seeing. And then, you know, go learn about that. Your learning never ends. So, yeah.
Mickenzee: I love this question because it always stems from a curiosity.
Brian: Absolutely.
Mickenzee: That's what it's all about. So our questions today were submitted by the third graders at Lincoln Elementary here in Madison, right off the top, we have a big question. How many types of trees are there in the world? And then narrowing it down. How many are in Wisconsin?
Brian: Ooh, so all these questions are fantastic because they really seem simple, you know, from just their context. But when you look at them, they become much more complex. For example, what is a tree?
Serviceberry in autumn (photo by karen_hine).
Mickenzee: Yeah. What counts as a tree?
Brian: You know, and then so people who are counting trees count them differently, you know, is a large shrub considered a tree. So for example, I would consider something like a service berry, which you might find around your school. A tree. Other people may consider that a shrub. Some people put a height restriction on it. So if it grows over 12ft. It's a tree. Other people, it's you know, is it the diameter of the of the trunk? Plus the height or whatever else. So the numbers vary. Let's put it that way. It's not a simple Google search, typically. But most experts there was a study that came out, probably what was it, 2023, 2024 where they really, really tried to get a good grasp on how many trees there are in the world. Because, as you know, we're losing species all the time, and we really need to get a baseline of what's out there. And they came up with we're somewhere around, like looking through all the literature and everything else that they were somewhere around like just over 60,000 different tree species.
Mickenzee: Oh my goodness.
Brian: Right? And those are the known ones. And then there's some debate about it. And another 4000, are they trees? Are they not? Whatever. Or are they, some species are, you know, getting really and but then there are, they believe, about 9000 trees we just haven't really pinned down yet. So in all total, if you add all those together, we're probably somewhere around 73, 74,000 different species.
Mickenzee: And that's a huge range too, between, you know, what they've got marked down versus what's in the gray area.
Brian: Right. Absolutely. Well and then the crazy thing would be too is like, where are all these trees. So you can imagine like on Antarctica, no trees. So that's kind of easy to count, you know. But then we look at South America and about 43% of those trees that we just talked about exist in South America.
Mickenzee: Oh my gosh.
Brian: Right. And so for the longest time, you know, we talked a lot about saving the rainforests and things like that. And then we're like, well why are we concentrating on the rainforest? You know, we've got forests here that need to be saved, preserved or whatever, which is all valid. But if you think about that biodiversity aspect, I mean, wow, 43% of the world's tree species, South America, Pretty crazy.
Mickenzee: That is a lot in one zone, yeah. Oh my gosh, I had no idea. And then what about in Wisconsin? Do we know.
Brian: Oh yeah. So this is another sort of what I would call a loaded question. So again going is it a tree? Is it a shrub or whatever? Then the question is are we talking native trees? What do we mean by native? Or are we just talking trees in general? So if we're looking at maybe like timbers, well I would say let's say our Wisconsin forest species, again, I don't have a spot on number for you because it's debatable, but I would say between 30 and 40. So not a huge, but between 30 and 40 species of trees. So in Wisconsin, for us now, if you were to walk outside your school right now, you could probably find 40 different species of trees. And you're like wait a minute, Brian. You said there were like, 30 to 40 in Wisconsin. Yeah, that'd be in the native our native forest species. Okay. But in our urban environments and in our yards and everywhere else, we plant non-natives as well. In fact, quite a few of them are at least, you know, maybe they're native to the US, but, you know, not in Wisconsin, or maybe they're not even native to the US. You know, they come from Europe or Asia or wherever else. My best guess on that currently is somewhere around 350 different species of trees here in Wisconsin.
Mickenzee: Wow.
Brian: And then ready for this. Now if you think of cultivars and varieties. So these are trees that let's say we've designed kind of as humans we've crossed trees or we breed them, kind of like clones. So they have specific shapes or attributes like flower color or something like that. So those would be more like our cultivars, for example, in crab apples. So crab apple in my 350 estimation there, you know, we're talking that was that was probably like if I were to put that in the inventory I would say malus species, because that's crab apples, because there's so many of them and they're hard to tell apart sometimes.
Mickenzee: Yeah.
Brian: But there are things like, Prairie Fire or Red Jewel or, you know, Crimson King or, you know, and those are all cultivars. And there are over 800 different cultivars of crab apples alone.
Mickenzee: Oh my gosh.
Variety of Apples (photo by Dennis Sylvester Hurd).
Brian: All right. Oh, and if you think about think about regular apples. Same deal. Right. You know, you've got Honeycrisp, Red Delicious, you know Golden Delicious, Pink Lady, I mean you can go on and on. So those are all different kinds of cultivars that are out there.
Mickenzee: Oh my gosh, that I didn't think that this question would be so huge.
Brian: I know right. So it's not an easy question to answer.
Mickenzee: Not easy at all.
Brian: Now when we think about diversity though, which is always a huge, huge thing for me when I'm educating my communities, when I go to a community and we look at their tree inventory and their inventory is going to mainly be based on public trees. So street trees, park trees, whatever. But this is somewhat reflective of the residential properties as well. But there are four species typically that are planted. Most commonly out of those 300, you know, potential things out there. Yeah. So we tend to plant a lot of maple, a lot of ash. Although that's changing, right, Because of the emerald ash for, honey locust and then linden trees. So those tend to be our most common. So if you walk outside, wherever you are now, I almost guarantee you if you point to your tree and say those four names, you'll be right.
Mickenzee: Is there a reason for that?
Brian: There. Well, yeah, there's a couple different reasons. One is, for example, Honey Locust. They're like a super, they're an ironclad tree, right? They can grow in a lot of really harsh environments. We put them in in our urban environments are extremely hard. Yeah. So I would like to say, like, sometimes sugar maples are a little there are native they're our state tree. Right. But they're a little bit picky about where they want to grow and thrive. Right. So if you look outside the library here, you know, they're not going to be happy in that little cutout that's between all the cement and that gets poured on with salt in winter and things near, and they're just not going out like that. Honey locust. Yeah, they'll be they don't designed pretty much. I mean, so that's one of the reasons why we see a lot of honey locust maple pretty similar. But the other thing with like maple is that they're familiar tree. A lot of times, and, you know, the nurse is going to tell you the most common tree that we tend to sell are something like the Autumn Blaze Maple or Freeman Maple. Because they have a beautiful fall color and people come in to the store and they're like, I want that maple with the beautiful red color because it's familiar to them. And they really like that. Okay. And also they tend to grow pretty well in those crummy, urban sites that we put them in. Lindens are kind of the same. That's more of a utilitarian tree that does pretty well, you know, a lot of those top sites. So, yeah, that's, and ash also was fantastic for those sites. But again, unfortunately, the emerald ash borer has taken its toll. So we aren't planting those anymore.
Mickenzee: Okay, that's so interesting.
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If you are interested in learning more about trees or getting involved with our programs, please head to our website as to why birds.org and check out the free lessons, games and activities like Meet A Tree, Leaf ID, or one of my favorites: Tree Tag. And also lucky for you, this is only part one of our tree episode, so please come back for part two. I posted them at the same time so you don't have to wait a single second. If you have a big nature question that you'd like to have answered, please have a grown up or teacher submit your questions to info at swibirds.org with the title questions for QuACK. Make sure to include your grade in the school that you attend so I can give you a shout out. Thanks for tuning in and I hope you join us next time on Quack!
Part Two
Hey, and welcome back to quack. This is part two of our tree episode with Brian Wahl. If you haven't listened to part one yet, I recommend starting there to get all the background on Brian and learn some really cool stuff about tree species around the world and in Wisconsin. Really very fascinating stuff. Okay, I don't want to keep you waiting any longer.
So here's part two.
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Mickenzee: Okay. All right. This third grader wants to know what is a mast crop and what animals would benefit from mast.
Brian: Oh, so that again is a great question. That one of was is kind of easier. So a mast crop in general, basically is when you have a tree and typically around here we're thinking more like the oaks. We don't have a lot of Beech here in Madison, but like, say along Lake Michigan. We do and they do. They mast as well. Pretty much any tree that's really producing a fruit or a cone or a seed or a nut can do mast.
And what is mast? That's a year, where they produce just a massive amount of nuts or acorns or fruit or whatever.
And when I say massive amount, it's like it's raining nuts or you're slipping on them because they're everywhere. And the crazy thing about a mast, a mast year is they are sort of cyclical. So they kind of happen like every 2 to 5 years ish. So, but we really don't know why.
Mickenzee: Oh. That's exciting.
Brian: Yeah. Like, we're not sure. We have ideas and theories. We think sometimes weather, I mean, whether definitely plays a role. We probably last year we were in a drought, so we probably didn't see a lot because it takes a lot of energy to produce. You can imagine all those seeds are just little nuggets of energy. And it takes a lot to put into those to make it a mast year.
But the crazy thing is, a lot of times, like if an oak tree in like Madison is, it's not usually an oak tree. It's like this oak tree, that oak tree, this whole grove of oak trees. Oh, and not only here in Madison, but all the ones in Milwaukee are doing the same thing.
Mickenzee: They all are together.
Brian: They are together. So how did they do that? Right? So that's the mystery. Again, a lot of theories behind it. Is it weather related, is it not? You know, is it maybe pheromones driven because, you know, trees do communicate by pheromones and hormones, just like our bodies do as well. Really cool side in the weeds here.
So, for example, in northern Wisconsin, we have a lot of, Aspen trees. Right. And so they sometimes get hit with, Oh, I can't think of the worm right now. It's one of the tent caterpillars. Right. And so they'll be munching all the leaves and all of a sudden, the front, the insects are moving.
Kind of. Let's say they're moving north. They stop. Oh, why do they stop? Because those trees have picked up on a signal from all the other trees that are getting damaged, and they've now started producing chemicals in their leaves that the caterpillars don't like. And so when they get there, they're like, whoa, what? What? This isn't good.
And so then, yeah. Isn't that crazy?
Mickenzee: Wow. That crazy. So wild. I had no idea that the plants could communicate in that way.
Brian: Oh my gosh. And there's a lot of new research out there. You know, I haven't read it all myself, but just. Yeah, how they communicate or underground. I've heard of like, the you may have heard of the wood wide web.
Mycelium connect different species and help trees communicate! (photo by Drew Brayshaw).
How you know, through the ground, through their roots? And then the mycelial connections, which are fungus, so little fungus like, connect various species through their roots. These associations really help trees, like, suck up more nutrients and exploit more water because they're finer. They can get into all the places where the roots can't necessarily, but they can also connect trees together and of different species.
And so we're not fully understanding exactly how, but there's definitely some evidence that there's some communication going on.
Mickenzee: Wow. And interspecies too is really, really cool.
Brian: Absolutely. So it just like gives me shivers to think about. So if anybody wants to study that I mean yeah, it is just that's why the nature natural world and science is so flippin cool. Again, we just keep finding out more and more stuff.
Mickenzee: Yeah, it's just uncovering even more that we don't know. We want to find out
Brian: Yeah. So tying that kind of back to the, the mast crop and you know what? What animals might benefit from that. This is where I, you know, if I were in a live presentation, I'd be asking the kids, well, who do you think you know?
Yeah, it's from that or or maybe doesn't benefit. Right. And so maybe this kind of comes to the leading theory as to why trees do mast crops. And the thought is that, you know, if a tree is going to, people will use the oak tree because it's easiest, right? We have acorns. So if an oak tree produces acorns, you know, of, let's say thousands of acorns that an oak tree is going to produce in an, in an average year.
Only a few of those are going to survive, and become an oak tree. And if you can imagine a lot of times if you have predators out there and the predators in this case, we like squirrels, for example, they love acorns. So if there's enough acorns for them to feed their family, you know, they're going to get to a certain population level and they're going to eat all the acorns.
And then all of a sudden, now we don't have any oak trees.
Mickenzee:Oh, yeah.
Brian: Because they've kind of they're going to kind of level out at this sort of like level of, hey, you got enough acorns for me to support, you know, my family. Cool. But we're not supporting you as an oak tree because, you don't get to have offspring.
So the prevailing theory is sometimes they'll be leaner years. And so those populations, then, of the squirrels might decline because there's not enough food for them. And then all of a sudden, boom, we do a mast year. Yeah. Guess What? The squirrels get to eat all they want, but there's extra.
Mickenzee: And there's still leftover.
Brian: Absolutely, if I go into a smorgasbord, you can't eat it all there, right. And so, again, there's food left over or there's acorns left over to then produce that next generation. And they can do that every 2 to 5 years. Well, great. You know, because in the time frame of a tree, that's not too long to wait, does the tree take an energy hit again from producing all those acorns at once?
Absolutely. But it gives itself, you know, 2 to 5 years to, you know, recuperate. And yes, it is producing acorns, you know, in between as well. And some of those may survive, but not as many as will survive during a mast year. So it's really thought to be a strategy of sort of this predator prey interaction to some degree.
Mickenzee: Population dynamics.
Acorn Weevil (photo by Andy Reago & Chrissy McClarren).
Brian: Absolutely right. And so who benefits. Right. So in a mast year. Well yeah. Mice, squirrels, deer, turkey, some woodpeckers and jays, you know, they go for the acorns. And again, this is just acorns. So other trees do this as well. And they may have, insects that feed off of them, but we often forget about well, what about the weevils that feed on acorns.
Absolutely. They have a bumper year or two. Right. And then who feeds on the weevils. Oh, you know, maybe some birds or whatever that we will becomes, you know, the Beatles. And then you know, going beyond that. So okay, so we have a bunch of chipmunks and a bunch of squirrels and a bunch of mice while the owls are happy.
Oh, right. The hawks are happy because they have a bigger food source. Unfortunately, when we have a lot more like mice and deer, we may also have more ticks. And so that could maybe negatively affect, say, us from like Lyme disease or something from that aspect. So yeah, these masting years, you know, ripple effects. Right. That just keep going on and on and on.
So you know, the food web and the ecological, you know, just entanglement I guess you can say, of just a masting tree are just phenomenal. Right.
Mickenzee: Right. There's such a huge impact. And you could trace it for thousands of species.
Brian: Absolutely.
Mickenzee: All right. And our last question today is why do leaves fall off the trees during the fall? We call it that but why is it?
Brian: We call it autumn. And no I mean it right. Aptly named fall for Falling Leaves. And so if you look at the question as in so fall for us here in Wisconsin, right. What's happening? What comes next? Winter. And if we think about it, you know, what is everything else doing in winter or what's not available in winter?
Traditionally, winters is very scarce for resources like food or water, things like that. For animals, for example, a bear. a bear goes into hibernation because it's waiting, you know, it actually stores up energy. And then it kind of goes asleep because I can't find enough water. I can't find enough berries because they're not out there.
So I'm just going to take the winter off and I'll wake up in spring. It's really no different with a tree. Right? So our leaves are out there. They're a pretty expendable part of the tree. The tree knows it's going to have them for a year and then and lose them. Those leaves, there are powerhouses while they're there.
Right? They're full of chlorophyl. And they are, photosynthesizing. Right. So they're creating the a lot of the food or the energy components that the tree needs to grow, but it's creating enough that there's a little extra left over. So it's also storing iy it's building for winter. When winter comes, winter is very harsh on leaves. They're very sensitive to not having enough water.
In fact, in the summer if you have a tree and it's not getting enough water, it starts to wilt just like any other plant. And those leaves can dry up and get crispy. Well, here in Wisconsin in winter, I don't know about you, but my skin gets extremely dry. I mean, it is very dry here in winter.
Yes, we have snow, which is water, but it's not really available to the plants. The ground is frozen, the roots are living and they're alive, but they aren't as active as they are in summer. So the big part of it is water conservation. And the other aspect of, you know, leaves are, are they're kind of expensive for the tree to put out there.
So they take energy to get out there and they take energy to maintain. And when it comes around to fall, you know, the light isn't we don't have as much light. The days are shorter. So are they producing as much food and energy? Nope. So they're costing a little bit more to be out there. And trees are very, very efficient.
So at some point they're like, oh, the cost benefit to having these leaves, the balance has tilted. Now it's more expensive to have the leaves than not. And so then what the leaves or the tree does is it forms what we call an abscission layer, which is a layer of cells between the stem of the leaf and, the branch.
And it then kind of closes off all the little vesicles, and the leaves get a little bit, you know, the attachment becomes weaker and eventually it falls off. And it's crazy too, right? Because not all trees lose their leaves at the exact same time. Right. Or it's over a period of time
Mickenzee: And not all the leaves drop at once.
Brian: Absolutely. Well, and so that's crazy too, right. So the ginkgo tree. The ginkgo tree, it's kind of fun because they lose all their trees in about 24 hours. All their leaves in about 24 hours.
Mickenzee: Whoa.
Brian: Boom. It's great. Just you know, if that's all you had, you have to work one day and you're done, right? So that's awesome.
Other trees will hold on like Norway Maples tend to wait a little bit longer in the season. Maybe that's because they're not native here. And you know, in their environment, that was the better timing cue for them. Other trees like oak trees will actually hold onto some of their leaves, and we're not really sure exactly why they do that.
And like Beech trees, for example, when they're younger, will hold on to their leaves. And one of the theories behind that, again, is deer, right? Are browsing on those buds because those buds are again packed with energy. Yeah. Right. So they can spring forth in spring. And so one of the theories is that, you know, like the leaves, the dry, crunchy leaves like, are around those little bud tips and they're not as palatable.
Right. So the deer like, what is this gross papery leaf thing? I'm not going to eat this. And so they go elsewhere, for some easier buds. And it's kind of cool because if you ever get to, like, again, the Lake Michigan coast where we have a lot of beech trees in winter, you'll see these yellow leaves in the understory.
Because of all the little baby beech tree is still have all their leaves. And it's pretty phenomenal. And it's also pretty beautiful.
Mickenzee: Yeah, that's really cool.
Brian: Again, not a simple question.
Mickenzee: These are great questions. Oh my gosh I feel like I learned so much today. Thank you to the students at Lincoln Elementary for submitting your nature questions.
This was a great episode. And thank you, Brian, for taking time to teach us.
Brian: Yeah you bet it's been fun.
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If you are interested in learning more about trees or getting involved with our programs, please head to our website at SWI birds.org and check out the free lessons, games and activities like me, a tree leaf ID or tree tag.
And if you have a big nature question that you'd like to have answered, please have a grown up or your teacher submit your question to info. swibirds.org with the title Questions for QuACK. Make sure to include your grade and the school you attend so I can give you a shout out. And thanks for tuning in and I hope you join us next time on Quack!
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Audio Editing and Transcription by Mickenzee Okon
Logo design by Carolyn Byers and Kaitlin Svabek
Music: “The Forest and the Trees” by Kevin MacLeod
