Street Trees and Soil Microbiota

Subscriber-only episode

In this Monday episode of Talking Trees, we uncover the hidden world beneath our feet—the soil microbiota that supports and sustains urban street trees. This episode dives into the crucial roles played by bacteria, fungi, and other microorganisms living in the rhizosphere and within tree tissues.

We discuss:

• How microbial communities affect tree nutrition, immunity, and resilience
• The constant biochemical communication between roots and microbes
• The importance of endophytes and rhizospheric organisms in supporting tree health
• Why moving beyond an anthropocentric perspective helps us appreciate the complexity of urban tree ecosystems
• How this knowledge can inform better urban tree care and soil management strategies

Street trees are not isolated individuals but hubs in a vibrant, living underground network. Recognizing their microbial allies is key to sustaining greener, healthier cities.

Background information:

• SUELO, ÁRBOL URBANO Y VECINOS INVISIBLES.pdf

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Chapters

• 0:00 Introduction to Talking Trees Episode
• 1:19 Understanding the Soil Microbiome
• 4:07 Bacterial Functions and Communication
• 6:31 The Rhizosphere: Tree-Microbe Marketplace
• 7:24 Tree Defense Mechanisms
• 10:15 Practical Soil Management for Arborists
• 13:24 Episode Wrap-Up

Transcript

Roger: 0:02

Talking Trees with Lillian Jad. Welcome to Talking Trees. Today we examine a 2020 article by Miguel Angela Sierra Castellanos that explores the intricate relationships between soil, urban trees and their unseen microbial partners. The study highlights how soil quality affected by contamination and degradation impacts tree growth, and it emphasizes the vital roles of bacteria fungi and endophytes in nutrition and defense against pests and diseases.

Jad: 0:39

Welcome back everybody Ready to dig into some more fascinating tree stuff, always Today we're fascinating tree stuff Always.

Jad: 0:45

Today we're going below ground. Okay, we're talking about the soil and the crazy world of all those tiny little creatures that are living down there that have such a huge impact on how healthy and strong our trees are. Right, this is a topic that can really change how you approach tree care, if you really can wrap your head around it. For sure, and to guide us on this journey, we're going to be looking at an article by Michelangelo, sierra Castellanos.

Jad: 1:11

Oh yeah, he's a biologist, a researcher A&D and arborist, and he starts things off with a really thought-provoking quote. He says without a little interaction with neighbors, life would not exist. So what do you think he means by that?

Lilly: 1:28

Well, I think he's really hinting at this idea that, you know, it's not just the trees that we see above ground that are important, it's this whole hidden world beneath our feet, all these microorganisms, the bacteria, the fungi, all these, you know, tiny creatures that we can't see with the naked eye. They're all interacting and they're all playing a crucial role.

Jad: 1:40

And for arborists. I mean, you know more than anyone soil is the foundation of life, for trees.

Lilly: 1:46

It's a living breathing system. It's providing nutrients, it's providing water, it's the lifeline for these trees.

Jad: 1:52

Yeah, and you know, I think about all the arborists working in urban environments. You've got pollution, you've got compaction, you've got limited space for the roots to even grow.

Lilly: 2:02

Yeah, All those factors are putting a lot of stress on the soil and ultimately that stress is going to impact the health and well-being of the trees.

Jad: 2:09

OK, so Cassianus talks about these neighbors, right, and he's referring to the microorganisms.

Lilly: 2:16

Yeah, so that's like bacteria, fungi, all that good stuff, all those microscopic organisms that are just teeming beneath the surface.

Jad: 2:21

It's wild to think that these little creatures were like the original inhabitants of Earth.

Lilly: 2:27

Oh yeah, I mean think about it. They were the pioneers shaping the very conditions for life as we know it.

Jad: 2:32

Yeah, billions of years ago.

Lilly: 2:33

Right. They were forming communities, exchanging resources, evolving together over eons, and it's those interactions that paved the way for more complex life forms.

Jad: 2:43

OK, so let's zoom in on bacteria for a second. I mean, they're everywhere, right Everywhere. What makes them so important for the health of our soil?

Lilly: 2:52

Well, for one, they're the most abundant organisms in the soil. I mean, they're just everywhere, and they're like tiny little recycling centers. They're constantly breaking down organic matter, releasing nutrients that the trees can use.

Jad: 3:05

So they're really key for that nutrient cycling, making sure that all those elements are available for the trees to take up.

Lilly: 3:11

Exactly, and what's even more amazing is that they communicate with each other what they use chemical signals.

Jad: 3:17

Wait, so bacteria talk to each other.

Lilly: 3:19

Yeah, it's like a microscopic language. How does that even work? Well, they release these chemicals and it signals their presence and it helps them coordinate their activities.

Jad: 3:29

Oh, wow.

Lilly: 3:30

And the concentration of those signals. It changes based on population density, so they can actually kind of adapt to their surroundings. That's amazing, like a whole social network happening beneath our feet.

Jad: 3:42

Right under our noses or our boots or whatever.

Lilly: 3:44

Yeah, exactly, and it's important for us to understand because those bacterial interactions, they have a direct impact on the availability of nutrients for our trees.

Jad: 3:53

Okay, so we've got these bacteria. They're breaking down organic matter, they're cycling nutrients, they're talking to each other.

Lilly: 3:59

Yeah. What else do we need to know about them as arborists? Well, there are different types of bacteria, and each one has its own specific role to play in the soil ecosystem. Okay. So, for instance, we have nitrogen fixing bacteria and they do something truly remarkable they take atmospheric nitrogen and they convert it into a form that plants can use.

Jad: 4:21

Wow.

Lilly: 4:21

Yeah, they're like little natural fertilizer factories Right, essential for tree growth.

Jad: 4:25

Yeah.

Lilly: 4:26

And then we have other bacteria that break down cellulose and pectin, which are the main components of plant cell walls. So they help break down fallen leaves, other organic matter, and they release those nutrients back into the soil.

Jad: 4:41

So they're kind of like completing that cycle of life and decay, Right exactly.

Lilly: 4:45

Okay, and then there's another really fascinating group of bacteria that Castellanos mentions, called endophytic bacteria, and these guys, they actually live inside plant tissues.

Jad: 4:55

Oh, wow. So they're not just in the soil, they're like in the tree.

Lilly: 4:58

They're living within the tree itself and they provide a range of benefits to the tree. They can help with nutrient transport. They can boost disease resistance. They can even produce those volatile organic compounds, or VOCs, that we talked about earlier.

Jad: 5:10

Oh yeah, the VOCs. Right, that's what gives plants their unique sense.

Lilly: 5:13

Right, exactly.

Jad: 5:15

So these endophytes, they're not just helpful, they're actually contributing to how the tree functions.

Lilly: 5:20

Exactly. They're more than just passive residents. They're active partners in the tree's growth and survival.

Jad: 5:27

Wow, and this whole idea of this close relationship between plants and microbes, it really comes to life in a special zone called the rhizosphere right, Absolutely.

Lilly: 5:35

The rhizosphere is the zone of soil immediately surrounding the plant roots. Think of it like a bustling metropolis of microbial activity.

Jad: 5:43

It's where all the action happens, so it's like a microbial city.

Lilly: 5:46

Yeah, a real hotspot for interaction between the plants and the microorganisms.

Jad: 5:50

So what's going on in this microbial metropolis?

Lilly: 5:52

Well, plant roots, they release these compounds called exudates.

Jad: 5:56

Exudates.

Lilly: 5:56

Yeah, and these are basically sugars, amino acids, other goodies that the roots are releasing Right and they're attracting and nourishing a diverse community of microorganisms.

Jad: 6:07

So it's like the roots are feeding the microbes.

Lilly: 6:09

Exactly, they're providing a feast and, in return, the microbes help with nutrient uptake, they protect against pathogens and they even improve the soil structure.

Jad: 6:18

Like a give and take.

Lilly: 6:19

It's a mutually beneficial relationship, a symbiotic dance between the roots and the microbes.

Jad: 6:24

This is blowing my mind. I'm starting to realize how much is happening under the surface now and how that's impacting everything we see above ground.

Lilly: 6:31

It's a whole other world down there.

Jad: 6:32

It really is. It's amazing. This whole concept of the rhizosphere really highlights how connected those roots are to the soil.

Lilly: 6:40

Oh, it's like an underground marketplace you know you've got this constant exchange of resources information.

Jad: 6:47

Wow, and speaking of information, Castellanos mentions that plants aren't just, you know, defenseless against all these threats like pests and diseases. Right, how do they protect themselves in this whole like microscopic battleground?

Lilly: 6:59

They've developed some pretty sophisticated defense mechanisms. You know they can produce these antimicrobial compounds called phytoalexins.

Jad: 7:08

Phytoalexins.

Lilly: 7:08

Yeah, it's a mouthful, but basically they act like chemical weapons against attackers.

Roger: 7:13

Oh wow.

Lilly: 7:14

And they can also release those VOCs that we talked about earlier Right, not just for scent, but also as alarm signals to nearby trees.

Jad: 7:22

So it's like a chemical distress call.

Lilly: 7:24

Exactly, and it's not just about warning their neighbors. Some VOCs can actually attract beneficial insects that will prey on the pests.

Jad: 7:32

Oh wow. So they're calling in reinforcements. That's so cool. It's incredible how these plants have evolved these like intricate defense strategies.

Lilly: 7:40

Yeah, it's amazing. And what's interesting is that those defenses are often triggered by mechanical damage. Oh, so like when we prune, okay, you're essentially, you know, making a cut, you're activating the tree's internal defense system.

Jad: 7:53

Yeah, that's a really good point. As arborists, we need to be thinking about that. You know, even routine practices like pruning can have these consequences for what's happening inside the tree.

Lilly: 8:02

Exactly. It underscores the importance of understanding that interconnectedness you know what we do above ground. It affects what happens below ground, and vice versa.

Jad: 8:12

And Castellanos really emphasizes that link throughout his article.

Lilly: 8:16

Right.

Jad: 8:16

He even says what is above is below.

Lilly: 8:19

Yeah, he's really highlighting that interdependence of the entire system, the energy that's produced through photosynthesis in the leaves. That's not just fueling the tree's growth above ground, it's also fueling the activity of the roots and the rhizosphere below.

Jad: 8:33

So it's like those leaves are acting like solar panels powering the whole operation, including all the microbes down below.

Lilly: 8:39

That's a great analogy. Yeah, the sugars that are produced during photosynthesis. They're transported down to the roots and released into the rhizosphere, providing nourishment for the microbes.

Jad: 8:50

So it's like sending down little care packages. Yeah, they're underground allies.

Lilly: 8:54

Right and in return the microbes. So it's like sending down little care packages, yeah, for their underground allies. Right and in return, the microbes release nutrients and other compounds that benefit the tree. It's a beautiful cycle it is, it's this beautiful cycle of give and take.

Jad: 9:03

Now you know we've been talking a lot about bacteria. Yeah, but what about fungi? I know they play a huge role in the soil ecosystem too.

Lilly: 9:10

They do, and while Castellanos doesn't go into detail about fungi in this particular article, their importance can't be overstated. You know mycorrhizal fungi, for instance. They form these symbiotic relationships with tree roots, extending their reach, helping them access water and nutrients more efficiently.

Jad: 9:29

So it's like an extension of the root system.

Lilly: 9:30

Exactly, they're expanding the tree's ability to gather resources of the root system. Exactly, they're expanding the tree's ability to gather resources and they create these vast underground networks connecting trees together, facilitating the exchange of nutrients and information. It's truly remarkable.

Jad: 9:43

That sounds like a whole other episode, right there, oh yeah, absolutely. Just on my coraisal fungi.

Lilly: 9:48

We could talk about fungi all day.

Jad: 9:50

For sure, but for now, let's bring it back to the bacterial interactions that Cassianos is highlighting. He really emphasizes that understanding these interactions can help us, as arborists, make better decisions about how we care for trees, and that's what I'm really curious about is how can we take all this amazing knowledge, all this fascinating stuff that's happening underground and actually apply it in the field?

Lilly: 10:13

Right, that's the key question, isn't it? Well, I think, first and foremost, it's about shifting our mindset.

Jad: 10:19

We need to stop thinking of soil as just dirt.

Lilly: 10:22

Yeah.

Jad: 10:22

And start recognizing it as a living ecosystem. It's a complex community that needs our support.

Lilly: 10:27

So it's not just about the tree, it's about that whole environment below ground.

Jad: 10:31

Exactly, and just like we would assess a tree's above ground condition, we need to be analyzing the soil. Soil testing is crucial.

Lilly: 10:39

Yeah, that makes sense. But soil tests, I mean, they can be kind of complex. Yeah, what should we be looking for specifically?

Jad: 10:45

Well, focus on indicators of biological activity. Okay, so things like organic matter content, microbial biomass, enzyme activity Okay, those parameters tell us how well the soil is functioning as a living system.

Lilly: 10:59

So we're not just looking at MPK, we're looking at the soil's overall vitality.

Jad: 11:04

Exactly, and once we understand the soil's condition, then we can tailor our management practices accordingly.

Lilly: 11:10

Okay. So, for example, if the organic matter is low, Right If organic matter is low. What can we do?

Jad: 11:14

Okay, so for example, if the organic matter is low. Right, if organic matter is low, what can we do? We can incorporate compost or other organic amendments to boost microbial activity. Okay, really, feed those microbes, and I'm guessing mulching plays a role here too.

Lilly: 11:24

Absolutely. Mulching is fantastic.

Jad: 11:26

Yeah.

Lilly: 11:27

Not only does it conserve moisture, it suppresses weeds, but it also provides a steady source of food for those soil microbes. Okay, enhances decomposition, nutrient cycling. Now let's talk about fertilization.

Jad: 11:39

Okay.

Lilly: 11:40

Conventional fertilizers, especially those high in nitrogen. They can actually harm the soil microbiome.

Jad: 11:47

Really, I thought nitrogen was like essential for plant growth.

Lilly: 11:50

It is, but too much nitrogen can disrupt that delicate balance of microbial populations. Okay, it favors the fast-growing opportunistic species while suppressing others, and that leads to a less diverse and resilient soil ecosystem.

Jad: 12:04

So it's like overloading a system with junk food.

Lilly: 12:07

Perfect analogy.

Jad: 12:09

Too much of a good thing can be bad.

Lilly: 12:10

Exactly. Instead, we should be focusing on building a healthy microbial community that can naturally cycle nutrients. So things like compost, biochar, cover crops these are excellent alternatives that enhance soil fertility without harming the microbiome.

Jad: 12:25

Yeah, that makes a lot of sense. Now let's talk about pruning. We talked about how pruning can trigger those defense mechanisms in a tree, right? So how can we minimize stress on the tree during that process?

Lilly: 12:39

Well, timing is crucial, you know. Avoid pruning during periods of high stress like drought or extreme heat. Okay, also make clean cuts to minimize the wound size and promote faster healing.

Jad: 12:50

All right, let's shift gears to pest management.

Lilly: 12:52

Okay.

Jad: 12:53

You know, instead of just relying on chemical pesticides, which can have all those unintended consequences for beneficial insects and soil microbes, right, what are some other approaches we can use?

Lilly: 13:02

Well integrated pest management, or IPM, is a much more holistic approach.

Jad: 13:07

Okay.

Lilly: 13:07

It involves monitoring pest populations, understanding their life cycles and employing a variety of tactics. Cultural controls, like choosing resistant tree varieties and ensuring proper site conditions those are your first line of defense. Biological controls, like introducing beneficial insects or nematodes, can help keep those pest populations in check. And then only as a last resort, when absolutely necessary, should we consider targeted pesticide applications.

Jad: 13:33

So it's like having a toolbox of options and knowing which tool to use for the right situation.

Lilly: 13:38

Exactly, and remember a healthy soil ecosystem with a thriving microbial community. That's a tree's best defense against pests and diseases.

Jad: 13:47

Wow, this has been such an amazing journey into this world of soil microbiology.

Lilly: 13:51

Yeah, it's pretty fascinating.

Jad: 13:52

And it's clear that you know we, as arborists, we have a responsibility to care for not just the trees but that invisible world that's supporting them. On that note, we wrap up this episode. We hope you've gained a newfound appreciation for that amazing world beneath our feet and how crucial it is for the health of our trees. Until next time, keep digging deeper.

Roger: 14:18

Thank you for joining us Today. We discussed Miguel Ángela Sierra Castellanos' 2020 article on the complex interactions between soil, urban trees and their microbial communities. The work provided valuable insights into how soil degradation and contamination affect tree growth and underscored the critical role of bacteria, fungi and endophytes in supporting tree nutrition and defense. We appreciate your attention and look forward to our next discussion.