Roger: 0:02

Talking Trees with Lily and Jad. Today we're diving deep into the world of tree assessment and the use of penetrographs to study the structure and extent of internal decay in tree trunks. Join us as we discuss this interesting method and discuss its principle, use and limits. This episode has something for everyone. Stay tuned.

Jad: 0:38

Hey there all you tree enthusiasts limits this episode has something for everyone. Using it. Interpreting those readings.

Lilly: 0:43

Lots of nuances, yeah, especially when you're dealing with all the different tree species out there.

Jad: 0:47

Yeah, so we're going to break it all down for you the science behind it, what can affect the readings and how to really understand what you're seeing so you can make the best calls for those trees you're working with.

Lilly: 0:57

Sounds like a plan.

Jad: 0:59

All right. So first things first. What exactly IS the resistograph? How does this thing work?

Lilly: 1:06

Well, in simple terms, it's like a tiny drill with a needle that spins.

Jad: 1:10

Okay, so we're drilling into the tree.

Lilly: 1:11

Yeah, but it's not just about making a hole.

Jad: 1:17

As that needle goes in, it's measuring the resistance it encounters, so it's sensing how hard it is to push through the wood Exactly.

Lilly: 1:20

And that resistance tells us about the density of the wood. Denser wood, more resistance.

Jad: 1:25

Makes sense, so harder to push through. Means denser wood.

Lilly: 1:28

Exactly, and there's actually research backing this up. Way back in 1988, a guy named the Len Now did a study and he found a strong correlation between wood density and the resistance measured by a 10 millimeter drill.

Jad: 1:41

Interesting. So the principles behind the resistor graph have been known for decades.

Lilly: 1:45

That's right, but of course technology has advanced a lot since then.

Jad: 1:49

Yeah Right. And we have more recent research too, like this paper by Eckstein and Sass from 1994.

Lilly: 1:55

Ah, yes, a classic. They did some great work highlighting all the other factors that can affect those resistograph readings.

Jad: 2:02

And that's what makes it so tricky right, it's not just about the wood density itself, there's all these other things that come into play.

Lilly: 2:07

Absolutely, and those can really throw you off if you're not aware of them.

Jad: 2:10

So let's dive into those other factors. What's the first one that comes to mind?

Lilly: 2:13

Well, one of the biggest culprits is moisture.

Jad: 2:16

Moisture. How does that affect things?

Lilly: 2:18

Think about it like a sponge A. A wet sponge is much denser and harder to compress than a dry sponge.

Jad: 2:24

Oh yeah, I see where you're going with this.

Lilly: 2:26

Same with wood If the tree's wet, you're going to get higher resistance readings.

Jad: 2:30

Even if the wood itself isn't that dense.

Lilly: 2:31

Exactly so you might think the wood's in great shape based on those readings, but it could just be because it's soaked.

Jad: 2:36

So if you're out there after a rainstorm, those resistograph readings could be misleading.

Lilly: 2:41

Absolutely, and Eckstein and Soss actually showed this visually in their paper. They compared resistograph profiles from moist and dry samples of poplar and birch.

Jad: 2:53

And what did they find?

Lilly: 2:54

Well, in the moist samples, those local density fluctuations, you know those peaks and valleys on the graph. They were way less defined compared to the dry sample.

Jad: 3:03

Wow, so moisture can really blur the picture.

Lilly: 3:06

It can. And then there's another sneaky factor that Eckstein and Sauce identified the drilling procedure itself.

Jad: 3:12

Wait, the way you drill can affect the readings.

Lilly: 3:15

You bet they found that it can create what they call artifacts on the profile.

Jad: 3:19

Artifacts Like fake signals.

Lilly: 3:21

Pretty much. They're not actual reflections of the wood structure, just corks of the drilling process.

Jad: 3:26

Okay, that's interesting. I never thought about that.

Lilly: 3:28

It's like if you're pushing a shopping cart it takes more effort to get it moving, initially right, and then, once it's rolling, it gets easier.

Jad: 3:35

Yeah, totally.

Lilly: 3:36

Same thing with the resistograph needle there's an initial resistance increase as the drill starts, so that first push is always going to be a bit tougher, exactly. And then there's a continuous increase as it penetrates deeper in a higher idle level after the drill exits.

Jad: 3:52

So all these things are happening just because of how we're drilling, not because of the wood itself.

Lilly: 3:57

That's the key. So as an arborist, you got to be able to spot these procedural artifacts so you don't misinterpret them as decay or some other issue.

Jad: 4:07

So how do you tell the difference?

Lilly: 4:08

Eckstein and Sauce figured that out too. They found that if you reverse the drilling direction, those artifacts will reverse as well.

Jad: 4:15

Oh, so if it's an artifact it'll flip-flop depending on which way you drill Exactly. That's a good trick to know.

Lilly: 4:20

It is Now so far. We've talked about moisture content and the drilling process itself both of which can throw off those resistograph readings. Right, and I'm guessing there's more there is. The next layer of complexity comes from the wood structure itself.

Jad: 4:34

Of course, every tree is different.

Lilly: 4:36

Yeah.

Jad: 4:37

So how does the wood structure play into this?

Lilly: 4:39

Well, different tree species have different anatomical makeups and that can really impact how those resistograph profiles look.

Jad: 4:46

Right Makes sense.

Lilly: 4:47

Yeah.

Jad: 4:47

So break it down for us how does this wood structure thing actually work?

Lilly: 4:51

Let's take oak as an example. It's a classic ring porous species.

Jad: 4:55

Ring porous right Now. Remind me what that means again.

Lilly: 4:57

No problem. Ring porous means that the early wood, the wood formed early in the growing season, has larger pores than the late wood.

Jad: 5:06

Okay, bigger pores in the early wood.

Lilly: 5:07

Yeah, and that creates distinct density fluctuations throughout the growth ring.

Jad: 5:11

So you get those peaks and valleys on the resistograph profile that correspond to those early wood and late wood bands.

Lilly: 5:17

Exactly, and in oak. The low density of that early wood often gives a clear indication of where the annual rings are. But there's a catch as the heartwood forms, those density differences can become less distinct.

Jad: 5:30

So it's not always as easy as just counting the peaks and valleys.

Lilly: 5:33

Nope Plus trying to identify those narrow rings near the center of the tree. The pith can be a real headache. The curvature in that area means the drill might be passing through multiple rings at once creating a real jumbled mess on the profile. So even with oak, a ring porous species, it's not always straightforward, not always. And then you have diffuse porous species like beech or poplar.

Jad: 5:57

Okay, so what's different about those?

Lilly: 5:58

Well, in diffuse, porous species the density difference between early wood and late wood is much less pronounced. So instead of those clear peaks and valleys, you get these abrupt density changes that create more subtle peaks on the readings.

Jad: 6:10

So it becomes even harder to pick out those annual rains.

Lilly: 6:12

You got it. And just like with the ring porous species, moisture content adds another layer of complexity.

Jad: 6:18

Oh, boy, so you really need to be on your toes with these diffuse porous species.

Lilly: 6:23

Definitely their resistograph profiles can be quite tricky.

Jad: 6:26

So we've got ring. Any other wood structures that we need to be aware of?

Lilly: 6:32

Well, Eckstein and Saas actually singled out hornbeam as a particularly challenging case.

Jad: 6:37

Hornbeam huh. What makes it so difficult?

Lilly: 6:40

Hornbeam has these wavy annual ring boundaries and large rays in its wood structure and those features cause the density fluctuations to be much less clear in the resistograph profiles.

Jad: 6:50

So it's like trying to read a blurry map.

Lilly: 6:54

Pretty much. It just goes to show how important it is to know your species when you're interpreting resistograph readings.

Jad: 6:58

Absolutely. It's not a one-size-fits-all tool. You got to take into account the specific tree you're dealing with.

Lilly: 7:03

Exactly. Now, speaking of accurate interpretation, let's talk about one of the most important reasons we use the resistograph detecting decay.

Jad: 7:11

Ah yes, the big question is there hidden rot lurking inside this tree?

Lilly: 7:16

And the good news is the resistor graph is very effective at detecting internal decay. We've got studies by Bemman and Klinger in 1993, and of course Eckstein and Soss confirming that.

Jad: 7:27

That's reassuring, but I remember reading that decay can actually cause resistance peaks on the graph, which seems counterintuitive.

Lilly: 7:35

I know right, You'd think decay would make the wood weaker, leading to lower readings.

Jad: 7:39

Exactly so. What's going on there?

Lilly: 7:41

Well, it turns out that decay often triggers a defense response in the tree Around the decayed area. The tree might occlude its vessels, basically blocking them off, or deposit gummy or hard substances to try and wall off the decay.

Jad: 7:55

So it's like the tree is trying to fight back and those efforts actually make the wood denser in that area, leading to those higher resistance readings.

Lilly: 8:02

You got it. It's another example of how interpreting the resistograph requires understanding not just the tool itself, but also the biology of the tree. Now this brings us to another important point for arborists Understanding the differences between the resistograph and another common tool, the increment borer.

Jad: 8:21

Ah, yes, the increment borer, the one where you actually get a core sample of the wood.

Lilly: 8:25

Exactly. Both tools have their pros and cons, and a key paper by Kirsten and Schwartzy in 2005 really dives into those differences.

Jad: 8:33

So what did they find? What are the key takeaways for us as arborists?

Lilly: 8:37

Well, one of their most interesting findings was about the development of mycelial plugs. Have you ever encountered those?

Jad: 8:43

I've heard of them. They're like those fungal barriers that can form in response to a wound right.

Lilly: 8:47

Right and Kirsten and Schwartzy found these plugs forming in many of the increment borer wounds but not in the resistograph wounds.

Jad: 8:55

Wait. So the increment borer, which makes a bigger hole, actually seemed to trigger this defense response in the fungus.

Lilly: 9:00

That's right. They observed that in many cases those plugs actually sealed off the increment borer holes, preventing the decay fungus from spreading further. But with the resistograph the decay fungus didn't seem to face that same barrier. That's surprising.

Jad: 9:14

I would have thought the smaller resistograph, the decay fungus, didn't seem to face that same barrier. That's surprising. Iowa thought the smaller resistograph wound would be less disruptive.

Lilly: 9:19

It's not always about the size of the wound, though. It's about how the tree and the fungus respond to it. It could be that the open nature of the increment borer hole triggers that plug formation as a defense mechanism.

Jad: 9:34

While the resistograph hole packed with sawdust might create a more stable environment for the fungus.

Lilly: 9:37

That's one theory. It's a complex issue, and this research shows us that choosing between the resistograph and the increment borer isn't a simple decision.

Jad: 9:45

You really need to think about the long-term impact on the tree.

Lilly: 9:47

Absolutely, and that's where the judgment of a skilled arborist comes into play Right.

Jad: 9:52

you gotta weigh the benefits of the information you get against the potential risks to the tree.

Lilly: 9:56

Exactly Now shifting gears a bit. Let's talk about cross-contamination. Anytime we're drilling into trees, there's that worry about spreading pathogens.

Jad: 10:08

It's a valid concern. We don't want to be accidentally introducing diseases.

Lilly: 10:12

Right, and Kirsten and Schwartz actually look at this specifically in their study.

Jad: 10:16

So did they find any evidence of cross-contamination?

Lilly: 10:19

The good news is they didn't find any evidence of the main decay fungi they were studying in Anodos hispidus or Bomes fomentarius being transferred between trees.

Jad: 10:29

Okay, that's a release.

Lilly: 10:30

However, they did find other fungi present on the tools, primarily deuteromycetes, and interestingly these were more common on the increment borer than the resistograph.

Jad: 10:40

That makes sense given the open nature of the increment. Borer wound.

Lilly: 10:43

Right, but thankfully those deuteromycetes aren't usually considered major threats to tree health in the same way those decay fungi are.

Jad: 10:50

So, while cross-contamination of those serious decay fungi doesn't seem to be a huge issue, it's still always best practice to sterilize our tools between trees, right?

Lilly: 11:00

Absolutely Can't emphasize that. Enough. Sterilize, sterilize, sterilize.

Jad: 11:05

Okay, so we've covered a lot of ground here Moisture content, drilling procedure, wood structure, decay detection, the differences between the resistograph and the increment bore and even cross-contamination. It's clear that the resistograph is a powerful tool, but it's not a magic bullet that gives us all the answers.

Lilly: 11:22

You said it, Interpreting those readings accurately requires a deep understanding of all these factors we've been talking about. It's about looking at that profile and seeing not just lines on a graph but the story those lines are telling us about the tree.

Jad: 11:35

And that story can be influenced by so many things.

Lilly: 11:38

It really is about piecing together all the clues to get a complete picture of the tree's internal condition.

Jad: 11:44

Let's shift our focus now to the practical side of things. How can arborists use all this knowledge to make informed decisions about tree care? And speaking of being a tree detective, let's shift our focus now to the practical side of things. How can arborists use all this knowledge to make informed decisions about tree care?

Lilly: 12:03

I think the key takeaway and it's something we've been circling around throughout this whole conversation is that the resistograph, it's just one tool in your toolkit.

Jad: 12:13

Right, it's not a replacement for all the other things we do as arborists.

Lilly: 12:16

Exactly. It's not going to tell you everything you need to know. You still need to use your eyes, your experience, your knowledge of tree biology, all of it.

Jad: 12:25

It's about putting all those pieces together, the resistograph readings along with everything else.

Lilly: 12:29

Exactly, and I think it's also important to keep in mind the long-term impacts of using these invasive tools, especially when we're only looking at a single point along the stem or branch.

Jad: 12:39

That's a really good point. Yeah, anytime we're drilling into a tree, we're creating a wound.

Lilly: 12:43

Yeah, and even though the resistograph needle is pretty small, those wounds still take energy for the tree to compartmentalize.

Jad: 12:50

So it's not a decision to be taken lightly.

Lilly: 12:52

Right. As arborists, we have to weigh the benefits of the information we get against the potential harm to the tree.

Jad: 12:58

It's a balancing act, for sure.

Lilly: 12:59

It is, and that's where experience really comes in. A seasoned arborist can look at the whole picture, the site, the tree's overall health, and make a judgment call about whether using the resistograph is the right move.

Jad: 13:12

So it's not just about knowing how to use the tool, it's about knowing when to use it.

Lilly: 13:17

Exactly, and if we do decide to use it, we need to do it responsibly. Choose our drilling locations carefully, minimize the number of holes and just be mindful of the tree's natural defenses.

Jad: 13:28

Absolutely. It's all about respecting the tree.

Lilly: 13:30

It is Now in terms of actually interpreting those resistograph readings. What advice would you give to arborists who are just starting out with the tool?

Jad: 13:38

Well, I think the first piece of advice is don't rely on the resistograph alone.

Lilly: 13:42

I was going to say the same thing. It's just one piece of the puzzle.

Jad: 13:45

You need to put it in context.

Lilly: 13:46

Look at the big picture, consider the species, the site history, any other symptoms you're seeing.

Jad: 13:52

Use all your senses, all your knowledge.

Lilly: 13:54

Right, and don't be afraid to consult with other arborists either. We've got a great community out there. People are always willing to share their expertise.

Jad: 14:02

Yeah, that collaboration is so important.

Lilly: 14:04

It is Bounce ideas off each other. Look at profiles together, learn from one another's experiences.

Jad: 14:11

So it's about continuous learning. Never stop growing as an arborist.

Lilly: 14:16

Exactly, and at the end of the day, I think that's what makes this field so rewarding there's always more to learn.

Jad: 14:21

Always something new to discover about these amazing trees. Absolutely Well, I think that about wraps it up for our deep dive into the resistograph. But I think the biggest takeaway is that the resistograph it's a powerful tool, but it's not a magic solution.

Lilly: 14:36

It takes knowledge, experience and a willingness to keep learning to really use it effectively.

Jad: 14:42

So, to all you arborists out there, keep exploring, keep learning and keep those resistographs spinning. And thanks for joining us on the Deep Dive.

Roger: 14:55

Thanks for joining us as we explore tree assessment and the use of penetrographs to study internal decay. We hope you found it insightful and enjoyable. Your interest and support help us care for trees more effectively. Thank you for your support and please share Talking Trees friends and leave us a review. Until next time, stay curious and keep exploring the natural world. See you in the next episode.