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[00:00:00] Michael Hawk: In nature, tree physiology is like the engine that keeps forest running smoothly. Similar to how human physiology keeps our bodies going. Just as we study how our body’s metabolism, respiratory system and other systems work to keep us healthy.

[00:00:12] Biologists can look at tree physiology to understand how trees grow, use energy and cope with the challenges in their environment. Today’s guest Dr. Lucy Kerhoulas is an associate professor of forest eco physiology at Cal poly Humboldt. She specializes in the forest physiology of Northwestern, California, which includes Redwoods, Douglas Fir, Oaks, and more. Today Dr. Kerhoulas explored various aspects of forest physiology, including how they adapt to different conditions such as fire and drought. She delves into the scientific tools, used to study how trees respond to environmental changes. Including measuring carbon isotopes and tree tissues to assess impacts of drought.

[00:00:50] And this understanding of carbon isotope preferences provides interesting insights into historical carbon levels in our atmosphere, dating back hundreds of years. In fact, this is sometimes called the smoking gun because it provides strong evidence of fossil fuel contributions to atmosphere at carbon. Dr. Kerhoulas also discusses how trees can share resources and signal each other during times of stress, possibly creating a cooperative environment within a forest. This was a jam packed discussion and I hope you enjoy it. So without further delay, Dr. Lucy. Kerhoulas. Lucy, thank you so much for making the time today.

[00:01:26] Lucy Kerhoulas: Yeah. Thanks for having me. I’m honored to be here.

[00:01:29] Michael Hawk: So I think we’re going to have a wide ranging conversation about trees and forests and forest physiology today. There’s lots of really interesting things that I’m only beginning to learn about. So I’m super excited to delve into this fascinating topic with you today. So maybe to kick us off though, I’d love to hear about how you got interested in nature in the first place.

[00:01:52] Did it start as a kid?

[00:01:54] Lucy Kerhoulas: Yeah, as a kid, I kind of always really liked nature and just being outside. And then in high school, I got really into nature through my science classes. I took an AP biology class and was just hooked and found the whole natural world really fascinating. Then in high school, I also started backpacking a lot in the summers.

[00:02:15] So that combination of kind of learning about nature through science classes and then being outside and spending a lot of time in nature just was a good combination that led to a lifelong love.

[00:02:27] Michael Hawk: So we were just chatting a little bit, a moment ago, and I know that your educational background, you’ve focused on things like redwood physiology and ponderosa pine ecophysiology. This word physiology and plants keep coming up together. So, can you tell me what led you to that line of study?

[00:02:46] Lucy Kerhoulas: Sometimes Ecology can be sort of wishy- washy. The answers to questions can be really complex. And one thing that I like about physiology is it’s a little bit more brass tacks. It’s these metrics that you can get really solid answers with.

[00:03:00] And so I like the combination of looking at ecology through a physiological lens. It can be a little bit less wishy washy and give you some kind of more concrete answers than sometimes in a purely kind of ecological approach you might not get. And my master’s work was looking at a little bit of the mechanics of how tall redwood trees work.

[00:03:25] So looking at water status kind of throughout the tree and their capacity for photosynthesis and respiration throughout the tree. And getting lost in the weeds of details of how these giant plants function. And then one thing led to another. The more you study a topic, it just reveals more and more questions and unknowns that you can get kind of hooked on following the breadcrumbs.

[00:03:51] Michael Hawk: Absolutely. I, I’m wondering here, for, I think for a lot of people, when you hear the word physiology, it’s probably through a context of people, so you started to touch on some of the things you look at in, in trees. So maybe you can tell me a little bit more you said that there are actually concrete metrics that you can look at.

[00:04:10] What are some of those metrics? And maybe we can even start with just a general definition of, physiology in this context.

[00:04:18] Lucy Kerhoulas: so with people you can go to the doctor and get all sorts of metrics blood workup, or they can take your blood pressure or your resting heart rate all these different things to get a sense of how healthy you are, what’s going on with you. Kind of the same story with plants.

[00:04:34] There are different things that we can look at to gain insight into how they’re functioning, how they’re responding to climate, how they’re responding to different management. You can take a class in forest physiology in college. It could be a long winded topic, but the kind of highlight ones that I work with a lot are looking at water status.

[00:04:52] How stressed, water wise is the plant? Because for terrestrial plants, water is often the most limiting factor, to their persistence. And so there’s a metric called water potential that we can use as a way to evaluate plant water status. And then other things that are helpful pieces of information to gather about plants are sort of their gas exchange.

[00:05:15] So through the leaves, there are these little pores called stomata. They’re like these microscopic gateways linking the plant to the atmosphere. And we can look at how fast gases are moving in and out of the stomata as a metric of vigor or productivity. So water is going out through the stomata, that’s the main way that the plant loses water.

[00:05:37] And then CO2 is coming into the plant through the stomata and that’s how plants do photosynthesis. So there’s sort of this trade off between losing water through your stomata but gaining, CO2 for photosynthesis. So we can look at kind of different ratios of that or different nuances of gas exchange in the plant.

[00:05:54] You can also stick these little probes into the stems of trees and measure how fast water is moving through the tree. Oh, that tree used this many liters of water per hour, this many liters of water per day, so we can measure water use kind of at that scale too.

[00:06:10] Michael Hawk: I didn’t know about that latter one. So actually I, kind of want to back up before I, delve too deeply into some of these measurement techniques and things like that, because I think that’s a a pathway that can go down a bunch of different branches. And so a question I sometimes like to ask people are if you’re trying to impress somebody about your field of study, like maybe you’re at a party or something like that, what kind of like fascinating or fun fact would you share to like, say, maybe get the, get that person hooked on what it is that you’re doing.

[00:06:41] Lucy Kerhoulas: It’s a good question. I think one thing that impresses people or kind of wows people is that plants don’t always get their water 100 percent from the soil via their roots. So there’s all these different crafty ways that have evolved in the plant kingdom for plants to get water in alternative pathways.

[00:07:00] So through the, like temperate rainforest, and it would be the same, a lot of the same things for tropical rainforest, but say, in the coastal temperate rainforest here in Northern California. The tall conifers, they’re bathed in fog a lot. And so they have all these neat ways of using that water locally in the canopy where the water is being intercepted.

[00:07:22] I think that’s really cool. So you’ll be hundreds of feet up in a tree. And there’s these adventitious roots coming out of the branches that are growing underneath wet moss mats, presumably absorbing water. There’s been studies that have shown that these trees can absorb water through their bark, which makes sense if their bark is kind of perpetually wet underneath these epiphyte mats, or mats of things like ferns and mosses.

[00:07:45] And then there’s been a lot of work, especially with redwood, showing how it can absorb water through its leaves and the pathways of that is it through the skin of the leaf, through the epidermis and the cuticle, or is it through the stomatal pores? Some of those nuances are still being figured out. We call it the SPAC, Soil Plant Atmosphere Continuum, which is the normal pathway that water moves through plants.

[00:08:09] They uptake water through the roots and then it moves. through the tree, up to the leaves for photosynthesis, and then out to the sky through the stomata. Thinking beyond the SPAC is something that can impress people at parties, I think.

[00:08:23] Michael Hawk: I only recently learned about how the Coast Redwoods had those special leaves that can actually intake water. I think that’s actually, if I’m not mistaken, a relatively recent discovery, like in the last couple of decades.

[00:08:35] Lucy Kerhoulas: Yeah, so redwood and redwood is not alone. Foliar uptake of water is a common water acquisition strategy in plants. But Redwood being the charismatic megaflora that it is kind of gets a lot of press about using fog water through its leaves.

[00:08:54] Like in maybe the early 2000s or so coming out of Todd Dawson’s lab at UC Berkeley, there was some work published that really showed very compellingly the foliar water uptake of Redwood. And then there’s been a flurry of papers since then that have looked in more detail at it.

[00:09:10] Recently, like in 2022 there was a cool discovery published in the American Journal of Botany led by Alana Chin. She’s another faculty member at Humboldt. And that paper showed that there was kind of this dimorphic, or two different morphologies dimorphic kind of leaf structure in redwoods, where you have this kind of divide and conquer or specialization of labor among leaves in redwood trees.

[00:09:38] So there are some leaves that are really good at photosynthesis, but don’t do that much water uptake. And then there’s other leaves that are rock stars at foliar uptake, but really don’t do that much photosynthesis. So kind of this specialization.Which suggests a long evolutionary history of using these atmospheric water inputs in this ancient species.

[00:09:56] Michael Hawk: right? You think about the climate where the Coast Redwood lives and you mentioned the fog and how many days per year they’re bathed in this fog. It sort of makes sense that they would figure out a way to utilize that, especially like what we call the Mediterranean climate, because it’s very dry in the summer, but they still get the fog in the summer.

[00:10:15] So it’s a, it’s another avenue to persist throughout the annual summer drought.

[00:10:21] Lucy Kerhoulas: I know, it’s really convenient that our dry season in this Mediterranean climate happens to have this fog water input off the ocean. I think that’s kind of what enables a lot of these trees to grow so tall.

[00:10:34] Michael Hawk: So I think we’re going to talk a little bit about what physiology tells us about climate change and potential climate change impacts. But since we’re talking about the fog and the redwoods here at the moment, I’ve seen some stories about how climate change may reduce the amount of fog that we see even currently, and then going forward.

[00:10:55] And so I’m curious your opinion on that, what do you see happening potentially with the Coast Redwood trees in the face of the change in climate and the impact that might have on the fog?

[00:11:06] Lucy Kerhoulas: There’s been a lot of stuff published saying that fog is on the decline. Some work has showed, I think, that over the last hundred years or so, it’s declined maybe by about a third. And if you talk to the old timers here on the North Coast they’re like, I remember when the fog was so thick you could swim through it and the streetlights would come on in the summer and they’d never go off kind of thing.

[00:11:27] Or the Mark Twain the coldest, I always mess it up, but something like the coldest Winter I ever knew was the summer I spent in San Francisco or something, just because the fog would always roll in. And so yeah, up here on the north coast in Humboldt, my husband and I are always like, shh, don’t tell anyone.

[00:11:44] It’s actually like great weather. Like Everybody thinks it’s rainy all the time and foggy all summer, but with climate change, it really has, is not like that. It’s beautiful weather a lot. So fog does just kind of anecdotally, seem to be really decreasing just from living here for 20 years and I think in the southern portion of the range Redwood only naturally occurs in this threshold of fog.

[00:12:06] It’s like a fog belt species. So it seems to be able to naturally establish only within this fog belt. With things like assisted migration you might be able to, pamper them along, planting them outside of the fog belt. They’re planted ornamentally all around the world. Redwoods, you see them in the Central Valley and stuff.

[00:12:24] So if you can get them established, they can do okay. But as far as their natural range and where they’ll naturally occur, we do seem to be really dependent on fog. In the southern portion of the range where the fog might be even more on the decline, I think this could have big effects on the southern or eastern edges of the species distribution.

[00:12:46] Michael Hawk: You mentioned some of the things that you measure, like gas exchange and water potential, and you know, even being able to put probes in to assess the water flow. So maybe run through a couple of those real quick. when you’re assessing gas exchange, how does that work?

[00:13:02] Because you need some kind of closed system or something where you have known quantities of gas and, and you’re able to measure it before or after. So tell me, what does that look like? How do you even do that?

[00:13:15] Lucy Kerhoulas: So there’s three different ways that are commonly used to measure gas exchange. So we can do it at the leaf level using two different tools. One is called a leaf porometer, One colleague I heard call it like it’s like the iPad of plant phys toys like it’s just really user friendly It looks like this little Pac Man thing and you clamp it on a leaf and it can measure using differences in humidity between the top portion of this little chamber that you clamp over the leaf and the bottom portion of this little chamber it can measure the difference in humidity and crunch the numbers in the little instrument and calculate how much water is leaving the leaf per meter squared per second.

[00:13:55] So it’s like millimoles of water per meter squared per second was leaving this leaf based on this little measurement that it takes for 30 seconds. And then you can scale that up if you know the leaf area or you can just look at it as a rate standardized across trees. So that would be looking at the stomatal conductance of water vapor.

[00:14:12] And stomatal conductance of water vapor is positively correlated and tightly tied to cO2 assimilation or photosynthesis. So we can use it as a proxy to think about how photosynthetically active that leaf might be. So the leaf porometer is really easy to use out in the field. You can tree climb with it and take measurements in canopies really easily.

[00:14:35] It’s light, it’s easy.

[00:14:36] Michael Hawk: You mentioned that you can kind of use it as a proxy, is it similar from uh, plant to plant? Like, is the ratio similar? Or is there like a table that you look up and you say, okay, for Redwoods, the ratio is X and for Black Oak, it’s Y.

[00:14:51] Lucy Kerhoulas: You’d probably want to do some, if you were really trying to get from stomata conductance of water vapor measurements, and you were wanting to relate that to photosynthesis, you’d probably in the lab want to measure how those two gas exchange rates compare to one another, and then you could apply that to your water conductance rates.

[00:15:11] Does that make sense?

[00:15:12] Michael Hawk: Yeah. Yeah. So there’s another step done in the lab to give you that conversion.

[00:15:18] Lucy Kerhoulas: Yeah.

[00:15:19] So that’s the next one. That’s called a LICOR 60, well, it’s the LICOR LI-COR 6400 portable photosynthesis system,

[00:15:27] I think. And

[00:15:28] Michael Hawk: oh yeah, I

[00:15:28] Lucy Kerhoulas: came out with the six,

[00:15:29] Michael Hawk: No,

[00:15:30] we all do.

[00:15:31] Lucy Kerhoulas: they recently came out with the 6800, which I would, I covet, but I don’t have. Um, so I have the 6400. I was lucky enough to inherit it with my professorship.

[00:15:41] But they’re like 50,000 dollars it’s kind of ridiculous. , I don’t think there’s a huge it’s huge demand, so they have to charge a lot for all their research and development. , anyway, through the LI-COR 6400, which is sort of a beast. it’s portable. It’s the portable photosynthesis system, but it’s kind of intense to do field work with it.

[00:15:57] Um, it’s heavy, it needs batteries. it’s just not as user friendly in the field, in my opinion, as the leaf porometer, but you can do a lot with the LI COR 6400 in the lab, and that will measure all sorts of gas exchange things. So, you have this little chamber that you put the leaf in and there’s all sorts of bells and whistles and accessories that you can buy for your LI COR, like a conifer chamber or a bryophyte chamber or a pine needle chamber.

[00:16:20] You can accessorize till your heart’s content with the LI-COR. , so you put your leaf in this little chamber and it can tell you how, and if you want more details I can go into the mechanics of how it works, but basically by comparing the leaf chamber gas concentrations to a known control chamber.

[00:16:40] They can crunch the numbers for how much CO2 is being assimilated or taken up by the leaf, how much CO2 is being released by the leaf in respiration, say if you put it in a dark chamber um, and it can also tell you how much water is leaving the leaf, so what the transpiration is doing.

[00:16:55] So you can get kind of three main measurements from the Li-COR . You can get CO2 uptake or photosynthesis, and you can get stomatoconductance of water vapor, and then by tweaking different conditions in the chamber, which you can set everything um, you can set or temperature, CO2 levels, you can tweak all these different things with the Li-COR , and see how the plant responds.

[00:17:16] You can also measure respiration, so kind of if you put the leaf in the dark, how much CO2 is it giving off, like what’s the metabolic cost of that leaf? There are some leaves that are more expensive than other leaves metabolically.

[00:17:27] Michael Hawk: Interesting. So what I’m trying to envision here is if you’re in the lab, are you, literally just putting singular or multiples of leaves into this testing device? Or are you using just maybe using small potted versions of the plants or how does that look?

[00:17:46] Lucy Kerhoulas: Yeah, you can get kind of mad scientist and there’s like an endless amount of different ways you could use the LI-COR. So, and there’s different leaf chambers, but say you are working with like a maple or something. You might have some maple leaves that you cut from a tree, like a big, foliar wand. It’s like this wand and there’s a bunch of leaves on it. And, a lot of times what you’ll do is make a cut hydro of that. So you take the wand and you cut the stem and then let it hydrate and so there’s no air in the line and now you have this really hydrated wand of foliage and you can say put one leaf in the Li-COR and take some measurements and then you can put a different leaf in the Li-COR and take some measurements and then maybe after three leaves take an average of that understanding that there’s differences in leaf structure and leaf physiology and so if you want an average assessment of that plant’s physiology you might take a couple measurements and average them.

[00:18:40] Michael Hawk: So the leaf continues. Yeah. So I guess the bottom line is the leaf continues to function similarly. as if it were still connected to the rest of the plant.

[00:18:49] Lucy Kerhoulas: Yeah, for a period so the cut hydros have in that example, the cut hydros have like a shelf life, know, how long will they be viable cut hydros? Or you could be using potted plants in a greenhouse, like set your Li-COR up in the greenhouse, and you have potted plants that say you’re exposing to some sort of plant torture experiment, right?

[00:19:06] Like we’re drowning these ones, or we’re shading these ones, or we starved these ones for nitrogen, and now we’re gonna put them in the Li-COR and like show me what you got, you know, what, what are you made of? What can you do after we’ve like done these different things to you?

[00:19:19] Michael Hawk: Yeah, that’s really interesting because I, imagine you could drought a plant or, you know, heat it up or, whatever. And, you know, one of the basic things I remember learning in Ecology 101 is how a lot of plants, when they’re heat or drought stressed, will close up their stomata.

[00:19:34] And I always assumed that you could just look at that, and see the stomata changing, but then you can replicate that in the lab as well, which I imagine would give you additional confidence in the other measures that you’re seeing too, uh, because you’re able to physically see this one behavior.

[00:19:52] Lucy Kerhoulas: I haven’t personally like visualized stomata closing and stuff, but um, I know like people do and can, but you can just also get a handle on what’s going on with the stomata via these measurements of gas exchange. You were asking how gas exchange is measured and so you have these two leaf level measurements and just really quickly there’s one other way that gas exchange can be measured and that’s at the forest scale using this thing called eddy covariance towers. And so basically, that is like you have a little LI-COR instrument basically at the top of the forest on this really tall tower, and using the same principle as the LI-COR in the leaf chamber, except it’s using these, packets of air that are moving over the forest. So you can see how much water is the forest transpiring. You can see how much CO2 is the forest uptaking and how much CO2 is the forest or oxygen letting go, releasing. So you can look at those gas exchange fluxes on a much larger scale using this other technique called flux towers sometimes you might hear about.

[00:20:50] Michael Hawk: So that makes me wonder when you mentioned eddies or packets of air, then I imagine you see kind of some variance throughout the day and you have to just kind of assess over a longer period of time to, draw conclusions.

[00:21:04] Lucy Kerhoulas: for the flux tower studies, those are usually many years. They’re really expensive to set up usually like a huge research project for a number of years. And um consideration You have to take into things like wind because that will affect kind of the footprint of air that the tower is is measuring, things like that. So it’s definitely a little bit more complicated than just clamping a leaf into a leaf chamber.

[00:21:25] Michael Hawk: Let’s jump in a little bit to climate and drought. Can you tell me? in a general sense, how trees tend to respond to a drought condition.

[00:21:33] Lucy Kerhoulas: drought is You know, a pretty big natural disturbance that we’re seeing, increasingly commonly in the Western United States right now. And really globally, I mean, droughts are are happening. not just, you know, the Western US, it’s a valid question. You know, how do trees respond to drought?

[00:21:50] Like, what’s going on? seen, especially in the Southern Sierras, really widespread drought related mortality. It’s important, I think, to understand how different trees will respond to drought and the different ways that this can affect forests. And also, if we understand how trees and different species will respond to drought, we can have a better idea of what might regenerate in the wake of widespread forest mortality, whether widespread forest mortality is coming from wildfires or wildfires. drought or, you know, bark beetles, And so there’s, different things that trees can do when faced with water stress, like when we’re thirsty, we can get up and move out of the sun or go get a glass of water. The trees, you know, are kind kind of stuck where they are. And so how do they handle it when the situation is drying around them and getting hotter. Um, and what we’ve been seeing recently with some of these droughts in the last 10 years or so is that not only are they dry, but they’re really hot. Um, so temperature is increasing a lot while the conditions are dry and plants really respond called VPD or vapor pressure deficit.

[00:23:00] And you can think of it as like how thirsty is the air, like how drying is the air, the drying or wicking capacity of the air. That’s kind of how the plant leaf experiences the world around it, is via kind of VPD. It’s a, it takes into consideration temperature and humidity. So it’s kind of like this silver bullet metric of describing The plant’s perspective of the world, and so, VPD increases exponentially with temperature just in the equation of VPD.

[00:23:28] So temperature is a really big effect on plant physiology.

[00:23:31] Michael Hawk: Is it right to think about VPD as kind of like almost a suction, applying to the plant. And the more water is being, pulled out of the plant, the more it has to be drawn up through the vascular system.

[00:23:43] Lucy Kerhoulas: exactly. Yeah when VPD is really high, the air is really thirsty. Like, it could hold a lot more water vapor than it is holding. And so it’s, it’s has a strong driving gradient to evaporate water out of leaves if the stomata are open. And so under drought conditions plants generally will close their stomata to conserve water.

[00:24:06] So if you picture water in the tree, like a rubber band, When the plant is really water stressed and VPD is really high there will be a lot of tension in that water column. So picture it like a rubber band and it’s being stretched really thin and really taut and eventually the cohesive nature of water, so these, hydrogen and oxygen molecules bonded together. Eventually, if there’s enough tension in that rubber band of the water column, it’ll snap like it overcomes the cohesive nature of water. And so when that happens, you now have cavitation or an air bubble in the tree vascular system, and it becomes really hard for the tree to slurp water from the root system up to the leaves. It’s like siphoning something with an air bubble in the line. And so if this happens enough throughout repetitive droughts, the hydraulic capacity of the stem of the tree can be really compromised. And so, a lot of times, plants will close their stomata to avoid cavitation, to avoid too much tension in the water column. Different species have evolved to have different thresholds of when they’ll close their stomata, when the water gets really stressed. So like juniper is really impressive, because it can keep its stomata open, even if that water column is under a lot of stress, and it the water column doesn’t cavitate.

[00:25:21] It has to do with kind of the wood anatomy, so we can see how wood, the different cells that make up the wood, the xylem cells, and the bordered pits, and all these different details about the wood anatomy, that can play into how resistant a species is to cavitation. And so this will play into its physiology.

[00:25:40] And I would say that’s kind of like the main approach that plants have when things get dry. It’s like kind of the majority of plants when stuff gets water stressed, they close their stomata to conserve water. And that’s awesome because you can serve your vascular system, but the plant then compromises uptaking CO2.

[00:25:58] And so this can lead to this kind of mechanism of tree mortality called carbon starvation. And it’s not this, you know, how a tree dies. What is the coup de gras, like the final blow that kills a tree? It’s not really like with us, we can say, Oh, yep, the heart stopped, you know, and the person is dead, but with plants, it’s not quite so cut and dry black and white.

[00:26:16] And so there can be different things that contribute to how a tree actually dies. But basically if they have to close their stomata for some, multi year drought to conserve water, then that means less and less CO2 is coming into the plant, and so they can eventually kind of deplete all of their carbon reserves and the tree can die of carbon starvation.

[00:26:35] On the other side of the spectrum, and it is a spectrum, it’s not binary. On the other side of the spectrum would be the strategy where the plant’s water stressed and it’s just going to be more risky and cavalier and leave the stomata open and flirt with hydraulic failure.

[00:26:50] So to keep CO2 coming in, the stomata stay open, photosynthesis continues, but the hydraulic system might cavitate and become very dysfunctional. And so that can be another way that trees respond to drought or drought induced mortality.

[00:27:04] Michael Hawk: When cavitation happens, is a tree able to recover from that?

[00:27:09] Lucy Kerhoulas: Yes, in certain species, they are able to repair embolisms, or these air bubbles, and so and there’s different kind of mechanisms of how that happens, but say, oaks come to mind as being really effective at refilling cavitated hydraulic lines. Generally speaking, the bigger the xylem cells the more vulnerable to cavitation. So oaks have huge vessels that like these big pipes that they move water through, as opposed to say like a conifer with these tiny little trachid cells. The more water you can move, like kind of the riskier it is in terms of cavitation.

[00:27:44] There’s like that trade off.

[00:27:45] Michael Hawk: Interesting. And then, I imagine the way this would show up from like a dendrology, dendrochronology standpoint would be slower growth during those drought years. Are there other metrics or other measures that can help you assess whether slow growth is related to drought or something else?

[00:28:03] Lucy Kerhoulas: So under drought conditions, that’s one of the things that we can do in dendrochronology, kind of using the study of tree rings. We can go back in time and assuming from what we can see in, in times where we know there was drought, yes, growth is generally reduced. So we’ll see big fat tree rings in years when whatever the limiting factor to growth was big.

[00:28:24] So usually it’s water in the Western US is the limiting factor for trees. And so you can go back in time in the tree ring record and look at these patterns of oh, look, here was really narrow rings, it was probably a drought. Or look, here was really fat rings. These were probably big, wet rainfall years. You can look at the stable isotope composition of tree rings, which we haven’t talked about, yet but basically, looking at the stable isotope composition of tree rings can also tell you things about what kind of water stress trees were under when they made that ring of wood.

[00:28:54] Michael Hawk: I wanted to get there. Tell me more. What are you looking at specifically when you’re looking at the isotopes in the tree rings?

[00:29:01] Lucy Kerhoulas: The enzyme that does photosynthesis, Rubisco, it’s in the ; leaves of plants. And when, through the stomata, CO2 is coming into the plant, rubisco is this enzyme in portions of photosynthesis where it will magically take the CO2 molecule from the abiotic world and bring it into the biosphere.

[00:29:19] It makes it into an organic sugar through photosynthesis. it’s a really big deal. In our universe, there’s multiple versions sometimes of different elements. So for carbon, like 99. 8 percent of carbon, as we know it out there is C12. It has an atomic mass of 12. But then there’s a heavy version of carbon called C13 that has an atomic masses of 13 because it has an extra neutron.

[00:29:46] And it’s a really small percent of the carbon out there, but Rubisco has a higher affinity for CO2 molecules with the C12 carbon atom, as opposed to CO2 molecules with the C13 carbon atom. And so when life is great and the stomata are open and lots of CO2 is coming into the leaf, Rubisco can be choosy.

[00:30:06] It can choose and select for CO2 molecules with the C12 atom. But when times get tough and stomata have closed and there’s less CO2 coming into the leaf, rubisco, beggars can’t be choosers, it has to settle for more CO2 molecules with the C13 heavy version of carbon. So we can go back in time and tree rings and look at the C13 signature and try to use that as a proxy for, oh look, the tree was under water stress during these times.

[00:30:35]

[00:30:35] Michael Hawk: Now as I understand it, looking closely at 12 and C 13 helps us understand a bit about climate change.

[00:30:42] Can you explain that?

[00:30:44] Lucy Kerhoulas: Oh, so the smoking gun? Yeah, so one thing that’s kind of interesting that plant physiologists have kind of put together with C13 research in tree rings is that because Rubisco discriminates against C13, plant material is by nature depleted of C13. It’s relatively light of C13. We know since the onset of the Industrial Revolution that our CO2 concentrations in our atmosphere have been increasing, and we can see this from the data that started being collected in the 1950s from the Mauna Loa Observatory, and then before that, we can see this rise in CO2 concentration in our atmosphere via ice cores. And so we can track this, and it’s yep CO2 concentrations are increasing in our atmosphere, and we think this is what might be contributing to global warming via the greenhouse gas effect. But some folks think, oh, I don’t know, I’m not totally convinced, the increasing CO2 concentration in our atmosphere, it might just be natural variation in our climate, if you go back into the 500 million years ago and look at stuff, we think that the, parts per million of CO2 were 3 to 9, 000 and right now they’re at 420.

[00:31:57] What’s the big deal? Maybe this is just natural variation. It has nothing to do with human activity. And so interestingly, what we can see though, via ice cores, the air trapped in them, going back in time, is that since the onset of the industrial revolution, we can see that the C 13 signature of our atmospheric CO2 has been changing since we started burning fossil fuels, which are largely plant based.

[00:32:23] And so we’ve seen, via burning fossil fuels, that the isotopic signature of our atmospheric CO2 has depleted. It’s become lighter because we’re burning material that already discriminated against C13 via rubisco, if that makes sense. If you have follow up questions, feel free, but that’s kind of, so some people have called this the smoking gun.

[00:32:47] This proves that the increased CO2 concentration in our atmosphere is from burning plant based materials that have this light c13 signature.

[00:32:57] Michael Hawk: That’s really interesting. You can see that signature and can you actually measure that signature, within the plant tissues as well?

[00:33:08] Lucy Kerhoulas: The atmospheric signature?

[00:33:10] Michael Hawk: I’m wondering if you can take a look at the C12 and C13 ratios that gives you an indication as to whether, a plant was drought stressed or not. Do you, can you see long term trends?, Looking at a broader time range within the plant tissues, like again, kind of a dendrochronology question, that also show this ratio changing because of the, atmospheric carbon changing over time.

[00:33:37] Lucy Kerhoulas: It’s a great question, you’ve got to be careful if you’re looking at long time series of isotopic signatures in tree rings because the atmospheric signature has been changing. So how people get around that is oftentimes they will flip the equations around and look at instead of looking at the actual delta signature of C13 entry rings, we will look at it as a metric called discrimination.

[00:34:04] So we can calculate how discriminatory rubisco was. So it takes into consideration at each annual step what the atmospheric CO2 concentration was, if that makes sense. And so we can, instead of looking at the actual signature of C 13, we’ll look at how, at the discrimination rate, at an annual basis. And that can remove that long-term trend of a shifting atmospheric CO2 signature.

[00:34:29] Michael Hawk: Got it. So do from your work, are there any other indicators of past climate conditions that, you can assess from looking at the trees? I mean, we’ve talked a little bit about tree ring widths and the carbon isotopes and so forth. Is there anything else that you can discern?

[00:34:45] Lucy Kerhoulas: Just to clarify the question, so what else can we tell about past climate from tree rings?

[00:34:51] Michael Hawk: From tree rings or your study of plant physiology? It’s a very broad question.

[00:34:57]

[00:34:57] Lucy Kerhoulas: There’s one other thing that you can tell from tree rings. It’s kind of cool that you can use to kind of go back in and understand climate better. So there’s a whole field called dendroclimatology, where you can use tree rings to try to reconstruct past climate. So the instrumental climate record that generally goes back to 1895 and then, if we make a relationship between tree ring growth and some climate variable like precipitation or maximum temperature or something like that, you can then once you’ve used the instrumental record to make a nice tight relationship between tree growth patterns for a certain species in a certain area and that climate variable, you can then use the tree ring record to go back like 5, 000 years and try to reconstruct that climate variable, which is pretty cool.

[00:35:42] Another thing you can do with tree rings that’s neat is you can look at not just the C13 isotopic signature, but you can also look at the O18 signature. So water being hydrogen and oxygen. You can look at the isotopic composition of oxygen in the wood of tree rings going back in time, and this can give you some insights into what water sources the tree was using. Was it using winter snow water, or monsoon rain water? Anyway, so you can try to go back and understand better about how plant water sources might have shifted over time.

[00:36:18] Michael Hawk: That’s fascinating. I didn’t know about that one at all.

[00:36:21] Lucy Kerhoulas: Yeah, and another thing you can do is look at the ratio, like within an annual ring, when you look at tree rings, there’s light wood and dark wood, and that’s, early wood that’s produced early in the growing season is light in color, and then late wood that’s produced late in the growing season as the tree is winding down for the dormant season, it produces a darker, generally narrower band of wood, and so you can look at the ratio of early wood to late wood, and how that maybe has changed over time and try to do things like reconstruct monsoon precipitation going back in time if we think late wood is being fueled by monsoon rain or something like that.

[00:36:58] So there’s different nuances that you can also try to tease out using tree rings about precipitation regimes going back in time. Plants can acclimate to climate in real time, so there’s like real time acclimation that could be happening within a plant’s life, or there’s longer term adaptations to climate that, can be the result of long term acclimations.

[00:37:20] Michael Hawk: So you described some of the kind of real time adaptations that trees have when they are heat or drought stressed. Are there trees that are more highly adaptable just in general, based on their evolutionary background or other factors that allow them to maybe have a wider, degree of capabilities?

[00:37:41] Lucy Kerhoulas: I’m not an expert in like looking at across plant taxa which species are more plastic in this but, in terms of their physiological responses, but just kind of broadly speaking, for sure, different plant species can acclimate physiologically. Sometimes the terms acclimate and Adapt are sort of used interchangeably, but here I’m using them in a specific way that acclimation is kind of real time, within the lifetime of a plant it could acclimate to a changing environment.

[00:38:11] say a neighbor tree gets chopped down, and all of a sudden there’s way more light hitting a tree. That tree could acclimate to that higher light environment by doing different things, whereas an adaptation might be happening over many generations. So anyway, just a clarification on those two terms that sound kind of synonymous.

[00:38:30] So within a species, depending on temperature, light, moisture conditions, depending on how those conditions varied when the plant or leaf was developing, there can be different physiological optima. So say for one species, you could have a plant, the same species, one plant, grew in a really hot environment, and one plant grew in a really In a much cooler environment, those, two plants, even though they’re the same species, they might have really different, temperature optima for doing photosynthesis. This shows kind of this plasticity with physiology. So I think within plants and certain taxa would be more plastic than others. I think taxa that are really widespread, say, like Douglas Fir or something that, occurs really broadly throughout the west. That probably has a lot of physiological plasticity to allow it to exist in all of these different environments, whereas a species that maybe has a very restricted range, let’s say like Port Orford Cedar or something like that, it probably, I would venture to say, is less plastic in terms of adapt, or in terms of acclimating its physiology to, different conditions.

[00:39:44] Michael Hawk: Would it be fair to say, just thinking about a spectrum here, we didn’t really talk about what happens if a tree is exposed to extreme wet conditions and how it reacts to that. If a tree is very drought tolerant, is it going to struggle, with wet conditions by and large, or is that just a totally separate relationship?

[00:40:06] Lucy Kerhoulas: That’s a good question. I haven’t really I pondered that one too much. I think when I think Of really wet soils and, the big thing that comes to mind for trees is increased wind throw. So like if the soils get really wet and are really saturated, it can cause root death. Say a big flooding event happens, there are some species that can tolerate that.

[00:40:25] Like redwood can tolerate kind of having submerged roots for long periods of time. But a lot of other species, the roots It’ll get anaerobic, like no oxygen if they’re flooded. And so that can cause root death, which could cause structural instability in the tree. And also if the soils are really wet, like a wind event, because the soils are usually really wet in winter, and there can also be heavy wind storms in the winter.

[00:40:48] And so that can cause a lot of tree overthrow, where trees are literally just blown over.

[00:40:53] Michael Hawk: I would love to learn about how trees respond to fire and what the range of responses might be, I’m not thinking about the types of fires that we’ve seen too many of lately where the trees literally burn to the ground, though I guess even then there’s a physiological response for some trees that can like stump sprout and, things like that.

[00:41:12] Lucy Kerhoulas: Yeah, there can be, we can think about fire ecology and trees in different ways, and I think there’s different fire strategies, just like there are different drought strategies. Again, the plant is stuck where it is, it’s just got to kind of deal with whatever comes its way as far as disturbances go, and so there might be different stomatal regulation strategies or different wood anatomy strategies to kind of deal with water stress. And similarly, there’s serotinous species, say like knob cone pine is one that comes to mind where you have these cones that are only going to open, under extreme heat, a fire will melt the resins and then the above ground portions of the tree just keel over, like they don’t even try to be fire resistant.

[00:41:55] That’s the strategy is the adults are going to burn to a crisp and in the wake of that and all of that newly revealed real estate after the fire, those seeds, the knob cone pine seeds are going to release and be the 1st on the scene, to establish and be the pioneers there. So you can have that strategy where you basically aren’t fire resistant and you’ll have some sort of, seed bank ready to go. Or, some tree species are sprouters. So after a disturbance like fire comes, the fire, or a lot of the oaks, the fire might burn the above ground portion of the tree to a crisp, but instead of having some, serotinous seed bank ready to go, these species will have underground reserves that are going to allow them to sprout.

[00:42:41] And they can really have an upper advantage, in a situation following fire like that because they’re not having to establish from seed, but they already are tapped into this kind of mature adult root system with all of its mycorrhizal symbionts, like they’re basically ready to go. And so they can really hit the ground running and take off. These sprouting species like redwood is a sprouter, a lot of the oaks, manzanitas, there’s lots of species that do this, a lot of the hardwoods.

[00:43:08] Redwood’s kind of unique in being a conifer that will do it. And then generally, kind of speaking, what are the physiological effects of fire on forests? a lot of it will depend on kind of the severity of the fire. fire has become a different beast in modern times than what a lot of these plants evolved with, so due to high fuel loads from fire suppression and high levels of drought related mortality, due to higher temperatures, a longer fire season with earlier snow melt happening in the early spring, it makes the fire season a lot longer.

[00:43:41] The fuels are drier because of droughts, so it’s kind of this perfect storm of fires in recent years have been. And so what can happen there is they’ll really sterilize the soil. So any sort of like seed banks that might have been there in the soil, they are burned to a crisp. And also a lot of the mycorrhizae, the different fungal symbionts that are in the soil, which are so important for a successful seedling establishment.

[00:44:08] And most terrestrial plants rely on these mychorizal symbionts depending on how intense and hot the fire is, it can just sterilize all of the soils, which can make forest regeneration really challenging. And then, couple that with the fact that the climate is really hot and dry right now. A lot of times reestablishing a forest in the wake of these fires can be really challenging. Oftentimes we’re seeing that many of the forests that burn at these high intensities and high severities that they are regenerating as shrublands rather than forests. And so we see a real shift in vegetation and community species compositions. There’s, kind of like with the mechanisms of drought mortality, there can be different ways that fire can kill plants.

[00:44:53] So generally speaking, I think exposure to 60 degrees Celsius and higher is going to kill the living tissues of plants. And there can be different adaptations, like really thick bark in giant sequoia or redwood that can try to, buffer the cambium, like the living portion of the stem from these high temperatures, but, heat from the fire especially if there was a lot of fuel that had accumulated around the base of a tree, that can increase the residence time of the fire at the base of the tree, and that can lead to increased mortality rates.

[00:45:28] Also, just with forest management, how it’s been for the past 100 years or so, there are a lot of ladder fuels where fire used to kind of roll through on the forest floor, and now, because of these different ladder fuels, a lot of times the fire is able to get into the canopy and burn the leaves, which can then be really hard for the plants to rally after that.

[00:45:48] Interestingly, there was just a paper that came out in Nature on redwood and some of the Santa Cruz fires using, C14 isotopes, looking at these. ancient trees and they burned to a crisp. It was a really intense fire in the Santa Cruz Mountains and I’ve never seen anything like it.

[00:46:05] Michael Hawk: Okay. I’d like to fill in a couple of things here. So we’ve mentioned 12 C 13 and now C 14. And yes, as mentioned earlier, these are all different carbon isotopes. And the number represented is the atomic weight. That’s the sum of the protons and the neutrons. So C 14 compared to the other two isotopes mentioned earlier, adds another neutron, giving it eight total, along with six protons. Now it actually has an unstable nucleus.

[00:46:34] So C 14 is radioactive and it undergoes predictable decay. So this property makes it useful for radio carbon dating. That’s a technique used to det