Episode 42: Can trees store water long-term in heartwood?

Episode 42: Can trees store water long-term in heartwood?
 

Traditionally, models of tree water consumption rely on evapotranspiration —a process describing the movement of water through the soil, plant, and atmosphere—as an indirect measure. However, these models may lack critical components, leading to inaccuracies in prediction. In this episode, we are joined by Lauren Tucker, a PhD candidate at Idaho State University, who discusses her team's groundbreaking research on how different tree species store moisture deep within their heartwood for long-term retention.

Notes

Lauren is a PhD candidate at Idaho State University focusing on plant physiological ecology. She received her bachelor’s and master’s in biological sciences from Cal Poly Pomona. She was a 2022, recipient of meters G.A. Harris Award. Her research has focused on long-term water storage in trees and its importance for whole tree water relations at both tree and ecosystem scales.

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Transcript:

BRAD NEWBOLD 0:00
Hello everybody, and welcome to We Measure The World, a podcast produced by scientists for scientists…

LAUREN TUCKER 0:07
So this has informed us a lot about daily use of storage, but it’s also a way that is kind of inferring how much water storage use happens in a day. You’re not directly measuring how much stored water is inside of the tree. And so a cutting edge method for that is using sensors that have been developed for being used to measure soil volumetric water content. And instead of putting them in the soil, you put them in the tree to measure tree volumetric water content.

BRAD NEWBOLD 0:37
That’s just a small taste of what we have in store for you today, We Measure The World explores interesting environmental research trends, how scientists are solving research issues and what tools are helping them better understand measurements across the entire soil, plant, atmosphere continuum. Today’s guest is Lauren Tucker. Lauren is a PhD candidate at Idaho State University focusing on plant physiological ecology. She received her Bachelor’s and Master’s in biological sciences from Cal Poly Pomona, and she was a 2022 recipient of METER’s GA Harris Award. And today she’s here to talk about her research into long term water storage in trees and its importance for whole tree water relations at both tree and ecosystem scales. So Lauren, thanks so much for being here.

LAUREN TUCKER 1:21
Yeah, thanks for having me.

BRAD NEWBOLD 1:23
So first, we’d love to get into a little bit of your own personal background, and just wanting to know how you got into sciences in general and eventually into your specialty of plant physiology and ecology, hydrology and all those things?

LAUREN TUCKER 1:36
Yeah so my involvement in science and my specialty in particular started in during my undergrad at Cal Poly Pomona in Pomona, California. It was a specific semester where I took two upper division courses simultaneously. I took a plant ecology course and a plant anatomy course. And I really enjoyed the way that the information from the courses kind of weaved together, where we were learning about different ways that plants respond to their environment, and also structural characteristics that they have in order to function and survive in their environments. And that plant anatomy course was taught by Dr. Frank Ewers, and I started doing undergraduate research with him after that course, which that research then evolved into my master’s research with Dr. Frank Ewers as my master’s advisor, where, for that research, I studied at the Southern California black walnut tree, which is a species that’s endemic to California, and I studied its response to drought, fire and biotic attack. And so, in the past, had been studied for its ability to re sprout after fire. And so you have a fire come through, and then re sprouts come up after and in a study by Otsub et at all in 2010 they found that fire re sprouts have like a hydraulic and a photosynthetic advantage compared to unburned adults. And so in during the 2012 to 2016 extreme drought that happened in California, it was observed to be re sprouting from a different disturbance, which appeared to be due to drought which caused the adults to die back, and then they re sprouted. And so that also appeared to be related to a canker disease that they were experiencing. And so I basically monitored those drought re sprouts, and I studied them and looked at their recovery after the drought, which they did appear to show, similar to fiery sprouts of the past, a hydraulic and photosynthetic advantage, but that really seemed to be associated with them receiving rainfall prior to that recovery. Also during my master’s, I had the opportunity to teach laboratory courses, and I really enjoyed teaching. I liked trying to get students to understand information and seeing kind of like the light bulb go off when they got it. So I graduated with my master’s degree in May of 2020, and then from there, I kind of learned from my master’s that I liked doing research in plant eco physiology, and I also like to teach. And so that gave me the ambition that I still have today, which is to be a university professor. So based on that, after my master’s, I looked for PhD programs, and I learned about how Dr Keith Reinhard, here at Idaho State University where I am now, was looking for a PhD student to start in January 2021, for a project that he had recently gotten funded through the NSF. And so I met with Keith through zoom, I visited ISU, and then I sent my application, and then I started as a PhD student here in January 2021, and now I’m in the fourth year of my PhD, and I’m planning to graduate next year.

BRAD NEWBOLD 4:32
Well, good luck, and congratulations on all of that as well. So let’s, let’s dive in then to, I guess, to your dissertation research and the affiliated National Science Foundation funded project that you’re working with here, looking at water storage in trees. Can you tell us a little bit about that project, just kind of how it got started, and what problems or what are you really looking at there?

LAUREN TUCKER 4:56
Sure so I can describe the overall, larger project. Project and the goal, and then I can kind of give a background into my research specifically for my dissertation, I’ll say that this is a NSF funded project that includes 11 people, and it’s collaborative. So it’s people from Idaho State University, University of Georgia as well as Duke University. And really the overall objective of this project is looking at long term water storage in trees, and kind of quantifying how much of this long term stored water exists in trees at different heights and different depths, so at a spatially and temporally intensive scale. And so a big portion of this project involves doing tracer experiments, where we inject a deuterated water tracer into the base of trees, and then we have in situ sampling going on at different heights in different depths, where we’re sampling water vapor, and then we’re sending it to a water isotope analyzer, a Picarro. And then we’re kind of tracking where does that tracer go after we inject it into our different species that we’re studying. And then also part of this is we’re studying or measuring sap flow and volumetric water content, also at those different heights and different depths. And so that kind of makes up what my dissertation research is. And so I plan to talk about that. And also, kind of like some results from the larger project, maybe sprinkled in as well.

BRAD NEWBOLD 6:17
Could you tell us a little bit about, I guess, kind of the technical side of the things you talked about, deuterated water. Can you explain a little bit what that might be for those in our audience that might not know the technical term for for heavy water, especially when you’re dealing with your isotope studies afterward, and how, how you’re able to track that, to trace that within, within the trees?

LAUREN TUCKER 6:38
With water, hydrogen and oxygen, those have different stable isotopes. And so what we see with hydrogen is that it’s heavier because it has two neutrons on each of those hydrogen and so that’s resulting in that heavier weight. And so hence why we call it that. And so how we basically trace that is that we send that water to a Picarro isotope analyzer, and it basically gives you kind of an idea of how much of that heavy water, or that deuterated signal is inside of the water. So it gives you an isotope concentration, essentially that allows you to measure it.

BRAD NEWBOLD 7:15
Got it, and so you’re taking those measurements you said you’ve injected at the base of the tree, and then you’re taking those measurements along the trunk of the tree is that is that how it’s going?

LAUREN TUCKER 7:24
That’s correct. So we have three different heights. We have a low height, a mid height and a high height of the tree. And then at each of those heights we have three different depths that were monitoring the isotope concentration of and so those three different depths being the shallow sapwood, the deep sapwood, and the heartwood of the xylem.

BRAD NEWBOLD 7:43
Before we move on. I was just wondering, just kind of as a background when it comes to long term storage of trees, like, how did that understanding come about in the first place, where we, where we were able to figure out or find out, or discover that trees actually store water within themselves, as opposed to just kind of transporting and transpiring water in general?

LAUREN TUCKER 8:04
Right so there’s a couple different ways that we’ve studied water storage and trees, I would say, from this standpoint of knowing about long term water storage in trees, this has come from tracer studies, where we inject the tracer into trees and then monitor it over time. This could involve taking it from collecting a leaf, say, and looking at the isotope concentration in the leaf after that injection. And it’s been found that in certain species, we can see that the tracer exists for weeks to months. So it’s existing for a long period of time. And so it’s kind of going against this idea of water goes up into the roots, up the stem, out of the leaves in a couple days time. But rather, it’s this idea that what we don’t think about often, which is that there could be radial or lateral transport going on in the tree as well, resulting in this long term storage.

BRAD NEWBOLD 8:57
Where exactly is is it being stored in relation to the transport areas of of the tree?

LAUREN TUCKER 9:04
So there’s evidence that, from these studies, or a few studies recently, that you can have water storage at deep depths in trees. So in in the xylem, you have your sapwood and you have your heartwood. And so recently, there’s been evidence that there could potentially be storage inside of the heartwood, or deep depths in trees. And so with those trees, you could have that deep storage. And maybe when drought or water stress conditions come along that can essentially be like your savings account, and you tap into that savings account when you need it. So maybe certain species are advantaged in the future, when water stress conditions take place, you also have storage that can happen in, say, like the sapwood, in the cells of the sapwood of the xylem as well.

BRAD NEWBOLD 9:55
Within your project, within your research, what exactly are you measuring? And how are you measuring it?

LAUREN TUCKER 10:01
As I mentioned earlier, I’m measuring sapwood or sap flow, sorry, and volumetric water content at multiple heights and depths in four different tree species that have different hydraulic strategies in two different environmental systems. And so to kind of give a backdrop as to why I’m studying this and why I’m taking those measurements, I’ll give an explanation. We’ve long known that substantial amounts of water can be stored inside of trees, and that that water in those storage compartments can contribute to transpiration, but most of our knowledge about water storage and trees comes at short term or daily time scales, where we know that at hourly to daily time scales, the stored water can contribute to transpiration. And when it comes to storing water, storage in trees, it involves specific methods, often using sensors in innovative ways in order to study it. And so one of the classic methods is that you are measuring sap flow at two different tree heights. You have it at the base of the tree, and then you have it up at higher heights, say in the canopy or the upper trunk. And then what you do is you look at the synchronization of sap flow at those different heights. And so what it’s been found is that in some species, you see that in the morning, when sap flow initializes or starts to increase, we see that it happens first at upper heights in trees before the lower height. So that’s suggesting that they’re not taking up water from the soil yet. Rather, they’re taking water from storage to support that sap flow at higher heights, and then vice versa. You can have instances where sap flow is greater at the base than at the upper height, and so that’s suggesting that there’s a refill of storage going on. So this has informed us a lot about daily use of storage, but it’s also a way that is kind of inferring how much water storage use happens in a day. You’re not directly measuring how much stored water is inside of the tree. And so a cutting edge method for that is using sensors that have been developed for being used to measure soil volumetric water content. And instead of putting them in the soil, you put them in the tree to measure tree volumetric water content. So this is a way where you’re looking at how much water is existing inside of the cells in the tissue, how much stored water is there. And with that measurement, you you’re making a direct, continuous measurement. And so you can look at stored water at daily time scales, as well as more seasonal timescales. And so we can see a daily time scales, when a sap flow goes up. We see in species that use stored water, volumetric water content goes down, so they’re tapping into that stored water to support transpiration. Or you can see, there’s evidence over a season that you can have species that will volumetric water content will kind of gradually decline, showing that they’re tapping into stored water to support sap flow and transpiration. So with that, what I wanted to do, or for my dissertation research, I’m kind of doing this at a spatially and temporally and intensive time scale. So I’m looking at this, these measurements at multiple heights in multiple depths. And so my four tree species are loblolly pine and Southern Red Oak in Raleigh, North Carolina, which represents my music site. And so I looked at that these measurements in the summer of 2021 and then I also have trembling Aspen and Douglas fir here in Idaho. And so I studied that at a site here in southeastern Idaho, representing my semi arid site in summer 2022 and so really, my goals are, I want to look at how does stored water use vary between these different species. What are the dynamics of stored water use over summer season? What can we see in changes as the summer season goes on? And then also to with this newer method of using volumetric water content to estimate stored water use. How does that compare to this more classic method of using sap flow to estimate stored water use in trees.

BRAD NEWBOLD 14:05
Just really quickly, let’s talk about the species. Any particular reasons why you chose those species was just easy, and that’s what was what was around. I see that, you know, when you’re talking about in your different locations, you have pine and oak in one and you have fir and Aspen in the other. We’re dealing with Evergreen versus deciduous. We’re dealing or, you know, if you want to get fancy, angiosperms and gymnosperms, that kind of stuff. So do you want to talk about a little bit? Because you you mentioned that that they have different hydraulic strategies. I don’t know if strategies the right word is that. Is that personification, from what you know, knew beforehand of the differences of those trees, why they were chosen, and then maybe we can get into a little bit of the results of what you were seeing in those differences?

LAUREN TUCKER 14:48
So it was a big motivation, was to have an angiosperm and a gymnosperm in both the music and the semi arid site, and so they have different wood anatomy, so that. Was inspiring that and that influenced that choice. There were also species that were, I guess, around and accessible in our different sites. So at our site in Idaho here, there was Aspen and Doug fir, and then there’s, you know, we had a good selection of red oak or Southern Red Oak and Loblolly pine at our site in North Carolina. And so I think that that kind of inspired those choices.

BRAD NEWBOLD 15:27
I also wanted to touch on you, you mentioned about using water content sensors, number one, because that’s a very unique application for using water content sensors. Again, like you mentioned, these water content sensors, especially the ones that we have here at METER. Our TEROS line are made for for soil, measuring soil water content. How did it come about where, where you decided, hey, let’s, let’s see if we can use these in measuring water content within within the trees?

LAUREN TUCKER 15:54
Using volumetric water content sensors in trees is a newer method, and it’s been developed and written a lot by Ashley Matheny. And so a benefit of using these volumetric water content sensors is that you can get at how is stored water different at different depths. And so that’s a big inspiration for using these sensors to understand stored water in trees. And so in the case of us, we had three different tree heights, a lower height, a mid height and upper height. And at those different heights, we had two depths. So we had a shallow depth and we had a deep depth. And how we use these sensors to get at looking at different depths is first we take a tree core of the tree, and then we take that tree core out, and we look at it. And what we looked for is we looked for the sapwood, Heartwood boundary. And so we were looking for what’s the depth of that sapwood, where does the heartwood begin? And so with those shallow sensors, we take the sensor and then we cut off the tines using a Dremel. And so for those shallow depth sensors, we made them the correct depth in order to measure and isolate just volumetric water content in the sapwood, and then with those deep depths, we made them long enough in order to measure for them to reach into the heartwood.

BRAD NEWBOLD 17:08
You’re actually physically modifying the sensor itself in order to be able to measure that. As opposed to cutting a chunk out of the tree and shoving it in, inserting it a little bit deeper you’re actually, yeah, modifying the the sensor. That’s that’s cool. That’s interesting. Have you come across any challenges with with either the installation process? Is there any special calibrations that you need to use with that? Have you gone about all that kind of stuff?

LAUREN TUCKER 17:37
Sure. So when it comes to the installation, what you do is you drill holes in trees, and then you kind of lightly hammer them in with a mallet. And so I guess when it comes to doing that installation process, I think a main challenge came with just setting up the sensors and having multiple sensors at different heights in different depths. And so I guess I’ll get into the challenges of, kind of just setting up the field setup, and then I’ll go back to talking about calibrations. So a goal with this was to have sensors at, you know, multiple heights and multiple depths. And we kind of did this at a scale that we don’t know before, or have knowledge of people doing this before, and one of the reasons is that it’s hard to get up to high heights and trees, and so you can do that in a few different ways. You can use a scissor lift, you can use a crane, you can arbor or climb the tree. And we ultimately decided to use a boom lift, because it would allow us to get from tree to tree easily. And so what came along with that was needing to select trees along the road, and also those had to be a level flat that had to be a level flat road, because you want to, you know, not be tipping the boom lift. And so that was fairly easy in our music site, but a little more difficult of finding the right trees in the right site, in our more mountainous, semi arid site here in Idaho. And then I think another challenge which came along with that was we have those sensors up at high heights, and we needed to make like wiring connections all the way to get from those sensors up high in the tree down to the data logger at the ground level. And so that could be one connection, that could be a few connections. And so I never had experience working with data loggers before my PhD. And then I kind of got this crash course in how to use them in wiring and figuring out sensors and figuring out when you have a NAN or not a number. And so generally, there was always, once we got all the sensors set up, was troubleshooting those NAN’s and those sensors that were giving issues. And so I would have kind of this process of elimination. And so first I’d be like, okay, is the program that’s running? Is that good? Are there any issues with the sensor itself? So I would take a data logger up to a sensor up on tree, and I’ll say that every time I did that, the sensors worked perfectly fine. More often than not, it was that you had those connections along the way and kind of troubleshooting the wiring and the connections, there was often a spot where there wasn’t a good connection. So I’d bring a data logger up to each collection on the way up or connection and make sure that it was working. And then eventually that would get me to having sensor, all the sensors, up and running. So I guess a challenge is getting up high, installing these sensors, troubleshooting the NAN’s. And so you might think, okay, you have eight trees. That’s not a very intensive sample size. But when you have all these different heights and depths at each height, it takes time and effort for each tree to set up. And so I guess that wasn’t really a challenge to problem solve. It was more of just like accepting the process that it’s taking a little bit longer than expected. Once we had that data right, you do need to calibrate the sensors, because you’ve cut down those sensors to those specific lengths. And so that’s kind of what we’re doing is we’re we’re making calibrations that calibrate the sensor to that specific wood type as well as to that length of sensor. So we needed a tree diameter or stem diameter segment that was essentially large enough to fit our sensors in. So unfortunately, that resulted in cutting down a tree of each of our species. And so in the case of North Carolina, that meant having those STEM segments or those chunks mailed to us here in Idaho. But in Idaho, it meant just going to a local site and finding some trees for that, and then we would bring them back to the lab, and we would submerge them in water for a few days, kind of just to get them as saturated as possible. Then set them up on a lab bench and let them dry down. And then, before, then, before you have them dry down, we would drill holes in the trees or in the STEM segments, install the sensors, let them dry down, and then periodically take measurements of the weight of the stem segment, and then also taking a reading of what’s the sensor VWC, and then we would look at the relationship between gravimetric volumetric water content of the stem with sensor VWC on the x axis. Determine that relationship. And then we would apply those equations to our data to get accurate VWC data for each different sensor length than each species. And so that was that was something to figure out in itself. Was just the proper method of doing that, doing it all correctly, to get accurate VW C data at the end.

BRAD NEWBOLD 22:33
That sounds exciting. It’s quite, yeah, like you said, it’s quite an intensive process to be able to get that to work, especially, yeah, when you’re trying to use, like you mentioned, a relatively new process and measurement system and trying to get it to work for for your particular research there, do you remember which, what the names of the sensors were that you were using?

LAUREN TUCKER 22:56
Sure, from METER, they were TEROS 10, and then we also had Campbell Scientific volumetric water content sensors. And so that was CS616, as well as CS650 if I’m remembering correctly, the other model. And so I would like to add that in 2021 that summer, I only had sensors at a shallow depth and a deep depth at the lower height. And then in 2022 I was awarded the GA Harris Fellowship. And so that allowed me, in summer of 2022 to have sensors at two different depths, at all of the heights for each tree. So that was really cool and useful to get those sensors from you guys.

BRAD NEWBOLD 23:42
Yeah, definitely, especially when you’re trying to track how, how the difference is, like you said, you’re dealing with spatial differences within within the tree itself. You definitely want to be able to to measure, like you said, at lower, mid and upper levels of the tree. That’s good into some of the results. What have been the results so far? Where are you learning? We’ve mentioned a little bit about the species that you’re working with. Can you talk a little bit about any significant variability in water storage capacity among these different trees, and maybe what factors are contributing to that?

LAUREN TUCKER 24:17
So I’m kind of in the depths of data analysis still for this, this portion of my dissertation research, but I can share what I know so far. And so at this point, I can talk about, how does stored water use vary between these different species, and what’s the dynamics of that stored water use over the summer season. And I can also kind of talk about how our measurements of stored water use using this volumetric water content sensors, this newer method kind of compares to those classic methods of using sap flow. I’ll start off by saying that in all four of the species that we studied, so that was loblolly pine, Southern Red Oak, trembling Aspen and Douglas fir, all of those species exhibited some fluctuation of volumetric water content. In the day to some extent. So that suggests that they’re all tapping into stored water, using stored water to some extent. But those fluctuations were considerably larger in the angiosperm species compared to the gymnosperm species, suggesting that the angiosperm species are using more stored water. And for our North Carolina site in summer of 2021 what we saw is that, as you would expect, the conditions were warm and they were human and humid, and there were numerous rain events that happened during that that period. And so with that, what we’d also expect is that the peaks of sap flow of our different trees were kind of maintained and consistent across the summer season. There wasn’t really any decline in sap flow. And what, what we do see, though, is that in our oak tree, which is showing those, you know, larger volumetric water content fluctuations, that after a rain event, those fluctuations would be smaller or small list, and then they would gradually in time, as things are kind of drying down a bit after that rain event, those fluctuations would get larger. And so based on that, I was excited in that I wanted to calculate what stored water use in a day and refill in a day using this volumetric water content data. And so how I did that is, for each day, I found the maximum VWC and I found the minimum VWC, and then to learn about stored water use, I took the difference between maximum minimum in a day, and then to learn about refill, I took the difference between the minimum VWC, and then the maximum VWC of the next day. And then what you can do is you can take that volumetric water content value, and then multiply it by the volume of the tree. So in the case of our shallow depth sensors, we’re multiplying it by the volume of the sapwood. But then in the case of those deep sensors, where we’re multiplying it by the value of the sapwood and the heartwood both what’s their volume together. And so based on that, as we would expect from what I said about the oak and the volumetric water content fluctuations, what we see is that if you look at water storage, use and refill over the summer season, we see that after those rain events, stored water use and refill is the smallest, and then in the days following, use and refill gets larger and larger gradually after that rain event. And then if we look at those values with environmental conditions and we relate them, we don’t see any relationships with soil moisture, but we do see that stored water use and refill increase with VPD. So there’s a relationship with the different heights and depths with mean daily VPD. And then, if we look in Pine while those stored water use and refill values are smaller and they’re not as large, and maybe the patterns aren’t clearly as noticeable, we do still see this pattern of smallest after the rain event, and then gradually gets larger. And then we do see relationships with VPD as well. And so that’s the North Carolina side our music site. And so I will also add that kind of this relationship of or what we’re seeing with the oak using more stored water than the pine, that’s also supported by our findings from our larger NSF project, from the tracer data, where we found that the tracer existed for longer periods of time, or longer period of time in the Oak, and we also see existing at deeper depths in the Oak compared to the pine. So suggesting that you do have kind of this radial and axial transport happening both together in our in our oak, resulting in it being more mixed in storage at deeper depths. But then in Idaho, what we see is that we do see a clear soil dry down occur or so decline in soil moisture, which we would expect, and we didn’t have significant rain events over our summer season. And if we look at and we relate stored water use and refill to those environmental variables. So in the Aspen first, I’ll talk about it’s interesting, because what we found is that there’s different patterns going on at the different depths. And so if we relate stored water use and refill with soil moisture, we see a relationship, and we see that at those deeper depths, that we see either no relationship at all with soil moisture, and so they’re pulling the same amount of stored water use on those drier days, or we actually see an increase as soil moisture declines, whereas at those shallow depth we are seeing that there’s a decrease going On as soil moisture declines in use and refill. And then, in comparison, in the pine we see less relationships of stored water use and refill with soil moisture and BPD.

BRAD NEWBOLD 29:52
Is there anything, anything else that that you’ve learned about the seasonality, I guess, when it comes to management of their water reserves? Yes, you’re talking about diurnal fluctuations, and we’re dealing with seasonal as well. Any any other, any other insights into into that?

LAUREN TUCKER 30:08
I think overall, we can say that angiosperms versus gymnosperms, we see that angiosperms are using stored water more than gymnosperms, and we’re seeing evidence that there can be deep stored water in those angiosperm species. And then I think that if we compare the music to the semi arid we’re seeing, that in those music environments, the species that we studied are more responding to rain events that happen over the summer, whereas in here, in our semi arid site in Idaho, we’re seeing more of a seasonal response as soil moisture dry downs. We see, dries down we see, we see changes happening in stored water use and refill.

BRAD NEWBOLD 30:54
What do you think your your findings could say about how, I guess, how long term water storage and trees might affect their response to drought conditions, so on a longer, you know, time scale. I mean, could this influence force resilience with with climate change, or just in in drought conditions in general, any any thoughts or insights into that?

LAUREN TUCKER 31:16
Yeah, I think in particular, with those results from trembling Aspen. I think it just shows that there could be certain species that are able to tap into deep, stored water in trees as conditions dry down. And so this could be a water use or type of strategy that these plants have that maybe could make them better off in the future if they’re experiencing drought conditions.

BRAD NEWBOLD 31:42
Along with that, you’re talking about, like direct measurement of long term water storage within trees. Are you also working with any kind of modeling aspects of this project or within your research?

LAUREN TUCKER 31:54
Yeah, so that’s kind of one of my next steps as part of my dissertation research. And so with modeling, I’m trying to get at more of the importance of stored water for whole tree water relations. And so I’m going to do that is I’m going to use a model that’s called the gain risk model, and so that’s been developed by Sperry. And what I plan to do is to extend this model to include stored water and so currently this gain risk model, what it does is it, it determines plant productivity based on this assumption that stomata want to regulate themselves in order to maintain a canopy water potential where you optimize carbon gain and also minimize the risk of hydraulic failure in the plant. And so currently, this gain risk model, it assumes that steady state is occurring in the plant, and it’s not incorporating that you can have stored water inside of the tree as an additional water source, in addition, in addition to the soil being a water source. And so what I plan to do is incorporate a species specific characteristic, which is called capacitance. And so capacitance is a measure of how much water is released per unit change in water potential. And so I plan to incorporate capacitance and then essentially turn capacitance on and off, both for observed conditions that we have, as well as conditions that are predicted in the future, and see how that affects plant productivity with this model, to learn more about the importance of stored water. And so what we can do is, if we’re doing this both Idaho and North Carolina, which I plan to do, we expect different climatic conditions to happen in these different regions with climate change, and so with Idaho, it’s predicted that week, and which is in the the Northwest, we predict that there’s going to be increased temperature and increased precipitation. So you might have an increase in precipitation, but when that’s coupled with an increase in temperature as well, what we expect is that there’s going to be precipitation, more in the form of rain rather than snow, and that you can have lesser snow pack, smaller snow pack, and maybe more increased melting of snow pack before the growing season. And so you won’t have the same recharge of the soil and the ground water that takes place. And so then during the growing season, we expect that our plants are going to experience more they could experience more water stress conditions. So we can incorporate these kinds of things into our model. And then, in contrast, in North Carolina, it’s expected that we’re going to have warmer, wetter conditions.

BRAD NEWBOLD 34:34
I think that there’s a lot of really cool implications for that. I know when you just mentioned in passing, dealing with groundwater recharge, that’s that’s a big, you know, a big item, especially in the arid west. You know, there’s stuff going on in California, like you’ve mentioned, there, in Idaho as well, all throughout the intermountain west as well. About, about, how do we, how do we measure that, groundwater recharge? How do we manage it? Pun not intended, but there’s a lot of down stream effects when it comes to having a minimized snow pack and and ground water recharge, and it’s, it’s then interesting to see how, how, then you know your trees that you’re looking at are going to react to to that as well. That, that leads me to to my next question about looking at at kind of like the ecosystem scale. So you’ve talked about how that you know these trees are managing their their water balance. What role at an at an ecosystem scale does long term water storage play in in the the overall forced water balance and and hydrology?

LAUREN TUCKER 35:35
We think that this stored water in trees is not really being it’s not being appropriately incorporated into current ecosystem models that we do. And so when we model ecosystem water balance, it’s important that we’re able to do it accurately, especially considering we want to be able to predict what’s the ecosystem water fluxes and storage like with both current and predicted changes in climate. And in a recent study by Scanlon et al in 2018 they essentially showed that if we look at water storage, at the water watershed scale that’s calculated from models, and then if we compare that to satellite estimates of water storage, we see that they don’t match up. We see that water storage at the watershed, watershed scale that’s calculated from these these computer models is underestimating satellite observations by up to a factor of around 20. And so one of the fluxes that these models incorporates or assesses is evapotranspiration. And so this is kind of consistent with our inability to simulate the fast dynamics of transpiration, where these models assume that it’s steady state in the plant. So what’s going in equals what’s going out. You kind of have this continuous sap flow that’s occurring in the plant, but we know that stored water is contributing to transpiration. So this transpiration is in part, controlled by that stored water in trees. And so we need more more of an understanding of how much stored water is inside of trees? Where is it stored? What’s the eco-physiological importance of the stored water for these forest environments? And so if we have a better understanding of that, we can have a better incorporation of stored water into these models, which would hopefully improve our ability to accurately model ecosystem water balance.

BRAD NEWBOLD 37:32
As you’re working along with your project and your research here, what do you hope might be the the implications or the impact or or even even the applications of your research when you you know, I guess you never finish right research, but, but when this project is done and your findings come out, and I guess, how, how might your research be able to help other scientists, others who are interested in in Plant Physiology, ecology or even forest management practices, especially in regions that might be vulnerable to water scarcity and those kinds of things?

LAUREN TUCKER 38:09
Yeah, so I think going back to that broader implication of being able to more accurately model ecosystem water balance, I think what I’m showing from this research is that it’s important to consider that you can have differences in stored water use and the dynamics of stored water use for these these plants that have different hydraulic strategies, and that you can have differences between a music environment and a semi arid environment. And so I think, I hope that the research will kind of emphasize the importance of considering those things and learning more about these things for incorporation in the future into models. And then I think, overall, I think this research also has implications just towards how we think about plant water transport in general, generally when we think about plant water transport, which I already kind of touched this on, this earlier, is that we think about water moving into the roots, up the stem and out the leaves. And that’s how we think about the soil, plant, atmosphere, continuum, and we don’t often think about that radial or lateral water transport. And so generally, it’s been regarded that the heartwood is dry and it’s immobile, that water that’s inside of it, and that there’s not really a hydraulic connection between the sapwood and the heartwood, but really you can have some species where the heartwood is similar water content as the sapwood, or it has even a greater water content than the sapwood. And so we might think, Well, that seems intuitive that if they have this Heartwood that’s wet, that they could pull water from that but that’s actually that’s not the current paradigm inside of when it comes to plant water transport. The current paradigm is that the sapwood and the heartwood are hydraulically separated. So I think with our findings from tracer studies, as well as studies using volumetric water content sensors at different depths. I think this is kind of changing the way we think about plant water transport and so kind of changing this traditional thought of water goes up and out of the tree in a couple days time. But really you can have in deep parts of the tree, you can have variable amounts of time that that water is inside of that deep depth, or in deep sapwood, or in the sapwood.

BRAD NEWBOLD 40:27
I guess, kind of to round things off. What do you see as the future of your research within this realm? Is this something where you’re, you’re definitely wanting to stick with long term water storage and trees? Is this kind of a a jumping off point to other things. What does it what does it look like for you here in the next little while?

LAUREN TUCKER 40:45
Yeah, I think this research is awesome, and it fits really well with my overall interest in in plant eco physiology, of understanding different strategies that plants can use in order to adapt and and respond to their environment. And so I remember when I learned about this project from Dr. Keith Reinhard, my advisor, I was just like, wow, water in the heartwood, and they can use that? That is so cool! And so I think I’ve just continued to have the same interest and and being like, wow, this is so cool. And I think it’s been really great being part of a group of researchers as part of this, this project where we have monthly meetings, or we had an annual meeting, and just talking with everyone about these findings and and how they kind of, they go together in some ways, you know where they meet or intersect in the middle of how one explains one phenomenon and one explains the other. I think it’s been really cool, and I can see myself doing research with this in the future, and continuing to think about water storage and trees and its importance.

BRAD NEWBOLD 41:54
Awesome, along with that, you’re dealing with new, new technology, I guess, in this, this novel way to measure water content in plants and trees. Are there any other, I guess, new technologies or methods that you might be excited to explore in your research?

LAUREN TUCKER 42:10
So there’s been a few times where I’ve thought, man, we can measure sap flow going up the tree. So we have these sensors that have been developed for this axial transport. How cool would it be if we could measure sap flow going radially? And have some kind of easy sensor that we could use we stick it in the tree, and then we have this continuous measurement of what’s that that water transport like radially, rather than just axially?

BRAD NEWBOLD 42:38
That’d be cool. I’ll pass it on to our R&D team.

LAUREN TUCKER 42:43
We need too.

BRAD NEWBOLD 42:46
We’ll put it on the backlog.

LAUREN TUCKER 42:47
I told Keith and my advisor a lot about about that. He’s heard me talk many times about radial transport. I’m like, we gotta find out a way to measure that Keith.

BRAD NEWBOLD 42:55
Right I mean, that would be cool. I mean, basically, right now you’re just, I guess you’re just measuring the difference between between what you’re seeing in the heartwood and the sapwood, and you’re just kind of, is that how, how you potentially could measure?

LAUREN TUCKER 43:10
It’s, it’s a little tricky, because we can have these sensors in the sapwood, and then those ones that are going into the heartwood, also Part of that length along the sensor is the sapwood, and so it’s an integrated value, right? So you always have to kind of consider, well, part of this measurement is from the sapwood as well, and so, but then you can also say, well, if I’m seeing significant differences between those deep sensors and the shallow sensors, it has to be because there’s something different going in the heartwood, going on in the heartwood, compared to the sapwood. And then we’ve tried to do various ways. We’ve done lots of, you know, thought, experience experiments of like, well, if we know about the sapwood, and we know about the heartwood, can we just take that, that sapwood off of that deep sensor value? And so we started to think about, like, Is it is it linear? So, like, if you have VWC, that’s that’s a long that that’s being measured along that sensor, like, and it’s integrated, can you kind of create this, like, linear relationship? And we, we found that we could make it linear. It’s just that, when I had all these different models and sensor types inside of the tree. When I actually tried to apply that, I didn’t get very, very believable values. Like maybe this isn’t right.

BRAD NEWBOLD 44:31
Yeah, we’re coming to the end of the time. Is there anything else that you would like to share about your studies, or where people might be able to find more about the work that you’re doing.

LAUREN TUCKER 44:41
Sure, if anybody wants to talk to me about my about these things, my email is [email protected] you can feel free to reach out to me.

BRAD NEWBOLD 44:51
Alright. Well, thank you, Lauren for joining us. It’s been a super fascinating discussion, definitely. Yeah, looking into all this novel applications of stuff that we kind of take for granted a lot of times here at METER when we’re dealing with our soil moisture sensors, water content sensors, and being able to see cool new applications of of those things. So again, thank you for joining us Lauren.

LAUREN TUCKER 45:15
Thank you so much for having me, this was fun.

BRAD NEWBOLD 45:17
And if you in the audience have any questions about this topic, or want to hear more, feel free to contact us at metergroup.com, or reach out to us on Twitter, @meter_env and you can also view the full transcript from today in the podcast description. That’s all for now, stay safe and we’ll catch you next time on We Measure The World.

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