Transcript:

BRAD NEWBOLD 0:00
Hello everyone, and welcome to Office Hours with the METER Group team. Today’s session will focus on canopy measurements and how to get them right, and we’re shooting for about 30 minutes of Q & A with our experts, Jeff Ritter and Chris chambers, whom I will introduce in just a moment. But before we start, one housekeeping item, if you’re watching this video and you think of a question you’d like to ask our science experts, we encourage you to submit your question on our website at metergroup.com, and then someone from our science and support team, we’ll get back to you with an answer via email. All right, with that out of the way, let’s get started today. Our panelists are application specialists, Jeff Ritter and Chris chambers. Jeff Ritter is the product manager for plant canopy and atmospheric monitoring instrumentation here at METER, he earned his master’s degree in plant physiology from Washington State University, where his research focused on leaf level gas exchange and the impact of plant biochemistry on the measurement of the global carbon cycle. Prior to working at METER, he held a research faculty position at Washington State University in the Department of Crop and Soil Sciences, Chris chambers operates as the environment support manager and the soil moisture sensor Product Manager at METER. He specializes in ecology and plant physiology, and has over 15 years of experience helping researchers measure the plant he specializes in ecology and plant physiology, and has over 15 years of experience helping researchers measure the soil, plant atmospheric continuum. Thanks for joining us today guys.

JEFF RITTER 1:28
Thanks Brad.

CHRIS CHAMBERS 1:29
Thanks Brad. Hi, Jeff how you doing?

JEFF RITTER 1:31
Doing well, how are you Chris?

CHRIS CHAMBERS 1:32
Pretty good. Ready to talk about some canopy science?

JEFF RITTER 1:35
Always yeah, I’m always excited to dive into, dive into plant physiology and canopy science, so where do we want to start?

CHRIS CHAMBERS 1:43
Well, it looks like we’ve got some questions about stomatal conductance.

JEFF RITTER 1:48
Great.

CHRIS CHAMBERS 1:49
So first off, I suppose we should define it for the new folks out there that are just diving into the measurement.

JEFF RITTER 1:57
Define stomatal conductance. Okay, so I think, you know, first we kind of need to define what a, what stomata are, right, and what they do, what the functions are.

CHRIS CHAMBERS 2:12
Start with the anatomy, okay, yeah, and so a stoma is a leaf pore that allows gas exchange within the leaf, right?

JEFF RITTER 2:22
Right, so, sometimes people talk about it like it’s analogous to the pores of your skin, which in some ways it is, but it is important to remember that it’s actually kind of more like your mouth, and that a plant has a lot of dynamic control over its stomata, where it can open and close it in response to the environment. And it does that to regulate, basically what it’s trying to take in and what what’s what’s leaving. So CO2 is coming in and water is leaving. And a plant can control its its stomata, how open they are, to control those two variables.

CHRIS CHAMBERS 2:22
And the critical function here, why everyone is interested in it, why you would want to measure stomatal conductance, is because it’s the plants minute to minute control over how much water it’s losing.

JEFF RITTER 3:13
Yeah, there’s a big trade off between a plant has to have its stomata open to take CO2 in and it needs CO2 for you know, a lot of its physiological functions to store energy, and when those stoma are open, it’s it’s losing water. So the plant needs to tightly control that. So you can tell a lot about a plant, and essentially, how happy that plant is, if you know something about a stomach conductance.

CHRIS CHAMBERS 3:37
And that’s why we like to measure it. And just to hammer home how critical it is plants might need to close their stomata to not just to conserve water, but even to prevent death. If the there’s not enough water in the soil, the stress on the plant pulling water out of the soil becomes greater and greater, and if that water column breaks, then the plant may not be able to move any more water. And that’s a really dire situation for it.

JEFF RITTER 4:13
Okay, so if you say you are able to measure the stoma conductance of a plant, what? What does that tell you?

CHRIS CHAMBERS 4:21
The important thing to keep in mind when you are looking at stomatal conductance data are the variables that affect it, right? And those are light, right? If there’s no light, like at night, stomata are typically closed there’s no photosynthesis to be done. So some plants, depending on what type of photosynthetic structure they have, they are they just close their stomata when there’s no light or a different light intensities water loss, right? So. Uh, typically, if you have a high VP D Day, VP D vapor pressure deficit, then the stomata are going to be closed to conserve water. Anything else? What am I forgetting?

JEFF RITTER 5:13
I think, I think that’s it in a nutshell. And I think the the real answer to that is that there’s so much that can affect stomatal conductance that you need more context than just a stomatal conductance number. You know, as you mentioned, there’s even different types of photosynthesis that certain plants can do that’s really going to impact that. So if you’re trying to measure stomatal conductance in a plant that uses Crassulacean acid metabolism.

CHRIS CHAMBERS 5:42
Like a cactus.

JEFF RITTER 5:43
Like a cactus, you need to know that before you measure it, otherwise, your numbers aren’t going to make any sense, right. So if you’re measuring stomatal conductance on an almond tree, then you try and compare it to a nearby grass species.

CHRIS CHAMBERS 5:43
Like maize.

JEFF RITTER 5:43
Yeah, you need the context of what, what a baseline is for those to really be able to interpret what that what that number means.

CHRIS CHAMBERS 6:05
Right and so here’s a question that comes up quite often, how does stomatal conductance relate to water loss? And that’s a question that isn’t necessarily intuitive, unless you look at the math and we can look at stomatal conductance as kind of how much water the plant can lose, right? So we typically express water loss as ET, and it’s going to equal the stomatal conductance times the vapor pressure deficit, right? And just to break this down, this VPD is basically the the tension on the water coming out of the plant, right? So what, what the gradient is to drive water loss. And this is kind of the stomatal conductance is basically the size of the pipe, right? So the difference in your gradient times the size of your pipe is going to equal the total water loss.

JEFF RITTER 7:17
So ultimately, then the stomatal conductance is the size of the pipe at which water can be lost. But you’re saying a measure of stomatal conductance is not the same as measuring how much water the plant is losing.

CHRIS CHAMBERS 7:32
Exactly.

JEFF RITTER 7:33
Okay.

CHRIS CHAMBERS 7:33
Exactly, and the way the plant controls how much water it’s losing, how it controls its stomatal conductance, how much water it can lose, is that you’ve got the density of stomata on a leaf surface, but also their state of closure.

JEFF RITTER 7:52
Right, okay, and you know, every species, every variety, even looking at the stomatal density in one variety grown in different environments. You know it’s going to be different. Development plays a big role in stomatal density. So you just need to be careful taking all that into account when you are measuring.

CHRIS CHAMBERS 8:13
Especially species because yes, every species has its own water management strategy, and stomatal conductance play a big role in that, in the regulation of those stomata.

JEFF RITTER 8:23
Okay, so maybe let’s get into some of the measurements that are needed. I think so you’ve outlined the general form of the equation that we need here with the simplified version of Fick’s law. But what do we need to actually measure stomata conductance? Ultimately, whatever you’re doing, you have to make a measurement of a couple of the parameters outlined in this equation. So whether that is measuring the total flux and then having some way at getting it individual concentration of both inside of the leaf and outside leaf, that can get you at conductance, you essentially need to be able to measure a couple of the parameters outlined in the simplified version of Fick’s law. So that’s, you know, there’s, there’s devices out there that will do it with a capacitance type, RH sensor, like the SC-1, like, like the METER Group makes an instrument called SC-1. There’s open path systems that will use infrared laser that measures water concentration. And so there’s a lot of ways. There’s multiple ways to do it. It just depends on what works for your your question, what you’re looking at, I kind of want to talk also about, because we’re talking about what parameters impact your measurement of stomatal conductance, what is, what’s the ideal situation that you should be measuring stomatal conductance?

CHRIS CHAMBERS 9:53
Um anytime that you want to know your plant water status, like in a in from minute to minute, or at a given type of time of day. Now, we’ve already measured some things that affects stomatal conductance, right, like light, like how much water the plant is losing.

JEFF RITTER 10:12
There’s so much that goes into it that attempting to be consistent, if you’re trying to compare one plant to the next, is going to be crucial, so, that’s right, and you can’t say there’s one ideal time to measure stomatal conductance necessarily, because it really comes down to what you’re asking. Obviously, if you’re interested in in pre-dawn measurements, that’s going to force your hand there. But otherwise, if you go out on a cloudy day, and it’s even the same time of day as it was yesterday, you know that cloud coverage is going to impact stomatal conductance, so it’s about attempting to establish a baseline and then seeing how plants respond to different environmental conditions.

CHRIS CHAMBERS 10:52
That’s right. And even within a plant canopy, a leaf that’s shaded all day or has been is at the bottom of the canopy and is always shaded, it’s going to have very different stomatal conductance numbers than something that gets full sunlight, and you could see this in plant morphology too and leaf morphology. The leaves at the top of a tree are generally smaller and have less mass than leaves at the bottom of the tree, and so your stomatal conductance will have will follow those kind of those similar patterns, where, if it’s cooler at the bottom of the tree and you get less light, you’ll probably have a little bit lower stomatal conductance, or could be higher stomatal conductance because it’s losing less water there’s, there’s just a lot of variables to balance there.

JEFF RITTER 11:47
So then let’s say we go out and we measure stomatal conductance, and this is one where we, you know, we, we’ve been making these measurements for a while. How do we interpret those like, what? What what does that mean? As far as stress is concerned, I’m interested in measuring plant stress, right?

CHRIS CHAMBERS 12:05
Well, it really depends on the species, right? Yeah, I think I’ve seen some. We have an application note about using the SC-1 in vineyards, and that has some specific numbers in there that kind of give you a guideline of how happy the plant is in any given any given stomatal conductance range. But generally zero to 50, the stomata are going to be pretty closed, right? The plants conserving water. It may not necessarily be stressed. It may have its stomatal closed so that it avoids stress. But it’s not very active right now. It’s not doing photosynthesis, and it’s just trying to protect itself from water loss.

JEFF RITTER 12:56
Yeah, I think that’s a good way to look at it. Is those numbers will tell you how actively that plant could be losing water, not necessarily, whether the plant is stressed from that, that’s up to your analysis, from, from what you know about what what condition that plant is in, stomatal conductance.

CHRIS CHAMBERS 13:14
Anytime you want to know leaf level water loss.

JEFF RITTER 13:17
Yeah, I mean stomatal conductance is, is extremely important anytime you are interested in looking at physiological properties of the plant. And we actually, even when you’re measuring something like photosynthesis and trying to look at what the plant is capable of yielding, that stems from understanding stomatal conductance to water vapor. And from there, you can get it to CO2 and, and use that in calculating photosynthetic uptake. So it’s important for water loss. We haven’t talked about it too much, but stomatal conductance is also important for temperature regulation for plant so if you are interested in heat stress, it’s important in those studies, you can, you can use heat of the canopy and relate that to stomatal conductance and how that’s stressed. So stomatal conductance is one of the major ways that a plant controls how it’s responding to its environment.

CHRIS CHAMBERS 14:12
And but that also is a great proxy for plant water status. Is just measuring the canopy temperature with an infrared thermometer right or something like that. So if you want something more canopy level rather than leaf level, then an infrared thermometer, and then you’re looking at the difference between the canopy temperature and the air temperature, right?

JEFF RITTER 14:12
Yeah.

CHRIS CHAMBERS 14:12
And you can kind of get a proxy for, not necessarily stomatal conductance, but general water loss of the plant canopy, right. How can the SC-1 be used to measure leaf transpiration? We touched on that a little bit.

JEFF RITTER 14:50
Yeah. So any, anytime you have stomatal conduct- stomatal conductance, and you want to get to transpiration, keep in mind that if. Talking about transpiration, we’re talking about the flux parameter in going back to the Fick’s Law of diffusion. So with stomatal conductance, we have one point, one, one measure of conductance. But there are others in that system that you need to account for to get the total conductance. So even if you have concentration of water inside the leaf and outside the leaf, and you have stomatal conductance, you still have to take into account a couple of other points of resistance, namely the boundary layer of your leaf itself, right. So you’ve got to take into account once that water vapor is able to be conducted through or diffuse through the stomata, what then happens at the boundary layer of that leaf with the atmosphere, which is going to be related to some characteristics of the leaf itself, as well as the wind speed. Wind is a huge, hugely important factor when it comes to water loss.

CHRIS CHAMBERS 15:57
And leaf level measurements do not generally scale up to canopy level measurements when you’re taking stomatal conductance or leaf level transpiration even, just the canopy resistance to water transport makes a big difference there.

JEFF RITTER 16:12
There’s not only complexity at the leaf level that needs to be taken into account if you want leaf level transpiration, but canopy architecture can be extremely complex, so scaling those processes can be can be difficult. So other measurements can be helpful. With that, something like LAI can help to understand canopy architecture, or at least biomass, and that can help. But you’re right, taking a direct scaling up of a leaf level measurement can be very difficult. So why is it important to have an accurate leaf temperature?

CHRIS CHAMBERS 16:49
In stomatal conductance?

JEFF RITTER 16:50
When measuring stomatal conductance because it’s?

CHRIS CHAMBERS 16:52
Well, that kind of depends on your method, right. If you are measuring if a if you’re using stomatal conductance to measure or to calculate evapotranspiration, leaf level evapotranspiration, then a leaf, an accurate leaf temperature, is critical to calculating the vapor pressure deficit, right? Because that vapor pressure deficit is basically your partial pressure of water vapor in the air minus the saturation vapor pressure of the leaf at that temperature. Or did I get that around? It’s one minus the other. Yeah, whichever one turns out to be positive, and so that leaf level temperature is critical for measuring the that saturation vapor pressure.

JEFF RITTER 17:53
And that, I think that kind of speaks to one of the underlying assumptions that we are making in these measurements, is that we assume that the inside of a leaf is at 100% relative humidity, right? Is fully the vapor pressure is that is at saturation, which means that we are able to simply take the leaf temperature and calculate a concentration of water right inside of that so if your leaf temperature measurement is inaccurate, that pool which, which is what we are using in Fick’s law, that, that pool size is going to be inaccurate as well, and throw that off. So it is important to get accurate leaf temperature measurements when you’re doing this. For that reason, and that on that assumption that we use that all across the board in physiological measurements.

CHRIS CHAMBERS 18:41
It’s a pretty common assumptions.

JEFF RITTER 18:43
It’s baked into a lot. Okay, so we’ve talked a lot about some leaf level measurements, talking about stomatal conductance. How can we scale that up? What we mentioned LAI, right? How do we, how do we scale up from the leaf level up to the canopy level?

CHRIS CHAMBERS 18:57
Well, Leaf Area Index is a pretty common proxy for canopy, for canopy size, right? And that leaf area index is the kind of the layers of leaves that you have. So if you have a area of ground that has, if you have one meter of ground area that has exactly one meter of leaf area above it, your LAI is one right. Say you’ve got three full leaf layers above that one meter area of ground, your leaf area index is three. And so leaf area index is frequently used as kind of a proxy for your canopy mass, and you can use that to you know you’ve got certain canopy parameters of assimilation or stomatal conductance or whatever and if you want to estimate that across across a larger area or across a canopy, then Lai is one of those variables that you can use to to to make that jump.

JEFF RITTER 20:15
Okay, so you’re saying, I need to go out to the forest and start picking up leaves and measuring them getting some LAI.

CHRIS CHAMBERS 20:22
Well, that’s something that’s where you can use something like the LP-80 some remote sensing indexes can give you an estimate of leaf area index, a sept- what’s called a septometer measures canopy light interception, and that is directly, directly related to the leaf area index.

JEFF RITTER 20:47
So there’s, there’s a couple of different techniques, and what you just mentioned there, where you’ve got some what we would call ground truthing techniques, where you’re going out to the forest and measuring it on the ground, see what it is as a point. And then with there’s also satellite based products out there too that allow you to take those ground truths measurements and scale them up to, you know, ecosystem level, or, you know, global, global levels. We can really take follow from a leaf level measurement and then scale that process up all the way across the entirety of the globe. So it’s pretty exciting. And it really just depends on what scale at which you are wanting to zoom in on. What question you’re asking.

CHRIS CHAMBERS 21:27
Exactly.

JEFF RITTER 21:28
We talked about, you know, some diurnal impacts on stomatal conductance. What, what sort of cycles do we see in LAI? You know, with stomatal conductance, we see a very strong diurnal pattern. There’s lots of things that go into that. If we are using Lai to scale up leaf level processes to a ecosystem scale, what sort of what sort of cycle parameters do we need to look at for that?

CHRIS CHAMBERS 21:57
Well, your canopy type, deciduous versus conifer, it’s going to make a big difference in that, like evergreen trees naturally, are going to have a consistent leaf area index all year round. But in deciduous forests or plants, it’s going to be more of a seasonal a seasonal development.

JEFF RITTER 22:19
And you know, LAI, even in agricultural species or short canopies, can be extremely interesting and telling so long as you have a way of consistently measuring those.

CHRIS CHAMBERS 22:31
And you can use it as a proxy for productivity, basically how much biomass you’re putting on, especially if you have plants that are not woody, because woody material generally indicates secondary growth and longer term, longer longer lived plants and all of the new biomass might not be going towards what we get for LAI either. How does canopy density influence the micro climate beneath it, particularly in terms of temperature and humidity. Less light generally means cooler temperatures.

JEFF RITTER 23:07
Yeah, and that can you know that impact the micro climate impacts everything else that that exists down there. So if you’ve got a dense canopy, that’s going to impact what other shade tolerant species can grow, and what sort of we haven’t talked anything about animals in this to this point, but that’s also going to impact what sort of animals are going to exist down there. So that then speaks to being able to not only measure something about canopy architecture, but then how that impacts the micro climate. You need to be measuring that micro climate, both above and below the canopy to understand how this impacts your ecosystem overall, right? We started at an individual stoma, and now we’re, we’re ending up in space looking down at the world from a satellite. So maybe, maybe that’s a good, good place to wrap it up.

CHRIS CHAMBERS 24:01
Yep.

JEFF RITTER 24:03
So.

BRAD NEWBOLD 24:05
All right, that’s going to wrap it up for us then. Thank you for joining us today. We hope that you enjoyed this discussion. Thank you again for all the questions that have come in. And again, if you have any questions that we did not answer, please contact us via our website, metergroup.com. Finally, subscribe to the METER Group YouTube channel to accept notifications, to see previous episodes of Office Hours and to get notified when future videos are available. Thanks again. Stay safe and have a great day.