Transcript

BRAD NEWBOLD 0:00
Hello, everyone, and welcome to office hours with the METER Environment team. Today’s session will focus on your burning questions about the LP-80 and LAI, and we’re shooting for about thirty to forty minutes of Q and 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 METER Group dot com, and then somebody from our science and support team will get back to you with an answer via email. Alright. With that out of the way, let’s get started. Today, our panelists are application specialists, Jeff Ritter and Chris Chambers. Jeff is the product scientist for plants, 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 is a product scientist at METER Group, and he draws on over fifteen years of experience as a support scientist and his background in ecology and plant physiology to improve instruments and to help METER’s clients collect the data they need to test hypotheses and make management decisions. Thanks for joining us, guys.

CHRIS CHAMBERS 1:43
Hello.

JEFF RITTER 1:44
Thanks, Brad.

BRAD NEWBOLD 1:45
Alright. Let’s get started taking some questions. And we’re gonna start with some general questions. And this first one, in general, what time of day should be best for taking LAI?

CHRIS CHAMBERS 1:59
Practical. Yeah. Jumping right into a practical Right off the bat. What I’ve seen, most peer reviewers prefer you to take them around midday, and a safe bet is from ten AM to two PM.

JEFF RITTER 2:14
Yeah. That plus or minus two hours around solar noon is your best option. Although I do oftentimes see that window, you can extend it depending on sky conditions. So when you have a lot of direct beam solar radiation, you want to stick to that plus or minus two hours around solar noon. In very overcast conditions, we’ve got a much more diffuse radiation. You can go out other times of the day and you’re going to get, good results.

CHRIS CHAMBERS 2:47
And really, if you’re not doing peer reviewed research, what you really want to avoid is at least that first hour and last hour of the day when you get really low zenith angles. And if you play around with the numbers a little bit in a sensitivity analysis, your numbers will be really good until those two those parts of the day.

JEFF RITTER 3:12
Yeah. And this is assuming, you know, we’re talking about using an instrument like the LP-80 or something else. It’s using an optical method for LAI. The biggest thing you have to consider the impact that sun’s angle is playing in that measurement. There’s a lot of assumptions built into those optical models, so you don’t want to violate that if you’re doing peer reviewed study. But if you’re looking at relative measurements, you’ve got some more freedom there, Chris was saying.

BRAD NEWBOLD 3:45
What other factors should people consider when they’re taking LAI? Specifically, this question is asking in a big field, but we can talk about other things as well.

JEFF RITTER 3:56
Yeah. If you’re talking about in a big field of a monocrop, or even if you’re in a forest stand where you’ve got, you know, a distribution of different species, there’s going to be some amount of heterogeneity from one side to the other, whether that’s based on how much water is in your field or other environmental conditions. And so making sure that you know what you’re looking for, whether you have different transects and you’re looking for differences in treatments, that’s going to play a role into how much you want to sample. So really, sampling is the biggest thing there, and we get the question a lot of how many samples is enough?

CHRIS CHAMBERS 4:42
All of them.

JEFF RITTER 4:43
All of the samples.

CHRIS CHAMBERS 4:45
All of the samples. With the l a with the LP-80 though, it has one big advantage going for it, and that is it’s really quick to take a reading. You’ll spend much more time getting from sampling point to sampling point than you will taking a reading. So really, good old straightforward brute force sample, take lots of samples, is the strategy with the LP-80.

JEFF RITTER 5:12
Yeah. I always recommend doing some exploratory sampling and looking at your variation afterwards, and that can help guide, you know, what number of samples you need for statistical rigor. Is really easy to take more samples than what you need for statistical rigor with LP-80, so it’s better to just take more, so long as they’re high quality, obviously.

CHRIS CHAMBERS 5:36
And one of the trickiest parts would probably be canopy shape, If you have many canopies are spherical, which is a chi parameter of approximately one. So if your canopies are all that similar shape or leaf area or leaf angle distribution, then, you probably can just, brute force your way through it. If you have different types of canopies that you’re interested in, like say strawberries versus maize, you probably want to have separate sampling strategies for those.

JEFF RITTER 6:08
Absolutely. Yep. And that’s something to consider. If you are in a big field, I typically would assume you’re working in, you know, some sort of a agricultural species, and if that’s the case, some of those do have fairly distinct leaf angle area leaf angle distribution parameters. So it’s important to keep that in mind because that will impact the numbers that come out of the model.

BRAD NEWBOLD 6:36
Alright. This next question is asking, how can they measure LAI manually from single plant leaves?

JEFF RITTER 6:44
So I have a couple of thoughts here. One, you can’t really do it with an instrument like the LP-80. That’s just not what it’s not what it’s designed to do. This is possible, however, with a couple of caveats or a couple of assumptions that you’re going to need to make. One, you need to get a leaf that is representative, a representative size of all the leaves in your canopy, or all the leaves of, you know, an individual that you’re looking at. And if you can get that leaf area and you can get an estimate of how many leaves you have either in a canopy or on an individual tree, and then, you know, how much ground area is your stand taking up, you can calculate leaf area index that way. Not a great way to do it. I’ve never actually attempted this myself.

CHRIS CHAMBERS 7:42
I’ve known some folks who’ve done it on sage plants.

JEFF RITTER 7:45
Yeah. Have they been successful?

CHRIS CHAMBERS 7:48
Yeah. They used it to, kind of tune their LAI, which you do have to bear in mind a couple of the differences. And it You kind of have to know what ground area your canopy is projected over for a single plant. So it still is good to identify a specific ground area over which you’re gonna sample the leaves. And yeah, then you’re just pulling leaves off and measuring their leaf area. Do be aware if you’re doing like a projected leaf area on really flat leaves, because three-dimensional leaves like needles might be a little bit trickier.

JEFF RITTER 8:28
But yeah, it has been done. And again, if you just have a single plant leaf, it’s gonna be really difficult to ensure that it’s representative in a way that makes your data meaningful. So again, you’re going to need likely more than one, sample there.

CHRIS CHAMBERS 8:46
It’s pretty exhaustive sampling, but it can give you some insight that you can’t get from the LP-80, which doesn’t distinguish between green leaf material and stems. Right. And it can be helpful. It’s pretty tricky to do.

BRAD NEWBOLD 9:02
Along those same lines, is it possible, or at least how can this individual measure LAI without using any kind of instrumentation?

CHRIS CHAMBERS 9:11
Well, if you’re gonna measure it, you have to have some kind of instrument, even if it’s a ruler and you’re measuring leaf dimensions.

JEFF RITTER 9:20
I think you’re right. I think a ruler is the that’s the minimum what you need to measure leaf area index. You’re gonna have to go out and collect leaves off the ground, and then figure out the leaf area. And, know, there are that kind of is the basis for how leaf area index was first, and it’s still measured in a lot of cases where they’ll have litter traps, where they collect fallen leaves. Typically, put them through some sort of a leaf scanner, so they don’t have to make individual measurements. But, a ruler would be the lowest cost option.

CHRIS CHAMBERS 10:00
But scaling this up to a canopy, really there’s so much opportunity for error in sampling that measuring canopy LAI is generally going to give you better results, and there’s plenty of methods out there to do that right now. And the LP-80 is a good one.

BRAD NEWBOLD 10:19
Alright. Let’s get into some plant specific questions here. This first one is asking, what about LAI measurement methods for cereal crops like maize or sorghum?

CHRIS CHAMBERS 10:30
Yeah. So one of the first things that I think about with these is how they have some of these crops have very distinct leaf angles, and sometimes it’s not static over the course of their life cycle, where you have a lot of cereal crops that start off with highly vertical canopies when they’re young, and then as they leaf out, they can become more and more horizontal. So if you are doing a season long LAI, you would expect that LAI to increase, but you want to make sure you’re controlling for any errors that are involved with the canopy architecture changing as they advance to their growing phase. Like a young maize plant, or young maize canopy might have a chi value, which is the parameter that captures leaf angle distribution in the LAI in the LP-80 model, it might be closer to point seven. If you’re looking at young maize plants as the canopy matures, then it’s probably gonna get closer and closer to one. Your sampling, might require some consideration for things like this too, as well as the life cycle, because row crops, you might need to incorporate your capturing the row variability as your canopy grows up, and that might be less and less important as the canopy reaches closure.

BRAD NEWBOLD 11:55
This next individual is asking about LAI for chickpeas because chickpeas have complex leaf structures and very small sized leaves.

JEFF RITTER 12:03
Yeah. What are your thoughts on chickpea?

CHRIS CHAMBERS 12:06
I don’t actually know very much about chickpea plants, to be honest.

JEFF RITTER 12:11
Yeah. I don’t either. I don’t foresee an issue again with enough sampling and even a complex leaf structure and complex angles kind of plays into the favor of having a spherical canopy architecture. So as long as you’re able to get the LP-80 under the plant to make the measurement, and with any of these, there’s going to be some amount of non leaf tissue that is intercepting light as well. So there’s going to be some amount of the actual plant area index and not just leaf area index in there. So, you know, some species are going to have more of that than others. I can’t speak specifically to chickpeas, but the small leaf size shouldn’t really impact things.

CHRIS CHAMBERS 13:01
The eighty centimeter wand of the LP-80 is actually excellent at capturing variability of sunflecks. And did you know the original instrument was called the Sunfleck?

JEFF RITTER 13:12
I did. I did. Although that was that was back in the eighties, so it was Yeah. A while ago now. Not one I personally have used in the field.

CHRIS CHAMBERS 13:24
So the, again, the canopy shape parameter, the chi value, that might require some consideration. And if you’re not sure, you can actually get out there and measure the leaf angle distribution to get the correct chi value for that.

BRAD NEWBOLD 13:40
And then this next question is asking about, the possibility of measuring LAI in climbing plants.

CHRIS CHAMBERS 13:46
LAI in climbing plants.

JEFF RITTER 13:48
Grapevines come to mind. What’s that?

CHRIS CHAMBERS 13:51
Grapevines come to Oh, grapevines.

JEFF RITTER 13:53
Yeah. My first thought is that the LP-80 or any optical method really is going to be difficult in a climbing plant. And I would Or rather a transmittent approach would be difficult with climbing plants just because of some of the assumptions in that model that you’ve got to violate to take this measurement. But I’m interested to get to get your thoughts on that.

CHRIS CHAMBERS 14:20
I’m What assumptions do you think they’re violating specifically?

JEFF RITTER 14:23
Well, you are looking at PAR transmittance through a canopy where the angle of the sun is coming to a horizontal surface. So where are we positioning our where are we positioned the LP-80 to catch that transmitted light? And how do we account for variations in zenith angle if the canopy is vertical? Good point. So I think if you’re in truly one hundred percent diffuse conditions, you could maybe make a relative measurement of this where sun angle doesn’t matter as much, and you’ve got light coming from all directions. But even then, it would be difficult, think, to make a good measurement of LAI with a PAR transmittance method like the LP-80.

CHRIS CHAMBERS 15:10
I think it depends partly on your leaf angle distribution. If you have, if you have If you’re capturing the leaf angle distribution for that model. And But also it is partly a sampling issue as well. Because you can turn off segments of the wand if you want to limit what area you’re capturing on the ground. So I don’t know that there are insurmountable obstacles. There are definitely there are definitely some challenges to consider, and your canopy shape is, I think we both agree that your canopy shape is gonna be a big one.

JEFF RITTER 15:48
Absolutely. And I think this sounds like a fun summer project. We can take bets on Hops. On how close we can get. There we go.

CHRIS CHAMBERS 16:00
Go measure some hops.

JEFF RITTER 16:02
Yep. We’ll get back to you.

BRAD NEWBOLD 16:05
The question here, is it appropriate to use an open canopy measurement of PAR transmittance to substitute for an above canopy measurement?

CHRIS CHAMBERS 16:13
Yeah. So I think this is asking, is it okay to walk into a clearing? Instead of having it truly above the canopy. And it really depends on how big the clearing is. If you have a small clearing and you are getting reduced PAR hitting your sensor because of the size of the clearing, then that is going to introduce some error into your study.

JEFF RITTER 16:40
Yeah. So I don’t have a exact answer of how big that clearing needs to be, but sometimes it’s gonna be just fine to go into a clearing. But if any obstructions or vegetation around you is impacting that measurement, then yeah, it’s gonna be a problem.

CHRIS CHAMBERS 16:59
What’s the rule of thumb for edge effects in general? Isn’t it like two times the canopy height?

JEFF RITTER 17:06
It depends on the measurement, honestly.

CHRIS CHAMBERS 17:08
Yeah, for sure.

JEFF RITTER 17:10
You know, it, yeah, I don’t know exactly for solar radiation what we suggest there.

CHRIS CHAMBERS 17:16
That might be a good starting point. I definitely wouldn’t take it as the definitive answer. But in lieu of any other starting point, try being at least two canopy heights away from any part of the canopy.

BRAD NEWBOLD 17:31
This individual asking, is hemispherical photography applicable for small shrubs? So for those that are maybe, like, half a meter in height, or is it only useful for bigger vegetation?

JEFF RITTER 17:43
Yeah. In my experience, it can be difficult with any low lying vegetation. You especially if that’s the only vegetation you’re seeing, you’re gonna get a lot of the picture is gonna be almost entirely dominated by open sky, and it’s gonna be really difficult to quantify that. But it’s not something I have a ton of experience with. So I don’t know if you have done much hemispherical photography with shrubs.

CHRIS CHAMBERS 18:11
Only on trees.

JEFF RITTER 18:13
Yeah. The only place I’ve really So I seen it. I think you can get at values with it, but the taller the better with that.

BRAD NEWBOLD 18:24
Okay. We’re gonna be moving on to some questions dealing with the LP-80 measurements themselves. And this first one says or asks, does the LP-80 measure true LAI or effective LAI, and how do you handle that distinction?

JEFF RITTER 18:40
Yeah. So, Chris, what are we what are we talking about here with true LAI and effective LAI?

CHRIS CHAMBERS 18:48
I’m not really familiar with those phrases. I assume true LAI means, you know, just direct sampling of the leaves.

JEFF RITTER 18:55
I guess, effective, I think apparent LAI sounds better than effective, but we can roll we can roll with effective LAI. But the main point, the main point I think from this is that the LP-80 doesn’t actually measure any leaf areas. Right? It is capturing the light intercepted by the canopy or transmitted through the canopy, right? However you wanna look at it. And then we use a model to estimate LAI.

CHRIS CHAMBERS 19:26
Yeah. So and that it’s an important distinction when we compare it to you know, if we’re talking about a true LAI, that would have to be in some form of measuring every single leaf in the canopy or being able to get at that at that value through destructive harvesting. And so rather than doing that, we are able to look at how much light makes through the canopy. We’re able to measure that extinction coefficient, and get to an LAI value through a model that way. And the point of LAI is that it’s a leaf area index. It’s unitless. It represents kind of this conceptual layering, of the canopy, whereas if you have an LAI of three, then you have say, like three meters squared leaf area of leaves over one meter squared of ground area. So getting at the true LAI or directly measuring it is extremely difficult to do well. Yeah. And what you lose in sampling from putting in all that effort, think is probably results in a higher error than estimating it.

JEFF RITTER 20:38
And how does this model compare to destructive LAI measurements? Where you are taking leaves off a tree or even ones that aren’t destructive, but litter traps.

CHRIS CHAMBERS 20:48
I mean, when you’re taking leaves that have fallen.

JEFF RITTER 20:52
Right. But then in that case, you are also, by the time you’re waiting for them to fall, there’s a lot of variability that creeps in there too, like, because you are, basing it off of some canopy area, right? So you can’t do that on like a one tree basis, you’re doing it on a canopy. You have variables like wind and where the leaves are falling in or out of. Leaf area changes as, leaves senesce and are dropped. So there’s definitely error involved with those measurements as well.

CHRIS CHAMBERS 21:30
That’s not to say that using a transmittance model isn’t without some caveats. And there are lots of different transmittance models out there. If you are in a very dense canopy, for example, if you are sampling actual leaves, the density of the canopy isn’t going to impact the measurement directly. But with the transmitting model, you can run into an issue of saturation of your measurement. Yep. That relationship between the amount of light that is transmitted and the LAI is not linear, at least, especially beyond a certain canopy density. So I think especially once you get up to LAI values of five or six, you start to see some of that nonlinearity. And then as you get higher and higher, eventually you’ll completely saturate and you’ll lose sensitivity to that measurement. But even a canopy LAI of five or six is very, very dense. I mean, what’s the highest LAI you’ve ever measured?

JEFF RITTER 22:31
You know, I’ve never measured in rainforest, so the highest I’ve ever measured is probably in five or six. But yeah, if you get into rainforest, you can get up into eight, nine, ten. What about you?

CHRIS CHAMBERS 22:46
Northern Idaho. Oh, East River Experimental Forest, eleven.

JEFF RITTER 22:50
Eleven. That is that is dark.

CHRIS CHAMBERS 22:53
Mixed conifer forest, it was dark. It was super dark. It was really neat though.

BRAD NEWBOLD 23:00
Okay. This next question, does the LP-80 include stems and branches in its LAI calculation?

CHRIS CHAMBERS 23:07
Everything is lumped in together. It can’t distinguish anything that’s casting a shadow or intercepting light is going to be included in the LAI calculation.

JEFF RITTER 23:16
Right. So you’re effectively getting a plant area index. The best case scenario is when you have a lot of leaves, the other tissues are rather insignificant in how much light they intercept. But if you go out and you stick this under a tree that’s dropped all of its leaves, you’re going to get some shading of your sensor anyway, so you’re going to get something that reports as an LAI even if there aren’t any leaves on the tree.

CHRIS CHAMBERS 23:48
And some other methods can do better than the LP-80 if you need to account for like stem biomass, in your canopy, I think hemispherical photos let you cope with that a little bit better. And then if you’re looking at something like a like a vegetation index that’s remotely sensed, then they are generally using the photosynthetic photosynthetically active pigments or wave bands.

BRAD NEWBOLD 24:14
Why does the LP-80 require diffuse light conditions for accurate measurements?

CHRIS CHAMBERS 24:18
It doesn’t necessarily require it. It does Calculates the fraction of direct beam. So there are consequences to your data, whether you’re sampling on a cloudy or diffuse radiated a cloudy day with diffuse radiation versus a clear day that has a lot of direct beam. But you can collect data and the model accounts for the fraction of direct beam.

JEFF RITTER 24:43
Yeah. With direct beam, you do run the risk of introducing more error into your measurement if you’re not careful, especially with things like your leaf angle distribution parameter. With direct beam, the angle of the sun plays a it has a really big interplay with the angle of your leaves. And so if you’re not correctly accounting for that, you’re going to get your extinction coefficient wrong. That’s not really a problem with diffuse light conditions, light conditions where the light’s coming from all different directions. So the question we get a lot then is, well, what if it’s somewhere in between? Because oftentimes you don’t have fully overcast days or completely sunny days. So what are some of the biggest problems you’ve seen on kind of partly cloudy days?

CHRIS CHAMBERS 25:36
You really need to be careful that you’re above and below canopy radiation measurements are taken concurrently or very close to each other in time. Because if you have changing cloud conditions and your above canopy reading is taken in a clearing when you get direct sun, and then you move under the canopy to take your measurement, then you’re going to have fairly significant errors in your dataset.

JEFF RITTER 26:02
Yeah. On a clear day, sky conditions, you know, they obviously change as the sun moves across the sky, but otherwise you’ve got more time when you have rapidly changing cloud conditions on partly cloudy days. It makes it more difficult for you to trust that your above canopy reading is static. So take them concurrently if you can.

CHRIS CHAMBERS 26:27
Concurrently is always the best, but sometimes, you know, depending on what you’re using, it’s just not possible.

JEFF RITTER 26:34
You might need to call a rain out, Dan.

CHRIS CHAMBERS 26:37
Yep.

BRAD NEWBOLD 26:38
Alright. So related to that last question, can LP-80 measurements be that are taken on different days be compared reliably?

CHRIS CHAMBERS 26:47
In the in the same light conditions, definitely yes. Yes.

JEFF RITTER 26:52
What do you think if you have a diffused day and you come back to the same canopy on a sunny day where there’s a high fraction of direct beam, you’re actually measuring a slightly different slice of canopy then, are you not?

CHRIS CHAMBERS 27:09
You are, and there is directional bias and more impact from things like clumping and leaf spacing that can play a role. So you can get around some of that with proper sampling, where you sample some, but with directional bias, you know, you are measuring a slightly different thing.

JEFF RITTER 27:28
By proper sampling, do you just mean more sampling?

CHRIS CHAMBERS 27:32
More proper sampling, I mean, knowing what your variation is on any given condition and being able to account for that through sampling.

JEFF RITTER 27:41
Brute force it. Get more measurements. Right. So you can compare them, but you are looking at a slightly different condition in your canopy because the angle of the sun is going to cause it’s gonna cause light transmittance in a different way. So how can you determine when the measurements you’ve taken should be thrown out?

CHRIS CHAMBERS 28:04
That’s a tricky one. Yeah. I don’t I don’t have the answer to that. The LP-80 does output the fraction of direct beam though, so that information is there if you see differences from one day to the other and you are concerned about Yeah, and I would say Differing light conditions.

JEFF RITTER 28:25
If you are measuring on clear sky days, just stick to that window of plus or minus two hours around solar noon. The closer you get to the sun being on the horizon, the more trouble you’re going to run into. You should Avoid any data taking around those times.

BRAD NEWBOLD 28:45
Next question is asking, how does sensor height above the ground as well as understory vegetation, how do both of those affect LP-80 LAI estimates?

JEFF RITTER 28:55
So I’ll tell you my first instinct kind of might portray a little bit of a bias. I would first think that it doesn’t affect LAI estimates, how high you are above the ground. But that’s not the case in everything that you measure actually.

CHRIS CHAMBERS 29:12
Depends if there’s canopy that you’re skipping over. That’s right.

JEFF RITTER 29:17
Yeah. So if you are measuring in a stand of trees, likely you are below all the vegetation whenever you measure. And so how high you hold the LP-80 isn’t going to impact your LAI estimates. But if you’re measuring in a short row crop, how high off the ground is going to matter because you’re going to have some below it. So you might want LAI for the top portion of your canopy, you might want to do a profile of LAI moving up and down. So that definitely can play a role. The thing that you need to be aware of is that you just need to be consistent with those measurements. You can’t just kind of throw it anywhere into your crop and then have it directly comparable if you’re not actually measuring the distance.

CHRIS CHAMBERS 30:12
Yeah, that’s right. And it depends on what you intend to do with the data. If you’re using LAI, or more frequently, the FPAR, the intercepted radiation to estimate biomass, then say if you’re looking at tree biomass, then you don’t want to include the understory. You want to try and just get the LAI of the trees. Right. It brings to mind this Sitka spruce stand I was in my youth when I was running around as a forestry technician, and it was a whole canopy of Sitka spruce, big huge Sitka spruce, and then another canopy of Devil’s Club. Miserable, hard to move through, but it was like two completely different canopies, and the LAI would have been completely different if you included the devil’s club.

JEFF RITTER 31:03
Yeah. And there’s always that question, if you are in any sort of environment that has multiple species, are you wanting or needing to get species specific LAI or biomass, or do you want it for the full canopy and everything that’s there? So if you need the full canopy and the entire biomass, then you definitely want to include the understory. But if you just want it for trees or a specific tree, then you need to be careful to know what height to measure at.

BRAD NEWBOLD 31:37
Alright. This next one is a combo question asking how critical is accurate above canopy PAR reference for LP-80 performance, and can it be used it meaning the LP-80, can it be used without an external reference sensor?

JEFF RITTER 31:52
Yeah. Having accurate above canopy PAR, it is important. You are looking at the difference essentially of measurement between the top of the above the canopy and below the canopy. So that’s when you’re looking at LAI, that’s the most important is getting an accurate delta value of PARs. We can get an intercepted PAR. If you’re looking at FPARs, other things that you need absolute PAR, it’s critical to have an accurate above canopy PAR reference. And if your reference sensor, your above canopy sensor is reading differently in the same light conditions than your below canopy, then you’re going to introduce some significant errors into your study. And that’s why our above canopy PAR, the reference sensor is used to calibrate the below canopy sensors, and always keep them on the same page as each other.

CHRIS CHAMBERS 32:48
So do you need to use an external sensor with LP-80? Sometimes. Sometimes?

JEFF RITTER 32:55
You absolutely have to keep your below canopy sensors calibrated. Yeah. At least in at least, you know.

CHRIS CHAMBERS 33:02
Yeah. So even though you don’t always need the external sensor connected to it during every measurement, you need a reference for that below canopy sensor. And it’s perfectly fine, especially on even light conditions where you’re not worried about light conditions changing from minute to minute to not use the external sensor, the reference sensor. Go out with just your wand, get an above canopy PAR in a clearing, trot back under your canopy, take a bunch of readings, and then, refresh and repeat.

BRAD NEWBOLD 33:36
Okay. Another combination question asking, how does sensor aging affect long term LAI monitoring? And then how often should the LP-80 calibration be verified?

CHRIS CHAMBERS 33:47
Yeah. Calibration question. I think that’s important. If we’re talking about needing a reference sensor, how do you know that you can trust your reference? And PAR sensors drift. This is known that you do need to recalibrate most radiation sensors on some kind of basis, right?

JEFF RITTER 34:07
We recommend, what do we recommend? Two years? Two years. Two years. And then just for the external sensor. So the wand sensors, the actual LEDs in the LP-80 itself, they are calibrated to the external sensor. So keep your external sensor calibrated every two years, then on a daily, weekly basis?

CHRIS CHAMBERS 34:29
I would, I mean, it’s a really simple process, so it’s good to do it daily if you Takes seconds.

JEFF RITTER 34:37
Yeah. Or if you get dramatic changes in the light quality in your environment. So a lot of the sensors for, that you have in the wand, they are sensitive to a different band of light than a full quantum sensor will be. And so if you get changes in light quality, you want to make sure that those are matched together, your reference sensor and your wand. It’s really easy to do. We recommend doing it frequently.

BRAD NEWBOLD 35:09
Alright. Are getting close to the end of our time here. We’re gonna finish with a couple questions covering interpretation and uncertainty of measurements. And so this question here, this next question is asking, what magnitude of LAI change is detectable above LP-80 noise? And then how does LP-80 error propagate into productivity or ET models?

JEFF RITTER 35:32
Yeah. That’s interesting. So we’ll get into how you use LP-80 data for ET models here in a second, but when, you know, if we’re first talking about the LP-80 noise or the error you can expect, that’s going to play a role in how much error then we get in our LAI measurements. And then, you know, if it’s It absolutely does propagate straight through whatever data product you’re using.

CHRIS CHAMBERS 36:01
It does. I’ve typically seen fairly low values though, maybe point two you know, LAI overall as far as, an error or noise floor for those. It probably depends on the level though, right? As you get closer to saturation, it’s going to be a little less sensitive.

JEFF RITTER 36:20
Yeah, but the overall noise is based on the model, you know, it’s going to be fairly low. It’s going to be a little bit higher when you get into direct beam conditions, lower for diffuse. So I would have to do some work to quantify exactly what that noise floor is. But I think the more interesting part is in how that plays into like an ET model. So if we are using The main way that you typically see LAI used in modeling evapotranspiration through like Penman Monteith is getting at crop coefficients. So you use your LAI value to get it a crop coefficient, which you then plug into ET. So you’re a couple of steps removed. And there are some important considerations or assumptions in that Penman-Monteith model, namely canopy closure. So if you’re talking about a noise floor of point two LAI, and you’re comparing that to what should be a closed canopy, it’s a fairly small drop in the bucket of overall what you’re looking at, and it’s a couple of steps removed. So, that error is going to propagate straight through, but it’s relatively small in comparison to how it affects your KC values there. So, yeah, because if you were working in a canopy that wasn’t fully closed, you’re already violating one of the assumptions of Penman-Monteith, but you are going to have more of an error propagation if you’re looking at canopy coefficients that way than if you have a very dense closed canopy. Right.

CHRIS CHAMBERS 38:01
And some models, particularly biomass models, they skip LAI altogether. That’s And this goes straight with the FPAR, your intercepted radiation. So it is important to know what your model needs, because you can not use an LAI model, which might introduce some error if you’re then going to plug that into another model, which has its own uncertainty. Right. So definitely, definitely be familiar with the parameters of the model that you’ll be using when you’re collecting your data upfront.

JEFF RITTER 38:33
Yeah. And if you’re looking at modeling ET, your meteorological parameters are going to introduce much more error if you’re not careful with those than the noise from your LP-80 derived crop coefficient.

BRAD NEWBOLD 38:47
Okay. And this is going to be our final question of the session here. When might LP-80 LAI be unsuitable for physiological interpretation?

JEFF RITTER 38:57
Brad, are you specifically asking us if we can use it to scale up leaf level measurements to canopy level?

BRAD NEWBOLD 39:05
Sure.

CHRIS CHAMBERS 39:06
Generally, generally that. It never matches up with the actual canopy measurements. So caution if you try to use it to assume a canopy is like a big leaf.

BRAD NEWBOLD 39:18
Alright. That’s going to wrap it up for us today. Thank you again for joining us, and we hope that you enjoyed this discussion. And thank you again for all the great questions that were submitted. And again, if you had any questions that we didn’t answer, please contact us via our website at METER Group dot com. Finally, subscribe to the METER Group YouTube channel and 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.

CHRIS CHAMBERS 39:55
Bye.

JEFF RITTER 39:56
Bye.