Eyes on Earth Episode 63 – ECOSTRESS and Post-Fire Recovery

JOHN HULT:

Hello everyone, and welcome to another episode of Eyes on Earth. We're a podcast that focuses on our ever-changing planet and on the people here at EROS and across the globe who use remote sensing to monitor and study the health of Earth. I'm your host for today, John Hult. Fires can be destructive or healthy for a landscape-often both. Fires have grown larger and more destructive in recent years, though, thanks to human activity, climate change, and a host of other factors. Satellite data helps us to monitor fire activity and model fire behavior and recovery in a host of ways, most often by tracking burn scars with the infrared and near infrared bands of the electromagnetic spectrum. But the study of post-fire plant life using remote sensing data goes further than fire mapping. Today's guest, Dr. Helen Poulos, is a plant ecologist with Wesleyan University who recently published a study on evapotranspiration in Arizona Pine Oak forest 5-7 years after severe fire, and learned that post-fire shrublands had surprisingly high rates of water use Dr. Poulos used data from the Ecosystem Spaceborne Thermal Radiometer Experiment on Space Station, which is also known as ECOSTRESS. ECOSTRESS data are available through NASA's Land Processes Distributed Active Archive Center or LP DAAC, which is located at EROS. Dr. Poulos, welcome to Eyes on Earth.

HELEN POULOS:

Thanks for having me. And I'm really excited to talk about fires and ECOSTRESS.

HULT:

Wonderful. Wonderful. Well, first let's back up a little bit. Let's talk about your background. How did you come to be interested in plant ecology? And when did remote sensing catch your attention as a tool for studying plant life patterns?

POULOS:

I spent a lot of time outside as a kid, so I camped a lot in coastal California with my family. And I'm an only child, so I ended up spending a lot of time alone in the woods. And I was always just fascinated by trees and understanding patterns of regeneration, and I was always filled with these questions about how the forests that I'm playing and spend time in as a child and an adult came to be how they are today, and what are the patterns and processes leading to our future forests? Graduate school was the first time that I started to understand that Earth observations could be used to understand processes that are going on right before our eyes. I really started to get excited about remote sensing. And one of the most fascinating things to me, I think, was to realize that I could take a measurement off of a leaf in my hands and scale that measurement to observations that were being taken from satellites,

HULT:

You were actually in the field using these instruments, this wasn't something that you were necessarily introduced to, or sort of like fell into in class? This wasn't like an actual, tactile experience.

POULOS:

So I took a remote sensing class when I was in graduate school at the same time as I was growing a bunch of plants from acorns that I had collected out in a greenhouse. And so I ended up taking leaf level measurements in one class, and then looking at those same properties of light reflectance from the Earth's surface and another class and remote sensing. These same properties were operating on something that I could actually touch on the ground and something that was orbiting the planet.

HULT:

Where they just handing out spectrometers at school? 

POULOS:

No, I wish.

HULT:

Was it finding one at a pawn shop or something? How did that work?

POULOS:

So I've really been interested in plant physiology for a long time. And so throughout my career, I've really tried to amass as many tools, analytical tools, as I possibly could, in order to understand what makes plants work under different kinds of environments and under different I've some stresses and after different types of disturbances. And so it just so happened. Taking two classes that were sort of at 180s in terms of thinking about how light reflects from objects. I was really interested in how to use the electromagnetic spectrum to understand patterns and processes on the surface of the Earth.

HULT:

And that's kind of what you've been doing since then, yeah?

POULOS:

I've been going ever since, and in fact, I just, just got a new spectroradiometer and I'm using it with students right here at Wesleyan to understand some of these same ideas.

HULT:

What happens when you hand your students these things and have them do what you did? Can you see the sort of flash across their face? Like the "oh, I get it now" sort of thing that you seem to have experienced?

POULOS:

It is this really neat thing, you know. You show students this slide of a picture of light rays reflecting onto, you know, a cross section of a leaf and explain to them that certain parts of the electromagnetic spectrum are absorbed by the leaf and others are reflected. Those wavelengths that are absorbed are wavelengths that the plant is harnessing and using for making food through photosynthesis. To think that you could look at that on an individual plant and then also observe it from space is one of those moments of like an "aha moment." At least it was for me. And I hope that my students feel the same way.

HULT:

Well, let's talk a little bit about what you do outside the classroom here. Let's talk about this study in Arizona. Where did the idea for this come from? Well, what made you turn to ECOSTRESS and what did you learn? 

POULOS:

I've actually been working in this Arizona study site in the Chiricahua Mountains of southeastern Arizona for about 20 years now. Originally my research questions were really focused around trying to understand 20th century changes in forest there due to fire suppression. So Smokey the Bear, people putting fires out people, excluding fire through grazing. And so the site, in the early 2000s, had experienced over a hundred years of fire removal or fire suppression. So there hadn't been a fire in the Chiricahua Mountains since the 1920s, essentially. Over the last a hundred years, the lack of fire caused an increase in fuel buildup, but not just surface fuels, but also live fuels. Lots of trees have regenerated due to the absence of wildfire. That was really my focus, trying to quantify those changes. Like what were the changes, and how far away from historical conditions have these forests become? Fast forward to 2011, one of the hottest years on record, with regionally synchronous fires throughout the Southwest. And we come to the Horseshoe Two Fire, a very, very large, very severe fire that burned almost the entire mountain range in the Chiricahua Mountains. This sort of signature fire event has really caused some very dramatic changes across the landscape. My colleagues and I were working after that fire to understand and quantify what the changes were. And then to start to understand some mechanisms that could be responsible for some of the patterns that we were seeing. We're seeing a big change in the vegetation complex. For centuries, we saw these frequent fires really promoting a mixture of species of both pines and oaks across the landscape. However, this very hot fire that burned at very high severity, which is quite uncharacteristic of historical fires that were usually frequent, low severity sort of creeping fires that moved across the understory. These high severity fires have caused a transition in the vegetation complex. And so places that have burned really, really hot, or if it burned at high severity, meaning that killed a lot of vegetation, what we're seeing is some conversions away from the mixed pine/oak forest. After the 2011 fire, in this landscape and across the west we're not seeing a lot of pine regeneration, when pine used to be prevalent prior to these contemporary fires. And so that's a big cause for concern. So I've started to look at that mechanistically, to try to understand what are the physiological processes that are going on that are explaining why oaks are doing well and pines, aren't doing very well. And ECOSTRESS is really just another lens for thinking about that. ECOSTRESS offers the opportunity for us to look at plant water cycling across the post-fire landscape and understand how wildfires really changed the hydrology of sites.

HULT:

You were looking at an area that hadn't seen a lot of large fire activity for years and years and years and years, thanks to fire suppression, maybe some other factors, and then a whole lot of fires happened at the same time, including a very large one. And what happened afterwards was significant. There were changes that maybe we wouldn't have expected and you were trying to figure out why? What's going on here?

POULOS:

Exactly. Why? And so ECOSTRESS is really one of those tools that lets me look at the why factor. 

HULT:

What did you find out with ECOSTRESS?

POULOS:

One of the great things about ECOSTRESS is that it allows us to understand how water moves from the soil to the atmosphere. Water can go from the soil to the atmosphere through one of two mechanisms: It can either be directly evaporated from the soil, into the atmosphere, or it can move through plants and out their leaves into the atmosphere. And so ECOSTRESS really allows us to think about how plant sweat is really influencing the hydrological cycle. Unlike most other studies of evapotranspiration, at my site, evapotranspiration in the post-fire landscape is really high. So you can imagine that if you burn all the vegetation down in a site that burned across a large landscape, that high-intensity, a hot fire, for most of these studies what we see is that there's a depression and evapotranspiration because there aren't any leaves to move water from the soil to the atmosphere, and that can be something that occurs for up to a decade. What we found using ECOSTRESS data in the sites that have had these oak shrublands come back in and recolonize these sites vigorously, is that evapotranspiration is really. And the hotter the fire or the higher the severity of the fire, the more, or higher, the evapotranspiration.

HULT:

And what does that mean for these landscapes, then?

POULOS:

In some cases that there's less water available downstream to end users, right? So we're talking about high-elevation mountain environments with lots of forests, but those mountains are really providing water to downstream and lower-elevation users in other communities. And also that means that there's less water in the soil to promote the regeneration and survival and success of other species that are there. What's happening is the water is going from the soil into the atmosphere and not into the soil, and then running downstream.

HULT:

I imagine this is something that could have a relatively significant impact on a place like Arizona?

POULOS:

Absolutely. And so not only are we concerned about the loss of important species that need to maintain higher water status like pines, but we're also concerned about, you know, how much water is available to local communities within the area.

HULT:

So really someone might look at this, just from the title and the abstract and think, "oh, well, she's looking at trees. And one set of trees came back and the other ones aren't there anymore. So what?" But the "so what" is really that this has impacts on users downstream, on ecosystem as a whole. There are a lot of sort of ripple effects of replacing one foundational vegetation type with another.

POULOS:

Yeah, absolutely. And it'll be interesting to see ... you know, we've just published our first paper from this project, but a second paper that we're working on is an experiment that we've actually launched in one of the canyons where we've put field sensors in, that are taking field of evapotranspiration data, and then we're going to be validating ECOSTRESS and seeing how good of a job ECOSTRESS does at predicting what's going on on the ground. And so it will be really interesting to see, because this year, in fact, was almost a record monsoon year for the Southwestern United States. So it will be interesting to see how hydrological cycling might differ in a year where we have really high rain.

HULT:

Let's talk about ECOSTRESS specifically now, because there are other remotely sensed evapotranspiration data sets and sources, other ways to do this. What was it about ECOSTRESS that made it suited for this job?

POULOS:

Most other remote sensing evapotranspiration products like MODIS have a 500-meter resolution for its pixel size. And so when we talk about ecological processes, especially in rugged landscapes like where I work, 500 meters is going to cover a vast proportion of landscapes. So it's not going to be necessarily very meaningful for understanding changes that occur over short geographical distances. So the 70-meter resolution pixel of ECOSTRESS really provides a major improvement to allow us to look at some of these processes using remote sensing with an ecologically relevant pixel size. So that's number one. Number two is that ECOSTRESS, because it's mounted on the international space station, it has a processing orbit. So what that means is that it can actually try to figure out how diurnal variation in plant water use occurs, right? So I used that word plant sweat. When plants start to get stressed out, moisture stress, or light stress, they close their stomates, right? The little holes for gas exchange in their leaves. And the timing of the closure of their stomates is actually really important in terms of understanding plant water stress. And so the fact that ECOSTRESS acquires imagery at different times of day actually allows us to understand temporal variation over the course of the day, in terms of how plants use water.

HULT:

So you have the 70-meter pixel size. You have a little higher resolution than what you would normally get, and you have really good temporal resolution. So you can see, you can get, a much better look at these cycles?

POULOS:

Absolutely. And so, for example, Landsat or most other satellite sensors, they acquire their images same time every day, local time. So Landsat, 10:30 a.m. local time, right? One of the things about this first set of results that we have coming out of this project is that in fact, by being able to compare morning versus midday evapotranspiration in these oaks, we actually showed that one of the reasons why these oaks are so successful is that they're able to keep their stomates open and continue to cycle water throughout the day into the afternoon. The pines on the other hand, shut down. And so if we didn't have ECOSTRESS, we wouldn't be able to see that that timing is actually a really important drought tolerance mechanism that these oaks have, and that's responsible for their post-fire success.

HULT:

So essentially, instead of getting a look at the same time of day, you can look at it through the day and that is unique. And that was informative in this case.

POULOS:

Yeah. So most plants open their stomates up around sunrise, maybe an hour after sunrise, they get going. It starts to sequester their carbon, doing photosynthesis. And then at about 11:30 or 12, when the conditions become hot and dry and lots of sunlight, that's when they shut down and they close their stomates, and they kind of go into a holding pattern til nighttime comes, and then they open their stomates back up. This timing factor is really a powerful component of ECOSTRESS. By being able to actually understand when these plants are starting to shut down under different types of stress tolerance. Not only for fire, but for many other applications, it's really quite useful. 

HULT: 

Why is it so important to understand post-fire ET patterns? We touched on this a little bit. What, what do you hope will be done with this research and where do we go next? 

POULOS:

One of the really interesting and exciting things about remote sensing in general is that it provides this opportunity for you to scale observations, right? And so I talked earlier in our discussion about scaling a leaf level measurement to space observations, but the same thing is true about most remotely sensed imagery. It offers this opportunity to understand patterns and processes across landscape. ECOSTRESS is really powerful in terms of post-fire recovery, right? Not just at my site in Arizona. That paper that we wrote this year was really about laying a foundation for thinking about how we could use ECOSTRESS as a tool for understanding post-fire recovery or trajectories of change. That can be really useful for landscape monitoring and for landscape management, focused on trying to maintain healthy ecosystems and healthy forests throughout the southwestern U.S and the West.

HULT:

It sounds like you have some more plans for more publications. You're looking a little deeper into these questions. What's coming up next?

POULOS:

One of the next things we'd like to do is to try to understand how contemporary wildfires are influenced by these hydrological variables. So throughout this conversation I've been talking about post-fire dynamics, right, and how we can use eco stress to understand post-fire recovery. But we can also use ECOSTRESS and the land surface temperature sensor itself to A, detect wildfires, because the land surface temperature sensor, like the VIIRS sensor, for example, provides a temperature sensor with fairly frequent visitation. But what you can also do is try to understand what kinds of factors were leading up to the fire and determine why a fire behaved the way it did. That's really what I'm currently involved in is trying to look at fire data from across a variety of different sites, to try to understand and use ECOSTRESS, to try to predict how different hydrological and topographical variables influence contemporary wildfires. So the idea then, of course, is to try to understand and use that information with managers to try to predict how fires might burn in particular landscapes and try to mitigate those risks.

HULT:

So we know that the trees that are coming back across this particular landscape are much thirstier, and that has implications for people who might actually use the water, need the water, but now we maybe need to think about what that's going to mean for the next fire to have that as a replacement?

POULOS:

The main thing is that by using remotely sensed evapotranspiration data, we can sort of understand certain landscapes or certain locations or certain vegetation types might be at risk of future wildfire. 

HULT:

So now I'd like to talk about your advice for someone who is interested like you are in the plants and the trees, and maybe he hasn't considered remote sensing.

POULOS:

Go take a remote sensing class, stat! No, I'm just kidding. No, I mean, it's interesting because students come to me saying that they want to use, they want to learn remote sensing, they want to use remote sensing. And, um, there's no time like the present to just get out there and do it, right? I mean, there's, there's definitely ... one of the challenges of course, associated with remote sensing is that there is a learning curve and there are analytical tools that you need to know and understand. And that's why I said, "oh, go take, go take a remote sensing class." But at the same time, I would say remote sensing offers this really amazing opportunity to look across landscapes. Go do it. Take measurements, learn about different images. I sort of stumbled upon ECOSTRESS myself. I'd used the more traditional Landsat and MODIS products, and then it's just sort of found out that there was this new sensor that had been launched that takes interesting ecologically relevant data. And so I got on the chain and became an early adopter for the imagery, and just basically taught myself how to use it. I would tell any young, ambitious person who wanted to learn remote sensing to go out, get an account at LP DAAC and start downloading some data, start working with it.

HULT:

Thank you to Dr. Poulos for coming on our show. 

POULOS:

Thanks so much for having me. It's been a great time.

HULT:

And a big thank you to the listeners for joining us, as well. You can find all our shows on our website at usgs.gov/EROS. That's U-S-G-S dot gov forward slash E-R-O-S. You can also follow EROS on Facebook or Twitter to find the latest episodes, or you can find our shows on apple or Google podcasts. This podcast is a product of the U.S. Geological Survey, Department of Interior.