KBIA's Trevor Grandin spoke with Assistant Professor of Biometeorology Jeff Wood about he and his colleagues recent study.
How did you and your colleagues go from this hyper micro scale to a more macro scale of forests in general wilting?
Yeah, the research that I'm involved with, a lot of it pertains to understanding what whole ecosystems do. How they function, how they respond to changes in weather and climate. And so that's kind of the level that I'm working at usually.
And we use a variety of tools to observe the breathing of the whole forest, and then we combine that with other measurements of individual trees or leaves or roots to try to get a finer scale picture and understand the forest from the bottom up, so the breathing of the whole forest. From the top down and then combining that with the bottom-up observations and combining those things to really try to develop a better understanding of how the whole forest functions and so that is kind of, I think, good context for understanding, kind of, how we came at this problem. You know, thinking about mechanisms that explain what you see individual plants doing and but then thinking about, well, how does this scale up to a whole ecosystem like a forest.
And when it comes to, talking about a forest itself wilting, can you kind of define the idea of a wilting forest, and maybe what that looks like to the naked eye?
So, this is going to be a bit different than if you think about the plant that you forgot to water because, you know, these are woody plants. And so, it's not like the tree stem tips over, right? So, the wilting idea has to do more with the leaves. And so, you know, seeing leaves that start to droop a lot more and become flaccid, much like you would expect on a small plant. So, it's a bit harder to see because, you know, the leaves are up in the canopy.
But another thing that occurred during the 2012 drought that we were studying, in particular here, that is kind of beyond the wilting is that the stress got so severe that a lot of the leaves at the top basically died really early and they started to look as if they were turning in the fall. But it was actually just all kinds of dead tissue. And so, they weren't making the chlorophyll that makes the leaves green. And so, it looked like the fall colors had started at the end of August that year. But it was really just that the top layer had died during the drought, so that would be if you were looking down on a very severely stressed forest, you may see that type of color as well.
And you mentioned the 2012 drought, what was so special about that [year], that you decided to go all the way back to 2012 to study that.
Yeah, we're very fortunate this has been a long-term project overall, you know, not this specific paper, but this effort to understand deciduous forests in mid Missouri. These observations started in 2004 and have continued since then. We have these automatic instruments that measure every second of every day, all the time. And so, we supplement that with additional measurements, that people have to go out and make, but we have a very rich historical data set. Some work had been done previously before my time on the project started to look at how changes from year to year in the variability of precipitation influenced the functioning of trees in the forest.
But these kinds of analysis were a bit coarser, looking at, you know, statistics over a whole growing season and how those might differ from year to year. In this case, we kind of got the idea to maybe go back and look at finer scales of time to look at the evolution of this very, you know, it was an exceptional drought, it was very, very dry. And so, it was a a very good situation to be able to look how things evolved over time because there was such a strong, what we would call, drought signal. And so, it made for a good case study to be able to test some of these ideas basically. And so, this kind of gives deeper understanding to some of those other results that I alluded to previously, like understanding, you know, why those were observed potentially.
And when it comes to testing the hydration of forests you mentioned bottom up. Do you start down with the roots or do you start up with the leaves and meet in the middle? How did you go about that?
Yeah, that's a that's a good question, actually. So yeah, we make a measurement and I apologize for the technical jargon here, but we'll start with that. We measure something called the leaf water potential, and, in particular, we usually measure this at pre-dawn. So that means someone has to go out to the forest, before the sun comes up, which around this time of year means very early. So I appreciate everybody who helps to make it happen. I've done it. We all take our turns, but you have to go out and collect the leaves. And what this measurement tells us is it gives us an indication of the water status of the system.
And in the system, in this case, when we sample a leaf from an individual tree, because at pre-dawn the trees have not been transpiring very much overnight. It means that the water from the soil all the way up to the leaf starts to get, if it's not at equilibrium, gets close to equilibrium. So the state of the water, from the leaf down into the soil, is similar and so by measuring the leaves, we can take it at pre-dawn, and we take advantage of that. So, it kind of tells us what the pulse of the tree is, of that whole tree, at that time. And by measuring for many trees of different species, you know, we get a sense of what that community of plants is feeling as far as water stress is concerned. So, it's a fancy name for measuring essentially the water status or the hydration level of the plant in a in a way.
Is it the idea that at night, it's not being beaten down by the sun, causing evaporation? So, it's right after it's asleep so it’s the perfect amount of time.
Basically yes, as far as the level of stress that that tree is experienced, it's as good as it's gonna get because the sun's about to turn on and go through a daily cycle of being exposed to intense light and hotter temperatures, and so getting a bead on, kind of, how this stress level of the trees changes through time, through the season, by measuring pre-dawn and then continuing overtime, we get a good sense of tracking how well are the trees doing.
And with those different species of trees, what is special about the place that the study was done, and can it be kind of extrapolated to different places?
Yeah, that's a that's a great question. You know this is a really interesting place for doing water relations research, particularly with respect to forests, because we have a lot of variability from year to year in rainfall. We also have a lot of variability within seasons. And when you have the input of water, being variable, combined with the fact that the soils are fairly thin, it means that the plants experience water stress more often, and it's a much more dynamic, frequent event for them to experience this. So, it's easier to observe it repeatedly over time, and so you get a good broad range of conditions going from no stress all year, there's some years where that happens, to ones where it's very severe, like in 2012 and everything in between.
And so, for really characterizing how these individual trees and how the whole forest behaves, it's a really good system for that. It becomes more difficult as you move further into the east, where things might be a little moister and you know soils might be a bit deeper and so there's more plant available water. And so, the exact way that we did this measurement from the top down, this forest wilting point, you need a strong drought to be able to determine it from the top down. We think there are ways to do it from the bottom up as well, and so we tested that using some measurements of the roots and the soil characteristics and that matched very well with what we derived from the top down. And so that is kind of a first order support for that at other sites where it might be tough to get at it from the top down, you could definitely get at it from the bottom up.
Reading through the study, the biggest kind of revelation was the idea that if this happens to a forest, if the wilting point occurs, then a forest could become a CO2 positive. Can you kind of talk about that and then also maybe what happens to a forest when that wilting point is hit, including that CO2 positive?
So the idea is when the actual water status of the plants goes below this wilting point, we're into a very severe situation indeed. So, in that year, this forest was a net CO2, carbon dioxide source for all of July and August, which is very unusual, right? Because you see, July and August, that's growing season, we should be storing lots of carbon. And the idea is, as the trees and the, more specifically, the leaves become progressively more dehydrated, they can no longer exchange gases with the atmosphere, so they can't take in the CO2.
Then also the other aspects of photosynthesis can be inhibited, so you get you start to get the physiology being inhibited and so the photosynthesis gets shut down but the plant is still metabolizing stored energy, you know, from prior months or even years potentially. And so, they're still respiring, you know, they're breathing in very little. You know, these trees are long lived organisms, right? It's very different than a corn field where the crop fails, “Okay, we start over the next year with new seeds,” and so, there can be legacy effects that are longer lasting.
So, when we start thinking about the drought impacts, it's acutely what might be happening. Then there's also the, well, how does this drought affect later years, right. And so, you know, this is kind of an interesting question that, you know, why are we having trees die after one drought and not after another and actually we observed elevated mortality or elevated death of trees in the year after the drought. It wasn't during the drought itself, it was a next-year phenomenon. So, we saw white oaks and black oaks dying at higher rates in the year after and the reason for this is the tree is a bit weakened during the drought. It has to change, you know, how it's allocating its resources, and so maybe it's no longer able to invest in defensive mechanisms. And so, the trees will become more susceptible to, say, pests, pathogens or insects after they've been previously stressed.
In terms of what you may hear of natural climate solutions or nature based solution, this is something to consider that in the context of climate change, you know, there's a lot of uncertainty into the persistence of the effect because of elevated risks associated with, you know, how climate affects the functioning of the trees and other things like fire, pests [and] pathogens. So, there's a lot of uncertainty because of that sort of collection of potential threats. But that doesn't mean planting trees is a bad idea, it's a very good idea.
So actually I should point out that that the forest at the end of August that year did get back out of the wilted state. There was recovery within that year. So even though the trees got into a very, very bad situation and were very stressed, the forest did display very remarkable recovery. There was the remnants of a hurricane that passed by and dropped a lot of rainfall, and the forest was very happy. And the CO2 sink recovered. So, after that the the forest started taking up carbon in a net sense again which was nice to see.
And interestingly, it actually was taking up about the same amount of carbon dioxide in a net sense as during wet years at that time. And that has to do with how, if you look at in an absolute sense say, how much photosynthesis was happening and how much respiration was happening, those were both lower in the drought year at that time versus the wet year. But the difference between the two was similar. So, like the whole metabolism slowed down because of stress and so during recovery the balance that we observed between those two broad groups of processes was similar to in a wet year. It's just those, individually, photosynthesis would have just been higher, so too would have been respiration. But because of how they both move around together in a net sense, the force was actually looking like it was doing pretty well. Which is kind of interesting.
What do you think the outcome is, long-term, thinking about climate change and droughts in Missouri, possibly occurring more often?
So, in terms of thinking about the results in a bigger picture one of the challenges really is projecting the future as far as how forests and other ecosystems are going to behave. And so, this is something that we can use to help build better models to make better projections. So, this is a way where we can confront the models and say, “Oh, are the models getting this right? Are they reproducing this behavior.”
And if they're not, then we can maybe get better clues as to how we might need to improve them in order to make, you know, better predictions of what's going to happen to the forest in the future.
You as a person, what do you hope this study brings forward? It being published, what do you, Dr. Wood, hope to see come from this study?
Yeah, that's an interesting question. And honestly, I guess it's one of the things where it often gets published and then it's on to the next thing quite often. And I hadn't necessarily thought too much about that. I mean, at a basic level, you always hope that your work is used by other people. And so hopefully other people, other scientists find it useful, and we can tell if that's the case after a few years if it's being cited by people.
But I think one that one of the big things, I think, is this idea of thinking about taking ideas up to understand the whole ecosystem[‘s] behavior and getting that scale jump, if you will, from leaves and whole plants up to the whole ecosystem and thinking about it in new ways.
This is a long-term research project, and this has been a great collaboration over the years between scientists who also provide funding support from Oak Ridge National Laboratory. So, that's a Department of Energy National Lab. And so, this research is sponsored through a project that they have with the DOE Office of Science through a terrestrial ecosystem, scientific Focus Area project. And so, to be able to have the great collaborators and be able to make these long term observations is really important to be able to ask some of these interesting questions and have the data that is needed to be able to answer them, and so, that has been a great collaboration. And then also other great co-authors from other universities, you know, research is a team game and so I think it's important to acknowledge that it wasn't just me sitting at a desk doing this. There's been a lot of great colleagues who I worked with on this project.
