Episode 24: Drought and disease in the Sierra
“The drought for a tree is like going on this really long fast. Without food or water for an extended period of time any human being and any tree is going to become weak and unable to defend itself.”
joan dudney
A conversation with Dr. Joan Dudney (UC Davis) about drought, disease, and pest interactions on forest communities in California's Sierra Nevada. Released June 11, 2021
guests on the show
Joan Dudney
Dr. Joan Dudney is a Davis H. Smith Postdoctoral Fellow based at UC Davis who studies global change impacts in terrestrial plant communities. Dr. Dudney combines novel methodological approaches with long-term observational and experimental data to disentangle the complex, interacting, and often nonlinear relationships between plant communities and global change drivers, including pests, pathogens, drought, and fire. Her research is grounded both by personal experiences and the challenges we face in an era of unprecedented socio-ecological changes. Dr. Dudney’s passion for the outdoors has led her to some of the most remote places in the lower 48, and has inspired a life-long commitment to ecosystem science and stewardship. Learn more about her work and photography here and follow her on Twitter @dudney_joan
Transcript
Mallika Nocco
Welcome to Water Talk. In today's episode we're speaking with Dr. Joan Dudney. Joan is a forest ecologist and a David H. Smith Conservation Research Fellow based at the University of California, Davis. Joan is looking at how drought in the Sierra can leave trees vulnerable to pathogens and she had a recent study that really quantified how complex some of these interactions can be. I was curious to learn more just in thinking about all of these interactions together, and really not thinking about drought, as I am often thinking about it as an irrigation scientist in this sterile environment and thinking about it in in an environment where there are plants, but then there are also, you know, there's diseases, and what does drought do? What does it really do to a plant, you know, how does being vulnerable impact these forests that many of us care so much about.
I think it's really interesting to listen to Joan in terms of those interactions, and also just the way in which she approaches her work, and how she is accessing and working in places that many of us don't have access to, and aren't able to go to. Joan has shared some of her amazing photography and art videography with us as well just related to this episode and some of the places that she speaks about. So, Sam, Faith, what are your thoughts on this episode with Joan?
Faith Kearns
My first real entree to Joan's work was through her photography on Twitter. I am just highly appreciative that she is out there making these multi day off trail treks to places that are just really inaccessible to most people. And as much as I would love to make it to some of these places, it is never going to happen. And so at least having these beautiful photographs is really one of the great aspects of her merging her science and art together and sharing it with the rest of us.
Sam Sandoval
She's able to make all these relationships between some pathogens, plant physiology, how we can be related with drought, fire. So all those relationships that happen or that we are thinking she is able to present it really well. I think this is an episode that is very timely for the drought and the future droughts or drought years that we're going to live. It is just timeless in terms of how climate change and the way that we are managing some of our forests are important today and in the future.
Mallika Nocco
Absolutely, Sam and that kind of made me think too about just how you know as scientists and as someone who's accessing these places, in some sense, being a field ecologist Joan is a witness. You know, scientists, in some circumstances such as these are bearing witness to these events. We did speak with Joan a little bit about what it's like to experience that grief, what it's like to bear witness to these extreme events as they continue to happen. So I thought that was really powerful and interesting, too. All right. So coming up next, our guest of the week is Joan Dudney. Hello, Joan.
Joan Dudney
Hi, thanks so much for having me.
Mallika Nocco
So I know that your work is very place based and it's grounded in the Sierras. Can you just tell us a little bit about your love of the Sierras and like how did you make that transition from recreation to conservation and science?
Joan Dudney
I wanted to do my PhD in the Sierras is because when I wasn't backpacking, I was directing restoration programs for this really cute nonprofit in the Bay Area called Grassroots Ecology, and I spent three years pulling out weeds trying to reverse the major impacts that land use change and urbanization was having on the local plant and animal communities. And after a while, it became super clear to me that reversing what has already been done to the landscape is super challenging, sometimes impossible. And I wanted to make sure that we didn't do the same thing to the Sierra Nevada. So I went back to school to figure out how we can be more effective stewards of the Sierra Nevada, and I realized that this is a lifelong endeavor, and no single PhD could ever answer that question.
Mallika Nocco
Well, that's cool. Well, speaking of that, it would be great if you could just give us an overview of the kind of things that you're working on now. For example, which tree systems do you focus your work on and which parts of the Sierra? The Sierra is so vast.
Joan Dudney
So I have this dream that I want to work in all parts of the Sierra and I'm sort of building that out. I started in Sequoia and Kings Canyon National Parks, which are in the southern tip of the Sierra Nevada. And then recently, I co led a research project that spanned the majority of the Sierra Nevada. So I kind of do research everywhere, now. Anywhere that I am just excited to understand something that's happening on the landscape a little bit more, I'll try to find funds and go out and explore.
Mallika Nocco
That's fantastic. I think that's pretty neat that you are trying to focus on so many parts of the Sierra. I just think back to like, what you were saying about how vast they were before, and now you're going all over them?
Joan Dudney
Yeah, totally, I have the skills now. I mean, it took me probably 10 years to really feel confident out there. And to bring a really large crew with me and now have the scientific chops that I can actually go out and collect the data that I want to collect, it took a while to get comfortable with it. But now I get to go out to these pretty incredible places that very few people have ever seen. They're really really remote, sometimes three days from any trail. So that's pretty rare, right. Like most of the time, you're hiking on a trail if you're in the outdoors, but we would hike for days off trail in a cross country passes, we're just navigating by descriptions of books and maps. And safety is something that we think about a lot, especially working with these crews for multiple summers. And you know, they are exposed to really extreme conditions. We've been out there during lightning events, for example. And when you're at the top of a mountain, sometimes my thought would be at the top of a mountain, and the trees are so short that they actually don't provide any relief. So you have to like, book it down to the bottom of the mountain to protect yourself. But it takes so long just to get out there that you don't want to leave. And so sometimes it'll be like, you know, the lightning will be right around. And we should have left, you know, half an hour ago, but we just want to get one last tree, you know. So yeah, we're definitely constantly navigating pretty extreme conditions.
I actually had one crew member who's this like, really big burly guy who went up to a pass and he looked at the path and he was like, absolutely not, I'm not going this way and has a bit of a meltdown. And then my crew and I had no choice but to go down with him and around the other way that he said was safer. As we were climbing up, it was so shale-y and so slippery, and we got to the top. And he looked at me and sheepishly and he said, you're right, this is way more dangerous. But you know, it's just interesting how everybody has a different comfort level with different types of conditions. So like, I feel the probability that you get hit is pretty low. But I know other people are terrified, like the minute they hear it in the distance they are down the mountain immediately. So yeah, you're constantly navigating these extreme conditions, but also, the crew’s own comfort levels with these different extremes.
One of my favorite places, it's called tuna. It is one of the most remote places in the lower 48 and you get to this low valley region, and you look up and it looks like a mountain range in between you and where you're trying to go. And once you get over that little sort of mountain, it's a lip, I guess. There's this huge lake and it's the most crystal clear blue lake I've ever seen, so deep, but so clear. And it's hard because nobody ever goes out there, you know, a few people a year maybe go out there. There's now sort of like these myths about how big the fish are in this lake, so I think people are excited about going into this region. It was almost like standing at the edge of that lake, the wind died down and you just really feel how desolate and remote this world is how tiny you are, and I think that sort of experience-it changes you. It really does. It gives you a permanent, different perspective on your own life and on your own sort of existence.
Mallika Nocco
Yeah. How long did it take you to get to this place?
Joan Dudney
Probably four days. So we had to go over two off trail mountain passes. And yeah, it was a pretty extreme place to navigate. But that's what I live for. I love it. It's so much fun. It's such an adventure to be able to go where almost nobody had gone before. To really trail blaze your own path. You are doing everything. It's you against the mountain and then of course you're responsible for your crew. But yeah, it's a pretty exhilarating thing I think to be fit enough, like I had to work out a lot, you know, to prepare for these types of extreme conditions, to have the comfort level to be able to, you know, be on these high ledges and feel like even though you have this crazy heavy pack that may feel a little imbalanced that you're going to make it, you're going to be fine. But yeah, it's an exhilarating thing.
Mallika Nocco
That's really cool. So we are on Water Talk, and we wanted to talk to you about a new study that just came out. And we often hear, at least I hear a lot. about how drought can leave trees vulnerable to pathogens and to pests. And it's something that is spoken of that the drought can impact trees in this way. But you quantify this and you went out and you really looked at these impacts for a specific group of trees, and a specific group of pathogens, and a specific group of pests. I was just wondering if you could tell us more about your new study.
Joan Dudney
So I was looking at a pathogen called White Pine blister rust, which is a mouthful, we'll call it blister rust from now on. And I was looking at how blister rust interacting with bark beetles can interact with drought. So maybe I'll just give a little bit of a backstory. But drought can induce vulnerability to pests and pathogens largely because trees become immunocompromised. So droughts for a tree is like going on this really long fast. You can imagine like without food or water for an extended period of time, any human being, any tree is going to become weak and unable to defend itself. So the same thing happens to trees during a drought. And basically what happens is they become literally immunocompromised, so they're less capable of using their immune systems to fight off attacks of bark beetles, or a pathogen. So in our systems and our immune systems, we have white blood cells, antibodies for a tree, they have secondary carbohydrates and chemical compounds called flavonoids. And this is the coolest thing ever. But actually, plasmoids are natural antioxidants for humans. And we actually eat them to build up our own immune systems.
Mallika Nocco
That's super cool, because I've heard of flavonoids as the reason that I get to drink a glass of red wine every night. Yeah, but I don't think I had connected that to trees, and I definitely had not connected that to drought.
Joan Dudney
That's really neat. So that's what we're trying to understand more of is how is it exactly that drought reduces the immune defense of a tree? And then how is it that bark beetles are able to multiply at the rates that they are able to during the drought? And then where do they go? Like what happens to those populations afterwards? I'm looking at a study right now that's trying to capture their movement after a drought. And I think what's happening is that in some cases, they're moving into higher elevation systems that may be less drought stress. And it's just sort of like this population boom that enables them to move up, whether or not the trees are really drought stress.
Mallika Nocco
So then, in the new study that you just published, you looked at the interaction of these drought stress, compromised trees, with the bark beetles, is that correct, and the blister rust - all of them together?
Joan Dudney
I compared trees that were infected with a pathogen to trees that were uninfected during the drought. So I was looking at what was the impact of the drought on trees that were infected and trees that weren't infected? What I found was, in general, we saw a lot more physiological stress in the trees that were infected with a pathogen, that's to be expected, and they were probably more likely to die during the drought. So actually, the overall effect of drought on this particular pathogen White Pine blister rust is probably not positive. Because if you're killing your host more quickly than the pathogen can't reproduce as quickly. And there's this other kind of crazy thing, it actually infects trees through the needle stomata - little plant cells at the top of the plant leave that are sort of the gatekeepers to the outside air. During drought, trees will often close their stomata to conserve water. And that also closes the gate to pathogen infection. So you kind of have this double whammy during drought, of negative for this pathogen that I'm studying. That's not to say that all pathogen impacts or drought impacts on pathogens are negative. There are other studies that show that drought can actually increase the spread of certain pathogens. So it's really kind of pathogen specific in terms of what the overall impact of drought will be.
Mallika Nocco
Okay, that's super interesting. So did you look at a few different species of tree with this pathogen? Or was it just one species?
Joan Dudney
I looked at one species interacting with drought and pathogens. And then I looked at four different species across the landscape. So what was also really interesting during the drought is that not all trees are actually drought stressed. So I couldn't really conduct a study of drought impacts on a tree that wasn't drought impacted, right. And that's because the trees at high elevation - the trees and the sub Alpine zone right here, treeline - you start hiking up into these, the top of the mountain, the density of trees declines, and you get these shorter stature trees. And those trees at that elevation actually did better, they did better during drought. Isn't that crazy? So like 150 million trees they estimate died in these low elevation systems in Sierra Nevada. But in the upper elevation systems, right around tree line, all of those trees actually grew more, they did more photosynthesis. And it was probably an overall positive effect for plant productivity and growth during this really extreme drought. So you have this incredible radiant of impact of drought in the Sierra Nevada from really negative to actually kind of positive, which makes it a pretty complex system to study pathogen interactions in.
Mallika Nocco
And I should just clarify for our listeners, we are talking about the California megadrought, correct? So when did you actually do the study? Like how long had they been under the drought? Or was it after the drought had passed?
Joan Dudney
Yes. So I did a couple of studies. So this is sort of like a combination of multiple studies that we're talking about. But the drought interaction with the pathogens was during the drought. And then a little bit after the drought, the way that I'm extracting data, I can actually look into previous years. So I started collecting in 2016, which is what some people would categorize as the end of the drought, though it is debated, actually, whether it was 2016, or 2017, or 2015. Yeah, I could look back into 2011 and compare post to pre drought conditions. And look at how the trees were responding because of the methodology that I was using, I didn't actually have to collect it at the exact same time.
Mallika Nocco
So this actually kind of I'm going to skip ahead a little bit because I feel like the right question to ask you now is what what are these methodologies like? What are some of the tools of your trade?
Joan Dudney
I use a number of different things, I combine long term data. So one of the exciting things about ecology right now is that we have a lot more long term data that we can use - long term data just means that over the past 20, 30 years - scientists have had funding to go out and repeat sample. So they've been able to go out to the same places and get similar data across multiple years. So those data are super awesome, and pretty rare still, but really, we're building that out in the ecological community. And then I combined those data with stable isotope analyses. I collect tree material, so it leaves tree rings. And then I can extract the stable isotope signature and look at how these trees were responding physiologically to changes in their environment. And then I sometimes combined that with remotely sensed data. Something I'm trying to build out right now is looking how to scale these field tests to the landscape. And the way that we can do that is by harnessing this amazing remote sense data and start to look at patterns both locally and at the landscape scale.
Mallika Nocco
So I guess the question that I had is, how does climate change and how do fires fit into all of this work?
Joan Dudney
Yeah, that's a question that I'm certainly going to probably spend the rest of my life trying to answer, especially around climate change. I think fire for me is less of a focus, although fire happens in my system all the time. And so just by proxy, I'm out there measuring the impacts of fire. So climate change is probably going to increase the severity and intensity of wildfires. In fact, I think we're already seeing that in California. I've grown up in California has never experienced this level of smoke and wildfire season before in my entire life. So it's definitely choppy and and these kinds of fires are having a major impact on our ecosystem and the interactions with pests and pathogens, actually.
A couple of really cool studies show that after a bark beetle outbreak - we thought you know, 100 and 50 million trees die, in part because of bark beetles - and you know, millions of tinder boxes just ready to burn and that dead dry fuel. Because of this you're more likely to see a high severity fire also probably more capable of burning, so your ignition probability is also a bit higher, but that's only probably within the first two or three years after a bark beetle outbreak. And then the probability of a high severity fire actually declines quite precipitously. So there are these really interesting dynamics that you get between pathogens, droughts, bark beetles and fire, that lead to these sometimes catastrophic fires, or sometimes actually decrease the the likelihood that the forest burns.
Mallika Nocco
I have a quick clarification question. So there was the drought, which happened first, the bark beetles come in first or the blister rust come in first, or was it at the same time?
Joan Dudney
Different times. Yeah. So that's another really interesting thing about all of these different actors in the system is that they act on very different timescales and spatial scales. So blister rust was introduced, an invasive pathogen was introduced 100 years ago by the father of conservation Gifford Pinchot. But that makes me very cautious when I think about, you know what my impact could be on the landscape and just inadvertently could have this major ramification for all of North American forests, like his tiny little decision to import new seedlings from Europe that happened to be infected changed the fate of American forests forever, right, we're losing white bark pine, which is this high elevation tree species in the IUCN endangered species list, it's potentially going to be listed under the Endangered Species Act in the United States. And that's because of one small action that we make as a human.
But anyway, these pathogens move through forests at a much slower pace than a fire or bark beetle outbreak will. Bark beetle outbreaks happen really quickly. Usually, within a couple of years, we'll have this catastrophic die back, but fires, obviously, within a couple months. So this specific pathogen arrived in southern Sierra about 50 years ago, and only now are we starting to see major ramifications for sugar pine and Western white pine. So these different temporal scales that these actors act on make it really complex. And yet you can't go out and measure trees within a couple years and know everything that's happened to that system, everything that will happen to that system, because of sort of these stochastic random things that are happening at different timescales.
Mallika Nocco
Wow, that's pretty interesting. That's super interesting about Gifford Pinchot. So I guess, how do you see this whole system of you know, climate change, potentially increasing drought, increasing fire? And then as well as these kind of different temporal impacts of disease like the blister rust, and the beetles? How do you think that this is all going to shift tree communities in this era or particular trees? Are there any trees that you're most concerned about? Are there any trees that you think are going to kind of win in this situation in this brave new world?
Joan Dudney
Yeah, I think we're already starting to see that. Who's winning who's losing. We found that just in the past 20 years, over 50% of the original trees that we measured 20 years ago had died. And so that's like one generation of sugar pine, the whole population could disappear from the landscape. So it's really fast for the southern Sierra, where I was looking in Sequoia King Canyon National Park specifically. And that's because of the combination of all of these factors we've been talking about: droughts, bark beetles, invasive pathogens, and actually fire is actually more of a nuanced story with sugar pine, because in northern Sierra, there are studies showing that fire can actually induce regeneration. So sugar pine really likes having open light availability, and fires provide that. So if you have seed trees nearby sugar pine can actually do better if there are some more frequent fire regimes. In the southern Sierra, we aren't finding that as much.
The combination of all of these factors are leading to a very rapid decline in sugar pine. I think that that's not the case for all the trees out there. Like I was saying before sugar pines declining, but in the high elevation white bark pine and foxtail pine are actually doing a lot better. There's been a general positive growth trend in the high elevations. And so we're probably going to see movement of species from the lower elevations. And we're going to see certain species that are more vulnerable to these compounding effects of disturbances, low elevations become even less common. So sugar pine, for example, yellow pine is probably not going to do well. And that's important because both of those trees are attacked by that aggressive bark beetle. So there's kind of cool thing where like the bark beetles and the pathogens select the community of trees that are out there. And if you happen to be a seedling that is impacted by a really aggressive bark beetle, and we're also seeing increased drought impacts, then that species probably more vulnerable than firs, for example, that don't have an aggressive bark beetle that attacked them. And so like the fate of these forests under climate change, is in large part determined by the pests and pathogens that attack.
Mallika Nocco
That's super interesting. So are the beetles just not interested in the fir? Or like, do they not eat them? Like are there certain trees that they absolutely will not attack?
Joan Dudney
That's right.
Mallika Nocco
I guess I said eat, but I don't know if there actually, are they attacking to eat?
Joan Dudney
Yeah, they are. Yeah, so most bark beetles have specific hosts that they attack. And then sometimes if the populations really large, like the mountain pine beetle, which is is the one that we always talk about, then they might attack secondary species species that they normally don't most of the time. Yeah, bark beetles, and pathogens. Pathogens can be more generalist but they'll attack specific trees and specific host trees. So yeah, there's sort of that mutualistic relationship that has a really big impact on the overall community and the biodiversity of that system.
Mallika Nocco
Wow, that's really interesting. So I really love how you merge art and science. One of the things that I've noticed is just you've been doing a lot of really cool aerial photography, using drones recently, how does art and how have drones specifically just enhanced your work.
Joan Dudney
I didn't really come to using photography or drone videography because I have like this artistic side that needs to be expressed. There's more that I was just out of these really cool places. And I just brought my cell phone with me. And then I had this friend of mine come out who was a real photographer and he took these incredible photos of me and the crew and the landscape. I was like, oh my gosh, I want to do that. That's awesome. What a great way to communicate my science. And so, I upgraded to like a point and shoot. And I've like slowly worked my way up to now I have a drone. And I have a pretty nice camera, thanks to the Smith Fellowship. But yeah, I think it's fun to sort of think about how you can integrate these other mediums into the work that you're doing to communicate.
Faith Kearns
Thank you. I've got one more question, which is just you know, as somebody who grew up in California, and now is doing this research and watching these ecosystems change just even in your relatively short lifetime, what it really feels like to watch that change, to sort of think about a 50% die off of sugar pines in the southern Sierra, and just how that hits you as a researcher?
Joan Dudney
Yeah, that's a really good question. Grief and loss is something that I think a lot of us are going through, and we don't talk about it a lot, you know, we go out there and we're going to find the answers and we're going to ask the best questions so that we can find the best solution and we don't talk about the emotional toll of watching these trees die. And even talking about it, I feel teary, you know, like it just is an emotional experience. And I connect, I just have such a reverence for these trees, they live, sometimes for over 2000 years, you know, they were around before the Roman Empire collapse, you know, like these are incredible beings right next to us. And so to lose them, it's a hard thing to see. So it certainly has an impact, I think on all of the research community, but I don't know that we have a good approach for how to deal with that or how to talk about it, and sort of, you know, work through that loss.
Mallika Nocco
I think that makes your art so special and that makes the photography that you're capturing important, because maybe they won't be around, you know?
Joan Dudney
Yeah, hopefully we can figure things out and do better. I have a lot of hope that we do have the technologies and we do have the ability to manage and live sustainably within these landscapes, we just need to focus on it, you know. We figure things out when we focus on it. And we actually really have intention around it.
Mallika Nocco
We always ask our guests if there is anything more that you just want people to know about your work? Or how everyone who is listening to this podcast, how can we support your work?
Joan Dudney
I think just going outside, connecting with nature is really important for the work that we do. If you understand that forests and the natural landscape around you is changing, and that you have a direct impact on that landscape, then maybe there'll be more policies on the ground I can then use to fund my research or go out and do the things that I do as a scientist, that has to be supported by a populace that cares, and that's invested and knows it's really important. So for me, it's going out and picking a wildflower in your backyard and appreciating that, looking at birds, climbing a tree, maybe going to a national park once in a while. Look at these incredible landscapes that we have that we live with all the time, you know that they they feel our impact every single day, all of the plants in California and Sierra Nevada feel the impacts of human beings on this earth every single day. We hardly ever connect the dots. We have such an impact on this community, yet we don't even realize what that impact is.
Mallika Nocco
Thank you. That's wonderful. I noticed that you did not mention to connect with the beetles.
Joan Dudney
Beetles are actually an important component of natural ecosystems, pathogens and beetles, they have co-evolved with these forests and actually sustain their adaptability. So the more you have to select out the weakened trees means that in the long run, we actually have more resilient, drought tolerant forests, it's just that when you combine that with the rapid change, environmental change, droughts, these really severe droughts and quickly rising temperatures, the system is just not used to that and so it can't function on the same level that it has been. But some pathogens are really important components of our forests and they make them healthy.