Transition Zone
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	In this episode of Where Two Deserts Meet, we sit down with Dr. Cameron Barrows of the University of California – Riverside. With his assistance, we break down what exactly it means for two deserts to meet in Joshua Tree National Park, what that truly looks like out on the landscape, and how it is changing before our eyes. 
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	Donovan:  Where Two Deserts Meet is an official podcast of Joshua Tree National Park. Joshua Tree National Park acknowledges the Serrano, Cahuilla, Mojave, and Chemehuevi  people as the original stewards of the land in which the park now sits. We are grateful to have the opportunity to work with the indigenous people in this place, and we pay our respects to the people past, present, and emerging who have been here since time in Memorial.   

Donovan: Hi, and welcome to Where Two Deserts Meet, a Joshua Tree National Park Official podcasts. My name is Donovan
Ian: and I'm Ian.
Donovan: And we're both park rangers here at Joshua Tree National Park, Where Two Deserts Meet is a podcast where we investigate topics that often require a bit more detail, and sometimes the help of an expert in the field to gain perspective.
Ian: You know, I had a visitor recently asked me about where the line between the two deserts is and Joshua Tree. They were looking for a spot where they could pull off that stand in both places at once kind of thing. But I realized I don't really have a solid answer for them. I usually refer to our park brochure where we have the transition zone labeled or just tell them to look for certain plants that grow in each desert.
Donovan: Yeah, that's still a great resource, but we've actually come to understand that it's a lot more complicated. At almost 800,000 acres, slightly bigger than the state of Rhode Island, Joshua Tree National Park is a large, vast open landscape of desert plants, animals, vibrant night skies, and fluctuating climate conditions. Joshua Tree National Park visitation has also skyrocketed to over 3 million visitors in the past few years, pulling in among the top 10 most visited national parks in the country. If you look at a general summary of Joshua Tree National Park in any sort of guide or travel blog, the first few lines usually start with Joshua Tree National Park, a place where two deserts meet, referring to the Mojave and the Colorado Deserts and the transition zone between them.
Ian: But wait, what even is a transition zone? If we're going to use that terminology, we should probably define it, right?
Donovan: Oh yeah, of course. But it's fairly hard to define. I have always been told that there's usually some good indicator plants that define where those boundary lines occur within the two deserts.
Ian: All right, I got this. If I recall correctly, the well-recognized Colorado Desert plants are the Paloverde and the Ironwood, which are often seen down near the south entrance of the park. Whereas in the Mojave, you would look for the famous Joshua Trees, which are usually seen on the north side and especially along Park Boulevard.
Donovan: Working in the visitor centers, I have actually seen a lot of confusion with this firsthand, especially visitors who come in through the south entrance of the park expecting to see a Joshua tree, not realizing that they're actually almost an hour south of the nearest Joshua tree.
Ian: Yep. That's definitely something a lot of us, park rangers at Joshua Tree, have experienced.
Donovan: But I personally think that driving through the transition zone between the Mojave and the Colorado Desert is a huge part of what actually makes Joshua Tree National Park so special. You drive around the corner from any direction, and suddenly new plant life and geological structures appear. For the visitors who are asking about this transition line, how do you usually describe to them where to go?
Ian: Well, if I'm at one of the north side visitor centers, I usually tell them to start driving down towards the south end of the park, and they will visibly start to see a shift as they get further and further down. But there isn't really a definite line to look for, mostly because the line is changing.
Donovan: Wait, how was it changing?
Ian: Well, if you came to the park a few centuries ago, the transition zone might have looked fairly different. It's changing because of more than just one factor, but I know just the expert to talk to about this, Dr. Cameron Barrows of the University of California Riverside's Center for Conservation Biology.
Ian: So, we've been driving about an hour now, and we've found ourselves on this beautiful campus. Where are we right now?
Donovan: We're actually at the University of California Riverside, but they're extension campus over here in Palm Desert. What's kind of funny though, is when you're driving on the freeway, you wouldn't really know that you're hitting Palm Desert because it's labeled as other desert cities.
Ian: Honestly, that's probably one of my favorite signs ever in the history of signs. It really does capture how people feel about the desert. There are a few big cities that everyone knows, like Palm Springs and Joshua Tree, but the other ones are just “other”…“other desert cities.”
Cameron: Hello. My name is Dr. Cameron Barrows and I'm an emeritus professor or researcher with University of California Riverside, just recently retired. I've been working in the desert for about 35 years or so. Primarily developing conservation programs for endangered and rare species, but more recently focused on how animals and plants are interacting with their environment and how that environment has been affected by humans and more specifically, climate change.
Ian: So, a project that I'm interested in hearing a little more about is one that is titled Managing Species in Transition Zones in the Face of Climate Change in Joshua Tree National Park. Can you tell us a little bit more about this and what spawned this project?
Cameron: Sure. Well, what spawned the project was, probably ten, twelve years ago, there was a research publication that indicated that Joshua Trees would be eliminated from the California landscape as a result of climate change. I somewhat jokingly went to the National Park Service and said, “before you change your name to Creosote Bush National Park, maybe we should look at this in a little bit more detail and focus the scale on the park itself.” The park loved the idea, but they wanted us to look at more than just Joshua Trees, but at the entire ecosystem. As we all know, Joshua Tree straddles both the Mojave and the Colorado Deserts. That's the transition zone between the lower hotter dryer Colorado Desert, and the somewhat cooler, somewhat moister Mojave Desert. What we were looking for is a couple of dozen species to see how each of those might respond to climate change over time. But it's affecting us right here where we live, and it's affecting us now. It's not something in the future, it's a current issue.
Donovan: Okay. So just to take us back, we've clearly established that there are two deserts within the park, the Colorado and the Mojave. But how exactly do you know which desert you're standing in while you're inside the park and where exactly should you look for that transition zone? Luckily, Dr. Barrows was able to provide us some key signs to look for.
Cameron: Well, the Colorado Desert has its own set of species, but there's some overlap. One of the primary species that goes both ways is the creosote bush, and turns out from our analyses, the creosote bush is probably one of the most resilient species to climate change. They don't seem to care all that much. They don't look all that great by the end of summer, but as soon as it rains, they pop right back again. So, that's good news. With regard to that, the Colorado Desert, the lower desert, some of the characteristic species are palm oases. So, the palm Oases within the park are part of the Colorado Desert. Another species that's characteristic of both the Colorado Desert and the Sonoran Desert are ocotillos. As you are transitioning into the Mojave Desert, you start picking up more yuccas, and especially the Joshua Trees. Ultimately at the higher elevations, you're picking up pinions and junipers as well.
Donovan: Plants are great indicators to use when identifying what desert you might be standing in. Now, of course, we've already started to establish that these transition zones might not be so clean-cut. Oases are typically found in the Colorado Desert, but can also be found in the lower elevations around the city of Twentynine Palms, which traditionally identifies as the Mojave Desert. This is a great reminder on how nature doesn't always identify with the lines that we draw on maps.
Cameron: Well, one of the quotes that I sometimes use is basically that the only thing constant about our earth is change and it's been changing for hundreds of millions and billions of years. On a geologic time scale, if you think about, when the first Native Americans, the indigenous people came to this region, which is anything from 15,000 to 20,000 years ago, somewhere in that range, it was a completely different place. It was much, much wetter, and much, much cooler during that period. There were camels, there were horses, there were… native horses, not introduced horses. There were mastodons and mammoths and giant ground sloths and a very, very different landscape than we see today. Junipers and the pines would've been more extensive. More oaks. Species would've made living in this region much easier for those first inhabitants that came here, but that was at the end of the last glacial, maximum of the Pleistocene. So, whether or not we're still in the Pleistocene or not, we'll have to wait another 20 or 30,000 years to see whether or not we shift back into another glacial period. But currently we're in an interglacial period, which is a drier, warmer period, and so as a result of that dramatic climate change, of course we don't have wild camels. We don't have native wild horses, we don't have mastodons, we don't have any of those mega type species that used to occur here; and what has replaced them with are things that have moved up from the south, from the tropics. Cactus, which they are an iconic species of deserts plants, actually their origin is in the tropics. If you go to Costa Rica or if you go to the islands in the Caribbean, cactus grew all over the place. Cactus are very happy in hot places, but they're also not too unhappy about being in a wet place. Except for, and this is the caveat, is that where they primarily occur in the tropics is as epiphytes at the tops of trees. So, capturing a fair amount of water as it's falling through the trees, but there's no soil in the trees, so their roots dry out really quickly afterwards. So that adaptation of dealing with both wet and dry, adapted them to living in a desert.
Ian: Considering what we know about how desert plants have adapted to these extremely harsh and various environments, we wanted to know how those landscapes are changing in the present. So, we asked Dr. Barrows, what are some factors as to why we are seeing such drastic shifts in desert plants as compared to the past.
Cameron: There's two real driving factors: climate change being one of them and the other one really is invasive species. Joshua Trees are in an interesting position in that they are in a pattern of air movement that comes from Los Angeles, and that Los Angeles air continues up to this point. This is one of the potentially good things about people adapting to climate change, if we reduced our impact on our use of cars in trucks and trains and things that burn fossil fuels. That air pattern that's coming from Los Angeles will be cleaner. Right now, it's really dirty. It's filled with nitric oxides of various chemical formulas and those nitrous oxides fall into Joshua Tree as well as much of the Mojave Desert. When they fall, if you were a farmer, you're thinking, how do I increase my crop yields? Well, let's add nitrogen to the soil. Those nitrous oxides are adding nitrogen to the soil. What's wrong is that Joshua Trees don't care about it and neither do bushes or junipers or pinions. They don't respond to that extra nitrogen at all, but what does respond to nitrogen is invasive species. Invasive grasses, invasive mustards, things like that, have moved in. On its face value, it’s really bad, but what makes it much worse is that those species carry fire. Fire has become, in recent decades, a component of the Mojave Desert, which it never used to. There was never enough fuel to allow a fire to move from one Joshua tree in the next Joshua tree or one creosote bush to the next creosote bush. Now those spaces between the trees and shrubs are filled with invasive grasses or invasive mustards, various types, and as a result of that, fires carry across the desert. Some of the listeners might be aware that Cima Dome burned just this past summer, and I think it was a lightning strike, which is a natural event, but it would've hit only one or two Joshua trees and that would've been the end of it, but because of the invasive grasses and other plants, the fire moved in and burned thousands, and thousands of acres, killing most Joshua trees in its path.
Donovan: The Cima dome fire that Dr. Barrows is referring to was a fire that burned about 44 thousand acres of Joshua trees in and around the Mojave National Preserve. Around August of 2020, over one million Joshua trees were burned in this fire. I got a chance to follow up with Drew, a Mojave National Preserve botanist, and he said that the dome fire was started by multiple dry lightning strikes. It was the same storm system that started much of the record breaking wildfires that we saw during 2020. Invasive grasses were just one factor that led to the size of this fire. Also, it was during an extended drought period and a heat wave this same week that Death Valley hit 130 degrees fahrenheit for the first time. Winds were strong and erratic contributing to unpredictable fire behavior. The science is clear that invasive grasses can be an issue when it comes to wildfires, but luckily, Dr. Barrow's research can assist with this issue.
Cameron: What my science is supposed to do is inform management and help them make decisions. Most of our land managers, whether it's the Park Service or the Bureau of Land Management or local nonprofits, land management is an expensive, under undertaking, and there's not a lot of extra money for that. The science that we do is try to inform the managers as to what the risk is if you don't do anything about this invasive species. And so, what is the impact of that species on the environment? Is it indirect competition with native species? In some cases, that's true. Is it changing the ecosystem so that the native species don't reproduce or they can't exist there anymore? Or is it facilitating fire? Or is it doing none of the above and, and just ignore it because the effort to get rid of it would not be worth the benefit you get from it. Deserts are defined by being dry and for the most part, warm or pretty hot. There are some deserts that are cold deserts, but they are still dry. The main factor that defines a desert is how much rainfall it gets. Temperature impacts that because the warmer it gets, the less of that rainfall actually gets to the plants. It evaporates away. In the Colorado Desert, which is a warmer desert, when we do get rainfall, the rain, a certain percentage of that evaporates and never gets to the plants, never gets to the animals. As it gets warmer, that percentage gets less and less and less. And the same is true for the Mojave Desert, so what we're finding in this transition is that the areas of what we would call the Mojave Desert, they have Mojave Desert plants like Joshua trees that are in that. Transition zones right near the Colorado desert, Joshua trees are no longer reproducing. You can drive through there, and if you're not looking carefully, you'll say, “well, there's a bunch of Joshua trees, what's the problem?” But if you do look a little carefully, you'll see that they're all the same age. They're all adult plants. There's no reproduction happening that's replacing those that naturally die from old age. And so what we're seeing as a result of climate change is in that transition zone with respect. Mojave Desert plants, those that are characteristic of the Mojave Desert are either dying, but more importantly, they're not reproducing in that transition zone or near that transition zone. So even as a visitor would drive by and say, “I'm seeing lots of Joshua trees, what's the problem?” You have to look more carefully and ask yourself if you’re seeing any little Joshua trees? Are you seeing the ones that are going to replace those big ones so that in fifty to a hundred years when our children or our grandchildren visit the park, that they'll have a similar experience? What is happening already is that it's much hotter and it's much drier, and that's already the been the case. I've been looking at drought cycles over the past roughly a hundred years, and in the first 75 years there was major droughts that were really debilitating to the plants and animals here. In the last 25 years, we've had three, and this year looks like it might be number four. If you extrapolate that out for the next a hundred years, then we are on the path of seeing twenty to thirty or more severe droughts affecting the plants. Even creosotes, which I said earlier are at this point seemingly resilient to the effects of climate change. They'll start to not be able to reproduce and they'll eventually not be on the landscape. The Joshua Trees are, this is happening quite quickly. It's also happening quickly with the Ocotillo, and we're seeing that across the board that many, many of the species that we consider iconic of desert habitats are able to handle the amount of heat and especially the amount of drying. When we talk about climate changes, there's another euphemism called global warming, but it's much more than that. It's really, at least in our deserts, global drying. If you lived on the east coast of the North America or in the Southeast, it's a very different situation. Climate change is going to make it warmer. Yes. But it's also going to make it much wetter. And so the flooding that we've seen in the east coast in the last four or five, ten years…that's climate change. And all those people that are pushed out of their houses…that's climate Change.
Ian: When talking about climate change, many have heard of what the research indicates for the future and it can be, frankly, overwhelming. However, change is possible, but we must start now. It's important to remember that people like Dr. Barrows aren't just focused on what could happen, but also what we can do to shift the trajectory.
Cameron: In the short term, the trajectory is very clear. It's getting much warmer and much drier, and that's impacting a lot of species. But what we're finding for many of them, maybe not all of them, for many species, there are places on the landscape that are going to be, at least for the shorter term, maybe the longer term, resilient to those effects. It could be north facing slopes, it could be a little bit higher elevation. What we refer to those as climate refugia, that these species will be able to sustain populations in those little patches. We've identified, for instance, within Joshua Tree National Park that if we get serious about climate change right now and start shifting out of a carbon-based economy and, and we get real serious about it, we'll still lose close to 75% of the Joshua trees. That's the current trajectory. The 25 or 30% that don't get lost are in this area that I call a climate refugia. They're resilient to a large extent, but not if we just choose to do nothing. If we say this is something we're going to let our great-great-grandchildren worry about, we're not going to deal with it ourselves or even our own children to worry about. We're not going to deal with it. If that's the path we take, then all the Joshua trees in the park are gone. It doesn't mean that they're gone extinct everywhere, but within the park, they're gone.
Donovan: In Dr. Barrow's research, he mapped out in detail ways in which the Joshua trees are shifting through the park, but he also noticed that there are certain areas in the park that Joshua trees are able to find a sort of refuge, often providing the necessary resources, plants and animals need in order to survive these areas, also known as Refugia can teach us a lot about how plants and animals are adapting to changes and potentially ways in which we can adapt ourselves.
Cameron: Yeah, I think the refugia are, the hope that we have. It’s one of the reasons that I started focusing on refugia as part of my research is that from the standpoint of a park manager or a land manager or somebody who cares about the environment. Climate change just looks overwhelming in terms of the loss of animals and plants, the loss of biodiversity, and because the sources of the change carbon-based economies are so pervasive the world. It's easy for a land manager to say, “well, I can't do anything about this. And these animals and plants, not only I'm being paid to take care of them, but personally I'm passionate about taking care of these animals and plants and I can't do anything about it.” I looked at that and said, “well, maybe there is something you can do about it.” If we do identify these refugia and we protect those refugia as much as we possibly can, which again means controlling invasive species so fire doesn't become an issue or invasive species that might be competing with the native plants. That gives those refugia more time to exist while the rest of us humans get our acts together and start really being serious about changing the way we treat our earth. What we think is likely to happen and we're seeing this already is that there is this boundary. It's a broad boundary. It's not a lot of time you see a line on the map and you think if you stand one foot on each side of the line, one of your feet will be in the Colorado Desert and your other foot's going to be in the Mojave Desert. That's not the case. It's a broad, broad zone, and the maps don't let you know that that's the case. You see the sharp lining. You think, “well, here I am,” but it's like you're moving from one county to the next county, and that's not the way it is. But if you think about it as the Colorado Desert plants, if climate change doesn't overwhelm altogether, they will start shifting into what is now the Mojave Desert. The Mojave Desert will look more like what the Colorado Desert looks like, and the Mojave Desert will have to shift to higher elevations. So within the park, highest elevation is about 6,000 feet, but most of the high elevations within the park are sort of the 4,000 to 5,000 zone. In that zone, the higher levels, like the 5,000 feet up to 6,000 feet, those are where those refugia you could be, especially if they're on north facing slopes and especially if they're at the western edge of the park because the western edge of the park probably gets double the rainfall that the eastern side of the park gets and a little bit cooler. That's where the refugia is going. They will be able to sustain populations at least for the next few decades, and maybe longer, hopefully. But then if you look at it broadly, you think, “well, then the Mojave Desert is going to be shifting up.” Mountains and so up towards Big Bear in the San Bernard Bernardino Mountains or on the other end of the spectrum, up towards Mount Jacinto. There's a lot of room to move when you look at it from that broader landscape, but when you look at Joshua Tree itself, 6,000 feet is the limit. There's no more habitat beyond 6,000 feet. We already are seeing species that have hit that limit and we won't see them anymore. There's a very few manzanita left. There's a horn lizard called the Blaine Fields Horn lizard, and right now they're just at that top, top level. A little bit more warming, a little bit more drying and they're popped off the top of the park, which again, doesn't mean they're extinct yet. It means that the only populations that are going to be able to sustain themselves as the ones that can move up the San Bernardino Mountains or up the Santa Santo or Santa Rosa mountains.
Donovan: Listening to Dr. Barrows, it's clear that things are changing. Plants and animals are having to adapt and learn at faster paces in order to survive. Luckily, wildlife and plants inside Joshua Tree National Park, are no strangers when it comes to adapting and overcoming. But it doesn't mean that there won't be effects from this long-term struggle. What's important is that there is still time to change things.
Cameron: This is a very good question and it's a very complex answer. One of the classes I teach is called Climate Stewards, and it’s the goal of the class to teach people to both understand climate change, but also feel empowered to do something about it. And that, I think, is the crux of things that people don't feel empowered to do anything about it. As a scientist, if another scientist or a knowledgeable person came and said, “this is happening and it's really bad, and here's the data that shows it”, that's what I want to see. I want to be able to look at that data and say, “yeah, could anything else explain that? No, it doesn't look like it, or it does”, and then I can either get on board or not get on board. What psychologists have found is that the science is crystal clear, and yet people are still not getting on board, so more doesn't actually help. And so what we need to do and what as park interpreters you need to do is to show them not only how it's going to infect them in terms of their experience in the park but that the park is this incredibly important national treasure, and it'll be changed in many of the species that we thought were protected. What the psychologist tell us anyway is that you really have to make it clear that it's not just that a Joshua tree is not going to be able to live where it used to live. This is something that affects every single living thing on earth, including us.
Ian: What we're trying to capture, right? We're trying to capture the diversity of both.
Donovan: With new insight into the ways in which the transition zone is changing, Ian and I went to the Ocotillo patch just off of Pinto Basin to see the effects of these changes. Just shortly after driving south passing White Tank campground, we noticed the Joshua trees were becoming smaller and less of abundant with not a single Joshua tree in sight. Once we passed Stirrup Tank Road, experiencing in person what Dr. Barrows was talking about, made everything feel very real. It made us think about how with three million visitors coming to the park each year, sometimes the park can feel a bit overwhelming, but it actually gave us hope. It gave us hope that if each of those three million visitors who come to Joshua Tree National Park connect with it and find out what it is that makes this place so special, then together we'd be able to make a difference. Of course, we can make individual choices in our own personal lives to help lower our impacts using less electricity, conserving water, reduced gas consumption. Throughout history, all great change not only came from individual action, but a collective movement. Increasing wildfires, drought, extreme weather, to see a decrease in the effects of climate change, we must be the change that we want to see.
Donovan: Where Two Deserts Meet is a production of Joshua Tree National Park, co-hosted and written by Donovan Smith and Ian Chadwick, produced and edited by Donovan Smith. We would like to extend special things to Dr. Cameron Barrows for talking with us, Sharon Lee Hart for letting us use her art piece titled Split as the Cover of Where Two Deserts Meet and Bar Stool for letting us use their songs, Slow lane, Lanky, Lockley Fells, and Feather Soft as the music for Where Two Deserts Meet. For more information, please visit our park website at www.nps.gov/jotr. Happy trails.