PubTalk 12/2006 - Mojave

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Detailed Description

Title: The Mojave National Preserve: Geology and Water Shape Desert Plant Communities 

  • One of the largest units in the National Park System, the Mojave National Preserve was established in 1994
  • The Preserve encompasses great sand dunes (including "singing sands"), young volcanic features, forests of Joshua trees, and fields of wildflowers 
  • Maps of the surface geology can predict the distribution of vegetation types
  • How do the different ways soils accept rainwater affect plant communities? 
  • Learn how USGS researchers are modeling the relations of geology, water, and desert plants 


Date Taken:

Length: 01:25:40

Location Taken: Menlo Park, CA, US


Good evening. My name is Leslie Gordon, and it’s my pleasure to welcome you again to the U.S. Geological Survey in our continuing public lecture series every month. Before I introduce tonight’s speaker, as always, I have an announcement because we want you to join us again next month and the next month and so forth. In January – so next – you know, the old joke, see you next year. After the first of the year, we have a great lecture lined up called The Hidden World Under the Golden Gate. And it’s a wonderful story of our scientists using a boat and what’s called side-scan sonar, have done some very – multibeam side-scan sonar – have done some very detailed mapping showing the bottom surface underneath the water under Golden Gate Bridge and in the bay. And just fantastic, beautiful pictures and a really interesting story about giant sand waves under the water. So that is January 25th, and we’ll be back to our last Thursday of the month routine in January. So please do join us for that lecture. For tonight, we’ve got kind of a triple feature on the bill. We will be showing a short movie – a short video, and then we have two speakers tonight. The video that you’re going to see was produced by Steve Wessels, who is a geologist on staff in the communications office here at the U.S. Geological Survey. It was done in conjunction with the National Park Service. And it’s called Mojave National Preserve – A Place to Explore. And I think you’ll really enjoy that, and it’ll set the stage as a nice introduction. After the video, you will have two speakers. Our first speaker will be David Miller. Got to get my notes here. Dave Miller is a geologist here at the U.S. Geological Survey. He’s been here in Menlo Park for 28 years. He received his Ph.D. in geology at UCLA in 1978. He’s studied geology all over the Intermountain West, especially in the deserts of Utah and California. And he’s currently studying desert ecology and soils as well as active tectonics in the Mojave Desert. So Dave Miller will be the first speaker after the movie. Our second speaker – we’ll get all this out of the way first. [laughs]

- [inaudible]

- Yes. Remind me. Which one’s Dave? Which one’s John? John Nimmo is a research physicist with the USGS here. He’s been with us since 1981. He’s the chief of the Undersaturated Zone Flow Project – he’ll have to explain what that is – since 1988. His Ph.D. in physics, specializing specifically in porous media, is from the University of Wisconsin. And for more than 20 years, he’s led field, laboratory, and theoretical investigations of unsaturated flow – and he will explain that – soil moisture, and contaminant transport. Together, they will give you the big picture of the Mojave – the geology, the water, the plant communities, and how the whole ecosystem fits together. So I hope you will enjoy tonight’s lecture, and we’ll start out with the movie. [Applause]

[video starts]

- Dry. Desolate. Forbidding. For many, the desert is land to be quickly crossed over, endured, or passed through. But look deeper. For there is much to discover and explore. Mojave National Preserve was created in 1994. It’s the third largest National Park Service site in the contiguous 48 states. 1.6 million acres. The preserve is located at the center of the Mojave Desert between Los Angeles and Las Vegas, bordered by Interstate 15, Interstate 40, and the Nevada state line. [Train horn] From Kelso Depot, in the heart of the preserve, you are surrounded by a mosaic of landforms and habitats, each with plants and animals uniquely adapted for life in the desert. There are numerous mountain ranges, sloping bajadas, and basin floor playas. And dramatic settings such as Kelso Dunes, Hole-in-the-Wall, lava flows, lava tubes, and cinder cones. And the largest and densest Joshua tree forest in the world. While exploring, you may find traces of the region’s rich human history as well. Enjoy a trek across the Kelso Dunes. In terms of total area, this is one of the largest dune fields in North America. The dunes are the product of sand blown to the south from Soda dry lake near Baker. The view from the top reveals the grand scale of the preserve – its basins and surrounding mountains. It’s a vision of raw beauty. Yet these are no ordinary dunes. Try running down the dune ridge and listen for a singing or humming sound. This is one of only a few places in the world where the dunes can actually produce an audible noise. As sheets of sand slip downward over the dune face, vibrating sand grains produce a deep, resonant hum. [Humming sound] Windblown avalanches of sand produce the same incredible sound. Savor your good fortune if you hear this unusual melody. [Humming sound] Between Baker the Kelso Depot, lava fields and cinder cones stand out dramatically. They are the product of volcanic forces active just a few thousand years ago. Some are a’a lava flows, formed like these seen today in Hawaii. The very steeply sided cinder cones are the product of explosive bursts of lava. Amidst the lava fields is a lava tube formed when the surface of a lava flow crusted over while the interior lava flowed on. A small cylindrical tube remains. A ladder into the lava tube offers access below. It’s a one-of-a-kind subterranean perspective of volcanic plumbing.

- .. the temperature of the …

- It’s cold down here.

- Watch your head. Watch your head.

- East of Kelso Depot is Hole-in-the-Wall. A campground and information center adjoin the unusual holey rhyolite cliffs that give the site its name. These, too, are volcanic in origin. Incandescent clouds of rock, ash, and gas spewed for miles from an ancient volcano. Resulting deposits trapped heat and gas, essentially cooking the rocks into the welded massive cliffs seen at Hole-in-the-Wall today. Gradual weathering by wind and rain has produced a somewhat eerie sculpted terrain. A trail passes through Banshee Canyon – a tight slot cut through the cliff walls. Embedded iron rings offer hand and footholds to hikers climbing through the canyon. [Birds chirping] East of Kelso Depot is the largest and densest Joshua tree forest in the world. Passing through this odd forest, you will also find creosote bush and numerous defensively armed plants, including prickly pear cacti and buckhorn chollas. These adaptations enable the plants to survive but can be painful to the careless hiker. Depending on rainfall and temperatures, spring flowers amongst the Joshua trees and across the preserve can be spectacular. Many consider these explosions of color the height of the desert experience. A special time when the dominance of harsh, climatic extremes is put on hold.

- I think this site was probably a little shallower than the other.

- Scientists are exploring many aspects of the preserve. A broad study of plants and animals is helping to clarify which organisms live where and how they manage to survive. The threatened Mojave Desert tortoise is of particular interest. They are protected by law after a sharp decline in numbers. Scientists are developing strategies to stabilize tortoise populations. Although they hibernate in cold months, the tortoises are active during spring and the cool hours of summer. Watch for them in or along park roadways. Since they are a protected species, do not touch or disturb them in any way. Questions about water have prevailed since the first humans set foot here. Evaporation rapidly sucks water from the terrain. This team from the United States Geological Survey is studying how water passes through the soil and becomes available to plants. These infiltration experiments use sensors in the upper few feet of soil to monitor the passage of water over time. [Thunder] [Sounds of rainfall] Understanding the dynamics of water in this place provides insight into the activity and timing of life. Here, water drives nature’s biological clock. Heavy rain from summer thunderstorms rapidly runs off. Seeming to do little with the fabric of life. But when gentle, extended winter rains fall, the payoff to plant and animal life is dramatic. A carpet of green, followed by explosions of wildflowers, mark the onset of a rich and productive spring. Food becomes abundant. A veritable nature’s buffet. But it’s a race to beat the heat. Animals pack away food and water before the seething summer begins. Summer days exceed 100 degrees, and summer nights drop into the 60s. It’s no surprise that life has adapted uniquely to this place. The dramatic mix of high-altitude mountain habitats, sloping bajadas, and basin floor playas, volcanic remnants, sand dunes, and ephemeral lakes and streams showcase the geological diversity of the area. Journeying even just a short distance from your vehicle allows you to experience the desert on its own terms. But the desert can be unforgiving. Drinking water and sun protection are a must for outdoor excursions here. Guidance on trails and other points of interest can be found at information centers. Immerse yourself in the region’s rich historical record, from ancient geologic processes acting over millions of years to more recent human history from Native Americans, through early life in Kelso. Mining. And the railroads. [Train horns] [Train engine running]

- The Joshua tree forest. Lava flows and cinder cones. Hole-in-the-Wall. And the Kelso Dunes are just a part of the rich offerings of Mojave National Preserve. It’s a geological wonderland with incredible desert survivors visible at every turn. Marvel at the irony that this seemingly barren landscape hosts an amazing diversity of life. Explore this strange new land for yourself and discover your own special places along the way.

[video stops]

- Okay. We’re going to move on. For a refresher course, my name is Dave. There will be a quiz at the end. This will be a live presentation. And what I’m going to do is sort of set the scene about geology in the Mojave Preserve, and in particular, why it’s important for understanding the ecology of the Mojave Preserve and the rest of the Mojave Desert. So this is Part I of a two-part talk that is overall titled, the Geology and Water Shape Our Desert Plant Communities. And there’s a large group of us that have been working on this topic. It’s been a really fun group to work with because it involves hydrologists and physicists and a wide variety of ecologists and information scientists and geologists. Been really fun getting to work with. The Mojave Preserve is the subject of the video we just saw. And I want to point out a few things on this Landsat image so that you sort of get the map sense of some of the things we just saw. First of all, over here on the – on the west side is Soda Lake. And it’s got a lot of unusual colors compared to much of the landscape here. And that’s because it’s a wet playa. It has springs emptying into it, and it has a lot of evaporative discharge of water. And so there are a lot of crazy salts in this particular place. So that shows up with a kind of a smorgasbord of colors. And down south of Soda Lake, there’s this – there’s this light-colored track that – it’s all eolian sand. So it’s a series of sand sheets and dunes culminating in Kelso Dunes over here, which was one of the focal points of the talk – of the video. There are high mountain regions here and over in this area. And something that some people don’t normally think of in terms of deserts – often we think of dunes and Saharan kinds of deserts. This is actually a rocky desert on the Mojave. And, spreading away from these high mountains are piedmont areas. These are broad alluvial fans that show up in a variety of colors here, both because of the kinds of rock that are – that are being shed into them and also some other factors that we’re going to talk about. The cinder cones area and the lava fields that we saw in the video are right in this area and really stand out as distinct in this landscape. The focus of many of our very detailed studies is right here, just northeast of Kelso Dunes. So when you see some of the slides that I show, and particularly the slides that John shows, it’s that area that has been studied very intensely. This one funny place right here in the – within the preserve is actually Mitchell Caverns State Park. And it’s remained a state inholding, and it’s a fantastic place as well as all the other places you heard of in the video. We are working with, and have worked with for a long time, a project in USGS that’s called Recoverability and Vulnerability of Desert Ecosystems. And this is a multidisciplinary project. It was designed from the get-go to involve all kinds of different scientists. And the very broad questions that we started to tackle, and are continuing to tackle, include vulnerability – namely vulnerability to disturbance. What parts of the landscape are especially vulnerable? Which ones may not be so vulnerable? Are the biota on the landscape as vulnerable as the landscape, et cetera. Those kinds of questions. And also the recoverability from disturbance. Given that most of the desert has been disturbed to one degree or another by the activities that we saw in the video, like mining and also things like ranching and military activities, et cetera, how well is that desert going to recover? Are there places that will recover just fine by natural processes? And other places where, if we gave it a boost and spent some money to help with the recovery, would it make a big difference? To answer almost all these questions, we really have to understand how the desert functions – how the biota in the desert function. And the particularly cool thing for me – and I suspect for John too – is that, as physical scientists, it’s turned out that the physical systems there are really, really important to study to understand how the biological systems work. So it’s kind of fun to have a home in answering ecological questions even though we’re pointy-headed physicists and geologists. Repeat photography has turned out to be a very neat approach to understanding recovery. And I’m just going to mention this a little bit. This isn’t something that either he or I have been engaged in, but very, very important approach. In this slide, two different pictures of the same place – the exact same place, carefully matched. They were taken nearly 140 years apart. And you can see that, in the upper-left one, the seat that was opened up and named Marl Spring was an important watering spot for a lot of the wagon trains and travelers going through the area. And it’s been developed, and the area’s fairly hammered. There aren’t too many perennial plants showing there. Not many shrubs. And it looks fairly barren. 140 years later, and at least part of that time with very benign impact – no grazing, no nothing. The road basically was shut down. The landscape has changed tremendously. These kinds of paired photographs provide a basis for describing quantitatively how much the landscape can recover by itself. But, of course, more information is needed than simply this view, but it provides a lot of information. This approach has been very, very valuable up in the Death Valley area and in that region of the northern Mojave in particular. So what I’m going to talk about mostly is how desert plants function. And the story is going to be very simple. It’s going to be the geology of the deposits under the plants – the geology that the plants are rooting in. And in particular, the soil development in those deposits. Now, I know a lot of people think of soil as just whatever the plants root in. To soil scientists and geologists, there’s a difference between the geologic deposit – say a sand dune that forms by geologic processes, and the soil that forms in it after it’s been stabilized. So if a dune stops moving, gradually, soils form in them. We call that process pedogenesis. And those are the soils we’re going to be talking about. So I’m going to be talking about geologic deposits, mostly in alluvial fans, so they’re alluvial deposits, and the soils that form in them over time. Quite a bit of study has gone into understanding what’s going on, basically, in the Mojave Desert and in most deserts. And water is the number-one resource that is – that is limiting in the desert. And it’s not just because it doesn’t rain or snow enough. It’s also because it’s extraordinarily hot, in the summers in particular, and evaporation is extremely high. So any precipitation that does make its way into the soils and is stored there temporarily tends to evaporate out very rapidly. So there’s not much water to begin with, and it evaporates very quickly. Nutrients are important too, but basically, you can have a lot of nutrients available for a plant, but if it doesn’t have water, it’s still going to croak. And obviously light and carbon dioxide are important for photosynthesis. And, in fact, too much light can be a problem and too much radiation. So plants have a variety of ways to adapt to these limiting conditions. And a couple of the plants that are really widespread that we’ll be talking about are the white bursage – and that’s in the upper left. It’s a relatively small plant. This particular ruler is a meter stick. And so what you’re seeing is about 70 centimeters, or 2-1/2 feet length of that stick. And the creosote bush down in the lower right is a much bigger plant. These have a really nice contrasting style in dealing with this limitation of not having enough water. The white bursage is what we call drought dormant. And when the soil moisture drops below a certain level, it will drop its leaves and simply go into dormancy until the moisture is once again available. It has a very shallow root system. In contrast, the creosote bush has a very deep, broad root system. It’s able to capture deep moisture. It’s evergreen. There are always green leaves on it. The green leaves may be kind of crinkled and shriveled and brown-ish during an unusually bad drought, but there always are leaves. One of the ways it deals with drought is it actually prunes limbs. So I’m not sure how clear it is, but there’s some limbs down here that just look like sticks without leaves on them. That’s actually a drought response. This picture was taken a year ago, and this plant is only partly recovered from a four- to five-year-long drought that – during which it dropped several of its limbs and focused all of its growth into a smaller area. So what have we learned over the last couple decades? And there have been several important researchers that have worked in here, and we’re following on their studies. One of the things we’ve learned is that, if you look at different geologic deposits, you can see that there are different plant species that inhabit certain deposits. And the plant cover – that is the amount of ground that’s covered by the canopy of the plant – it’s a measure of the biomass – also varies on different deposits. And I’m going to go into this a fair bit. And lastly, that soil horizons that develop in the geologic deposits are really important for understanding the soil moisture budget. Okay. Here’s a beautiful scene from Dante’s View in Death Valley. The Panamint Mountains in the background and the Hanaupah fan in the foreground. Why geologists think this is beautiful is not only because of its intrinsic value, but also because, in that big alluvial fan, you can see many different colors and textures and patterns. And each of those tells a story to a geologist. So one of the things that I’d like to convey to you is that, to a geologist, this looks like a way to make a map of that fan that indicates different kinds – different parts of the fan. Some parts are darker and smoother. Others are deeply dissected and lighter-colored. And it turns out that the active washes are the really light part in these deeply entrenched stream systems. So there’s a story to be told here, and it’s easy to put on a map if we can understand what the patterns are trying to tell us. When we look up close, one of the starkest contrasts in the desert is the desert pavement in the foreground, where there are virtually no perennial plants. They’re just – every now and then, you’ll get a crop of annuals that come up after a certain kind of springtime rainfall. But, in general, they’re devoid of plants. And then behind, even though it’s desert, there’s a relatively lush growth of plants out there, and those are mostly creosote and white bursage in this particular case. So we’re on an alluvial fan here, and we’ve got a huge contrast in the plant cover and also the species of plants. So the question is, why is that? Well, the geologist looks at this and they say, well, I can map these as two different deposits. So let’s understand something about those deposits, and maybe then we can use that to help understand why the plants care about which deposit they’re on. So our way of looking at the deposits is – I’m going to run through real briefly here. We basically have an old deposit here – and I’ll describe what these patterns mean later. And, at some point, a stream cuts a channel through the side of it and carries part of the deposit away. And then it backfills some more deposit. So that’s a younger deposit because it’s inside under the older one. And then it’s, again, cut out, and a younger deposit is formed, and so on. So we can start at the active wash down here, and we can step up these terraces and feel pretty confident that every time we step up a terrace, we’re going to an older deposit. Now, it turns out that, as we go through the stepping, that we go through changes in surface characteristics and plants too. And I’ll get back to that in a minute. One of the things we’ve done is we’ve worked through the eastern Mojave Desert mapping the surficial geology by this scheme is that we’ve collected sort of reconnaissance-level information about the plants, the surface characteristics, the soils – a whole variety of things. And so we can, based on hundreds of observations, start to ask questions about just what do the plant abundances look like? So this is a graph of the cover of creosote bush – the amount of land covered as a function of the age of the deposits. So this is the same age sequence in the last – the last picture that goes from young to intermediate to old. And you basically see that, in the active wash, there’s very little creosote bush, but that’s a function of the frequent flooding and the abrasion by the flood deposits that creosote can’t tolerate. And then, as you get older, in this – in this sequence of deposits, there’s less and less cover by creosote bush. There’s less plants out there, and they’re smaller. So this tells us there’s a very strong relationship between age of an alluvial fan deposit and creosote bush cover. It turns out that it’s about the same for ambrosia, with a couple twists. Ambrosia is the white bursage. And for many other plants, there’s a strong relationship between age of deposit and the plant happiness, if you will. So let’s go back here and look at this diagram and ask the question of, what’s changing with the age? I mean, the plant doesn’t care how old a deposit is, right? Well, it turns out that, after a deposit is stabilized at this point, there’s no more water flowing on it. There’s no more flooding. You know, nothing’s going on. Dust from the atmosphere starts to infiltrate into these fairly porous gravelly sandy deposits. And that dust accumulates progressively with age. And so we can see that progressive change in the deposit with accumulation of silt, some clay, and a fair bit of very fine sand. And, along with the pedogenic processes – the soil horizonation processes, there’s a transformation in the upper half-meter or meter, and sometimes even as deep as 2 or 3 meters in the deposit with time. So what we see is that, when we get back into these older deposits that have strong desert pavements, they also have very strongly layered soils beneath them. They have a – what we call a vesicular A horizon, which is a silt cap. And it is a very unfriendly thing for plants, apparently, because it doesn’t like to let water through into the soils below. And then there is the B horizon, which tends to be more clay-rich in these old deposits, and that has its influence on the soil moisture availability. So a hypothesis was put forward about a decade ago that it is the soils that progressively increase with age in the deposits that are controlling the plant characteristics that I just showed you in that last graph. I’m going to take a little divergence here because that’s sort of the point where John’s going to step in and start telling us exactly how the soil-water relationships work. But first, I want to tell you a little bit about surficial geologic maps because we’re going to use those again later on in this talk. Or at least John will. For surficial geologic maps, what we’re trying to do is show you what we – what I showed you in the picture of the Hanaupah fan, which is basically for this depositional environment being alluvial fan right here, we can put every one of those pieces of the fan into one of these boxes that represents the age that is the soil development in the fan. We can do the same for dry wash deposits, for playa deposits, for lake deposits, or eolian sand and the like. So when we make our geologic map, we’re basically assigning a label to every area [coughs] – excuse me – every piece of the ground out there – every part of the landscape – somewhere in this matrix. And then you can use that information. These are digital geologic maps. They can be used in a predictive sense if we understand, for instance, how the plants interact with the soils in any one of these deposits. So we made a surficial geologic map of the Mojave National Preserve, and this is a – kind of a small version of it. It’s hard to see the detail, but once again, it shows deposit types, the ages of the deposits. It also shows the erosional areas – impediments and the – and the hillsides of the steep mountains that are being eroded. All the various symbols out there indicate places that we have got quantitative data of one sort or another that helps to sort of anchor our maps and helps us to understand the map information. We zoom into the Kelso area. We saw the Kelso Depot in the video, and it lies right here near this junction of the roads. Kelso Dunes would be just down on the fringe of the map here. And much of the study area that John is going to talk about is in this piedmont setting right here below the Providence Mountains. So the geologic map, you can see many different units are displayed there. They convey a fairly complex suite of attributes that we will use in a – in a predictive product towards the end of this. And I would like to summarize the geology part of this by reiterating that desert ecosystems are water-limited. The plants care. The plant species and cover definitely vary with geology. And we think that the reason that there is that variation is that the soil moisture availability for the plants co-varies very strongly with the soil development characteristics. And finally, the surficial geology, particularly when you make a digital map, provides characteristics that you can use to make predictions, including attributes of the soil moisture budget. So that’s that. And now we’re going to set it up for – what’s the name of our next speaker?

- John.

- All right. [laughter] Not everybody’s sleeping.

- Thank you, Dave. I’ll be describing a set of field experiments that we’ve done exploring the interaction of the soil and water. The red light’s off. Can people hear okay? Okay. Some of the questions we want to answer. How fast can water infiltrate in the soil? How deep does it go? How long does it stay at the relatively shallow depths where roots are and the plants – so the plants can make of use it? And how far and how fast does it travel sideways once it infiltrates in one place? To get some of the answers – this is the basic scheme of our research. Select sites with features of interest. So these – and we are particularly concerned, of course – as you know from Dave’s talk, with the different degrees of soil development. So different ages of soils, more or less developed. At the – at the selected sites, we’ll install sensors in the – in the ground that measure soil wetness, measure soil water content, and the pressure of the water in the pores of the soil. And with those installed, we’ll put water on the soil in a controlled way and measure what happens. This is, of course, based on the diagram that Dave showed you. The different soil units that exist in this area of the globe piedmont, from younger to older, of course. And the ones where we have – we have chosen three of these for our main field experiments. This is where you see the large red circle. So, on these three different units, we have done extensive experiments. So we’ve chosen, for one thing, the active wash deposits. Just a few years old. Recently deposited. An intermediate soil has some of the results of added silt and perhaps clay from – that would be blown in by the wind. And then, a much more highly developed soil deposit with a distinct desert pavement at the surface. And different soil horizons, which I’ll show pictures of, that have different textures and different structures of the soil material. So all of these factors in the different age of the soil, or degree of development, can affect the water and how it – how it – how much flows in, how much runs off, what happens to it in the ground. This is our Site number 1, the active wash site. You can see it’s mostly a coarse, sandy material. And here, where we’ve dug it out, you can see some evidence of the individual layers that were deposited by events of sediment being washed down the stream bed. So these depositional events might not happen every year, but every few years, there will be a large enough storm that it moves sediment and deposits a new layer. So this is – what we expect is a fairly favorable environment for – favorable medium for infiltrating water quickly. Our second site – soil of intermediate age – has, at the surface, it has the beginnings of a desert pavement. You can see some stones. They’re not as tightly interlocked as they will be a few thousand years later. And, directly under this, there’s just sort of the beginnings of some clay and silt accumulation forming a finer – beginnings of a finer textured layer at the surface. Below that, to eye, it appears actually quite uniform. When we were out there digging in it, it seemed like pretty much the same stuff as you went down deeper and deeper. There are some coarse particles. You can see in this picture that looks a little deeper some stones, and there’s a fair bit of gravel mixed in with the – with the sandier materials, even though it’s not as – not all completely obvious in this picture. Our third site is a very highly developed soil. It’s got a beautifully mosaiced desert pavement at the surface and a relatively thick vesicular A horizon. This layer – at this site, it’s perhaps 10 centimeters or even more in thickness. It has essentially no gravel – no sand except maybe the very finest sand. Mostly silt and some clay. And it is this layer Dave mentioned that – or, of the layers he mentioned, this one in particular is likely to be an impediment to the infiltration and downward movement of soil moisture. Below this layer, we have a number of different layers. If you look at this level here, there’s a lot of really coarse stuff. A lot of stones. A little bit less of that here. A little bit less of that down below. So we’ve got the soil horizonation that has been occurring here. Different levels within the soil acquiring different characteristics. These different characteristics, of course, acting on the water, they not only have their own unique effect, like we expect fine particles to be restrictive to water flow, but the simple fact that there are layers – that there are layer contrasts – acts as an impediment to downward flow and also to promote sideways flow, as water can spread along these layer surfaces and between them. We look at the particle size distributions. We did this by – we take samples from different depths here down to about 1 meter, and carefully measure their particle size distributions. The way these graphs work is, a point on the curve here tells you how much – what fraction of the soil – what fraction of the bulk soil – in this case, I’m pointing at something like 40% – is smaller in diameter than the value here. Where these are in – these are in microns, but a convenient reference is 1,000 microns is 1 millimeter. So, at this 1 millimeter point, that’s like a very coarse grain of sand. So you can see the active wash and also the Holocene soil look somewhat similar. There is a little bit of difference with depth. Actually, more surprising to me, more in the wash than in the Holocene soil at these shallower depths. So there’s a little difference in the depths, and it’s mainly – you see most of the material is in the – in the sandy ranges. A little bit here, less than these – at the shallower depths, some greater amount of finer particles. In the older soil, rather different. Here you have a lot of variation with depth. These curves from the different depths are – they’re much more spread out – almost unique for each depth. And some of them have a very large fraction of fine material. Now, this is a picture of our experimental setup right at the – at the moment of – after we’ve had our frantic activity getting ready, this is the moment of calm just before we’re ready to start infiltrating water. So what you see here is an artificial pond. This is 1 meter in diameter. Where we’re going to add water – add water from this – this is a 425-gallon tank on the back of a pickup truck. We parked it uphill as far as we could go. So we’re going to let that hose feed the infiltration pond for a certain time. We have instruments installed, which you can see sticking up out of the ground various places. These would be soil moisture sensors at different depths. And, in addition, we have a number of electrodes for electrical resistance measurement. These are – they’re little – they’ll be easier to see in a picture I’ll show a little later, but metal spikes pounded into the ground connected by these – connected electrically by these yellow cables to the – to the electronics and computer we have taking data. Now, I mentioned the instruments in the boreholes. This is just kind of the basic scheme of these instruments. Here’s our pond, and in two different directions, we’ve got a series of three boreholes. And each of these – well, different distances from the pond. But the – in each borehole, we’ve got several instruments. I won’t go into all the terminology here, but these are – these are different devices that measure the water content and the water pressure. The pressure is important because it’s an indication of – it’s actually a negative pressure. It’s an indication of how much work does a plant have to do to suck water out of the soil. If pressure is lower, it has to work that much harder. Now, I mentioned electrical resistance measurements. This is actually an old technique, although we’ve applied it in a very new high-tech sort of way. The basic idea is that you have your steel electrodes – metal spikes in the ground. Say you have a set of four of them. Apply a current – an electrical current to these. Current travels through the ground with two other electrodes. You measure the voltage across them that tells you an electrical resistance. And that resistance, in effect, is resistance – electrical resistance of this soil material that’s down here in this vicinity at some shallow depth below. So what does the resistance tell us? Well, I’m sure most of you know, water is much more electrically conductive than, say, dry sand particles. So, in the soil, the water tends to be the most conductive component that’s there. So, if the resistance – electrical resistance is lower, than that’s an indication it has probably a much higher water content. So we do this as an indication, then, of what the water content is. Very – fairly precisely between saturation and air-dry soil. Now, we have these electrodes – we have a total of 48 electrodes in two crossed lines here. And these are – can be – we can apply our voltages and read the current – or, apply current and read voltages in different combinations. So we’ll read various sets of – sets of four along each of these lines. Gives us an idea of the resistance at points along here. We measure them in combinations that are further spread apart. Say apply current all the way across here, measure voltage here. And this tells us – gives us an indication of resistance at a deeper depth. So we accumulate a large data set from these of electrical resistances that we work with with, naturally, a very complex mathematical algorithm – computer code that applies the tomographic method – looks at these tomographically to give us an image – a two-dimensional image of what the water content – how the water is distributed in the soil – just, what points it’s wetter, what points it’s drier. I’ll be showing some of these in just a moment. This is our experiment with the pond filled. You can see the electrodes here – some of them. The pond we filled to a depth of 5 centimeters and maintained that for two hours and 20 minutes. We did this – we did exactly the same amount of time at each of our three field sites so the points in time could be – or, data from different points in time could be directly compared. During the infiltration, this is – this is showing how much water infiltrates in how much time. So it’s the total volume of water infiltrated here as a function of time. And, in all three of these sites, it actually goes fairly steadily. This doesn’t happen with all soils, but here it’s a pretty constant rate. Very steadily, from our constant height of water in the pond, constantly flowing into the soil. The total amount that infiltrates, as we expected, it’s greater in the active wash than the old soil. And the intermediate soil falls somewhere in between. That makes good sense. One thing that surprised me in this data is that, from this amount of water to this amount of water, it’s only a factor of 4. So 4 times as much water went in at the active wash as in the old soil. And that – to my intuition, thinking about soils and water, that surprised me. Because I think of the active wash as – well, it’s that coarse, sandy material. The water ought to go really fast through that. And the developed soil has that beautifully consistent fine-textured horizon near the surface that I expect is a major impediment to flow. So differences happen to be less than I expected. And I think what that tells us is we’ve got to – we’ve got to look more closely. We’ve got to consider that, in active wash, it’s not a uniform medium like we might want to think. The first picture I showed of it, you saw the different depositional layers – some evidence of that. It could be that the slight differences in texture of different depositions, that that creates enough of a layer contrast that it forms an impediment to downward flow. So it could be that slows down the flow from what it would otherwise be. In the old soil, we’ve got this fine-textured material at the surface. But that’s got some amount of clay in it. It’s got roots going through it. The clay has the effect of, it expands – it swells when it’s wet. Contracts a little when it dries out. This tends to form very fine cracks in the soil. So we probably – in the old soil, we probably have a large number of very small cracks and holes that are hydraulically effective, even though they’re not so apparent to the eye when we’re looking at it. Now I’ll show some of these results from our electrical resistance tomography. The next few slides are images that we made from the data at our active wash site. So this is the one where we had the most infiltration. And what we have here is a two-dimensional picture in depth down to a little more than 1 meter. Here’s our pond. And we go 3 meters to each side of the pond. So we’re looking at electrical resistance in the subsurface here. Now, what these colors show is the change in electrical resistivity. Change from what it was before we started the experiment, where the soil was relatively dry. So where it’s changed negatively – this blue color – lower resistance, that indicates a greater water content. And, in this picture, a few minutes after we started infiltrating, it shows just what we expected. That directly under the pond, there is decreased resistance, indicating some water flowing into the soil. Now, I’m going to go forward in time and show a sequence of images. This is a few minutes later. You can see that it’s spread deeper. There’s more water deeper. Also it’s got what we would call a rather broad, diffuse wetting front. That is, the water doesn’t – as it’s moving downward, there’s not a – not a real sharp change from wet to dry, but it’s spread out a little. And we’ll see that develop. During the period of infiltration, it gets bigger. That is, the wetted area gets bigger. Here, this is the last image we have before the infiltration stopped. So this shows the maximum extent of the – of the water in the soil. The next one is just a few minutes later. And already you can see water starting to drain out. And a few hours later, much of it’s gone. Go a few more hours. And bigger time intervals. This is a day later. And here, two days later, we can see evidence that the great bulk of the water has gone deeper. It looks like it’s gone much deeper than 1 meter. However, there’s still a fair bit of this yellow color left. Yellow means a 50% decline in resistivity. So that’s some amount of soil moisture. The soil would be slightly moist still in this region, even though most of the water has gone down below. Now, I have a couple slides showing comparisons of the youngest soil and the oldest soil. These are done at equivalent points in time of the experiment. So both of these are right at the end of infiltration. So this is the picture you’ve seen before. Here in the older soil, obviously there are – there are significant differences. One thing, of course, we have only 1/4 as much water that’s infiltrated as here. It has not gone nearly as deep. In fact, it seems to be – it’s perhaps held up above this level of half a meter or so. Doesn’t – not much of it ends up going lower than that. If we look at the sideways movement, it’s probably at least as much in this old soil as in the – in the young soil. So here impediments to vertical flow, but probably more horizontal flow. And also, you can see this is – this is more lopsided. The soil is apparently not as uniform as in the – in the active wash. So we can get features – as we often think of soil developing in horizontal layers, but there are other differences that happen also. Various features cause the soil to go one way or another – can cause the water to go one way or another. Now, this is a very important slide. This is four days later. At four days after the infiltration, the active wash site, we have the – basically the picture we’ve seen before. Most of the water is gone. It’s gone below. A little bit left at these shallow layers. However, in the older soil, there are pockets of quite significant soil moisture that are retained in the – in the shallow layers. So there’s also still not much evidence of water moving much below this 1 meter depth. So it looks like there’s a pretty effective barrier to flow somewhere around this half-meter to 1-meter layer. Also these – the materials near the surface – finer-textured materials are likely more attractive to the water, can hold onto it more firmly. So you have pockets of material with what we’d call greater water capacity that can hold onto that water. Now, what does this mean to the plants? These are – I’m going to talk about the same two plants that Dave mentioned – the creosote bush here on the left and the white bursage. And this picture I took at our field site number 2 where we happened to have both of these plants in abundance. Had both of them right there. So you can see the size difference. The creosote bush is much larger than white bursage. And root systems go along with that. As Dave mentioned, the creosote bush has a deep and broad root system. Grows deep and spreads quite a ways horizontally. The white bursage has a much shallower and less extensive root system. The other thing that’s obvious here is this one’s got nice, green leaves. This one’s dormant. So this picture was taken in March. Happened that we’re still in the dormant phase of the white bursage. But the creosote bush always has some leaves. In fact, we have to think in terms of an evergreen plant. Even in the hottest, driest days of summer, it’s got to have some soil moisture – some – not much because the creosote bush has adapted to small quantities of soil moisture, but it’s got to have at least a little bit to bring up to those leaves to keep them alive. So where do they grow best? Here – first, let me point out a couple more pictures. This is the creosote bush you saw before. This is a couple months later in the year when it’s in bloom. The ambrosia at its dormant – is dormant phase. And also here in full flower at some later time. Now, this figure shows from – data from Hamerlynck and others. Study done in the same general area as our experiments. They looked at how much – how much plant matter you have of these species. So sort of the density of creosote bush per unit area of land, the density of the white bursage per unit area of land, and where is it more dense or less. And what they found is, that – oh, they also – they looked at different ages of surfaces. Just as we have done – some young, intermediate, and old, in both cases. So you see the trend Dave was noting here for the creosote bush, it seems to like the young surfaces. Surfaces get older, and you have less creosote bush that grows there. The ambrosia – the white bursage is rather different. It doesn’t vary as much from one type of surface to another. And it’s actually – seems to grow best in the oldest soil. So let’s think about what differences might cause that to happen. I’d like to keep that in mind and look back again at this picture, after four days of redistribution of water, that if you – okay, imagine a creosote bush on this surface. The creosote bush has deep roots, broad-spreading roots. Roots probably go much below 1 meter, so our instruments didn’t go below 1 meter, but the water’s probably – there’s probably some water there that it would have access to. The ambrosia, on the other hand, shallow roots would not have access to water for very long in this type of medium. In the old soil, consider here. Well, the creosote bush with its deep roots don’t give it – perhaps don’t give it too much of an advantage. It would have some shallow water to make use of. The white bursage, however, okay, it’s – I would guess these pockets of moisture, they’re probably pretty much ideally suited for the ambrosia, or white bursage, to make use of. So it – with soil wetted in this condition, it might come out of its dormancy, make use of the – of the water in the shallow layers white it’s there. You think about how it would evolve from here. If we’d been able to keep our instruments going for a very long period of time, this picture would probably change very slowly. Very slowly dry out as plants use the water, and as it – as it drains downward. Here, similar sorts of things. Water near the surface is likely to be – likely not to last as long. It can evaporate directly out of the soil as well as be used by essentially all the plants, shallow-rooted as well. And so this – it’s got water here now. But this is not likely to last a real long time. So that might be fine for the white bursage. It can go dormant when this shallow water is gone. The – but the creosote bush would need to rely on some water source. It needs to get water from a deeper reservoir. The deeper soil is a better place for water to be a little more stable for a longer period of time. So we use this type of information, as Dave was saying, in mapping quality – or, predicting quality of habitat. This is the sort of graph that we expect to be making a lot more of in the – or, it’s a map we expect to be making a lot more of in the future. It involves the geologic information that Dave described, plus other information about the – about the soil water, the particle size distributions of the soil. And what it tells us is, for these different areas, what’s the – what’s the potential for growing a certain plant? In this case, it’s the – this is strictly the creosote bush. It’s telling us where – okay, where it’s blue, that means potentially a quite dense cover of creosote bush. It doesn’t mean it’s there now. It just means this model predicts that’s good creosote bush habitat at some of these alluvial fans up here. The red color means very low – essentially zero coverage of creosote bush. So that’s – I suppose these are the sand dunes here where we wouldn’t expect to find much. And the – if we look at the area where we’ve done our experiments, this globe piedmont, this area right here, you see there are a lot of smaller patches that you can find areas that are relatively poor for creosote bush. These would be probably the – mainly the older soil materials. And very nearby soil surfaces that are – that are at least medium good for growing creosote bush. So this is the direction we’re aiming at to be able to, by knowledge of the geology, the soil water, the water requirements of the plants, to be able to figure out and portray what areas of the land’s surface make good or poor or medium-quality habitat for the plants. And, of course, well, for the plants, that’s what the animals rely on. Good plant habitat is essential to the tortoises, the lizards, and all the other – the birds – all the other components of the ecosystem. So we’ve seen desert soils have – they have great variety and features that develop over time. Time being – here, the relevant time period that we’ve been studying is, like, in tens of thousands of years. Developmental features strongly affect the behavior of water in the soil. And so both the soil development and water behavior are critical to the desert plants and to the health of the ecosystem as a whole. Thank you. So …


So if you have questions for Dave or myself, we’d be happy to answer them.

- I’m going to ask you, as usual, to please use the microphones. Not only so all of us can hear, but we record these lectures. And if you don’t speak into a microphone, the question doesn’t get recorded. And Dave and John will – if you can remember which one’s which, will be happy to answer questions. If you’re not able to get up to a microphone in either one of the aisles, wave at me, and I’ll bring you the microphone. There’s a first question up there.

- Hi. What effect has global warming had on the desert, if any?

- You want to take that?

- Okay. I’ll take that. That’s a tricky question. [laughs] Probably a big effect. And there’s some predictions, but they’re – let me back up a little bit and describe what it is that we know and we don’t know. We do know that, in the desert, in past climate changes, that there have been big changes in the – in the plant communities. And we could describe what those are. So, to a certain extent, that provides some predictive capability. But it’s only – it only can be used to predict if we can predict very faithfully what the climate change in the future is going to be. And models right now tend to not be very good for the desert communities in particular. And they don’t tend to be very location-specific. So there’s some general information about the Mojave Desert area that indicates it’s going to be warmer. So the growing season will be longer, and plants will respond to that. There is ambivalent information on precipitation. It may be wetter. It may be drier. The models aren’t very faithful in predicting that. I think one of the really important questions is, how fast these changes take place. Because changes have been taking place always. And the plant communities are adapting to that, or they wouldn’t be there. The animal communities also. One of the things that I find personally to be kind of alarming, is that it looks like some of the changes in temperature are going at a pace that’s more rapid than probably has been witnessed by any of these plant communities and animal communities. There’s extraordinarily rapid change in temperature that’s going on right now. And it’s possible that the plant communities will not be able to use their normal means to adapt to that, such as moving to higher elevations up the mountain gradients to try to maintain their position in a certain temperature regime and that sort of thing. It may be that the plants and insects and animals get out of sync in terms of pollination times, migration times, food source availability times. And if they can’t adapt fast enough during this rapid climate change, there could be disastrous consequences. But I don’t think we know specifically enough to be very specific in the answer to that question. It’s a really important topic.

- Are there aquifers in the location that you’ve studied that are of any significance? Or aquifers in the greater Mojave Desert that you have knowledge of?

- Yes, they very much are. You know, our talks have been focusing on most of the landscape out there that you might call dry. And the aquifers are very deep. And they aren’t an influence. The plants cannot make use of the – of the position. Maybe this would be a good time to talk about saturated versus unsaturated, which is a promised …

- And an estimate of the depth of the aquifers, if you would.

- Yeah.

- Yeah. Okay. In an aquifer, we’re talking about the ground at deeper depth than we’re looking at, where the soil would be saturated, or the soil, sediment, or whatever – gravel, rock – whatever material is there, it’s entirely water. What makes that happen is gravity. Gravity is going to pull the water downward, and it’ll accumulate at some – at some level and completely fill the pores between sand grains, the pores in rocks, or whatever. As we – between the land surface and the top of the aquifer, we – in most situations, there’s what we call a water table. So that’ll be a zone where – well, it’s a transition layer where the top of the zone where we have saturation, above that, we have the unsaturated zone. What this means is that there’s both air and water in the pores of the medium. So your average garden soil, that’s in the unsaturated zone. And it’s – it makes for very different – very different physical interactions to have water as well as air in the pores. Very – it’s very complex how the water pressures relate to water movement, changes in water content. So that’s my main specialty of study. In the – also in applications, or usefulness, of these different zones, they’re quite different. The unsaturated zone, of course, is where we have the plants, the biosphere, most of our – we and other creatures on this planet, while we’re interacting mostly with the – with the unsaturated zone. Whereas, for the aquifer, greater depth, naturally we use that to get water out of. We drill wells. Drill wells down into the aquifer. And by the way, aquifer – when we use the word “aquifer,” it implies that, well, the properties of the material there are suitable for a lot of water to flow to wells, where it can be extracted pretty easily. So, in effect, like, if the material were a coarse sand, that typically makes quite a good aquifer. Water flows easily. It’s a dense rock, less so. But it would still be a saturated zone.

- And aquifers are important. There are places where there’s springs, and there’s some riparian zones along some of the bigger streams. And historically, those have been huge. Because almost all the immigration route to cross the desert have followed those. The Indians knew where all the springs were, and the animals out there – the desert bighorn sheep and other large animals know exactly how to get to the various water sources.

- And the – well, the saturated zone here would come at various depths. Over much of the desert, it would be quite deep – say tens of meters or more – quite deep. Because there’s not much water. Where there’s a spring, that means, well, essentially the aquifer is very shallow and comes to the – comes to the surface there. So the – about this area – for aquifers where there – there are some wells in this vicinity. Not many because there’s not much water being added to the system here. So it’s not practical to draw a lot of water out. Although, in some desert locations, there is extensive use of groundwater. And it can cause problems of subsidence and lowering water tables. Here in the – in the preserve, of course, there’s not much water being used. However, at the – there is a well near the Kelso Depot, and in fact, that’s where we got permission from the park service to meet somebody there, and they unlocked it and – the water tap. And that’s where we filled our 400-gallon tank to do our experiments. [laughter] So it’s actually very good water too. Although, if you’re in – if you’re in the town of Baker, where we’d sometimes stay – if we’re not camping but in a motel, we’d be in the town of Baker. And, well, my personal experience, the water in the motel tastes salty. So it’s …

- Yeah. It’s terrible. [laughs]

- It’s not sure where it’s from. [laughter] But it’s not a high-quality aquifer.

- Hi. Yeah, is the word “soil” a well-defined technical word? Or is this something that we just know and generally – I think of soil as having organic material and all that kind of stuff. And secondly, is there an evolutionary process to go from this young soil to the old-age soil? Is that – or is this – are these just samples, and you’ve dated them? Or is there a change from one to the other, and how does that occur?

- Okay. Maybe we can split that question, if I do the first part, and you do the second. The definition of “soil.” This is something that took me several years out of grad school, where I studied physics, to catch on to what soil scientists call “soil.” The word “soil,” it has different meanings to different people. And, yes, it has a very technical meaning. And it depends on what field of study you’re in. [laughter] Here – let me stick to what the soil scientists – the ones that are more concerned with plants and so forth, what they would call soil. It’s different from what soil mechanics people call soil. People who deal with soil for, how strong is it to put buildings on – different definition. Anyway, soil – well, let me compare that also with the – what I think of as the general popular definition, which is, well, soil is granular material that’s at the surface of the ground. It’s particulates. It’s – you can dig in it. It’s not solid rock. It’s not bedrock. But it’s particulate matter. Now, the soil scientists would have a very – well, they have a more restrictive meaning. It becomes soil when it has been subject to, well, some of the developmental processes we’ve talked about. That, if there is – say there’s a landslide. And a whole bunch of fresh rock falls into place, breaks up into pieces, covers the ground. Traditional soil scientists, they wouldn’t consider that soil. They might think of it – they’d think of it as what they call parent material. That is, some material that will become soil. So material like that, eventually plants will grow on it. Rain will fall. Water will be there. There will be chemical reactions. Particles of different sizes will be brought in and moved around. There will be cracks and – cracks that develop. Organic matter is very important – gets absorbed. It changes dramatically the properties – both the mechanical properties and the – and the water properties. Also the nutritive properties for plants. So the soil scientists refer to – they speak of soil as this – the upper part of the unsaturated zone that these processes have been at work in. So where that’s – where that gets to be, in my own work, where that’s potentially a point of confusion is that we often study the unsaturated zone at considerable depths. In arid regions, it can be quite important. If you go down, say, 10 meters down, what’s the – what are the – what’s going on with the water there that might be recharging an aquifer, for example? And so that’s important to us. Material from 10 meters down. But soil scientists don’t call that soil. We can call it sediments, but not soil. Soil would be closer to the ground – the material that the plants are actively growing in. So we’ve got these different definitions, and you have to pay attention and speak what people know.

- And one of the specific points I think you made was, does it require organic material to make it a soil. And the desert’s probably a good place to address that because there’s very little organic material in the deposits that everybody – all the soil scientists in the world would agree are soils. So, while organic material is a real common component of soil, and it’s one of the components that you want to watch really carefully for a variety of soil property reasons, it’s not a requirement. The desert materials basically are so strongly oxidizing that organic material doesn’t tend to last very long. And there is a progressive development of soil. That’s a standard concept in soil science, and geologists and soil physicists all embrace that. So it is a – there is a steady progression in soil development with time.

- Over 30,000 years?

- Yes. Absolutely. Over millions of years.

- Yeah.

- I’m curious about the life expectancy of those two plants that you were dealing with. Do you have any idea how long they last?

- I don’t know enough about plant physiology to be confident in my answer, but I’ve heard various ecologists that I’ve worked with describe the lifetime of – the life expectancy, if you will, of many of those desert plants as being multiple decades, or, you know, maybe as long as a century. There are a couple ringers here, though. The creosote bush has been described as cloning. That is, growing outwardly as successive rings of the same genetic individual through time. And there have been some assertions that, in fact, one individual that’s an extraordinarily wide ring might – and has been dubbed “king clone” – might be as old as 10,000 years. And I think that there’s some good reasons to be cautious about those sorts of interpretations. But some of the – some desert plant species are capable of cloning. And so then you have to have special definitions of how old an individual is.

- Thank you.

- Any more questions for …

- We’d be happy to talk to individuals up here too. But maybe we ought to close the questions at this time.

- All right. Well, I’d like to not only thank John and Dave and our Mojave video – Steve Wessels – he’s not here, but thank you. It was wonderful. We all got to take a trip down to the desert, I think. And I thank all of you for joining us tonight. Have a happy holiday season, and we’ll see you in 2007 at the end of January. Thank you.


[Inaudible background conversations]