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>> A little background on this project among other projects as many
or most of you aware we try to provide [inaudible] support to do projects
such as this [inaudible] research mode.
There are competitive for the most part, and actually this project was one
of the first ones that we did this way.
Judy has just recently [inaudible] less
than a year ago finished her PhD [inaudible] March [inaudible].
See I was right, it's [inaudible].
Dr. Bob Graham at the University of California Riverside.
She's the one that actually did the work on this project and used it to complete her PhD.
She is now actually at The Richard Stockton College of New Jersey,
and I'll let Judy go ahead and introduce herself [inaudible] carry on.
>> Okay, thank you [inaudible].
So first of all, I wanted to say a big thank you to Larry West for inviting me
to the webinar today and to Sean McVay [phonetic] for helping me to get set
up with the [inaudible] everything.
And first off, this is the research that I did for my dissertation at the University
of California of Riverside, and I worked with my advisor, Bob Graham,
and as Larry West mentioned, we got funding from the National Soil Survey Center
but also supplemented this funding with funding from NRCS in California.
We also got a small research grant from the NSF and supplemental funding
from the UCR Agricultural Experimental Station.
[ Background Noise ]
So the vesicular horizon, as the title says is a critical component of arid and semi-arid land,
so I like to start out with this slide showing the worldwide distribution
of arid and semi-arid land.
You can see all the orange shaded areas are designated as extremely arid, arid,
or semi-arid, and in total these occupy about a third
of the earth's land surface [background noise].
And my focus was on the vesicular horizon, which is just the very surface layer of arid soil.
So these are either surface or near surface horizons.
They're very often associated with desert pavement.
Let's see if I can get the pointer.
So the desert pavement is the surface layer of gravel, so you can see a nice example
of on this slide, but they're not always associated with desert pavement.
Sometimes they're beneath either a physical or a biological surface crust.
The vesicular horizon itself is shown here.
It's the thin layer beneath the desert pavement in this photo, and it's usually less
than 10 cm thick, so it is a very thin horizon, and the most prominent morphologic feature
of this horizon is the predominance of vesicular pores, which are these bubble-like pores
that you can see throughout the horizon.
They are nearly spherical in shape, and they're discontinuous, so they're very poor
at conducting water, and they have an appearance like air bubbles, so they're actually formed
by bubbles that get frozen in place as the soil dries.
So the air bubbles get trapped when the soil is wet
and then get stuck in place when the soil dries.
Other distinguishing features of the vesicular horizon include the characteristic structure,
which is typically columnar structure that parts to platy structure although I'll show a lot
of examples of these horizons throughout the presentation, and you'll see in some
of them the columnar structure is a much more prominent feature and then
in other cases the platy structure is more prominent [background noise].
So first of all, the vesicular horizon is very often associated with desert pavement.
So this is a nice example of, here we go.
This is a nice example of the desert pavement.
This is a very prominent feature of the desert landscape,
and some of its distinctive features are first all you notice it's dark in color.
So that black color comes from desert varnish, which is a coating that forms on rock fragments
that are exposed for long periods of time in the desert.
The desert pavement is also characteristically very smooth
in topography and has a low vegetation cover.
When you look up close at the desert pavement, you can see it's basically a mosaic
of interlocking rock fragments that form this surface coating.
And then you cut into the desert pavement and you can see it's actually just a monolayer
of rock fragments, and immediately beneath the desert pavement there's a relatively gravel-free
zone, which is often where we find the vesicular horizons.
And the way that this gravel-free zone forms is through [inaudible] material,
stuff that gets caught beneath the rock fragments and actually lifts them up.
This is a major mechanism of younger pavement formation.
For this I mentioned, vesicular horizons are not always associated with desert pavement.
This is a paper that was published earlier this year showing vesicular horizons
that form beneath biological soil crust.
This is an example of typically the moth lichen form of crust,
which has a rough microtopography.
These surfaces often have a tendency to trap dust.
That dust accumulates beneath the [inaudible] surface crust,
and then once you get a thick enough [inaudible] dust that can actually result in a layer
that forms the vesicular horizon [background noise].
So we can think of the method of vesicular horizon formation as a 3-step process.
Although these 3 steps in the process occur somewhat separately and somewhat simultaneously,
so you can't really think of it as a process.
So, the main components are first eolian depositions.
So what this does is it enriches the surface layer of the soil in eolian silt and fine sands,
which are the major texture that conduces to the formation of the vesicular horizon.
The second step is that there has to be some sort of field that forms at the surface
that cuts off the above-ground atmosphere from the soil atmosphere.
And this can be either a layer of rock fragments,
like we have with the desert pavement,
or it can be either a physical or a biological surface crust.
The third step in the vesicular horizon formation is that you have cyclic wetting
and drying of the soil surface that results in the entrapment
of air bubbles that form the vesicular pores.
So this is a figure showing each time the soil becomes wet, air bubbles get trapped,
and then as the soil dries, that air expands and leaves imprints that form the vesicular pores.
Also in these wetting and drying events, you get shrinking and swelling of the soil material,
which results in the [inaudible] cracking that forms the columnar structure.
And other thing that can happen is as these pores expand and merge with each other,
the pores grow to such a size that they can't support the weight of the soil
and they collapse, which is the major mechanism by which platy structure forms
in these horizons [background noise].
So one of the interesting things about vesicular pores is that they can actually be destroyed
and recreated, and this has been known for a long time.
You can actually take a sample of the vesicular horizon and crush it
up so you destroy all the vesicular pores, and if you expose that material to repeated wetting
and drying in the laboratory, you can actually get the pores to reform.
So here this is a study from 1971 showing the vesicular pore free forming in the lab.
This is after 4 wetting and drying cycles.
You see just some very tiny pores starting to form at the surface and then over 8 wetting
and drying cycles, 14 and 25, you see the continuous growth of the vesicular pores.
[background noise] So the significance of these in the arid landscape, first I wanted to go
over just a little bit about the land use in the desert region.
So this is a map of land use in the Mojave Desert portion that's in California.
You can see there is a very large area of land that is owned by the Bureau of Land Management,
several military bases, which are in purple, and then a lot of national park land,
that's in green, so we have a lot of relatively undeveloped land in the desert.
But there are several increasing land uses.
One is the expansion of urban population.
So examples of cities in the desert are Las Vegas.
The population of Las Vegas increased by over half a million between 2000 and 2010.
The population of Phoenix increased by nearly a million in that period.
One of the main recreational uses of the land is off-road driving.
This is one of the off-road vehicle areas in southern California, Johnson Valley.
It's about 800 km square and gets about 200,000 visitors per year.
And these are images of the land surface.
This is an alluvial fan surface immediately outside of the off-road vehicle area and then
within the off-road vehicle area.
So you can see all these linear features are the tire tracks.
So you can see that off roading can have a really dramatic impact on the soil surface.
This is looking at military land use.
This is a study that looked at a military training ground that was used
in the World War II era and then haven't been since, so this is a study in 2004.
They went back and looked at some of the track scars that had persisted since World War II era
and since 2000 [background noise].
And one of the really rapidly expanding desert uses is that millions of acres
of desert land are being approved for large scale solar and wind farms.
This is an illustration from the LA Times of one
of the big solar arrays that are being constructed.
This is along the I-15 freeway right on the boundary between Nevada and California.
So this is actually that same [inaudible] from the LA Times illustration.
I was driving by and stopped to take a picture of them constructing the solar array.
So you can see there's a lot of dust being produced from the soil
at this site due to the construction.
And this can be related to vesicular horizons because they are a surface horizon that's formed
in eolian material that when they're disturbed they have a tendency to release a lot of dust.
This is a study from an off-road vehicle area demonstrating the length
between vesicular horizons and dust emissions.
Here we're looking at dust emissions with increasing driving speeds,
and each of the different lines here represents a different type of surface
that the author has characterized, and what I want you to notice is that one
of the most emissive surfaces was actually the desert pavement surface.
And the reason why the desert pavement surface emits so much dust is
because it is associated with the vesicular horizon.
And thus emissions from desert soil can have wide-reaching ecological impact.
This is a study linking dust emissions from desert soils
to the snow pack in surrounding mountain ranges.
So what they found in this study is that when the snow pack becomes coated in dust,
which is what we see with the red line compared to clean snow that doesn't have dust on it,
there's a decrease in the albedo of the soil.
This means that more light is absorbed, and less light is reflected and this leads
to faster melting of the snow [background noise].
So going back to the desert ecosystem, this is an image showing the contrast
between what we call the shrub component of the landscape and interspace.
So this is one of the basic ecological principles of the desert ecosystem.
So there are these 2 distinctly different components of the landscape.
We have distinctly different soil properties.
So these are, different terminologies are used to describe this.
Some people call it the shrub and interspace.
Sometimes it's the canopy and intercanopy.
Another terminology is dune and interdune.
This refers to the fact that shrubs have a tendency to trap dust
and produce what are called coppice dunes.
They tend to have micro highs around the shrubs and micro lows between them.
The shrub areas also sometimes referred to as resource islands or islands of fertility.
So basically all describing the same phenomenon that the shrubs tend to grow in clusters,
and when a new shrub is established, it tends to establish
in these clusters instead of in the interspace areas.
So this is a study from the Journal of Range Management highlighting this contrast
between the 2 parts of the landscape,
and here they're highlighting the microtopography associated
where you have coppice dunes and then micro lows in between the coppice dunes.
And what they're showing is the difference in the surface morphology
between these different positions in the landscape,
and this part of the illustration is showing the vesicular pores.
These little illustrations of bubbles are representing the vesicular horizons.
And what you'll notice is that the vesicular horizon occurs
in the interspace part of the landscape.
It does not occur beneath the shrubs.
Another important principle relating to the vesicular horizon is
that it has a very strong impact on the surface hydrology of desert ecosystem.
This is a study from the Desert Research Institute.
This is showing infiltration rates measured on surfaces with increasing soil age.
So this is a chronosequence of alluvial fan surfaces in the Mojave Desert.
And if you look at just the yellow bars, this is where they removed the desert pavement
and measured infiltrates and rate directly into the surface horizon,
which in the older [inaudible] surfaces was a vesicular horizon.
And what you'll notice is that with increasing surface age as the vesicular horizons develop,
there is a decrease in infiltration rate.
And I should point out this is on a logarithmic scale, so this is a really dramatic decrease
in infiltration that is taking place.
What the red bar are showing is that when the vesicular horizon was removed
and they measured infiltration directly into the underlying horizon, which was either a B horizon
or an AC horizon, there was no longer a decrease in infiltration rate with increasing age.
So this was isolating the vesicular horizon as the feature of the soil
that is controlling infiltration.
This is another study, which shows a similar principle in a slightly different way.
So, here, again we're looking at infiltration rate with increasing soil age.
Here [inaudible] they look separately at the intercanopy soils, so the soils that tend
to have vesicular horizons and at the undercanopy soils,
which do not develop vesicular horizons.
And what you see is that in the intercanopy there is a decrease in infiltration
with increasing age, but in the undercanopy soils,
you don't see that same trend [background noise].
Another photo illustrating the surface hydrology of desert ecosystems.
This is soil taken during a rain storm in the Mohave Desert.
[inaudible] desert pavement that has vesicular horizons underneath it.
And you can actually see the water ponding and running off
from the areas in between the shrubs.
So what's going on is that we have low infiltration rates between the shrubs
in the interspace areas and high infiltration rates beneath the shrubs,
so there tends to be runoff and concentration of water in the shrub islands,
so this helps to support the natural pattern of plant growth in the desert landscape.
This is just an aerial photograph,
so you can think about the surface hydrology of these systems.
So all of the dark areas are the desert pavement.
So this darkness is the desert varnish.
This is an alluvial fan surface.
So we have washes dissecting the surface and shrubs growing in those washes,
and then each of these little white spots is a shrub island.
So you can see we have a large area of desert pavement that tends to promote runoff and then
that runoff concentrates in the washes or in the shrub islands where the plants grow.
So getting to what I did for my research.
The first question that I had when I started doing this research was how am I going
to quantify vesicular horizon development.
How am I going to look at vesicular horizons in one part of the landscape and compare them
to another part of the landscape.
So I came up with 2 approaches.
The first was the simpler approach,
which I could directly calculate based on field observations.
And the advantage to this was that I could also use existing data and apply this technique
in order to quantify vesicular horizon development
from a [inaudible] field description.
So I started out with some morphologic descriptions, and I extracted 3 values.
The first was the thickness of the vesicular horizon in centimeters.
So this will be the thickness of the zone in which vesicular horizons were the predominant,
sorry vesicular pores were the predominant type of pore.
The second one is the vesicular pore quantity class,
so whether there were a few common or many vesicular pores.
And the third was the vesicular pore size class, whether they were very fine,
fine medium, coarse, or very coarse.
I then assigned a numerical value to the size class and the quantity class
and then calculated what I called the vesicular pore term, so this was just the sum
of the quantity class plus the size class for each of the different size classes
that occurred, and then I set down the scale from 0 to 1 by dividing
by the maximum possible value for the vesicular pore term, which was 220, and then I multiplied
that value by the thickness of horizon to calculate the final vesicular horizon index.
So in order to test this out, I calculated the vesicular horizon index based
on data from a chronosequence study.
This was Eric McDonald's dissertation at the Providence Mountain chronosequence,
which is in the Mojave National Preserve.
And what I saw was that there was a nice increase in the vesicular horizon index
with increase in age, which shows that with increasing development of the vesicular horizon
that was reflected in an increase in the vesicular horizon index.
[inaudible] oldest surface in the chronosequence and the authors of the study documented
that that purpose has been eroded, so we also see that with erosion and degradation
of the vesicular horizon there's a decrease in the vesicular horizon [inaudible].
In addition to just my field morphology, I also wanted some way
that I could collect more detailed data on the core morphology.
So the approach I used here was CT scanning or computed tomography.
So this shows what is called a slice of a CT scan,
so this is basically a virtual cross section through the sample, and it's not a photograph,
it's a map of the density inside the sample which is based on the rate of x-ray attenuation.
This analysis was done at the University
of Texas high resolution x-ray computed tomography facility, which is the facility
that specializes in CT scanning of all types of field object samples [background noise].
So based on the CT scan, I had used image analysis software to separate
out the lower density zones, which were the cores, and then also separated
out the higher density zones, which were the gravel.
But the really nice thing about computed tomography is
that you don't just get a single cross section through the sample,
you actually get a continuous series of virtual slices, which can be put together in order
to build a 3-dimensional model of the pores.
So what I found when I looked at these 3D models is
that there was actually 3 different major classes of pores
that I saw in a vesicular horizon.
These were the vesicular pores, which are the very spherical shaped pores.
Sometimes they're ovoid, sometimes they're spherical, but they're the roundest pores.
We also saw a lot of Vughs, which are similar to the vesicular pores
in that they're discontinuous from each other, but they tend to be more irregular in shape,
and then I also described a number of interconnected pore networks,
and what I mean by this, these are basically Vughs
that had short interconnecting channels in between them.
[ Background Noise ]
Next I had these 3 different types of pores that I observed in the samples.
I needed some way that I could extract the data from the CT scans without having to go through
and look at each individual pore in the thousands of pores per sample
and hundreds of samples that I was analyzing.
So what I came up with was I derived the volumetric form of a method that's used
to analyze pore shape and [inaudible] sections.
It's called the lobation ratio.
And basically it's an index of how spherical a shape is based on the ratio
between its surface area and its volume.
So what I did was I went through one of my samples
and I actually identified the pores visually as vesicles, Vughs, or interconnected,
and then for each of those that I had identified, I calculated the lobation ratio
and looked at the correspondence between the lobation ratio
and which pore shape it fell into.
And so I found that I can separate the pores into vesicles which had a lobation ratio
than 0.79, a Vugh, which had intermediate lobation ratio, and the interconnected pores,
which had a lobation ratio between 0.58.
Then, once I had the pores classified into what shape they were,
I could calculate 3 different values from the CT scan.
The first is the percent of the sample volume that was occupied by pores of a certain shape,
and what the formula is showing is that it was a gravel corrected volume,
so it is taking into account how much of a sample was gravel.
The second value I calculated was the number of pores per sample volume.
This was also sometimes referred to as number density, and this corresponds more
with the quantity of the pores in the sample.
And then the third value I calculated was the geometric mean of pore volume.
So this is corresponding with the size of the individual pores.
So, going on to part 2.
So now that I had my technique for analyzing the vesicular horizon, the first question
that I looked at was what is the geographic distribution and range
of characteristics of the vesicular horizon.
[ Background Noise ]
So first this map is showing a rough estimation
of the worldwide distribution of vesicular horizons.
So the background gray map, this is that same map that I showed you on the first slide.
This is the USDS map of arid and semi-arid land,
and then each of these yellow points is an example of the scientific study
that documents the presence of vesicular horizons in different parts of the world.
So we can see that vesicular horizons occur on every continent on earth.
They're deep in a study of vesicular horizons in Antarctica,
and they're mainly associated with arid and semi-arid lands.
In the US, I was able to use the NRCS databases to get an even better look
at the distribution of vesicular horizons.
So the first thing I did was I went through the official series descriptions and found all
of the soil series that had vesicular horizons in them.
So there was a total of 1092 soil series with vesicular horizons,
and when I added up the total area mapped for these soil series in the SSURGO database.
This added up to 157,000 km square.
This map is showing the distribution in the [inaudible] database,
so all of the gray areas are areas with map units that have one of the soil series
of vesicular horizons as a major component.
So here we can see a little bit where the soil vesicular horizons occur.
You can see they mostly follow the outline of the basin and range province [background noise].
Next, again using the NRCS databases, I looked at the texture of the vesicular horizons.
So, this is showing the laboratory textures applied on the textural triangle,
and you can see they are somewhat concentrated in the silt corner.
The vesicular horizons do tend to be silt rich because they are concentrated,
because they are formed in eolian material.
But there is a wide range of textures that occur in vesicular horizons.
This next graft, this is looking instead at the field-determined textures,
and there were 5 most commonly described field-determined textures
in the vesicular horizon.
These were loams, sandy loams, fine sandy loams, very fine sandy loams, and silt loams.
And here what I did was I calculated my vesicular horizon index for each
of the 5 textural classes that commonly occurred in the vesicular horizon,
and we see that there is a slightly higher vesicular horizon in [inaudible] silt loams.
This is the median.
But it wasn't a statistically significant difference, so we do see pretty good development
of vesicular horizons across the textures.
Next, these are chemical properties
of the vesicular horizons, again from the NRCS databases.
First, organic carbons.
The organic carbon content, the median content was 0.7%.
Electrical conductivity median was 1.2.
Exchangeable sodium percentage was 2%, so these were mostly nonsaline, non-sodic soil.
The pH median was 8, and the calcium carbonate percentage, the median was 4%.
The interquartile range was from 0 to 12%, and I should point out there were a lot of 0's
in this data set, so there are a lot of vesicular horizons
that contain no calcium carbonate, which means they don't need calcium carbonate
in order to form the vesicular horizon.
Next, this is another map derived from the NRCS databases.
Each of the points on this map is either a type location for an official series of friction
that has a vesicular horizon
or it's a [inaudible] description that has a vesicular horizon.
So with this point data what I did was I overlaid it with the [inaudible] region,
which divide the basin and range province up into 4 different ecological zones.
So the top 2 ecological zones are the northern basin and range and the central basin and range.
These are sometimes lumped together and called the Great Basin.
These are considered the cold desert.
They have higher precipitations, lower temperature,
and tend to be dominated by sagebrush vegetation.
Next we have the Mojave Desert and the Sonoran Desert.
These are considered the hot deserts.
They have lower precipitation, higher temperature,
and tend to be dominated by creosote bush.
[background noise] These are photographs taken in 3 of the ecoregions,
the central basin and range, Mojave, and Sonoran.
So you can see these are all shrub-dominated landscapes.
They all have this characteristic shrub and interspace landscape component,
but we do have a slightly higher shrub cover when we look at the central basin
and range compared to the Mojave Desert and even lower in the Sonoran Desert.
So what I did with this data was once I had my points grouped by [inaudible] ecoregion,
I calculated the vesicular horizon index for each of the different ecoregions,
and what I found was kind of an interesting trend.
I found that there was actually a higher vesicular horizon index,
and this was a statistically significant difference,
in the cold desert compared to the hot desert.
This was kind of the opposite of what I expected to find.
I felt since vesicular horizons were associated with desert they would be best expressed
in the most arid parts of the desert, in the Sonoran and Mojave
where we have the lowest rainfall and highest temperatures,
but they're actually better expressed in the cold desert [background noise].
In addition to looking at the databases, I also went out and collected samples from the Sonoran,
Mojave, and central basin and range desert, and with the samples that I collected,
I used the CT scanning procedure.
And here this this graph is showing the percentage
of the sample volume that's occupied by vesicles and Vughs.
So here this is showing them together, and we saw that there was just like we saw
with the database, we saw that there was a greater percentage of vesicles and Vughs
in the soil from the central basin and range compared to the Sonoran and Mojave Desert.
And so now you see what the data looks like, look at what the soils actually look
like in these 3 different ecoregions.
So these are photos that I took in the Sonoran Desert.
So you can see, we see pretty weak expression of the vesicular horizons here.
The vesicular pores are very small, probably hard to distinguish in these photographs,
but in some of the sites we did see really strong expression of columnar structure
in the vesicular horizon, which you can see nicely in this photo from Portside, Arizona.
These are photos from the Mojave Desert.
So now we still have [inaudible] vesicular horizons
but those vesicular pores are a little bit larger and easier to distinguish.
And again, some of the sites have fairly weak structure, but I just also saw some sites
that had very strong expression of the columnar structure.
And last, these are samples from the Great Basin Desert,
and now these horizons tended to be thicker.
The vesicular pores are larger, and these 2 photos show a really nice illustration
of the 2 types of structure that you'll see in the vesicular horizon.
So the first photograph you see a really nice example
of a vesicular horizon with a strong platy structure.
The second photograph you see a vesicular horizon
with a greater expression of the columnar structure.
So what is the reason for the difference in vesicular horizon expression
between the different ecoregions.
So in order to think about this, I broke it down into what factor
of soil formation is controlling this trend.
So the first possibility is that this is controlled by climate.
So remember we have higher precipitation and lower temperatures in the cold desert,
and we know that vesicular horizons are formed by repeated wetting and drying cycles.
So it could be that because there is more frequent precipitation there's more wetting
and drying cycles, which means that there is more opportunity
for vesicular pores to form in the cold desert.
So this is showing that average annual precipitation frequency expressed as number
of events per year across the 4 different ecoregions.
The other possible controlling factor is parent material.
So vesicular horizons are formed in eolian parent materials.
And one of the theories of how vesicular horizons form is that there was,
during the transition between the Pleistocene and Holocene era, there was a drying
of pluvial lakes, and those pluvial lakes released a large pulse of dust,
which was distributed across the landscape forming vesicular horizons.
So here we see the map of Pleistocene pluvial lakes.
So we have Lake Bonneville that shrunk down to form the Great Salt Lake,
Lake Lahontan that covered much of Western Nevada, Lake Manly that covered the floor
of Death Valley, and if you look at the distribution of these pluvial lakes,
they are much more widespread in the northern part of the basin and range province,
so it could be that because we have a greater extent of these pluvial lakes
in the northern part of the basin and range there were more sources of dust,
which means we have better expression of vesicular horizons.
So, it's possible that this trend we see is one of the climate or the parent material
or it could be a combination of both.
[background noise] So the third part of my presentation deals with the effects
of disturbance on vesicular horizon.
So my question here was, what effects does disturbance
of the soil surface on vesicular horizons.
One of the reasons why I was interested in this question was because as I showed you
with the laboratory results at the beginning of the presentation,
the vesicular pores can actually be disturbed and reformed on a relatively quick time scale,
so one possibility is that the disturbance took very little effect
because the vesicular pores regenerate so quickly.
Another possibility is that they don't recover quickly enough
when we do see disturbance effects.
[background noise] So, since I knew that vesicular horizons [inaudible] difference
between ecoregions, I divided my study size up among 3 of the different ecological zones.
I had 5 sites in the Sonoran Desert, 5 in the Mojave, and 5 in the central basin and range.
[background noise] And I looked at 3 different features
that displayed disturbance in each landscape.
One was tire tracks.
So I compared soil samples that I collected from tire tracks with soil samples collected
from beneath undisturbed desert pavement.
I also looked at coppice dunes.
So remember, the coppice dunes typically don't have vesicular horizons,
but when I was in the Great Basin I saw a lot of vesicular horizons
that had been buried beneath coppice dunes, so what was happening was
that a shrub was establishing, building a coppice dune, and the vesicular horizon
that had previously been at the surface was becoming buried.
And so I was interested in this because this could reflect during different climatic periods
when you've had expansion of vegetation on the landscape,
what effect this would have on the vesicular horizon.
Does the vesicular horizon persist through these periods when vegetation expands,
or it is destroyed by that process.
The third thing that I looked at was disturbance plots that I actually set up in order
to control the form of disturbance and the exact time that I was getting for recovery
of the vesicular horizon, so I was looking at 2 forms of preexisting disturbance
where I didn't have very good experimental control, and then I had the third form
of disturbance where it was maybe less realistic but I had really good experimental control.
So this is an example of one of the tire tracks that I examined.
Here I collected a sample from within the tire track and then
from beneath the nearby desert pavement.
And these are slices from CT scans showing the difference between the vesicular horizons.
[inaudible] vesicular pores in those with tire tracks and undisturbed samples
but maybe a little smaller in the tire tracks.
There was also a pretty pronounced structural difference
between the desert pavement and the tire tracks.
So this first photograph is this is beneath the undisturbed desert pavement,
so here I removed the desert pavement class and then took a photo of the surface
of the vesicular horizon immediately beneath the desert pavement.
And here this is the surface and the tire tracks, so what you see is a flattening
of the top of these rounded columnar [inaudible] [background noise].
And these are the data extracted from the CT scans.
So this first line shows the data just per the vesicular pores,
so these are just the pores that are rounded in shape.
On the 3 types of data I am showing here, this is the percentage of the sample volume
that is occupied by vesicles, then the number density of the pores and the percentage
of the quantity of the pores, and then the geometric mean of pore volume
that corresponds with the size of the pores.
And what we see when we compared the pores I classified as vesicles based on their shape,
there is actually very little difference between the tire tracks and the undisturbed.
They had similar total volume, similar number, and similar size.
This is the data for the pores I classified as Vughs.
With the Vughs again, we didn't see much difference
between the tire tracks and the undisturbed sites.
They were similar in total volume, similar in number, and similar in size.
Where we did see the difference was in the interconnected pores.
So these are Vughs that have short interconnecting channels.
In this case, we saw that there was a smaller total volume of the interconnected pores.
They were similar in number, but they were smaller in size.
So the next disturbance feature that I looked at was the coppice dunes.
So, again, these are, we didn't have vesicular horizon with the surface of the coppice dunes.
These were vesicular horizons that had become buried beneath the coppice dune.
So I would sample beneath one of the coppice dunes
and then beneath the surrounding desert pavement, and these are slices from CT scans.
And here just looking at these slices
of CT scans you can actually see there's a much more pronounced difference
than we saw with the tire tracks.
We started out with much larger vesicular pores, and these are collapsing [inaudible] structure
in this CT scan from beneath the coppice dunes.
This is one of my field photographs.
So here you see the coppice dune sediments above the vesicular horizon that I sampled,
so this was a vesicular horizon that had become buried when the coppice dune was established.
And data from the CT scans.
Again, this is the data just for the vesicles, so these are the rounded pores.
The vesicles beneath the coppice dunes were smaller in total volume.
They were similar in number, but they were smaller in size,
so what we're seeing is a collapse of the large vesicular pores that form small vesicular pores.
Same trend with above, these were smaller in total volume,
similar in number and smaller in size.
With the interconnected pores, we see a slightly different trend.
There was actually a decrease in the total volume of interconnected pores and an increase
in number but a decrease in size.
So they're collapsing but more of them are forming, so you have a larger total volume.
So if we look at the CT scans, these interconnected pores I want to point
out are these lateral interconnections between Vughs.
I did see some root channels growing through these samples
that I collected beneath the coppice dunes but not a lot.
Most of these interconnected pores were these lateral [inaudible].
So next, looking at the data from the 1-year disturbance plots.
So this is demonstrating how I set these up.
So first what I did was I removed the desert pavement class and set those aside.
Then I made a small excavation just through the depth
of the vesicular horizon and I removed that material.
I crushed it up and added it back to the excavation, put the desert pavement class back
on the surface, and left these out in the field for a year and came back one year later
and looked at the recovery of the vesicular horizons.
These are photographs from one of my field sites in the Great Basin.
So here this is before disturbance, what the vesicular horizon looked like.
It was a few centimeters thick, had fairly large vesicles and Vughs
and then a vesicular horizon index of 3.7.
Following disturbance, we did see at the site pretty good recovery of the vesicular horizons ,
ut the pores are much smaller and much thinner now, and they're about 2 cm thick and the VHI
that I would compute for this is 0.5, so we see a decrease in the vesicular horizon index.
This is pre and post disturbance vesicular horizons from a site in the Sonoran Desert.
So here, this is one of the sites that had really strong columnar structure.
The pores occurred in that zone now were a few centimeters.
They're not as large as the pores that you see in the Great Basin.
So the VHI started out as 2.9.
Then following disturbance and 1 year of recovery, I came back,
and post disturbance the vesicular pores were really only forming in this really thin layer,
only a few millimeters thick, but what was interesting at this site was that we started
to see some little bit of recovery of the columnar structure.
So here the VHI of the post-disturbance soil was 0.2,
so a really drastic decrease in vesicular horizon index.
One of the interesting trends that I observed when I looked at the post-disturbances soils was
that the post-disturbance vesicular horizon index showed a really strong relationship
with precipitation.
So 9 of my field sites were close enough to a climate station that I could use weather data
from the recovery year in order to calculate the number
of precipitation events that occurred at the site.
So when I looked at the relationship between the number of precipitation events
and the vesicular horizon index of the recovered vesicular horizons,
I saw that there was a linear trend with an r square value of 0.88, so there seemed to be
that the main factor controlling how the degree
to which the vesicular horizons recovered was the number of times
that it rained over the course of the year.
And we also see a relationship to the ecoregion, particularly in the Sonora Desert.
We had a very low number of precipitation events,
which led to very low vesicular horizon index.
So, based on this trend, we can predict how long it would take the vesicular horizons
to recover at these field areas.
So what I used was the vesicular horizon index of the predisturbance soils.
So what's our goal, what are we trying to get to to call recover.
And then the average number of precipitation events per year.
So based on this equation, which is [inaudible] the range of the data that I observed,
so it could have some issues with the prediction, but what I found was
that the recovery times were predicted to be between 2 and 11 years
for recovery of the vesicular horizons.
So this recovery time would generally be shorter
in the Great Basin were we have more precipitation
and the longer recovery time would be needed in the Sonoran Desert
where you have very very infrequent precipitation.
So the way that I sampled these in the field was a little bit different
from the other disturbance features, so now instead of sampling an undisturbed site
and a disturbed site, instead I was sampling an undisturbed site, then disturbing that site,
and sampling after disturbance, and actually after disturbance and then 1 year of recovery.
And so these are slices from the CT scans.
So before disturbance, you can see we have a much thicker zone
with vesicular pores, and the pores are much larger.
Following disturbance, we see some vesicular pores starting to form,
but they are in a very thin, thin layer, and they're much smaller in size.
So if we look at the data, this is the data for the vesicles.
So we saw that there was a smaller total volume of vesicles.
They were actually greater in number following disturbance, but they were smaller in size.
And this is actually the same thing that is seen when you look
at recovery of vesicular pores in the lab.
So, in the initial stages of vesicular pore formation, you actually have a larger number
of pores that are smaller in size, and then as the pores grow they merge with each other,
and because they're merging with each other, that decreases the number
of pores but lead to growth of the pores.
With the Vugh-shaped pores, we saw that they were also smaller in total volume,
similar in number but smaller in size.
And then with the interconnected pores, they were also smaller in total volume,
similar in number, and smaller in size.
So what I found across the 3 parts of my study.
The first question I asked was how can we measure vesicular horizons,
and I came up with 2 systems.
The first was the vesicular horizon index, which can be computed from field data.
So this is really easy to work with and can be applied to existing data.
The second approach that I used was analysis of pores using computed tomography.
So this allowed me to get really detailed data on the size and shape of the pores.
[background noise] The next question was what is the geographic distribution and range
of the characteristics of vesicular horizons.
So in terms of geographic distribution,
I found that they cover 157,000 km square in the western United States.
They have a wide range of chemical properties and textures.
They're not highly restricted to certain soil properties, and they're best expressed
in the cold desert relative to the hot desert.
And my third question was what effect did disturbance
of the soil surface have on vesicular horizons.
I found that disturbance both decreased the vesicular horizon index
and altered the pore morphology, and I found that the rate
of recovery was most highly dependent on the frequency of precipitation.
So that concludes my presentation, and I guess we can move onto the question and answer.
[ Background Noise ]
>> All right.
Well thank you Judy.
I've got 3 questions that have been submitted, and again I just remind people
that you can submit a question by going to the Q and A,
like at the top of the live meeting frame, and I'll queue up these questions.
So the first question is from Susan Perez [phonetic].
She says she was having any trouble viewing them and slow loading,
but I think that's been resolved.
I'll go to the next question.
I have a 2-part question.
One, is the pavement on this desert floor useful for anything.
In other words, could it be processed for making roads?
Do you want to tackle that one before I go to the second part.
>> I guess I'll answer that question first.
I'm not sure physically if it's useful for making roads,
but I think it's providing a service by its presence at the surface
because it's protecting the surface from being eroded.
So, it has an ecological service in its function.
So what's the second question?
>> The second part of it is, if this pavement were to be harvested for a helpful use,
if there is one, what would that mean for the vesicular horizon?
We see the disappearance of the horizon and have better infiltration,
or would it just result in more wind erosion?
>> I think, yes, both, you would see destruction of the vesicular horizon and wind erosion.
[ Background Noise ]
>> And I'll go onto another question from Tom Wright [phonetic].
Vesicular horizons are also observed in surface horizons dispersed with sodium.
What relation would these horizons have with your study?
>> Um, so some of the vesicular horizons both at my field sites
and in the database were sodic soils, so they weren't all along sodic
but did occur both in sodic and nonsodic soils.
So I think the dispersed condition probably does promote vesicular horizon formation
because in order for the pores to form you have to have weak aggregate stability
and also [inaudible] that crusting process where you have to have some sort of barrier
between the atmosphere and the soil.
Sodic soils are susceptible to crusting, so if you have formation of a surface crust
that can promote the formation of vesicular pores.
[background noise]
>> Okay. And then there's a comment here from Nelson Rolong [phonetic]
about vesicular horizons are a common feature in the Chihuahuan Desert in Texas and New Mexico
and then less extension in the high plain without presence of columnar structure.
>> Um hum.
>> So I think you mentioned that it could have a couple different structures
with the presence of vesicular pores
>> Yes, so you were saying that they didn't have expression of columnar structure?
>> Correct, without presence of columnar structure.
>> Yeah, so in different sites you'll see better and weaker compression of the structure.
And yeah, I focused entirely on the basin and range province.
I don't know a lot about the characteristics outside of that zone.
On the databases, I was really surprised at how few vesicular horizons show
up in the Chihuahuan Desert, and I don't really have a good explanation for why that is,
but it's interesting to hear from someone who's familiar
with that area that they do occur there.
>> And then a question from Char Waltman [phonetic],
would the vesicular pores be considered the result
of modern climates or possibly paleoclimates.
How old might these horizons be?
>> So they can be linked to paleoclimate in that this idea of how the vesicular horizons form
through this big pulse of dust that occurred during the Pleistocene, the Holocene transition,
so any period of very active eolian activity can be linked to formation of vesicular horizons.
So some of them are very old horizons.
I think especially in areas where you see really thick vesicular horizons,
those probably reflect several periods of very high eolian activity.
But they can also form very quickly,
so the really thin ones [inaudible] probably [inaudible] recent climate.
>> And a somewhat related question from Mike Domeier [phonetic], what is the amount
of precipitation required to be a precipitation event,
and what would be the depth of the associated wetting front?
>> Um, I, I have to look back at my notes.
I think used a [inaudible] as 5 mm, so [inaudible] 50% [inaudible]
that would be letting the soil soak to a 1 cm depth, so I figured anything smaller
than that would be really insignificant, so that's what I used as the cutoff.
[background noise]
>> And then a question from Joseph Deyer [phonetic], does disturbance, say grazing,
affect the concentrating effect of rainfall and therefore range condition?
[background noise]
>> Sorry, can you repeat the question.
>> Does disturbance, say grazing, affect the concentrating effect
of rainfall and therefore range condition?
>> Um [background noise] so yeah, I think the grazing could through compaction lead
to the concentration of runoff so it could function similarly
to the desert pavement that we see.
I didn't look very much at the effects of grazing, although the other impact is
that it could lead to greater erosion of the vesicular horizon and so counteract the effects.
[background noise]
>> And a question from Kip Paris [phonetic],
have any studies been done relating the microbial populations and the health
of the vesicular horizon, does the higher VHI equate to more rigorous, or excuse me,
more vigorous microbial environments that could affect fertility and other characteristics.
>> Um, [background noise] that's a hard question to answer, so I did,
actually one of the experiments that I did was looking
at whether microbial respiration played a role in the production of the air bubbles
that formed the vesicular pores, and so the results of that were kind mixed.
I found that the vesicular pores can form after you sterilize the soil,
so clearly biological activity is not required for vesicular pores to form,
so when I tried to simulate greater respiration by adding glucose to the soil,
I felt better formation of vesicular pores.
So there could be a microbial component that leads to a better expression
of vesicular horizons, but it's not necessary.
And then the other link that we see to the microbial system is this link
that sometimes we see vesicular horizons forming beneath biological soil crust.
So the biological soil crust is another feature that can serve as the surface field
that cuts off the soil atmosphere from the above-ground atmosphere
and leaves the entrapment of air in the pores that form the vesicular pores.
>> And I think we'll [inaudible].
>> I'm not sure if I answered the question, but that's what I know
about the relationship [inaudible].
>> And then one last question, which is kind of a 2-part question, is there a relationship
between the normal wetting front and undisturbed vesicular horizon thickness.
>> Um [background noise], that I'm not sure about, so, um, I guess the closest
that I could have, comment that I could have to that is when I looked at the relationship
between precipitation and recovery of the vesicular horizon following disturbance,
when I looked at the data as number of events, there is a much stronger relationship
than when I looked at the data simply as depth of precipitation, so that would seem to imply
that there's not a really strong relationship to wetting concept,
but I can't think of any other specific study that's addressed that.
>> And then the followup to that question is do you thing that there's an interaction
of biological soil crust in vesicular horizons like there is with plants?
>> Um, um, there are documented observations
of vesicular horizons forming beneath biological soil crusts, so they are associated in that way.
I'm not sure if you're asking whether the vesicular horizons tend to shed water
and concentrate water that is used by the biological soil crust.
In that case, I'm not sure, but I think in the case
of biological soil crust they require much less water than the plants do
and so just a small amount of rainfall
without it being redistributed is what causes the crust to form in the interspace.
[ Background Noise ]
>> [inaudible] last.
>> Hi.
>> Can you shed some data about infiltration
through the vesicular horizon, the materials [inaudible] .
>> Um hum.
>> Did you try making any infiltration measurements when you disturbed things
and put them back [inaudible] recovery [inaudible] ?
>> Yes, I did [inaudible].
And I was a little bit surprised by the results.
So following recovery there was pretty much no change in infiltration.
So the infiltration was restricted both before and following disturbance.
So what that indicates to me is that the material
that forms vesicular pores promotes a very low infiltration rate,
but it's not the actual presence of vesicular pores that restricts infiltration.
It's not because you have vesicular pores, it's because you have materials that are conducive
to forming vesicular pores, I guess is my interpretation, but yeah,
I thought that following disturbance that you might see an increase in infiltration
because we think [inaudible] vesicular pores as being discontinuous from each other,
so they tend to restrict infiltration, but that's not what I found.
>> Interesting.
>> Does that answer the question?
>> Absolutely.
>> With that I think we'll conclude our webinar.
I want to thank our speaker today, and again this webinar has been recorded
and it will be made available as an archive on our soils.usda.gov web page at search
for webinars, presentations, and training sessions.
Thanks all.
>> Thanks Judy.
>> Thank you.
>> Well done.