We've descended to about 1,600 meters, little over 5,000 feet, and
we have lost the tree vegetation.
So we're now in classic desert scrub.
The desert scrub in different parts of the Mojave Desert depends on what kind of
geomorphic setting you're in.
Here, we're on a hill slope, which is a pretty typical thing in the Mojave,
because of its topography.
And we have a variety of things like blackbush, ephedra and other species that
are tolerant of dry conditions, but not necessarily of salient conditions.
If you get down into the flats often, then you're in an environments like playas.
Where there's a lot of evaporative water loss, and accumulation of salts.
And so the vegetation there, we'll see is going to be a little bit more on the salt
tolerant end of the spectrum, with species like shadscale and maybe pickle weed.
The two other geomorphic settings we're going to take a closer look at later today
are alluvial fans and dunes.
Alluvial fans occur on hill slopes, and dunes are sort of a special
wind driven feature of the deserts of California with their own flora.
Desert soils, in general, develop slowly, they're thin, they're low in nutrients.
And then you also get these unusual features of deserts' soils that have to do
with the interacting forces of wind which plays a big role in these systems.
And other kinds of erosion.
You can see the surface here covered almost entirely with rocks.
This is called desert pavement.
And it can develop to such a strong degree that you don't get plants anymore because
nothing is able to penetrate the nearly solid surface of stones
pushed together side by side.
Desert pavement is formed by a combination of erosion
of silt and other soil particles that are overlying subsurface rocks.
And also by the wind deposition
of silt that over time can lift up rocks to the surface.
A lot of wind transported particles in the desert are really fine.
And so
another feature of some desert soils that you can get, especially in older soils,
is over time a deposition of a fine layer of silt over the top of the existing soil.
That silt is so fine that moisture from the soil, as it's rising,
sometimes will leave air bubbles trapped underneath that fine layer of silt.
You can develop something called a vesicular horizon.
It's like a layer of air that then makes it very difficult for
water to penetrate down beneath it into the soil.
And so, vesicular horizons can combine with desert pavement to create conditions
that are really impossible for plants to get a foothold in.
For plants in the desert to reproduce, and
effectively outcast can be challenging because they're few and far between.
And the vectors that could move their pollen for them may also be few and
far between.
One of the iconic species or genera of Californian deserts are the yuccas.
And they include a variety of species ranging from small ones to these big
tree like Joshua trees.
And the yuccas are a classic example
of an evolved mutualism to overcome that challenge in the desert.
So, what the different yuccas do, and of 27 species,
at least 20 of them are known to have single moth associates,
is that they form a mutualism with a particular moth species.
And this even extends down within the species level.
The different varieties of Joshua tree each form a one to one mutualism
with a particular moth.
And the moth is moving pollen from the flowers of the yuccas.
And then is in turn getting services from the yucca,
because it lays its eggs in that flower.
And the eggs of the moth develop into larvae, which can feed on some of
the seeds in the developing fruit, and survive to reproduce as moths.
So, the reason that there are these close one to one associations
from the plant's point of view, is that that way they can ensure pollen transfer
to another individual that they can successfully reproduce with.
From a moth's point of view each yucca's flower
is a slightly different size and shape.
And the moths have ovipositors, these long blade like,
ovipositors through which they lay their eggs.
So, if you match the right moth to the right yucca, then the ovipositor
will put the eggs at just the right distance from the developing seeds.