Grasslands

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En provenance du cours de University of California, Santa Cruz

Ecosystems of California

15 notes

University of California, Santa Cruz

Ecosystems of California

15 notes

This course surveys the diversity, structure and functioning of California’s ecosystems through time and the ways they have influenced and responded to human activities and stewardship. Topics include ecosystem drivers such as climate, soils, and land use history; human and ecological prehistory of the state; comparative marine, freshwater, and terrestrial ecosystem dynamics; and managed ecosystems such as range, fisheries and agriculture in California. The course also emphasizes important skills to understand as a scientist or consumer of scientific information, including data collection, natural history, and writing.

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Lowland Systems

Grasslands11:57

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Erika Zavaleta
Professor
Ecology and Evolutionary Biology

[MUSIC]

I'm Erica Zavaleta, and this is ecosystems of California.

Today we are at Jasper Ridge Biological Preserve, the research reserve for

Stanford University, and

it's located just east of the Coast Range, which you can see behind me, and

just west of Stanford University and Palo Alto at the heart of the Silicon Valley.

It's July now so most of the plants in the grassland are either dormant or dead for

the summer.

Today we're talking about grasslands, and

the reason that these iconic golden hills got their name

are that these grasslands are adapted to California's summer droughts.

So, every year, these plants either die or grow dormant under ground for

the most part for the summer when it's not raining.

They stay golden because they don't decompose right away, it's too dry.

And so, it’s only in the next fall and winter when the rains return that

this gold will turn to gray, it'll return to the soil as organic matter and

some residual nutrients and the hills will green up until the following spring.

Jasper Ridge's grasslands are typical of many of the state's interior grasslands.

A lot of these grasslands are fragmented by agriculture, urban development,

and suburban sprawl.

So all of them have ecology that reflects that sea around them.

Jasper Ridge is dominated in it's grasslands

by invasive exotic species that have been in the state for a really long time.

Things like these wild oats from Europe, that are annual grasses as

opposed to perennial species and that have shorter annual phenologies.

They dry up earlier in the year.

They have shallower roots.

So how did this place look a few hundred years ago?

We don't really know for

sure but it was probably, instead of so many annual grasses,

dominated by more big tufts of native bunch grasses like purple needle grass.

And, or by more wildflowers like Poppys and

native Tar Weeds, Lupins, and native Lotuses.

It could be that because all those other kinds of plants have deeper roots and

are able to stay green longer into the summer,

the California's golden hills were really golden for

a much shorter period of time before all these annual grasses took over.

This is a native tarweed.

It's called the tarweed because it's got this sticky stem.

You can see that it's still green even though most of the plants

around it are dead.

And that's because it has these deep roots that allow it to keep reaching ground

water longer into the summer.

This is a native lotus from the genus.

Its a nitrogen fixing legume so

it has an association with bacteria on its roots that form nodules on the roots and

can take nitrogen that wouldn't normally be available to plants, nitrogen straight

out of the atmosphere, and convert it into usable forms like ammonium and nitrate.

And you can see this lotus is also still green even though the plants around it

are mostly dead.

So again, we have these native plants that are able to tap into deeper stores of

water and stay green longer into the summer.

Jasper Ridge is also the site of a lot of research over the years that's built

our knowledge of California grassland systems and

help build ecological theory more generally.

We're at the Jasper Ridge global change experiment right now,

which is a manipulative experiment that is looking at how different climate and

atmospheric changes interact with each other to reshape a whole ecosystem.

So, these plots of grasslands are exposed to different levels of

warming with these heat lamps, elevated carbon dioxide in the atmosphere,

which is emitted through these emitter tubes which are full of little holes.

Elevated rainfall, which comes out of this sprinkler heads and

then nitrogen deposition or nitrogen pollution which is applied to

two out of four of these quarter plots every year.

And so the project is looking at how changes that we expect to see happen

worldwide in the next several decades are affecting ecosystem level dynamics.

Like water budgets, carbon storage and the balance of productivity above and

below the ground.

And community level questions, what kinds of plant, animal, and

microbe species increase or decline in response to these changes.

And the grassland is a great model system for this kind of study because the plants

are small, thousands of individuals can fit into a single one of these plots.

It's diverse, we get a lot of different species in each of these plots,

even though the plots are pretty small, and they have mostly annual life cycles.

So we can see what responses are like over many generations by following the plots

for just several years, whereas if we were trying to work in a forest, for

example, to get several generations will take longer than the amount of time it's

expected to take for the world to get that much warmer.

Experiments like these are cool because they let us get a cause and effect.

They let us look at the interactions among different factors that we're not

going to find together on the natural world through observational studies.

And they also give us a chance to control things like the temperature,

and the nitrogen deposition rate

in ways that give us a clearer sense of what the effects of those changes

are against the backdrop of all the natural variation and noise in the system.

So, they're a great complement to observational studies where we look over

time at how the real world is responding to things like climate change and

atmospheric pollution.

The experiment is set up with 36 of these plots, and each one has a different

combination of warming and elevated carbon dioxide, so it's replicated.

There are nine plots that get each combination of high warming,

low carbon dioxide, high levels of both,

low levels of both, and high carbon dioxide, low warming.

Doing them over and over like that allows us to see how the plots

that are experiencing the same treatment differ from one another, and to see

whether those differences are smaller than the differences between those plots and

the ones that are getting a different combination of warming and carbon dioxide.

It's also a nice way to isolate the different effects of warming and

carbon dioxide.

And then to look at whether when you put them together,

the whole is the sum of the parts or you get unexpected

synergistic interactions between elevated carbon dioxide and warming.

That you wouldn't get just by adding up the separate effects

of those two changes alone.

Let's take a walk up to a different part of the reserve,

to look at a different kind of grassland on the ridgetop of Jasper Ridge itself.

So Jasper Ridge Biological Preserve is an island in the sea of urban and

suburban development.

And that means there's a steady stream of exotic species, plot yields of seeds,

coming into the reserve.

So that's why most of the grasslands look like this.

You've got yellow star thistle, you've got wild oats,

Italian rye grass and different bromes, soft chess

and that's typical in California, but if you come with me up to here,

you can see that something changes pretty dramatically.

We've got all of a sudden a grassland that's shorter plants, more green,

different species, it's less productive.

And what's happening at this boundary

is that we've reached the edge of an outcrop of serpentine rock.

Serpentine rock is mantle rock, it's rock that bubbles up along the faults that dot

the Bay Area from deep in the mantle, and it's got this strange chemistry and

this very low capacity to hold nutrients and to hold water.

That creates refugees for native California grasses and other plants and

animal species that have been excluded by these more competitive

exotic invaders elsewhere.

So let's take a little time to visit the serpentine grassland and

see some of what's there and what's not.

So these serpentine islands are scattered like an archipelago across

hundreds of kilometers in the Bay Area, and north and south of here.

And we talked down the hill about how grasslands in California have

been a great model system for studying ecosystem dynamics.

At a very different scale, these archipelagoes of serpentine islands have

been a great model system for studying landscape level, phenomena, and processes.

So for example,

there used to be a butterfly, that lived here at Jasper Ridge.

It was called Edith Bay Checkerspot Butterfly.

It's now a federally protected species.

And it still exists, at just a couple of serpentina Islands south and east of here.

But that butterfly used these islands of serpentine and jump from

island to island from time to time so that the little populations were connected.

Professor Paul Ehrlich and others used Edith's Bay Checkerspot butterflies and

the populations on these different islands of serpentine rock as a model

system to study the dynamics of meta populations, which are series of

linked populations that act as sources and syncs for each other.

And, even though the butterfly is gone from here,

this is still potentially an island that it could colonize in the future.

The serpentine grassland, because it's got these rocky,

poorly developed soils, is a refuge for

native California plant species like this squirreltail, which is a grass.

And this rosinweed, which is a native forbe.

Both of these are native species, but they're not endemic to the serpentine.

There are many other things out here that we can see in the winter and

spring that are endemic, and only occur on the serpentine grassland.

But there are many other native things like this that used to be present

throughout a much broader span of grassland types.

And that now are able to find refuge here

from those really tall competitive annual invaders from Europe that have

come to dominate the non serpentine grasslands in the state.

So animals play really important roles in shaping grassland dynamics, too.

This is an old gopher hole.

The gophers in California, they turn over anywhere from a fifth

to a third of the area of the grassland like this every year.

And where they make their little gopher mounds are places where there's fresh,

fertile soil for different kinds of plant species to establish then you might

get in the more compacted maybe nutrient core matrix around them.

So as you look around the grassland,

particularly here in the serpentine, you can see that there are these patches

of different things popping up right next to each other and that's a lot of factors.

It's little places that are shallow or deeper soils or places that are mounds and

swales.

But a lot of that is driven by animal activity.

The gophers, which are big and really engineer this system, as well

as the actions of harvester ants and a whole host of other invertebrate species.

[MUSIC]

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