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Lecture 4c - Vegetation response to climate change
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Biosphere 2 Science for the Future of Our Planet
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Desert Plants, Climate, and Changing Landscapes
How can many different species coexist in once place? In Module 4, go beyond Biosphere 2 to UA Tumamoc Hill—the world’s oldest long-term desert ecology research site near the University of Arizona. Learn how living and nonliving components of the environment are related, how different species coexist, and how climate variation affects Sonoran Desert plants. With Dr. Larry Venable, UA Ecology & Evolutionary Biology.
Meet the Instructors
Kevin E. Bonine, PhDDirector of Education and Outreach
Biosphere 2
In the last two segments,
I've talked about things that create species coexistence and biodiversity,
and how different functional traits of organisms
are responsible for how they respond differently to the environment.
And today, I want to extend that discussion and talk
about vegetation responses to climate change,
what's happening and why.
So, first, I'll speak generally about climate change.
The earth's climate has been changing for a long time,
it's been changing since the beginning of the earth.
Sometimes it's been hotter than today,
sometimes colder, sometimes wetter sometimes drier,
and vegetation and the plants have changed along with the climate.
So, the tropical rainforests have
moved all over the world or the ice sheets have come down,
lots of different things have happened.
A cool example is about 50 million years ago,
there was no polar ice and much of the world was a tropical rainforest.
So today, Australia is almost entirely desert.
50 million years ago it was entirely tropical rainforest.
And if you wanted to see a trap of a rainforest,
you didn't have to go to Costa Rica or the Amazon,
you could just go to Iowa,
because most of North America,
most of the United States was a tropical rainforest.
So this is an interesting long term perspective.
But humans, as practical organisms,
we're a little bit more concerned about what's happening today, yesterday,
coming decades, interpreted in light of what's happened in
previous decades or a few centuries this way forward and back in time.
And what's been going on at the present and for the last couple of hundred years is that
the CO2 levels have been increasing in the earth's atmosphere due to carbon emissions.
And along with that,
temperatures have increased as well.
So, plants respond to these changes,
the ongoing and future climate change in a variety of ways.
Well, increase CO2, plants kind of like that.
CO2 is plant food.
Plants take carbon dioxide and water and they use it to
make energy and structures and whatnot.
So plants actually grow faster under high CO2.
Plants kind of like warmer temperatures too.
They don't like, when it gets cold they can't photosynthesize as fast,
because the chemical reactions operate faster at high temperatures.
But, there's a problem which is there's sort of
an optimal temperature that any particular plant has,
there's a great variety among plants.
And if you get too far beyond the optimum,
they can't tolerate the temperature and the plants will die.
So, warmer is better up to a certain point and then no longer any good.
Then, another thing that's happening with climate change
is that there's changes in where precipitation is occurring.
Some places are wetter,
some places are drier than they used to be,
and they predict that's going to go on in the future.
So for plants, well,
they are wherever they are.
So, they may not do that well in
a new precipitation regime which is wetter or drier than what they're used to.
Another prediction of climate change
is there's going to be more cyclones, fires, and storms,
and these all constitute disturbances to plant community,
might knock down trees or burn up trees but a certain amount of this is fine.
It's always been there and plants can adapt to it.
So, communities come back from some disturbances.
And in fact, you may even have higher species diversity because
the species that come in early after a fire or after
a windfall of trees are
different than the species that eventually compete and take over the gaps in the forest.
And so, you may actually have more species with
a certain amount of disturbance happening here and there now and then.
But the plant communities don't
necessarily come back from extreme disturbances very easily.
And one thing complicating all of this is
that climate change isn't the only thing that plant communities have to deal with.
There's a lot of other things going on.
There's habitat degradation from human activities,
there's habitat loss from agricultural conversion and urban growth,
and there's an increasing rate of
exotic species that are arriving to continents where they've never used
to be due to
the increased global transport and movement of people and goods around the world.
So you might have a big disturbance or hurricane going through Florida,
and okay, before it can respond in its normal way,
the native species come back,
maybe there's an exotic species that's been waiting in the corridors,
and it comes back instead and so,
it deviates the community into something different and perhaps less desirable.
Okay. So how do plants deal in general with environmental changes?
But one thing plants do is they have something called phenotypic plasticity.
So they just change their structure or their physiology to a different condition.
So the same genotype can look
and behave differently depending upon what environment it grows up in.
And so, in this example of plants growing
in 23 degrees centigrade and this is its temperature,
this is its photosynthetic temperature optimum here.
It's lower than if the plant was raised in a 43 degrees.
So the plant has its, like I said,
higher temperature is better up to a certain point and then it goes down.
But depending upon the conditions the plant was growing in,
it may be able to shift its photosynthetic Optima.
But they can only do this to some certain extent.
There's plasticity which is a great thing,
but then there's limits and eventually
plants may have to move if the environment changes too much.
So this is a diagram showing like,
this might be where a species occurs,
it's got these different populations.
Then, the climate gets warmer,
and so maybe these southern populations are no longer tenable,
they go extinct but there's maybe some new populations that can get established.
In the north it wasn't possible before.
So this is range shifts and plants.
Range shifts are well known to already be occurring with climate change in
a variety of plant species both in rain shifts in latitude and rain shifts in elevation.
Elevation in particular has been well studied.
I think because you can study elevation in one place.
You go to a mountain range you can study it where 100 feet
or 100 meter elevation change is equivalent to going north,
I don't know how many kilometers.
So, there tends to be a little bit more evidence for elevation.
But one thing that people have found repeatedly
is that treeline on mountains has been moving up.
So tree line is where the limit of where forests can grow,
and beyond which you've just got these little alpine plants.
And it's been going up and all around
the world in different places where people have studied it.
In the Alps in Europe, the alpine plants,
the ones that are up in the alpine area,
the individual species have been moving up in elevation from one to four meters a decade.
So this is in response to the global warming that's occurred
in the last two, three, four decades.
There's also documentation of latitudinal changes in plant species range.
So like that diagram I had possibly moving north.
This is an example of a shrub abundance in the Arctic tundra in Alaska.
This picture was taken in 1949,
and there's not as many of these dark shrubs.
And then in year 2004,
the shrubs have really increased.
So these shrubs are moving north and into the tundra.
Well, so these are all different ways that plants can respond to climate change,
but it's not clear how fast plants can move
and if they can move as fast as their environments can move.
And again, we've got to think of these complications like habitat fragmentation,
maybe is there direct order even for the plants to be able to move.
Another thing plants do is they change their timing of their lifecycle.
So, Phenology is a word that means
the timing of lifecycle events like flowering time for example.
And plants are changing their phenology.
A case study that's really interesting is Rocky Mountain wildflowers which have been
blooming up to a month earlier than they did 40 years ago.
So it has been a shift in earlier flowering time in these Alpine Meadows.
And so, this is interesting.
It's not always the best for them because for example,
the date of the last hard frost hasn't changed.
So some of these early flowering plants are freezing,
also the pollinators haven't quite caught on.
So it's a little bit of a lack of synchrony with the pollinators.
The bumblebees spend the winter in the ground and they are not emerging
a month earlier and the hummingbirds aren't
coming back from their southern migration a month earlier either.
So there's some pollination problems for
these early flowers that are starting to flower a month earlier.
So for the rest of the segment,
I want to talk about a case study where we think we know what's happening and why.
And you've gotten some of the background in the previous two segments.
So this is desert annual plant communities and how they are responding to climate change.
Work done here in Tucson,
Arizona at the Desert Laboratory on Tumamoc Hill and we have
permanent plots for censusing winter annual plants and have been doing it since 1982.
And so we now have detailed population data on what plants are increasing and decreasing.
We measure their germination, their survival,
their reproduction, the persistence of seeds in the soil seed bank.
So this is a 120 year precipitation data for Tumamoc.
Well, you can see every year there's more or less,
there's wet years, dry years,
it's a lot of variation.
The blue line here is a ten year moving average.
So you can see these trends,
it was dry in the 1920s,
and there was dry in the 1950s,
and it was pretty wet by 1980,
but since we set up this experiment measuring the desert annuals in 1982,
it's been getting progressively drier.
We're about down past the 220 year window,
we're starting to kind of move out.
It looks like of
a precipitation status quo that's been within this range of variation that has occurred.
So it's kind of cool, there's a little quick variations up and down
and there's a sort of decadal trends too,
and then I think we may be seeing some climate change.
We started at least at a very wet position.
Temperature, again, and since we started monitoring these desert annuals,
has just been getting hotter and hotter,
and in fact it's kind of now the running mean and some of
these extreme years that are outside of the historical envelope of temperature.
Well, so what's happening to the desert annuals during
this period that we've documented of drying and heating?
And what's happened is that there hasn't been any net change in the species diversity.
They are doing just fine.
Decades of warming and drying, same species diversity.
The community is dynamic,
so some species that were very common in the 1980s are now rare,
and then some species that were rare in the 1980s have now become common.
So there are some shifts,
but the species diversity is doing just fine,
but we want to know then is,
why are these species shifting?
Why are some species becoming rare?
What's causing population changes in these species?
And we studied in a study of 13 species, common species,
we found that the dominant driver for five,
was variation in the germination fraction.
In this drawing, half of the seeds germinate, half of them didn't.
Well, germination fraction varies from year to year and for five of
these species the variation germination factor was
the best predictor of whether population went up or down.
For another five species,
the best predictor of whether population went up and down was their survival
after they germinated to reproductive state. So, did they survive?
Or did they not survive?
And then for the final three species,
it was the number of seeds that the surviving plants produced.
That was the thing that best correlated with population increases and decreases.
Now all three of these, germination, survival,
and reproduction were significant contributors
to population change for half of these species.
So we've got this range of species in which the whole life cycle to varying degrees in
different species is impacting them as to whether they're increasing or decreasing,
but again they're not all increasing and they're not all decreasing.
So, what's going on? And what's causing
these population changes is that the plants are responding to the changes in weather,
like temperature and rainfall,
and they're responding through these functional germination traits,
and which are giving rise to
a greater or lesser germination for specific species in different years.
So this is the germination input and it's due to the environment interacting with
these functional traits making the plants germinate more or less.
Similarly, the survival and reproduction are controlled by the functional traits
like water-use efficiency and growth rate which are interacting with these variables.
Maybe the rainfall after germination,
there wasn't any or maybe there was plenty for a while and then it stopped.
So all this variation was filtered through these functional traits to give
rise to differences in survival and reproduction.
So the question then is,
what traits distinguish the species that are
doing fine and are increasing with this drying and heating?
And what traits are associated with the ones that aren't doing as well?
Well, it turns out the species that are
increasing in the decades of heating and drying are the ones that have the traits
associated with fast germination and high water-use efficiency.
And the opposite is true for population decline of species with
slow germination rates and
high population growth rates are the ones that are not doing as well.
Okay, so that's kind of a nutshell of what why some species are going up
and down even though diversity has been maintained constant,
but now I want to focus in a little bit more on this trade-off between growth rate.
Some species have a high growth rate and some have high water use efficiency.
This was demonstrated studying
the common species that we've got good data on and there's lots
of them out there and probably account for 70 percent of the plants at our site.
So we wondered, what happened if you added more rare species to this?
What would happen was that would the trade off hold up?
And what we found is that as you add more and more rare species,
they don't necessarily follow the rules.
So this is the trade off line again and the circle sizes to how,
the big circles mean the species is abundant,
and the small circles mean they're rare.
So all the big circles are pretty close to this trade off line,
but all the things way off the line are the rare plants.
So there's rare species in our community and they're not following the rules.
So. This community seems to consist of
a changing core of dominant species that obey this community functional trade off,
but they're embedded within this matrix of
infrequent species that don't necessarily obey the trade off.
And so what's the deal with these rare species?
They're kind of mismatches to the environment at the Desert Laboratory on Tumamoc Hill.
Most of them are more common somewhere else,
maybe where it's hotter or wetter,
but they represent a reserve of species that aren't doing too well,
but that they might have high success in possible future environments.
So what we think that is going on is we've just entering this novel climate envelope,
and so far the structure and diversity of the community
has been resilient even though some shift in the dominant players,
but will a community be able to absorb further environmental change?
And what are the limits to this resilience?
And I don't know,
but I'm guessing that they might do okay at least for a while because
these desert annuals have very fast population dynamics
that can grow populations quickly, decline.
And so maybe they can keep up with the rate of change in climate,
and there are dozens of these misfit species who may hold
the key to community resilience in the face of coming climate change.
There's just things there, now,
that don't do well now, but maybe they'll do better when the climate changes.
Okay, so that's the end of this particular segment.
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