Introduction to ecosystems
Introduction to ecosystems

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Introduction to ecosystems

2.1 The carbon cycle

Dr Vince Gauci describes how carbon that plants have fixed from the atmosphere moves through an ecosystem and eventually is returned to the atmosphere. Carbon can be stored for long periods in the natural environment.

When you've watched the video think of some examples of places where carbon is stored.

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The peatbog problem

DR. FRED WORRALL
Our largest store of carbon in the country are not our forests, it's our peat bogs. The peat bogs of the UK store more carbon than the forests of Britain and France combined.
JULIETTE MORRIS
How significant a part can peat bogs play in helping to tackle global warming?
DR. FRED WORRALL
Oh, oh tremendous. The amount of carbon stored in our peat lands in the UK is the equivalent of 21 years of total UK CO2 output. So that's all the CO2 from cars, power stations, everything.
JULIETTE MORRIS
Gosh.
DR. FRED WORRALL
There's 21 years worth. And that's a relatively conservative estimate. So if we damage these areas, we're going to be contributing to our CO2. But also, this stuff has been growing here for thousands of years, and there's no reason why it couldn't keep growing for another 6,000, 8,000 years. This has been growing here since the last ice age, so it can keep on growing. And that means it can keep on storing carbon and keep on taking carbon out of the atmosphere. So if we manage these well, they will actually help us solve our problem. If we manage them badly, they will contribute to our problem.
JULIETTE MORRIS
Keeping across all these carbon movements is Fred's colleague, Bob Baxter.
JULIETTE MORRIS
So how does it work?
DR. BOB BAXTER
Well, what it's got is two major components that you can see here. One is simply just measuring. As air passes through the prongs of the system, the concentration of carbon dioxide in the atmosphere at 10 times per second. So very rapidly. And coupled with that, we have basically wind coming across the landscape, bouncing across the landscape if you like. We need to know the wind speed, so we use something called a sonic anemometer, which are the prongs that you can see on that system there. People at home may recognise the cup anemometer, which you see on weather stations often spinning around, telling wind speed. But that's just in one direction, just in the horizontal. We need to know whether the air is moving up from the land or down to the land.
JULIETTE MORRIS
And that then obviously tells you which direction the carbon's going in.
DR. BOB BAXTER
Yes, exactly.
JULIETTE MORRIS
Ultimately, what's going to happen to all your research?
DR. BOB BAXTER
So, what we're trying to do here at the present time is get a long run of information day by day. Early days at the moment in terms of what is happening certainly, but through modelling, through trying to predict into the future, then we're trying to use this as a baseline information of a number of years trying to predict what is happening in terms of global warming.
DR. VINCENT GAUCI
At the Open University, we're also researching greenhouse gas emissions from carbon-rich wetland ecosystems. Bob Baxter uses a micro meteorological tower to integrate over very large scales. But if you're interested in the very fine scale, like we are, all you need is this. It's called a chamber and it's used to trap emissions from the soil so we can analyse what they are. So you place a chamber on the soil's surface, and here we've got a nice soggy, peaty soil surface, which should be producing lots of methane, and you define the volume of the chamber by placing a lid onto the top there and sealing it. It's simple but effective. With the chamber in place, we can take an initial sample. This should show a composition that is similar to the ambient local atmosphere. It's our baseline sample. And then we wait, taking samples over time. 20 minutes later, I come back and take my third and final sample of the volume. I would expect to see a much larger concentration of methane in the chamber. The samples are analysed in the lab so we can calculate the rate at which methane is being produced and get an idea of what's happening to an important carbon store. This kind of field work looks at methane emissions as they are today. But in the lab, we can travel back in time and that's what Ph.D. student Carl Boardman is doing. Inside these sealed units, Carl has a selection of peat cores from a bog and a fen, two different types of peat land ecosystems that produce methane. The units are linked to gas cylinders so Carl can precisely control the makeup of the air inside. And he's particularly interested in the level of CO2.
CARL BOARDMAN
An experimental CO2 level is approximately half modern day concentration. And now that's significant because approximately a half modern-day concentration is equivalent to what was present 21,000 years ago, which was the last glacial maximum. So what we're trying to do is trying to recreate the CO2 concentration in the atmosphere back them.
DR. VINCENT GAUCI
So the air in Carl's experimental cabinet is the same mix of gases as the air would've been at the height of the last ice age, so we can find out how the availability of CO2 would've effected methane emissions back then. The way he samples and records methane emissions is similar to what I was doing out in the field, but a little more high tech. With the chamber in place, Carl can see a readout of the methane emissions immediately.
CARL BOARDMAN
What we're looking at now is a continuous readout of methane emissions coming from the peat core. On the X-axis, we've got time. On the Y-axis, we've got methane concentration in parts per million. The flat line before 800 seconds is the ambient methane concentration. About 800 seconds is when the chamber was put onto the peat core. When the chamber has been put onto the peat core, what you can see now is a linear increase in methane concentration with time that's coming from the peat core. The main reason why we're doing it is because current research is based upon modern day parameters; so when these studies or these models try to extrapolate and go back in time, they're actually constrained by the fact that they're using these modern day relationships. Well, hopefully, the results that we get from this experiment will constrain the models that are currently out there.
DR. VINCENT GAUCI
So, in a modern lab, like the one where Carl is conducting his experiments, you can really push or manipulate or constrain the system you're investigating to find out how it works. Same thing's happening with earth's climate system, where the carbon balance is being perturbed by human emissions. Now, we can really mimic these cycles and these perturbations in the laboratory, and that really helps us to find out what's going on out there in the real world. The cycle of carbon is the key to life on Earth. Plants absorb carbon as CO2 through photosynthesis, and its re-released over time through decomposition. But the balance of carbon is also important in regulating climate. So, as our climate changes, it becomes more and more important for us to understand both the balance and cycle of carbon. And that will help us to understand what will happen in the future.
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Life on earth is carbon based. A key feature of ecosystems is the passage of carbon through the system as part of the carbon cycle. Solar energy is captured in the leaves of plants and drives the incorporation of carbon into organic molecules. Carbon dioxide, in effect, combines with water to produce simple molecules. The process is called photosynthesis and in this video Sir David Attenborough describes it as the very basis of life.

How does the availability of light, water, carbon dioxide and nutrients affect the productivity of an ecosystem?

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How plants make food

DR. VINCENT GAUCI
The fundamental material of all living things on our planet is carbon. Now this starts out as an inorganic, molecular gas in the atmosphere, carbon dioxide or CO2. So to get the carbon into an ecosystem you need the process of photosynthesis. It's a process that is unique to plants and certain micro-organisms, but it benefits almost every living thing on earth. Photosynthesis is how plants make their food, using a simple set of ingredients. Sir David Attenborough described it as the very basis of life. So let's leave it to him to explain how it works.
SIR DAVID ATTENBOROUGH
Air seeps into the leaves from pores on their surface. It circulates within them and reaches tiny granules that contain a green substance, chlorophyll. This is the key facilitator that uses the energy of the sun to bond carbon dioxide to hydrogen derived from water and produces carbohydrate - sugars and starches. These dissolved in sap are then carried from the leaf into the body of the plant, even during the night when the leaf factory has shut down. Come the dawn, the sun re-appears and the process starts up again.
DR. VINCENT GAUCI
So photosynthesis is the fundamental process driving the production of material in ecosystems. Light, water, nutrients, and CO2 are all key ingredients in driving that level of productivity. If you were to reduce any one of those key ingredients, that would result in a loss of productivity in a plant, like this tree here. Those four key ingredients, together with temperature, are known as environmental variables. Each one of them can affect photosynthesis and as they are unevenly spread through space and time, there can be dramatic differences in productivity across the globe and over different time scales. The availability of light varies throughout the day as the earth spins on its axis. At the Poles there can be almost constant daylight in the summer months, but most of the planet experiences a diurnal cycle - night and day, darkness and light.
PROFESSOR DAVID GOWING
Well, light is key to photosynthesis because it's the source of energy, and therefore it determines the rate of which photosynthesis can proceed. This means that productive ecosystems need to be in full sun, like a forest canopy. Beneath the canopy where light is attenuated by the canopy above it, then plants can operate at a much lower rate and take in much less carbon per unit time. Too much light can be a problem because if leaves can't access carbon dioxide quick enough to use that energy in photosynthesis, the excess energy becomes a problem for the leaf. It may even damage the photosynthetic apparatus. So this is an issue at the top of the forest canopy, where the leaves are in full sun. And plants have come up with a whole range of adaptations to cope with that, including pigments to try and absorb the excess light and photo respiration, where they actually respire the carbon they have just fixed to produce carbon dioxide to soak up that excess energy.
DR. VINCENT GAUCI
At the equator, light is available for twelve hours a day, all year around. That steady supply of energy, combined with high levels of rainfall, make the tropical rain forests highly productive. Temperatures are high all year, so water is available in liquid form. It doesn't get locked away as ice during the winter. In other parts of the word there is a different mix and cycle of environmental variables. In temperate regions, where the seasons are more pronounced, production takes place in spring and summer, when there's the most sunlight and the warmest temperatures. This cycle of production through the seasons can vary year by year. And we can see a record of variations in tree rings, with the wider rings showing warmer summers. In dry, arid areas of the tropics it's not light that's the issue. It's the availability of water that limits production. Perhaps the most powerful way to show how water limits production is to look at what happens when a desert gets wet. And that happens on a huge scale in the area of the Kalahari Desert known as the Okovango Delta. These remarkable images were filmed by the BBC programme "Plant Earth". As water flows into the delta, the landscape is transformed. With the water comes life. Plants can once again produce carbohydrates through photosynthesis, converting CO2 and water. What was once a barren desert is now a wildlife oasis. This is an extreme example of water limitation and the relief water can bring to a dry ecosystem. But to fully understand the effect of water on productivity, we need to understand the biochemistry of photosynthesis.
PROFESSOR DAVID GOWING
Water is essential for taking carbon dioxide out of the air because plants exchange water vapour for carbon dioxide when their stomata are open. And so a lack of water can really limit the amount of carbon a plant takes in. In environments where you have a real lack of water then plants have come up with alternative photosynthetic pathways to cope with the problem. A group called the C4 plants concentrate carbon dioxide by having specialist cells that take it in, store it, and pass it to photosynthetic cells which are located deeper within the leaf. And by doing that, they're supplying those specialist cells with enough carbon dioxide that they can utilise light to its full without the risk of excess light damaging the apparatus. C4 plants tend to occur in dry environments in high light, so savannahs would be the most typical natural environment for them. And plants that have evolved in that sort of savannah environment are quite often used as crop plants such as maize, and sugar cane, and sorghum. They all use the C4 photosynthetic pathway, which means they are extremely productive if supplied with light. Taking that one step further, an even more extreme adaptation is cacti and succulents that only take up carbon at night. And they store the carbon as organic acids in their big, fleshy cells, for later use during the following day where it's re-released as carbon dioxide and normal photosynthetic pathways take over.
DR. VINCENT GAUCI
So light and water can influence and limit productivity. The same is true of CO2, which is vital for photosynthesis. Now the concentration of CO2 doesn't vary over the planet. On a fine scale you can have concentration differences that are sufficient to affect productivity. In a dense canopy, CO2 can be used up faster than it's being replenished from the atmosphere. Manmade emissions can also have an effect. So light and water, carbon dioxide and nutrients, all directly influence the productivity of any one ecosystem. These limiting factors, together with temperature, are known as environmental variables. But that's only half of the cycle. In the next film we will look at the other half - decomposition.
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