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0:00
♪ Music ♪ In this first of two lectures on ecosystem ecology, Dr. Whendee Silver presents an overview of the field and some of the key currencies within it. She defines ecosystem ecology as the study of the organisms and the abiotic environment and the interactions between the two in an area defined by the strength of the connections across components of the system. She notes that ecosystem ecology focuses on stocks, flows, and pools of the basic components of life, energy, and nutrients.
0:31
She uses the example of the carbon cycle as one physical process that is conceptualized and measured within ecosystem ecology and one that has contemporary relevance in the context of understanding global climate change. She notes that the conceptualization of cycles and impacts in ecosystem ecology functions at the human timescale rather than the geologic timescale, in part because of measurement and data issues. Let’s start with a pop quiz. First question is what is ecosystem ecology?
0:56
And I’m just gonna give you a couple minutes quickly right off the top of your head, don’t think about it too deeply, what do you think it is? I would define it as the study of organisms, the abiotic environment, and their interactions in a defined area where the connections within the area are stronger than the connections across the system boundaries. So what Brian said about drawing a box, that’s kind of what we do. We look at the interactions within a given area and those
1:21
interactions are stronger than the interactions across. That’s not to say that there aren’t interactions across the boundaries, there are. What’s biogeochemistry? I always have to ask people this because I was a Leopold fellow; Leopold program is this program that tries to teach scientists how to communicate with everybody else and the first thing they say is don’t call yourself a biogeochemist. Nobody knows what a biogeochemist is, so I’m asking you what’s biogeochemistry?
1:49
And if you can tell me, great I’ll write it down. The study of biological, chemical, physical, and geological processes and reactions that govern the environment. The whole concept of biogeochemistry and why I think it’s an important part of ecology and ecosystem ecology is it brings in these other scientific disciplines. Ecologists used to be, and maybe still are, accused of having physics envy, maybe because of this theory issue that physics has these laws
2:16
and these theories and ecology has always struggled with these laws and these theories.
2:21
Biogeochemists bring in a lot of those kind of fundamental laws and theories into the realm of ecology based on physics, biology and chemistry, and geology. Okay, these are really quick. Where does energy, where does the energy in ecosystems come from? The sun, great, thank you. What are the essential macronutrients for life? N, P, K, calcium, sulfur, and magnesium. Those are the fundamental nutrient, nutrients that most organisms, I would say all organisms,
2:54
microbes are a little weird, but all organisms need to survive; kind of important and ecosystem ecologists and biogeochemists spend a lot of time focusing on pools and flows of these in addition to carbon. And then the last two questions are things that I just want you to know and I think everybody should know. What are the three big greenhouse gases? CO2, nitrous oxide, methane. Water vapors are really big greenhouse gas, but we kind of don’t count it very much because
3:22
the big water vapor fluxes, at least at a global scale, are pretty much driven by natural processes even though humans are altering the water vapor, you know, the water vapor dynamics globally, we tend to focus on CO2, nitrous oxide, and methane.
3:37
Can you give their hundred year global warming potential? How many people know what a global warming potential is?
3:44
Okay perfect. One? Is that really? One. Okay, global warming potential is like the radiative forcing, it’s how much heat it can generate relative to CO2, so how potent are these greenhouse gases relative to CO2? We do it relative to CO2 because CO2 is the biggest greenhouse gas. Nitrous oxide is 298 times more potent per molecule over a hundred years than CO2, methane is 34 times more potent. Those numbers change over time as the concentration changes in the atmosphere and the physics
4:18
and the chemistry changes, but it’s important to know CO2, big greenhouse gas, the one that most people focus on; nitrous oxide, also increasing dramatically rapidly, 298 times more potent per molecule, methane 34 times more potent. From a socio-environmental perspective, there’s a lot of interesting CO2 questions, but God there’s tons of nitrous oxide and methane questions, and people really need to be paying attention to those other ones as well.
4:46
I want to start off just going on a little bit with definitions. So we kind of defined what ecosystem ecology is. Ecosystems, an ecosystem is a physical construct; it’s both physical and conceptual.
5:06
And it describes the interactions of a set of organisms, biota, the biota that most of the rest of ecology is focusing on is a single focus and we look at the interactions among that biota, but also with the abiotic environment. Ecosystems can be any size and they can occur anywhere in life. Ecosystem ecologists generally focus on large blocks of land or water, but ecosystem ecology has recently been applied to other kinds of systems like the human body,
5:37
it also, just the gut, people are looking at the gut ecosystem, the gut can be defined as an ecosystem with energy flows and material in there, and I guess if you were to define the gut as an ecosystem, the body would become a landscape. Landscapes are places where ecosystems interact, so where you get that exchange across an ecosystem boundary that interacts with another ecosystem type, where you got all those other interactions going on internally,
6:05
so we’ve got all these different systems in the body, they interact, that could be looked at as a landscape. Yellowstone National Park is also a landscape, a classic landscape that’s been well studied. Most ecosystem ecologists who work on land, terrestrial ecosystem ecologists define ecosystems as a watershed. You don’t have to, there are other, there are other ways of defining ecosystems, but watershed is very common; similar kinds of features with distinct boundaries.
6:37
In aquatic systems, ecosystems tend to be lakes, ponds, pools, river reaches or similar kinds of designations in oceans, but being a terrestrial ecosystem ecologist, most of my examples come from land, the same concept should apply elsewhere. Now I’m gonna draw an ecosystem. Usually I stand there and I try to be an ecosystem, but I’m realizing that it’s getting harder and harder; I probably need to have all of you crawling on the ground as microbes cause microbes really rule the world.
7:06
So here’s our ecosystem, we’ll start with a tree
7:13
and
7:15
of course the tree is more than just the above ground parts that we see, and if I were to really draw this to scale
7:25
and be honest it’ll actually, it would look like this.
7:37
And which is one of the reasons why I had you read the Richter and Billings paper is because ecosystem ecologists have recently realized that ecosystems extend far down, often to where the bedrock is and now maybe we’re learning that it’s not just the edge of the bedrock, but all of this is supplying materials and energy that flow through the environment. We look at energy flow,
8:13
and energy comes into ecosystems from the sun, primarily from the biotics perspective, and it’s fixed in the plants via photosynthesis; I abbreviate photosynthesis, PS.
8:31
We also have a lot of energy exchange in ecosystems because these plant parts, which are not now connected, there we go, these plant parts die and produce organic material, which, some of which ends up in the soil and that feeds the vast majority of organisms that we have in the world, which are microbes; microbes rule the world, they absolutely rule the world. We are shells for microbes; we are ecosystems; we are, probably like I said, probably landscapes
9:02
filled with microbes that are making a lot of the decisions that we think that we’re making. In fact, they’ve done a really good job of, evolutionarily, of figuring out how to let us think that we’re making the decisions when really they’re making the decisions. In terrestrial ecosystems, the vast majority of the biomass, the vast majority of the diversity, the vast majority of nutrient and energy flows is happening through the microbial biomass, most of which is living in the soil,
9:27
although of course it also lives in the aboveground parts of the ecosystem as well as in the air; there’s a lot of microbes that are transported through air. Okay, so we’ve got sun coming in producing or helping to produce this organic material that ends up in the soils. These little microbes here, we’ll draw different shapes cause they are different, eat that organic matter they are called heterotrophs and they release CO2 back into the atmosphere; these guys take in CO2,
9:59
they release oxygen, they produce carbon;
10:04
there's nitrogen, calcium, magnesium, potassium, and sulfur all cycling around here as well, so these are all going around
10:18
and there are things that also leave ecosystems, some of them leave in gaseous form –
10:29
oxygen, CO2, nitrous oxide, methane CH4, and a whole host of other gases that leave, we see leeching out of ecosystems as well, so in liquid form, these can be these mineral nutrients, they can be dissolved organic carbon, they can also be materials that are leaving, so soils can leave either in runoff or erosion go into aquatic systems and out into oceans and places like that, and we have export of organisms, this is my organism, there we go. They can also leave ecosystems and
11:15
come into ecosystems, right. So ecosystems are defined by these units of land, generally, or water, and we look at the cycling of materials, materials being these nutrients – water, organic matter, and energy being the sun and also carbon. I also should mention that there’s a group of microbes called chemolithotrophs
11:45
that can also use this stuff for energy. Again, the vast majority of focus has been right up here near the skin of ecosystems, but there’s all this stuff down here that probably is playing a really important role, and this again is where scale becomes important. We tend to focus on human timescales; . ecologists have a very human timescale bias. Evolutionary ecology of course is a bit longer, but geology also has an evolutionary, geologic timescale focus and there’s probably a lot
12:17
of really important dynamics that are going on here, they just don't tend to happen on a human timescale, so we’ve kind of ignored them, but this whole chemolithotrophic energy cycle that’s going on down here does connect with what’s going on in the surface over long periods of time How do we measure this stuff or what units of measure do we use?
12:40
We tend to, as Brian said, tend to divide things into pools and fluxes; I would say pools and concentrations versus fluxes.
12:51
So a pool would be how much carbon or nutrient is stored in the aboveground biomass or how much is stored in the below ground biomass or how much is stored in the fish, the fish species or fish communities or fish whatever’s larger? I guess communities is as large as you can go with fish. How much is stored in those organisms, right? That would be a pool, a concentration, and so pools could be how many grams per meter squared, kilograms per hectare, metric tons per hectare, so on and so forth.
13:30
We also have concentrations like micromoles per milliliter of water or in solution, micrograms per gram in soils or in plant tissues, milligrams per kilogram, which is the same unit, which is also called parts per million, just cause you’ll see those sometimes in literature, it’s good to know that they’re the same thing, so micrograms per gram, milligrams per kilogram, parts per million are all the same unit, a percent, right. Those are concentrations, so we tend to look at
14:08
pools and concentrations on the one hand and fluxes on another, and fluxes are per unit, time, right, so you take micrograms per gram per day or you could do kilograms per hectare per year. So you could ask the question, how much nitrogen is being deposited in ecosystems through anthropogenic activities? We might measure that in kilograms, kilograms of nitrogen per hectare of land area per year, okay, so that’s a flux. So ecosystem ecologists tend to measure pools
14:50
and concentrations and fluxes and differentiate them. The characteristics of pools and fluxes or pools, concentration, and fluxes largely determines the biotic potential of an ecosystem. So, for example, if you had a tropical forest, I do a lot of work in the humid tropics; there’s abundant sunlight, there’s warm temperatures, lots of primary energy coming in, lots of water, fairly large nutrient stocks because the soils are very deep and have been weathered for very long periods of time,
15:20
large sections of the tropics never experience glaciation, so they’ve just been sitting there weathering, doing their thing, getting their soils deeper and deeper and deeper over time. So fairly large nutrient stocks, that drives high net primary productivity;
15:42
another abbreviation you’ll see a lot NPP, net primary productivity, which is that, the net growth rate, right, how much biomass we see growing? How many kilograms of biomass per hectare per year? Where is my flux? Kilograms per hectare per year, okay, or metric tons per hectare per year.
16:02
Contrast that tropical forest, which is one of the most productive ecosystems we have on this planet, with arctic tundra. So in an arctic tundra we have a lot less sunlight, right, during the year, right; lot less sunlight, so a lot less energy coming into that ecosystem. We also have a lot less nutrients in soils because it’s cold a lot, there’s a lot of permafrost, it’s hard for microbes to live, there’s not a lot of organic acids being produced relative to a
16:33
tropical forest, that’s not gonna weather this sub-straight quite as quickly, so essentially that tundra is growing in a much smaller pot than the tropical forest is. Tropical forest in a big pot, lots of nutrients, things are really pumping – plants grow big, plants grow fast. Tundra - much smaller pot, lot less light, lot less energy, things are going a lot slower, looks really different than a tropical forest, so the pools, concentrations, and fluxes
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really control the structure and function of the ecosystem.
17:08
All of those factors feed back then to shape what that ecosystem looks like. What are the species composition? What’s the biological potential of that ecosystem? What are the potential interactions in that ecosystem? So as opposed to taking the biota-centric perspective of cascading down and maybe this gets it the bottom-up, top-down controls. Ecosystem ecologists look at the world more from well, what’s the biotic potential in that ecosystem based on
17:37
the energy, the nutrients, and the rates of flow? The climate and environmental conditions that are controlling that as well as the organisms that are there in and of themselves, okay.
17:51
Even though these two ecosystems, it’s an important to point, the two ecosystems do the same thing, so they’re all cycling energy in the form of sunlight and carbon, they’re all cycling the same set of water and nutrients, right, they have the same stuff. They have that same suite of nutrients that they’re using, in fact, all plants do everything pretty much exactly the same way and in some ways I started out as a plant ecologist and I thought plants were really interesting
18:19
and then I realized wow, plants are doing everything exactly the same way. In some ways that’s a wonderful simplifying principle; they all photosynthesize, photosynthesis is just one process, there’s essentially, I mean, there’s some, a couple of slight variations on this, C4, C3, CAM. Pretty easy, right. When you get into microbes, you realize wow microbes have completely different physiologies; they do things fundamentally differently, so microbes have evolved ways to do things differently.
18:49
Plants, however, which produce a lot of the biomass on the surface and are really key for capturing that sunlight and bringing that energy in, pretty much do things the same way with the same suite of nutrients, and because we’re so dependent on that plant life, we pretty much physiologically do the same thing. And higher organisms with essentially the same suite of nutrients. The difference is how much you have and how quickly is it going? And a lot of that is controlled
19:18
by the physical environment. Okay. Ecosystem ecology builds on the composition, structure, and behavior of populations in communities, ecophysiology, biometeorology as well as geology, geochemistry, atmospheric chemistry and physics, oh and evolutionary biology cause deep time matters. So to do ecosystem ecology really well, you have to not be afraid to explore these other disciplines. I think that’s probably easy for people in this room, you guys have gone way beyond that and are exploring
19:47
social science and economics and law and all these other areas. Ecosystem ecology in and of itself has tried really hard and I think it’s continuing to branch out into these other disciplines, so when Margaret said ‘I want you to pick a lens or a theory to teach people about ecosystem ecology, I sat and banged my head against the wall for a while trying to figure out well what would that be. We really need a whole semester, but in the end I decided to pick one that I think
20:19
most everyone can relate to and that’s the carbon cycle because it really impacts our lives very directly. Why is carbon important? Why would I pick the carbon cycle? It’s the building block of life.
20:33
It’s the primary resource for energy capture for plants, right, they take in CO2. Plants by the way are called autotrophs, right, they, I think of them as automatic. They take, they use sun and CO2 as their primary resources of energy, that’s their primary form of carbon is CO2. And they produced simple substances or, and complex substances from that sunlight. Carbon also becomes the key currency for heterotrophs, organisms that use complex and simple carbon compounds for growth.
21:09
We are heterotrophs, together with most other organisms that eat organic substances. The process of photosynthesis and respiration are redox reactions and this is something I just have found super fascinating in my career and I promise I’m not going to get into it. It is kind of complex biogeochemistry, but at the same time I find it really fascinating that we can boil very complex ecology and chemistry and physics that an ecosystem took global scale down to two really important
21:47
processes – oxidation and reduction. So plants take CO2 out of the atmosphere and they reduce the CO2 to carbon compounds to make us. That’s what we are, reduced carbon, and heterotrophs oxidize that carbon for energy, so CO2 is being pulled out of the plants, pulled out of the atmosphere into the plants for energy, right, and we oxidize that for energy, so it’s the two ends of the energy spectrum and release that CO2 back to the atmosphere, all happens at the cellular level,
22:24
it’s a huge amount of energy exchanged, so really it all boils down to oxidation and reduction. Carbon is important because it’s the building blocks, carbon is important because it drives these really important oxidation and reduction processes that impact acidic global scale, and it also is important
22:45
because atmospheric CO2 concentrations are increasing. This is the famous Keeling curve with CO2 concentrations here on the y-axis and time on the x-axis. This is a really cool study that was started in the ‘50s by Dave Keeling, who put a CO2 analyzer on the top of Mauna Loa volcano to measure CO2 concentrations. Why did he pick Mauna Loa volcano? Because Hawaii’s out in the middle of the ocean and it has some of the cleanest, cleanest air anywhere.
23:20
It’s High Mountain, it’s not surrounded by other land masses. There is air coming over from China and there is some, some exchange with other places, but generally it’s relatively clean air, so what he was doing was getting a measurement of the well mixed atmosphere. And what did he find? And people who follow through this work, Pieter Tans primarily, over time is that CO2 concentrations in the atmosphere are increasing, okay we all know that, we all know that
23:46
CO2 is increasing in the atmosphere. This is a really important change in the global biosphere that’s affecting all of us. Why is CO2 increasing? Well it’s increasing because we’ve unbalanced or we’ve altered the balance of the carbon cycle. So the way the carbon cycle works, right, is CO2 comes into plants via photosynthesis, some of that carbon that gets fixed into plant biomass ends up getting trapped in the soil. Lots of it goes back to the atmosphere through a
24:20
heterotrophic activity, but some of it escapes that heterotrophic activity. It either gets trapped in the soil, on a soil surface, or in a little soil clod, we call those aggregates, or in the oceans or in some of the streams, it might sink too fast or it might end up in an anaerobic microsite, a place where there’ not enough oxygen for organisms to break it down very efficiently and so then it gets, it sinks and it gets buried and soils and sediments, and it gets compressed and it gets heated
24:49
the further down it gets and so you have that plant or that animal matter that changes from being recognizable to being kind of this black goo, which we call oil and gas. This is the breathing of the biosphere, this is seasonality. Plants and microbes are a hundred percent responsible for this, so at the global scale on the top of Mauna Loa volcano we can see plants going ‘ahhh’ and microbes going ‘whoo.’
25:19
Right, and it’s primarily the northern hemisphere, which is where most of the land mass is. Okay, pretty cool. All happening because of a redox reaction and cells. So we’ve unbalanced the carbon cycle by taking all that goo, that organic goo that got trapped and has been trapped and gets trapped for millennia and instantaneously releasing it through oxidation; that oxidation being burning of fossil fuels, right, so in this case it’s not just microbial oxidation,
25:52
but it’s also the oxidation that we figured out how to do by just lighting a match, right, or inventing a motor. So that releases a lot of it, we also get some through land-use change, so land-use change especially deforestation facilitates microbial respiration in soils; it makes it go faster, it’s warmer and wetter; warmer and wetter makes chemical reactions go faster, so it marches forward, that carbon gets chewed through more rapidly, more completely, more efficiently goes into
26:22
the atmosphere as opposed to getting stored in soils, right. If we could slow this down, we could definitely have an impact on the CO2 concentration in the atmosphere, but relative to fossil fuels, it’s kind of small. Okay. We have exchange going on, some gets trapped in the land on an annual basis because plants regrow, carbon gets stored in soils, we have an ocean sink due to ocean chemical processes of CO2 coming in, dissolving and sinking to the bottom,
26:53
so we do have these background processes that are going on. One of the most, probably most scary things and there are other people in this room who understand this better than I do,
27:04
with climate change is the fact that this ocean sink is shifting. This is something we depend on dramatically for taking CO2 out of the atmosphere and the more acidified that the ocean becomes and the warmer that the temperature gets, the less efficient it is at pulling that CO2 out. People, I think a lot of people don’t know that. The bottom line here is that we’re changing the balance of the carbon cycle and that has, you know, global significance, and ecosystem ecologists have
27:31
a lot of, a lot that I think we can contribute to understanding this problem, and as I'll talk about tomorrow maybe contributing solutions as well. Just to give you an example of where ecosystem ecology comes into this, there’s some evidence from the literature that suggests that trees in some parts of the world are growing faster; trees are growing faster. Here’s some of that data. So this is accelerated growth rates of Eastern Trees. This is aboveground biomass, so the stems
28:03
and the leaves and the branches of the trees in metric tons per hectare, so it’s a pool, and then this is time here, stand age in years, and we would expect as trees grow especially if they were younger trees and people know in East, right, that a lot of the East was deforested when European settlers came, so they’re still growing back. We would expect that those trees to get bigger, but this purple line here shows the rate at which we would expect those trees to get
28:34
bigger just based on age alone. This is where we go into the realm of modeling that Brian described. We have some data, we’ve now produced a model, we meaning the ecological community, not me, produced a model with some degree of confidence that suggests that this would be the rate at which trees would gain biomass with time, age, and you can see that a lot of these lines are steeper than would be expected, and one hypothesis is that these trees are growing faster because we’re increasing
29:06
a fundamental resource for those trees; we’re increasing atmospheric CO2. So we could make up a thesis statement that atmospheric carbon dioxide concentrations through its role in photosynthesis is the dominant controller of ecosystem carbon storage. We might ask a question, what is the ecosystem response to elevated carbon dioxide? And then we could hypothesize, carbon dioxide is the key resource for plant growth and thus increased concentrations will stimulate
29:33
plant growth, and greater plant growth will lead to greater overall carbon storage in the ecosystem. This would be really nice if this were the case, right, because if plants are going to grow more in response to elevated CO2, then maybe we have a built in at least partial solution, right, plants would pull more CO2 out of the atmosphere and maybe we don’t need to worry quite so much; we just need to make sure that these plants grow. How do we do that?
30:01
Well I like to first come up with a conceptual model, a conceptual framework and there’s a good conceptual framework that exists for us to do this. We could try to measure the net carbon balance of an ecosystem, net ecosystem production, NEP. And the equation that we could derive for this is that net ecosystem production equals GPP, which is gross photosynthesis, the total amount of carbon uptake by plants and then we have to account for that carbon loss, right, cause if it all doesn't stay
30:34
all that CO2 comes into the atmosphere, plants themselves respire, we call that autotrophic respiration, R-auto, we have to subtract that out. GPP minus R-auto is NPP, net primary production and then we also have to, if we’re going to ecosystem scale, we have to subtract out the heterotrophic respiration, so how much carbon gets released from the rest of the ecosystem, the heterotrophs, the guys who are eating the carbon, that’s already fixed. That make sense?
31:03
So NEP is the total amount of CO2 coming in minus the CO2 that’s respired by the plant and the CO2 that’s respired by the heterotrophs. Well how do you measure that? It’s a challenge,
31:18
especially if we’re talking about forests. How do you go out and measure that? Well this is what we were talking about over the coffee break, one of the things that I find really exciting about ecosystem ecology is that the tools have improved dramatically over the last decade and I really feel like we’re being able now to start to deal with some of these scaling issues. It used to be that we would go out and measure, and we still do, as many trees as we can
31:43
with armies of undergraduates and other people, who would all stand and figure out what 1.3 meters well they call it diameter at breast height, but breast height is not breast height on everybody, so breast height for me is about nose height. The 1.3 meters above the ground, they’d measure all these trees, we would use models, allometric equations to try to take that diameter and sometimes the height measurement and turn it into biomass and figure out how much carbon
32:09
is here, but of course it took you five years to measure this entire stand in the ecosystem, right, and by then its changed, so then you've got to go out and do it again, and there are whole armies of undergrads that are doing this, actually all over the world, trying to get a handle on this, but what your spending all your time and energy measuring trees in this way and trying to extrapolate from limited information, you’re not going to get very far.
32:34
So one of the newer techniques that’s been used, and newer because its been around for over a decade, couple decades I guess, is the Eddy co-variance technique. Eddy co-variance measures the CO2 in and the CO2 out of ecosystems. This is an Eddy co-variance tower and what it is, what you do is you put a tower up in a place, they work better in flat places than they do in hilly places, but we’re learning how to deal with hills through again a lot of spatial modeling,
33:03
but what you do is you put wind sensors up and down these towers that measure wind direction and speed in three dimensions and you put a CO2 sensor on the top, CO2 analyzer on the top, and you take CO2 measurements along that tower as well, and so what it does is it measures the, the change in wind direction, so is wind going up, is air moving up or is air moving down or is it just going around? And you measure CO2 concentrations along that gradient,
33:39
you also tend to measure water vapor because it also helps us correct for a lot of the variability in here. And you can use that information to figure out how much CO2’s getting taken up by the canopy versus how much CO2 is leaving the whole ecosystem. So it gives us carbon in versus carbon out in a given footprint and a footprint is how far is that tower able to extrapolate out from the tower base, and again we use physics and spatial modeling to figure out what that tower footprint is and we
34:10
combine it with field measurement by placing tracers out here in the woods and we’ll do a little release of gas, of a certain kind of gas, we’ll put that up on the, the sensor up on the tower and figure out, okay, how much of that gas do we measure? And we’ll be able to figure out what our footprint is and of course the footprint changes a little bit over time, but it gives us a map, an area of land that we’re measuring all the carbon in and all the carbon out.
34:34
The technique is really good for getting the net carbon balance of that ecosystem, it’s not good for discerning mechanisms and so we need to get out there also and do things like a lot of what my lab does which is try to understand well how much of the CO2 that’s leaving is coming from autotrophic respiration in the plants versus heterotrophic respiration in the soils or other sources of heterotrophic respiration. That should be coupled with ground based not round based measurements,
35:01
ground based measurements and models. So again we use a lot of assumptions in these techniques, but we use them to try to help put bounds on our understanding of how much carbon is going in and how much carbon is going out. So let’s assume that we can do now a pretty good job of plots of land understanding how much carbon’s coming in, how much carbon’s coming out, we couple that with ground based measurements, we get a pretty good idea of the dynamics.
35:26
What does this have to do with elevated CO2? When we’ve got this national experiment that’s going on all around us all the time, it’s really hard to control for, so it’s hard to see a treatment and an effect, right, it’s hard to understand well what’s being driving by CO2 versus other things when we’re bathed in CO2 everywhere. So the way that we’ve gotten around this as a community is by doing CO2, elevated CO2 experiments. This is an example of one of those approaches.
35:55
This is the Free Air CO2 Enrichment experiments or FACE experiments, and the way FACE works is that you put, they’re usually round, these ones are a little square, but here’s an example of round ones; a bunch of types essentially that you array in a circle or a large square or rectangle, and you pipe CO2 through those and that CO2 bathes the ecosystem and you can control the concentrations of CO2, so mostly people, we're now at about 400 parts per million in the atmosphere of CO2.
36:32
Most people are looking at 500 or above and they’re adding CO2. It’s circling around in here and the plants within this and microbes and other organisms are experiencing elevated CO2, whereas these control sites, some of these are control, some of them are treatments, don’t, so when people do this they actually, some places are actually piping atmospheric air to try to control for the effect of piping air onto the environment, right, so we have treated air
37:00
and untreated air, and we can make those ground-based measurements and use Eddy co-variance to try to understand what the impacts of these are on carbon uptake, growth, net primary productivity, heterotroph respiration, and all of our other measurements. FACE experiments are incredibly expensive and have other whole host of issues associated with them. There are other experimental approaches that people could use, they’re not as satisfying – one is open top chambers where people surround
37:28
single individual plants, and of course we all know that single plants are not forests or a grassland or a crop, and they also have a lot of surface area, a lot of area around that’s disturbed relative to the actual organism that you’re studying; they tend to heat up, they create all kinds of other issues, but for certain kinds of questions, those experiments work. And then of course there are these big macrocosm approaches, this is biosphere 2, its been built in the desert in Arizona,
38:00
which was built as a big experiment to try to, I guess the original goal was to try to figure out whether or not they could build an artificial environment that people could eventually live in on other planets. The idea is, is that you create, you know, anywhere from a micro to a meso to a macrocosm where you control the concentrations. There are pros and cons to all these methods. The pot scale studies are really too small, the plants become root-bound, there’s no
38:32
ecosystem interactions or competition or processes that Brian laid out this morning. The chamber or soil monolith experiments use artificial light and humidity and temperature levels, there’s often, again, no larger ecosystem processes, and limited to a few or low stature plants. The Free-Air Exposure Studies have field conditions and the plants do compete for light, water, and resources; it’s extremely expensive, big tractor trailer trucks come with large containers
39:04
of CO2 to plug in and you need these things very frequently, right, so incredibly expensive and that CO2, a lot of that CO2 ends up getting advected away, meaning it blows away, right, so if the winds blowing particularly strong, the CO2 goes in, the CO2 goes out, and we’re not measuring the up and down quite so much, so that’s a problem. You have to set your control points at, you know, 10 to, plus or minus 10 to 20 percent because we can’t control that they’re not sealed,
39:35
so we can’t control it very well, expensive to replicate, and there are sampling effects meaning that you’ve got these plots that you’ve established, you spent all this money on, you can’t do sampling that’s going to effect your measurement. So, for example, if you wanted to harvest some trees to figure out the chemistry of that biomass any tree you take out of your CO2 experiment site is going to, it’s going to be one less tree you're measuring plus you’ve created a disturbance, right, you've
40:07
removed an individual that didn’t remove itself naturally. Anytime you poke holes in the soil to get soil-carbon measurements, you’re changing the soil conditions for the remaining organisms. And so anytime you have these long-term studies with plots, and something to keep in mind, you know, what’s the sampling effect going to be and at some point is the sampling effect going to overwhelm the treatment effect, it’s something that we all really need to consider.
40:34
That said, this has been done in a lot of sites, this is a map of elevated CO2 studies, I don’t think all of these are FACE sites, but the vast majority are FACE sites and what you should notice is that most of them are in the northern hemisphere and actually Europe and the U.S.; I know of one in the tropics, there may be another one that started now, there was one in Panama. Tropics has a lot of biomass, they cycle a lot of carbon, certainly affected by elevated CO2 just like
41:02
everybody else, but very little representation, so we’re getting a lot of information on certain ecosystems in the U.S. and a few ecosystems in Europe and not a lot of information elsewhere. So what have we learned? Well plant growth, net primary productivity can be enhanced per unit of leaf area, so as canopies close and we get more and more leaves in ecosystems, we see that with elevated CO2, we can see more growth per unit of leaf area in some tree species, but not in others.
41:40
Okay, so this is a graph of the relative response to enriched CO2, the LAI’s, Leaf Area Index, and the elevated CO2 plots over Leaf Area Index in the ambient CO2 plots, and you can see that there’s this, you know, one would be that there’s no effect and you can see that there’s quite a wide range of responses. We see that total aboveground plant growth, NPP, can be enhanced in some tree species and not in others. Another study aboveground at primary productivity,
42:11
again, they’re not doing a lot of work on roots, there’s been some work done on the below ground portions of plants, but it’s so destructive to sample it that you can’t sample it very often, you can't sample it very much without running into that sampling effect. So what you see is in a couple of plant species here that the blue bars, which are the elevated CO2 are higher than others, but, you know, not in all cases. So if we go back to this map, you know,
42:35
some of these sites are in, you know, northern, actually not even really, yeah, so there’s some kind of hardwood forests, there’s some pine forests and some crop fields, there’s some, you know, other coniferous forests and grassland sites in various places. Within each one of these, there are a certain species composition that occurs at those sites, so in, actually I think the Duke forest they were plantation species and some places there there're natural species that have been occurring there.
43:06
When you do a lot of these studies and there are some overlaps in species, you begin to see some of those trends and I think, I think what we really need to get to, which I’m going there now, is it’s not all about identity, right, it’s about what they’re doing. ♪ Music ♪
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