Wisconsin Public Television University Place  Erika Marin-Spiotta Landscape Legacies Of Land Use  September 19, 2012 UP769   cc >> Welcome, everyone, to Wednesday Nite at the Lab. I'm Tom Zinnen. I work here at the UW Madison Biotechnology Center. I also work for UW Extension Cooperative Extension, and on behalf of those folks and our other sponsors, including the Wisconsin Alumni Association, Wisconsin Public Television, and the UW Madison Science Alliance, thanks for coming to Wednesday Nite at the Lab. We do this every Wednesday night, 50 times a year. Tonight, it's my pleasure to introduce to you Erika Marin-Spiotta. She's an assistant professor of geography and a faculty affiliate here at the Nelson Institute for Environmental Studies. She's also with the Department of Soil Science and the Latin American, Caribbean and Iberian Studies program here at UW Madison. She studied biology and political science as an undergraduate at Stanford and received a PhD in ecosystem science from UC Berkeley. After completing a postdoctoral fellowship in geography at UC Santa Barbara, she moved to Wisconsin in 2009. 

Her research applies interdisciplinary methods to examine the effects of human-driven changes in land cover and land use on biodiversity and the cycling of biologically active elements through the environment. Much of her research focuses on understanding carbon sequestration in terrestrial ecosystems. Dr. Marín-Spiotta also serves on the leadership board of the Earth Science Women's Network and works towards increasing diversity in science. Please join me in welcoming Erika to Wednesday Nite at the Lab. [APPLAUSE] >> Thank you very much. Is the volume okay in the back? Okay. Well, I apologize for the technical difficulties. I am just relieved that it did not happen in front of 150 undergraduate students. You're a lot more patient than they are, so I appreciate it. But we got it. 

I think my computer was just tired from working all day. So today I'm going to talk to you about some of the research that I've been doing for the last 10 years, and now some people in my lab have picked up. So it's kind of exciting to be working at the same place, and I think, hopefully, one of the points that you'll get at the end of today is just the importance of going back to the same sites and just measuring the same things over and over and kind of having more of a long-term record of what's happening on the landscape. And I will be talking about changes to the landscape. I'm going to be talking about a lot more recent changes to the landscape driven by human activities, especially agriculture. So, a lot of the work on the effects of human activities on the landscape was actually pioneered here in Wisconsin. So one of the first, I think he was a professor in the botany department, Curtis, actually was looking at forest fragmentation in Wisconsin, and this is a map showing you where he did the research that I'm showing right here, which is -- County which is just a little bit south of Dane County highlighted in the red on the map over there. And, basically, by looking at historical records of land tenure and historical records of forest cover, and then, more recently, looking at some aerial photographs, he was able to document really significant changes in the extent of forest cover. Really dramatic decreases in the extent of forest cover that occurred with European colonization and agricultural expansion in most of the Midwest. And this is just showing you data here for this one county. 

So you've got, for 1831, the dark color is forest cover. So most of the county was covered in forest. 1882, you already see significant decrease in forest cover. 1902 and 1950, just really just a few trees left here and there. So, very dramatic visualization of this habitat fragmentation in a very short period of time. Just 10 years from 1882 to 1992,well 20 years, and then from 1902 to 1950. Just very fast dynamic changes to the landscape. And there actually people here on campus that are still monitoring these forest fragments and other fragments in the state of Wisconsin and looking at the effects, what happens to these forests as they shrink? And so habitat loss is one of the main drivers of biodiversity loss across the globe. And one of the important things to note is that habitat loss and habitat protection are not equally distributed globally. And so there are certain parts of the globe, certain types of ecosystems and biomes that are more threatened than others, and I've highlighted them in the red boxes. 

We've got, on top, temperate grassland, savannas, and shrublands. So temperate grasslands are prairies. That's what we had here in part of the Midwest as well as temperate forest. In the black, we have habitat converted. So 45.8% of temperate grasslands have been converted to another land use, and most of that has actually gone to agriculture. And in the white boxes on the right, we have what percentage of that habitat worldwide is protected. And we have just 4.5%. So, really critical zone for thinking about conservation and biodiversity loss. I'm not actually going to talk about temperate grasslands. I'm going to talk about the next two boxes. 

So, number two, tropical, subtropical, dry broadleaf forests where 48.5% of the habitat covered by this type of vegetation has been lost already. This is actually the highest percentage of all biomes that has been lost, number one. And only about 7.6% of that habitat is currently protected. So pretty serious case. And then I'm also going to talk about tropical, subtropical, moist broadleaf forests where about 32.2% of the habitat at the global scale has already been converted., but there is more of that area that is currently protected. So a little bit of a better story. And agriculture is the main driver of land cover change globally. So, if you remember, if you've ever taken biogeography, you learn about the different biogeographic realms, but basically pretty much everywhere neotropical, Australasian, Indomalaya, Afrotropical, and Paleoarctic, you've got an increase in the amount of area that's cultivated and in pasture in the red and the yellow, and then a decrease in whatever the natural vegetation was of that landscape. So pretty significant changes, and the main drivers are agriculture. I teach biogeography so I teach my students about the global distribution of the biomes, and probably you're all familiar, you've all seen a map like this looking where you have the distribution of your tropical rainforests, your deserts, boreal forests where you have tundra. 

And there's actually now a push to actually try to put people into this map to try to think about, okay, well this is where, if we're just worried about climate and precipitation, this is where we'd expect to have our boreal forests. That's where we would expect to have our tropical rainforest. But, actually, humans are changing the landscape, and we're changing the geographic distribution of these different types of biomes as well as the species that are on these landscapes. So now there's this new idea of anthropogenic biomes where it's actually not classifying our landscape not in terms of what the vegetation would be there if there were no humans on the plant, because there's not much vegetation left like that, but actually thinking about what does the landscape look like now. So we've got our dense settlements. So we have our urban anthropogenic biomes. We have our village types of biomes, rice village, rain fed villages, crop and pastoral villages. Crop land is a really important type of land cover. And many different types of crop lands, whether they're irrigated or they're rain fed, as well as range lands and forested. But there's, increasingly, this recognition, which we've known for a while but for some reason ecologists and biogeographers didn't really want to work too much or think too much about the effect of humans on the natural landscape, but we realize that it's very difficult to study these systems and understand the response to, say, climate change, to fire or other types of disturbance without understanding how humans have changed these systems. 

So, much of our global landscape actually looks like this. And this, actually you might recognize this if you've taken a flight from O'Hare to Madison. So very patchy, very fragmented, very human-dominated landscapes. But there are some places in the world where, actually, agriculture is not the main mode of economic production of the local people. So there's actually been areas of the world that are seeing a change in this landscape. So going from an agricultural landscape to more of what we might think of a natural landscape. So we're seeing a lot of post-agricultural forest growth, and we've seen some of that already in northern Wisconsin as well, especially with areas that used to be logged and now are no longer logged, we're seeing forest regrowth. So there's an increase in forest in many different parts of the world for many different reasons. One other area of the world where this has been studied a lot is New England. So the northeastern part of the US. 

And this actually showing you data from one of the counties in Massachusetts that has seen an incredible increase in forest cover. So, at the top you have 1830. All those little green patches are little forest fragments. So that's kind of what Curtis was looking at in Wisconsin with agricultural fragmentation of the landscape. And in 1985, most of that county has grown back into forest. And this is at the same time as we've seen an increase in population density. So it's not that there are no people on the landscape. There are a lot more people in 1985 than there were in 1830, but they're concentrated. They're concentrated in urban areas. And so these agricultural landscapes have been allowed to grow into a forest. 

And typically, if you study ecology, you study, there's this predictable pattern of species replacement with succession after a natural disturbance. So, after a fire, after a wind, anything that creates a disturbance in the forest, depending on your forest, you can expect to see different species coming in over time. And this is a classic textbook example of secondary succession, this predictable sequence of species after disturbance But what happens when the forest is growing back on an agricultural landscape? We actually can predict what species are going to come in because the landscape has been completely transformed and altered by humans. And so that's what I'm really interested in. And there's a lot of people looking at these landscapes that are occurring on human dominated areas and calling them novel ecosystems. And we call them novel ecosystems because, typically, they're composed of different species combinations that might not have ever existed there. So we sometimes have new species. Sometimes we have the same species, but they're proportion is completely different. So while we might have had a forest dominated by aspen in the past, now we have a forest that's dominated by sugar maple. So, it looks really different from what was there on the landscape. 

And there are two ways that we can get these novel ecosystems. One is we can have our wild or our natural non-human altered landscapes, however you want to call them, and through degradation or invasion of some species, instead of that disturbance, instead of following that predictable sequence of recovery, of secondary succession, that system ends up going in a different trajectory. And so we end up in a new state in this novel ecosystem. Another way that we can get there is through the abandonment of intensive agriculture. And this idea of novel ecosystems is that human activities, this degradation and invasion is because of human activities. So humans have kind of introduced this element of surprise into these ecosystems. And so we can't predict what's going to occur on that landscape. And if we can't predict what's going to occur on that landscape, we can't predict how it's going to behave. We can't predict if it's going to still be a good habitat for the type of species that's endangered or threatened. We don't know if it's going to still be a good carbon sequestration project. 

We don't know if it's still going to have the same hydrological services that the original forest was providing. So, there's this idea that past human activities on the landscape basically have just kind of knocked this site off from its predictable pattern into a different state, and we don't know where it's going or what that state might look like in terms of biodiversity or species composition, nutrient cycling or carbon sequestration. And this is what a lot of my research focuses on and the research that I'm going to share with you today. So, what do we know about some of these novel ecosystems? There's been quite a bit of research on them recently. So we know that they have very different species combinations or species assemblages from what was there before when we know what was there before. We don't always know what was there before. Sometimes we compare it to the closest site that we think looks like what might have been there before. In many places, they're dominated by non-native species. Especially in tropical forests, you can have, in forests that are pretty species-rich, these new post-agricultural ecosystems are dominated by exotic species. So we don't know. 

Sometimes you have very different species combinations that have never coexisted before. So what effect is this going to have ecologically? But we also know that they're not entirely bad systems. Some of them can still be very species rich. Some of them could still provide habitat for their other species, for birds. We also know that this different species composition is very much determined by what the previous land use was. And so, in Puerto Rico where I do a lot of my work, you can actually walk through one of these post-agricultural forests, and by looking at the trees around you, you can tell whether you're standing on a former pasture, a former sugarcane plantation, or a former coffee plantation because there are certain species that are more associated with that land use. And we don't necessarily understand why those species are there. Some of them, for example in abandon coffee plantations, there were some species that were planted as shade, and so obviously they're there once the coffee plantation gets abandoned. But other species, we don't understand why but there's some association with that previous land use. And we know that this effect of the past human activity persists even through a catastrophic disturbance like a hurricane. 

So in many months, many of our sites in Puerto Rico get hit by hurricane. And when the forest grows back, and sites that have gone through five or six different hurricanes, we still see a legacy of a land use. So you still see differences in the species composition based on what the previous land use was. So very persistent. We know that some of these sites behave differently in terms of nutrient cycling. Some of them have much faster nutrient cycling rates. Some of them have slower nutrient cycling rates. And we know that the legacy of land use leaves an effect on the concentration of different elements but also this spatial variability of different elements. So, if sometimes when sites were plowed before and they grow back into a forest, you'll actually see a different, both vertical distribution of you nutrients as well as spatial distribution. And we're actually doing some work in southeaster Colorado looking at abandoned gardens in former World War II Japanese American internment camp. 

And there were people gardening these soils for only two to three years, and then the site was abandoned and it got covered back in dust and natural vegetation, and we can still detect the legacy of those gardens not only in the concentration, but also in the spatial distribution of those nutrients 60 years later. And we also know that some of these sites can gain carbon. They can be good for carbon sequestration. And some of them can lose carbon and actually contribute more carbon to the atmosphere. So, I'm going to talk about our case study where I've done a lot of work in Puerto Rico, which is an island in the Caribbean. You can see it, it's a pretty small green dot in the Caribbean right there. It's a small island. And in the 1930s and 1940s, most of the island looked like what you can see in the picture here. So completely denuded. And this is actually the picture of the introduction slide. 

So completely denuded hill slopes. Eighty percent of the forest on the island had been completely cut down for pasture, for sugarcane, for coffee, for subsistence agriculture. But now we've seen a lot of that forest grow back. So if you look at the picture on the right, and I know it's a little hard to see, but if you look at the tops of the mountains, it's a lot darker green. That's forest. That's all forest that has grown back, especially at the mid-elevations and higher elevations. And, actually, if you look at the forest cover, in the mid-1940s 13% of the island was covered in forest. Mid-1990s, we've bumped up to 42%. So, really nice increase of forest cover. Very similar to what was happening in New England. 

And one of the main drivers for this were socioeconomic changes in the island in the 1940s and especially the 1950s where there were, at this point, Puerto Rico was already kind of a free-associated state of the United States. And there were incentives for, the federal government wanted to industrialize and modernize the island, and, basically, help the island progress economically. And so there were a number of incentives for industry to move to the island, and there were a number of incentives for people living on the island to migrate to the continental US for work. So there was a really large-scale rural to urban migration and then island-wide to mainland migration which basically ended up in the abandonment of pretty much almost all of the agriculture on the island. There's still a little bit in the south where you can see some agricultural fields, but most of the island, most of the agricultural production was abandoned. And this lead to this natural reforestation as the land was abandoned. It helps that you're in a tropical region where there's a lot of precipitation, high temperature, and so productivity and regrowth was really fast. >> Is that an artificial lake? >> That is a reservoir. >> Reservoir. >> Yes. 

And so this is looking at one of our sites. In the back, you can see the reservoir, and there's a house. There's still people living on this landscape, but they don't use the land anymore for agriculture, especially once the older generation passed. The younger generation is working in the cities, working in the towns, and so this used to be a pasture and now it's covered in a secondary forest. And one of the advantages of working in Puerto Rico is that we have an incredible knowledge of the history. We have incredible knowledge of the geography of the island, and so we already have maps. I didn't have to create these maps. These maps already exist of changes in land use over time. And we also have aerial photographs from the '30s, '50s, '70s, '90s, and more recently that allow researchers to come up with maps like this. So in 1951, hard to see but very little forest. 

Most of those different colors are all different types of agricultural crops. 2000, most of the island is covered in either the light green or the dark green. And we actually know the age of these forests too. So we've got age classes 1 to 9, 10 to 20, 20 to 30, 50 to 60. So it's an incredible place to do this kind of research because there's so much great information. And, actually, let's see if this works. My sites are all located right here in this dark green patch. And if any of you have been to Puerto Rico, the El Yunque National Forest is right here and San Juan is right there. I'm a geographer so I have to point out where we are on the map. So our research questions were, now what do these secondary forests look like? What is the species composition? Does it look like anything that we thought the forest looked like in the past? We have some patches that we were pretty sure were never deforested, so we have a pretty good baseline of what the native species composition was. 

And what happens to carbon? There's this whole idea of reforestation growing back forests to sequester carbon. Well, here we have a natural experiment. Do these secondary forests sequester carbon? And I'm also really interested in the soils, and I'll convince you why in a few slides. And we actually, so looking at changes in above ground composition, looking at carbon, looking at the soils has started us thinking about what are the links between these changes in above ground plant community and how might that be effecting the microbial communities that are actually driving some of these changes in soil carbon. And this where my students come in. This is our new project which is really cool. So we used the land use chronosequence approach because we have all these sites of different ages that we could actually identify with this aerial photograph. So when I first started working here, we had our active pastures, then we had secondary forests that were regrowing on pastures abandoned 10, 20, 30, 60, and 80 years ago. So really nice kind of scale study system to be able to look at changes over time. And then we had our primary forest as far was know and we think were never cut down. 

Of course, there's some caveats to using this approach because I'm not waiting 80 years to see what happens to the forest. I'm taking different forests of different ages and assuming that the differences I see between these ages are representing what might happen at the same site over time. And so there are some assumptions there which are easier to take if you try to control for some of the environmental variables. So all of these sites are the same type of wet subtropical forest, same rainfall, same soil type, same oil type, same elevation, and they're all really close to each other. So we're trying to control for this environmental variability in order to be able to look at these changes over time using, basically, space for time. >> What is MASL? >> I'm sorry? >> MASL? >> Meters above sea level. Thanks for asking that. And then this was actually really unique opportunity to study these changes in secondary forest because we were able to find three replicate sites of all of these land uses in order to try to understand some of the heterogeneity that we might see on the landscape. And now we've gone back to these sites, and of course these sites are older. So now we have 20-year-old, 30-, 40-, 70-, and 90-year old forests, and so we're actually also able to detect some real-time changes in these sites as they've aged 10 years as well. 

So our first question was looking at this tree species composition and the forest structure. So what do these forests look like? And we actually found that some forest traits recovered really quickly. So after 20 years the forests were very similar to the primary forest in certain aspects in terms of their stem density, in terms of their biomass, so how much wood biomass was on the sites, we had pretty high species richness in our forests And after 20 years there was no difference in species richness. We had 56 species from 38 genera, 26 families. So these aren't Amazonian lowland forests, but they're pretty species rich for a small island. Eleven out of the top most important species, so these are the species that were more dominant in terms of their size but also more dominant in terms of how many stems were located in each site, were native species. So that's not too bad. We don't have these completely bizarre forests composed of exotic species that were never there before. But still, there were eight species that only occurred in the canopy of the primary forest and not in the secondary forest. So still different composition. 

But the good story is we also found some species endemic to Puerto Rico, which means that they only exist in Puerto Rico in the understory. So small, little plants in these secondary forests. So maybe, with time, they'll actually make it into that canopy as big trees. So we put together all the different tree species composition, and we're interested at looking at similarities. And I'm just going to explain really quickly how to interpret this graph. And basically here, what you want to look at is you want to look at how close dots of the same color are to each other. So I've got the 10-, 20-, 30-, 60-, and 80-year-old secondary forests, and then we have the primary forest. So the closer the dots are to each other means the more similar they are to each other, and the farther away they are means the more different they are. And we actually found some pretty nice clumping based on their successional stage. We have our youngest forest, 10-, 20-, 30-year-old forest clumping pretty close to each other. 

Then our 60- and 80-year-old forest and our primary forest. And you'll ask, well, what's up with that little red dot? That 20-year-old forest over there? And that actually, that site had been a pasture before, but it was also closer to one of the houses and they had also planted a lot of fruit trees. So we had mango trees, we had orange trees, we had lemon trees that didn't exist in any of the other sites, which kind of says okay, well, our analysis works because the one site that we know is really different is pretty far apart from the rest of them. And so a question is, well there's differences between this forest, but they kind of look like they're approaching the primary forest. You could imagine that with time they're all going to march towards what the primary forest are looking at. And, actually, that's where we're going to be able measure because we've gone back five years later after I made the original measurements and we're resurveying all the forest sites. So we're going to measure changes in species composition to see are they actually kind of all marching down in that direction? Do we see how fast are the trees growing? We know we've already lost a lot of trees, so we're going to be able to measure tree mortality and changes in biomass. And this has been a pretty unique long-term data set for these types of post-agricultural forests, especially since we've been able to go back and measure them. I'm glad we didn't wait too much longer because some of the trees were swallowing up some of the tags that we use to identify the trees. That's one of the advantages of working in a tropical forest. 

Very dynamic. There's some trees that actually spit out their nails and the tags altogether, too. So it's going to make looking at the data set pretty interesting. But we're excited about that. So I have a new master student who just went this summer, in July, to measure those trees again. Okay, so carbon. We know there's changes in above ground species composition, but I'm really interested in carbon. And one of the reasons I became really interested in carbon originally as a grad student was thinking about maybe we can use carbon sequestration as kind of a way to try to conserve or protect or manage some of these tropical forests. If we can show that they're really good at sequestering carbon, then maybe people will actually pay attention to them, especially these post-agricultural forests where people are like, that's not a real tropical forest because it was a pasture 20 years ago. But, actually, they can still be pretty species rich and have a lot of native species. 

So what's going on with the carbon? And the carbon story is interesting because we don't always know exactly how carbon is going to behave either. We understand above ground carbon a lot better than we do below ground. So by above ground I mean, basically, how the trees are growing. So we start off with our total carbon in our forest before it gets deforested. When you remove a forest and turn it into a grassland, then obviously you're going to decrease the amount of carbon that's stored above ground. And then during some time, you have the site and your agriculture use, and then once it gets abandoned, if a forest grows back on it, then you're going to be increasing biomass in your trees, and so your carbon numbers go up. But we don't actually know whether we're going to recover the same amount of carbon that was there before the disturbance. We might have changed the fertility in the soil. We might have changed the microclimate. We might have changed the species. 

So we might not actually recover the forest that was there. We might actually have the same type of carbon. And I said I would tell you why soils are really important, and, actually, they store more carbon than above ground biomass. So soils, at a global scale, store two to three times more carbon than is in the atmosphere in all plant vegetation and more than is in the surface ocean as well. So really important reservoirs of carbon. And this carbon is actually part of this soil organic matter which is really important for soil fertility, really important as a nutrient source, and contributes to all of these properties of the soil that lead to really high productivity as well. And they also sustain a very diverse food web. Actually, soils are the frontier now of biodiversity prospecting and looking at genomic material in soils. They're probably the most diverse place on this planet. And they're fun to dig around in as well. 

But we understand a lot less what's happening to soils with land use change because there was a lot of different factors that affect how much carbon gets stored in soils. And, actually, a lot of my research is focusing on looking at some of these biological and physical and chemical process that affect how carbon pools respond to land use change. But I mentioned soils store a lot of carbon. So we're starting off with as much carbon in the soil than we have in our above ground biomass. Typically with deforestation, you see an initial decrease. Although sometimes, depending on the land use, you might see an increase. And then after you abandon agriculture and forest grows back, what happens to the soil carbon? Well, sometimes it increases. Sometimes it increases a little bit. We still don't really know exactly what controls the direction of soil carbon, and we also don't really know the temporal component. We don't know how long it takes for soil carbon to recover if it ever does recover. 

Okay, so let's go back to our sites. And we actually found that above ground, our secondary forest stored more carbon than our primary forest. And that's because we had a species replacement. Our primary forest had a lot of palms, which are non-woody species so there's a limit to how big they can get, and our secondary forest was dominated by woody species. And so this simple replacement of the different types of trees affect how much carbon was stored above ground. Not at all what we had expected. And below ground, also not at all what we had expected, I was sure I was going to prove that reforestation sequestered carbon in soils. We found no change in the stored carbon stocks. So very large differences between above ground and below ground. And so I actually went into the literature and said, okay, how common is this response of no change in soil carbon with secondary forest age? And so we combed through a whole bunch of studies in the literature and analyzed more than 400 secondary forests and plantation forests where we know their age across the entire tropics and standardized their soil carbon. 

I'm showing you data for 0 to 30 centimeters with forest age and not a very impressive relationship with time. It's barely, it's significant but if you look at the predictive ability of the equation, it's very, very low. So not a very strong relationship of soil carbon and increase in forest age. So at least our data, and our data are the bigger red blobs, actually can see we don't have a lot of information of older forests. So our sites are nice in that we're adding some data for some of the older forests. >> How do you tell which part... [INAUDIBLE] >> So, you dig pits or you take a core and you bring them back to the lab and you grind them up and then we combust them in an elemental analyzer. And so, basically, all organic carbon, this is total carbon. All of these sites, because they're really acidic, we don't have any inorganic carbon, but basically, all the carbon in our sample gets converted to carbon dioxide. And then we have a detector that detects the concentration of carbon dioxide. 

So that's how we figure out how much carbon is in there. And so we see the above ground and below ground pools respond really differently. This shows you how important soil carbon was at these sites. The blue bars at the bottom are soil carbon, and the green is the total above ground biomass. Especially in our younger forests, most of the carbon is in these soils. We found no difference in the soil carbon pool, but I refuse to believe that nothing was going on below ground. There's got to be something going on. And, actually, one of the advantages of working in the tropics is that forest trees and pasture grasses have very different isotopic compositions. So the chemical compositions of the carbon in their material is very different. And you can use that as a tracer to look at the sources of pasture or forest in your sample. 

So you can take a piece of soil in your 30-year-old forest, and if you know what the composition, the isotopic composition was of the forest and of the pasture, you can actually tell what proportion of your soil sample came from the forest and what proportion came from the pasture. And so we actually were able to detect changes in the composition of the -- soil carbon using this method. And so I've got the amount of carbon isotope composition on the X axis. Over on the right, we have sites that are dominated by pasture. Over on the left, we have our forests and by depth. So we dug down to a meter and took samples every 10 centimeters. And this is just showing you the averages for all of our sites for all of our pits. And out pastures' soils at the surface show strong input of pasture grasses. And, over here, we have our primary forest in the light blue. These are the oldest forests that show this forest composition. 

And what's really neat is if you just focus on the top 10 centimeters, as you move from right to left, you're increasing in forest age. So what's happening is our forests are adding a lot of carbon to their soil as we have greater production of forest, but at the same time, we're losing some of the pasture derived carbon. So there's kind of been a replacement as well of the type of carbon that's in the soil. And so the net effect at these sites was that there was no change. If you just measure total carbon, you would have said there's no change at all in the carbon. And what's happening is that we're having a lose of the pasture derived carbon which, by doing chemical analyses, we then see it's easier to degrade than this new carbon that's coming in. And so we're seeing a loss of that carbon. And the soil carbon pool, we've already seen it's made up of different compositions, but it's also made up of different pools that respond to land use differently. So that are more or less sensitive to land use. And so a lot of the work we do in my lab is trying to break apart this black box of soil and say, okay, can we figure out what components of this organic matter pool are really resistant to decomposition and which ones turn over really quickly? And we do that by looking at the association of organic matter with mineral surfaces. 

So we've got our organic matter, our free-floaters around here. And this is basically dead roots, dead leaf litter. Then we have stuff that's protected inside aggregates or inside these crumbs of soil that are protected from decomposition because microbes can't access them. And then we have the stuff that's really protected that's associated with these mineral surfaces. And this picture is a little misleading in terms of it's hard to tell what proportion these different pools make in the soil, and different types of soil will have more or less of these different pools. So a lot of our agricultural soils in Wisconsin have most of their carbon in this pool outside the aggregates and inside the aggregates. At my site in Puerto Rico, 80% of the carbon was in this really heavily protected carbon pool which kind of explains why we might not have seen really large differences in the carbon. And we actually see differences in the distribution of the physically unprotected, the stuff outside here, from the primary forest to the pastures and an increase in the secondary forest So even though there's no change in the bulk pool, soil organic carbon is really dynamic. And we did see differences in the chemistry and differences in the distribution. And we use a technique very similar to radiocarbon dating that archeologists use, but actually, we take advantage of the bomb peak during nuclear bomb testing that actually increased the amount of radiocarbon that's in the atmosphere. 

And so, if you look in this figure here, this is looking at the atmospheric radiocarbon concentration for the Caribbean going back to 1500 and pretty stable until we had this big peak during nuclear bomb testing, which is kind of blown up here, no pun intended. Did not mean to. [LAUGHTER] But, basically, we can use this as a tracer to look at the incorporation of this carbon because it has different radiocarbon concentrations. We can still use radiocarbon to come up with the age of the carbon. We found that even though there is no difference in the bulk carbon pool in these sites, land use had a pretty significant effect on the age of these carbon pools. And so in the pastures where we have less of that physically unprotected carbon because the inputs are easier to degrade, the leftover soil was a lot older than in the forest. So pretty significant differences in the turnover of the sites, but after 20 years, it was all the same. So just like after 20 years, we found tree biomass and tree structure, so your stem dynamics or stem number, same after 20 years, really similar patterns in the soil where there's no difference in the age of the carbon after 20 years. So kind of interesting. I don't know exactly what's magical about those 20 years. 

We might be able to tell because we have one of the 10-year-old sites is turning 20, and our 20-year-old site has turned 30. So maybe we'll see some difference there. And now, we've brought in the new generation of scientists. We brought in my grad students, and, actually, in the brown in the middle, Peyton, and then this summer we got some funding from NSF to actually go to Puerto Rico and take two students from Puerto Rico universities to the field and take them there where they collected their samples. They actually participated in a summer research program back here on campus. So they were with us for the whole summer which is really neat. Neither of them had been to this part of the island or these mountains, and they haven't really known this history of reforestation of the island either. So that was pretty neat. So we've been looking at microbial communities, and we're not yet using any very fancy genomic or metagenomic methods because we haven't gotten all the data yet. So I don't have much data to show you. 

But there's also a lot more, they're pretty expensive and they also don't really give us the kind of information we want. So we're really interested in looking at the function of microbes, not just whether you have a thousand different types of species or not. So we're using a a method called PLFA which stands for phospholipid fatty acid analysis. We're basically measuring the chemical composition or the lipid composition of the membranes of different microorganisms. And it gives us an idea of what they do in the environment. So, are they methanogenic? Do they like anaerobic conditions? Do they like aerobic conditions? Are they decomposers? Are they nitrogen cyclers? And so it gives us some broad scale analysis which helps us look at some patterns, and then when we see something really interesting, then we can go actually look at the DNA and the RNA and try to figure out what they are. And the problem with working with tropical soils is that they have so many different types of bacteria and fungi that it just gets overwhelming and we have no idea what they do. And we're interested in trying to figure out how these forests are behaving. And so, we took samples and we've been going back the last three years and collecting samples twice a year because microbial communities could change pretty quickly. We want to make sure we don't just pick a really bizarre day when we went and sampled. 

And we've actually found something really interesting that we did not expect in these wet tropical forests is that, actually, even though mean annual rainfall is really high, there's actually are some seasonal differences which I did not expect to see in these sites. And so we're actually seeing, we've got our communities, and this, again, the closer they are to each other, the more similar the data is; the farther apart they are, the less similar it is. And so we've got clumping of all of the sites in the wet season versus the dry season. So now we're starting to think about, okay, what's the effect of rainfall? And rainfall is actually predicted to change pretty significantly in parts of Puerto Rico, and some parts of Puerto Rico are probably going to dry out a lot. So what effect does that have on these microbial communities? And we find, actually, we also measure microbial activity, and we're also finding very large differences in microbial activity between the wet and the dry season as well which we didn't quite expect. Some forests we do expect to see really large seasonal differences and other ones we don't. But even though we see differences or even with the differences in season, we're also seeing some really large differences in successional stage very similar to what we saw in the above ground trees. And so even in the dry and the wet, we see clumping of our 10-, 20-, 30-year-old forests just as we did with the trees. And we have our older forests together and then we have our pasture communities separate as well. So we're seeing some really interesting kind of similar patterns in above ground and below ground community composition. 

>> What units are you using in your graph? >> It doesn't have units. And so, it's kind of this multi-dimensional space, and you're not really supposed to pay attention to the number on the axises as much as how they are separated along the Y axis or, sorry, the X axis or the Y axis. >> So it's a principal component? >> Yeah. Yeah. This is a principal component analysis. Yeah. And, actually, we found that the one advantage, again, of this long-term monitoring, going back to the sites over and over and measuring the same things, is that we found some really rapid shifts in the microbial community. So, in our July 2010 wet season, we had our pastures over here. And then when we went back the next wet season, our pasture community had actually migrated over here toward the 10-, 20-, and 30-year-old secondary forests were. And at first, we were like, what's going on? We have pastures versus forests, why do we have similar communities? And that's actually driven just by one pasture site. 

And here what I've done is I have all the data but I've highlighted the pasture sites. We have three pasture sites, one, two and three, in the diamonds here. And then here are the 10-year-old forest sites. So there's this one pasture site that suddenly has become a lot more similar to our 10-year-old forest site. And we said okay, well, let's look up what pasture site that is. Let's look at the data. And this is what it looked like July 2010. I know it's hard to believe that's a pasture, but it's a pasture. It's not a very well grazed pasture, but it's still a pasture. And this is what it looks like in August 2011. 

So you can see a lot more woody vegetation. So we're actually seeing this reforestation in real-time which is really cool. We don't have to go find a site that's 10 years old. We're seeing it in real-time. And we're seeing the microbial communities respond to that in the time span of one year which is really fast too. And we've never known how long microbial communities respond to land use change. And, actually, one of our original ideas for another project was to cut down a piece of secondary forest that people we're going to cut down for biofuel anyway and track changes in the microbial community over time. And everybody said there's no way you can do that for a dissertation because you're not going to be able to detect changes in two to three years, which is the length of a dissertation. And we've been able to detect them in one year which is pretty cool. And this was what the pasture, I was going to say what the forest looks like right now. 

And so we're actually going to have to go and start measuring trees at this site as well because it's no longer a pasture. Okay, so what have we learned from these sites? We've found that forest structure composition and species richness recover in 20 years. So pretty quickly. But tree composition remains the same even after 80 years And we'll actually see over time, and our plan is to go back every five years or at least every 10 years to these sites and measure changes in composition over time and see if we see them approaching the older secondary forests. And what's happening to those smaller trees, those endemic species in the secondary forests? Are they actually growing up and surviving to become bigger into the canopy? We found the species replacement effected biomass carbon accumulation. We found that the bulk soil carbon turnover in this physically unprotected soil carbon turnover recovered in 20 years. So also really quickly. Inputs of the new forest carbon were balanced by losses of old pasture carbon. Something really interesting is happening in these successional stages where our microbial communities are showing similar patterns as the tree communities, the plant communities. We're interested to try to pursue that further and try to understand exactly what's going on there. 

Is there something that's going on with the microclimate of the soil or something that's going on that suddenly changes in the litter chemistry or changes in the soil chemistry? And we're finding microbes are responding really quickly to season, but especially to these changes in vegetation. So that's going to be really fun to be able to detect over time. And so this is one of those sites where we are going to invest into getting some genetic data to really look at what might be those microbial communities that are responding so quickly, microbial species that are moving the community in a different direction. And that kind of led us thinking about, okay, well, this is looking at pasture reforestation, what happens when you have different successional trajectories? And so we actually started a new project looking at sugarcane abandonments. So we've actually moved to another Caribbean island in the US Virgin Islands, St. Croix. And this is our tropical, subtropical, dry, broadleaf forest that I said has experienced the greatest habitat conversion of any type of biome on the globe, and it also has a really low percentage of habitat that's protected. So really important for biodiversity. And this is a place where season is really important. These are seasonal forests. So they're actually adapted to drop all their leaves in the dry season, and then they flush green in the wet season. 

So this is the same site in February and July. And I actually, other than living in deciduous forests in the US, I'd never seen a tropical forest like this before. Where we were working Puerto Rico was really, really wet, and the first time we went to St. Croix was in February. I was just like, oh, my gosh. Everything is dead and so desolate. And then we went back and were like, whoa, okay, it's a seasonal forest. It's very much alive. And so it'll be interesting, we have data for these sites, I'm not going to show you because we don't have time, but we've been doing the same measurements at these sites two different seasons and we're going to have a few more years of different seasons. And one other interesting thing about working in St. Croix is that Puerto Rico is small, St. Croix is even smaller. You can basically go from one end of the island in an hour. 

It's really small. And urbanization is a really big threat to these forests here. And this is a map of different forest covers on the island. I apologize to those who may be color blind. I didn't make this map. But green is a forest. These regions here in the yellow are our urban areas. And we have a lot of secondary forests. It's pretty much people's backyards, right behind grocery stores and the high school. One of our sites is right across from the high school. 

So there's some other interesting questions there about real human pressures on these ecosystems. And so, what we're looking at is we're looking at different successional trajectories. So, the island was pretty much 80% covered by sugarcane for over 200 years, and then the last sugarcane plantation shut down in the '60s, 1960s. So pretty recently. So we have some sites that went from sugarcane into pasture, and were in pasture for a really long time, and then in the last 10 or 20 years the economy is not doing so well in St. Croix. There's been a lot of agricultural abandonment, and so we've got our young secondary forest, 10-20 years old. We have some sites that went from sugarcane, in the 1960s were completely abandoned. They weren't even converted to pastures. And so they went straight into our older secondary forests. And then we have some land that went from sugarcane into a tree plantation that was managed by the Forest Service. 

And so we were going to look at these pretty different species composition because our young secondary forests are basically 99% one species, this exotic nitrogen-fixing species. And we actually can trace when it was introduced to the island because it was introduced as agricultural fodder. And our old secondary forest, which established before this one species because so widespread, are pretty species diverse. So we have, again, these very different plant communities which will allow us to look at how the microbes are responding to that and how soil carbon is responding to that. And we though, oh, great. And we know there's this plantation which is also the same age as these secondary but we know has one or two species as well because the Forest Service managed these sites and planted these species. What we didn't know when we actually went to visit the site was that it's no longer being managed as a plantation because it go trashed during the most recent hurricane. And so now it's a secondary forest, but it has a very different species composition than this secondary forest. So it will be interesting to look at these different successional trajectories, and we also have some sites that went straight into pasture and are still in pasture, and then we have different species composition. So this project just got started with a new PhD student. 

So we're excited about that as well. And I already mentioned this, so I'll skip through that. But, yeah, we're interested in looking specifically at climate because we're finding that climate is really important for determining species composition as well as carbon stocks. And, actually, in the Caribbean seems to be more important than former land use. So we're actually interested in comparing these wet tropical forests to the dry tropical forests, the seasonally dry tropical forests. And so, with that, we've got a lot of collaborators in Puerto Rico and St. Croix. These are my three grad students who've been working on this. Peyton's been doing all the soil microbiology in Puerto Rico, Emily is starting to do work in St. Croix, and John, this summer, was doing the tree surveying. So it was actually fun. We all got to go to the field and hang out in a bunkhouse and sleep in bunk beds this summer while we were doing field work. 

But it was pretty neat to take them back to my sites. And thank you very much for your time. [APPLAUSE]