Video
Fire, Climate, and Land-Use in the Ancient Southwest
Transcript
Karen: Good afternoon, everyone, and welcome to ArcheoThursday. My name is Karen Mudar and I'm your host. I'm an archeologist in the NPS Washington Archeology Program which is sponsoring this series. This fall and winter, we are exploring the Anthropocene and climate change in archeology. We're about halfway to our speaker series after taking a break for Thanksgiving. I hope that everyone had a pleasant holiday despite unsettling weather, at least on the East Coast. Two weeks ago, we heard a great talk by Shelby Anderson who's been carrying out research at Cape Espenburg and Krusenstern, in Cape Krusenstern National Monument in Northwestern Alaska.
She's been carrying out a research program to hunter/gatherer persistence from a technological perspective and to study social networks in socio-economic organization in ancient Alaskan hunter/gatherer groups. Her talk focused on the ways that new data and settlement patterns and social networks can be used to evaluate and refine our existing models of late Holocene human environment interaction in the Arctic. I thought that the first half of Shelby's talk gave us a nice overview of some of the research that she's carrying out in that region of Alaska.
I was particularly intrigued, however, by the second part of her talk, which was about ongoing efforts to assess and mitigate the impacts of rapid climate change on arctic archeological sites. Shelby and her team have developed a thoughtful and robust set of protocols for assessing the degree of danger to archeological resources and for prioritizing sites for mitigation, and she had tested her data collection instrument in the field and it appears to me to be farther along in development than anything else that I’ve seen. I think that it could be adapted to other types of sites in other parts of the country as well.
If you're interested in this topic and didn't get a chance to attend the webinar, Shelby's talk is recorded and posted on the ArcheoThursday webpage of the NPS training and development website, and I hope that some of the cultural resource climate change folks saw it, too, because I think it has great application potential.
The next two webinars, we're turning our attention to another part of the climate change spectrum in another part of the country. We're going to be looking at fire regimes and climate history in the Southwest. Next Thursday, Craig Allen, an ecologist at US Geological Services is going to talk to us about a research program that he's been carrying out at and around Bandelier National Monument. I attended a lecture that he gave a couple of years ago in which he talked about the history of the Bandelier landscape and the way in which land field practices had contributed to soil erosion. This research documented the scale of archeological site degradation as a result of this erosion and he conducted experiments to devise simple ways to stop the erosion and stabilize the soil.
It was a really good presentation that had a lot in it for archeologists and I anticipate that he'll give - I hope that he'll give - a similar talk next week, but no matter what he talks about, I know that it will be really interesting. He has a lot of insight and a great deal of enthusiasm for his topic.
Today, however, I'm pleased to introduce Christopher Roos who's going to be talking to us about research that he's been carrying out also in the Southwest, in Arizona, on fire and fire regimes and changing vegetational communities. He's an environmental archeologist at Southern Methodist University and he has primary research interests in the long term interactions between climate change, human land use, and landscape fires. He’s been investigating the ways that Native American communities in the pine forests of the Southwest have adapted to various climate regimes and, today, Chris will bring archeological, dendrochronological, and paleo-ecological information together to examine human and climatic interactions in Arizona's fire-prone forest over the last millennium in order to illuminate pathways for more resilient, fire climate society relationships in the present and the future.
Thanks for being with us today, Chris.
Chris: Thank you, Karen. It's my pleasure to be here and have a chance to share some of the work that I've been doing. I hope that there maybe some tidbits of information that people in the audience can find of value or at least find to be of interest. What I like to talk about today is work that I did primarily associated with my PhD work while I was a student at the University of Arizona and use that as kind of a lead into work that I'm doing now in the Southern Jemez Mountains in northern New Mexico. Basically, the southern part of the landscape that Craig Allen will probably be talking about next week.
Fire is interesting from an anthropological standpoint because fire scholars have wrestled for the better part of the century with very simplified questions about the relationship between people, fire, and the landscape, really only asking the question about whether or not fire in the environment was a natural phenomenon or if it was in fact cultural, and as the fire folks, and I know there are some fire folks in the audience, probably know from the history of fire management policies in the US, this was a framing part of the debate as well.
Today, we have a much better idea about the long-term historical context of fire. Wildfires have been present just about as long as we've had land plants on earth. The oldest charcoal as evidence of fire, an incomplete combustion of plant biomass, dates to the late Silurian more than 400 million years ago, so fire has been an incredibly potent ecological and evolutionary force in earth's history.
Also very recently, we've been pushing back the period of time in which we know that humans and human ancestors manipulated and used fire. Work by Francesco Berna and others from Wonderwerk Cave in South Africa have now documented the presence of In-situ combustion deep in these occupied caves more than 1.1 million years ago, and this may well stretch further in the past. This has an important place in human history and evolution, too, and, of course, Richard Wrangham, an evolutionary anthropologist from Harvard University, has hypothesized that the use of fire for cooking may have been in a really important role in shaping human biology for potentially for the last 2 million years.
As Steven Pyne, my colleague and historian at Arizona State University, has put it, we have this actually unique unity on earth that we have planetary conditions that are suitable for fire, plenty of biomass, plenty of oxygen, plenty of ignition sources, but also that humans are simultaneously kind of fundamental fire creatures and has probably played a really important role in shaping our evolutionary history, so to frame research questions as if the only thing that's meaningful to ask is whether or not fires are human or natural really puts the blinders on and keeps us from asking more meaningful questions, that it's both and it has been both for a very long time.
I should say in the exposition now, but please feel free to stop me if you have any questions about anything that I'm pointing out on the slide as I go along. I'm happy to pause. Now the contemporary fire problem, as we wrestled with it in the western US but also in other parts of the world is one in which we viewed it as a clash of culture and nature, of human values and properties and livelihood, with inherently fire prone landscapes, and we've tended to view this primarily from an ahistorical lens. The role that history that plays in this is largely, for the landscapes where this appropriate anyway, has largely been about documenting the divergence between contemporary fire regimes and historical fire regimes, and that humans have either contributed to that or are caught in between.
This overlooks the fact that many of these landscapes in the Western US and especially the Southwestern U.S., where I do my work, have been to human communities for millennia, and these fire-prone settings have posed challenges and opportunities for the ancestors of the modern American Indian communities who call these landscapes home for time immemorial and on Archeological time for at least last few millennia, and in some cases, at least 13,000 years.
Archeology has a unique place within this because we have many decades of experience wrestling with questions about the relationship between human societies and climate change, how human societies have coped with and adapted to climate regimes of different sorts and the dynamics between them, and we have a much better way of thinking about these problems if we integrate the human questions about adaptations to different climate regimes and climate dynamics with the paleo-ecological and paleo-climatic concerns for the role of climate as providing biophysical controls on fire regime dynamics and changes, perhaps, in fire severity.
To do this, I've tried to integrate those concerns in a set of research questions, so I have those posted on the screen right now. Rather than asking whether or not Southwestern fire regimes are natural - they are - or whether or not people affect them - they do - but to ask how Southwestern forest with people respond to those climate changes in ways that are different than those either without people or, probably more appropriately phrased, with a lighter human footprint on them? What can we learn from these histories to improve the resilience of these forests today or to, more specifically, improve their sustainability in the context of contemporary climate change that in the Southwest is involving, probably, increased duration and severity of droughts.
These are not archeological questions by any means and archeology alone can't really help us answer these questions, so to address these questions well, we need three different data sets to be integrated into an interdisciplinary research program. On the one hand, we need to have a good idea about variation in fire-relevant climate. I'll get to that in a minute. It's more than knowing whether it's warm or cold or dry or wet but to understand in a temporarily explicit fashion how those climate drivers that influence the key properties of the fire regime have changed over time. We need to, obviously, have some idea about the variation over time and space and human land use, human populations. We can know this from archeology but also from ethnohistory and ethnography, including, of course, how people used or may have used fire in their livelihoods.
Then, finally, we need some evidence for variation in fire regimes themselves that match up on temporal and spatial scales with the archeology and the climate reconstruction, and so over the course of this talk, I'm going to describe the data sets that we built together to try to address these questions in Eastern Arizona.
To make this a little bit tractable, I've restricted myself to Southwestern ponderosa pine forest. It's an ecosystem that I know well and love, but also happens to be one of the ecosystems with the best characterized fire regimes in the world, thanks in large part to the ability to date fire events using tree rings to the year, and in many cases actually, to the portion of the growing season of the tree, so for at least the last few centuries, we know this fire regime very well. Here I'm just going to match up the broad distribution of ponderosa pine forests in the Southwest and the two study areas, my eastern Arizona area and my New Mexico study area. It doesn't include ponderosa pine forest in the sky islands of southwestern Arizona.
From those tree ring studies, we have a really good idea of both about the fire regime and its climate relationships. Prior to inadvertent and active fire suppression over the last 100, 120 years, in all southwestern ponderosa pine stands that have been sampled, we have fires that occurred very frequently, every 3 to 15 years or, a more general rule of thumb, every 5 to 10 years, but these are fires of a very specific type. These are fires that burn in surface fuels, like depicted in the upper left here, that don't burn into the canopy. Because of some unique or some specific fire adaptations that ponderosa pine has as a species, including very thick bark to insulate the trunk and self-pruning branches, these trees were able to survive fires,
once they've grown to a certain height because their canopy is out of the reach of the flame lengths, and that in a few conditions like on slopes where you might have localized accumulations of fuels or eddying as fire moves up-slope. Fires may get hot enough to kill the cambium locally on a tree and as that bark sloughs off, the tree tries to regrow and heal that scar, and so we call it a "fire scar," and on this figure here in the bottom, you could see a really great illustration of a tree that's recorded 22 fire scars over the course of a couple hundred years.
In terms of the seasonality, we also have a good sense about when during the growing season these happen. These typically happen in the arid fore-summer, before the monsoonal rains start in early to mid-July, so between the period of time when the moisture from cyclonic winter storms has stopped and snows have melted if there was snow present, but enough time for any understory fuels and grasses to dry out and carry a fire, but also really importantly are these inter-annual relationships between the occurrence of fires and multi-year moisture patterns, particularly those associated with El Niño southern oscillations.
It looks like, in statistical analysis after statistical analysis, fires in these settings occur after multiple wet years in which those fine surface fuels of grasses and forbs can grow in sufficient abundance and continuity to be able to carry a fire, and then the fires typically happen in the dry year that follows those. These patterns of wet, wet, dry correlate really strongly with particularly widespread regional fire years.
Now, of course, it's not how fires happen in these settings today. Many of you probably know this well. We see this, very unfortunately, depicted on the news seemingly every summer for the last few years where now fires get into the canopy, get into the crown of forest and burn over very large areas. In some cases, like parts of the Jemez Mountains that Craig will talk about next week with such continuity that there are thousands of hectares without a seed source for the trees to return, so these are basically being wiped out in large patches through these stand-replacing fires, these crown fires, and this is an old graph from the increasing representation of crown fires from Coconino National Forest and the observant member of the audience will note that it seems to more or less track the global temperature anomaly.
In fact, some people have looked at this and thought perhaps this is symptomatic of global climate change and maybe this is something that would be happening anyway as a result of anthropogenic emissions and contemporary global warming, but the story is not quite so simple. Those frequent surface fires in the Historic Period maintained a very open park-like structure. In fact, the ponderosa pine forests historically have been described as "parklands" in which the tree density was very low and the potential to carry fires through the crown was very low, in addition to the difficulty of actually getting any fire from the ground into the canopy.
Now to illustrate what has to happen to the structure of those forests and the role that climate or land use can play in altering that structure to create this vulnerability to crown fires during severe droughts, so frequent surface fires maintain open canopied forests. This is a photo from outside of Flagstaff from 1909. You can see that the trees are not particularly densely spaced, that the canopy is relatively high, there's not much in the way of herbaceous vegetation on the ground, this area has already been pretty extensively grazed for a few decades.
If you remove fire, remove those surface fires that would burn along the forest floor, increase the period of time between which these happen, you accumulate fuel on the surface, heavy fuels, and if you also have multiple years of wet conditions, these are ideal for germinating seeds and recruiting a lot of conifers into the understory and ultimately, into the canopy, this is what creates conditions for what we call "doghair thickets," and many of you are probably familiar with these. This is in 1949. All of these young trees and these two trees here in the middle are the reference points. All of these germinate around the same time, the relatively wet period a couple of decades before this, but they're all very small and competing with each other for moisture as well as competing with the mature trees.
You add a few more decades to this and you can see how they're still stunted in terms of their stem, but they're reaching now into the canopy. At this point in time, these trees are very vulnerable to drought stress because they're all competing with each other for water, but they're also capable of carrying fire from the surface into and through the canopy, so if you add drought conditions to these doghair thickets, you have conditions for crown fires, and continuing, carrying or running crown fires.
Those often are stand-replacing but if we look at their occurrence over the last few decades, it turns out that when they do return to forest, and sometimes particularly if there's a seed source nearby, they do return to forest, they don't return to open ponderosa parklands, they actually return to doghair thickets, so these fires, when they occur in these doghair thicket conditions, provide opportunities for tipping points and moving from one alternative meta-stable condition of ponderosa pine forests into others, either a permanent shrub field - this is happening in northern New Mexico where these fires, particularly repeated fires, are turning some arid landscapes into oak, Gambels oak shrub fields - or into grasslands or meadows.
We can represent this. This is a conceptual metaphor using a stability landscape to represent this and the role that fire and certain types of fire, in particular, play in maintaining those alternative meta-stable states. With the historic, relatively high frequency, low severity surface fire regime that characterized Southwestern ponderosa pine forests for the several hundred years before fire suppression in the 20th century, the internal feedbacks basically drove any particular stand represented by the ball towards the bottom of the basin on this landscape. The surface fires as a key ecosystem dynamic were driving the feedback for open-canopy forests that were very difficult to actually alter to alternative meta-stable conditions.
If you increase the period of time, and now the specific interval here is not the main point. This is actually something that we're actively investigating. I've thrown up 50 to 75 years since the occurrence of large crown fires in Southwest seemed to happen between 50 and 75 years after late 19th century's suppression. I changed the topography of that stability landscape where the feedback are different. Now the feedback drive movement toward higher density of ponderosa pine forest that are more vulnerable. The threshold between the alternative meta-stable conditions are now lower and it takes less extreme conditions to move a previous ponderosa pine forest into grassland or shrub field conditions.
Working within this resilience framework, we can hypothesize that humans might actually provide some enhanced resilience by providing functional redundancy of sorts to the natural ignition source of lightning. Humans are not limited to a particular season in which they can ignite fires nor are they limited in their ability to put fires where they want fires to be, so if fuels are fragmented such that widespread fires can occur, they can overcome this if they so choose with more ignitions, whereas lightning as far as I know is pretty agnostic on the subject.
This is kind of a framing hypothesis for this work is trying to figure out whether or not, over the course of last millennia, there's evidence that human uses of fire might provide this functional redundancy and resilience during climatic conditions that might have favored crown fires at certain points in time. The first point of order then is to figure out whether or not there were periods of particular vulnerability to altered stand density and crown fires on the basis of climate patterns over that period of time. To do this, taking advantage of the strong relationships between surface fires in ponderosa pine forests, those patterns of wet, wet, dry, so there's wet antecedent years that produce the fine fuels and then the dry fire year to build a multiple regression model for Southwestern climate from paleo-precipitation reconstructions from tree rings.
Basically, using analogous methods to the reconstruction of paleo-climate from tree rings, I've calibrated the paleo-precipitation record with the well represented fire scar record instead of calibrating tree ring width to modern climate data, and so I've used basically a 200 year window of greatest sample depth for fire scars to build the model and use that for calibrating an integrated paleo-precipitation reconstruction using Henri Grissino-Mayer's El Malpais precipitation reconstruction for west-central New Mexico and the Salzer and Kipfmueller on paleo-precipitation reconstruction for the southwestern Colorado Plateau that spans about 1400 years of the latest Holocene.
The resulted reconstruction, once it's re-transformed, is surprisingly strong. This is a very simple model - there are only three variables in this multiple regression, and for that window between 1700 and 1900 those 3 variables explain about 50% of the variance in the proportion of ponderosa pine forest that burned in any given years, so you can see that in this graph, the orange is the predicted portion of stands burning in the given year, the black is what's actually reported in the fire scar data, and you'll notice that from 1700 through, really, about 1880, the relationship's really strong, but after that point, a lot more fires are predicted than actually occurred, and you might ask, "Well, what happens?" Sheep happens.
By the 1890s, there were more sheep in New Mexico than there were people. New Mexico was the envy of Wales. I don't know that for sure, but I can imagine so. The sheep and the cattle were effectively outcompeting fire for consuming those fine fuels, and that for much of the end of the 19th century and the early 20th century, this is the primary fire suppression effect is the competition between grazers and fire, and grazers won, but if we look back at the last 1400 years, here at the top, we’ve got the annual reconstruction, you'll note that this is 1748 which is well known as the largest fire year in the fire scar record that we have.
A comparable year in the 11th century, but if we look at the variance and how frequent these regional fire years are, and that's plotted here on the bottom, the black line are years in which more than one standard deviation of the mean value were predicted to burn in that year and the very large years are more than two standard deviations, so this looks like the frequency of these events. I believe this is in the 25 year moving average, and I want to highlight a couple of areas here that indicate periods in the 8th century but also the late 14th and 15th century when those climate conditions, those frequencies of wet, wet, dry patterns that are really good for widespread surface fires kind of broke down.
I don't have a really good explanation for that but those are indicated here at the top, that there are prolonged, at least fire-free intervals at the regional scale during those windows of time, and the second one is one that's particularly interests us as archeologists because there's a lot that's going on in the Southwestern cultural world in the 14th and 15th centuries.
In other words, that's a period of time in which we would expect that some of the Southwestern forests, and perhaps many of them, were in conditions more like this model B here in which there were conditions suitable for increased stand density and maybe greater vulnerability to crown fires. That gives us something to aim for in terms of evaluating the human contribution to fire regime change over the last thousand years, and I chose to focus in eastern Arizona for a couple of reasons. One, the University of Arizona has a long history of doing research here, but also this was an area that had been burned really severely by, at the time, what was the largest fire in Arizona's history, the Rodeo-Chediski fire and this is those two fires very early in their burning.
This is something that had basically some very local importance in the communities that we were working with, but also from the work of the Forest Service as well as the University of Arizona, we had a good sense of the gradients of land use histories, so these are indicated here in the sampling location. This yellow, I mark as controls, and that they've been surveyed intensively by archeologists that didn't disclose any evidence for residential architecture. There's some artifact scatters, so they were used certainly but not used the same way as the parts of the landscape where people were living, and these are basically densities of residential sites, but also a gradient in time.
We have the yellow control water sheds that were never lived in over this last thousand years, these central watersheds, Day Wash and Willow Wash where people lived in these episodically for many centuries but they depopulated these areas relatively early, and the Forestdale Valley in the east that had a very long prehistoric occupation and was also an important location for Western Apaches when they moved into the landscape.
To just talk you through what the archeological history is like, for the Pueblo 2 and Pueblo 3 period from about AD 1000 to 1275 here, this is the period of most dispersed settlement and residential structure. People were cultivating maize, beans, and squash and living in small hamlets or even small villages. This is one of the largest ones in the Forestdale Valley, that's Tlakii ruin that has maybe a couple of dozen rooms and a great Kiva. These communities were connected with one another as well as communities on the Colorado Plateau, but that's if they lived year round in these Ponderosa Pine Forests settings.
Now after 1275, many of you probably know there's a great population reorganization on the Colorado Plateau. In the Mogollon Rim area here of eastern Arizona, there are populations that move in from northern Arizona. Some of the local populations may also leave. One thing that happens is that the remaining populations and the new populations aggregate into large pueblos, so they're far fewer actual settlements but there have many more people living in them, and these also seem to be multi-ethnic communities and are pretty unstable, so the one on the left here, Bailey Ruin, it's located at the Day Wash-Willow Wash water shed boundary. It was only occupied for less than 50 years before it was depopulated whereas this is a large village in the Forestdale Valley was occupied all the way until 1400, maybe a little beyond.
Now after 1400, at least until the late 16th century, this is an area that was almost certainly Tierra Despoblada, the Coronado Expedition goes just east of here and his guides refer to this area as a depopulated landscape. They're unaware of anybody living on this landscape, but by the late 16th century, we've got both archeological and ethno-historical evidence for Apaches or in the ethno-historic descriptions, Apache-liked peoples living in this area, and there are a number of clan origin locations for Western Apache clans in the vicinity of the study area, particularly around Forestdale Valley, and they're there until the establishment of reservation in 1870 and the Forestdale Valley is still on the Apache Reservation.
Our understanding of the use of fire as a tool for ancestral Puebloan peoples is not very precise. We have some hypotheses from the archeological literature, including from some work by Alan Sullivan who presented in this series last year. What he refers to as the burn-plot hypothesis, or what has come to be known as the burn-plot hypothesis, in which he suggests that these types of landscapes, in large part because the soils derived from the sedimentary rocks are relatively nutrient poor would have benefited from recycling nutrients by burning off understory plants and pine dust to make phosphorous and potassium, in particular, more available to crops.
This is not slash and burn, the way we think of that, especially for tropical landscapes and in part because the ponderosa forest were probably very open anyway, but he sees this as a way in which people may have used fire and added additional fire to these landscapes. He's also very recently, or more recently, advocated that other ancestral Puebloan groups including those in the Grand Canyon area where he works used burning of understory plant communities to actually promote those disturbance-loving species that produce greens and seeds that were of economic value. Although it's not really clear whether that would be done after harvest or maybe early in the season to promote a green-up, we're not really sure.
Ultimately, there's evidence for hunting. This actually comes from some ethnographic work that suggests that fire was used by some historic Pueblo populations to drive small game as well as large game.
With Apaches, we have a much better picture and a lot of this has been summarized pretty well in Winifred Buskirk's dissertation that was published in the '80s by the University of Oklahoma press. Some of these is from his own works, some of these is summarized from earlier work. Western Apaches used fire in a variety of ways. For agricultural land use, clearing new fields, cleaning irrigation ditches, cleaning up detritus after harvest, they explicitly burnt after harvest to create an ash-bed effect, basically recycling those nutrients for the next year. They use it in hunting in a variety of ways. They use it to promote wild seed collecting areas or promote wild tobacco, and even used it to promote re-sprouting shrubs so they produce nice straight switches for making baskets.
Now, the ways in which fire histories are usually reconstructed are either through tree rings which are very precise spatially and temporally but suffer a very serious age attenuation effect, and you saw from that plot I had earlier about the fire scar chronologies that I used, as you go further back in time, beyond say 1650, the records starts to get very thin, and so to look at this full thousand record, I wasn't going to be able to use fire scars to be able to do this, especially since the landscape that I was working had been heavily logged historically.
The other was in which paleo-ecologists have approached long term fire histories is through sedimentary charcoal studies, basically using charcoal, the byproduct of incomplete combustion of biomass, as a proxy for fire within the sedimentary catchment area. This is often done from lakes, ponds, bogs or wetlands which is an environment that is not very common to make a real serious understatement in the dry ponderosa pine forest. So, to be able to match a fire history record to the appropriate spatial and temporal scales, I adapted sedimentary charcoal techniques and built in a few other fire related proxy data sets to a different type of sedimentary environment,
in this case, looking at the accumulating reaches, what are called "alluvial channel fans" of discontinuous ephemeral streams, really focusing on the sheetflow reaches of these, where the products of wildfires might be mobilized off of the slopes and transported by streams, and then the sheetflow reach where sediments are accumulating and will accumulate with those. That's the idea. I felt confident that this was a strategy that was likely to work although the sedimentary environment is certainly much more complicated than in a lake or in a bog, that streams carry, in some cases, large volumes of post-fire erosion products, and that these were places where we would likely expect to see those.
What I did, and I'm a geo-archeologist by trade, is to walk these drainages, we would see now probably that these study units are hydrological basins, walking along entrenched streams, clearing off cut banks, describing alluvial and soil stratigraphy, taking a wide variety of samples including continuous samples for charcoal and some geo-chemical indicators and other small, undisturbed blocks, you can see here in these little blue blobs to make soil thin-sections to be able to look at the microstratigraphic contexts of some of these indicators. I did this across four different watersheds, two of which were control watersheds in which there was no evidence for residential life, but they were undoubtedly used, one with a short occupation, where that Bailey Ruin was that I showed you that was depopulated by 1325, and one with a long occupation including a long Apache occupation, the Forestdale Valley.
I built in some replication, so in the Forestdale Valley, I actually did three chronologically overlapping sequences to help improve the chronology but also to make sure that the proxies were not being affected by the more complex sedimentary dynamics of these stream valleys. And then to make sure that I understood what these proxies meant in relationship to certain types of fire activity, I collected analogous samples from aggrading channel fans after the Rodeo-Chediski fire but also from a smaller crown fire in the area, the 1974 Day burn, to be able to make sure that it had an appropriate frame of reference. You can think of these as standard samples.
The proxies that I'm working with from these sedimentary environments include charcoal but also phosphorous, and these landscapes that are framed by mostly sand stones, mud stones and shales, these are generally nutrient-poor as I mentioned. Phosphorous is mostly bound up in plant material and at low temperatures is liberated and re-mineralizes in the ash, so this was a way of getting at relatively low temperature combustion independent of the charcoal. Also looked at palynology, sort of reconstruct the plant communities and basically the fire impacts. I'm not going to talk about carbon isotopes today, that's a little bit more complicated record, but this relates a bit to what fuel burned as well as perhaps seasonality, although that's a little less clear.
Then combine those with some sedimentological and post-depositional studies to understand the context and how they were altered, looking at grain size and some other petrologic properties using soil thin-sections, what archeologists and geologists refer to as "soil micromorphology" to see the difference between very quickly accumulating here on the left sections where the bedding structures are preserved, and you can see how charcoal is imbricated within some of these finely laminated beds to those in which petogenic processes act to mix samples very quickly after they're deposited, as you can see here on the right, as well as for superposition of features that let us know or allow us to corroborate the chronological reconstructions about the upbuilding of these alluvial channel fans.
I won't show you all those records because I would probably put you all to sleep with a bunch of squiggly lines if I did that, but I do want to show you what one of these looks like. This is from the upper reaches of Forestdale Valley. You could see a bunch of young ponderosa pines on a very young overflow channel fan. This is the active channel here, I'm standing on top of this terrace which is where the sequence is from. You can see that the stratigraphy is not particularly impressive. It's characterized by these very weakly expressed alluvial soils that are the A, Bs, or ACb horizons here. These are what we call "fluvents" or alluvial soils, weakly developed alluvial soils with some evidence for a thicker, accumulized growth here at the top and welding of soils here in the middle, indicative of soil or accumulation rates.
The first thing that I did is plot all of the data stratigraphically. You can see changes in grain size and organic matter. Charcoal, I did continuously at 2 centimeter intervals where with most of the other data, I did episodically horizon by horizon. You can see the stable carbon isotopes, carbonates, the petogenic indicator, and phosphorous, one of our fire proxies. We converted this to time using radio carbon chronologies. In the Forestdale Valley, I actually used all three locations to build - hey have the same soil stratigraphy in these units - to build more precise soil dates to build age-depth models to be able to turn these into time series.
This is what that looks like as a time series over the last thousand years with time moving from left to right. We have phosphorous concentrations here in the bottom. This is how much phosphorous is present in fresh alluvium today, so how much is moving through the system in the absence of fire. This is charcoal, this is a proxy from grain size here on the back so that you can see how depositional energy varied. This all 3 taxa aggregated together in green, yellow are shrubs, and then orange are basically weeds, wildflowers, grasses and other herbaceous plants, and this "Zea" refers to pollen from the domesticated Zea mays - corn, as we know it.
The way I looked at this was really in cultural time periods, so the Pueblo 3 to Pueblo 4 period from about 1150 to 1400 in the Forestdale Valley, we have occupation and we kind of have peak accumulations of charcoal though it is episodic, not continuous. We have peak values here that also coincides with the deposition of domesticates, indicating cultivation of domesticates and gardening and farming. Phosphorous also is at its peak, so this kind of corroborates that there's a lot of fire activity although not necessarily continuous fire activity, in this part of the basin in the context of farming.
The other part of the pollen assemblage which is really interesting is that this is basically identical to early historic period of ponderosa pine forest and the relative breakdown of trees, shrubs, and grasses, so all these fire is clearing out forests, it's happening in the context of a more or a less natural forest. Our interpretation of this is something akin to Al Sullivan's burn-plot hypothesis in addition, probably, to natural fires happening elsewhere on the landscape.
Now, when people leave in 1400 and move elsewhere, there's a decline in charcoal despite still having good conditions for charcoal deposition and a decline of phosphorous, so there's less fire happening on the landscape, although there's still fire happening on the landscape, and the pollen assemblage indicates that the plant species aren't really changing, that the plant communities aren't really changing, although of course there's no gardening taking place, so we interpret this really as evidence of what the natural fire regime was like, in fact this is very similar to what happens in the last few hundred years in those control water sheds.
Now after the late 1500's, when Apaches are likely in the area, we see a really big change in the plant communities, although these are percentages, so it doesn't necessarily mean that the trees are less abundant. You could have the same impact if those disturbance species become much more common or much more prolific in their pollen production. Interestingly, the charcoal doesn't necessarily change much, although the phosphorous indicates that there is more fire happening on the landscape. This is a bit of a curiosity unless you think that these fine or basis fuels are what's burning. These produce fewer particulate byproducts that combust more completely and that they would actually produce less charcoal, then you could actually make sense of these 3 lines of evidence.
In fact, that's consistent with what we know from Apache ethnography. They relied really heavily on those disturbance plants for greens in the spring, a particularly lean period for Apaches, and seeds later in the season, so important that they actually during the reservation period would broadcast them even though these are wild around their camp so they'd make sure that they had them. We think this is evidence for a lot of Apache burning in addition to natural burning to promote those economically important wild plants, and then of course charcoal and phosphorous decline in the very top of the record with fire suppressions.
You can plot out those inferences over time and here, we got the Forestdale Valley in blue with agricultural burning until 1400, a decline to exclusively natural fires when people leave, and then wild plant burning by Apaches before fire suppression here in the end, and this is plotted on top of the precipitation anomaly used in my primary construction, so wet, dry, these are in standard deviation units and this is the frequency of regional in black, moderate and large regional fire years in gray, so this is the frequency of climate-predicted fire activity on the bottom.
In the Bailey watershed, we actually lack the early part of the record. It starts off with about a half a meter deposit that's almost identical to the deposit that formed after the 1974 Day burn. The 1974 Day burn was a crown fire that removed about half the canopy in the watershed and about half of it was still intact, so it still had quite a bit of pine pollen and pine trees locally, but half the trees were taken out, very thick charcoal deposits that were distinctive associated with that along with some other chemical indicators, and so that record actually starts with evidence of a crown fire, but it didn't take out the forest entirely and so we have evidence for natural fires in its wake and there's some evidence for Apache use and even gardening in the 17th and 18th century and that before fire suppression in the end of the 19th century.
Those control water sheds, again, similarly, we don't have the early part of the record, but the start of the record in the 1400's is accumulation entirely in the absence of a pine forest. This is actually a denuded landscape full of shrubs and weedy species, and pines don't actually show up again until the late 1600s. Our inference here is actually this experienced a high severity crown fire in which actually the forest was locally replaced and it took a few centuries before the forest was able to regenerate there.
These point to - in different settings - the Bailey Ruin watershed and the control watersheds, crown fires that either replaced the ponderosa pine forest or put a hole in it without taking all of it away during the 1400's, and this is actually consistent with our climate model of increased vulnerability to these types of fires with decreasing antecedent fire conditions followed by a prolonged wet period that allowed for altered stands. Note that the Forestdale Valley had anthropogenic burning in addition to natural fire through both of those periods, so this is a landscape that did not experience that reduction in fire activity. Only those that were depopulated by 1325 experienced both of these processes and became more vulnerable to crown fire as a result.
The conclusions are that it does look like anthropogenic burning in addition to natural, lightning-started ignitions may help foster maintenance of the more resilient, open-canopied pine forest in the context of climate conditions or other conditions that might favor increasing periods of time between fires and altered stand conditions, which seems to provide good evidence that we should be engaged in the prescribed burning program that we are and may actually need to do more of this if maintaining those type of open canopy forests is our goal. If that's our goal, restoring the structure of those forests and expecting lightning to do all the ignition work for us, particularly in light of the modern wild land urban interface expansion, may be unreasonable.
The forest maybe too fragmented to be able to carry fires from lightning ignitions. We may need to supplement it to do that but, of course, the situation's never quite so simple and there's a lot of evidence from areas as disparate as California and Sub-Saharan Africa for non-linear relationships between population density and fire that as population densities get higher, you actually have a decrease in fire activity as the human landscape fragments the natural one. You can see this here in Africa too where people are using this for the livelihood as the population density gets higher both in terms of the number of fires and the area burned decreases.
The next stage for us is actually to look at an ancient wildland-urban interface in the Southern Jemez where, in an area that's smaller than my eastern Arizona study area, we have an order-of-magnitude -larger population prehistorically, probably using fire much the same way but we're curious as to whether or not in this landscape, with these very large pueblos - any one of these is as large as the entire population of my Mogollon Rim study area - have inadvertent suppression effects. They are living at high elevation. They needed a lot of fuel for heating their homes as well as timber for building their homes, and land for growing crops, and maybe there were inadvertent fire suppression effects that made parts of this landscape more vulnerable. We don't know. That's one of the hypothesis that's driving our research, and so that's what we're investigating now.
I want to leave you - so let's bring a close to this - with an anecdote from Southwestern ethnography that is kind of a side note in the ethnographic work of Leslie White among the Acoma Indians in Western New Mexico. In the 1930s, I can't remember the day off the top of my head, Leslie White was at Acoma and we have the great fortune that he happened to be there for a particular Kachina ceremony that he called "Torotzo lights the fires." This is a ceremony that only took place every 5 to 10 years.
This was organized by the Corn Clan and they met at the beginning of July or the end of June to decide whether or not it was an appropriate year to do this, and then within the following week, they would organize young masked Kachina impersonators to go in the cardinal directions from the Pueblo as far afield as 20 kilometers from the Pueblo and light fires on the landscape and light them all way back to the Pueblo in which they would have a big community festival. And a few years later, he wrote up the oral tradition that describes the origin for that particular Kachina ritual, andhe described it thus: “At some point in time, in the legendary past the leaders of the Corn Clan came across Torotzo, this Kachina spirit, and three of his compatriots setting fires out near Grants, New Mexico, not too far from the El Malpais basalt flows.
The Kachinas were concerned that they might misinterpret what they were doing, so they said, "Wait, wait, wait, wait. We're not doing this to destroy the earth, we're doing this to heat it up, to make it more productive." This immediately resonated with the leader of the Corn Clan because he invited Torotzo and the other Kachinas to come live in Acoma Pueblo and, in fact, to assume the leadership of the Corn Clan because they thought this was so important. And, for a time, they lived there but they ultimately had to go back to their mountain homes, and so the Corn Clan was responsible for maintaining this and this indicates, in large part because of the scale of this if for no other reason, that knowledge of fire, it's ecological context, its frequency, its seasonality were intimately known by the people who lived on these landscapes for many, many, many generations.
They understood this very well, although even in my talk, we've treated this in a very kind of functionalist way, those sets of functionalist and spiritual symbolic divisions were not so contrastive for the peoples who lived there at the time, as well as for their descendants. It's entirely possible that, by expecting there to be any difference at all, we're missing a big part of the human-fire relationship, so I think we have a long way to go to work with native people and continue to build our relationships with the archeological past, the oral traditions about the past, paleo-climate and paleoecological and paleo-fire records to learn everything that we can from a millennia of human experience in these fire prone settings to try to live in the best way that we can with these types of fire-prone forests in an era of climate change ourselves.
I want to thank you guys for your time. Sorry I went a little bit over and I'm happy to entertain any questions if you guys have any.
Karen: Chris, thanks for a very interesting talk. There's a lot there to think about. Do we have any questions or ... No questions, no comments? Chris, I wanted to ask you about the wet, wet, dry intervals that you were speaking of prehistorically. Is this related to El Niños?
Chris: Yes. One of the challenges for understanding the origin for that anticipated decline in surface fire activity in the 14th and 15th century is that most El Niño reconstructions, because at least in the more recent past, in the last 3 or 400 years, it's definitely related to El Niño, absolutely, but on longer time scales, the reconstructions of El Niño are kind of all over the place. Some reconstructions have a really high amplitude variation in El Niño/La Niña switching during that period of time, which I would expect actually drive more frequent surface fires, because it'd be really good for producing fuels and very good for producing a kind of dry year suitable for spreading fires.
Others indicate maybe a weakened El Niño amplitude during that period of time, which would align a little bit more with what we see here. Certainly, in the annual reconstruction, what is driving that change is that there is a reduction in the year to year variation of moisture on a more decadal timescale.
Karen: Okay. Thanks for that. What are your future plans?
Chris: We've done three years of field work in the Jemez Mountains now, and now my grad students and I are furiously working on the samples. We're doing much the same kind of work although we have probably twice as many localities across that landscape, some of which have much longer records. I have a 40,000 year record of very localized fire and vegetation history from the Banco Benito Rhyolite Flow in the Valles Caldera National Preserve that includes some evidence for heightened fire activity during the human occupation in the last few hundred years that may also be associated in that case with agricultural use. Thus far, it actually looks like our hypothesis about inadvertent fire suppression in the denser ancient wildland-urban interface may not be correct.
It looks like in the areas at least where I've got complete data so far, or at least maybe it's more complicated is what I should say, in these agrarian landscapes, there's a lot of fire associated with evidence for agriculture, including Zea mays, pollen, and the pollen of some of those - lots and lots of fire happening on these landscapes. It looks a bit like the burn-plot hypothesis, but in the areas that are a little bit more residential, there's very little evidence for fire, and so it looks like there may be a mosaic, although we haven't found clear evidence yet about what the impacts of that mosaic were. We have a big modeling component. Rachel Loehman is one of the co-PI's on this. She's with the USGS now in Alaska and she's a dynamic ecological modeler, and so she's trying to take the information that we have and scale it up.
We have these kind of watershed-scale data points and really, the consequences of this might play out in more complicated ways at the landscape-scale, so she's modeling forest dynamics and fire dynamics under different types of land use and climate conditions to see if we can tune those towards our paleoecological information to figure out, "Okay, what likely actually happened? What would be the consequences on the larger landscape of this kind of mosaic of suppressed fire here and enhanced fire there?"
Karen: I was thinking if there was any way to, if you're able to tease out when you have low ... Other than the density of charcoal, are there ways to determine whether or not you've got like low density fire versus something that's very intense?
Chris: Yes.
Karen: I know - I just listened to your talk - but I mean in the archeological record, is there any way to tell whether or not they're burning their fields off versus something more extensive?
Chris: That's a really good question. One way that I've tried to get at that is by focusing on the stable carbon isotopes of the charcoal fraction itself. Zea mays is a C4 plant. It uses a relatively young photosynthetic pathway that is less discriminating against the heavier forms of carbon and CO2, so it shows up distinctly in the ratio of the stable carbon isotopes. So far, I haven't seen that and it could be because I'm not sampling fields directly themselves or that by mass, the vast majority of charcoal is still coming from the C3 pine forest that it's swamping out the relatively low biomass signal from the corn. That is a part of the story that we're still working on.
There is another to get way to get at fire intensity. I'm working with a colleague, Andrew Scott who's on this Thank You slide and in fact, it's his slide but I stole it for the backdrop here, this really cool flower, he's a geologist who has been working with his students and post-docs over the last couple of decades adapting a microscopy technique from coal petrology for reconstructing the combustion temperature of charcoal using its quantitative reflectance under a very narrow wavelength of visible light in oil microscopy. Basically, the higher the temperature, the more light is reflected back because the cell walls that them increasingly homogenized and they reflect more light back.
One thing that we're doing on the New Mexico project is making thin sections, polishing them and then doing reflectance measurements of the charcoal in situ on those blocks. I don't have any results to tell you about that other than to say we've done some work from contemporary fires to make sure that this works in wildfire settings the way it does in experimental settings, and it does. It does seem to work.
Karen: If you're burning off agricultural fields, then you would have a lower fire?
Chris: Yeah, you would expect those to be lower temperatures.
Karen: Okay. The temperature fire and you would expect to see it reflected in this set of analogies.
Chris: Yeah.
Karen: Interesting. Do we have any more questions? Any comments? All of our fire people on the line here?
Jim: Christopher, this is Jim Kendrick. I just want to thank you for a great presentation and really interesting work out there.
Chris: Thank you.
Karen: Jim, you used to work in the Southwest, didn't you?
Jim: I did. I was the archeologist at El Malpais National Monument and I went to school at SMU.
Chris: Hey!
Jim: Yeah!
Chris: We're celebrating our 50th anniversary this year!
Jim: I know! It's fantastic!
Chris: If you get a chance, my colleague here at the department, who also happens to be my better half is organizing our departmental Facebook page to spread the word about the 50th anniversary and sharing all sorts of great photos of students and faculty from years past. Come Like us on Facebook if you're on Facebook.
Jim: Excellent. Thank you.
Karen: One of our colleagues too is very supportive of the webinar series. Pei Lin Yu is also an alumni of SMU. Anybody else? Any other comments?
Jim: Christopher, this is Jim again. You mentioned the prescribed burning strategies that we currently have in place. What I was most concerned with out West was not too much the prescribed burns themselves but doing them when the fuel loading seemed to be at abnormally high levels, and another thing, this is kind of a comment, that I noticed out there in the archaeology is that I rarely saw fire effects on any of the archaeological assemblages. I was wondering if you noticed that, too, and if that gives us any indication of the, I guess, fire regimes or intensity and such. Probably not as much as the fire scars but it always interests me that I rarely saw any fire effects on archeological assemblages.
Chris: Yeah, that's a really great point. A colleague of mine who's the director of Heritage Research in archeology for the Valles Caldera, Anna Stefan and Rachel Loehman are working on a project, doing some experiments partly related to fuel loads and burn intensity actually in the Jemez Their project is called I think "The Arch-burn Project." Just to explore this a little bit, in at least contemporary landscape fuel settings, to understand what consequences, for one thing, contemporary fire management strategies might have on cultural resources. Also, maybe to better understand why very often we have not seen fire effects, I think maybe depending on what the resource context is like, and one thing that we're learning from the fire scar work we're doing on the Jemez is that there's very little fuel on top of those large Pueblos to burn ...
We don't see evidence for spreading surface fires again for usually about 40 to 50 years after depopulation, and those are still probably only happening in relatively light fuel loads, and I think that's probably an important part of that is that when fires happened in the past on at least many of these residential sites, they're probably happening in very light fuel loads so that they were probably burning over very quickly and at relatively cooler temperatures. Whereas now, you're right, the fuel loads are really, really high, and so in the Jemez, they've done some real serious fuel management in advance of a recent prescribed burn that burns really hot, so it ended up producing very heavy and relatively coarse thick fuel loads on the surface that smoldered in a way that dust and grassy fuels don't.
I think that's actually where some of the Arch-burn study locations were, so, we'll get some really interesting information to come out of that. I know that in terms of post fire erosion, oh my gosh, last time it was epic! One of my localities was just downstream from there and it was totally transformed by head cutting and really massive deposition of these drapes of charcoal-rich fan deposits.
Jim: You've had a pinon dieoff up there, right? Are you too high for that?
Chris: We're up above that, yeah.
Jim: Okay. Interesting. Very interesting. Thank you.
Chris: My pleasure. Thank you.
Karen: This is a good time for me to put a plug for Jay Sturdevant’s talk who I think is going to be talking about some of his research on the effects of fire on different artifact Types. I’m sorry that he's not on the line today, unless he is and has something to contribute to the discussion of the artifacts, the lack of burning evidence for artifact assemblages from this particular area. No, Jay?
All right. We will hear him at a later date and please join us, everybody, for a talk also looking at fire in the Southwest, next week with Craig Allen, so thank you very much, Chris. It was a great talk.
Chris: Thank you. It was my pleasure.
Karen: See you next week, everyone.
Jim: Okay. Bye. Thanks, everybody.
Chris: Bye.
Description
Christopher Roos, 12/4/2014, ArcheoThursday
Duration
1 hour, 11 minutes, 8 seconds
Credit
NPS
Date Created
12/04/2014
Copyright and Usage Info