Trees with a Story to Tell: Stahle’s Tree-Ring Work in the Amazon

Bob Whitby: Hello and welcome to Short Talks From the Hill, a podcast from the University of Arkansas. I’m Bob Whitby, a science writer at the University. Today we’re talking with Dave Stahle, a Distinguished Professor in the Department of Geosciences and who’s done important work in field of dendrochronology. Welcome Dave.

David Stahle: Thank you.

BW: First of all, let’s start with a little definition of exactly what is dendrochronology.

DS: Dendrochronology, tree time, tree ring dating, it is the most accurate and precise dating method in geochronology. It’s a slick dating method that we use to determine the age of ancient archeological sites, or early historic buildings, or living trees. We can determine the age of the living trees with dendrochronology. Of course you can count the rings to determine that, but with dendrochronology we’re actually using the imprint of climate variability on the annual growth rings.

BW: That work you did in the Ozarks and other places. You’ve recently done work in the Amazon. Tell us about that and why you’re interested in this kind of research in the Amazon.

DS: Well paradoxically, the most biodiverse forests in the world are the tropical rainforests of Amazonia or the tropics, the global tropics, annual ring formation is extremely rare in these tropical forests. I mean, tree rings are decidedly a mid-latitude temperate climate phenomenon, where you have, you know, summer and winter, and so the trees go dormant in the winter, in fact, you know there’s no distinct seasonality of temperature in Amazonia or most tropical forests. So most of these incredibly biodiverse forests, which in a case of Amazonia, lowland Amazonian forest may have something on the order of 16,000 tree species native to Amazonia, that’s only an estimate, a scientific estimate, but taxonomically 6,727 species have of trees have been described in the lowland Amazon, and most of them, probably 99 percent of them, do not form reliable annual rings. That said, there is precipitation seasonality in parts of the tropics including parts of Amazonia, where there’s a wet season and a dry season, and that can be so pronounced as to be associated with annual ring formation in a small subset of native tree species. And so we’re trying to find that needle in a haystack, those few native tree species that do reliably form annual rings, are long-lived, like centuries long and are sensitive to precipitation variability. That’s what we’re after in the Amazon.

BW: What is known about the long-term climate in the Amazon, and how are you adding to that record?

DS: Well, not much of course, not on a not on a high-frequency, inter-annual basis. I mean there are paleo-environmental records, paleo-climatic records from the glaciers that border the Amazon and Andes and from speleothems, cave formations and so forth, but these are not highly resolved nor exactly dated climate records. And of course there’s pollen data as well, lacustrian data, but for the high-frequency, inter-annual variability of precipitation and temperature or streamflow, there’s really nothing prior to the instrumental record for all of Amazonia. And you know Amazonia, of course, is a big place and it’s one of the global centers of deep convection in the atmosphere. It affects climate over tropical South America and more broadly globally, so it’s exceedingly important from a global climate perspective, and of course it is the largest river on earth. In fact, you know the mean annual flow of the Amazon is 209 cubic meters per second. That’s dwarfs all other rivers of the world. In fact, the flow of the Amazon is greater than the combined flow of the next seven largest rivers on earth. It’s 12 times the flow of the Mississippi, so we’re talking about a prodigious hydrological resource.

BW: Your work extended the record considerably. What is it showing? What are you finding?

DS: Well we’ve found a couple of, three or four tree species, that are useful for dendro-climatology. Our greatest success so far as with been with cedrella odorata, which is a commercially valuable tree species native to Amazonia and widespread across the Amazonian tropics. It doesn’t always form really high quality annual rings, but in along the Rio Peru in the eastern Amazon we have found forests with cedrella in it that do form excellent annual rings. We’ve built two exactly dated tree-ring chronologies. They are correlated with wet season precipitation totals, and they are weakly correlated also with the discharge of the Amazon River, but their principle value is as precipitation proxies for the eastern Amazon.

David Stahle

BW: This research involved you going to Brazil and being in the Amazon. Tell us what that was like.

DS: Well it’s a lot of fun, I mean this is a tremendous environment. Many challenges confront the Brazilians and the Amazon, of course, development, deforestation and potentially anthropogenic environmental change, but still vast areas is intact, triple-canopy tropical rainforest, and the principal avenue of access is by the rivers. I mean it has historically been river travel upon which the Amazon basin was settled.

BW: How does this affect our understanding of climate change?

DS: Well, climate varies on all time scales, for known and unknown reasons. It’s definitely… that’s out big objective. You know the Amazon suffered a tremendous drought in 1925. It was the worst year of drought in the instrumental record. It also… the precipitation data, such as they are for the Amazon, and the streamflow data, as they are for the Amazon, exhibit some changes. They exhibit decadal variability, multi-decadal variability. The precipitation and discharge data exhibit, in the last 30 years, this increasing seasonal amplitude, that is the greater difference between the dry season totals and the wet season totals. And so we can see variability in the modern record. We’re not sure of the cause of this variability, and so if we can reconstruct precipitation, and possibly streamflow, then we, for centuries past, we can place the modern era of instrumental, hydro-climatic record observations into a longer perspective and perhaps uncover some of the dynamics responsible for this variability.

BW: Is your work in the Amazon done? Are you going back?

No it’s not done. We’re in the middle of a project funded by the National Science Foundation, and we hope to obtain additional funding for this work. The fact it’s huge. The legal Amazon, as it’s called, the drainage basin of the Amazon River, is almost the size of the continental United States. For the continental United States we have on the order of 2,000 tree-ring chronologies that scientists have developed over the last hundred years of tree-ring research in the United States, and those provide pretty fair coverage of this temporal and spatial variability of moisture in the United States. Well we need something like that for the lowland Amazon as well. Right now we have two. And we need 2,000. Of course, we won’t get that in my lifetime, but that is the goal, the goal is to provide a network of exactly dated tree-ring chronologies, 200 to 400 hundred years long, that can be used to reconstruct not only to temporal history of rainfall and streamflow, but also the spatial variability as well. It’s a big job. There’s a lot of interest in Brazil and in other countries in the Amazon to try to develop these tree ring records and extend the instrumental record deeper into prehistory.

Matt McGowan: Music for Short Talks From the Hill was written and performed by Ben Harris, guitar instructor at the University of Arkansas. For more information and additional podcasts, go to KUAF.com or researchfrontiers.uark.edu, the home of research news at the University of Arkansas.