Ancient human impacts in Amazonia – the debate continues…

How well can you ever really know 5.5 million km2 of hyperdiverse forest? The Amazon region, and this question, are at the heart of several ongoing debates in the natural sciences – why are there so many species? how much carbon can the forest store? how much did ancient humans impact the forests, and can we still see their effects today?

How fully can you really know a forest as huge and diverse as the Amazon?

This last question was the subject of a paper by Caroline Levis and colleagues published in the prestigious journal Science in March this year, which caused significant ripples within and outside the scientific community. In a nutshell, the article found that tree species humans have cared for are five times more likely to be forest ‘hyperdominants’ than you’d expect. Past human impacts also help explain where these species are found now, accounting for up to 20% of the variation in their distribution (for comparison, environmental factors explained up to 30%). The paper’s conclusion, as picked up by the media, is that “modern tree communities in Amazonia are structured to an important extent by a long history of domestication by Amazonian peoples.”

Theobroma cacao – the tree cocoa beans come from – was one human-influenced species in the Levis et al study

So, debate settled? Not quite. Responding to the paper some critics pointed out that the potential influence of soil nutrients hadn’t been adequately considered; others suggested the data showed that human effects were only significant within 20km of archaeological sites; and yet another argued that most of the human-influenced trees are so short-lived that pre-colonial individuals would be dead by now. Furthermore, another study published around the same time showed these Amazon forest plots are disproportionately close to places highly affected by ancient humans, so maybe it was inevitable that the sampled areas would still be showing their influence.

This, perhaps, goes to the heart of the problem with trying to uncover the human history of all Amazonia: there isn’t much detailed region-wide environmental data, the non-random tree plots cover a vanishingly small fraction of the forest, and there are many archaeological sites we may never find. Reflecting on the paper and its findings at a recent TPRG meeting, it was sobering to consider how this can limit the conclusions of wide-ranging studies like this.

It was also striking how, when it comes to ancient human impacts on forests, people can draw very different conclusions from the same data. As I embark on my research into the effects of past humans and climate change on southern Brazil’s Araucaria forests, these will certainly be important things to bear in mind!



The original paper by Caroline Levis and colleagues can be found here: Some scientists have responded to the paper in its eLetters section (, and Christopher Dick posted his thoughts on his research group website ( The paper on the link between forest plots and areas of ancient human impact was published by Crystal McMichael et al., and can be found here:

Where Palaeoecology leads us 2: A&E


Figure 1.  The Hospital, for most people, a dreaded place. For us, a place of revelation


This is the sequel to the post about “Guns” where we talk a little bit about where palaeoecology leads us when we follow our quest for investigating the past using lake/bog sediments.


An unusual patient

If you are worried about the consequences of working in palaeoecology, I can assure you that my recent visit to A&E was not because of an accident (and was not an accident either). This was a very well planned visit to the hospital to become more familiar with my beloved Brazilian patients, my bog cores.


Having taken over 110 Russian cores from numerous bogs across southern Brazil, between Frank Mayle and I (check out the Je Landscapes Project website to know more), I needed a quick non-destructive technique to visualize the internal structures of the cores so I could select the best ones for the project. I therefore decided to take x-rays of these half-metre cores. This relatively low-cost technique allows us to identify any key lithological changes through the core which are not apparent to the naked eye. The differences in density are seen in the resulting images as shades of light and dark. The lighter the colour of the image, the denser the sediment is. That is why features such as clastic material and tephra layers appear light, in comparison with organic peats which are usually dark.



So I arrived at the hospital with two oversized suitcases (Figure 2), completely filled with sediment cores (I am glad to say that I didn’t have to rush there). Carrying the cores this way allowed me to navigate easily through the labyrinthine hospital and get to the subterranean x-ray room.



Figure 2. Just arrived at the Hospital


The X-ray manager was very helpful, and we worked together as a great team, with me unwrapping the cores and placing them on the plate, taking notes and hiding behind the x-ray shield; while he was pressing the button and inputting the information into the computer (Figure 3). It was a relief to be on the other side of that shield.



Figure 3a. Our view to the patient. Behind the shield



Figure 3b. Not sick looking, one of the bog cores being analyised


The cores just about fitted on the x-ray plate (in diagonal) (Figure 4). I am glad I checked that beforehand! If you are considering taking x-rays of your cores, its important to call the hospital beforehand to make sure they have the plate size you need. It seems that most of the x-ray facilities only have the 23cm length plate, which is no where near large enough for a half-metre Russian core.



Figure 4. X-ray of one of the bog cores


I am pleased with the results. The images allow me to distinguish internal structures and therefore enable me to select the best cores (some cores look fine from the outside, but with the x-rays I can see some small gaps in the sediment). On the other hand, the lithological changes revealed by the x-rays enable me to cross-correlate overlapping and duplicate cores. A particularly useful further step is to undertake grey scale analysis of the x-ray image, which can reveal even greater detail (Figure 5), especially when correlating with XRF and magnetic susceptibility results.



Figure 5. Grey scale analysis performed with ImageJ in one of the cores


This was my first experience of undertaking x-ray analysis of bog cores, and I must say that I am very pleased with how useful this technique has proven to be.


By Macarena Cárdenas





Where Palaeoecology leads us 1: Guns



Our quest for understanding the past, using sediments pushes us to find ways to extract the information in what is sometimes quite an exotic manner, as well as making us visit unexpected and fun places (soon to come, a blog post about the places we have visited lately in our field trips). Here is an example of the fun things we get to do.


No blood involved

One fun activity we have been doing is: holding a gun. Not any gun, but a galactic-looking laser-shooting one. I am talking about the portable x-ray fluorescence (XRF) analyser (Thermo), a fantastic piece of technology that allows us to analyse a wide range of elements from the Periodic Table (Figure 1). The gun is very easy to use, with touch-screen and flexibility to customise it, depending of the nature of your sediments. Beware that, because of the nature of this piece of kit, you will need to undertake training in radiation. And: make sure that if you are using it in hand-held format for long periods, that you have strong biceps!



Figure 1. The portable x-ray fluorescence (XRF) analyser (Thermo) in action. Russian core being measured.



Results from this technique have several applications that go from cross-correlating cores within a site to understanding the past environmental characteristics and deposits of the sediments. The fast processing time and resolution available with this method (1cm) enable results to be obtained very quickly, and allow one to understand the nature of the sediments in a non-destructible way (which is very much appreciated when you have only small volumes of sediments at your disposal with Russian cores!). The results can be imported into the computer and easily opened in an excel spread sheet (Figure 2).



Figure 2. Excel spreadsheet with XRF results from a bog core


Next steps will be correlating the XRF results with other analyses, such as magnetic susceptibility and pollen analysis. I am really looking forward to seeing what these multi-proxy analyses reveal.


By Macarena Cárdenas