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Being able to determine changes in the environment, and how humans may have adapted to those changes, is an increasingly important aspect of archaeological study. In this month’s ‘Science Notes’, we explore a recent project, led by Dr Amy Styring from the University of Oxford, which has looked at whether carbon-isotope analysis of hazelnut shells might be able to be used as a proxy for determining levels of forest cover, as well as foraging strategies, in the past.
The research is based on the premise of the ‘canopy effect’, by which plants that grow in dense, well-shaded forests have lower carbon isotope (δ13C) values than those growing in open spaces, due to the lower light levels leading to lower rates of photosynthesis. As hazelnuts are common finds on prehistoric sites, the team aimed to see whether hazelnuts were as susceptible to the ‘canopy effect’ as other plants, and hence might serve as a good target for future studies of environmental archaeology.
To test this hypothesis, the team gathered hazelnuts from trees growing in varying light levels at three locations in southern Sweden over two years. They then analysed the variation in their carbon-isotope values, and the relationship between these values and the light levels the trees were exposed to. They found that there was a strong and significant relationship between nut- shell δ13C values and light level, with nutshells from hazel trees growing under more ‘closed’ canopy having lower δ13C values. The team then tested whether nutshell δ13C values alone could accurately predict canopy density, finding that while the model they developed was good at identifying hazelnuts that had been grown in either completely open or completely closed conditions, it was less good at predicting which nutshells had grown in ‘semi-open’ conditions, such as on the edge of forests or in dense hedgerows. They also found that meteorological conditions during the two sampling years did not appear to have a sizeable impact on δ13C values. The team could not rule out, however, whether more extreme changes in weather might have an effect.
An artistic reconstruction illustrating typical landscape changes from the time of early humans (bottom image) to the present day (top image).
Having established a relationship between canopy density and hazelnut shell carbon-isotope values, the researchers next inves- tigated the δ13C values of hazelnut shells from archaeological sites also found in southern Sweden. They selected shell fragments from four Mesolithic sites and from 11 sites ranging from the Neolithic to the Iron Age (covering a total time period of around 9,000 years), some of which had been occupied in more than one period. Among the nutshells from Mesolithic contexts, they found an even split between nuts from open and closed canopies, suggesting that people had access to, and exploited, a range of different environments. This appears to be in keeping with pollen analysis in southern Sweden during that period, which indicates that the landscape was dominated by broadleaved forests with smaller areas of open land. After this period, from the Neolithic through to the Iron Age, there appears to be a steady increase in the δ13C values of hazelnuts, suggesting that over this period they were being gathered from increasingly open environments. By the Iron Age, the vast majority of nuts appear to have been obtained from open wood pastures. This trend is consistent with pollen records, too, which indicate that, between the Mesolithic and Iron Age, tree cover in southern Sweden decreased from around 80% to 30%.
Highlighting the impact of their findings in a paper recently published in Frontiers in Environmental Archaeology, the team said: ‘On Mesolithic and Neolithic sites, where hazelnut shells are abundant, nutshells from discrete palimpsests can help to characterise the microhabitats that were exploited by different groups. In later periods, nutshells with δ13C values indicative of growing in closed canopy environments can identify sites that were in the proximity of more closed woodlands.’
Text: Kathryn Krakowka / Image: illustration by Nils Forshed, given to Dr Per Lagerås and shared with Dr Lagerås’s kind permission
