Ecosystems

Reverse archaeology reveals new insights into global warming

Often archaeologists will examine differences in vegetation to gain information on buried sites. Now, some botanists have reversed this and are using archaeological sites to learn about the effect of nutrient differences on plants.
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Despite the name, life is tough for plants in Greenland. They have to be able to cope with the seasons at their most extreme, with long nights in the winter, a short growing season in the summer and perishing cold once the winter returns. On top of this, they often have to work with a paucity of nutrients in the soil, but not everywhere in Greenland suffers the same nutrient deficiency. There are 6000 archaeological sites in Greenland, each one a distinct place where scientists can track human activity. And so can plants, as humans have left extra nutrients in the soil. Angela Luisa Prendin and colleagues examined a time series of ring widths of Salix glauca L. collected at nine archaeological sites and in their natural surroundings along a climate gradient in the Nuuk fjord region Southwest Greenland. The data gathered allows them to see nutrient availability’s effect on a plant’s response to global warming.

Greenland is turning green, but it’s not turning green evenly. A warming climate is changing what plants grow and how they grow. Greenland has more woody growth than before, but it’s unclear how plants will spread and adapt as the temperature rises. Water availability will affect plants’ ability to adapt, as will the movement of pests like insects. Another variable is in the soil. Plants need access to nitrogen and phosphorus, so understanding how plants react under better nutrition is helpful, and this is where archaeology helps.

Three salt-bleached wooden crosses stand among spiky brown tussocks of grass that scratch a living in the rock and gravel. Some snow lies on the foreground. Beyond the graveyard lies a fjord, much of it covered in ice. Beyond that a low snow-covered hills.
Three graves at Qoornoq, Nuuk Fjord, Greenland. Image: Canva.

One way to find archaeological sites in Greenland is to study the vegetation carefully. Plants on archaeological sites are distinct compared to their neighbours, thanks to the waste humans have left behind. The waste from humans or their animals creates hotspots of nitrogen and phosphorus. Grasses can have double the biomass aboveground than their off-site relatives, thanks to the boost from these nutrients.

Taking a look at the soil reveals that phosphorus on-site can be double or even six times higher than off-site. Nitrogen also has an isotopic ‘fingerprint’ that scientists can track due to human activity. If you have a schedule of these archaeological sites, you also have a list of places of botanical interest. Prendin and colleagues returned to nine of these sites and their surroundings to study Greyleaf willow (Salix glauca L.). 

The sites they chose cover four and a half thousand years of human activity. While the Vikings arrived and lived in Greenland from around 985 to 1350 CE, the earliest inhabitants are known to archaeologists as the Saqqaq culture, dating from 2,500 BCE to 800 BCE. The other sites in the study date from the Dorset Culture, 300 BCE to 600 CE and the Thule culture, 1300 CE to the present.

To see how plant growth was changing, Prendin and colleagues studied the growth rings in Greyleaf willow. Measuring the width of the rings told the botanists how well the shrub grew in a given year. The team took the samples from sites in the Nuuk Fjord region, from close to the ice cap along the fjord to the sea.

The botanists found that there was indeed an increase in growth rates over the past few decades. They attribute this to both warmer temperatures and also a longer growing season. The team write that, “the highest growth rates and the strongest correlation with summer temperatures were found at the drier inner fjord sites, suggesting that the observed variation in S. glauca growth across the East–West transect is driven especially by variation in temperature.”

The plants on the archaeological sites seem to be more responsive to these temperature variations than the off-site plants. This could be due to the increased nutrients. However, Prendin and colleagues also mention other possibilities. Archaeological ruins might act as snow traps, holding a store of water for longer to tide plants over the drier summer months. The disturbed soil also holds water better, so some of the reasons for greater responsiveness may be physical and not chemical.

While the study has lessons for understanding future greening in the arctic, the authors also note their study has archaeological importance. The sites are currently protected by the vegetation above them. If this changes, maybe in composition with more shrubs with deeper roots, the preservation of the sites could also change. This study shows that relying on off-site vegetation as a proxy for soil disturbance will not be accurate, as the off-site samples won’t have benefitted from the increased nutrients.

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Prendin, A.L., Normand, S., Carrer, M., Bjerregaard Pedersen, N., Matthiesen, H., Westergaard‐Nielsen, A., Elberling, B., Treier, U.A. and Hollesen, J. (2022) “Influences of summer warming and nutrient availability on Salix glauca L. growth in Greenland along an ice to sea gradient,” Scientific reportshttps://doi.org/10.1038/s41598-022-05322-8

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