From: David A Gabel, ENN
Published July 27, 2011 12:05 PM

Tree Rings Affected by More than Just Climate

To this point, tree rings have been amazingly useful for gathering information on past climates. A two hundred year old tree can carry a world of information in its rings. In temperate regions, where seasons differ greatly, each ring denotes one year of growing. For longer growing seasons, the rings are wider. For shorter growing seasons, they are narrower. Droughts or floods have an enormous impact on the amount a tree grows, thus affecting the rings. But what else can affect the thickness of growth rings? A research team in Norway has just released a study which shows that sheep and other herbivores have a greater impact on trees than previously known.


The researchers, a team from Norway and Scotland, have conducted a series of experiments in the mountains of Norway, between Oslo and Bergen. They fenced off a large area on the mountainside and divided it into sections. Each section held a set density of domestic sheep every summer. For the past nine summers, this has been done. After the ninth summer, cross sections of 206 birch trees were taken and the tree ring growths were measured.

The sizes of the tree rings were compared with the number of sheep in that particular section and the climate of that given year. The goal was to disentangle the relationship between the sheep browsing and climate.

According to Dr. James Speed, lead author from the NTNU Museum of Natural History and Archaeology, "We found tree ring widths were more affected by sheep than the ambient temperature at the site, although temperatures were still visible in the tree ring records. This shows that the density of herbivores affects the tree ring record, at least in places with slow-growing trees."

"Our study highlights that other factors interact with climate to affect tree rings, and that to increase the accuracy of the tree ring record to estimate past climatic conditions, you need to take into account the history of wild and domestic herbivores. The good news is that past densities of herbivores can be estimated from historic records, and from the fossilized remains of spores from fungi that live on dung."

However, Dr. Speed emphasizes that this research mostly pertains to highland environments. "This study does not mean that using tree rings to infer past climate is flawed as we can still see the effect of temperatures on the rings, and in lowland regions tree rings are less likely to have been affected by herbivores because they can grow out of reach faster," he explains.

The study is published in the British Ecological Society’s journal, Functional Ecology.

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