For more than 100 years, tree rings have served as a reliable way to estimate past climate conditions because in warmer years, which usually mean better growing conditions, tree rings tend to be wider. But herbivores that nibble away at trees also have an effect on tree ring size — a phenomenon that was known but never measured. Now, researchers from Norway and Scotland report the significant effect of grazing sheep on tree ring size over a nine-year period.

The researchers, whose findings are published in the current issue of the journal Functional Ecology, created nine fenced-off areas in the mountains of southern Norway. Some contained no sheep, some had about 10 sheep per square mile and some had about 30 sheep per square mile. Although sheep prefer to eat grass and herbs, in the mountains this is sometimes not enough, and they turn to shrubs and small trees for nourishment.

After nine years, the researchers sampled cross sections of more than 200 birch trees in the enclosed areas and measured the trees’ ring widths. “What we found is that the number of sheep had a very large impact,” said James Speed, an ecologist at the Museum of Natural History and Archaeology in Trondheim, Norway, and the study’s first author. “This shows how important the number of herbivores is.”

In areas where there were no sheep, the radial growth in the tree rings was double that areas with few sheep. And it was three times the growth seen in areas with the most sheep.