The widths of tree rings have long been used as a proxy for temperature. However, ring width is controlled by precipitation and fertilization by carbon dioxide, as well as by temperature. To further complicate interpretations, a Norwegian study shows that grazing by sheep and other herbivores can also have a significant effect on tree ring width. Tree ring proxies figured prominently in the Climategate scandal phrase “Hide the Decline.”
James D. M. Speed, Gunnar Austrheim, Alison J. Hester and Atle Mysterud (2011), ‘Browsing interacts with climate to determine tree ring increment’, doi: 10.1111/j.1365-2435.2011.01877.x, is published in Functional Ecology on 27 July 2011.
According to the press release for the paper: “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.”