Dating objects based on tree rings — dendrochronology if you want to get technical — requires a fairly advanced understanding of how the environment interacts with the trees in order to be accurate. It's fairly common for scientists to make inferences about ancient climate based on the condition of tree rings at certain points in time. Except now it turns out that roving animals may have more to do with tree ring formation than the weather.
New research published in Functional Ecology has shown that sheep have more of an impact on tree rings than the climate. Over a period of nine years, scientists in Norway and Scotland allowed sheep to graze around one set of birch trees, and protected another. Nine summers later, they chopped down the trees, and analyzed how thick the rings were.
Contrary to the accepted belief, the climate wasn't the biggest influence on the tree ring thickness. While the ambient temperature still altered how large the ring was, grazing herbivores slimmed it down the most.
While this doesn't invalidate analyzing ancient climates through tree rings, it does make it more complicated, as you need to attempt to take into account numbers and types of domestic animals that may have affected the outcome.
Photo by Matt Huffman